handbook of combustion (online) || combustion synthesis

17Combustion SynthesisStefania Specchia, Elisabetta Finocchio, Guido Busca, and Vito Specchia

17.1Introduction

Since prehistoric times,mankind has employed exothermic reactions for its survival,from the burning of wood for warmth production to the preparation of food. Also, theenergy from exothermic processes has been used to modify the properties ofmaterials: it was discovered more than ten thousand years ago that heating a pieceof clay in fire would convert it into a ceramic item with remarkably differentproperties and potential use. After all these centuries, modern technologists havedeveloped techniques that allow them to sinter net-shape bodies consolidated frompowders in furnaces to produce, for instance, ceramic shields aimed at protectingspacecraft. For as different as these applications can appear, they rely on a commonprinciple: application of external heat to rearrange chemical bonds and shift thematerial properties in the desired direction. Considering that the rearrangement ofatomic bonds can release significant amounts of energy itself, it is attractive to usethis intrinsic energy directly to produce valuable materials.

The synthesis of solids with desired structures, composition, and properties hasbeen and continues to represent a challenge for chemists, materials scientists, andengineers. The formation of solids by the ceramic method is controlled by thediffusion of atoms and ionic species through reactants and products and conse-quently it requires several successive steps such as grinding, pelletizing, andcalcination of reactants (e.g., to synthesize oxides and carbonates) for longer dura-tions (with respect to so-called soft chemical routes) at high temperatures. Recently,several attempts have beenmade to eliminate the diffusion control issues entailed bysolid synthesis by using various innovative synthesis strategies. Among these,methods based on �combustion synthesis� have attracted remarkable interest. Theseprocesses make use of highly exothermic redox chemical reactions between metalsand non-metals, the metathetical reaction between reactive compounds, or theyinvolve redox compounds and mixtures.

The term �combustion� covers awide range of processes, fromflaming (gas phase)to smoldering (heterogeneous) and explosive reactions. Numerous technologically

Handbook of Combustion Vol.5: New TechnologiesEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

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useful oxides –more than 1000 to date (refractory oxides, magnetic, dielectric, semi-conducting, insulators, catalysts, sensors, phosphors, etc.) – and non-oxidematerials(carbides, borides, silicides, nitrides, etc.) have been prepared [1, 2].

Significant interest has been generated by the �combustion synthesis� (CS) ofmaterials because among their advantages there are low energy requirements andshort synthesis times. Since this process entails reaching high temperatures, onlythermodynamically stable phases can be prepared, yet high heating and cooling ratescan provide the potential conditions for the production of metastable materials withunique properties. In addition, owing to intensive volatilization of impurities at thevery high temperatures in the reacting system, the products of combustion synthesisare frequently even purer than the starting reaction mixture.

In CS, the exothermicity of the redox chemical reaction is used to produce usefulmaterials [3]. Depending upon the nature of the reactants, that is, elements orcompounds (solid, liquid, or gas), and the exothermicity, CS is usually described asself-propagating high temperature synthesis (SHS), low-temperature combustionsynthesis (LCS), solution combustion synthesis (SCS), flame synthesis (FS), gel-combustion (GC), sol–gel combustion (SGC), emulsion combustion (EC), andvolume combustion (VC).

CS means the synthesis of compounds in a wave of chemical reaction (combus-tion) that propagates over a starting reactive mixture owing to layer-by-layer heattransfer. Practically, it is an exothermic redox process in which the reaction betweentwo or more solid reactants or gaseous and condensed reactants takes place in a self-sustaining regime leading to the formation of solid products of a higher value [3]. It iswidely well-known in catalysis that nanosized catalysts possess extremely highactivity and selectivity [4, 5].

Among several production methods – such as hydrothermal sol–gel method [6],high-energy milling [7], the citrates method [8], reactive grinding [9], freeze dry-ing [10], and copolymer synthesis [11] – CS allows effective low-cost production ofnanomaterials with the desired phase compositions thanks to its relative mediumheating temperatures (350–600 �C), fast heating rates, and short reaction times, withthe advantages of:

1) use of relatively simple equipment;2) use of relatively cheap reactants (like nitrates);3) exothermic, fast, and self-sustaining reaction;4) formation of high purity productswith various size and shape, tunable, to a certain

extent, with the synthesis conditions;5) adaptability to covering processes on various structured substrates via in situ CS.

Notably, thehigh temperatures reachedduring the exothermic step ofCSpurge thepowders of any volatile impurities adsorbed or present in the reactants, affording verypure products. Remarkably, the high temperature gradients, combined with rapidcooling rates in the combustion wave, may form uniquemicrostructures that are notpossible to achieve by conventional methods of powder metallurgy.

Historically, SHS was firstly described in 1967 by Merzhanov, Skhiro, andBorovinskaya [12]: they outlined self-sustaining reactions used to synthesize many

440j 17 Combustion Synthesis

ceramic and intermetallicmaterials. From the early 1970s up today, SCS has receivedlot of attention due to its economical features and ease of up-scaling for industrialnanomaterials production. In 1998 the Institute of Structural Macrokinetics andMaterials Science (ISMAN) from the Russian Academic of Science founded theInternational Journal of Self-propagating High-temperature Synthesis (from 2007 man-aged by Springer), which is completely dedicated to such a process, covering both thefundamentals of SHS processes (chemistry and technology of SHS products andadvanced materials) and related fields (kinetics and thermodynamics of high-temperature chemical reactions, combustion theory and related modeling, applica-tions of SHS products in engineering, and R&D work).

Several review articles on CS of nanomaterials illustrate the benefits and advan-tages of such a process and its recent trends [1, 2, 13–23].

17.2Theory

17.2.1Self-Propagating High Temperature Synthesis (SHS)

Synthesis of refractory materials, such as borides, carbides, nitrides, intermetallics,and composites, continues to be the main goal of SHS [1]. These materials can beprepared by igniting pellets of the respective metals and non-metals with a suitableheat source. Following ignition, the combustion reaction is self-propagating with anadiabatic temperature in the range 1200–2700 �C. In general, the chemical equationfor the elemental reaction can be written as:

mX þ nY !XmYn ð17:1Þwhere X can be Ti, Zr,Hf, V, Ta, Be, and Si and acts as fuel (metal) whereas Ycan be B,C, N, S, and Si and acts as oxidizer (non-metal).

The high ignition temperature required, above 1500 �C, can be attained by meansof laser radiation,with a resistance heating coil, an electric arc, and so on. Innovationsin the field of SHS are aimed especially at lowering the ignition temperatureand using as reactants metal oxides or halides instead of finely divided metalpowders.

Mechanical activation and field activation processes have been employed withthe aim to lower the ignition temperature. Interestingly, field-activated combustionsynthesis has been used to activate low enthalpy formation reactions [24] and it hasbeen shown [25] that the wave velocity can be enhanced by applying the field ina direction perpendicular to wave propagation, thus helping to complete thereaction and favoring a decrease in particle size of the product. It is generallyagreed that SHS processes are time- and energy-saving and often an increase inthe reactivity of the products is observed. All SHS processes yield porousmaterials, therefore novel techniques that combine it with densification are alsobeing considered.

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17.2.2Solid-State Metathesis (SSM)

While SHS reactions involve metals, non-metals, and oxides, SSM (solid-statemetathesis) is based on rapid, low-temperature-initiated solid-state exchange reac-tions between reactive halides with alkali metals [26]. Owing to the exothermicity ofthe process, temperatures above 1000 �C can be reached in about 300ms. Schemat-ically, the process can be summarized as follows:

MHx þ xANyðignitionÞ!MNz þ xAHþðx � y--zÞN ð17:2ÞwhereM¼metal, H¼halide, A¼ alkali metal, and N¼non-metal or metalloid. TheSSM reaction can be initiated either by mere mixing or grinding, or by using a hotfilament. Once initiation occurs –which is very often related to one of the precursorschanging phase or decomposing, thus enabling a greatly increased surface contact –the reaction becomes self-sustaining and rather high temperatures can be reached ina very short time.

A major limitation of SSM processes is the need for anhydrous halides, whichmust be handled in a dry box and stored under an inert atmosphere.

17.2.3Flame Synthesis (FS)

FS differs from the typical SHS process in that all the reactions take place in the gas-phase and form fine powders (often nanoscale as in carbon soot from hydrocarbonsflames, fumed silica, titania, etc.). The potential advantages of this method overnormal solid/solid and solid/liquid SHS processes are its continuous operatingmode, as opposed to the batch mode of the latter, and the higher products purity.

Such gas-phase combustion of FS has been used to prepare fine particle metalnitrides, carbides, borides, silicides, and refractory metals (Ti, Ta, Zr). Nanosizesilica, titania, and fullerenes have been prepared using the corresponding metalhalides and hydrogen/air or hydrocarbons/air flames [27–29].

17.2.4Solution Combustion Synthesis (SCS) of Oxide Materials using Redox Compoundsand Mixtures

For this process, a different approach is considered, with the view of producing eithersimple or complex oxidematerials.With the use of combustible precursors and redoxmixtures, low-temperature-initiated (<500 �C) reactions can occur, which are self-sustaining and yield voluminous fine oxide particles in a few minutes.

Compounds like (NH4)2Cr2O7, which contain both oxidizing (Cr2O72�) and

reducing (NH4þ ) groups, if properly ignited can decompose to yield voluminous

green Cr2O3 [1]. The exothermicity of the reaction is due to the oxidation of NH4þ to

molecular nitrogen and water by the dichromate ion, which is, in its turn, reduced to


Cr3þ . The smoldering (flameless) combustion causes the evolution of gases thatresult in the voluminous, fine Cr2O3 powder.

Other mixed oxides with spinel structure [30], ferrites [31], and cobaltites [32] havebeen prepared starting from similar precursors. Their exothermicity is, however, notenough to actually sustain the combustion reaction and an external heat source isneeded to complete the decomposition.

Other precursors containing a carboxylate anion, hydrazide, hydrazine, or hy-drazinium group were found [33] to ignite at temperatures in the range 120–350 �Cand to decompose, yielding fine particle, large surface area oxides. The intenseexothermicity was attributed to the oxidation by atmospheric oxygen of atoms groupsor molecules like COO�, N2H3

�, N2H4, present in the precursor, to form CO2, H2O,and N2. However, if the preparation of oxide materials starting from this class ofcompounds is an attractive option, some limitations must be accounted for: severaldays may be needed for the preparation and very small quantity of material can beattained. In addition, not all metals form complexes with the hydrazine carboxylateligand and consequently this method cannot be used to prepare high-temperaturesoxides such as chromites, alumina, and so on.

An alternative method to use combustible redox compounds relies on redoxoxidizer–fuel mixtures like gun-powder (KNO þ C þ S) or solid propellant(NH4ClO4 þ CTPB þ Al) that, if ignited, undergo self-propagating combustion.Several advanced materials, including aluminates [34], chromites [35], ferrites [36],alumina [37, 38], and zirconia [39–41], have been prepared via a SCS process that isvery appealing for its simplicity. Several useful materials, ranging from catalysts [42],phosphors [43], pigments [40], and refractories [44, 45], have been synthesized bymodifications of this technique.

At a practical level, synthesizing virtually any oxide powder via SCS involves arelatively simple procedure. As a first step, an aqueous solution containing suitablemetal salts and an organic molecule that can properly work as the fuel in the redoxmixture must be prepared. When brought to temperatures in the range 300–600 �C,the solution reaches ebullition, becomes dry and in a matter of minutes the mixtureignites, thus setting off a highly exothermic, self-sustaining and fast chemicalreaction, which results in a dry, usually crystalline, fine powder. Generally, nitratesare chosen as the metals precursors: not only are they fundamental for the method,the NO3

� groups being the oxidizing agents, but also their high solubility in waterallows a sufficiently high solution concentration. The fuel can be chosen amongvarious organic compounds, like urea, glycine, hydrazine, or precursors containing acarboxylate anion. Urea seems to be the most convenient fuel to be employed, giventhat it is cheap and readily available commercially. These fuels are a source of C andH, which on combustion form CO2 and H2O, releasing heat; moreover, they formcomplexes with the metal ions, facilitating homogeneous mixing of the cations insolution. The exothermicity of the redox reaction allows peak temperatures that varyfrom700 to 1500 �C [15, 46].Dependingupon the fuel used, thenature of combustiondiffers from flaming to non-flaming (smoldering) type.

What makes this process interesting for potential application on a larger scale isthat the energy (heat) necessary is basically provided by the exothermic reaction itself

17.2 Theory j443

and hardly any external supply is required. In fact, metal nitrates can simplydecompose upon calcinations into metal oxides, by mere heating to or above theirdecomposition temperature. Subsequently, the so-formed oxides may take part inother reactions to form other compounds. With such a procedure, though, acontinuous supply of heat from outside should be necessary to maintain the systemat the appropriate temperature, whereas the mixture of nitrates and organic mol-ecule, suitable to serve as fuel, can be ignited at a relatively low temperature and thefollowing reaction provides the heat required to complete the process. Anotheradvantage also lies in the very short residence at high temperature, which reduces theoccurrence of sintering. In addition, because the reagents are mixed in an aqueoussolution, the system possesses a good chemical homogeneity, which allows for quickreactions in all the processing mass.

SCS entails interesting advantages over other combustion-based methods:

1) being a solution process, it allows a very tight control over the homogeneity andthe stoichiometry of the reaction products;

2) it is possible, if desired, to incorporate impurity ions in the oxide hosts so as toprepare materials of industrial interest (pigments, phosphors, catalysts, etc.);

3) the process is fast and no special equipment is needed, whichmakes it preferableover SHS methods;

4) the rapidity of the method may allow the formation of metastable phases.

DuringSCSprocess,oncetheinitialexothermicreactionmixture is ignitedbymeansof an external thermal source (ignition time is on the order of a few minutes) a rapid(typically0.001–0.1ms�1)high-temperature (1000–3000 �C)reactionwavepropagatesthrough the heterogeneous mixture in a self-sustained manner (see Figure 17.1),leading to formation of the final products without involving any additional externalenergy [1–3].

The high combustion temperature associated with a SCS application is related tothe enthalpy change between the reactants and products [15, 16]. During the CSreaction there are three important temperature points (see Figure 17.2) that mayaffect the course of the reaction and final product properties: (i) initial temperature(T0), which is the average temperature of the solution reactant mixture before the

Figure 17.1 Typical explosive reaction during solution combustion synthesis (SCS) (a) and thespongy look of the formed powder (b) after synthesis. Oven set at 450 �C.


reaction is ignited in the propagation mode; (ii) ignition temperature (Ti), whichrepresents the point at which the CS reaction is dynamically activatedwithout furtherexternal heat supply; and (iii) the actual combustion temperature (Tc), which is themaximum temperature achieved normally under non-adiabatic conditions. Con-sidering an exothermic SCS reaction of a reactant mix, at an initial temperature T0,the reactants need to be heated from T0 to Ti.

Attention should be now focused on trying to understand the mechanismgoverning the SCS process and the role played by the fuel. Interestingly, differentfuels have been used for specific classes of oxidic materials [2]. They all seem to servemainly two purposes:

1) they are the source of carbon and hydrogen that, on combustion, form CO2 andwater and liberate heat during the reaction;

2) they can form complexes with the metal cations, thus enabling good mixinghomogeneity in solution.

Depending on the fuel used, the adiabatic effect can vary, aswell as thenature of thecombustion – flaming or smoldering (non-flaming). Urea has received most atten-tion, which is not surprising since it is readily available at a cheap price and has a veryhigh exothermicity. Research should now be directed towards a deeper understand-ing of how the nature of the fuel can influence the particle size and microstructure.What appears interesting is that by using precursors like glycine, or acetates,combustion can be controlled, to a certain extent, and the use of potentiallycarcinogenic hydrazine-based fuels can be avoided.

Based on propellant chemistry [47], CO, H2O, and N2 are the most stable productsof the CS reaction with respect to other theoretically acceptable combinations thatmight be considered, including the formation of nitrogen oxides, CO, and so forth.The overall SCS reaction by using, for example, ametal nitrate as oxidizer and glycine(CH2NH2COOH) as fuel, can be written as follow:

Figure 17.2 Temperature versus time during the SCS of ceria-zirconia powder; fuel-to-oxidizerratio W¼ 1; the effect of the oven temperature 450 and 600 �Con SCS is shown.

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MnðNO3Þn þ59nWC2H5O2Nþ 5

4nðW�1ÞO2

!MnOn=2 þ 109

nWCO2 þ 2518

nWH2Oþ 12n

�59Wþ 1

�N2 ð17:3Þ

where Mn is a n-valent metal and W is the so-called �elemental stoichiometriccoefficient,� or, less properly, fuel-to-oxidizer ratio. W is the ratio between the totalvalencies of fuel (glycine in the example) and the total valencies of oxidizers (nitrates).Under stoichiometric conditions,W is equal to 1: the initial mixture does not requireatmospheric oxygen for complete oxidation of fuel.W< 1 means oxidant-rich (or fuellean) conditions (oxygen isnot a reagent but a product),whereasW> 1 represents fuel-rich conditions. Increasing the amount of fuel,W> 1, leads to an increase of not onlythe heat release but also in the gas-phase production, which is an important factor incontrolling the product specific surface area [17] and the spongy morphology of theprimary obtained solid. Specifically, this scheme involves a reaction between fuel andsolid oxidizer, which usually are dissolved in water. The aqueous solution allowsmixing the reactants at the molecular level, thus permitting precise and uniformformulation of the desired composition on the nano-scale. The solution must beuniformly heated by an external energy source (e.g., an oven). After water evaporation,the temperature of the formed sol–gel viscous media rapidly increases, achievingignition temperature (Ti), at which reaction spontaneously initiates over the entirevolume, leading to the formation of solid product with the desired phase composition.The maximum combustion temperature (Tc) can be as high as 1500 �C, and processduration is around 10 s. The high reaction temperature ensures high product purityand crystallinity. This feature allows us to skip an additional step, high-temperatureproduct calcinations that typically follow the conventional sol–gel approach, to achievethe desired phase composition.Moreover, short process duration and the formation ofvarious gases during SCS inhibit particle size growth and favor synthesis of nano-sizepowders with high specific surface area. The characteristic phase transformations(melting or decomposition of oxidizer) taking place during SCS are typically respon-sible for the initiation (explosion) of rapid chemical reactions. The important role ofthe environment has been proved by many authors [48–51]. Even in the case ofstoichiometric solutions (W¼ 1: there is enough oxygen in the oxidizer for completefuel burn-out), the interaction with oxygen in air occurs at different stages of the SCSprocess. In general, a larger amount of gas-phase products leads to a smaller size(higher specific surface area) in synthesized solid particles. It is possible to control theprocess by changing the fuel-to-oxidizer ratio, or using complex fuels and/or addingeasily gasified inert precursors [e.g., ammonium nitrate NH4(NO3)]: combustionconditions depend, in fact, on the chemical nature of the reactive solution formed [2].For example, even if the systems have comparable energy for product formation, NH2

groups appear to have higher activity than the OH group, which in turn ismore activethan COOH. This explains why glycine, which contains the NH2 group, is a morereactive fuel than citric acid, which contains only OH and COOH groups [20].

The synthesis of the ceria-zirconia catalyst, starting from metal nitrates of Ce andzyrconyl and using glycine as fuel, is reported as an example. The overall reactions in


the general form can be written as:

CeðNO3Þ3 þ 2ZrOðNO3Þ2 þ349

WC2H5O2Nþ 172

ðW�1ÞO2

!CeO2 � 2ZrO2 þ 689

WCO2 þ 859

WH2Oþ�72

þ 179W

�N2 ð17:4Þ

The precursors and fuel, dosed in the stoichiometric amount, were dissolved indistilled water and the resulting solution, thoroughly stirred to ensure completedissolution of all reagents and high homogeneity, was then transferred in a ceramicdish and placed into an electric oven set at 450 �C. After water evaporation and asignificant increase in the systemviscosity, themixture frothed and swelled, until a fastand explosive reaction took off, and large amounts of gases evolved. The heat releasedin the fast reaction allowed formation of the ceria-zirconia powder as a foamystructure, which is easy to be crumbled. The whole process was over after 5–6min,but the time between the actual ignition and the end of the reaction was less than 10 s.Figure 17.1 shows the typical explosive reaction, characterized by attractive fire-works,during the synthesis of ceria-zirconia powder, togetherwith thefinal obtainedproduct.

By detecting the temperature versus time during the SCS process by means of athermocouple inserted in the ceramic dish within the starting solution, the mainsteps of the transformation described above, can be easily recognizable through T0,Ti, and Tc detection (Figure 17.2): at the beginning, an almost constant temperature,corresponding approximately to the boiling point of the solution, is reached; a rampof moderate slope can then be seen, probably due to the boiling point elevation withcontinuous water evaporation. When a thermal level sufficient to trigger the mainreaction is reached, a dramatic increase is recorded, resulting in a marked temper-ature peak. Notably, the peak temperature measured by the thermocouple does notcorrespond to the value actually reachedwithin the reacting system.Owing, in fact, tosome delay of the thermocouple signal in the case of a sudden change in the systemtemperature and to a not perfect contact of the thermocouple tip, especially aftercomplete water evaporation, the values recorded are significantly lower than thoseactually reached by the reacting mixture. Similar behavior has also been describedby other authors [15, 19, 49]. The effect of oven temperature on igniting the SCSreaction is visible in Figure 17.2: when synthesizing ceria-zirconia from a stoi-chiometric mixture, an increase in operating temperature of 200 �C causes thereacting system to reach ignition significantly earlier.

Figure 17.3, instead, focuses on the effect of fuel-to-oxidizer ratio: the bettercondition in terms of intimate contact throughout the reacting mass, between fueland oxidizer, is the stoichiometric one (W¼ 1). The farther from the stoichiometriccondition, the longer the timeneeded for ignition to occur. This happens especially infuel-lean conditions (W< 1), for the lower combustion rate of the fuel dispersed in thelarger oxidizer mass. However, notwithstanding the increased heat release potentialof the system, the ignition delay occurs to a lesser extent also in fuel-rich conditions(W> 1), most likely due to mass transfer control of the external oxygen needed tocomplete the fuel oxidation.

17.2 Theory j447

Ascanning electronmicroscopy (SEM) imageof theobtainedpowder (Figure17.4),with W¼ 1 and oven temperature set at 450 �C, shows the typical spongy structure,characterized by high porosity and elevated specific surface area (74.6m2 g�1).

The quality of oxide materials obtained by SCS is high enough to allow surfacestudies by transmission/absorption IR spectroscopy [52]. This technique is possibleusing self-supporting pressed disks of the powders if they are constituted by lowparticle size, thus giving rise to small light scattering. Figure 17.5 shows the FT-IRspectrum of a pressed disk of ceria-zirconia prepared by SCS.

Figure 17.3 Temperature versus time during SCS of ceria-zirconia with glycine; the effect of thefuel-to-oxidizer mol ratio (W) is also shown; oven temperature: 450 �C.

Figure 17.4 SEM image of ceria-zirconia powder obtained by SCS at 450 �C,W¼ 1.Measured BETarea: 74.6m2 g�1.


An �absorbance� as low as 0.3 is measured near 1200 cm�1, providing evidence ofthe very high transparency of the material. The small slope of the baseline – growingtowards higherwavenumbers, due toRayleigh scattering,whichdepends on the forthpower of radiation frequency and wavenumber as well as on the particle size of thepowder – is also a confirmation of how fine the powder is. Below 900 cm�1 theabsorbance of this sample grows sharply up to the cut-off of the radiation due toabsorption associated to the bulk metal–oxygen stretching modes of the solid. Thismay be better seen in curve (b) in Figure 17.5 from the spectrum of the powderdispersed in a KBr pressed disk (KBr is a bindingmaterial transparent tomedium-IRradiation). The skeletal spectrum,with themaximumat 410 cm�1 and the shoulder at650 cm�1, is typical of cubic ceria-zirconia [53] without any relevant presence of themonoclinic or tetragonal phases [54].

The spectrum of the pure ceria-zirconia disk shows a well-defined band with amain maximum at 3673 cm�1, a shoulder at 3710 cm�1, and further component at3649 cm�1, due to the OH stretching mode of surface hydroxy groups. The positionof these components corresponds to those of some OHs observed on zirconiapolymorphs (3670 cm�1 [52, 55]), and of OH groups observed on ceria (3710,3645 cm�1 [52]), suggesting that such hydroxy groups may be due to ZrOH andCeOH species. The spectrum found [56] is similar to that reported by Daturi et al. forCe0.5Zr0.5O2 [57]. The surface cationic sites may be characterized by FT-IR using theadsorption of probe molecules such as carbon monoxide [52]. In fact, CO adsorbs atlow temperature over surface metal sites and the position of its C–O stretchingreveals the nature (oxidation and coordination state) of surface metal centers.

The main band arising from CO adsorbed at low temperature on ceria-zirconia(Figure 17.6) is detected at 2163 cm�1, with an evident weak shoulder at 2184 cm�1.The maximum shifts continuously to higher frequencies upon outgassing at in-creasing temperature, showing, however, the existence of a third component near

Figure 17.5 FTIR spectra of: (a) pressed disk of pure ceria-zirconia powder after outgassing at500 �C; (b) KBr pressed disk of the same powder.

17.2 Theory j449

Figure 17.6 FTIR spectra of CO adsorbed on pure ceria-zirconia powder disk at low temperature(�133 �C) under outgassing in the range �133 to �73 �C.

2174 cm�1. The position of this band is typical of CO coordinated over electron-withdrawing cationic sites acting as Lewis acid sites, such asCe4þ andZr4þ ions. COadsorption on Zr4þ surface sites is likely responsible for the high frequencycomponent of the absorption. In fact, CO adsorption on zirconia usually gives riseto bands near 2175 and 2190 cm�1. Thus, the mainmaximum ismostly due to Ce4þ

carbonyls. CO adsorption on ceria gives rise to bands in this range [52].The effect of the fuel-to-oxidizer ratio has been widely studied in the literature. An

example canbegivenbyprevious results of the authors [58],where LaMnO3perovskitewas prepared by SCS, varying the fuel-to-oxidizer ratio and the fuel type. In particular,urea [CO(NH2)2], glycine (CH2NH2COOH), b-alanine [H2NCH2CH2COOH], andglycerol (CH2OHCHOHCH2OH) were employed, and the oven for the SCS was setto 350 �C. The overall reactions can be written, respectively, as follow:

LaðNO3Þ3 þMnðNO3Þ2 þ 4WCH4ON2 þ 6ðW�1ÞO2

! LaMnO3 þ 4WCO2 þ 8WH2Oþ�52þ 4W

�N2

ð17:5Þ

LaðNO3Þ3 þMnðNO3Þ2 þ83WC2H5O2Nþ 6ðW�1ÞO2

! LaMnO3 þ 163

WCO2 þ 203WH2Oþ

�52

þ 43W

�N2 ð17:6Þ

LaðNO3Þ3 þMnðNO3Þ2 þ85WC3H7O2Nþ 6ðW�1ÞO2

! LaMnO3 þ 245

WCO2 þ 285WH2Oþ

�52

þ 45W

�N2 ð17:7Þ

LaðNO3Þ3 þMnðNO3Þ2 þ127WC3H8O3 þ 6ðW�1ÞO2 !

! LaMnO3 þ 367

WCO2 þ 487

WH2Oþ 52N2 ð17:8Þ


Table 17.1 DH values of SCS reactions of LaMnO3 with different fuels at W¼ 1.

Fuel DH per molof LaMnO3

synthesized(kJ) mol�1

DH per unitreacting mass

(kJ kg�1)

Mass ratio of fuelto oxidizer

Mass ratio ofproduced gas

to total products

Urea �2176 �2925 0.476 0.675Glycine �2301 �3270 0.397 0.656b-alanine �2359 �3650 0.283 0.626Glycerol �2535 �3831 0.313 0.634

The syntheses were carried out maintaining constant the ratio between nitrates(oxidizers) (equal to 1, corresponding to the desired LaMnO3 compound) while thefuel-to-oxidizers ratio (W) was varied from 0.5 to 3. The different fuels considered areexpected to affect the reaction based on the different enthalpic release theiruse entails: Table 17.1 shows the DH values corresponding to the SCS reactionswith the four considered fuels.

Depending on W, though, differences in the combustion synthesis regimes wereobserved: a relatively slow and basically flameless reaction at very lowW (smolderingcombustion synthesis), and an extremely fast reaction (explosive-type) occurringnearly simultaneously in the whole reacting mass under stoichiometric and slightlyfuel-rich conditions. Finally, atW> 1.5, features recalling a typical SHS behavior canbe seen, characterized by the reaction initiated locally and propagating as a com-bustionwave in a self-sustainedmanner through the reaction volume. XRD analyses,(Table 17.2) showed the formation of LaMnO3 (generally obtained in its orthorhom-bic form) over a rather wide range of operating conditions, but the degree ofcrystallization was higher for aW close to 1, except for urea. Very fuel-lean mixturesoften furnished an amorphous phase, whereas at higher values of W traces oflanthanum oxynitrate LaONO3 were found for glycine, b-alanine, and glycerol.

By using urea in stoichiometric conditions instead, no visible reaction took placefor LaMnO3. Non-stoichiometric conditions were needed for a proper combustionsynthesis reaction. A fuel-to-oxidizer ratio larger than 2, at an oven temperature of500 �C, proved to be effective.

Based on the results obtained with XRD analysis, it is possible to build-up themapof the operating conditions at which LaMnO3 is formed (see Figure 17.7 for glycine).This kind of map is typical for each synthesized material and fuel used. Figure 17.7shows that at low fuel concentration no crystalline perovskite was formed, and thathigher oven temperatures led to easier achievement of a perovskite crystal structure.However, in case of glycine, when too high a concentration of organic fuel was used acarbonaceous residue remained after the synthesis. BETsurface area measurementswere made for most of these conditions. Table 17.3 shows some results: thesynthesized matter always has a rather good specific surface area. BET values forLaMnO3 were lower when urea was employed as organic fuel. Higher oven tem-peratures seem to have a positive effect on BETarea, at least for not too highW values,but the gain is not dramatic. It is thought that this small BET increase is due to a faster

17.2 Theory j451

Table 17.2 XRDphase characterization of LaMnO3 powders prepared by SCSwith different fuels, atdifferent W values; oven temperature: 350 �C.

Fuel W Phases present (XRD)

Urea 0.5 Amorphous1 Amorphous1.5 Amorphous2 Amorphous (traces of LaMnO3)2.5 LaMnO3

3 LaMnO3

Glycine 0.5 Amorphous0.7 Amorphous (traces of LaMnO3)0.9 LaMnO3

1 LaMnO3, traces of LaONO3

1.1 Weakly crystallized (LaMnO3, LaONO3)1.5 Weakly crystallized (LaONO3, LaMnO3)2 Weakly crystallized, carbonaceous residues

b-Alanine 0.5 Amorphous0.9 LaMnO3

1 LaMnO3

1.1 LaMnO3

1.5 Weakly crystallized (LaMnO3, LaONO3)2 Weakly crystallized, carbonaceous residues

Glycerol 0.5 LaMnO3

0.9 LaMnO3

1 LaMnO3, traces of LaONO3

1.1 LaMnO3, some LaONO3

1.5 Weakly crystallized (LaONO3, LaMnO3)2 Weakly crystallized, carbonaceous residues

Figure 17.7 Operating conditions map [oven temperature versus fuel-to-oxidizers ratio (W)] forLaMnO3 perovskite synthesis by SCS with glycine.


reaction rate, which promotes the nucleation of grains with respect to their growth.BETvariationwithW for the various fuels seems to depend on two contrasting effects:(i) the increase in temperature with possible sintering outcome; (ii) a partialsegregation and subsequent charring of fuel during the reaction, which could bepartly responsible for the presence of residual carbon and the consequent BETenhancement. Therefore, as shown in Table 17.3, the BET trends with W may bealmost different depending on which of the two phenomena is prevailing. Moreover,TG tests coupled with MS carried out on SCS prepared materials showed that thosesynthesized from both very fuel-lean and very fuel-rich mixtures underwent asignificant weight loss upon heating. These results suggest that for fuel-leanmaterials there is incomplete oxidizers decomposition and confirmed for thefuel-rich ones the presence of residual carbon.

Figure 17.8 presents the characteristic microstructure of LaMnO3 powders pre-pared by SCS with glycine by varyingW: agglomerates involved mostly thin smoothflakes, whose surfacewas perforated by a large number of pores.WithW¼ 2, though,conglomerates appeared to be no longer thin flakes but waffles with a more complexhigh surface area network structure. Similar results were obtained using b-alanineand glycerol. In particular, it was interesting to observe how a greater reactionDH inthe case of glycine and b-alanine, with respect to urea, resulted in a more intenserelease of heat and how this more explosive-like process affected the average poresize.As shown inFigure 17.9, the use of urea reduced thepore size by about one order

Table 17.3 BET values of LaMnO3 powder synthesized by SCS by using different organic fuels, fuel-to oxidizer ratios (W), and oven temperatures.

Fuel BET specific surface area (m2 g�1)

W¼ 0.5 W¼ 1 W¼ 1.5 W¼ 2Oven T (�C)

350/500 (�C) 350/500 (�C) 350/500 (�C) 350/500 (�C)

Urea — 2.4/3.1 3.2/3.8 3.5/3.9Glycine 15.1/15.8 16.6/16.9 32.1/32.4 24.6/24.8b-Alanine 17.6/20.1 14.8/15.2 27.9/30.2 27.7/30.0Glycerol 15.8/16.0 9.6/10.1 9.9/10.2 16.3/16.5

Figure 17.8 SEM images of LaMnO3 powders synthesized by SCS at 350 �C with glycine atvarious W.

17.2 Theory j453

of magnitude. Figure 17.10, instead, shows a TEM image of LaMnO3 prepared withglycine at 350 �C, W¼ 1: perovskite crystals, with pores of nanometric dimensions,are clearly visible.

Combustion synthesis also allows the preparation of excellent catalysts such as, forexample, noblemetal containing complex catalysts. Very high dispersion of themetalcan be obtained. For example, the addition of Pd nitrate to the starting solution allowsthe production of very fine Pd clusters on the support:

LaðNO3Þ3 þMnðNO3Þ2 þ 2ZrOðNO3Þ2 þPdðNO3Þ2 þ569WC2H5O2Nþ 14ðW�1ÞO2

!PdðLaMnO3 � 2ZrO2Þþ 1129

WCO2 þ 1409

WH2Oþ�112

þ 289W

�N2 ð17:9Þ

SEM investigations revealed the presence of Pd clusters on LaMnO3�2ZrO2

(Figure 17.11). Also in this case, surface characterization can be performed byFT-IR spectroscopy of probe molecules. Figure 17.12 shows the spectrum of a purepowder pressed disk of a Pd/LaMnO3�2ZrO2 catalyst prepared by SCS, after out-gassing at 500 �C. Again the small particle size of the powder results in very hightransmission of the IR light (absorbance 0.12 at 1250 cm�1). Residual carbonates are

Figure 17.9 SEM images of LaMnO3 synthesized by SCS at 500 �C with urea; W¼ 2.5.

Figure 17.10 TEM image of LaMnO3 synthesized by SCS with glycine at 350 �C; W¼ 1.


responsible for the couple of bands at 1475 and 1397 cm�1, usually found underthese conditions also at the surface of LaMnO3 obtained with conventional prepara-tions (usually giving rise to poorly transmittant samples, due to high particlesize [59]), according to its strong basicity. The low temperature adsorption of CO(curve b and the inserted curve c in Figure 17.12) produces a band centered at2158 cm�1, mostly due to CO adsorbed on La3þ ions, and a weaker one at 2104 cm�1

typical of CO on top of highly dispersed zero-valent Pd metal centers. This resultconfirms the good dispersion of the noble metal obtained with the combustionsynthesis method.

Figure 17.11 Backscattered SEM image of Pd/LaMnO3�2ZrO2 powder synthesized by SCS; Pdclusters with diameter less than 100 nm are visible.

Figure 17.12 FT-IR spectra of: (a) pressed disk of Pd/LaMnO3�2ZrO2 powder after outgassing at500 �C; (b) following CO adsorption at low temperature; (c) enlargement of CO stretching region.

17.2 Theory j455

17.3Applications in Research

Natural gas (NG) combustion is a typical application widely used for energeticpurposes and for automotive traction, to reduce the emissions of the greenhouse gasCO2 compared to other fossil fuels. A typical SCS application in research is thedevelopment of catalysts for NG combustion; SCS is, in fact, a very interestingtechnique considering its simple adaptability for in situ catalysts deposition onstructured supports, as the outcome of engineering industrialized or semi-indus-trialized processes. Another appealing application, in the field of low-temperaturepolymer electrolyte membrane fuel cells (PEM FCs), is the almost complete removalof CO fromH2-rich gas resulting fromhydrocarbon fuels reforming. COpreferentialoxidation (CO-PROX), in fact, can lead to a reduction of the CO concentration in theH2-rich gas down to at least 10 ppmv or below; this enables the cleaned-up reformateto be fed directly to PEM FCs, thus avoiding the poison effects on Pt-based anodeelectrocatalysts and assuring a long life-time for PEM FCs stacks. Catalysts for CO-PROX reaction can be prepared and deposited by in situ SCS in microstructuredsystems for on-board vehicles applications.

An overview is nowgiven for the following applications, particularly studiedwithinour research group:

1) in situ SCS of Pd/LaMnO3 catalysts on honeycomb wall flow monoliths forcompressed NG (CNG) vehicles exhaust treatment;

2) in situ SCS of Pd/CeO2�2ZrO2 catalysts onfibermetal burners forNG combustionin domestic boilers.

3) in situ SCS of Pt/Al2O3-3A zeolite catalysts on metallic micro-channel plates forCO preferential oxidation in fuel processor applications.

17.3.1In Situ SCS of Pd/LaMnO3 Catalysts for Compressed NG (CNG) Vehicles ExhaustTreatment

The transition to NG is an integral part of the ongoing process of decarbonizationof fuels. The concept of NG as an automotive fuel began around 1930. Researchhas proved that it can be used safely. Many countries are known to use CNG asan automotive fuel. These include USA, Canada, UK, Italy, Thailand, Iran, Australia,and New Zealand. In the 1980s, other Asian and South American countriesembarked upon CNG programs, namely, India, Bangladesh, Indonesia, Pakistan,and Argentina.

Information on available engines and equipment shows that with suitable im-provements in emissions it is possible to meet the most stringent future regulatoryrequirements. In many countries the overall costs of NGV operation, includingcapital, maintenance, and fuel, are much less than the total cost of runningconventionally fuelled vehicles. Payback periods, sometimes less than two years,will vary with local situation – especially when favorable government fuel taxes andfinancial incentives are available.


Although NG is widely recognized as the cleanest hydrocarbon fuel, and CNGengines have been found to lead to very low pollutant emissions, unburned CH4 isharder to oxidize than gasoline-derived unconverted HCs. The strong greenhousepotential of CH4 (more than an order of magnitude higher than that of CO2) forcesabatement of CH4 emissions fromNGengines [60]. Catalytic combustion of CH4 onhoneycomb converters similar to those used for the treatment of gasoline engineexhausts is the way to go.

Particular demands are imposed on the catalysts for CH4 combustion: they mustresist thermal and mechanical shocks and exhibit high activity, which is not trivialowing to the high stability of CH4 molecule and the low temperature of the exhaustsof CNG vehicles (seldom exceeding 500 �C). Commercial catalysts are mostly basedon c-Al2O3-supported Pd catalysts [60], having a three times higher noble metalloading than conventional three-way catalysts. A possible solution is represented bynanostructured Pd-perovskite catalysts with lower Pd content than conventionalones, deposited by �in situ SCS� over ceramic honeycombs to produce reliablecatalytic converters.

Experimentally, a layer of c-Al2O3, thermally stabilized with 10% by weight ofLa2O3, was deposited on cordierite monoliths (Corning honeycombs with a celldensity of about 600 cpsi; length of 40mm; diameter of 20mm) via an in situ SCStechnique. Themonoliths were repeatedly dipped in an aqueous solution containingthe suitable amount of precursors, Al(NO3)3�9H2O and La(NO3)3�6H2O, and urea.Subsequently, the monoliths were placed in an electric oven set at 500 �C, where thesynthesis reaction was completed within a few minutes. Several deposition cycleswere repeated until a load of 25% by weight with respect to the original monolithweight was achieved. Calcination for 2 h in calm air at 700 �C stabilized the obtainedcoating layer.

The LaMnO3 active phase was added by SCS as well, by dipping again the Al2O3/La2O3-layered monolith in a solution containing La(NO3)3�6H2O, Mn(NO3)2, andurea. After impregnation with the perovskite-precursors solution, the monolith wasplaced again in the oven set at 500 �C and subsequently calcined at 700 �C for 2 h incalm air to improve crystallization of the LaMnO3 phase. The cycle was repeated untila loading of 15% by weight with respect to the Al2O3/La2O3 layer was attained.

Palladiumwasthendepositedonmonolithsbydippingit intoanaqueoussolutionofPd(NO3)2, which was further diluted so as to eventually have a 1% or a 2% by weightfinal Pd load with respect to the Al2O3/La2O3 layer. The monoliths were then kept at500 �Ctopromote stabilizationofPdand the formationoffinelydispersedPdclusters.Calcination at 700 �C for 2 h was repeated at the end of the Pd-deposition step.

The layer adhesion on the channel walls was verified as excellent throughultrasonic bath: the overall catalyst loss was less than 1% in terms of catalyst weight.Physicochemical characterization was performed on sections of the catalytic mono-lith; in particular, SEM-EDAX analyses revealed the morphology and composition ofthe deposited catalytic layer: as shown in Figure 17.13, a very homogeneous and thinlayer of catalyst was deposited on the cordierite channels walls, with a very spongystructure, likewise the powder catalysts; a sign that SCS is easily reproducible overstructured supports. EDAX analyses confirmed the presence of the LaMnO3 phase.

17.3 Applications in Research j457

Catalytic combustion experiments were carried out in a steel reactor heated in asplit tube furnace with a heated length of 60 cm. Homogeneous reaction tests werealso carried out with the reactor equipped with a bare monolith to evaluate the onsettemperature of gas-phase reactions. The catalytic monolith was sandwiched betweentwo mullite ceramic foam disks. A thermocouple, inserted along one of the centralchannels, was used to control the temperaturewithin themonolith. The gaseousflowrates were regulated bymass flow controllers. Lean inlet conditions were fixed (0.4%CH4, 10% O2, N2 balance), to simulate the exhaust gases of a CNG vehicle. Thetemperature gradient produced in the axial direction was not significant. The GHSVspace velocity was set to 10 000 h�1. Figure 17.14 shows a scheme of the reactorcontaining the catalytic converter. The composition of the product stream in terms ofCH4, CO2, CO and O2 was monitored by continuous analyzers. Figure 17.15 showsthe main obtained results, in terms of CH4 conversion versus temperature: allLaMnO3/c-Al2O3-La2O3 supported catalysts significantly shifted the combustion

Figure 17.13 SEM images of cordierite monolith with LaMnO3 deposited by in situ SCS: front viewof the channels (a); LaMnO3 layer on the channel wall (b).

Figure 17.14 Scheme of the reactor employed to test the catalytic monolith for CNG emissionsabatement and detail of the reactor with the catalytic converter.


temperature range towards temperature values more than 350 �C lower than thosetypical of the bare monolith combustion. The half-conversion temperature wasreached at 450 �C for the catalyzed monolith without Pd, compared with 750 �Cachieved for the bare monolith. Interestingly, the addition of 2 wt% Pd to c-Al2O3/La2O3-LaMnO3 shifted the results back to 350 �C, in agreement with several reportedstudies dealing withmechanistic details of the superior activity of Pd catalyst [60–62].Even more interestingly, the combination of half-the-above amount of Pd withLaMnO3produces an equivalent performance, whichhelps in reducing catalyst costs.

17.3.2In Situ SCS of Pd/CeO2�2ZrO2 Catalysts for NG Combustion in Domestic Boilers

In accordance with modern climate policy (e.g., the Kyoto protocol [63]), theincreasing use of NG in all applications requiring a source of thermal energy haspromoted a widespread effort to develop new highly efficient and clean combustionappliances [60, 64]. In the light of numerous stringent European regulations beingproposed or adopted as regards NOx and CO emissions from domestic appliances, agreater penetration of low-NOx burners into the global boilers market is expected.

In the last decade pre-mixed combustion within porous media has been the objectof extensive experimental and theoretical research [58, 64–66], especially in the lightof its remarkable potential in enhancing the efficiency of heat transfer and reducingthe impact on the environment related to pollutant emissions: CO2 and unburnedhydrocarbons (HC) arewell known as greenhouse gases, while CO andNOx are toxic,even in very small concentrations.

What makes this technology potentially preferable over free flame combustion isthe possibility of attaining amore intense heat exchange by radiation and conductionfrom the solid burner to the heat sink, with a consequent increase in the overallthermal efficiency and a reduction of flame temperature, which enables a more

Figure 17.15 Methane conversion versus temperature over different catalysts deposited oncordierite monolith (10 000 h�1 GHSV; 0.4% CH4, 10% O2, N2 balance).


limited production of thermal NOx. This technology entails further advantages, suchas the possibility of a more compact design of the heat exchanger and combustionchamber, a very large flexibility in hosting the burner inside the appliance (since theflame is controlled by fuel gasmomentum rather than natural convection, the burnercan fire upwards, downwards, sideways, and so on), lower air excess levels, and thecapability of being shaped in a way that properly fits any required geometry. In thistechnology the fuel/airmixture is fed to a porousmetallic panel, where combustion isignited by sparking and depending on the operating conditions – basically specificfeed flow rate, excess of air with respect to the stoichiometry, structure of thecombustion chamber – the flame front can be stabilized at the balance positionwhere the laminar flame speed equals the supplied gas speed. Such premixedburners have found application as retrofit for fire-tube boilers [67], as heatingelements for process heaters in the petroleum industry [68], for refinery storagetank heaters, and for commercial warm-air furnaces [69]. More recent applicationsare in paper drying, annealing furnaces of glass industry, baking furnaces in the foodindustry, and kitchen stoves [70].

A major goal and challenge for modern NG burners for domestic boiler applica-tions is a wide power modulation range that can satisfy the demands of differentusers, ranging from hot sanitary water production, which requires on average 25 kWper apartment, down to the 2–3 kW necessary for heating purposes of medium-sizeapartments with efficient thermal insulation. At the same time, this would entail adecrease in the number of start-up and switch-off cycles, which can cause high energyloss, significant CO emissions, and material stresses due to thermal shock. In thiscontext, notwithstanding the great advantage coming with the low NOx emissionsover the large modulation range, non-catalytic premixed burners generally sufferfrom high and almost unacceptable CO and HC emissions at low Q values (e.g.,200–400 kWm�2, which correspond to the lowest power values of the modulationrange); the comparatively low flame temperatures occurring in the �weak� radiantregime significantly affect the completeness of methane combustion.

Performance of the radiant premixed burners could be improved by adoptingperovskite-based catalysts, which are attractive because of their low cost, thermo-chemical stability at comparatively high temperature (900–1100 �C), and catalyticactivity [71]: such catalysts increase the fuel flow rate fraction burnt within or justdownstream the burner deck, thus maximizing the heat fraction transferred byradiation, cooling the flame temperature, and improving the combustion complete-ness with lower CO and unburned hydrocarbon level.

For such a purpose, a deposition technique based on in situ SCS has beendeveloped for application of the catalyst on metal fiber burners. Based on the resultspreviously obtained on powders [60, 64, 72], the most promising Pd/CeO2�2ZrO2

catalyst was employed. For its deposition on the FrCrAlloy fiber burner, the optimumoperating procedure to guarantee both good adherence of the catalytic layer to themetallic surface and sufficiently high specific surface area of the deposited catalystlayer was determined; the rapidity and low cost characteristics of the SCS route werepreserved, too. First, the FrCrAlloy supportswere kept at 1200 �C for 10minunderO2

flow (0.5 vol.% in N2) so as to favor regular growth on the fiber surface of a-Al2O3


grains into a uniform protective layer, moreover characterized by an external surfacemorphology able to ensure a good adherence of the catalytic phase to be deposited onthe metallic mat [73]. Subsequently, starting from a solution containing Ce(NO3)3,ZrO(NO3)2, and glycine, an in situ spray-pyrolysis SCS was adopted to develop thecatalyst on the metal fiber panels made of FeCrAlloy, and therefore produce catalyticburners. The aqueous solution of precursors was sprayed over the surface of theFeCrAlloy panels, previously heated at 400 �C. Owing to in situ pyrolysis SCSoccurring on the hot panel�s surface, catalyst formation was obtained. The panelswere then returned to the hot oven to stabilize the coating. The spray deposition cyclewas repeated several times to achieve the desired catalyst load (namely, 2% w/w).Finally, for further stabilization and complete crystallization of the catalytic phase, theburners were calcined at 900 �C for 2 h in still air. The Pd deposition was performedbymeans of further spray-pyrolysis runs over the ceria/zirconia layer, by employing adiluted Pd(NO3)2 aqueous solution. Calcination in air at 600 �C followed, to promotefull decomposition of Pd(NO3)2 into the oxidized form, PdO. Figure 17.16 shows thein situ spray-pyrolysis SCS process on the FrCrAlloy panels.

To check the effectiveness of the in situ spray-pyrolysis SCS-based depositiontechnique, SEM analyses were carried out on the as-prepared burners to evaluate theadherence quality of the catalytic layer and to verify whether the highly porous

Figure 17.16 In situ SCS spray-pyrolysis of Pd/CeO2�2ZrO2 catalyst on FrCeAlloy fiber mats:during synthesis in the oven (a); after the calcination step (b).


morphology observed on catalytic powders was maintained after the deposition onthe metal fibers. As revealed in Figure 17.17, a highly corrugated and porous catalystlayer was formed, assuring an optimum gas–solid interaction for the heterogeneouscatalysis. In Figure 17.17c, the core of the FeCrAl alloy fiber can be easily distin-guished from the outer CeO2�2ZrO2 layer, the thickness of which is about 2–3 mm.

Tests under realistic operating conditions were performed on a partially modifiedcommercial condensing boiler rig for domestic application, mounting a cylindricalcatalytic burner (production of hot water for sanitary and heating purposes). A bareburner was also tested as a reference counterpart to assess the effectiveness ofcatalyzed burner. CH4was fed to amodulating electrovalve, able to vary its volumetricflow rate (max power output: 30 kW). Air coming from a blower wasmixed with CH4

in a Venturi positioned so that a proper mixing was achieved before entering theburner. The cylindrical fiber-mat burner, fitted horizontally in the combustionchamber, fired through the heat exchanger coils. The burner diameter was approx-imately 6 cm.A schemeof the combustion chamber used for the experimental runs isrepresented in Figure 17.18, together with a picture of the catalytic burner. Tests were

Figure 17.17 (a)–(c) SEM images of FeCrAlloy fibers catalyzed with Pd/CeO2�2ZrO2 byin situ SCS spray-pyrolysis; (c) a catalytic layer approximately 2–3mm thick is appreciable(fiber cross-section).

Figure 17.18 Scheme of the domestic boiler employed to test the activity of the catalyticmetal fiberburner (cylindrical geometry) and details of the catalytic burner.


carried out over a wide range of operating conditions by varying the nominal power(Q) from 10 to 28 kWand the air excess (Ea) from 2 to 50% (i.e., l from 1.02 to 1.50).A K-type thermocouple was used to measure the temperature on the downstreamsurface of the burner deck. The flue gas composition (O2, CO2, CO, and NO) wasmonitored by means of a multiple gas continuous analyzer.

Figure 17.19a shows the CO concentrations attained in the flue gases for bothburners. When the air-to-fuel ratio approaches stoichiometric condition, the non-catalytically assisted combustion is strongly penalized, given that the reduced oxygen

Figure 17.19 (a) CO and (b) NO emissions (dry gases) versus excess of air (Ea) at two differentheating values and for both bare and catalyzed burner.


partial pressure can be a limiting factor for the conversion of CO into CO2, while inthe presence of the catalyst those unacceptable carbonmonoxide emissions could belowered significantly. Independently from the power input, at low air excess valuesthe catalytic burner was able to reduce CO concentration to values close to 100 ppmv,whereas its bare counterpart produced CO at concentration of more than 50% ascompared to bare counterpart. The beneficial effect of the catalyst was slightly lessevident at higher air excess values and higher power output. Considering the NOemissions (Figure 17.19b), the contributions of the catalyst to the combustion wasless evident, independent of the air excess and power output: the NO emissions fromthe catalytic burner were only slightly lower than those of the bare counterpart.

With the catalyst promoting the complete oxidation of CH4, an increased com-bustion heat can be released within the porous medium, thus enhancing its radiantoutput. As a consequence, a reduced amount of heat is carried away by convectionfrom the �cold� flue gases, with a further rise of the system overall thermal efficiency.If the catalyst can stabilize the combustion reactions deeper inside the metal fibermat, the gas phase is cooled by heat exchange with the porous medium that radiatesmore intensively. Thermal NOx emissions, generated according to the Zeldovichmechanism [74], are expected to be lower under those conditions corresponding tolower flame temperatures.

17.3.3In Situ SCS of Pt/Al2O3-3A Zeolite Catalysts on Metallic Micro-Channel Platesfor CO Preferential Oxidation in Fuel Processors

Fuel processors (FPs) are a viable alternative for clean energy generation [75, 76]. Atpresent, the variety of potentially commercial applications for FCs technology rangesfrom portable/micro-power and transportation to stationary power for buildings anddistributed generation [76, 77]. In particular, PEM FCs possess a series of profitablefeatures that make them leading candidates for mobile power applications (auxiliarypower units – APUs – for vehicles) or for small stationary power units (SPUs): lowoperating temperature, sustained operation at high current density, low weight,compactness, long stack life, fast start-ups, and suitability to discontinuous opera-tion. The ideal fuel for low temperature PEM FCs is pure H2, with less than 10 ppmCO, as dictated by the poisoning limit of the Pt anodic electrocatalyst. Size, weight,and actual costs of PEM FCs, lack of H2 production plants and distribution nets,together with technical limitations to store H2, make it difficult to stock up on thevehicle the necessary H2mass. Therefore, H2-rich gas will likely be generated on siteand ondemand, by reforming available gaseous/liquid hydrocarbon fuels, owing alsoto the existing fuel storage capacity and distribution nets [76]. When H2 is generatedfromHC fuels, a complex systemof production by reforming and reformate clean-up(called fuel processor unit, FPU) is necessary, to feed the PEM FCwith a CO-free H2-rich gas. Depending on the reformer process (reaction with steam, autothermalreforming, catalytic partial oxidation, cracking), the operating conditions, and thetype of fuel, the outlet stream from the reformer contains 3–10% of CO, which isfurther converted into H2 and CO2 via the water-gas shift reaction. The sequence of


these two catalytic processes produce a H2-rich stream containing 40–65% H2,0.5–1% CO, 15–20% CO2, 15–30% H2O, and 0–25% N2. The CO concentration isfurther reduced to less than 10 ppm in a CO clean-up system, via the CO-PROXreaction or CO selective methanation (CO-SMET).

The CO clean-up for small-scale applications is usually carried out by catalyticreaction systems; alternative technologies such as pressure or temperature swingadsorption, normally used in industrial-scale plants, add too much space to FPUs.Microstructured plate reactor technology has been tested and experimentally eval-uated because it has several advantages over conventional technology such as fixedcatalyst beds –lower pressure drop and enhanced heat and mass transfer arecharacteristic features of microstructured reactors [78]. When the microstructuredmetal sheets heat exchangers is coatedwith catalyst, the heat generated by exothermicreactions may in fact be removed very effectively, thereby improving the thermalmanagement of the reactor [79].

Pt-Al2O3 catalysts have been studied extensively and found very active towards COoxidation [80, 81]; Pt-supported zeolite catalysts were proposed thanks to theirenhanced selectivity [82]. Previous studies of our group showed that the bestperformance, in terms of catalyst activity and selectivity, together with an almostcompleteCOremoval in the presence of at least 50 timesH2 concentration, comparedto the CO one, was obtained with a Pt-3A zeolite based catalyst, thanks to the specificstructure of 3A zeolite support [83]. A catalyst for CO-PROX reaction consisting of 2%Pt over a mixed support 3A zeolite/c-alumina was prepared and deposited onmicro-channeled metallic plates via a slightly modified in situ SCS technique. The goal wasto realize a microstructured reactor for fuel processor applications. The main aim ofresearch in the field of fuel processors is to reach, over a sufficient wide temperaturerange (suitable for the CO-PROX reactor control), the almost complete conversion ofCO, with at the same time complete utilization of O2 added to the reactor; the lattercondition is required to be sure to employ the lowest O2 excess rate, which in anycase is essential for ensuring the desired CO removal, thus maintaining as low aspossible the H2 oxidation losses. Anyway, the limitation of H2 oxidation is not themandatory issue in the CO-PROX step, if compared with the compulsory require-ment of avoiding the CO poisoning effect on the low-temperature PEM FCelectrocatalyst.

A test reactor has been designed for development of the catalyst coating tech-nique [84]. The microreactor was composed of a casing for the metal plates stack,heated by six electrical cartridges and sealed by graphite gaskets. Feed and productgas temperaturesweremeasured by thermocouples introduced in tubes sealed at oneend by welding and placed at the inlet and outlet of the plates stack. An equal flowdistribution in the stack of plates was achieved by a specific inlet diffuser [85]. Theplates (50� 50� 1mm) had 49 channels each (0.26mm width). The reactor wasequipped with a six plates stack. Figure 17.20 shows the microreactor and plates.

Before depositing the catalyst on the microchannel plates, the latter were washedfor 30min with a solution of K2Cr2O7, H2SO4, and distilled water to eliminatepossible organic contamination on the plate surfaces during machining; they thenwere then rinsed in distilled water. Afterwards, the precursors solution/suspension


was prepared: Al(NO)3�9H2O, urea, andPt(NH3)4(NO3)2 (asAl2O3 andPt precursors)were mixed in an aqueous solution, then commercial 3A zeolite fine particles,K12[(AlO2)12(SiO4)12]�H2O, were suspended in the solution, to add the thirdcatalyst component. Alumina/Pt precursors and zeolite were dosed to obtain asupport composed of 50wt% zeolite and 50wt% c-alumina, and 2wt% of Pt onthe 50-50wt% mixed support. After a few minutes stirring the suspension on aheating plate, to ensure proper homogeneity, the suspension was deposited insidethe microchannels of six plates stacked together with clamps by using the �infusionpump� method (i.e., by filling the plate channels with a syringe). Any excess of thesuspension was eliminated by spraying with compressed air at low flow rate. The sixplates were subsequently transferred into an oven kept at the constant temperature of500 �C, until the fast and highly exothermic SCS reaction took place, to allowformation of the catalytic layer on the microchannel plates. The whole processextended over 5–6min, but the time between actual ignition and the end of reactionwas less than 10 s. Finally, compressed air was used to remove excess catalyst not wellstuck onto themicrochannel walls [84]. The deposition procedure was repeated untila catalyst load of 0.5mg cm�2 was achieved. SEM investigations (Figure 17.20c) onthe catalyzed microchannel plates revealed that the inner surface of the micro-channels was covered homogeneously and the channel ridges were clean of catalyst.

The as-prepared catalyticmicrochannel plateswere then tested in themicroreactor.The operating conditions for catalytic activity tests on the microreactor were asfollows: GHSV¼ 2500 h�1; synthetic feed mixture: 1 vol.% CO, 2 vol.% O2, 18 vol.%CO2, 5 vol.% H2O, 37 vol.% H2, He balance (O2 to CO feed ratio l¼ 2O2/CO¼ 4).The obtained results, shown in Figure 17.21, proved that the performance of thestructured catalyst deposited into microchannel was quite satisfactory: the almostcomplete CO conversion (CO residual concentration less than 2 ppmv) was achievedin the temperature range 149–167 �C, with a contemporary complete O2 conversionand a selectivity of 25%. The roughly 20 �Crangewhere complete CO conversionwasobtained proved to be sufficiently wide to assure an optimal control and regulation ofthe microreactor.

Figure 17.20 Microreactor for CO-PROX tests (a); metal microchannel plates (b); SEM picture ofthe catalyzed microchannel plates (c).


17.4Outlook

In situ SCS has been employed successfully for the synthesis of Pd-perovskite-basedand Pd/Pt-oxide-based catalysts for NG combustion in three different applications:abatement of HC/CO emissions from CNG fuelled vehicles, completion of NGcombustion inhousehold appliances, and almost completeCO removal fromH2-richgas stream for PEM FCs applications.

The main steps for the preparation of powdered noble metal/mixed oxides-basedcatalysts via SCS can be summarized as follow:

1) preparation of a solution containing the precursors of the metal nitrates togetherwith the sacrificial organic fuel (based on the proper stoichiometry); a slightheating up to 50–60 �C could be necessary to favor the complete and fastdissolution of the compounds;

2) precursor solution pre-heating in an oven at 300–500 �C in calm air to ignite andfavor the achievement of the powdered catalytic material;

3) soft grinding of the so-obtained powder;4) powder stabilization through calcination in air;5) eventual addition of noble metals via incipient wetness impregnation, if not done

directly at point 1., by adding the noble metal nitrates directly in the precursorssolution.

SCS offers interesting advantages:

1) the preparationmethod is extremely fast; in less than 4 h it is possible to obtain thedesired catalysts;

Figure 17.21 CO and O2 conversion and selectivity versus temperature for 2% Pt/(alumina þ zeolite) catalyst tested in the microchannel reactor.

17.4 Outlook j467

2) energetic costs for the synthesis are limited thanks to the energy given by theinternal oxidation of the organic fuels used as sacrificial agents; commercially, theagents are quite cheap (e.g., urea);

3) use of a simple oven to reach the solution ignition temperature saves energy;4) the procedure is easily adaptable to direct deposition of catalytic materials on

structured supports as ceramic or metallic monoliths, foams, tissues, mattresses,and so on, making easy its industrialization. Such a direct deposition method isusually called in situ SCS. In fact, once prepared, the precursors solution can bedeposited onto the structured supports by infusion, immersion (when micro-channels are present), or by spraying (when a rough surface, or a mattress, ispresent). The catalytic layer strictly anchored to the support can be easily obtainedby placing the infused/immersed/sprayed support in an oven to start up theexothermic synthesis reactions.

In situ SCS is easily industrialized, thanks to its main advantages described above.The main steps for the preparation of structured catalysts via in situ SCS can besummarized as follows:

1) formetal supports: support washing and degreasing with acid solution (K2Cr2O7/H2SO4 or C2Cl4 solution), followed by rinsing in distilled water to eliminatepossible organic contamination on the plates surface during machining; thispoint is not necessary for ceramic supports.

2) preparation of a solution containing the precursors of the metal nitrates togetherwith the sacrificial organic fuel (based on the proper stoichiometry); a slightheating up to 50–60 �Ccould be necessary to favor complete and fast dissolution ofthe compounds;

3) precursors solution deposition inside/over the microstructured supports byinfusion or immersion or spray methods;

4) elimination of excess solution by spraying with compressed air at low flow rate;5) catalyst layer development by SCS of the impregnated supports in oven at

300–500 �C;6) spraying with compressed air to eliminate the excess not well stuck to the catalyst;7) catalytic layer stabilization through calcination in air;8) repeated deposition procedure to reach the requested catalyst load;9) eventual addition of noble metals via incipient wetness impregnation, if not done

directly at point 1., by adding the noble metal nitrates directly in the precursorssolution.

A series of continuous conveyor belts, ovens, and spraying or infusion nozzles canbe in fact designed to realize a continuous industrial process.

In view of the rapidity of in situ SCS method for structured catalysts preparationand of its relatively low cost, in terms of starting materials and energetic expense,such a technique represents a very promising and cost-effective alternative to moretraditional processes for catalytic systems preparation proposed in the recent past,such as deep coating or wash-coating [86, 87]. For these procedures, in fact, thedeposition of an additional c-alumina layer, before the catalyticmaterial deposition, is


necessary to favor anchoring of the latter on the support, thereby requiringthe preparation of the slurry catalyst/c-alumina before the deposition cycles (usually,this means at least 48 h of wet-tumbling to assure proper homogeneity and viscosityof the slurry).

A drawback of this technique is that formation of NOx is possible during thesynthesis [88]: metal nitrates can undergo a partial thermal oxidation just before themain reaction is ignited, thus releasing nitrogen oxides, and organic fuels containingnitrogen atoms, like glycine and b-alanine, can decompose, generating NOx. Ap-parently, as the process is very fast, little time is given for thermal decompositionprior to ignition, and only small amounts of NOx are expected to be released.Doubtless, NOx emissions can, anyhow, become an issue when scaling up theprocess to the industrial level. In that case a small NOx abatement reactor by selectivecatalytic reduction with ammonia might be envisaged.

17.5Summary

Combustion synthesis (CS) is becoming one of themost important ways to produce awide range of advanced porous ceramic, metallic materials and nanostructuredcatalysts. Among various possibilities ofCS, the solution combustion synthesis (SCS)technique is an attractive alternative for the production of particular materialscompared to the more conventional and expensive processes. SCS is, in fact,characterized by exothermic, fast, and self-sustaining reactions, formation of highpurity products with various size and shape, relatively easy procedures, use ofrelatively simple equipment, and cheap reactants. Thanks to these main character-istics, SCS is easily tunable to complex systems to produce directly in situ structuredcatalysts. Three successful examples of in situ SCS have been reported:

1) A series of experimental tests on ad hoc preparedCNGexhaust gas after-treatmentconverters have demonstrated a superior activity towards CH4 conversion bycoupling the effects of lanthanum manganite and Pd active species. This shouldentail a reduction of the overall catalyst costs compared to conventional Pd-onlybased catalysts currently employed on CNG fuelled vehicles.

2) A series of experimental tests on ad hoc prepared catalytic premixed burnersfor household applications have displayed the lower environmental impact mainlyin terms of CO, compared to the bare counterpart, when a ceria-zirconiacatalyst with Pd as active phase was properly deposited over the burner. Thecatalytic burner was able, in fact, to stabilize the combustion process withinthe porous medium to a greater extent, thus maximizing the heat fraction trans-ferred by radiation (which resulted in an increased overall thermal efficiency),cooling theflame temperature (with a beneficial effect onNOx concentrations), andenhancing the degree of completeness of CH4 combustion (lower CO emissions).

3) A series of experimental runs on an ad hoc prepared catalytic microchannel platesreactor for CO-PROX as the final step of CO removal fromH2-rich gas streams forPEM FCs applications were satisfactorily carried out: starting from a synthetic

17.5 Summary j469

reformate gas (1 vol.% CO, 2 vol.% O2, 18 vol.% CO2, 5 vol.% H2O, 37 vol.% H2,He balance), the catalytic reactor performed almost complete CO removal(CO residual concentration less than 2 ppmv) in the temperature range149–167 �C, with a concomitant complete O2 conversion and a selectivity valueof 25%. The obtained temperature range was sufficiently wide to assure optimalcontrol and regulation of the microreactor.

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