handbook of heterogeneous catalysis (online) || catalytic combustion

11.6.1 Introduction 2411

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11.6Catalytic Combustion

Pio Forzatti∗, Gianpiero Groppi, and Cinzia Cristiani

11.6.1Introduction

Catalytic combustion can achieve stable and effectivecombustion over a wider concentration range and at lowertemperatures than in conventional burners, based on gas-phase radical chemistry. Because of this it is commonlyused for the abatement of gaseous emissions in manymanufacturing and energy-conversion processes.

During the past few decades, catalytic combustion hasalso been explored as a primary control method in theproduction of heat and energy with ultra-low emissions ofNOx , CO, and unburned hydrocarbons (UHC). Catalyticcombustion in gas turbines (GT) has attracted particularinterest, and today such turbines represent the preferredenergy conversion technology in medium- and large-scale power stations. Likewise, major opportunities areforeseen in the small-scale distributed power generationsector.

NOx emissions are a major concern of GT systems.In the United States, emission regulations are basedmainly on air quality standards fixed by federal laws.Although NOx standards are now usually attained, NOx

is deemed to be a precursor of ozone, the standardsfor which are not agreed in several areas. In order tocope with these problems, a permit strategy is appliedby local authorities which requires the adoption of a BestAvailable Control Technology (BACT) in attainment areas,or a Lowest Achievable Emission Rate (LAER) technologyin non-attainment areas, typically in combination withemission fee/credit systems. A similar regulation systemis currently applied in the European Union (EU).

To date, several NOx control technologies for GTs havebeen developed, based on both primary and secondarymethods. Dry low-NOx (DLN) burners based on leanpremixed combustion guarantee NOx emission levels of

References see page 2424∗ Corresponding author.

2412 11.6 Catalytic Combustion

between 20 and 25 ppm, though several GT manufactur-ers claim that the system can be designed and operateddown to 9–10 ppm. Any further reduction may be pre-cluded by flame-stability problems. In order to meet themost stringent emission regulations, many installationsinclude a selective catalytic reduction (SCR) unit whichallows a further reduction in NOx emission levels.

Catalytic combustion has been demonstrated, on acommercial basis, to reduce NOx emissions below3 ppm while maintaining CO and UHC emissions below10 ppm, without the need for expensive exhaust clean-up systems. In addition, a catalytic combustor reducestypical DLN problems such as risk of blow-out and flameinstability.

A cost analysis commissioned by DOE [1] consid-ered GT of small (5 MW), medium (25 MW) and large(150 MW) sizes, and compared the following technolo-gies: water/steam injection; DLN; conventional, low-Tand high-T SCR; SCONOX (a secondary control methodbased on NOx adsorbers); and catalytic combustion. Theresults clearly indicate the economic advantage of primarymethods, including catalytic combustion as opposed tosecondary clean-up measures (SCR and SCONOX). Thedisparity of cost impact is particularly large for small-scalegas turbines that are deemed most suitable for the dis-tributed generation market which, in turn, is threatenedby the most strict environmental regulations.

Although the potential of catalytic combustion has beenrecognized for more than 30 years, only recently has thistechnology been proven commercially viable and finallymarketed, albeit to a limited level.

This chapter provides a review of the status and ofthe perspectives of catalytic combustion for GTs. Initially,details of the base concepts associated with the systemrequirements are presented, after which the design ap-proaches based on lean and rich catalytic combustion areillustrated, together with reports of performances demon-strated in full-scale and field tests. Details of the mostrelevant characteristics of PdO-supported catalysts and

of transition metal-substituted hexaaluminates that havebeen most extensively considered for lean combustionapplications are outlined, along with those of the no-ble metal catalysts adopted in rich combustion systems.Finally, brief details are provided of the use of mathe-matical modeling as a tool for the design and analysisof catalytic combustors, and the perspectives for thistechnology outlined.

11.6.2Base Concepts and System Requirements

As illustrated in Fig. 1, in a conventional system a fractionof the air delivered by the compressor is mixed with thefuel, typically natural gas. The mixture is then combustedin a flame, such that the hot gas produced expands anddrives the turbine.

Flame stability requires adiabatic combustion temper-atures to be as high as 1600–1800 ◦C, but these mustbe reduced to 1100–1450 ◦C, by means of cooling bypassair, before the hot compressed gas is delivered to theturbine, in order to avoid damage to the inlet blades. Atsuch temperatures, and within the tens of millisecondsresidence time required for complete burn-out of fuel andCO, significant amounts of NOx are produced, mostly viathe Zeldovich thermal mechanism [2].

In a catalytic burner, the combustion is ignitedand stabilized under ultra-lean conditions, which resultin adiabatic temperatures close to those allowed fordelivering the hot compressed gas to the turbine. Thus,the need for bypass air is minimized and the formation ofthermal NOx is almost prevented due to an absence of ahot combustion zone. The reduction in NOx emission hasbeen reported as being even larger than expected from thelower combustion temperature if a significant fraction ofthe fuel is oxidized on the catalyst surface [3]. This effecthas been attributed either to a reduction in the formationof prompt NOx , in view of the decrease in CH radicalsin the gas phase due to fuel complete oxidation on the

Fuel Fuel

300–400 °C 300–400 °CHC

By pass CC

Air AirExhaust Exhaust

Output Output

1100–1400 °C1100–1400 °C

Catalyst segments1600–1800 °C

C T C T

(a) Flame combustion (b) Catalytic combustion

Fig. 1 Comparison between conventional (a) and catalytic (b) gas-turbine systems. HC: homogeneous combustor, CC: catalyticcombustor, C: compressor, T: turbine.

11.6.3 Design Approaches 2413

catalyst surface [4], or to a reduction in the formation ofNOx due to the release of H2O produced at the catalystsurface. Indeed, a beneficial effect of increased H2Oconcentration on the reduction of NOx emission undertypical GT operating conditions has been reported [5].

The operating constraints of GT systems (Table 1)pose severe requirements to the catalytic combustor. Airis delivered by the compressor at temperatures whichtypically range from 300 ◦C to 450 ◦C, depending on loadconditions and the nominal pressure ratio of the machine.

Upon accurate fuel/air mixing, ignition must occurreadily and fuel conversion should proceed rapidly tocompletion, while the gas reactants heat up to theadiabatic combustion temperature. Due to high gasvelocity (10–30 ms−1 at the combustor inlet) and sizeconstraints of the combustion chamber, the process mustbe completed within a few tens of milliseconds, while theoverall pressure drop (including mixing) must be keptbelow 5% of the turbine inlet pressure in order to preventsignificant energy efficiency losses.

These characteristics of GT operations result in thefollowing requirements:

• Highly active catalysts able to ignite the combustionof natural gas at temperatures as close as possible tothose at the compressor discharge must be employed.Indeed, to fill the gap between compressor dischargetemperature and catalyst ignition temperature, ahomogeneous preburner is needed, but this mayproduce significant amounts of NOx .

• Materials with high thermal stability able to hindercatalyst deactivation by sintering, phase transformationand volatilization, as well as to secure mechanicalintegrity upon thermal shocks, are required. In fact,strong temperature excursions are experienced duringstart-up and shut-downs, and particularly during theload trip of the turbine. To prevent overspeeding anddestruction of the turbine in this case, the fuel feed is

Tab. 1 Design criteria and operating conditionsof gas turbine combustors

Design criteria

Emission targets NOx < 5 ppmCO < 10 ppm

UHC < 10 ppmPressure drop <5%Catalyst durability 8000 h

Operating conditions

Inlet temperature 300–450 ◦COutlet temperature 1100–1400 ◦CPressure 1–2 MPaMass flow rate 100–200 kg m−2 s−1

Residence time 10–30 ms

immediately shut off while air continues to flow; thisresults in a temperature decrease of several hundreddegrees in less than 1 s. Under typical GT operationsa lifetime of the catalyst section in excess of over8000 h must be guaranteed; this would correspond toa yearly replacement during the scheduled inspectionof hot combustor parts. Thus, catalyst stability againstpoisoning by air-borne and fuel-borne contaminantsmust also be considered.

• Ignition of gas-phase reactions is needed in orderto secure complete fuel conversion and CO burnoutwithin the imposed residence time constraints. Theonset of mass transfer limitation would preventcomplete fuel conversion and CO burnout in thepresence of catalytic reactions only, unless the reactoris largely oversized.

• The design of a structured catalyst configuration able tocope with mass transfer and pressure drop constraintsis required.

11.6.3Design Approaches

Different design concepts have been proposed to matchthe severe requirements of catalytic combustors. Amajor classification criterion is based on the combustionstoichiometry of the catalyst section, as this has adominant effect on the selection of catalytic materialsand on the operating characteristics of the combustor.In the following, only those configurations based onlean catalytic combustion will be described; the peculiarcharacteristics of rich catalytic combustion will bedescribed later.

11.6.3.1 Fully Catalytic CombustorThe first design concept was an attempt to fully exploitthe potential of catalytic combustion by completing theprocess in a single catalyst section. In such configuration,a preheated, premixed fuel air stream is fed to thecatalyst section. Ignition occurs at the catalyst walls, whichrapidly achieve the adiabatic reaction temperature, and thereaction rate is controlled by gas–solid mass transfer ofthe fuel. The heat released at the catalyst surface results ina progressive increase of the temperature in gas phase, andeventually causes ignition of homogeneous combustion,allowing for rapid fuel burnout within or immediatelyafter the catalyst section. One major problem with thisconcept is the development of catalytic materials able towithstand that thermal stresses resulting from operationat temperatures slightly higher than those required atthe turbine inlet. In addition, complete fuel conversion is

References see page 2424


difficult to achieve in catalyst sections of reasonable sizeat high flow velocities characteristics of modern GTs [6].Research efforts which continued for almost two decadesfailed to yield satisfactory results [6–10] and accordingly,in view of the trend towards higher turbine inlettemperatures, this concept was abandoned. Subsequently,novel hybrid design approaches were pursued, the aimbeing to keep the temperature of the catalyst section wellbelow that of the exit combustor.

11.6.3.2 Fuel StagingOne method of controlling the catalyst temperature is toreduce fuel concentration and, consequently, the adiabaticreaction temperature. This can be achieved by splittingthe fuel feed, partly to the catalyst and partly to adownstream homogeneous section where combustion iscompleted, being stabilized by the hot gas stream fromthe catalyst section. This fuel staging concept was firstdeveloped by Toshiba Corp. [11, 12], and by CRIEPI andKansai Electric [13, 14] in Japan. The fuel–air ratio fedto the catalyst was adjusted to keep the adiabatic reactiontemperature below 1000 ◦C, which was considered as athreshold limit to prevent catalyst deterioration. Specificefforts were devoted to optimize mixing of the fuel beingfed to the downstream homogeneous section with thehot stream coming from the catalyst; such mixing iscritical to avoid NOx formation according to thermal andprompt mechanisms. For this purpose, in addition toa specific design of the catalyst section and of the fueldistribution system, air staging was adopted. Typically,65% of the overall air flow was fed to the catalyst, 30%to the homogeneous section through fuel–air premixingnozzles, and 5% for cooling and leaking. The concept wasproven up to a scale equivalent to one combustor of a10-MW multi-can type GT. At base load conditions, NOx

emission below 5 ppm (@ 16% O2) and CO and UHC

<10 ppm were obtained with a pressure drop of less than4% [15].

However, the need for careful control of the complexfuel–air distribution pattern remains a major drawbackof this design concept.

11.6.3.3 Partial Catalytic Hybrid CombustorCatalytica Energy System and Tanaka Kikinzoku KogyoKK have developed a configuration in which allthe fuel–air mixture required to achieve the desiredcombustor outlet temperature is fed to the catalystsection. Here, combustion proceeds only to partial fuelconversion (≈50%), and is completed in a downstreamhomogeneous section [16]. The catalyst wall temperatureis kept well below the adiabatic reaction level by means ofa proprietary catalyst design based on: (i) the temperatureself-regulation characteristics of PdO–Pd system in CH4combustion; (ii) the use of metal monolith supports withinternal heat-exchange capabilities between nearby activeand passive channels; and (iii) the use of a diffusionbarrier on the top of the active catalyst layer.

The peculiar features of the PdO−Pd catalyst will bediscussed later.

Metal monoliths with internal heat-exchange capabili-ties are obtained by assembling single-side coated flat andcorrugated sheets, as illustrated in Fig. 2 [17, 18]. Heatgenerated at the catalytic wall of the active channels isefficiently transmitted by conduction through the thinmetal foil, and then dissipated in the gas flow on boththe catalytic and the non-catalytic sides. This allows thewall temperature to be kept well below the adiabatic reac-tion temperature. The structure can be adjusted in orderto tune the fraction of active channels in the monolithcross-section, as shown in Fig. 2.

The deposition of an inert porous diffusion barrier onthe top of the catalyst layer can significantly hinder the

Passive channel

Passive channel Passive channel

Passive channelActive channel

Active channel Active channel

Active channel

Active coating

Metallicwall

(a) (c)

(b) (d)

Fig. 2 Examples of monoliths with internal heat exchange [18] and different fraction of active channel.

11.6.4 Commercialization Status and Perspectives 2415

Air

Fuel

Fuel

Pre-mix Homogeneouspost-combustion

1st catalyticstage

2nd catalyticstage

Pilotburner

Fig. 3 Schematic configuration of the XONON combustor.

rate of mass transfer of reactants to the catalyst surface,but at the same time only negligibly affect the rate ofheat transfer to the gas phase. The effect is equivalent tothat observed with fuels, such as higher hydrocarbons,the mass diffusivity of which in air is considerably lowerthan the thermal diffusivity of the fuel air mixture (Lewisnumber >1). Such unbalancing of heat- and mass-transferrates results in a significant decrease of the catalyst walltemperature.

A typical combustor configuration is shown schemat-ically in Fig. 3. The catalyst section consists of an inletstage with high activity, designed to minimize ignitiontemperature and to operate at low to medium wall temper-ature. There is also one or more subsequent stages whichare less active than the inlet stage, designed to operate atmedium-high temperature and to provide an outlet gastemperature sufficiently high to guarantee rapid ignitionof the gas-phase combustion in the downstream homo-geneous section, where fuel conversion is completed andthe gas stream heats up to the adiabatic combustion tem-perature. The use of flame-holders [19] or other meansof hydrodynamic flame stabilization [20] of homogeneouscombustion has also been proposed to secure a moreefficient burnout of CO and UHC within the limitedresidence time (10–15 ms).

The design features noted above result in the operatingwindow shown graphically in Fig. 4.

The inlet temperature must be high enough to allowrapid ignition of the catalytic combustion. The gapbetween compressor discharge and light-off temperatureis compensated by means of a homogeneous preburner.The adiabatic combustion temperature must be highenough to guarantee complete burnout of CO and UHCin the downstream homogeneous section, within a veryshort residence time. The upper bounds of the operatingwindow are fixed in order to prevent unacceptablecatalyst deterioration, which can occur either gradually bythermal ageing or catastrophically in the case of flashbackignition of gas-phase combustion within the catalystchannels. As the flow pattern in the monolith catalyst is

CO andUHC

emissions

Catalystoverheating

2nd stage

1st stage

Operating window

Inactive catalyst

Adiabatic temperature

Inle

t tem

pera

ture

Fig. 4 Operating window of the XONON catalytic combustionsystems [22].

segregated radially, operation within the window mustbe guaranteed at each cross-sectional location, whichstresses the requirement for very fine fuel–air mixingand a uniform gas temperature distribution at the catalystinlet [21, 22].

The partial catalytic hybrid combustor concept has beendeveloped to a commercial stage by Catalytica EnergySystem, with the trademark of XONON cool combustion.The present status and perspectives with respect tocommercialization are described in the following section.

11.6.4Commercialization Status and Perspectives

Following extensive sub-scale and full-scale, full-pressuretesting [23–25], a Kawasaki M1A-13A gas turbine of 1.4MWe was fitted with a XONON combustor. A RAMD(Reliability, Availability, Mantainability, Durability) testcampaign was carried out at Silicon Valley Power in SantaClara, California. The test was completed in June 2001



and the following project objectives were successfullyachieved: an average emission of NOx < 2.5 ppm (@15%O2) was maintained for more than 8000 h with COand UHC emissions at full load below 6 and 4 ppm,respectively. Emission levels were certified within theEnvironmental Technology Verification (ETV) programof the USA Environmental Protection Agency [26].Commercial levels of GT unit overall availability (timein operation/total test period) of 90.5% and a good(98.5%) catalyst reliability [time in operation/(total testperiod – time due to scheduled shut down or to otherfailure of the machine)] were obtained.

A second 8000-h RAMD campaign performed withthe same machine was concluded in May 2003 [27, 28].Design and material improvements allowed loweremission levels (1.2 ppm average NOx ; average COand UHC < 1 ppm), a higher system overall availability(94.5%), and 100% reliability of the catalyst module.

Following the successful outcome of these RAMDcampaigns, two commercial installations of KawasakiM1A-13X GTs equipped with XONON combustiontechnology are currently under operation: at SonomaDevelopmental Center in Elridge (CA) since November2002, and at San Luis Obispo (CA) since December2003. In the second case, the system was assembledin a cogenerative configuration package with a 23.4%(1.42 MWe) electrical efficiency and a 53.7% heatefficiency at 15 ◦C air inlet temperature [29].

The application of XONON technology is currentlyunder investigation on a General Electric GE10-1 singlecan machine rated at 11 MWe, a size well suited to small-scale distributed generation facilities [30]. At full load, thismachine operates at a fairly high adiabatic combustiontemperature (1350 ◦C), with a nominal pressure ratioof 16 which guarantees a reasonable electric efficiencyof 30.8%. In January 2005, a 50-h campaign, including20 start-up runs, was performed at the Nuovo Pignonemanufacturing site in Firenze, Italy with a machineconnected to the electric grid. A reliable, robust andrepeatable control logic of the turbine start-up sequencewas defined. At load conditions, the system has proven tobe fairly operable, with NOx emission @ 15% O2 below7 ppm in the 50 to 75% load range, below 5 ppm in the75 to 90% load range, below 2.5 ppm in the 90 to 100%load range, and with CO and UHC emissions @ 15% O2below 9 ppm in the 50 to 100% load range. The diffusivepreheating burner was responsible for significant NOx

formation at low-medium load, while NOx emission afterpreburner flame-out were always below 0.5 ppm over theentire load range. Although significant NOx reduction canbe achieved by optimization of the preburner, emphasishas been given to the need of a further decrease inthe light-off temperature, particularly upon prolongedageing. Besides, oversensitivity to fluctuations of local

fuel–air ratios at the catalyst inlet was reported. Atemperature spread in excess of 150 ◦C at the catalystoutlet was observed, despite fluctuations below ±5%being guaranteed by the multiple venturi mixer. Thismay affect operability, as some of the temperaturemeasurements closely approached the trip limits.

At a more preliminary stage, the XONON technologywas also tested under the nominal conditions of a large105 MWe GE MS9001E GT [21, 31]. Other testing activi-ties to be performed in the frame of the Advanced TurbineSystem funded by Department of Energy are described inRef. [32].

In addition, R&D programs based on the hybrid con-cept have been carried out by other GT manufacturers asSiemens Westinghouse [33] and Alstom Power Technol-ogy [34].

The following comments summarize the main con-clusions from bench-scale and field tests, together withresults from commercial operations.

A technology based on the catalytic combustion of nat-ural gas under lean conditions has been commercializedfor a small 1.5 MW Kawasaki machine, and this may findniche market applications. Promising indications of com-mercial operability have also been obtained for a GeneralElectrics single can engine GE10-1, a 10 MW-class GT.Considering the disparity in cost impact between catalyticcombustion and secondary clean-up technologies (SCR,SCONOX) in small-scale systems, these results providegood market perspectives in distributed power generation.

With regards to the application of catalytic combustionto large-sized GT units within centralized power stations,two main specific concerns must be considered:

• The design, development and operation issues associ-ated with the integration of a catalytic combustor into aGT are certainly more demanding for a large multi-canengine than for a single-can engine.

• Firing temperatures in excess of 1400 ◦C, which havebeen already reached in the most advanced largeGTs [35], pose serious risks of catalyst overheating,considering that a substantial fraction of the fuel mustbe oxidized within the catalyst.

11.6.5Fuel-Rich Catalytic Combustion

After having been explored since the pioneering yearsof catalytic combustion as a method to reduce fuel NOx

formation [3] and to process different liquid fuels [36],air staging design concept based on fuel-rich catalyticcombustion has recently attracted a novel interestfollowing the results obtained by Precision CombustionInc. (PCI). The base configuration of the rich catalytic lean

11.6.5 Fuel-Rich Catalytic Combustion 2417

Air

Fuel

Catalyticreactor

Mixer

Pre-mix

Homogeneouscombustor

Fig. 5 Base configuration of the rich catalytic lean (RCL) burn technology [38].

(RCL) burn technology developed by PCI is illustrated inFig. 5.

Air from the compressor is split into two streams:(i) primary air is premixed with the fuel and then fed tothe catalyst which is operated under O2-defect conditions;(ii) secondary air is used first as a catalyst cooling streamand then mixed with the partially converted stream fromthe catalyst in a downstream homogeneous section wherethe ignition of gas-phase combustion occurs and completefuel burn-out is readily achieved. The control of catalysttemperature below 1000 ◦C is achieved by means of O2starvation to the catalyst surface, which allows the releaseof reaction heat to be controlled by the mass transfer rateof O2 in the fuel-rich stream, and of backside cooling ofthe catalyst with secondary air. In order to handle bothprocesses, a catalyst/heat exchanger module has beendeveloped; this comprises a bundle of small tubes that arecoated externally with an active catalyst layer, and with thecooling air and fuel-rich stream flowing in the tube andin the shell side, respectively [37].

Stabilization of the gas-phase combustion in thedownstream homogeneous section can be achieved bya thermal effect, associated with heat released in thecatalyst section and/or by a chemical effect associatedwith H2 production under fuel-rich conditions. The lowercombustion temperature, allowed by stabilization effects,results in a significant reduction in NOx emissions,while CO and UHC are maintained at ultra-low-level.However, to minimize NOx emission, the homogeneousignition delay downstream of the catalyst section shouldbe long enough to allow effective premixing of secondarycombustion air with catalyst effluent, so as to minimizelocal over-temperatures. The relative importance ofthermal and chemical stabilization effects, as well as theignition delay, are mainly controlled by the temperatureand composition of the catalyst effluent which, in turn,can be tuned by catalyst design. According to Ref. [38], infull-pressure tests the operation of the fuel-rich catalystwas selective to full oxidation products (CO2 and H2O)and thermal stabilization was preferred.

The RCL burn technology has been tested at nominalconditions of a single (of twelve) injector of a 7.2 MWeT70 Solar turbines machine with 1121 ◦C turbine inlettemperature, 17 MPa nominal pressure at full load.

Two configurations have been designed: a catalytic pilotburner, which replaces the existing diffusion flame orpartially premixed pilot of the DLN combustor [39]; or afull catalytic burner [38].

The catalytic pilot burner processes only a fractionof the fuel, and is targeted to retrofitting applicationswith minor combustor modifications. Test results indicatethat in order to achieve an effective stabilization of thehomogeneous combustion, 18–20% of the fuel/air mustbe processed in the catalytic pilot, which is a muchhigher fraction than the typical 2–5% processed in aconventional pilot burner. Under such conditions, thetest results demonstrated single-digit (<5 ppm @ 15%O2)emissions of NOx and CO with low acoustics have beenobtained at 50% and 100% load conditions.

In the full catalytic burner, all of the fuel is processedwithin a RCL burn module which replaces a conventionalpremixer/swirler arrangement in the DLN combustor.This configuration requires major design modificationof the combustion with respect to the pilot burner, buthas provided better emission performances: NOx , CO andUHC emissions below 3 ppm, 10 ppm, and 2 ppm @ 15%O2, respectively, with negligible combustion acoustics(<0.15% peak-to-peak oscillation of mean combustorpressure).

The RCL burn technology was also adjusted to a SaturnT1200 engine, nominally rated at 750 kWe. Four parallelRCL burn injectors were installed to a modified single cancombustor. An engine start-up procedure was developedbased on a state-of-the-art control logic. A homogeneouspreburner was used to ignite the catalyst at idle conditions,and this was then turned-off and the catalyst remainedignited at any investigated load conditions. The engine wastested up to 60% load, providing NOx and CO emissionsbelow 2.5 ppm and 10 ppm @ 15% O2, respectively, withUHC below 3 ppm.

Results obtained in full-pressure tests demonstrated thefollowing advantages of rich fuel over lean fuel catalyticcombustion:

Low light-off temperature. Operation under deficientO2 exhibits a higher catalytic oxidation rate than under anexcess of O2, which in turn results in a lower ignition



temperature. Besides, much wider ignition/extinctionhysteresis is observed, and upon light-off the catalystremains ignited down to temperatures well below thepractical range of relevance to GT operation (down to215 ◦C in the case of the low-pressure Saturn T1200).Accordingly, no homogeneous preburner is needed atsteady state during partial and full load operation.

No risk of flashback in the catalyst section, thanks to O2

depletion in the fuel-rich stream and an absence of fuelin backside cooling air.

Tolerance to high turbine firing temperature and topoor fuel–air mixing. For a given geometry of the catalystmodule, and with a fixed air flow rate and air stagingratio, the catalyst temperature was proven to be ratherinsensitive to the overall fuel–air ratio (i.e., adiabaticcombustion temperature) of the combustor [38]. Thereason for this is that the heat release and heat removaldepend mainly on the gas–solid mass flow rate of O2

in the catalyst side and the air flow rate in the back-cooling side, respectively, and scarcely depend on fuelconcentration.

Fuel flexibility. Sub-scale tests at 1 MPa proved theviability of RCL technology to natural gas, landfill gas(70% CH4, 30% CO2), refinery gas (70% CH4, 30% H2),prevaporized diesel n ◦2 and gasoline, or blast furnacegas with a low heating value [40]. This was achievedwithout significant design changes, as O2 mass transferrate scarcely depend on the fuel type.

11.6.6Catalytic Materials

Structured catalysts are used in GT applications becauseof severe pressure drop constraints combined withrequirements of rapid rates of gas–solid mass transfer.They consist of a honeycomb substrate which provides therequired geometric and mechanical characteristics, andof an active washcoat layer which ensures high activityand appropriate morphological properties.

11.6.6.1 Honeycomb SubstrateAlthough extruded ceramic honeycombs were extensivelyinvestigated in the earlier stages of their develop-ment [6, 9], their use has been progressively abandoned,due mainly to inadequate resistance against thermalshocks. Most ceramics will fracture during the suddentemperature drop associated with the fuel cut-off duringturbine trips.

On the other hand, following the development of hybridcombustor configurations which prevent operation of thecatalyst module at temperatures above 900–1000 ◦C,the major drawback of metallic monoliths – that is, thelimited maximum operating temperature – has been

overcome. Accordingly, honeycombs made from metalfoils have been adopted in GT catalytic combustors, inview of their excellent thermal shock resistance andthermal conductivity properties [9].

MeCrAl (with Me = Fe, Ni) alloys doped with smallamounts of Y (<0.1%, w/w) or other reactive elements(e.g., Zr, Hf) have been adopted for the fabrication ofmetal monoliths. These alloys segregate a stable α-Al2O3

scale upon calcination at 900–1100 ◦C by migration ofthe Al content (4–6%, w/w) to the surface. The α-Al2O3

scale protects the alloy substrate from oxidation at hightemperature and humid conditions, and also providesadhesion for the active washcoat layer. Stacked patternsof corrugated sheets are interlayered with flat sheets toavoid nesting, and are then assembled into spiral rollsor columnar packing to form the honeycomb structure.Herringbone corrugation patterns can be used to avoidnesting, without need of flat spacers and to provide sometortuosity to the flow pattern [19, 41].

In order to obtain adjacent active and inactive channelswhich provide internal heat exchange capability (seeSection 11.6.3.3), the metal foils are generally coated ona single side with the active catalyst layer. Deposition ofthe active layer is typically obtained by using sprayingmethods [18, 42–44].

11.6.6.2 Active Catalyst LayerThe combustion catalysts that have been most exten-sively investigated for GT applications, based on leancombustion concepts, are PdO-based systems and metal-substituted hexaaluminates.

11.6.6.2.1 PdO-Based Catalysts Supported palladiumoxide is the catalyst of choice for GT combustors fuelledby natural gas, in view of the following properties:

• Maximum activity in CH4 combustion, which resultsin minimum light-off temperature

• Unique temperature self-regulation features associatedwith the reversible PdO−Pd transformation

• Good thermal stability.

Such key features are strongly interconnected via thecomplex behavior of supported palladium. It is well knownthat CH4 combustion activity depends markedly on theoxidation state of palladium. A typical conversion curveobtained in temperature-programmed combustion (TPC)experiments during heating/cooling cycles is shown inFig. 6.

As reported by many authors [45–49], a large conver-sion hysteresis is observed between heating and coolingbranches, and this is associated with the reversible trans-formation of highly active PdO to poorly active metallic

11.6.6 Catalytic Materials 2419

1.0

CH

4 co

nver

sion

0.8

0.6

0.4

0.2

0.0

200 300 400 500 600 700 800 900

Temperature / °C

Fig. 6 Temperature-programmed combustion test in the an-nular reactor at high gas hourly space velocity (GHSV).Feed composition: 0.5% CH4, 2% O2, 1% H2O He bal-ance; GHSV = 106Ncm3 g−1 h−1; h/c rate: 15 ◦C min−1; catalyst:10%Pd/La2O3(5%)−Al2O3. Arrows indicate heating and coolingramps.

Pd [45, 46]. Indeed, the temperature ranges where a nega-tive apparent activation energy is observed, are associatedwith PdO thermal decomposition in the heating branch,and with PdO reformation in the cooling branch. Thedetailed features of the PdO–Pd reversible transforma-tion and its relation to CH4 combustion activity are quitecomplex, and remain the subject of debate. An extensivestate-of-the-art discussion is provided in Ref. [50], but inthe following we summarize the main features of practicalrelevance. The position and the amplitude of the activityhysteresis depends mainly on the partial pressure of O2

and on the nature of the support material. Thresholdtemperatures of both decomposition and reformation ofPdO markedly increase with O2 partial pressure. Data onthe effect of the support material are listed in Table 2.

The threshold temperature of thermal decompositionof PdO does not depend significantly on the support,

Tab. 2 Temperature of onset of PdO decom-position (TD) during the heating ramp, and thatof the reformation (TR) of PdO from Pd duringcooling in thermal cycling in air for PdO sup-ported on different alumina- and zirconia-basedmaterials

Support TD/ ◦C TR/ ◦C

Al2O3 795 690La2O3/Al2O3 800 690CeO2/Al2O3 795 755La2O3/CeO2/Al2O3 800 750ZrO2 800 725YSZ 800 715

and corresponds well to the value predicted by thermody-namics [10, 51]. On the other hand, the support markedlyaffects the threshold temperature of PdO reformation dur-ing cooling, which is significantly lower than that of PdOdecomposition. In particular, oxides with high oxygenmobility such as CeO2, ZrO2 and YSZ present a markedlyhigher threshold temperature of PdO reformation and ac-cordingly a much lower PdO decomposition/reformationhysteresis. Although the mechanism of such hysteresishas not yet been fully clarified, it has been proposedthat passivation by chemisorbed O2 on the Pd surfaceoccurs at high temperature, which hinders the formationof bulk PdO [47]. It has also been suggested that supportswith high oxygen mobility would most likely promote thenucleation of bulk PdO from the passivation layer [50, 52].

The variations of CH4 combustion activity associ-ated with the PdO−Pd reversible transformation areresponsible for the unique thermostat ability of palladium-supported catalysts. Indeed, under adiabatic combustionof CH4 the catalyst stabilizes at the temperature of PdOdecomposition predicted by thermodynamics for the ac-tual oxygen partial pressure (850–900 ◦C at typical GTconditions) – that is, well below the adiabatic reactiontemperature [16, 53]. Temperature oscillation problemshave been reported by some authors [14, 54, 55], whichare likely associated with the dynamics of the PdO−Pdtransformation and of the hysteresis in the combustionactivity. The use of ZrO2-based supports, a preferred ma-terial for GT combustor catalysts [44], has been claimedto eliminate oscillation problems.

The light-off performances of palladium catalystsare determined by the kinetics of CH4 combustionover the highly active PdO phase. There is generalagreement in the literature [56–61] that this reactionexhibit zero-order dependence on O2 concentration andfirst-order dependence on CH4 concentration. StrongH2O inhibition has been reported typically with a −1negative order at low temperature. Few studies haveindicated that H2O inhibition is still present up to 600 ◦C,although with a less negative reaction order, higher than−1 [49, 61, 62]. Additional controversial reports have beenissued on the effect of CO2 which has been indicatedeither to inhibit CH4 combustion, particularly at highCO2 concentration [58, 59, 63], or to not exert any effectat all [53, 60].

Apparent activation energies in the range of 70 to90 kJ mol−1 are typically obtained in dry (i.e., no H2Oin the feed) combustion experiments when assuming asimple first-order kinetics in CH4 combustion. However,it has been emphasized that such activation energies mustbe corrected to higher values by properly accounting forH2O inhibition [60].



Several authors have proposed that CH4 combustionover PdO occurs via a redox mechanism [64–67]. Methaneactivation through assisted H extraction is generallyregarded as the rate-determining step, although thereis not a general consensus on the nature of the adsorptionsites. Besides, desorption of H2O by decomposition ofsurface hydroxyls has been reported to play a key role inreaction kinetics at temperatures below 450 ◦C [49, 68].

The structure sensitivity of CH4 combustion over PdOis also a widely debated issue [50]. Ribeiro et al. [58, 69]carefully reviewed data reported in earlier studies andsuggested that the wide scattering of turnover frequencies(TOFs) was mostly due to spurious factors such as neglectof H2O inhibition, presence of contaminants originatingeither from Pd precursors (e.g., Cl− [70]) or from thesupport (e.g., SiO2 [71]), or poor control of the Pd oxidationstate. Precise measurements of PdO surface area isanother major problem in the accurate determinationof TOF. Conventional chemisorption techniques requirethe prereduction of PdO particles, but this may resultin a major modification of morphology. Measurementsof labeled 18O exchange have been recently proposedfor this purpose [72, 73]. By using this technique onsamples obtained from single crystals of Pd metal, Ribeiroand coworkers [69] confirmed that the oxidation of Pdmetal to PdO during catalyst pretreatment results in amarked increase in the active surface area due to surfaceroughening [72, 74]. Besides, CH4 combustion was foundinsensitive on amorphous PdO layer grown on differentcrystal faces of Pd metal. Unfortunately, this approachcan hardly be applied to real supported catalyst as it hasbeen found that PdO facilitates the exchange of 18O withthe support, the exchange increasing with decreasing sizeof PdO particles.

Finally, it is worth emphasizing that the collectionof kinetic data on CH4 combustion under conditionsrelevant to practical applications (high temperature andhigh reactant concentration) is a difficult task. This isbecause the results are typically biased by the onsetof temperature gradients associated with the strongexothermicity of combustion, and also by the impactof diffusion-limiting effects associated with the veryhigh reaction rate. Novel structured catalytic reactors,obtained by depositing thin catalyst layers onto asupport with well-defined geometry, and assembled ina configuration capable of minimizing mass-transfereffects and efficiently dissipating the reaction heat, havebeen developed for this purpose. Examples of thesestructured reactors (the annular reactor [47, 61] and platecell reactor [75]) are shown in Fig. 7.

Structured reactors equipped with measurement sys-tems able to provide spatially resolved concentration andtemperature profiles have also been developed [76].

The durability of catalyst performances under the harshenvironment of a GT combustor is another key issue inthe development of this technology. In addition to thethermomechanical issues discussed above, volatility andsintering of the active catalytic species are major concernsin this respect. On the other hand, poisoning by sulfurand other contaminants has been recognized to have aminor effect on catalyst performances, due mainly to thehigh temperatures of this application [8, 28].

Due to very high gas hourly space velocity (GHSV)of >107 h−1, an extremely low limit of vapor pressure(<10−4 Pa) has been fixed as a rough criterion to matchthe 8000 h−1 constraint of catalyst life in GT combus-tors [77]. Estimates performed considering all relevantspecies (metals, oxides, hydroxides, oxyhydroxides) un-der the oxidizing and water-containing atmosphere ofGT combustors showed that Pd is able to match suchconstraints up to about 1000 ◦C, while most of othercomponents (including Pt) fail [77].

The PdO−Pd transformation plays also a key role in thesintering behavior, since coarsening of palladium parti-cles occurs more readily upon thermal reduction [58, 78].Sintering of Pd metal particles under conditions rele-vant to the GT combustor have been investigated [78, 79].The results showed that sintering occurs, via an Ostwaldripening, up to an average particle size of hundredsof nanometers. It has been observed that the reoxi-dation of large metal particles results in a significantdecrease in crystal size [52, 74]. However, such fragmenta-tion – which originates from the large difference of crystalstructure and density between tetragonal PdO phase andfcc Pd metal phase – occurs via the formation of multiple,incoherent PdO domains [74], and does not result in areal redispersion of the palladium aggregates.

It is worth noting that, in order to match the combinedrequirements of high combustion activity and durabilityunder harsh operating conditions, catalysts with a highPd loading (about 10%, w/w) must be adopted [16, 80].

11.6.6.2.2 Metal-Substituted Hexaaluminate (HA) CatalystsHA materials containing transition metal ions in thestructure have been extensively investigated for GTapplications, in view of their excellent thermal stabilityand catalytic activity [9, 81–83].

These materials have the general formula ABM′xM11−x

O19−α , where A and B are large cations such as Ba,Sr, Ca, La; M′ is a transition metal ion such Cr,Mn, Fe Co, Ni, Cu; and M represents for Al. Suchmaterials crystallize in two hexagonal structures, β-Al2O3

and magnetoplumbite, depending on the nature of thecomponents (Fig. 8).

The structures consist of spinel blocks, containing theM and M′ ions, while the mirror planes contain the large

11.6.6 Catalytic Materials 2421

Titanium plateCatalytic surface

To GC or MS

ThermocoupleAlumina platecovered with Pd

Heaters

Gas mixture in

Annular duct

Thermocouple

Ceramictube

Catalyticcoating

Gas outlet

Thermocouple

Gas inlet

Quartztube

Oven

(a) (b)

Fig. 7 Examples of structured catalytic reactors for kinetic measurements. (a) Annular reactor [47, 61]; (b) plate cell reactor [75].

cations. The alternative stacking of spinel blocks andmirror planes results in the peculiar hexagonal plate-like crystallites, characterized by a strong anisotropyalong the c-axis direction, that are responsible forthe high thermal stability of these systems [81, 82].Transition metal cations partially replacing Al3+ ionsprovide the significant methane combustion activity ofthe catalyst [82, 84].

Mn-substituted HA catalysts are most active. Mn en-ters the structure at low concentration, preferentially intetrahedral Al sites with dominant oxidation state +2 and,at high concentrations, in octahedral Al sites with dom-inant oxidation state +3 [85]. The catalytic activity is alsoinfluenced by the composition of the mirror plane: highmethane combustion activity has been reported by Araiand coworkers for Sr0.8La0.2Mn1Al11O19 [86]. This activ-ity is comparable to that of BaMn2A11O19, and is slightlylower than that measured over LaMg0.5Mn0.5Al11O19 [87].It has been suggested that the incorporation of a divalentcation such as Mg2+ stabilizes the structure, thus favor-ing the incorporation of Mn as Mn3+, which is highlyactive [87].

These materials have been prepared by both hydrolysisof the alkoxides [82] or by coprecipitation from solublesalts of the constituents by using NH4OH or (NH4)2CO3

as precipitating agent [88]. Monophasic samples withsurface area in the range 10 to 15 m2 g−1 have beenobtained upon calcination at 1300 ◦C [82, 88].

Recently, the synthesis of nanosized HA has beenproposed via a reverse-microemulsion preparation thatis reported to be effective in controlling the hydrolysis andpolycondensation of the alkoxides of the constituents.Using this preparation route, the nanoparticles crystallizedirectly to the desired phase at the relatively lowtemperature of 1050 ◦C, and maintain surface areas inexcess of 100 m2 g−1 after calcination at 1300 ◦C for2 h [89–91].

11.6.6.2.3 Rich Combustion Catalysts Few studies havebeen published on catalyst materials directly related to richcatalytic combustion for GT applications [55]. However,the bulk of these data is available on the catalytic partialoxidation of methane and light paraffins, which has beenwidely investigated as a novel route to H2 production.Mixed oxides (Ni, Co, Fe) and noble metals (Pd, Rh,Ru, Pt, Ir) have been reported as active in the partialoxidation of methane to synthesis gas [92]. Although theobserved performances (degree of methane conversionand yields of CO and H2) vary with the catalysts andthe operating conditions, all reported data share thesecommon features: (i) the product mixtures consist of CO,H2 and CO2 and H2O; and (ii) all reported conversionsand selectivity to CO and H2 are below or approach thevalues corresponding to the thermodynamic equilibrium.

The direct reaction of methane partial oxidation alwayscompetes with total oxidation reactions, that are alsoresponsible for O2 consumption, whereas steam anddry reforming and C-forming reactions are also to beconsidered. All reactions are catalyzed by the materialswhich are active in partial oxidation, but different scalesof reactivity for the catalysts can be estimated from theexperimental data. Total oxidation prevails at the light-offof the fuel-rich stream over most of the catalysts, butprecious metals are more active than transition metals.

Veser et al. [93] investigated the light-off behaviorof C1−C4 alkanes over different noble metals as afunction of the fuel-to-air equivalence ratio, φ. Theirresults (see Fig. 9) showed that the surface ignitiontemperature generally decreases with increasing φ dueto site competition between oxygen and hydrocarbon onthe catalyst surface and the higher sticking probability ofO2 on noble metals. A similar behavior has been reportedby several other groups [55, 94–96].



m

m

s

c-axisD = 200–300 nm

Hexagonal plate-like crystallite

d = 20–30 nm

(a) (b)

Fig. 8 β-Al2O3 (a) and magnetoplumbite (b) structures. Large solid circles = A and B cations; small solid circles = M and M′ cations;open circles = oxygen ions. m = mirror planes, s = spinel blocks.

700

600

500

400

300

200

1000 0.2

Φ/(I + Φ)0.4 0.6 0.8 1

T/°

C

Pt

CH4

C2H6

C3H8

C4H10

Fig. 9 Ignition temperature of light alkanes over a Pt catalyst as afunction of the combustion stoichiometry [93].

As a result, a rich catalytic combustor exhibits betterlight-off performances than a lean catalytic combustor.Veser et al. [93] also found that ignition temperaturecorrelates well with the C−H bond energy of thehydrocarbon, in line with the crucial role of activationof the first C−H bond proposed in the literature [97, 98].

For steam and dry reforming, the following scale ofreactivity exists over precious and transition metals [99,100]: Ru, Rh > Ni > Pd, Pt, Co, Fe.

It has been noted that there is substantial similaritybetween the orders of activity of the metals for partial ox-idation of methane and the orders of activity of the samemetals for steam and dry reforming of methane [101].Given the relative reactivity of the catalysts, selection ofthe catalyst and of the operating conditions can result indifferences in the mechanism for CO and H2 formation.Rh is extremely active both in the direct and secondaryreactions, and this explains why rhodium is the catalyst ofchoice for the ultra-short contact time applications whichhave been extensively studied by Schmidt and cowork-ers [102–104]. Under ultra-short contact times, Pt appearsless selective to H2 than Rh, and this can be explained bya lower intrinsic activity towards direct partial oxidationand secondary steam and dry reforming reactions.

It must also be noted that the short-contact time reactorsare typically operated under adiabatic conditions withoutlet temperatures in the order of 700 to 1000 ◦C. Undersuch conditions, with respect to other noble metals, Rh isbelieved especially stable due to a low vapor pressure andan increased resistance to carbon formation, even undersevere operating conditions. The use of low-surface-areaoxides such as α-Al2O3 and ZrO2 as support materialshas been reported to improve the catalyst stability bylimiting the coarsening of Rh particles while avoidingincorporation of Rh within the oxide structure [105].

Basini et al. [106] have published the results of methanepartial oxidation runs in a pilot-scale reactor operating athigh pressure and short contact time, showing stable

11.6.8 Status and Outlook 2423

activity (almost complete conversion of methane and over90% selectivity to CO and H2) for over 500 h on stream.

In addition, operability for 20 000 h with bench-scaletesting has been recently claimed [107].

11.6.7Modeling of Catalytic Combustors

Mathematical models represent a powerful tool in thedesign and analysis of catalytic combustors for GTs.Single-channel models have been customarily used todescribe catalytic combustors, which appears reasonable,considering that global adiabatic conditions and uniformdistribution of the variables at the catalyst inlet sectionare approached in reality.

In principle, many physical and chemical phenomenathat occur within the reactor need to be considered,including: (i) heterogeneous reactions in the catalyst andhomogeneous reactions in the gas phase; (ii) heat, massand momentum transfer by convection and diffusion inthe gas phase and at the gas–solid interface, that areaffected by entrance effects; (iii) mass diffusion in theactive catalyst layer; and (iv) radial and axial heat transferin the solid phase through the active washcoat and theceramic or metallic substrate by conduction and radiation.Additional complexity should be added to account for heatexchange between adjacent active/inactive channels [108].

Models differ in both physical and chemical complexity.The most comprehensive physical models are based onthe solution of the Navier–Stokes equations, consideringboth axial and radial dispersion, momentum, and energytransport [109]. As these models are computationallyexpensive, an alternative is represented by modelsbased on the boundary layer equations that retain adetailed mass and heat transport description to andfrom the channel wall, wherein the axial diffusivetransport in the gas phase is neglected in view of thehigh gas velocity. The momentum balance can also beincluded in these models. The models based on theboundary layer equations represent a significant reductionin computational expense compared to Navier–Stokesformulation [109].

Plug-flow one-dimensional (1-D) models, that are basedon the lumping of the variables over the cross-sectioninto appropriate average values, have also been used forpreliminary design purposes, despite the fact that theyprovide only an approximate description of the light-off performances and of heterogeneous–homogeneousinteractions. These models are relatively simple to writeand easy to solve, but they require reliable correlations formass- and heat-transfer coefficients. These correlationsare not well established in particular for transitionaland turbulent flow conditions that prevail under most

operating regimes of catalytic combustors for GTs underpressure. For example, strong laminarization effectsassociated with rapid heat release from the catalytic wallhave been recently identified [110].

For what concerns the description of the chemicalkinetics, it is worth noting that the catalytic and gas-phasecontributions to fuel conversion are decoupled in thehybrid configuration of catalytic combustors. Althoughthe results of some investigations have been published[111] a reliable detailed machanism that could capturethe relavant and complex features of PdO catalysts is leanCH4 combustion is not yet available. Accordingly, thesurface chemistry is usually described by simple kineticsthat can also incorporate catalyst aging.

For rich combustion of CH4 over Rh catalysts,a few detailed surface mechanisms have been pro-posed [112–114]. However, such detailed surface chem-istry is still under debate [110], and simple molecularkinetics can be useful for modeling purposes [115].

On the other hand, gas-phase chemistry is usuallydescribed by detailed and well-established mechanisms(see e.g., Ref. [116]).

Mathematical models have long been employed forseveral purposes, including: (i) the design of the cat-alytic and homogeneous section for different combustorconfigurations (e.g., catalyst length and channel size,volume of post-combustion chamber, operating win-dow); (ii) the analysis of the combustor performances(e.g., conversion performances, emissions, pressuredrops, T-profiles, thermal stresses); (iii) the extrapo-lation of laboratory data (e.g., light-off predictions,conversion performances); and (iv) fundamental stud-ies providing insight into the role of the differentphysico-chemical phenomena occurring inside the com-bustor. Examples for the use of mathematical modelingfor design and analysis purposes may be found inRefs. [117–127].

11.6.8Status and Outlook

Following major R&D efforts during the 1990s, catalyticcombustors for GTs have finally reached the stage ofcommercialization. For both economic and technicalreasons, catalytic combustion appears today particularlyattractive for GTs with single-can engines in the 1.5 to10 MWe power size range, which provides good marketperspectives in distributed power generation.

From a technical viewpoint, a number of improvementswould be desirable in the areas of catalyst ignitioncapability, so as to reduce NOx contribution from thehomogeneous preburner, and of catalyst robustness and



durability which should extend towards a lifetime targetof 20 000 h. Fundamental studies aimed at filling theknowledge gap with regards to the complex behavior ofPd-based catalysts, and also to develop high-temperaturecombustion catalysts, are expected to provide an importantcontribution to such technical improvements.

Novel concepts based on rich catalytic combustionoffer wide opportunities with respect to most of above-mentioned issues, including: flexible integration indifferent machines; low temperature ignition ability;tolerance to fuel concentration and temperature non-uniformities; and fuel flexibility.

As a final comment, it must be mentioned that, beyondthese technical issues, the true potential for catalyticcombustion in GTs will depend on emission regulations.Only powerful environmental drive towards ultra-lowNOx limits will offer opportunity to further technicaldevelopments in catalytic combustion, and to a widermarket penetration.

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11.7Catalytic Routes to Hydro(chloro)fluorocarbons

Zarah Ainbinder†, Leo E. Manzer∗, and Mario J. Nappa

11.7.1Background

The first commercial catalytic system, SbCl5, for theproduction of chlorofluorocarbons (CFCs) was basedon the pioneering work of Swartz [1] during the 1890s.DuPont developed and commercialized CFCs during the1930s. Since then, these marvelously stable, non-toxicand non-flammable compounds have found their wayinto many aspects of modern lifestyle [2]. Refrigeration,air conditioning, energy-conserving foams, cleaning ofelectronic circuit boards, and firefighting are just a few ofthe applications of CFCs. However, during the 1980s, theincredible stability of CFCs was scientifically linked to thedepletion of the Earth’s ozone layer by the NASA OzoneTrends Panel. This created a major challenge for industrialcatalytic scientists and engineers to identify, develop, andcommercialize an entirely new family of products thatwere environmentally safer, yet still satisfied the needs ofsociety.

∗ Corresponding author.

The CFC replacements need to be non-toxic, non-flammable, and have significantly lower, or zero ozone-depletion potentials. Many organic or aqueous-basedsystems, that do not contain chlorine or fluorine,have been developed for some applications, whereasothers will still use hydrochlorofluorocarbons (HCFCs)and hydrofluorocarbons (HFCs). Unlike hydrocarboncatalysis, the presence of hydrogen, chlorine and fluorinein the same molecule creates a very large number ofisomeric possibilities. As a result of many years ofcareful study, the early list of >800 potential candidateshas been narrowed down to less than a dozen viablemolecules and their blends and azeotropes [3]. Manyof these were commercially manufactured as societyapproached the January 1996 phaseout date of theMontreal Protocol.

This chapter reviews some of the literature reported forthe synthesis of CFC alternatives. It is not possible to becomprehensive in the space available, so the referencesare selected to be informative and to outline the extensiveand exciting types of heterogeneous catalysis available.A more comprehensive review has been publishedelsewhere [4].

11.7.2Catalytic Transformations

Carbon – fluorine bonds can be prepared by a variety oftransformations through the use of both homogeneousand heterogeneous catalysis. Although this chapter ismeant to cover only heterogeneous catalysts, that doesnot diminish the value of homogeneous systems.

11.7.2.1 Addition of HFHCFC-123, HCFC-124, and HFC-125 have been foundto be useful as refrigerants, either individually oras azeotropic mixtures with other components. Thesecompounds can be prepared by contacting hydrogenfluoride and tetrachloroethene (PCE) [Eq. (1)] withselected catalysts.

HF + CCl2=CCl2PCE

−−−→ CHCl2CF3

123+ CHClFCF3

124+ CHF2CF3

125(1)

Vapor-phase catalysts include Cr2O3/MgO/Al2O3 [5],Cr+3/AlF3 [6], and Zn/Al2O3 [7]. The production of HFC-125 can be minimized if a catalyst comprising a selectedmetal (e.g., Cr, Mn, Ni, or Co) supported on a high-fluorinecontent (>90% AlF3) alumina is used [8].

A convenient method of preparing HFC-152a, useful inrefrigerant blends, is by the reaction of vinyl chloride (VCl)with HF [Eq. (2)]. An 86% conversion of vinyl choride with

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