This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication.

Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available.

You can find more information about Accepted Manuscripts in the Information for Authors.

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Accepted Manuscript

Catalysis Science & Technology

www.rsc.org/catalysis

View Article OnlineView Journal

This article can be cited before page numbers have been issued, to do this please use: V. Halperin, G. E.

Shter, V. Gelman, D. M. Pesel, M. Mann-Lahav and G. S. Grader, Catal. Sci. Technol., 2014, DOI:

10.1039/C4CY01341G.

Catalysis Science & Technology RSCPublishing

ARTICLE

This journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1

Cite this: DOI: 10.1039/x0xx00000x

Received 00th January 2012,

Accepted 00th January 2012

DOI: 10.1039/x0xx00000x

www.rsc.org/

Catalytic activity of electrospun Ag and

Ag/carbon composite fibres in partial methanol

oxidation

V. Halperin, G. E. Shter, V. Gelman, D. M. Peselev, M. Mann-Lahav and G. S. Grader*,

We report on the catalytic activity of Ag metal fibres and composite Ag/carbon fibres synthesized by

electrospinning, in partial methanol oxidation (PMO) to formaldehyde. Uniform mats, containing fibres

with 300-600 nm in diameter were prepared from a stable suspension of Ag nanoparticles (NP), AgNO3,

PVA, water and ethanol. Thermal decomposition of the "green" mats was studied by TGA/DTA/MS in

inert, strongly and mildly oxidizing atmospheres. Special heat profiles were developed to obtain

metallic Ag as well as composite Ag/carbon (~10 wt. % carbon) fibre mats catalysts. The effect of

sintering conditions on the morphology and catalytic activity were studied. Using a catalyst weight of

45-70 mg and a methanol WHSV in the range of 60-90 h-1

, the highest conversion and selectivity for

metallic Ag fibres at 550°C were 94.2% and 82.4%, respectively. Under the same conditions, the

catalytic activity for Ag/carbon mats was found to improve yielding a conversion and selectivity of 95.8

% and 84.1 %, respectively. Large space time yields (STY) of 56 h-1

and 64 h-1

were obtained for metal

and metal/carbon cases, by optimizing the sintering conditions. A maximal WHSV and STY of 540 h-1

and 386 h-1

respectively were achieved with Ag/carbon composite catalyst at 95% conversion and 550°C.

In summary, we demonstrated the feasibility of electrospinning technology in producing highly active

catalysts containing Ag and composite Ag/carbon fibres for partial oxidation of organic molecules. We

also demonstrated the superior behaviour of the metal/carbon composite fibres over the metallic silver

fibre and powder analogues.

Introduction

Partial methanol oxidation to formaldehyde (PMO) is an

important industrial process with global annual production

(1996) estimated at 8.7∙106 metric tons1. Overall formaldehyde

consumption is expected to grow at annual rate of 5% in 2011-

20162. The most common catalyst used in PMO is silver.

Although the PMO process is not new, it is still the focus of

intense research as evidenced in recent publications3-7. Due to

the large formaldehyde production scale, one of the potential

routes to increasing the energy efficiency and catalyst lifetime

is reduction of the high temperatures (600-720°C) used

industrially1,2. Therefore, improvements in catalyst activity and

selectivity, as well as the possible lowering of the operating

temperatures will result in overall significant decrease in energy

consumption and production costs of this process.

In recent years much progress has been made in controlling

the phase composition, size and structure of nanometric

catalysts, allowing for highly selective and efficient catalysts8.

However, one of the obstacles of using micro-porous materials

in heterogeneous catalysis is the diffusion limitations inside the

pores. Diffusion coefficients decrease with decreasing pore

diameter and increasing molecular weight of both reactants and

products. For reactants or products molecules with

characteristic dimensions larger than pores diameter the

transport is limited. The deposition of either large molecules or

coking near the pore entrance during the reaction can deactivate

the whole pore8. One of the common ways to overcome these

limitations is the use of hierarchically porous materials. Fibrous

catalysts can be engineered to have hierarchical porosity,

ranging from micro-porous fibres to the meso- and macro-

porous structure of the fibrous mats.

Electrospinning of Ag containing fibres for catalysis

applications has been reported in the literature. Silver

nanoparticles (NP) dispersed in carbon fibres produced by

electrospinning of Ag nitrate and polymer containing precursor

Page 1 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

ARTICLE Journal Name

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012

with subsequent reduction were tested for catalytic activity in

styrene epoxidation9,10. Electrospun polyacrylonitrile (PAN)

fibres were immersed in Ag nitrate solution followed by

reduction and tested in methylene blue reduction process11.

Silica fibres with dispersed Ag NP were prepared by either

electrospinning the Ag nitrate containing precursor12 or

immersing the activated electrospun silica nanotubes in Ag

nitrate solution with subsequent reduction13. The Ag NP/silica

fibres were tested in catalytic reduction of 4-nitrophenol13 and

methylene blue12. Additionally, Ag/NiO fibres were obtained

by using metal nitrates containing precursors followed by

calcination14. Finally, pure Ag fibres were produced by

impregnation of electrospun polymer fibres followed by their

thermal degradation and organics removal15.

In general, most methods for electrospinning of composite

or metal Ag fibres above are based on precursors including

polymer and Ag-salt or Ag-complex or, alternatively,

metallization of polymer or ceramic electrospun fibres matrix.

The chosen model reactions for catalytic activity tests in most

above mentioned cases were conducted at low temperatures that

are insufficient for the PMO process discussed herein. Recent

studies by our group focused on nanostructured Ag based

materials with organic doping for enhanced catalytic properties,

including their deposition on nanofibrous TiO2 support16-18. The

hybrid catalyst consisting of thermally activated Congo-red

nanoparticles entrapped within Ag deposited on TiO2 fibres was

shown to considerably outperform other studied catalysts in

every parameter of PMO16.

In this study we describe the production of Ag and

composite Ag/carbon fibre mats and study their catalytic

activity in the PMO process. The fibre mats were produced by

electrospinning, using Ag nanoparticles suspension as a

precursor. This type of precursor is fundamentally different

from the precursors described earlier: firstly, it is heterogeneous

system including solid part of Ag-nanoparticles and liquid part

(polymer in solvents with small additives of salts). Secondly,

the skeleton of the final fibres was formed by relatively large

(~50nm) initial nanoparticles in contrast to very small

nanoparticles formed from salt solution during reduction. The

last factor prevents the formation of very large and non-uniform

Ag particles during sintering at high temperatures in this

catalytic application. In addition, this technique allowed

obtaining a high Ag loading (~70 wt.%) in the “green”

electrospun fibres. Further thermal treatment with varied

atmosphere and temperature regimes facilitated the synthesis of

fibrous catalytic material with desired structure and

composition. The high Ag loading used in this work has several

key advantages: firstly, the "green" fibres can be processed to

obtain either metal or composite metal/carbon fibres without

modification of electrospinning parameters or precursor

composition by varying only the thermal treatment conditions.

Secondly, the lower polymer content in fibres results with

significantly lower shrinkage during thermal treatment, hence it

allows retention of the fibrous morphology during sintering.

The main question addressed in this research is whether the

catalytic activity demonstrated thus far with Ag containing

fibres, can be improved using the highly Ag loaded electro-

spun fibres developed herein. In particular can residual carbon

intentionally preserved in the fibre matrix prevent Ag particle

aggregation and growth – that lead to degradation of the

catalytic activity? In this paper we demonstrate that Ag and

composite Ag/carbon fibres mats give rise to enhanced

performance in high temperature gas-phase PMO processes. In

addition, we show the interplay between the processing

parameters and the resulting morphology and catalytic activity.

Experimental

General description

The experimental work can be divided into 4 main parts: a)

preparation of liquid precursor for injection, b) the

electrospinning process resulting in "Green" Ag/polymer fibre

mats, c) thermal treatment of fibre mats under different

atmospheres (Air, Ar, O2/Ar) resulting in "white" Ag or

Ag/carbon fibre mats, and d) catalytic activity measurements in

partial methanol oxidation (PMO) using "White" Ag fibre mats

and "White" Ag/carbon composite mats as catalysts.

Throughout the paper the term "Green" mat designates the fibre

mat obtained after electrospinning and drying, while "White"

mat designates the electrospun fibre mats which were heated at

high temperature for organics burn-out and sintering.

Chemicals

AgNO3 - (99+%, Aldrich), Ag nanoparticles suspension -

(35 nm, 25%, IoLiTec Ionic Liquids Technologies GmbH,

Heilbronn, Germany), Polyvinyl alcohol (PVA) - (205000

g/mol, Aldrich), Ethanol - (Absolute, Carlo Erba), Methanol –

(Absolute, Bio-Lab ltd), Formaldehyde (A.G., Frutarom,

Israel), water – Milli-Q purified.

Precursor preparation

Precursor preparation procedure included the following

main steps:

1) A solution of 1.2 g PVA in 6 g of water was mixed with

magnetic stirrer at 70°C for 1 hour followed by homogenization

on roller at room temperature (RT) for 24 hours resulted in a

transparent high viscosity solution;

2) 7.2 g of this solution were mixed with 9.2 g of 28 wt.% Ag

nanoparticles suspension in ethanol and 0.8 g of 50 wt.%

AgNO3 water solution. The mixture was stirred on roller at RT

for 12 hours to yield a final precursor – stable

AgNP/AgNO3/PVA/Water/Ethanol suspension with weight

ratio Ag to PVA ~2:1.

Electrospinning

The electrospinning system consisted of high voltage supply

(SL40P60, Spellman, Hauppauge, New York, USA), a

grounded vertical rotating aluminium collector with controller

(EP-613, O.Fi. electronics ltd., Haifa, Israel) and feed system

with syringe pump (KDS100, KD Scientific, Holliston, MA,

Page 2 of 11Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

Journal Name ARTICLE

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 3

USA) and injector. The precursor solution was fed at 0.5 ml/h

rate (SFR). The working distance between the needle and the

collector (WD) was 15-18 cm, while the applied voltage was in

the 20-25 kV range. Typical electrospinning duration was 6

hours. After electrospinning, the resulting "green" Ag/PVA

fibre mats were vacuum dried at 70°C for 12 hours in order to

remove the residual solvents.

Thermal treatment

"White" mats were obtained by thermal treatment of dried

"green" mats. The heating was performed in a horizontal tube

furnace (CTF, Carbolite). The gas atmosphere was adjusted by

flowing either air, or oxygen and Ar through mass flow

controllers to achieve desired O2 concentration. Gas was fed

into the furnace at a flow rate of 200 ml·min-1. Samples were

placed on dense alumina substrate. The gas mixture of 1.5

vol.% O2 in Ar was used to obtain “white” Ag metal fibre mats.

Argon gas was used to obtain “white” composite Ag/carbon

fibre mats. Stepwise profiles were based on TGA-DTA analysis

and are shown in Fig. 1a, b for both metal and composite

metal/carbon fibres.

Fig. 1 Thermal treatment profiles: a) in 1.5% O2 in Ar, b) in pure Ar

Catalytic Activity

The Experimental Setup for measurement of the PMO

process was described in detail earlier18. The catalytic reaction

was carried out at different temperatures, gradually decreasing

from 550 to 150°C, by employing the “methanol ballast

process” version, in which only air and pure methanol are fed

into the reactor without extra water in the reactant mixture. The

weight of Ag fibre mats as catalyst was typically 45-70 mg. To

keep constant catalyst bed volume of 1 ml, all the samples were

diluted with quartz powder. The CH3OH concentration in the

reaction mixture was kept constant at 500 mg L–1. The total

pressure in the reactors was held at 1.5 bar. The feed gas

mixture flow rate was 8.35 L·h–1, with corresponding methanol

WHSV in the range of 60-90 h-1. In the context of this study,

the WHSV (weight hourly space velocity) represents maximal

weight of methanol which can be converted to formaldehyde

during one hour per weight of catalyst at constant conversion.

The conversion and selectivity are defined in Eq.1 and Eq.2,

respectively.

Eq. 1 ���������� ��

����� ���������� 100%

Eq. 2 �������������

���������� ��� 100%

Conversion and selectivity determinations were carried out

after 2 h of continuous process at each reaction temperature to

ensure that the operating conditions of the reaction are

stationary. A full PMO run consisted of 6 steady state

measurements at different temperatures.

Thermal Analysis and material characterization

About 40 mg of samples were analysed by simultaneous

thermal gravimetric analysis (TGA) and differential thermal

analysis (DTA) at ambient pressure (Setsys Evolution 1750,

Setaram, Caluire, France). The TGA/DTA measurements were

carried out from 25 to 800 °C with a heating rate of 5°C/min

under Ar or air or Ar/O2 flow of 60 ml/min using a 100 µl

alumina crucible. The evolved gases were analysed on-line by a

quadrupole Mass Spectrometer (MS, ThermoStar GSD320,

Pfeiffer Vacuum, Asslar, Germany). The obtained data were

treated using the Calisto Processing software (AKTS and

Setaram). Morphology and microstructure were investigated by

high-resolution scanning electron microscopy (ULTRA plus;

Zeiss, Zurich, Switzerland). The catalytic system evolved gases

were analysed using a gas chromatograph (7890A, Agilent

Technologies, Santa Clara, CA, USA) with flame ionization

detector (FID) and Wcot fused silica column (Varian) kept at

40°C.

Page 3 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

ARTICLE Journal Name

4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012

Results and Discussion

Metallic fibres: preparation and characterization

Preparation of metallic fibre mats requires complete

removal of the organic species which make about 30 wt.% of

the "green" fibres. Specifically, during thermal treatment of the

"green" Ag/AgNO3/Polymer fibre mats in oxidative

atmosphere, polymer oxidation can be realized by two oxidants:

internal AgNO3 and external gaseous oxygen. Thermal analysis

experiments in air show minute water desorption of ~0.5% and

a small endothermic peak at 95°C (Fig. 2a).

Fig. 2 Thermal analysis of green fibres in air: a) TGA-DTA signals and b) MS signal

of effluent gas.

A multistep oxidative decomposition follows the water

desorption, with three main exothermic peaks up to 450oC. The

gaseous products of this oxidation stage were detected by on-

line mass spectrometry and MS profile included NO, N2O,

NO2, CO2 and water (Fig. 2b). Based on the evolution of NOx

gaseous by-products (Fig. 2b, m/z=30), polymer oxidation by

Ag nitrate occurred at 165°C, leading to an additional weight

loss of 4%. This reaction is apparently accelerated by PVA

melting and observed in narrow temperature range. The next

stages of polymer oxidation started around 210°C with a

dominant participation of external oxidant (O2 from air) and

characterized by several steps of weight loss (Fig. 2a) and

strong exothermic peaks. The largest weight loss was detected

in the range of 230- 375°C and the total loss was ~ 25.5%.

Mass spectrometry (MS) profiles reveal massive formation of

water and CO2 with only minor NO and NO2 contribution. At

the final oxidation stage in the range of 375 - 450°C (Fig. 2a)

the dominant evolving gas was CO2 (Fig. 2b) which indicated

the combustion of carbon or partially decomposed polymeric

residues. At 450°C the polymer was fully oxidized by internal

Ag nitrate and external oxygen with a 30% total weight loss.

The thermal treatment conditions of the “green” fibre mats

significantly affect the resulting "white" fibres morphology.

Fast polymer oxidation can have two main negative effects: a)

massive gas release can damage the fibrous morphology; b) fast

rates of heat generation can lead to local overheating, causing

aggressive fibres sintering along with undesired particles

growth leading to formation of large aggregates and non-

uniform structure. Therefore the attempts of thermal treatment

in air resulted in very non-uniform and agglomerated "white"

mats with undesired morphology. To avoid the above negative

effects, a stepwise treatment profile under Ar/oxygen gas

mixture with lower O2 content than air was implemented.

Fig. 3 Thermal analysis of green fibres under 1.5% O2 in Ar: a) TGA-DTA signals

and b) MS signal of effluent gas.

The TGA-DTA-MS analyses in air and in Ar with 1.5% O2

were significantly different. Firstly, the rate of weight loss in

Ar-1.5% O2 shown in Fig. 3a was substantially slower

Page 4 of 11Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

Journal Name ARTICLE

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 5

compared to the process in air (see Fig. 2a). Secondly, the

organic burnout under 1.5% O2 was incomplete even at 800 °C,

whereas under air the weight loss was over by 450 °C. In

addition, under 1.5% O2 the internal oxidation due to the

nitrates occurs at a higher temperature than in air (above

~200°C vs. 165 °C) as confirmed by MS data on release of

nitrogen containing gases (Fig. 3b). The above three factors are

important for preserving the desired nanostructure of the fibres

during thermal treatment. The developed profile of this process

shown in Fig. 1a includes three soaks at 200, 250 and 300°C

with heating rate of 150°C/h and cooling rate of 300°C/h under

gas mix flow of 200 ml·min-1. This gradual profile allowed

lowering of the residual carbon to values of 0.5-0.7 wt.% (by

TGA, EDS) and a slow release of gases, preserving the fibrous

structure in the sintered mats seen in the HRSEM images in

Fig. 4.

Fig. 4 The effect of sintering on the morphology of the fibres: (a, b, c) “green”

fibres, and (d, e, f) “white” fibres after sintering in 1.5% Oxygen atmosphere (1 h

at 300°C).

Based on Fig. 4, the diameter of the "green" fibres was in the

range of 300-600 nm. The diameter of the sintered "white"

fibres decreased significantly to value of 150-400 nm. In

summary, sintering the fibres in low oxygen pressure (1.5% O2

in Ar) allowed preservation of the fibre structure, giving rise to

well-connected Ag particles without visual organic binder.

Composite metal/carbon fibres: preparation and

characterization

One of the undesired phenomena during usage of

polycrystalline Ag fibres at high temperatures is particle growth

and formation of hard agglomerates. In the present case these

phenomena occur both at the stage of initial thermal treatment

and during the PMO catalytic process, leading to catalyst

deactivation. A key element of this research was to determine

whether separation of the Ag NP inside the fibres by a carbon

residue formed from the decomposed polymer, can inhibit the

sintering and grain growth, thereby preventing the associated

catalyst deactivation. As shown in the following section, the

benefits of composite Ag/carbon fibre mats were proven. In

addition, we believe that the mats with composite Ag/carbon

fibres would be more flexible and mechanically more stable

compared to pure metallic fibres mats.

Thermal treatment of “green” Ag/AgNO3/PVA material

under Ar was chosen as a way to obtain composite Ag/carbon

fibre mats for use as catalysts in the PMO process. To develop

an optimal profile of thermal treatment in Ar, a TGA/DTA test

in flowing Ar gas was run with "green" mats as shown in Fig. 5.

Fig. 5 The TGA and heat flow signals of “green” fibres vs. temperature under Ar.

Endothermic water desorption at ~90°C (WL= 0.8-0.85 %) and

a sharp exothermic PVA oxidation by Ag nitrate in the range

~160-190°C (WL=4.6%) were detected. These two effects were

also observed in TGA/DTA profiles under Air and Ar/1.5% O2

(Figs. 4, 5), however the data in Fig. 5 proves beyond doubt

that these effects are not dependent on the external oxygen. The

shallow endothermic effect at 211°C can be attributed to

AgNO3 melting and it was not found in TGA/DTA test under

oxidative atmosphere (Fig. 2, 3). In the previous case the

endothermic melting was masked by strong exothermic process

of organic oxidation by the external oxidant. The next

exothermic peak starting at ~295°C represents further polymer

oxidation by AgNO3 occurring after melting of both PVA and

AgNO3. This process leads to a dominant part of the weight

loss from 4.8 to 21% in the 210 – 450°C range. After ~450°C

the weight loss rate was slow and approximately constant up to

800 °C. Weight loss at 800°C was 22.4% - significantly lower

than was detected in the case of heating in air (30%, Fig. 2):

this difference was attributed to residual carbon, uniformly

distributed between the particles of Ag. The presence of carbon

was confirmed by EDS where its content was found in the

range of 10±3 wt.%. This weight is slightly larger from the

expected residual carbon content predicted from the TGA data

Page 5 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

ARTICLE Journal Name

6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012

of ~ 8%. The EDS data should be regarded as a qualitative

indicator rather than a quantitative measure.

Although the actual weight of residual carbon content is not

high, it is significant in terms of volume: for fibres with 8 wt.%

of carbon (based on the TGA data), the calculated carbon

volume is ~ 30 vol.%. This value justifies considering the final

fibres as a composite Ag/carbon material. On one hand, the

carbon serves as a physical barrier between the Ag particles that

prevents the Ag from aggregating and undergoing aggressive

sintering. On the other hand, the carbon may cover the active

sites on Ag surface and lower its catalytic performance in the

PMO process. Based on the TGA-DTA results above, the

heating profile included slow heating rates and 120°C and

300°C soaks for 30 and 6 min respectively, The morphology of

a composite fibre is presented in Fig. 6 a-d, where Ag particles

are coated and mostly separated by uniformly distributed

carbon. As mentioned above, the carbon residue of ~ 8 wt.% at

650°C, was effective in maintaining exceptionally small Ag

particle size, in the 50 nm range as seen in Fig. 6d.

Fig. 6 HRSEM images of composite Ag/carbon fibre mats after sintering at 650°C.

The carbon residue and the degree of Ag particle sintering

can be controlled with heating temperature and duration. It is

well known, that increase of either of these parameters results

in more active sintering. The TGA results in Fig. 5 for heating

in Ar clearly showed that carbon content in fibres decreased

with temperature. The morphology of the composite "white"

fibres obtained at different temperatures after a 1 h soak at 450

- 650 °C (see profile in Fig. 1b) is shown in Fig. 7.

Fig. 7 HRSEM images of composite Ag/carbon mats at sintering temperatures of

a) 450°C, b) 550°C and c) 650°C.

The temperature range was chosen from TGA data (Fig. 5) and

it corresponds to the stage when decomposition and

carbonization of polymer is completed. The two main

observations from these images are as follows: a) the absence

of Ag aggregates usually formed at high temperatures; b) a

similar average particle diameter of 110 nm over a significant

temperature range. It is direct evidence of the positive role of

the carbon as inhibitor of aggressive sintering and

agglomeration of the Ag particles. In the same time, the fibrous

structure with poorly sintered Ag particles was preserved, by

virtue of the carbon as good binder in the composite. Finally,

the composite fibre mats were found to be more flexible and

more robust during their handling.

Ag fibres - PMO catalytic activity

Methanol conversion and total selectivity towards

formaldehyde increased as a function of the reaction

temperature as shown in Fig. 8a.

Fig. 8 The PMO catalytic activity of “white” Ag mats after 2 h firing at 300°C in

1.5% O2: a) activity vs. temperature and b) activity at 550°C vs. the PMO run

number.

A sharp increase of the conversion and selectivity values

was measured in 250-500°C range, reaching a plateau above

550°C. The second run typically showed improved performance

compared to the first run, with the results slightly decreasing

during later runs as seen in Fig. 8b. This pattern can be

attributed to primary catalyst activation during the first cycle of

Page 6 of 11Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

Journal Name ARTICLE

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 7

the reaction. The maximal total selectivity and conversion of

82.4% and 94.2% respectively were found for 2nd and 3rd runs.

A slow deactivation process started during the subsequent runs

(Fig. 8b). The main contribution to catalyst deactivation

appears to be the sintering, grain growth and densification of

the fibrous structure.

The performance of prepared catalysts was compared in

terms of space-time yield (STY) which is defined by the mass

of formaldehyde produced in 1 hour (g h-1) per mass of Ag (g)

in the catalyst, as expressed by Eq. 3:

Eq. 3 ��� ����� !"#$%&#$

�'(∙*+,-./

The STY represents simultaneously the joint effect of

WHSV, conversion and selectivity and is therefore a practical

and convenient parameter for comparison of different catalysts

normalized by their weight. The effect of firing time at 300°C

on STY is shown in Fig. 9 for the for Ag mats during the

second PMO run. The optimal firing time was found to be 2

hours corresponding to an STY maximum of 56 h-1 in the PMO

process.

Fig. 9 The STY of Ag metal fibres at the second PMO run vs. the sintering soak

time at 300°C.

The effect of the PMO process on the morphology of the Ag

mats is seen in Fig. 10. The initial "white" mats morphology

(Fig. 10a), shows that the fibrous structure was retained after

firing for 1 hour at 300°C in 1.5% O2. However, during six

PMO runs the fibrous structure is completely transformed into a

porous yet strongly aggregated material with large primary

particles (Fig. 10b). Basic particle size is 5 to 15 µm that are

fused into a porous network. The formation of large connected

particles during the PMO runs is a result of the close proximity

of the Ag nanoparticles to each other in the original Ag mats,

which enables the massive diffusion and particle aggregation

seen in Fig. 10b.

To study the drastic change in morphology of the Ag fibre

mats during the PMO process (seen in. Fig. 10), we changed the

firing time of the soak at 300oC during the initial heat treatment

in 1.5% O2. The morphology of the Ag mats after 2 h and 5 h

firing is seen in Figs. 10c and 10e, respectively. The fibre

structure was lost in both cases, yet the particles formed after 2

h of firing are considerably smaller and more compact than

their 5 h analogue.

Fig. 10 The effect of firing time at 300°C in 1.5% O2 and PMO runs on the fibre

mat morphology : a) “White” mat after 1 hr and b) same mat after 6 PMO runs,

c) “White” mat after 2 hrs and d) same mat after 2 PMO runs, e) “White” mat

after 5 hrs and f) same mat after 2 PMO runs.

The grains in 5 h sample (Fig. 10e) have transformed into a

porous structure with particles of ~ 2-8 µm and relatively small

(20-30 µm) prolonged aggregates of 4-6 particles. The effect of

the PMO process on the two initial "white" mat structures is

shown in Figs. 10d and 10f. The structure formed after 5 h

firing limits the diffusion of the Ag during the PMO process

and leads to a lower sintering effect. So although some

aggregation is seen in Fig. 10f, the structure is relatively stable

compared to Fig. 10e.

The sample sintered for 2 hours at 300°C soak (which was

shown to be optimal in terms of STY) possesses high porous

mix structure: the fibres, the particles of 1-4 µm and small

aggregates of 2-3 particles (Fig. 10c). This structure has

transformed during the PMO reaction into a highly porous non-

fibrous morphology with relatively small both grains and linear

aggregates of 4-5 particles (Fig. 10d). It is important to point

out that all structure elements of this material are significantly

smaller compared to those in 5 h analogue described above.

The larger contact area arising from the smaller aggregates in

the 2 h case (Fig. 10d) can explain the better catalytic

performance as evidenced by the larger STY (Fig. 9). The

lower performance of the mats sintered with soak at 300°C for

less than 2 h can be attributed also to polymer residues. They

might hinder the reaction in few different ways: decreasing the

catalyst active surface area, consuming the oxygen from the

reagents mixture, generating local overheating and thus

promoting undesired grain growth.

Page 7 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

ARTICLE Journal Name

8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012

Composite Ag/carbon fibres - PMO catalytic activity

In the process with composite Ag/carbon catalyst, the

selectivity and conversion as a function of reaction temperature

are shown in Fig. 11 a,b. The general trends are very similar to

those observed in the metal fibres seen in Fig. 8 a,b with a

sharp rise in conversion and selectivity above 250°C and

activity stabilization after the first run.

Fig. 11 The PMO catalytic activity of “white” Ag/carbon mats after 1 h firing at

550°C in Ar: a) activity vs. temperature and b) activity at 550°C vs. the PMO run

number.

The optimal catalyst thermal treatment condition was found by

measuring the STY as a function of temperature. As shown in

Fig. 12, a sharp maximum in the STY of 64 h-1 was achieved

with mats sintered in Ar for 1 h at 550°C.

To explain the STY behaviour in composite fibres the

complex effect of carbon on the catalytic performance must be

discussed. The direct and immediate effect of carbon is to block

the active sites on some of the Ag surface and lower the

catalytic performance. Another effect is the possible carbon

exothermic oxidation in the reaction mixture, lowering effective

reactant concentration and leading to local overheating.

Fig. 12 Composite Ag/carbon fibre mats STY vs. the sintering temperature.

A third important effect of carbon is maintaining a physical

barrier between Ag particles in fibres, preventing aggregation

and sharp particle growth and, in turn, delaying the sintering-

related deactivation. Surely, other effects of carbon in the

vicinity of the Ag catalyst grains such as the "adsorb and

shuttle"19 or “spillover”20,21 mechanisms may play a role here as

well. These multiple opposing effects give rise to the maximum

in STY as function of sintering temperature seen in Fig. 12. As

seen in the TGA data for fibres heated in Ar (Fig. 5), the mass

monotonically drops with temperature above 450°C due to

carbon loss. Therefore, as the amount of carbon in composite

mats decreases with increasing sintering temperature, more

active Ag sites are exposed for catalysis, potentially leading to

an increase in catalytic activity. So what causes the sharp drop

in the STY values from 64 h-1 at 550°C to 35 h-1 at 650°C in

Fig. 12? This phenomenon may be due to the dynamics of pore

formation and their behaviour during thermal treatment and

subsequent PMO process. The morphology of the Ag/carbon

composite fibres after 2 PMO runs is shown in Fig. 13 for

fibres that were sintered at 450°C, 550°C and 650°C.

Fig. 13 HRSEM images after 2 runs of PMO of composite Ag/carbon fibre mats

treated at: a) 450°C, b) 550

°C and c) 650

°C.

Unlike the previous case of Ag metal fibres, where the original

fibre structure was totally lost in the PMO process (Fig. 10),

remnants of the original fibre structure are clearly visible in this

case due to the carbon acting as a sintering inhibitor (Fig. 7). As

the gaseous products of polymer decomposition evolve from

the treated fibres, a porous carbon mass is created. So long as

there is sufficient carbon to prevent the Ag massive particle

growth and aggregation, the STY increases with increased

exposure of the Ag catalytic centres. However, as the

temperature rises, the carbon content may drop below a critical

level, where the Ag particles are free to grow. This in turn can

lead to decreased porosity and smaller exposed surface that

Page 8 of 11Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

Journal Name ARTICLE

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 9

give rise to an overall lower catalytic activity (Fig. 12) at higher

temperatures. Clearly, the structure obtained after PMO of the

650°C mat (Fig. 13c), is more dense and compact than that from

the mats treated at lower temperatures.

The weight hourly space velocity (WHSV) measurements

(Fig. 14a) were conducted in the 400-550°C temperature range

during the second run.

Fig. 14 Comparison of the a) WHSV and b) STY of CR@Ag powder

16, Ag deposited

on TiO2 nanofibres16

, 1.67 % CR@Ag deposited on TiO2 nanofibres16

and

Ag/carbon fibre mat sintered at 550°C.

Methanol conversions were kept constant at intermediate 70%

(at 400°C) and close to maximal 95% (at 500°C and 550°C)

values by varying the feed rates. The WHSV values achieved

were 140 h-1 for 95% conversion at 500°C, 370 h-1 for 70%

conversion at 400°C and, finally, 540 h-1 for 95% conversion at

550°C and flow rate of initial gas mix of 807.9 normal ml·min-1

(mcat=0.045 g). Comparison of these results with submicron

powder of CR@Ag, investigated earlier17,18, shows a significant

advantage of composite Ag/carbon fibre mats, with 17.5-22.5

fold increase in WHSV. The corresponding STY values were

97.5 h-1 for 95% conversion at 500°C, 129 h-1 for 70%

conversion at 400°C and, finally, 386 h-1 for 95% conversion at

550°C. These values are also significantly higher in comparison

to powder-like catalyst (Fig. 14b).

The comparison of achieved WHSV and STY using

Ag/carbon mats as catalyst with values recently published for

Ag/TiO2 and CR@Ag/TiO2 nanofibres16 is also presented in

Fig, 14a,b. While the results obtained in the present work are

comparable with those in16 at temperatures 400-500°C, the

considerable advantage of Ag/carbon mats was found for

550°C: STY of 386 h-1 vs 320 h-1 and 273 h-1 for doped silver

and undoped silver deposited on TiO2 nanofibres. In addition, it

should be noted that the fabrication technique using

electrospinning is much simpler and more scalable than the

nanofibres preparation in the above publication16.

Finally, all above STY values (97.5, 129, 386 h-1) in

temperature range of 400 - 550°C were found higher than a

corresponding STY value of 23 h-1 calculated from a reported

results for the industrial process conducted at 700°C with

commercial Ag powder catalyst22. This significant difference is

probably due to the larger Ag surface accessible for catalysis in

the present work.

Conclusions

This paper summarizes the synthesis of Ag metal fibres and

composite Ag/carbon fibres by electrospinning and their

catalytic activity in a partial methanol oxidation (PMO) process

to produce formaldehyde. Uniform mats, containing fibres with

300-600 nm in diameter were prepared from a stable

suspension of Ag NP/AgNO3/PVA/water/ethanol. Thermal

decomposition of the "green" mats was studied by thermal

analysis (TGA/DTA) and mass spectrometry (MS) in different

atmospheres: inert (Ar), strongly oxidizing (air) and mildly

oxidizing (Ar/1.5% O2). Based on the thermal analysis data,

specific heat treatment profiles were devised to obtain metallic

Ag fibre mats as well as composite Ag/carbon fibre mats that

were subsequently used in the catalytic testing.

Metallic fibre mats were obtained under mild oxidation

conditions at an optimal temperature of 300°C at various

soaking times. The morphology of the Ag mats changed

dramatically with time from highly porous with sub-micron

fibres at 1 hour at 300°C, to porous network of fused Ag

particles at 2 hours and finally, to a strongly sintered large Ag

aggregates at 5 hours. The highest catalytic activity at methanol

WHSV in the range of 60-90 h-1 was found for samples treated

for 2 hours: the methanol conversion and selectivity towards

formaldehyde were 94.2% and 82.4%, respectively. A large

space time yield (STY) of 56 h-1 was obtained.

Alternatively, thermal treatment in Ar was used to prepare

the composite Ag/carbon fibrous mats. For treatment in inert

atmosphere, the maximal sintering temperature was varied from

350 to 650°C, for 1 h. The residual carbon content in composite

fibres was on the order of 10 wt.%, consistent with the TGA

data. The presence of uniformly distributed carbon prevented

aggressive sintering during heat treatment and PMO process.

The composite mats sintered at 550°C showed the highest

catalytic activity. The achieved WHSV values were 140 h-1 for

Page 9 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

ARTICLE Journal Name

10 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012

95% conversion at 500°C, 370 h-1 for 70% conversion at 400°C

and, finally, 540 h-1 for 95% conversion at 550°C. The

corresponding STY values of 97.5 h-1, 129 h-1 and 386 h-1 at the

above three temperatures.

These results demonstrate a considerable improvement over

the previously published laboratory results obtained with

submicron Ag powder17,18, undoped and doped Ag

nanoparticles deposited on the TiO2 nanofibres16 as well as

commercial Ag powder catalyst22.

In summary, we demonstrated the effectiveness of

electrospinning technology in producing highly active catalysts

containing Ag and composite Ag/carbon fibres for oxidation

processes as demonstrated on partial methanol oxidation.

Acknowledgements

Supported by a Tashtiot Infrastructure grant of the Israel

Ministry of Science (Grant 3-6474), and by joint grant from the

Committee for Planning and Budgeting of the Council for

Higher Education and by the Center for Absorption in Science

(Israel) under the KAMEA program. Authors acknowledge the

support of Russell Berrie Nanotechnology Institute (Technion,

Israel), the Grand Technion Energy Program (GTEP) and the

Adelis Foundation for renewable energy research.

Notes and references * Department of Chemical Engineering, Technion –Israel Institute of

Technology, Haifa, 32000, Israel. Fax: +972-4- 8295099, Tel: +972-4-

8292008, E-mail: grader@ce.technion.ac.il.

1 G. Reuss, W. Disteldorf, A. O. Gamer and A. Hilt, in Ullmann's

Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH &

Co. KGaA, 2000.

2 H. R. Gerberich, G. C. Seaman and Updated by Staff, in Kirk-Othmer

Encyclopedia of Chemical Technology, John Wiley & Sons, Inc.,

2000.

3 D. Nagaki, C. J. Peterson, E. Bowden, J. T. Chapman and S. Mueller,

Integrated process for the production of acrylic acids and acrylates

comprising methanol oxidation to formaldehyde and aldol

condensation of formaldehyde with acetic acid, Celanese

International Corporation, USA, 2014.

4 T. E. Jones, T. C. R. Rocha, A. Knop-Gericke, C. Stampfl, R. Schlogl

and S. Piccinin, Phys. Chem. Chem. Phys., 2014, 16, 9002-9014.

5 T. C. R. Rocha, A. Oestereich, D. V. Demidov, M. Havecker, S.

Zafeiratos, G. Weinberg, V. I. Bukhtiyarov, A. Knop-Gericke and R.

Schlogl, Phys. Chem. Chem. Phys., 2012, 14, 4554-4564.

6 N. S. Andryushina, A. L. Stroyuk, O. O. Ustavytska, Y. I. Kurys, S. Y.

Kuchmy, V. G. Koshechko and V. D. Pokhodenko, Theor. Exp.

Chem., 2014, 50, 155-161.

7 C. Baltes, G. Kolios, P. Resch, U. Wegerle and T. Maeurer, Oxidative

dehydrogenation of methanol to formaldehyde over silver-containing

knits, BASF SE, Germany, 2012.

8 E. L. Hu, S. M. Davis, R. Davis and E. Scher, in Nanotechnology

Research Directions for Societal Needs in 2020, ed. nonymous ,

Springer, 2011, p. 445-466.

9 H. Liu, J. Bai, S. Wang, C. Li, L. Guo, H. Liang, T. Xu, W. Sun and H.

Li, Colloids Surf. Physicochem. Eng. Aspects, 2014, 448, 154-159.

10 H. Liu, J. Bai, Q. I. Wang, C. Li, S. Wang, W. Sun And Y. Huang,

Nano, 2014, 09, 14500411-14500417.

11 C. Zhang, Q. Yang, N. Zhan, L. Sun, H. Wang, Y. Song and Y. Li,

Colloids Surf. Physicochem. Eng. Aspects, 2010, 362, 58-64.

12 A. C. Patel, S. Li, C. Wang, W. Zhang and Y. Wei, Chemistry of

Materials, 2007, 19, 1231-1238.

13 Z. Zhang, C. Shao, Y. Sun, J. Mu, M. Zhang, P. Zhang, Z. Guo, P.

Liang, C. Wang and Y. Liu, J. Mater. Chem., 2012, 22, 1387-1395.

14 H. Wu, D. Lin, R. Zhang and W. Pan, Chem. Mater., 2007, 19, 1895-

1897.

15 F. Ochanda and W. E. Jones, Langmuir, 2007, 23, 795-801.

16 Y. Aouat, G. Marom, D. Avnir, V. Gelman, G. E. Shter and G. S.

Grader, J. Phys. Chem. C, 2013, 117, 22325-22330.

17 Y. Binyamin, G. E. Shter, V. Gelman, D. Avnir and G. S. Grader,

Catal. Sci. Technol., 2011, 1, 1593-1599.

18 G. E. Shter, H. Behar-Levy, V. Gelman, G. S. Grader and D. Avnir,

Adv. Funct. Mater., 2007, 17, 913-918.

19 Y. Sagatelian, D. Sharabi and Y. Paz, J. Photochem. Photobiol. A.,

2005, 174, 253-260.

20 M. Boudart, M. A. Vannice and J. E. Benson, Z. Phys. Chem., 1969,

64, 171-7.

21 H. W. Kohn and M. Boudart, Science, 19 145, 149-50.

22 A. Aicher, G. Lehmann, N. Petri, W. Pitteroff, G. Reuss, H. Schreiber

and R. Sebastian, Preparation of formaldehyde, US Patent 4,584,412,

1986.

Page 10 of 11Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G

A feasibility of electrospinning in producing of highly active Ag and Ag/carbon fibres based

catalysts is demonstrated in methanol partial oxidation process (PMO).

Page 11 of 11 Catalysis Science & Technology

Cat

alys

isS

cien

ce&

Tech

nolo

gyA

ccep

ted

Man

uscr

ipt

Publ

ishe

d on

03

Nov

embe

r 20

14. D

ownl

oade

d by

Tec

hnio

n -

Isra

el I

nstit

ute

of T

echn

olog

y on

03/

11/2

014

15:5

6:25

.

View Article OnlineDOI: 10.1039/C4CY01341G