Selective CO oxidation over Fe 5 (PO 4 ) 3 (OH) 5 supported Pt catalyst: Kinetic and mechanistic studies

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1 Indian Journal of Chemistry Vol. 53A, April-May 2014, pp Selective CO oxidation over Fe 5 (PO 4 ) 3 (OH) 5 supported Pt catalyst: Kinetic and mechanistic studies A Hari Padmasri a, *, P Anil Kumar b, S Naveen Kumar b, A Venugopal b & Sooboo Singh c a Department of Chemistry, University College for Women, Osmania University, Koti, Hyderabad , AP, India b Inorganic and Physical Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad , AP, India c School of Chemistry, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa ahpadmasri@gmail.com Received 16 January 2014; revised and accepted 13 February 2014 Iron hydroxyphosphate as support for Pt catalysts is explored for preferential oxidation of CO using simulated reformate gas mixture in the temperature range C. The CO oxidation activity and selectivity at low reaction temperatures are enhanced on addition of H 2 O but decrease at high temperatures due to the reverse water gas shift reaction. X-ray diffraction patterns of the supported and unsupported catalysts show very weak diffraction lines due to the Fe 5 (PO 4 ) 3 (OH) 5 phase. The chemical composition of this phase on the catalyst surface has been confirmed by X-ray photoelectron spectroscopy results. Keywords: Catalysts, Supported catalysts, Platinum catalysts, Oxidation, Carbon monoxide oxidation, Iron hydroxyphosphate The production of hydrogen by steam reforming of hydrocarbons and alcohols generate far less greenhouse gas emissions than conventional fuels. Studies pertaining to hydrogen production by reforming of renewable resources such as biomass, bio-ethanol, and bio-glycerol have been reported 1,2. However, the product stream is inevitably contaminated with 1-2% of CO, which is detrimental for hydrogen fuel cell applications. Hence, removal of CO from the reformed gas mixture is imperative. The possible methods for the removal of CO are adsorption, reduction or oxidation. Among these, selective CO removal by catalytic oxidation appears to be the most amiable approach since adsorption processes typically require unacceptably large volumes of adsorbents. The reformate gas mixture usually consists of 75% H 2, 1-4% CO, 20-21% CO 2 and the balance is made up of nitrogen 3,4. CO can be oxidized by water gas shift reaction (WGSR) and preferential oxidation (PROX). However, it is difficult to remove CO completely from the reformate gas by WGS catalysts, thus, eliminating its use in the fuel cell applications. On the other hand, complete removal of CO from the H 2 rich reformate gas is possible by preferential oxidation catalysts. Hence, the development of PROX catalysts has become the prime focus in recent times. Influence of H 2 O and CO 2 has been studied by Schubert et al. 5 during the PROX of CO over Au/Fe 2 O 3 catalysts. The selective CO oxidation in the presence of both H 2 and CO 2 is demanding. PROX requires the injection of a stoichiometric amount of O 2 into the gas stream for CO oxidation, and at the same time a fraction of H 2 is also consumed under the reaction conditions 6,7. Among the several catalysts reported, Pt supported systems are claimed to be efficient for PROX 8. Pt supported on ceria 7, alumina 9, silica 10, and zeolites 11 have also been tested for the PROX reaction. It is reported that partially reducible support materials such as Fe 2 O 3 and TiO 2 for Au and non-reducible support materials, Al 2 O 3 and SiO 2, for Pt are active and selective for low temperature CO oxidation reaction 9. Recently, low temperature active Pt supported on a mesoporous silica catalyst (FSM-16) showed noticeable PROX activity 15,16. Iron based noble metal catalysts are highly active and selective for low temperature CO conversion reactions either by WGSR 9,17,18 or by selective oxidation 12, In this study, we have studied the iron hydroxyphosphate synthesized by us earlier. 23 A simulated reformate gas mixture, i.e., 1.8% CO/N 2, 64.1% H 2 /N 2, 23.5% CO 2 /N 2 was used in this investigation. We report results obtained for the

2 512 INDIAN J CHEM, SEC A, APRIL-MAY 2014 preferential oxidation of CO together with reactions involving forward and reverse water gas shift activities, carried out separately using Pt supported on iron hydroxyphosphate [Fe 5 (PO 4 ) 3 (OH) 5 ] with Pt loadings of 1.0 and 2.0 mass%. The physicochemical characteristics of the Pt supported catalysts are explained by BET-surface area, XRD, DT/TGA, AAS, XPS, TPR/TPO and CHNS analyses. Materials and Methods The support material Fe 5 (PO 4 ) 3 (OH) 5 was prepared by the precipitation method as reported by us recently. 23 The fresh and used forms of the catalysts were characterized by BET-surface area, powder XRD, TPR, DT/TGA, AAS, XRS and CHNS analyses. 23 Catalytic activity A multi-purpose online micro reactor interfaced to a HP-5890 thermal conductivity detector (TCD) gas chromatograph was constructed. The products, CO, H 2, CO 2 and O 2, were analyzed using a 60/80 carboxen SS column-1000 (380 mm in length, 3.2 mm OD and 2.1 mm ID). For catalytic activity measurements, standard conditions were maintained except for reverse water gas shift studies. Prior to the PROX measurements, the catalysts were stabilized using a reaction gas mixture with composition, 1.8% CO/N 2, 64.1% H 2 /N 2 and 23.5% CO 2 /N 2 and 10.6% O 2 /N 2 (air) by raising the reaction temperature at intervals of 2 C/min up to 240 C. All the gases were supplied by Inox Gases (Mumbai, India). The flow rate of H 2 O and the gases were maintained using a B. Braun Perfusor Secura (GS) infusion pump and Aalborg (DFC 26) digital mass flow controllers respectively. After stabilization of the catalysts, PROX activities were measured and analyzed at 40 C. For the evaluation of the catalysts with different weight loadings of Pt, the following experiments were carried out in this study, viz., preferential oxidation of CO using air in presence of H 2 and CO 2 (PROX), effect of water on the PROX activities, reverse water gas shift reaction with CO 2 and H 2 (CO 2 : H 2 1:3 molar ratio), reactions were also carried out using 1.8% CO/N 2, 64.1% H 2 /N 2, 23.5% CO 2 /N 2 and 10.6% O 2 /N 2. The carbon monoxide and oxygen conversions were defined as follows: % Conv. of CO = ([CO] in - [CO] out / [CO] in ) 100 % Conv. of H 2 = ([H 2 ] in - [H 2 ] out / [H 2 ] in ) 100 % Conv. of O 2 = ([O 2 ] in - [O 2 ] out / [O 2 ] in ) 100 Finally, the selectivity of the CO oxidation in the presence of H 2 and CO 2 was calculated from the oxygen mass balance as follows: % CO select. = {(0.5 [CO 2 ] out /[O 2 ] in - [O 2 ] out )} 100 Results obtained in this investigation were reproducible within 5%. Results and Discussion Poorly crystalline Fe 5 (PO 4 ) 3 (OH) 5 phases at diffraction lines, 2θ = 28, 26.6, 59.8 are observed in both the supported and unsupported Pt catalysts. Catalytic activity over Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalysts A reformate gas mixture with composition, 1.8% CO/N 2, 64.1% H 2 /N 2 and 23.5% CO 2 /N 2 has been used for catalytic activity measurements in the selective oxidation of CO. Compressed air (22% O 2 in nitrogen) was used as an oxygen source. Venugopal et al. 24 reported the PROX activities over Pt, Pd supported on Al 2 O 3 catalysts and optimized the reaction conditions with respect to O 2 /CO ratios at different reaction temperatures using air as an oxygen source. In this study, the PROX measurements have been carried out separately for various Pt loaded catalysts under similar and optimized reaction conditions to investigate the effect of reaction temperature as well as the influence of Pt loading. The optimized O 2 /CO ratio of 1.25 is maintained constant for all the measurements and the gas hourly space velocity (GHSV) was fixed at L (h g cat ) -1. It is important to note that neither methanation activity nor methanol formation is observed with any of these catalysts. The activity over 1.0 wt% Pt catalyst (Fig. 1a and 1b) revealed that both CO and O 2 conversions increased linearly with an incremental change in the reaction temperature up to 120 C. Above this temperature, the CO conversion decreased drastically up to 240 C. On the other hand, 100% O 2 conversion was observed from C. The 2.0 wt% Pt catalyst exhibited similar tendencies, however, maximum CO conversion (Fig. 1a) was obtained at 100 C but decreased from that point onwards up to 240 C. During the stabilization of the catalysts, a maximum CO conversion of 98.6% was achieved at 128 C over 1.0 wt% Pt catalyst. In contrast, the 2.0 wt% Pt catalyst showed a 100% CO conversion for a temperature as low as 96 C. It is clearly evident that not much variation in the selectivity is found over both catalysts (Fig. 1c) at low reaction temperatures between C. However, a slight difference was

3 HARI PADMASRI et al.: SELECTIVE CO OXIDATION OVER Fe 5 (PO 4 ) 3 (OH) 5 SUPPORTED Pt CATALYST 513 Fig. 1 Catalytic activity and selectivity as a function of reaction temperature for CO and O 2 over 1.0 and 2.0 wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5. [(a) CO conversion; (b) O 2 conversion; (c) CO 2 selectivity. Catalyst wt.: g, React. temp.: C. GHSV: L h -1 g -1 ; Gas comp.: 1.8%CO/N 2, 64.1%H 2 /N 2, 23.5%CO 2 /N 2, O 2 source: air; O 2 :CO ratio: 1.25:1 (mole ratio)]. observed at high reaction temperatures up to 240 C. A similar observation was made by Yin et al. 25 over Pt-Fe/Al 2 O 3 catalyst during the preferential oxidation of CO, at and above a reaction temperature of 100 C. It is also observed that an increase in Pt loading lowered the reaction temperatures to attain maximum CO conversions. Similar to this observation, the selectivity decreased slightly with increase in Pt content. Therefore, it can be concluded that a higher loading of Pt would catalyze H 2 oxidation rather than CO oxidation above 120 C. However, the advantage of higher concentration of Pt is the high CO oxidation rates at lower reaction temperatures. From the fuel cell unit operation perspective, the catalyst should work in more real environmental integrated conditions. Selective removal of CO from reformate gas mixtures (obtained by steam reforming of alcohols) by catalytic oxidation is prone to have different types of side reactions. The possible reactions during the selective CO oxidation in the presence of H 2, CO 2 and H 2 O are: CO + ½O 2 CO 2 [PROX]... (1) H 2 + ½O 2 H 2 O [H 2 oxidation]... (2) 2CO CO 2 + C [Boudouart reaction]... (3) CO + 3H 2 CH 4 + H 2 O [Methanation]... (4) CO 2 + H 2 CO + H 2 O [RWGSR]... (5) CO + H 2 O H 2 + CO 2 [WGSR]... (6) CO 2 + 4H 2 CH 4 + 2H 2 O [Methanation]... (7) These are the problems associated with the side reactions during selective CO oxidation. Of the above mentioned reactions, only reactions (1) and (6) are desired ones and proper precautions relative to reaction conditions should be undertaken to prevent them. The main factors that could influence the undesired reactions are high reaction temperatures, excess O 2, nature of the support material and the type of precious metal. One of these undesired reactions is CO 2 hydrogenation to form CO and H 2 O known as reverse water gas shift reaction (RWGSR). In separate experiments carried out in this investigation, RWGS activity was observed at above 160 C. Similar observations are reported by Manasilp et al. 26 during the selective CO oxidation over Pt supported on alumina prepared by sol-gel method at a reaction temperature of 170 C. They also found that increasing the reaction temperature up to 210 C drastically enhanced CO formation by the reverse water gas shift reaction. The other reason for the decrease in CO oxidation activity at high temperatures was presumed to be the result of simultaneous H 2 oxidation, which is also thermodynamically feasible under these reaction conditions. Under identical reaction conditions using the required amount of H 2 along with CO and CO 2, the amount of H 2 oxidized was measured. In general, the possible loss of H 2 is most likely to be due to the reactions between H 2 and O 2, CO methanation and CO 2 hydrogenation (RWGSR). Hydrogen oxidation was also observed at reaction temperatures as low as C but was more pronounced above 120 C. Hence, decrease in CO conversion was accompanied by H 2 oxidation. The partial loss of H 2 is also attributed to the reverse water gas shift reaction at high temperatures. The decrease in CO conversion over 2.0 wt% Pt catalyst at high reaction temperatures may be due to the RWGS reaction or undesired H 2 oxidation. This is emphasized in a rate diagram shown in Fig. 2a where an increase in reaction temperature increases the rate of RWGS. The reverse is true for the WGS rates as

4 514 INDIAN J CHEM, SEC A, APRIL-MAY 2014 shown in Fig. 2b. This implies that the larger crystallites of Pt (assuming bigger Pt crystallites over 2.0 wt% Pt compared to the lower loading) over 2 wt% Pt catalyst are responsible for the undesired H 2 oxidation as well as hydrogenation of CO 2 to form CO. The support material was found to be inactive for PROX activity under the reaction conditions applied in this investigation. Effect of water on preferential oxidation of CO Addition of water during CO oxidation was carried out at 100 C over both the Pt loaded catalysts. The CO conversion increased from 72% to 80% in PROX conditions in the presence of H 2 O over 1.0 wt% Pt catalyst, while a slight decrease in conversion from 99% to 94% was observed over the 2.0 wt% Pt catalyst. There can be several reasons for the enhanced activity shown by the 1.0 wt% Pt catalyst through the addition of water. One possible explanation is that water increases the CO conversion rate by water gas shift reaction selectively to CO 2. Another reason could be the formation of formate groups in the presence of water which participate in the reaction and increase the rate of CO oxidation as proposed by Kahlich et al. 9 In this study, over Pt/Fe 5 (PO 4 ) 3 (OH) 5, the WGS activities decreased upon increase in reaction temperature, revealing that the active sites are more prone to PROX than the WGS reaction above 120 C. The forward and reverse water gas shift activities over 2.0 wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst are shown in Fig. 2a and 2b. Reaction under different conditions The selective CO oxidation and H 2 oxidation activities were measured under differential flow conditions in the presence of large amounts of CO 2 over 1.0 wt.% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst. The catalyst was diluted in α-al 2 O 3 in order to ensure the differential flow conditions. The Arrhenius plot (Fig. 3), which relates ln(rate of CO oxidation) to inverse reaction temperature, indicates that the apparent activation energies for both CO and H 2 oxidation reactions to be E a = 70.3 kj/mol and E a = 94.0 kj/mol respectively. Although H 2 oxidation competes with CO, the E a for CO oxidation is otherwise low as compared to the E a obtained for H 2 over the Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst system. The experimental error in the evaluation of activities under differential flow conditions is ±2%. Catalytic activity during the stabilization period The T ½ (T ½ : temperature at 50% CO conversion) values obtained during the stabilization of the catalysts using the simulated reformate reaction mixture are 96 C and 64 C for the 1.0 wt% Pt and 2.0 wt% Pt catalysts respectively. A decline in CO oxidation activity with increase in Pt loading and reaction temperature suggests that the systems with Fig. 2 (a) Reverse water gas shift rates measured over 2.0wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst. [Catalyst wt.: g, React. temp.: C; CO 2 :H 2 = 1:3 (mole ratio); Total flow rate: 9.03 L h -1 g -1 ]. (b) Water gas shift activities over 2.0wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst. [Catalyst wt.: g, React. temp.: C; Gas comp.: 1.8%CO/N 2 ; flow rate: 7.12 L h -1 g -1 ; CO:H 2 O: 1:7.8 (mole ratio)]. Fig. 3 Arrhenius diagram of 1/T versus ln(rate of CO oxidation) and 1/T versus ln(rate of H 2 oxidation) over 1.0 wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst.

5 HARI PADMASRI et al.: SELECTIVE CO OXIDATION OVER Fe 5 (PO 4 ) 3 (OH) 5 SUPPORTED Pt CATALYST 515 lower Pt loading, i.e., 1.0 wt% and at moderate reaction temperatures, i.e., C are ideal conditions for the complete removal of CO from the reformate gas mixture by selective oxidation. However, compromise on 5-10% H 2 losses at high reaction temperatures alerts the research on these catalyst systems. Further studies are in progress to minimize the H 2 oxidation as well as CO 2 hydrogenation, which are the routes identified in this investigation over 2.0 wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst for the decrease in selectivity on CO oxidation (Fig. 1c) by the RWGS activities. Conclusions Catalytic studies carried out over the pure support did not show any significant activities either for CO oxidation or H 2 oxidation up to a reaction temperature of 240 C. For the 1 wt% Pt loaded catalysts, increase in reaction temperature up to 120 C increased the oxidation of CO, and above 120 C both conversion and selectivities decreased. The forward water gas shift rates over 2.0 wt% Pt/Fe 5 (PO 4 ) 3 (OH) 5 catalyst decreased with increase in reaction temperature, while the opposite is true for the reverse water gas shift rates. Addition of water has significant effect on the activity and selectivity of these catalysts. The apparent activation energy E a corresponding to CO oxidation is 70.3 kj/mol and 94.0 kj/mol for H 2 oxidation. Finally, it can be concluded that these catalyst systems have potential for use in hydrogenation of carbon oxides at high reaction temperatures. References 1 Adhikari S, Fernando S D, Filip To S D, Bricka R M, Steele P H & Haryanto A, Energy Fuels, 22 (2008) Huber G W, Iborra S & Corma A, Chem Rev, 106 (2006) Holladay J D, Hu J, King D L & Wang Y, Catal Today, 139 (2009) Song C S, Catal Today, 77 (2002) Schubert M M, Venugopal A, Kahlich M J, Plzak V & Behm R J, J Catal, 222 (2004) Mariño F, Descorme C & Duprez D, Appl Catal B: Environ, 54 (2004) Pozdnyakova O, Teschner D, Wootsch A, Kröhnert J, Steinhauer B & Sauer H, J Catal, 237 (2006) 1. 8 Cohn J G E, US Patent No. 3,631,073, Kahlich M J, Gasteiger H A & Behm R J, J Catal, 171 (1997) Mergler Y J, van Aalst A, van Delft J & Nieuwenhuys B E. Appl Catal B: Environ, 10 (1996) Andorf R, Maunz W, Plog C & Stengel T, US Patent No. 5, 955,395, Schubert M M, Hackenberg S, van Veen A C, Muhler M, Plzak V & Behm R J, J Catal, 197 (2001) Kahlich M J, Gasteiger H A & Behm R J, J Mater Electrochem Syst, 1 (1998) Avgouropoulos G, Ioannides T, Papadopoulou Ch, Batista J, Hocevar S & Matralis H K, Catal Today, 75 (2002) Fukuoka A & Ichikawa M, Top Catal, 40 (2006) Fukuoka A, Kimura J, Oshio T, Sakamoto Y & Ichikawa M, J Am Chem Soc, 129 (2007) Venugopal A, Aluha J, Mogano D & Scurrell M S, Appl Catal A: Gen, 245 (2003) Andreeva D, Tabakova T, Idakiev V, Christov P & Giovanoli R, Appl Catal A: Gen, 169 (1998) Liu X, Korotkikh O & Farrauto R, Appl Catal A: Gen, 226 (1998) Kotobuki M, Watanabe A, Uchida H, Yamashita H & Watanabe M, J Catal, 236 (2005) Basinska A, Nowacki A & Domka F, React Kinet Catal Lett, 66 (1999) Schubert M M, Kahlich M J, Gasteiger H A & Behm R J, J Power Sources, 84 (1999) Anil Kumar P, Hari Padmasri A, Naveen Kumar S, Venugopal A & Singh S, Indian J Chem, 53A (2014) Venugopal A, Subrahmanyam M & Prasad K B S, 16 th CATSYMP and Indo-German 1 st Symposium, Hyderabad, India, February Yin J, Wang J, Zhang T & Wang X, Catal Lett, 125 (2008) Manasilp A & Gulari E, Appl Catal B: Environ, 37 (2002) 17.