Nanogold mesoporous iron promoted ceria catalysts for total and preferential CO oxidation reactions

1 Nanogold mesoporous iron promoted ceria catalysts for total and preferential CO oxidation reactions Tomás Ramírez Reina1*, Svetlana Ivanova1, Vasko Idakiev2, Tatyana Tabakova2, Miguel Angel Centeno1, Qing-Fang Deng3, Zhong-Yong Yuan3, José Antonio Odriozola1 1 Departamento de Química Inorgánica e Instituto de Ciencias de Materiales de Sevilla Centro mixto Universidad de Sevilla-CSIC, Avda. Américo Vespucio 49, 41092 Seville, Spain. 2Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 11, Sofia, 1113 Bulgaria. 3Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, China (*) corresponding author: t.ramirez-[email protected].uk Abstract Herein, a series of highly efficient gold based catalysts supported on mesoporous CeO2-Fe2O3 mixed oxides for CO elimination reactions have been developed. The materials have been fully characterized by means of XRD, Raman and UV-Vis spectroscopies among other techniques. We identify the Ce-Fe synergism as a fundamental factor controlling the catalytic performance. Our data clearly reveal that the CO oxidation activity is maximized when the electronic and structural properties of the support are carefully controlled. In this situation, fairly good catalysts for environmental applications as for example H2 streams purification for fuel cell goals or CO abatement at room temperature can be designed. Keywords: Gold catalysts, mesoporous cerium oxide, low temperature CO oxidation, PrOx, H2 purification. 1T.R.Reina actual address: Department of Chemical Engineering, Imperial College London. London SW7 2AZ, UK 2 1. Introduction Fuel cells have the potential to become an important energy conversion technology. The most successful fuel cells are powered by hydrogen [1]. Recently, research efforts directed toward the widespread and commercialization of fuel cells have accelerated the interest to develop a hydrogen-based economy to reduce oil use and decrease pollution [2]. However, hydrogen is difficult to store in a vehicle and to distribute from a centralized production facility. As a result, many efforts have been focused on the conversion of more readily available fuels to hydrogen, either on board a vehicle or for stationary applications. Currently hydrocarbons are employed to obtain hydrogen. The conversion of hydrocarbons to hydrogen is effected mainly by reforming [3]. However reforming processes produce carbon monoxide in association with hydrogen and carbon monoxide leads to a deactivation of the electrodes used in the fuel cell [1,3]. Therefore, CO abatement reactions such as CO oxidation [4], water gas shift (WGS) [5] and preferential CO oxidation (PrOx) [6] are of crucial relevance for the fuel cells and hydrogen economy development. The WGS (CO + H2O CO2+ H2) not only reduces CO concentration but also produces hydrogen. Nevertheless, the WGS is an equilibrium-limited reaction making mandatory further clean-up processes for achieving the desirable levels of carbon monoxide in the gases feeding fuel cells (<10ppm). In this sense, the preferential oxidation with air of the pre-cleaned reformate (PrOx) represents a rather cheap and effective solution to abate the ultimate CO traces [7]. As CO and H2 oxidation may take place simultaneously, selectivity towards CO oxidation is one of the key aspects that should be targeted when facing the PrOx process. In addition, since the PrOx 3 reactor will be feed with the pre-cleaned reformate gas, a suitable PrOx catalyst must tolerate high amounts of CO2 and H2O present in the reforming stream [8]. Gold catalysts have been broadly employed in such oxidation processes due to their high activity especially at low temperatures [9-14]. The activity of gold based catalysts at low temperature is an added value feature since normally their activity window matches the operation temperature of the fuel cell avoiding the conventional heating exchange steps. On the other hand, the performance of gold-based catalysts is known to be sensitive to the preparation method, the nature of the supports, the size of the gold particles and the interactions between the gold and the support. It is the later effect the one attracting most recent interest [15]. The presence of structural defects in the support surface as for example, oxygen vacancies, plays a determinant role in improving gold metallic dispersion influencing positively the CO oxidation activity [16- 19]. In addition, an adequate support should be able to avoid gold particles agglomeration, as well as to participate actively in the reaction mechanism by redox cycling of the support metal ions [20]. For all above, ceria-based materials are very attractive for supporting gold particles due to its ability for undergoing fast reduction/oxidation cycles since its intrinsic redox behavior (Ce4+/Ce3+). This ability results in the formation of oxygen vacancies whose concentration is directly related to the catalytic activity [21]. In addition the number of oxygen vacancies available for oxidation reaction could be improved, either by specific surface area increase, e.g. the use of mesoporous ceria [22], by ceria deposition on the high surface supports to increase the surface/volume relation [23] or by doping ceria with different oxides [24]. Earlier studies found that Au/ceria catalysts are very promising for the PrOx reaction [25-29]. We have recently reported that ZnO, ZrO2, Fe2O3 and Eu2O3 are efficient dopants of Au/ceria system enhancing 4 oxygen mobility, ceria reducibility, and in the end the catalytic performance in the PrOx process [28, 29]. Considering the above mentioned ways to increase the oxygen vacancies population, a detailed study of a series of gold based mesoporous mixed oxide catalysts (Au/CeO2-Fe2O3) is proposed. The samples are tested in the direct and preferential CO oxidation reactions in the present work, with main efforts focussed on achieving a further insight of the Ce-Fe interaction and its manifested relevance on the catalytic behaviour. 2. Experimental 2.1 Catalysts preparation Mesoporous Ce-Fe composite oxides were prepared by surfactant-assisted method previously reported [30]. At room temperature, 6 mmol of cetyltrimethylammonium bromide (CTAB) was dissolved into 200 mL distilled water, then calculated amount of Ce(NO3)3•6H2O and Fe(NO3)3•9H2O were added under vigorous stirring. After stirring for 0.5 h, 0.2 mol/L sodium hydroxide solutions were gradually added into the above solution until the pH value of the mixed solution was 10, and then further stirred for about 12 h. The final suspended solution was aged at 90 ºC for 3 h, washed with hot water several times, dried in the oven at 110 ºC for 6 h, then calcined at 400 ºC for 4 h. The as prepared series contains 6 samples where the Ce/Fe molar ratio varies between 0-1. More precisely the supports corresponding to the following formulas are considered: Ce1Fe0O2, Ce0.8Fe0.2O2, Ce0.6Fe0.4O2, Ce0.4Fe0.6O2, Ce0.2Fe0.8O2, Ce0Fe1O2, which for briefness will be labeled for example Fe0Ce10, Fe2Ce8, etc. 5 Gold catalysts were prepared by deposition-precipitation method in a "Contalab" (Contraves AG, Switzerland) reactor under full control of all parameters of preparation (pH, temperature, stirring speed, reactant feed flow rates, etc.) as reported elsewhere [31]. Briefly, gold was deposited as Au(OH)3 onto mixed oxides thoroughly suspended in water via chemical interaction between HAuCl4·3H2O and K2CO3, under vigorous stirring, while keeping constant pH 7. After filtering and careful washing, the precursors were dried under vacuum and calcined in air at 400 °C for 2 h. The nominal gold amount was 3 wt. % Au. 2.2 Catalyst characterization The actual gold loading was measured using atomic absorption spectroscopy (Varian Vista MPX). X-ray diffraction (XRD) analysis was carried out on a X`Pert Pro PANalytical instrument. Diffraction patterns were recorded using Cu Kα radiation (40 mA, 45kV) in a scanning range of 3 – 80º (2θ) using step size of 0.05º and step time of 240 s. The diffraction peaks of the crystalline phase were compared to those of standard compounds reported in the JCPDS Date Base. The average crystallite size was calculated from the peaks width using Scherrer’s equation. N2 adsorption–desorption isotherms were collected at liquid nitrogen temperature using a Quantachrome NOVA 2000e sorption analyzer. The specific surface areas of the samples were calculated following the multi-point Brunauer–Emmett–Teller (BET) procedure. The pore-size distributions were determined from the adsorption branch of the isotherms using the Barett– Joyner–Halenda (BJH) method. Before carrying out the measurement, each sample was degassed at 200 oC for more than 6 h. 6 The UV-Vis spectra were recorded on an Avantes spectrometer model AvaLight-DH-S-BAL and using BaSO4 as reference. All the spectra were collected in a diffuse reflectance mode and transformed to a magnitude proportional to the extinction coefficient through the Kubelka- Munkfunction F(α). The Raman spectra were recorded in a dispersive HorivaJobinYvonLabRam HR800 Microscope, with a He-Ne green laser (532.14 nm) working at 5 mW, and with a 600 g.mm-1 grating. The microscope used a 50x objective and a confocal pinhole of 1000 µm. The Raman spectrometer was calibrated using a silicon wafer. 2.3 Catalytic activity and selectivity CO oxidation The oxidation of CO was carried out in U-shape glass flow reactor at atmospheric pressure. The catalysts were pre-treated in a 30 mL·min-1 activation flow of 21% O2 balanced in He (from room temperature to 400 ºC, 5 ºC min-1). After the activation a reactive flow (3.4% CO and 21% O2, balanced in helium) was passed through the reactor at room temperature. The total gas flow was fixed to 42 mL·min-1 and the quantitative analysis was carried out with Blazers OmnistarBentchop mass spectrometer calibrated using certified gas mixtures of CO and CO2 in helium. The weight hourly space velocity of the catalytic test was 31.5 L·g(cat)-1h-1. The catalysts were tested in the reaction flow at room temperature until the steady state was reached. Then, the systems were heated to 400 ºC at 5 ºC·min-1. To study in details the catalysts behaviour as a function of Fe-content the same reaction was carried out at temperatures below 0ºC. The reactor, loaded with fresh and activated sample, was immersed into cooling bath composed by 7 liquid N2 and acetone. Once the temperature stabilized, the reactive flow was flushed through the reactor. The temperature was increased slowly to the room temperature and the quantitative analysis of the reactants and products was carried out on a Blazers OmnistarBentchop mass spectrometer. For the low temperature CO oxidation, all the samples were studied in the temperature range -80 – 170 0C. Preferential CO oxidation (PrOx) PrOx reaction was carried out at atmospheric pressure on a cylindrical fixed bed reactor of steel stainless 9mm inner diameter), with 100 mL·min-1 of reaction feed composed by 1% CO, 1.5% O2, 10% CO2, 10%H2O, 50% H2 and He as balance. For each test 100 mg of catalyst sieved between 100-200 μm was load. Prior to the catalytic measurements, the samples were treated in 100 mL·min-1 flow of 21% O2 in He, at 300 ºC for 1 h. Water vapor was added in the dry gas stream via syringe pump Gilson 307. Products and reactants were separated and quantified by on-line gas chromatography (Agilent 6890) equipped with HP PLOT Q and HP-5 columns and TCD detector. The CO conversion was calculated according to Eq. (1) where COin is the amount of the CO in the inlet and COout that of the outlet 𝐶𝑂 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%)= 𝐶𝑂𝑖𝑛−𝐶𝑂𝑜𝑢𝑡 𝐶𝑂𝑖𝑛 𝑥 100 (1) The selectivity towards CO2 formation was calculated with the Eq. (2). O2in corresponds to oxygen in the inlet and O2out to that in the outlet. 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (%)= (𝐶𝑂𝑖𝑛 − 𝐶𝑂𝑜𝑢𝑡 )𝑥 100 2 𝑥 (𝑂2𝑖𝑛− 𝑂2𝑜𝑢𝑡) (2) 8 3. Results and discussion 3.1 N2 Adsorption isotherms The isotherms of all Ce-Fe composite oxides and catalysts (Figure 1A, C) present the characteristic profile of typical mesoporous materials. An increase in N2 adsorption volume, located in the p/po range of 0.7–1.0 is observed. The pore volume of all Ce-Fe mixed oxides is higher than that of pure CeO2 and Fe2O3 (Table 1). The gold addition provokes slight decrease of the pore volume, suggesting that the material kept its mesoporous structure after the gold deposition. The pore size distribution (Figure 1B, D) follows the same trend. The pore size of the solids appears to be affected by the sample composition. Gold deposition causes insignificant decrease of the pore size for Au/Fe2Ce8 and Au/Fe4Ce6, whilst a slight increase is observed for the other catalysts. This increase in the pore size was previously observed and ascribed to the nucleation of gold nanopartilces inside the pores of the support expanding the mesopore spacing [32, 33]. The catalyst`s specific surface area is also influenced by the support composition (Table 1). The increase of the nominal Fe concentration causes a gradual decrease of SBET whereas gold deposition affects unimportantly the specific surface area. Indeed as shown in Figure 2A, the specific surface area reaches a maximum for both gold catalysts and supports at Fe2Ce8 composition. Rather similar trend was found for the catalysts gold uptake (Figure 2B). The intermediate formulations Fe2Ce8, Fe4Ce6 and Fe6Ce4 allow higher capacity for gold incorporation. As indicated by Moreau et al. gold uptake strongly depends on the point of zero charge (PZC) of the support where gold is anchored, always considering the pH of the media during the synthesis procedure [34]. For basic pH values, taking into account the adsorption of the anionic species, lower the PZC of the support lower the uptake, an effect confirmed by the lower gold loading for pure hematite sample in comparison to ceria (5-6 vs. 6-7 ) [35]. For all the 9 mixed samples, however, the gold uptake is always superior to the hematite and ceria pure supports. It could be due either to the increase of the specific surface of the oxides or to a second reason which might explain the gold enrichment observed for those samples. As commented in the introduction part, the use of appropriate dopants usually leads to the formation of oxygen vacancies on the ceria structure. These vacancies are viewed as electron rich sites where gold particles tend to nucleate [28]. Therefore, the enhanced gold uptake observed for the doped ceria samples could suggest higher concentration of oxygen vacancies on these mixed systems compared to the monometallic compositions. 3.2 XRD The XRD patterns are presented in Figure 3A, B, for the supports and gold catalysts, respectively. For the supports, all the diffractions could be assigned either to face-centered cubic fluorite structure of ceria with cell parameter a = 0.541 nm (# JCPDS File 034-0394) or to hematite, α-Fe2O3, (# JCPDS File 033-0664). The progressive addition of Fe2O3 to ceria causes broadening and shifts of the ceria diffractions toward higher 2theta angles, the later suggesting a solid solution like formation, proof being the Fe2Ce8 sample, for which the absence of the hematite phase is accompanied by the ceria lattice parameter decrease. The decrease of the lattice parameter originates the contraction of the unit cell due to the possible substitution of Ce4+ (0.97 Å) by Fe3+ with quite lower ionic radius (0.64 Å) and reveals formation of a cubic CeO2-like solid solution for this sample. Starting from Fe4Ce6 sample, weak diffractions related to hematite, appeared in the XRD pattern. For this sample the coexistence of Ce-Fe solid solution and Fe2O3 oxide could be imagined. A further increase of iron oxide concentration leads to an increase of the relative contribution of this phase, and consequently decreases that of ceria. For 16 Regarding the selectivity, Figure 7B shows the selectivity curves obtained during the PrOx test. All the catalysts presented a similar trend, the selectivity towards CO oxidation decreases when the temperature increases. The latter is in line with the most of the literature data involving gold based catalysts for the CO-PrOx and could be explained in terms of the competitive H2/CO oxidation processes [49-51]. Carbon monoxide oxidation is favoured at low temperature and gold based catalysts are very promising systems in this regard allowing high CO oxidation rates no matter the gas stream (direct or preferential CO oxidation environments). Nevertheless, when the temperature increases, H2 combustion becomes energetically more favourable and starts to control the selectivity of the overall process. In a recent publication we analysed in details the CO/H2 competition via pulses experiments reaching the conclusion that at low temperature CO is able to displace H2 from the surface and reach the actives sites of the catalyst and subsequently react. However, this ability of CO to remove hydrogen from the catalyst surface decreases with the temperature and therefore the CO oxidation diminishes in favour of the hydrogen oxidation marking the selectivity of the preferential CO oxidation [52]. However, the selectivity of this type of materials could be notably improved by adjusting some reaction parameters as for example O2/CO ratio and/or the space velocity [50]. Overall, the catalysts developed in this study can be categorized as rather efficient solids for the preferential CO oxidation in H2 rich streams and in the presence of CO2 and H2O. Very importantly the best CO oxidation activity is reached at low temperature (100-120 ºC) which represents a temperature window that matches pretty well with the working temperature of the PEM-FCs. 17 Conclusions A series of nanogold based catalysts rather useful for environmental applications have been designed in this work. The prepared catalysts exhibit fairly good activity in the total and preferential CO oxidation reactions. The activity of these materials is guided by the support composition which is the key parameter in the catalytic design. Our data indicate that mesoporous CeO2-Fe2O3 mixed oxides are ideal supports for gold nanoparticles. In particular ceria-rich compositions like Ce0.8Fe0.2 O2 seems to be optimum to achieve the best activity. From the Ce-Fe intimate contact some relevant features of the material, as for example the population of oxygen vacancies and the d-band structure, are altered affecting positively to the catalysts performance. As an additional advantage, the catalysts developed in this study are very effective in the CO- PrOx reaction under realistic post-reforming streams. Moreover, their activity is maximized at low temperatures matching rather well with the typical operation temperatures of the polymer electrolyte fuel cells converting our systems in an interesting alternative to be further explored in the field of hydrogen economy. Acknowledgements Financial support for this work has been obtained from Junta de Andalucía (TEP-8196) and from the Spanish Ministerio de Economía y Competitividad (ENE2012-374301-C03-01 and ENE2013-47880-C3-2-R) co-financed by FEDER funds from the European Union. The Bulgarian team gratefully acknowledge the financial support by the Bulgarian National Science Fund (Projects DFNI E-01/7-2012 and E-02/2-2014). 18 References [1] R.J. Farrauto, Y. Liu, W. Ruettinger, O. Ilinich, L. Shore, T. Giroux, Catal. Rev. 49 (2007) 141–196. [2] M.A. Hickner, H.Ghassemi, Y.S.Kim, B.R. Einsla, J.E. McGrath Chem. Rev. 104 (2004) 4487-4612. [3] F. Joensen, J. R. Rostrup-Niesen, J. Power Sources 105 (2002) 195-201. [4] S. Royer, D. Duprez, ChemCatChem, 3 (2011) 24-65. [5] K.C. Petallidou, A.M. Efstathiou, Appl. Catal. B. 140–141 (2013) 333–347. [6] N. Bion, F. Epron, M. Moreno, F. Mariño, D. Duprez, Top. Catal. 51 (2008) 76–88. [7] F. Mariño, G. Baronetti, M. Laborde, N. Bion, A. Le Valant, F. Epron, D. Duprez, Int. J. of Hydrogen Energy 33 (2008) 1345-1353. [8] G. Avgouropoulos, M. Manzoli, F. Boccuzzi, T. Tabakova, J. Papavasiliou, T. Ioannides V. Idakiev, J. Catal. 256 (2008) 237-247. [9] R. Burch, Phys. Chem. Chem. Phys. 8 (2006) 5483-5500. [10] B. Hammer, J.K. Nørskov, Nature 376 (1995) 238-240. [11] A. Sandoval, L. Delannoy, C. Méthivier, C. Louis, R. Zanella, Appl.Catal. A 504 (2015) 287–294. [12] S.A.C. Carabineiro, S.S.T. Bastos, J.J.M. Órfão, M.F.R. Pereira, J.J. Delgado, J.L. Figueiredo, Appl. Catal. A 381 (2010) 150-160. [13] S.A.C. Carabineiro, A.M.T. Silva, G. Dražić, P.B. Tavares, J.L. Figueiredo, Catal. Today 154 (2010) 21-30. [14] S.A.C. Carabineiro, A.M.T. Silva, G. Dražić, P.B. Tavares, J.L. Figueiredo, Catal. Today 154 (2010) 293-302. [15] D.L. Trimm, Appl. Catal. A 296 (2005) 1-11. [16] X. Du, D. Gao, Z. Yuan, N. Liu, C. Zhang, S. Wang, Int. J. Hydrogen Energy 33 (2008) 3710-3718. [17] G. Avgouropoulos, J. Papavasiliou, T. Ioannides, Catal. Commun. 9 (2008) 1656-1660. [18] S.A.C. Carabineiro, N. Bogdanchikova, P. B. Tavaresc, J.L. Figueiredo, RSC Adv. 2 (2012) 2957-2965. 19 [19] M.A. Soria, P. Pérez, S.A.C. Carabineiro, F.J. Maldonado-Hódar, A. Mendes, Luis M. Madeira, Appl. Catal. A 470 (2014) 45-55. [20] A. Corma, H. García, Chem. Soc. Rev. 37 (2008) 2096-2126. [21] T. Tabakova, F. Boccuzzi, M. Manzoli, D. Andreeva, Appl. Catal. A 252 (2003) 385-397. [22] Z.Y. Yuan, V. Idakiev, A. Vantomme, T. Tabakova, T-Z. Ren, B-L. Su, Catal.Today 131 (2008) 203–210. [23] L. Pastor-Perez, R. Buitrago-Sierra, A. Sepulveda-Escribano, Int. J. Hydrogen Energy 39 (2014) 17589-7599. [24] C. Pojanavaraphan, W. Nakaranuwattana, A. Luengnaruemitchai, E. Gulari, Chem. Eng. J. 240 (2014) 99–108. [25] M.M. Schubert, V. Plzak, J. Garche, R.J. Behm, Catal. Lett. 76 (2001) 143-150. [26] W. Deng, J. De Jesus, H. Saltsburg, M. Flytzani-Stephanopoulos, Appl. Catal. A 291 (2005) 126-135. [27] E. Ko, E.D. Park, K.W. Seo, H.C. Lee, D. Lee, S. Kim, Catal. Today 116 (2006) 377-383. [28] W.Y. Hernández, M.A. Centeno, F. Romero-Sarria, J.A. Odriozola, J. Phys. Chem. C 114 (2010) 10857- 10865. [29] T.R. Reina, S. Ivanova, M. I. Domínguez, M.A Centeno, J.A Odriozola, Appl. Catal. A 419- 420 (2012) 58-66. [30] J.-L. Cao, Y. Wang, T.-Y. Zhang, S.-H. Wu, Z.-Y. Yuan, Appl. Catal. B 78 (2008) 120-128. [31] T. Tabakova, F. Boccuzzi, M. Manzoli, J. W. Sobczak, V. Idakiev, D. Andreeva, Appl. Catal. B 49 (2004) 73-81. [32] M.I. Domínguez, M. Sánchez, M.A. Centeno, M. Montes, J.A. Odriozola. Appl. Catal. A 302 (2006) 96–103. [33] Z. Konya, V.F. Puntes, I. Kiricsi, J. Zhu, J.W. Ager, M.K. Ko, H. Frei, P. Alivisatos, G.A. Somorjai, Chem. Mater. 15 (2003) 1242-1248. [34] F. Moreau, G.C. Bond, Appl. Catal. A 302 (2006) 110–117. [35] G. A.Parks, Chem. Rev. 65 (1965) 177–198. [36]V.V. Pushkarev, V.I. Kovalchuk ,J. L. d'Itri, J. Phys. Chem. B 108 (2004) 5341-5348. [37]Z. Wu, M. Li, J. Howe, H. M. Meyer III, S. H. Overbury, Langmuir 26 (21) (2010) 16595– 16606. [38] R. Q. Long, Y. P. Huang, H. L. Wan, J. Raman Spectroscopy 28 (1997) 29-32. 20 [39] S.H. Shim, T.S. Duffy, American Mineralogist 87 (2001) 318-326. [40] V. Petrovsky, B.P. Gorman, H.U. Anderson, T. Petrovsky, J. Appl. Phys. 90 (2001) 2517- 252. [41] M. Schwidder, M. S. Kumar, K. Klementiev, M. M. Pohl, A. Bruckner, W. Grunert, J. Catal. 231 (2005) 314-330. [42] A. S. Reddy, C. Y. Chen, C. C. Chen, S. H. Chen, C. J. Lin, K. H. Lin, C. L. Chen, S. C. Chang, J. Mol. Catal. A: Chem. 318 (2010) 60-67. [43] N. Pestryakov, N. Bogdanchikova, A. Simakov, I. Tuzovskaya, F. Jentoft, M. Farias, A. Díaz, Surface Science 601 (2007) 3792–3795. [44] M.M. Schubert, S. Hackenberg, A.C. van Veen, M. Muhler, V. Plzak, R.J. Behm, J. Catal. 197 (2001) 113-122. [45] Y.-F. Han, Z. Zhong, K. Ramesh, F. Chen, L. Chen, J. Phys. Chem. C 111 (2007) 3163– 3170. [46] M. C. Kung, R. Davis, H. H. Kung, J. Phys. Chem. C 111 (2007) 11767-11775. [47] D. Gamarra, A. Martínez-Arias, J. Catal., 263 (2009) 189–195. [48] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301-309. [49] A. Luengnaruemitchai, S. Osuwan, E. Gulari, Int. J. Hydrogen Energy 29 (2004) 429-435. [50] T. R. Reina, E. Papadopoulou, S. Palma, S. Ivanova, M.A. Centeno, T. Ioannides, J. A. Odriozola, Appl. Catal. B 150-151 (2014) 554-563. [51] H.C. Lee, D.H. Kim, Catal. Today 132 (2008) 109-116. [52] T.R. Reina, C. Megias-Sayago, A. Perez, S. Ivanova, M.A Centeno, J.A Odriozola, J. Catal. 326 (2015) 161-171. 21 Table 1 Comparison of textural properties of Fe(10-x)Cex and Au/Fe(10-x)Cex catalysts. Catalysts Au contenta (wt. %) Surface area (m2/g) Pore volume (cm3/g) Average pore size (nm) d111 Aub (nm) α220 CeO2 c (nm) Fe0Ce10 - 86 0.105 2.3 - 0.5412 Fe2Ce8 - 118 0.281 9.2 - - Fe4Ce6 - 104 0.340 12.6 - - Fe6Ce4 - 73 0.371 20.1 - - Fe8Ce2 - 68 0.351 19.8 - - Fe10Ce0 - 46 0.157 8.1 - - Au/ Fe0Ce10 2.95 69 0.104 4.7 6 0.5422 Au/Fe2Ce8 3.23 109 0.254 8.0 5.5 0.5370 Au/Fe4Ce6 3.68 93 0.315 9.9 5.8 0.5363 Au/Fe6Ce4 3.35 60 0.295 18.6 6.2 0.5367 Au/Fe8Ce2 3.20 51 0.264 24.1 6.2 0.5392 Au/ Fe10Ce0 2.45 39 0.151 9.2 n.dd - aAnalyzed by atomic absorption method. b average particles size, calculated from Scherrer’s equation and (111) crystallographic plane cunit cell parameter, calculated from the (220) crystallographic plane and Bragg’s equation d not detected (< 5 nm) Table 2 Low Temperature CO oxidation reaction rates inmol CO mol Au-1s-1 and TOFs values for the gold catalysts at a) 0 ºC and b) 25 ºC. Solid a)reaction rate (x 102) b)reaction rate (x102) a)TOF (s-1) b)TOF (s-1) Au particle size (nm) Au dispersion (%) Au/ Fe0Ce10 1.09 2.16 0.05 0.08 6 22 Au/Fe2Ce8 5.83 8.13 0.24 0.34 5.5 24 Au/Fe4Ce6 4.38 7.12 0.19 0.31 5.8 23 Au/Fe6Ce4 1.38 3.56 0.07 0,15 6.2 21 Au/Fe8Ce2 1.29 3.70 0.06 0,14 6.2 21 Au/ Fe10Ce0 0.21 0.38 0.01 0,02 5* 26 * For this sample Au diffraction peaks were not observed so that 5 nm was assumed as average gold particle size. This assumption is made based on the detection limit of the XRD and aiming to avoid a possible overestimation of the TOF. Figure 1 N2 adsorption-desorption isotherms and pore volume distribution of the prepared materials; A and B) Supports; C and D) Gold based catalysts. (The reader is referred to the web version of the article for interpretation of the color information in this figure legend.) Figure 2 A) specific surface area as a function of Fe loading; B) gold uptake as a function of Fe content. 22 Figure 3 X-ray diffraction patterns of the studied samples. A) Supports; B) Gold based catalysts; C) Au (111) region. Figure 4 Raman spectra of the Ce-Fe mixed supports. A) all the supports together; B) detailed spectra of the Fe10Ce0 sample; C) detailed spectra of the Fe0Ce010 support. Figure 5 UV-Vis study A) supports spectra; B) subtracted spectra: FeXCe (10-X) – Fe0Ce10 ; C) gold based catalysts spectra. Figure 6 Catalytic activity in the total CO oxidation. A) Typical light-off run from room temperature to 400 ºC; B) low temperature CO oxidation activity. Figure 7 Catalytic screening in the preferential CO oxidation (PrOx) A) CO conversion; B) selectivity towards CO2 23 Figure 1 010 20 30 40 50 60 70 0,000 0,005 0,010 0,015 0,020 Fe0Ce10 Fe2Ce8 Fe4Ce6 Fe6Ce4 Fe8Ce2 Fe10Ce0 dV/dD (cm3/g/nm) Pore diameter (nm) B 0,0 0,2 0,4 0,6 0,8 1,0 0 50 100 150 200 250 300 Volume adsorbed (cm3/g) Relative pressure (P/P0) C 010 20 30 40 50 60 70 0.000 0.004 0.008 0.012 0.016 dV/dD (cm3/g/nm) Pore diameter (nm) Au/Fe0Ce10 Au/Fe2Ce8 Au/Fe4Ce6 Au/Fe6Ce4 Au/Fe8Ce2 Au/Fe10Ce0 D 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 Fe2Ce8 Fe4Ce6 Fe6Ce4 Fe8Ce2 Fe2O3 CeO2 Volume adsorbed (cm3/g) Relative pressure (P/P0) A 24 Figure 2 0 2 4 6 8 10 30 40 50 60 70 80 90 100 110 120 supports gold based catalysts A S BET ( m2x g-1) Fe content 0 2 4 6 8 10 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 B Gold Uptake (%) Fe content 0 2 4 6 8 10 30 40 50 60 70 80 90 100 110 120 supports gold based catalysts A S BET ( m2x g-1) Fe content 25 Figure 3 10 20 30 40 50 60 70 80 A CeO2 Fe2O3 Fe10Ce0 Fe8Ce2 Fe6Ce4 Fe4Ce6 Fe2Ce8 Fe0Ce10 Intensity, a.u. 2 , degree 10 20 30 40 50 60 70 80 B CeO2 Au Fe2O3Au/Fe10Ce0 Au/Fe8Ce2 Au/Fe6Ce4 Au/Fe4Ce6 Au/Fe2Ce8 Au/Fe0Ce10 Intensity, a.u. 2 , degree 35 36 37 38 39 40 41 42 C Au (111) Au/Fe10Ce0 Au/Fe8Ce2 Au/Fe6Ce4 Au/Fe4Ce6 Au/Fe2Ce8 Au/Fe0Ce10 Intensity, a.u. 2 , degree