One-Step Synthesis of AuCu/TiO2 Catalysts for CO Preferential Oxidation

Catarine Santos Lopes Alencar Ana Rita Noborikawa Paiva Julio Cesar Martins da Silva Jorge Moreira Vaz Estevam Vitorio Spinacé About the authors

Abstract

Au/TiO2 (1wt% Au), Cu/TiO2 (1wt% Cu) and AuCu/TiO2 (1wt% AuCu) catalysts with different Au:Cu mass ratios were prepared in one-step synthesis using sodium borohydride as reducing agent. The resulting catalysts were characterized by X-ray diffraction (XRD), X-ray Dispersive Energy (EDX), Transmission Electron Microscopy (TEM) and Temperature Programmed Reduction (TPR) and tested for the preferential oxidation of carbon monoxide (CO-PROX reaction) in H2-rich gases. EDS analysis showed that the Au contents are close to the nominal values whereas for Cu these values are always lower. X-ray diffractograms showed only the peaks of TiO2 phase; no peaks of metallic Au and Cu species or oxides phases were observed. TPR and high-resolution TEM analysis showed that AuCu/TiO2 catalysts exhibited most of Au in the metallic form with particles sizes in the range of 3-5 nm and that Cu was found in the form of oxide in close contact with the Au nanoparticles and well spread over the TiO2 surface. The AuCu/TiO2 catalysts exhibited good performance in the range of 75-100 °C and presented a better catalytic activity when compared to the monometallic ones. A maximum CO conversion of 98.4% with a CO2 selectivity of 47% was obtained for Au0.50Cu0.50/TiO2 catalyst at 100oC.

Keywords:
AuCu/TiO2 catalyst; carbon monoxide; preferential oxidation; hydrogen

1. Introduction

Hydrogen gas is produced predominantly by combining the methane steam reforming and the gas-water shift reactions resulting in a hydrogen-rich mixture containing about 1% of carbon monoxide (10.000 ppm of CO). This CO level is not sufficiently low for application of hydrogen in Proton Exchange Membrane Fuel Cell (PEMFC) and in ammonia synthesis because CO poison the catalysts used in these processes11 Mishra A, Prasad R. Review on preferential oxidaiton of carbon monoxide in hydrogen rich gases. Bull Chem React Eng Catal. 2011;6(1):1-14. http://dx.doi.org/10.9767/bcrec.6.1.191.1-14.
http://dx.doi.org/10.9767/bcrec.6.1.191....

2 Liu K, Wang A, Zhang T. Recent advances in preferential oxidation of CO reaction over Platinum Group Metal Catalysts. ACS Catal. 2012;2(6):1165-78. http://dx.doi.org/10.1021/cs200418w.
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3 Huang S, Hara K, Fukuoka A. Green catalysis for selective CO oxidation in hydrogen for fuel cell. Energy Environ Sci. 2009;2(10):1060-8. http://dx.doi.org/10.1039/b910696k.
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4 Park ED, Lee D, Lee HC. Recent progress in selective CO removal in a H2-rich stream. Catal Today. 2009;139(4):280-90. http://dx.doi.org/10.1016/j.cattod.2008.06.027.
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-55 Saavedra J, Whittaker T, Chen Z, Pursell CJ, Rioux RM, Chandler BD. Controlling activity and selectivity using water in the Au-catalysed preferential oxidation of CO in H2. Nat Chem. 2016;8(6):584-9. http://dx.doi.org/10.1038/nchem.2494.
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. Therefore, the purification of the hydrogen-rich mixture is necessary and one process that has been considered very promising is the preferential oxidation of CO in hydrogen-rich mixtures (CO-PROX reaction), because it can dramatically reduce energy and hydrogen losses when compared to the processes currently used in the industry such as CO methanation and pressure swing adsorption (PSA). However, the catalysts for CO-PROX reaction should be active and especially highly selective, as it should selectively oxidize CO and avoid hydrogen oxidation11 Mishra A, Prasad R. Review on preferential oxidaiton of carbon monoxide in hydrogen rich gases. Bull Chem React Eng Catal. 2011;6(1):1-14. http://dx.doi.org/10.9767/bcrec.6.1.191.1-14.
http://dx.doi.org/10.9767/bcrec.6.1.191....

2 Liu K, Wang A, Zhang T. Recent advances in preferential oxidation of CO reaction over Platinum Group Metal Catalysts. ACS Catal. 2012;2(6):1165-78. http://dx.doi.org/10.1021/cs200418w.
http://dx.doi.org/10.1021/cs200418w...

3 Huang S, Hara K, Fukuoka A. Green catalysis for selective CO oxidation in hydrogen for fuel cell. Energy Environ Sci. 2009;2(10):1060-8. http://dx.doi.org/10.1039/b910696k.
http://dx.doi.org/10.1039/b910696k...

4 Park ED, Lee D, Lee HC. Recent progress in selective CO removal in a H2-rich stream. Catal Today. 2009;139(4):280-90. http://dx.doi.org/10.1016/j.cattod.2008.06.027.
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-55 Saavedra J, Whittaker T, Chen Z, Pursell CJ, Rioux RM, Chandler BD. Controlling activity and selectivity using water in the Au-catalysed preferential oxidation of CO in H2. Nat Chem. 2016;8(6):584-9. http://dx.doi.org/10.1038/nchem.2494.
http://dx.doi.org/10.1038/nchem.2494...
.

Au nanoparticles supported on TiO2 (Au/TiO2 catalyst) showed good CO conversions and CO2 selectivity for CO-PROX reaction in the range of 20 to 100oC; however, the procedure used to prepare Au/TiO2 catalysts has a significant influence on the catalytic performance, which is a result of Au nanoparticles sizes (should be smaller than 5 nm) and Au-TiO2 interactions66 Leal GB, Ciotti L, Watacabe BN, Loureiro da Silva DC, Antoniassi RM, Silva JCM, et al. Preparation of Au/TiO2 by a facile method at room temperature for the CO preferential oxidation reaction. Catal Commun. 2018;116:38-42. http://dx.doi.org/10.1016/j.catcom.2018.07.021.
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7 Klyushin AY, Jones TE, Lunkenbein T, Kube P, Li X, Havecker M, et al. Strong metal support interaction as a key factor of Au activation in CO oxidation. ChemCatChem. 2018;10(18):3985-9. http://dx.doi.org/10.1002/cctc.201800972.
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8 Panayotov DA, Morris JR. Surface chemistry of Au/TiO2: thermally and photolytically activated reactions. Surf Sci Rep. 2016;71(1):77-271. http://dx.doi.org/10.1016/j.surfrep.2016.01.002.
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9 Wang Y, Widmann D, Behm RJ. Influence of TiO2 bulk defects on CO adsorption and CO oxidation on Au/TiO2: electronic metal-support interactions (EMSIs) in supported Au catalysts. ACS Catal. 2017;7(4):2339-45. http://dx.doi.org/10.1021/acscatal.7b00251.
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10 Lakshmanan P, Park JE, Park ED. Recent advances in preferential oxidation of CO in H2 over gold catalysts. Catal Surv Asia. 2014;18(2-3):75-8. http://dx.doi.org/10.1007/s10563-014-9167-x.
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11 Qiao B, Liu J, Wang Y-G, Lin Q, Liu X, Wang A, et al. Highly efficient catalysis of preferential oxidation of CO in H2-rich stream by gold single-atom catalysts. ACS Catal. 2015;5(11):6249-54. http://dx.doi.org/10.1021/acscatal.5b01114.
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-1212 Tabakova T. Recent advances in design of gold-based catalysts for H2 clean-up reaction. Front Chem. 2019;7:517. http://dx.doi.org/10.3389/fchem.2019.00517.
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. Sangeetha et al.1313 Sangeetha P, Zhao B, Chen Y-W. Au/CuOx-TiO2 catalysts for preferential oxidation of CO in hydrogen stream. Ind Eng Chem. 2010;49(5):2096-102. http://dx.doi.org/10.1021/ie901233e.
http://dx.doi.org/10.1021/ie901233e...
prepared Au nanoparticles supported on CuOx -TiO2 (x from 1 and 10 wt%), where the support was prepared by impregnation of Cu(NO3)2 in TiO2 (Degussa P25) and treating at 350ºC. The deposition of Au nanoparticles was carried out by the deposition-precipitation method obtaining nanoparticle sizes of about of 2.5 nm. The resulting Au/CuOx-TiO2 catalysts were more active than the Au/TiO2 catalyst with CO conversions close to 100% and CO2 selectivity values of 60 to 80% in the temperature range of 50 °C to 100 °C. Duh et al.1414 Duh FC, Lee DS, Chen YW. Au/CuOx-TiO2 catalysts for CO oxidation at low temperature. Modern Research in Catalysis. 2013;02(01):1-8. http://dx.doi.org/10.4236/mrc.2013.21001.
http://dx.doi.org/10.4236/mrc.2013.21001...
also prepared a series of Au/CuOx-TiO2 catalysts with various Cu/Ti ratios by incipient-wetness impregnation and Au was supported by deposition-precipitation. It was observed that the addition of CuOx in Au/TiO2 catalyst enhanced the activity significantly for CO oxidation at low temperature, which was attributed to the interactions among Au, CuOx and TiO2. Recently, Qi et al.1515 Qi C, Zheng Y, Lin H, Su H, Sun X, Sun L. CO oxidation over gold catalysts supported on CuO/Cu2O both in O2-rich and H2-rich streams: necessity of copper oxide. Appl Catal B. 2019;253:160-9. http://dx.doi.org/10.1016/j.apcatb.2019.03.081.
http://dx.doi.org/10.1016/j.apcatb.2019....
prepared nanosized CuO and Cu2O materials and followed by the deposition of Au nanoparticles. The catalytic activities of the resulting AuCu catalysts for CO-PROX showed a significant enhancement when compared to CuO and Cu2O materials. It was concluded that CuO not Cu2O species play a critical role for the CO oxidation and that the cooperative effect between CuO and Au nanoparticles enhanced the activity of Au/CuO catalyst when a strong interaction between them occurred.

In this work we prepare Au/TiO2, Cu/TiO2 and AuCu/TiO2 catalysts in one-step synthesis by co-reducing the Au+3 and Cu+2 ions with sodium borohydride in the presence of the TiO2 support. The catalysts were characterized and tested for CO-PROX reaction.

2. Experimental

2.1 Synthesis of Au/TiO2 catalyst (1.0 wt% Au)

A suspension containing 346.5 mg of TiO2 (P25 Evonik) and 45 mL of deionized water was prepared. Then 6.12 x 10-4 L of tetrachlorouric acid (HAuCl4) solution (2.88 x 10-2 mol L-1) was added and stirred for homogenization for about 10 min. After this, 2.02 x 10-4 L of NaBH4 solution (2.643 x 10-1 mol L-1) was added and the resulting mixture remained under stirring for 24h. Then, the solid was then separated by centrifugation, washed with excess water and dried at 70 °C.

2.2 Synthesis of Cu/TiO2 catalyst (1.0 wt% Cu)

The procedure was similar to that described above, but using 5.50 x 10-4 L of Cu(NO3)2 solution (1 x 10-1 mol L-1).

2.3 Synthesis of AuCu/TiO2 catalyst with different mass ratios

The procedure was similar to that described above but using HAuCl4 solution (2.88 x 10-2 mol L-1) and Cu(NO3)2 solution (1 x 10-1 mol L-1) in the desired proportions and 5.14 x 10-4 L of NaBH4 solution (2.643 x 10-1 mol L-1). The pH of the synthesis solutions were in the range of 4.5 and 5.

2.4 Characterizations

The semi-quantitative chemical analysis of the catalysts were performed by Energy-dispersive X-ray spectroscopy (EDS) on a Philips Scanning Microscope model XL30 with 20 kV electron beam equipped with an EDAX model DX-4 micro analyzer.

Transmission electron microscopy (TEM) was performed on a JEOL Transmission Electron Microscope, model JEM-2100 (200 kV). The particle size distributions were obtained by measuring de diameter of more than 100 nanoparticles from the micrographs.

X-ray diffraction (XRD) was performed on a Rigaku Multiflex diffractometer using Cu Kα radiation source (λ = 1.5418Ȧ) with 2θ scan between 20º and 90º, with 0.06º step and time per step of 4s.

Temperature Programmed Reduction (TPR) experiments were performed on Quantachrome ChemBET Pulsar using 50 mg of catalyst in a U-shaped quartz cell and H2 consumption was measured using a thermal conductivity detector (TCD). Initially the catalyst was treated in a flow of N2 (50 mL min-1) at 200 °C for 1 h and after cooling to room temperature the catalyst was exposed to gas containing 10% vol H2/N2 at a flow rate of 30 mL min-1and heated to 750ºC at 10ºC min-1.

2.5 Catalytic tests

The catalytic tests were performed in a fixed bed reactor (U-tube) using 100 mg of catalysts in the temperature range between 25 °C to 150 °C using a gas composition containing 1 mol% of CO, 1mol% of O2 and 98 mol% of H2 (O2/CO volumetric ratio of 1, λ =2) and a flow rate of 25 mL min-1 (spatial velocity of 15000 mL gcat-1 h-1). The reaction products and unconverted reagents were quantified using a Gas Chromatograph Agilent HP 7890A with TCD and FID (methanation of CO and CO2) detectors. To evaluate the performance of each catalyst, CO conversion and CO2 selectivity were calculated according to the Equations 1 and 2:

C O c o n v e r s i o n : C O i C O f × 100 C O i (1)
C O 2 s e l e c t i v i t y : 0.5 × C O 2 f × 100 O 2 i O 2 f (2)

3. Results and Discussion

The semi-quantitative EDS analyses of the catalysts are shown in Table 1.

Table 1
Chemical composition and average particle sizes of the catalysts.

In a general manner, it was observed that the amounts of Au and Cu determined by EDS increase with the increase of the nominal values; however, the values determined for Au are close to the nominal values, while for Cu these values are always smaller than the nominal ones. This could be explained by the EDS analysis having been performed in a semi-quantitative way and/or or that not all Cu species were deposited on the TiO2 support as the mother liquor was slightly colored after centrifugation.

The X-ray diffractograms of the TiO2 support and Au/TiO2, Cu/TiO2 and Au0.50Cu0.50/TiO2 catalysts are shown in Figure 1.

Figure 1
X-ray diffractograms of the TiO2 support and the catalysts (A = peaks of Anatase phase, R = peaks of Rutile phase, * peaks position of Cu (fcc) phase and + peaks position of Au (fcc) phase)

For all catalysts it was observed the diffraction peaks of the support TiO2 P25, which has about 80% of anatase phase with 2θ: 25.36º, 37.89º, 48.14º, 54.03º, 55.18º corresponding to the planes (101) , (004), (200), (105), (211) (110), (101), (211) and 20% of rutile phase with 2θ: 27.4º, 36.1º and 54.4º corresponding to the planes (110), (101), (211)66 Leal GB, Ciotti L, Watacabe BN, Loureiro da Silva DC, Antoniassi RM, Silva JCM, et al. Preparation of Au/TiO2 by a facile method at room temperature for the CO preferential oxidation reaction. Catal Commun. 2018;116:38-42. http://dx.doi.org/10.1016/j.catcom.2018.07.021.
http://dx.doi.org/10.1016/j.catcom.2018....
,1616 Jiang X, Manawan M, Feng T, Qian R, Zhao T, Zhou G, et al. Anatase and rutile in evonik aeroxide P25: heterojunctioned or individual nanoparticles? Catal Today. 2018;300:12-7. http://dx.doi.org/10.1016/j.cattod.2017.06.010.
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. However, it was not observed the presence of Cu0 and Au0 having a face-centered cubic structure (fcc) where the diffracting planes are (111), (200), (220) which correspond to 2θ: 43.24º; 50.35 °; 73.96 ° for Cu (#PDF 4-784) and 38.17 °; 44.37 °; 64.55 ° for Au (#PDF 4-836)1717 Zhao D, Xiong X, Qu C-L, Zhang N. Remarkable enhancement in Au catalytic utilization for liquid redox reactions by galvanic deposition of Au on Cu nanoparticles. J Phys Chem C. 2014;118(33):19007-16. http://dx.doi.org/10.1021/jp500908e.
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nor the presence of copper oxide phases such as CuO or Cu2O1818 Choi D, Jang D-J. Facile fabrication of CuO/Cu2O composites with high catalytic performances. New J Chem. 2017;41(8):2964-72. http://dx.doi.org/10.1039/C6NJ03949A.
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. This could be due to the low content of these metals or to the average diameter of the crystallites (< 5 nm) resulting in low intensity and broad peaks that in the presence of well-defined and high intensity crystalline peaks of TiO2 support make their identification difficult1919 Yang Y-F, Sangeetha P, Chen Y-W. Au/TiO2 catalysts prepared by photo-deposition method for selective CO oxidation in H2 stream. Int J Hydrogen Energy. 2009;34(21):8912-20. http://dx.doi.org/10.1016/j.ijhydene.2009.08.087.
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.

The transmission electron micrographs of Cu/TiO2, Au/TiO2 and AuCu/TiO2 catalysts are shown in Figure 2.

Figure 2
Transmission electron micrographs and histograms of a) Cu/TiO2, b) Au/TiO2, c) Au0.50Cu0.50/TiO2 and d) Au0.75Cu0.25/TiO2 catalysts

For Cu/TiO2 catalyst (Figure 2a) it was not observed in the TEM micrographs the presence of black dots while for catalysts having Au (Figure 2bd) they were observed having average sizes in the range of 3-5 nm (see Table 1). Figure 3 shows a high-angle annular dark-field scanning transmission electron microscopy (HAADF/STEM) image, EDS mapping and line-scan of Au0.50Cu0.50/TiO2 catalyst.

Figure 3
High-angle annular dark-field scanning transmission electron microscopy (HAADF/STEM) image, EDS mapping and line-scan of Au0.50Cu0.50/TiO2 catalyst

HAADF/STEM image showed bright dots of small sizes (average 3 nm) what is coming from the differences between metals and support element atomic numbers contributing to a high contrast in the image showing that metal nanoparticles are dispersed on TiO2 support. The EDS mapping and line scan confirmed that Au is exclusively located in the position of bright dots while Cu is also located at these positions in close contact with Au and distributed over the TiO2 surface.

The temperature programmed reduction (TPR) profiles of TiO2 support and Au/TiO2, Cu/TiO2 and Au0.50Cu0.50/TiO2 catalysts are shown in Figure 4.

Figure 4
H2-TPR profiles of TiO2 support and the catalysts

The H2-TPR profile of the TiO2-P25 support showed no reduction peaks in the studied temperature range as already reported in the literature for temperatures from ambient to 800 °C2020 Li J, Lu G, Wu G, Mao D, Guo Y, Wang Y, et al. Effect of TiO2 crystal structure on the catalytic performance of Co3O4/TiO2 catalyst for low-temperature CO oxidation. Catal Sci Technol. 2014;4(5):1268-75. http://dx.doi.org/10.1039/C3CY01004J.
http://dx.doi.org/10.1039/C3CY01004J...
. The H2-TPR profile of Cu/TiO2 catalysts showed an intense peak at about 150 °C attributed to the reduction of CuO to Cu(0)2020 Li J, Lu G, Wu G, Mao D, Guo Y, Wang Y, et al. Effect of TiO2 crystal structure on the catalytic performance of Co3O4/TiO2 catalyst for low-temperature CO oxidation. Catal Sci Technol. 2014;4(5):1268-75. http://dx.doi.org/10.1039/C3CY01004J.
http://dx.doi.org/10.1039/C3CY01004J...

21 Li K, Wang Y, Wang S, Zhu B, Zhang S, Huang W, et al. A comparative study of CuO/TiO2-SnO2, CuO/TiO2 and CuO/SnO2 catalysts for low-temperature CO oxidation. J Nat Gas Chem. 2009;18(4):449-52. http://dx.doi.org/10.1016/S1003-9953(08)60144-9.
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22 Kang MY, Yun HJ, Yu S, Kim W, Kim ND, Yi J. Effect of TiO2 crystalline phase on CO oxidation over CuO catalysts supported on TiO2. J Mol Catal Chem. 2013;368-369:72-7. http://dx.doi.org/10.1016/j.molcata.2012.11.021.
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-2323 Oros-Ruiz S, Zanella R, Prado B. Photocatalytic degradation of trimethoprim by metallic nanoparticles supported on TiO2-P25. J Hazard Mater. 2013;263:28-35. http://dx.doi.org/10.1016/j.jhazmat.2013.04.010.
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and a small peak at about 375°C that could be ascribed to the reduction of CuO nanoparticles having little or no interaction with the support2323 Oros-Ruiz S, Zanella R, Prado B. Photocatalytic degradation of trimethoprim by metallic nanoparticles supported on TiO2-P25. J Hazard Mater. 2013;263:28-35. http://dx.doi.org/10.1016/j.jhazmat.2013.04.010.
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,2424 Boccuzzi F, Chiorino A, Martra G, Gargano M, Ravasio N, Carrozzini B. Preparation, characterization and activity of Cu/TiO2 catalysts. J Catal. 1997;165:120-39.. It was also observed on H2-TPR profile of Cu/TiO2 catalyst a peak at about 575 °C. Ramírez and Gutiérrez-Alejandre2525 Ramírez J, Gutiérrez-Alejandre A. Relationship between hydrodesulfurization activity and morphological and structural changes in NiW hydrotreating catalysts supported on Al2O3-TiO2 mixed oxides. Catal Today. 1998;43(1-2):123-33. http://dx.doi.org/10.1016/S0920-5861(98)00141-2.
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observed a peak at about 570 °C in the TPR profile of pure anatase TiO2 support. Kang et al.2222 Kang MY, Yun HJ, Yu S, Kim W, Kim ND, Yi J. Effect of TiO2 crystalline phase on CO oxidation over CuO catalysts supported on TiO2. J Mol Catal Chem. 2013;368-369:72-7. http://dx.doi.org/10.1016/j.molcata.2012.11.021.
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reported for CuO supported on pure anatase TiO2 phase two peaks at 140 and 470 °C that were attributed to reduction of Cu+2 to Cu0 and to the reduction of anatase phase, respectively; however, for CuO supported on pure rutile TiO2 phase only one peak at around 120 °C was observed. Zhang et al.2626 Zhang C, He H, Tanaka K. Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature. Appl Catal B. 2006;65(1-2):37-43. http://dx.doi.org/10.1016/j.apcatb.2005.12.010.
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described that no peaks were observed between 25 and 500 °C in the TPR profile of an anatase TiO2 support; on the other hand, the TPR profile of the Pt/TiO2 catalyst showed two peaks at 80 and 360 °C that were attributed to the reduction of PtOx to metallic Pt and to the reduction of the surface capping oxygen of TiO2, respectively. Thus, the peak observed at 575 °C in the TPR profile may result from the reduction of the surface oxygen of anatase phase of TiO2 P-25 support that is favored by the interaction with Cu species. The H2-TPR profile of Au/TiO2 catalyst did not shown reduction peaks below 100 °C suggesting that Au was predominantly found in the metallic form2727 Pérez P, Soria MA, Carabineiro SAC, Maldonado-Hodar FJ, Mendes A, Madeira LM. Application of Au/TiO2 catalysts in the low-temperature water-gas shift reaction. Int J Hydrogen Energy. 2016;41(8):4670-81. http://dx.doi.org/10.1016/j.ijhydene.2016.01.037.
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taking into account that pre-treated (reduced) Au catalyst do not shown any peak of reduction2828 Magadzu T, Yang JH, Henao JD, Kung MC, Kung HH, Scurrell MS. Low temperature water-gas shift reaction over Au-supported on anatase in the presence of copper: EXAFS/XANES analysis of gold-copper ion mixtures on TiO2. J Phys Chem C. 2017;121(16):8812-23. http://dx.doi.org/10.1021/acs.jpcc.6b11419.
http://dx.doi.org/10.1021/acs.jpcc.6b114...
. However, it was observed a very small and broad peak at about 225 °C that could be related to ionic Au-species interacting with TiO2 phase2727 Pérez P, Soria MA, Carabineiro SAC, Maldonado-Hodar FJ, Mendes A, Madeira LM. Application of Au/TiO2 catalysts in the low-temperature water-gas shift reaction. Int J Hydrogen Energy. 2016;41(8):4670-81. http://dx.doi.org/10.1016/j.ijhydene.2016.01.037.
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,2828 Magadzu T, Yang JH, Henao JD, Kung MC, Kung HH, Scurrell MS. Low temperature water-gas shift reaction over Au-supported on anatase in the presence of copper: EXAFS/XANES analysis of gold-copper ion mixtures on TiO2. J Phys Chem C. 2017;121(16):8812-23. http://dx.doi.org/10.1021/acs.jpcc.6b11419.
http://dx.doi.org/10.1021/acs.jpcc.6b114...
. For Au0.50Cu0.50/TiO2 catalyst it was observed a peak at about 150 °C attributed to the reduction of CuO to Cu(0) and a small and broad peak at about 180 °C that could be due to ionic Au-species interacting with CuO and TiO2 phases. In the region of temperature between 350 and 450 °C two small peaks are observed for Au0.50Cu0.50/TiO2 catalyst, which could be a result of the interaction of Au and Cu species with TiO2 support and a peak at about 575 °C resulting from reduction of TiO2 support. Thus, it could be inferred from these results that AuCu/TiO2 catalysts prepared by this methodology exhibit most of the Au in metallic form while most of the Cu is in oxide form (CuO). In fact, by analyzing the results of H2-TPR and microscopy it could be inferred that Au and CuOx species interact with each other and with TiO2 support.

The catalytic performances of the Cu/TiO2, Au/TiO2 and Au0.50Cu0.50/TiO2 catalysts were studied in the temperature range of 20 °C to 150 °C (Figure 5). No previous treatments were done in these samples before catalytic tests and the results shown correspond to the second cycle of the catalytic reactions.

Figure 5
(a) CO conversion and (b) CO2 selectivity in function of the temperature for Cu/TiO2, Au/TiO2 and Au0.50Cu0.50/TiO2 (1.0% CO, 1.0% O2, 98.0% H2 and a space velocity of 15000 mL gcat-1 h-1)

Cu/TiO2 catalyst showed low CO conversions (below 20%) until 100 °C increasing to 75% at 150 °C. The CO2 selectivity showed a maximum value of 70% at around 120 oC. Au/TiO2 catalyst showed a maximum CO conversion of 55% at 75 °C; however, CO2 selectivity values were very low (around 20%) in all range of temperature. For all AuCu/TiO2 catalysts prepared with different contents of Au and Cu the maximum CO conversions occurred at 100 °C. In addition, the CO conversion values increased with the increase of Au content reaching a maximum value of 98.4% for Au0.50Cu0.50/TiO2 catalyst (Figure 5a) and, after that, these values began to decrease (92.0% for Au0.75Cu0.25/TiO2 catalyst) as the amount of Au was increased further. Conversely, CO2 selectivity values increased with the increase of Cu content and the values varied between 35% and 55% at 100 oC. The CO2 selectivity value for Au0.50Cu0.50/TiO2 catalyst was 47% (Figure 5b) reaching a maximum value of 55% for Au0.25Cu0.75/TiO2 catalyst and after that decreased to 50% for Au0.10Cu0.90/TiO2 catalyst. Thus, AuCu/TiO2 catalysts showed to be more active for CO-PROX reaction than Au/TiO2 and Cu/TiO2 catalysts in the temperature range of 20 °C to 100 °C, as already observed for CO-PROX reaction1313 Sangeetha P, Zhao B, Chen Y-W. Au/CuOx-TiO2 catalysts for preferential oxidation of CO in hydrogen stream. Ind Eng Chem. 2010;49(5):2096-102. http://dx.doi.org/10.1021/ie901233e.
http://dx.doi.org/10.1021/ie901233e...
and for CO-oxidation at low temperature1414 Duh FC, Lee DS, Chen YW. Au/CuOx-TiO2 catalysts for CO oxidation at low temperature. Modern Research in Catalysis. 2013;02(01):1-8. http://dx.doi.org/10.4236/mrc.2013.21001.
http://dx.doi.org/10.4236/mrc.2013.21001...
.

Figure 6 shows the CO conversion as a function of Au content (wt%) and nanoparticle sizes for AuCu/TiO2 catalysts prepared with different contents of Au and Cu. It could be seen that there is a relationship between Au content and nanoparticles sizes where the maximum CO conversion was observed for the sample Au0.50Cu0.50/TiO2 with similar quantities of Au and Cu and that`s where a smaller size of the Au nanoparticles was observed.

Figure 6
CO conversion at 100 °C in the function of Au content (wt%) and nanoparticle sizes for AuCu/TiO2 catalysts with different Au and Cu content (1.0% CO, 1.0% O2, 98.0% H2, 100 °C and a space velocity of 15000 mL gcat-1 h-1)

The long-term stability test results for Au0.50Cu0.50/TiO2 catalyst is shown in Figure 7 showing CO conversions above 90% and CO2 selectivity values in the range of 45-50%, which remained stable throughout the period evaluated showing the stability of the catalysts under the applied reaction conditions. Similar results were observed by Sangeetha et al.1313 Sangeetha P, Zhao B, Chen Y-W. Au/CuOx-TiO2 catalysts for preferential oxidation of CO in hydrogen stream. Ind Eng Chem. 2010;49(5):2096-102. http://dx.doi.org/10.1021/ie901233e.
http://dx.doi.org/10.1021/ie901233e...
in long test time using as catalyst Au/CuOx-TiO2 (1wt% of Au and a CuOx:TiO2 ratio of 4.8:95.2) at 80 °C for 49 h. A maximum CO conversion of 95% with a CO2 selectivity of about 65% was obtained using a gas composition of 1.33% CO, 1.33% O2, 65.33% H2, and 32.01% He and a space velocity of 30000 mL gcat-1 h-1.

Figure 7
CO conversion and CO2 selectivity in function of time for Au0.50Cu0.50/TiO2 catalyst at 100 °C (1% of CO, 1% of O2 and 98% of H2, spatial velocity of 15000 mL gcat-1 h-1)

Sangeetha et al.1313 Sangeetha P, Zhao B, Chen Y-W. Au/CuOx-TiO2 catalysts for preferential oxidation of CO in hydrogen stream. Ind Eng Chem. 2010;49(5):2096-102. http://dx.doi.org/10.1021/ie901233e.
http://dx.doi.org/10.1021/ie901233e...
also observed an increase of maximum CO conversion and CO2 selectivity for CO-PROX reaction comparing Au/TiO2 and Au/CuOx-TiO2 catalysts and attributed this increase due to the presence of Au(0) and CuOx species, where CuOx-TiO2 was proposed to be a supplier or storage of oxygen. Wang et al.2929 Wang A, Liu XY, Mou CY, Zhang T. Understanding the synergistic effects of gold bimetallic catalysts. J Catal. 2013;308:258-71. http://dx.doi.org/10.1016/j.jcat.2013.08.023.
http://dx.doi.org/10.1016/j.jcat.2013.08...
studied the synergistic effects of different Au bimetallic alloy catalysts in low temperature CO oxidation and observed for AuCu alloy catalyst that a phase segregation occurs during CO oxidation forming a Au@CuOx hybrid structure (nano or even sub-nano CuOx supported on Au nanoparticles) resulting in interfacial sites between them. FTIR studies of CO adsorption showed that CO adsorbed on Au(0) while CuOx species were responsible for providing active oxygen in the same way as reducible oxides do. Recently it was shown that CuO not Cu2O species played a more critical role for CO oxidation at low temperature and that CuO and Au(0) species enhanced the activity of Au/CuO catalyst only if a strong interaction occurs between them1515 Qi C, Zheng Y, Lin H, Su H, Sun X, Sun L. CO oxidation over gold catalysts supported on CuO/Cu2O both in O2-rich and H2-rich streams: necessity of copper oxide. Appl Catal B. 2019;253:160-9. http://dx.doi.org/10.1016/j.apcatb.2019.03.081.
http://dx.doi.org/10.1016/j.apcatb.2019....
.

4. Conclusions

Active, selective and stable AuCu/TiO2 catalysts was prepared by a simple one-step methodology for CO-PROX reaction. The AuCu/TiO2 catalysts exhibited good activities and selective in the range of 75-100 °C and presented a better catalytic activity when compared to the monometallic ones. The analyses showed that Au is predominantly found in its metallic form while Cu in its oxide form and that Au(0) and CuOx species are in good interaction with each other and with TiO2 support.

5. Acknowledgments

The authors gratefully acknowledge support from FAPESP and SHELL Brasil through the ‘Research Centre for Gas Innovation – RCGI’ (FAPESP Proc. 2014/50279-4), hosted by the University of São Paulo, and the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. FAPESP / Shell Proc. no 2017/11937-4 (CINE); FAPESP Proc. no 2014/09087-4, and CNPq Proc. no 304869/2016-3 are grateful for financial support. Centro de Ciência e Tecnologia dos Materiais (CCTM) – IPEN-CNEN/SP and LNNano-CNPEM (JEOL JEM 2100F) are greatly acknowledged for the use of TEM facilities.

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Publication Dates

  • Publication in this collection
    23 Oct 2020
  • Date of issue
    2020

History

  • Received
    29 Apr 2020
  • Reviewed
    17 Sept 2020
  • Accepted
    27 Sept 2020
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