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

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.


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 processes [1][2][3][4][5] . Therefore, the purification of the hydrogenrich 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 oxidation 1-5 .
Au nanoparticles supported on TiO 2 (Au/TiO 2 catalyst) showed good CO conversions and CO 2 selectivity for CO-PROX reaction in the range of 20 to 100 o C; however, the procedure used to prepare Au/TiO 2 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-TiO 2 interactions 6-12 . Sangeetha et al. 13 prepared Au nanoparticles supported on CuO x -TiO 2 (x from 1 and 10 wt%), where the support was prepared by impregnation of Cu(NO 3 ) 2 in TiO 2 (Degussa P25) and treating at 350ºC. The deposition of Au nanoparticles was carried out by the depositionprecipitation method obtaining nanoparticle sizes of about of 2.5 nm. The resulting Au/CuO x -TiO 2 catalysts were more active than the Au/TiO 2 catalyst with CO conversions close to 100% and CO 2 selectivity values of 60 to 80% in the temperature range of 50 °C to 100 °C. Duh et al. 14 also prepared a series of Au/CuO x -TiO 2 catalysts with various Cu/Ti ratios by incipient-wetness impregnation and Au was supported by deposition-precipitation. It was observed that the addition of CuO x in Au/TiO 2 catalyst enhanced the activity significantly for CO oxidation at low temperature, which was attributed to the interactions among Au, CuO x and TiO 2 . Recently, Qi et al. 15  In this work we prepare Au/TiO 2 , Cu/TiO 2 and AuCu/ TiO 2 catalysts in one-step synthesis by co-reducing the Au +3 and Cu +2 ions with sodium borohydride in the presence of the TiO 2 support. The catalysts were characterized and tested for CO-PROX reaction.

Synthesis of Au/TiO 2 catalyst (1.0 wt% Au)
A suspension containing 346.5 mg of TiO 2 (P25 Evonik) and 45 mL of deionized water was prepared. Then 6.12 x 10 -4 L of tetrachlorouric acid (HAuCl 4 ) 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 NaBH 4 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.

Synthesis of AuCu/TiO 2 catalyst with different mass ratios
The procedure was similar to that described above but using HAuCl 4 solution (2.88 x 10 -2 mol L -1 ) and Cu(NO 3 ) 2 solution (1 x 10 -1 mol L -1 ) in the desired proportions and 5.14 x 10 -4 L of NaBH 4 solution (2.643 x 10 -1 mol L -1 ). The pH of the synthesis solutions were in the range of 4.5 and 5.

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 H 2 consumption was measured using a thermal conductivity detector (TCD). Initially the catalyst was treated in a flow of N 2 (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 H 2 /N 2 at a flow rate of 30 mL min -1 and heated to 750ºC at 10ºC min -1 .

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 O 2 and 98 mol% of H 2 (O 2 /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 CO 2 ) detectors. To evaluate the performance of each catalyst, CO conversion and CO 2 selectivity were calculated according to the Equations 1 and 2:

Results and Discussion
The semi-quantitative EDS analyses of the catalysts are shown in Table 1.
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 TiO 2 support as the mother liquor was slightly colored after centrifugation.
The X-ray diffractograms of the TiO 2 support and Au/ TiO 2 , Cu/TiO 2 and Au 0.50 Cu 0.50 /TiO 2 catalysts are shown in Figure 1.
For all catalysts it was observed the diffraction peaks of the support TiO 2 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), 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 welldefined and high intensity crystalline peaks of TiO 2 support make their identification difficult 19 .
The transmission electron micrographs of Cu/TiO 2 , Au/ TiO 2 and AuCu/TiO 2 catalysts are shown in Figure 2.
For Cu/TiO 2 catalyst (Figure 2a) it was not observed in the TEM micrographs the presence of black dots while for catalysts having Au (Figure 2b-d) 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 Au 0.50 Cu 0.50 /TiO 2 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 TiO 2 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 TiO 2 surface.
The temperature programmed reduction (TPR) profiles of TiO 2 support and Au/TiO 2 , Cu/TiO 2 and Au 0.50 Cu 0.50 /TiO 2 catalysts are shown in Figure 4.
The H 2 -TPR profile of the TiO 2 -P25 support showed no reduction peaks in the studied temperature range as already reported in the literature for temperatures from ambient to 800 °C 20 . The H 2 -TPR profile of Cu/TiO 2 catalysts showed an intense peak at about 150 °C attributed to the reduction of CuO to Cu(0) 20-23 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 support 23,24 . It was also observed on H 2 -TPR profile of Cu/TiO 2 catalyst a peak at about 575 °C. Ramírez and Gutiérrez-Alejandre 25 observed a peak at about 570 °C in the TPR profile of pure anatase TiO 2 support. Kang et al. 22 reported for CuO supported on pure anatase TiO 2 phase two peaks at 140 and 470 °C that were attributed to reduction of Cu +2 to Cu 0 and to the reduction of anatase phase, respectively; however, for CuO supported on pure rutile TiO 2 phase only one peak at around 120 °C was observed. Zhang et al. 26 described that no peaks were observed between 25 and 500 °C in the TPR profile of an anatase TiO 2 support; on the other hand, the TPR profile of the Pt/TiO 2 catalyst showed two peaks at 80 and 360 °C that were attributed to the reduction of PtO x to metallic Pt and to the reduction of the surface capping oxygen of TiO 2 , 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 TiO 2 P-25 support that is favored by the interaction with Cu species. The H 2 -TPR profile of Au/TiO 2 catalyst did not shown reduction peaks below 100 °C suggesting that Au was predominantly found in the metallic form 27 taking into account that pre-treated (reduced) Au catalyst do not shown any peak of reduction 28 . However, it was observed a very small and broad peak at about 225 °C that could be related to ionic Au-species interacting with TiO 2 phase 27,28 . For Au 0.50 Cu 0.50 /TiO 2 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 TiO 2 phases. In the region of temperature between 350 and 450 °C two small peaks are observed for Au 0.50 Cu 0.50 /TiO 2 catalyst, which could be a result of the interaction of Au and Cu species with TiO 2 support and a peak at about 575 °C resulting from reduction of TiO 2 support. Thus, it could be inferred from these results that AuCu/TiO 2 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 H 2 -TPR and microscopy it could be inferred that Au and CuO x species interact with each other and with TiO 2 support.
The catalytic performances of the Cu/TiO 2 , Au/TiO 2 and Au 0.50 Cu 0.50 /TiO 2 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.
Cu/TiO 2 catalyst showed low CO conversions (below 20%) until 100 °C increasing to 75% at 150 °C. The CO 2 selectivity showed a maximum value of 70% at around 120 o C. Au/TiO 2 catalyst showed a maximum CO conversion of 55% at 75 °C; however, CO 2 selectivity values were very low (around 20%) in all range of temperature. For all AuCu/TiO 2 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 Au 0.50 Cu 0.50 /TiO 2 catalyst (Figure 5a)  and, after that, these values began to decrease (92.0% for Au 0.75 Cu 0.25 /TiO 2 catalyst) as the amount of Au was increased further. Conversely, CO 2 selectivity values increased with the increase of Cu content and the values varied between 35% and 55% at 100 o C. The CO 2 selectivity value for Au 0.50 Cu 0.50 /TiO 2 catalyst was 47% (Figure 5b) reaching a maximum value of 55% for Au 0.25 Cu 0.75 /TiO 2 catalyst and after that decreased to 50% for Au 0.10 Cu 0.90 /TiO 2 catalyst. Thus, AuCu/TiO 2 catalysts showed to be more active for CO-PROX reaction than Au/TiO 2 and Cu/TiO 2 catalysts in the temperature range of 20 °C to 100 °C, as already observed for CO-PROX reaction 13 and for CO-oxidation at low temperature 14 . Figure 6 shows the CO conversion as a function of Au content (wt%) and nanoparticle sizes for AuCu/TiO 2 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 Au 0.50 Cu 0.50 /TiO 2 with similar quantities of Au and Cu and that`s where a smaller size of the Au nanoparticles was observed.
The long-term stability test results for Au 0.50 Cu 0.50 /TiO 2 catalyst is shown in Figure 7 showing CO conversions above 90%   13 also observed an increase of maximum CO conversion and CO 2 selectivity for CO-PROX reaction comparing Au/TiO 2 and Au/CuO x -TiO 2 catalysts and attributed this increase due to the presence of Au(0) and CuO x species, where CuO x -TiO 2 was proposed to be a supplier or storage of oxygen. Wang et al. 29 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 CuO x supported on Au nanoparticles) resulting in interfacial sites between them. FTIR studies of CO adsorption showed that CO adsorbed on Au(0) while CuO x species were responsible for providing active oxygen in the same way as reducible oxides do. Recently it was shown that CuO not Cu 2 O 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 them 15 .

Conclusions
Active, selective and stable AuCu/TiO 2 catalysts was prepared by a simple one-step methodology for CO-PROX reaction. The AuCu/TiO 2 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 CuO x species are in good interaction with each other and with TiO 2 support.

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.