Pt-Au/MOx-CeO2 (M = Mn, Fe, Ti) Catalysts for the Co-Oxidation of CO and H2 at Room Temperature

A series of nanostructured Pt-Au/MOx-CeO2 (M = Mn, Fe, Ti) catalysts were prepared and their catalytic performance for the co-oxidation of carbon monoxide (CO) and hydrogen (H2) were evaluated at room temperature. The results showed that MOx promoted the CO oxidation of Pt-Au/CeO2, but only the TiO2 could enhance co-oxidation of CO and H2 over Pt-Au/CeO2. Related characterizations were conducted to clarify the promoting effect of MOx. Temperature-programmed reduction of hydrogen (H2-TPR) and X-ray photoelectron spectroscopy (XPS) results suggested that MOx could improve the charge transfer from Au sites to CeO2, resulting in a high concentration of Ce3+ and cationic Au species which benefits for the CO oxidation. In-situ diffuse reflectance infrared Fourier transform spectroscopy (In-situ DRIFTS) results indicated that TiO2 could facilitate the oxidation of H2 over the Pt-Au/TiO2-CeO2 catalyst.

co-oxidization of H2 and CO at low temperature [5], but the gas hourly space velocity (GHSV) (20,000 h −1 ) was relatively low. At high GHSV, the oxidation of H2 was strongly inhibited by the presence of CO [6]. Therefore, the simultaneous removal of H2 and CO at room temperature at high GHSV still remains challenging. CeO2 enhanced the oxidation reactions due to its high oxygen storage capacity and redox property [24]. Corma et al. [15] pointed out that nanocrystalline CeO2 with Ce 3+ ions could adsorb and activate O2, thus enhancing the catalyst reactivity. Ordered CeO2 support with higher surface area could lead to the better dispersion of active sites and also boost oxygen transfer to active platinum species [25]. For the CO and H2 oxidation, surface diffusion and spillover enhanced oxidation reaction on Pt/CeO2 [26][27][28] and Au/CeO2 catalysts [8,29]. Therefore, CeO2 nanospheres with meso-structure were promising supports for Au and Pt catalysts. Fe2O3 [30][31][32], TiO2 [33,34] and MnO2 [35][36][37][38] were proven to be excellent promoters because of their high oxygen storage capacity and redox property. In addition, preparation methods showed significant effect on the catalytic performance of catalysts [6]. Reduction treatment improved the catalytic activities of Pt catalysts [12] and urea was an excellent precipitant for Au catalysts [39].
According to the above-mentioned understanding, a series of nanostructured Pt-Au/MOx-CeO2 (M = Mn, Fe, Ti) bimetallic catalysts were prepared by the reduction-deposition precipitation method and their performance for the co-oxidation of CO and H2 under the GHSV of 500,000 h −1 at room temperature were evaluated. Physical and chemical properties of the Pt-Au/MOx-CeO2 (M = Mn, Fe, Ti) bimetallic catalysts were characterized. Based on the characterization, the relationship between the structure and the catalytic performance has been elucidated. Figure 1 presents the activities of the Pt-Au/MOx-CeO2 catalysts for the catalytic co-oxidation of CO and H2. For Pt-Au/CeO2 catalyst, conversions of CO and H2 are 93% and 25%, respectively, and then gradually decrease. It is attributed to the CO accumulation on Au and Pt active sites. CO can be completely removed while the conversion of H2 is low over Pt-Au/MnO2-CeO2 and Pt-Au/Fe2O3-CeO2 catalysts. It is encouraging that 100% conversions of CO and H2 are obtained at room temperature over Pt-Au/TiO2-CeO2 catalyst. However, the conversion of H2 decreases over Pt-Au/TiO2-CeO2 catalyst due to H2O accumulation on the Pt and Au active sites [7]. The oxidation of CO is suppressed by H2O when the H2O content is over 200 ppm [40]. The inhibiting effect of H2O is due to the competitive adsorption between H2O and CO molecules on the surface twofold coordinated oxygen site [41]. On the other hand, a competitive adsorption between H2O and O2 molecules also exists due to the accumulation and occupation of H2O on the Pt and Au active sites [7].    Figure 2 presents the X-ray diffraction (XRD) patterns of the CeO 2 support and Pt-Au/MO x -CeO 2 catalysts. All these samples show typical cubic CeO 2 diffraction peaks (JCPDS 43-1002). The diffraction peaks ascribed to MO x , Pt and Au species are absent, which indicates that MO x , Pt and Au species are highly dispersed on the support. Figure 3 shows the transmission electron microscope (TEM) images of the CeO 2 support and Pt-Au/MO x -CeO 2 catalysts. It can be found that CeO 2 support presents nanosphere that is comprised of many small particles with a crystallite size of 5 nm. The contents of Pt and Au species in energy dispersive spectrometer (EDS) results of the Pt-Au/MO x -CeO 2 catalysts are close to the theoretical values (1 wt %). Chemical composition and textural properties of Pt-Au/MO x -CeO 2 catalysts are seen in Table 1. Compared with the X-ray photoelectron spectroscopy (XPS) results presented in Table 1, contents of Pt and Au species in EDS results are higher, indicating that parts of Pt and Au species are distributed on the surface of the CeO 2 nanoparticles. Brunauer-Emmett-Teller (BET) surface areas of Pt-Au/MO x -CeO 2 catalysts decrease due to the introduction of MO x . The dispersions of metal on Pt-Au/MO x -CeO 2 catalysts are very close due to the same preparation method.  Figure 2 presents the X-ray diffraction (XRD) patterns of the CeO2 support and Pt-Au/MOx-CeO2

Physicochemical Properties of Catalysts
catalysts. All these samples show typical cubic CeO2 diffraction peaks (JCPDS 43-1002). The diffraction peaks ascribed to MOx, Pt and Au species are absent, which indicates that MOx, Pt and Au species are highly dispersed on the support. Figure 3 shows the transmission electron microscope (TEM) images of the CeO2 support and Pt-Au/MOx-CeO2 catalysts. It can be found that CeO2 support presents nanosphere that is comprised of many small particles with a crystallite size of 5 nm. The contents of Pt and Au species in energy dispersive spectrometer (EDS) results of the Pt-Au/MOx-CeO2 catalysts are close to the theoretical values (1 wt %). Chemical composition and textural properties of Pt-Au/MOx-CeO2 catalysts are seen in Table 1. Compared with the X-ray photoelectron spectroscopy (XPS) results presented in Table 1, contents of Pt and Au species in EDS results are higher, indicating that parts of Pt and Au species are distributed on the surface of the CeO2 nanoparticles. Brunauer-Emmett-Teller (BET) surface areas of Pt-Au/MOx-CeO2 catalysts decrease due to the introduction of MOx. The dispersions of metal on Pt-Au/MOx-CeO2 catalysts are very close due to the same preparation method.   Figure 2 presents the X-ray diffraction (XRD) patterns of the CeO2 support and Pt-Au/MOx-CeO2

Physicochemical Properties of Catalysts
catalysts. All these samples show typical cubic CeO2 diffraction peaks (JCPDS 43-1002). The diffraction peaks ascribed to MOx, Pt and Au species are absent, which indicates that MOx, Pt and Au species are highly dispersed on the support. Figure 3 shows the transmission electron microscope (TEM) images of the CeO2 support and Pt-Au/MOx-CeO2 catalysts. It can be found that CeO2 support presents nanosphere that is comprised of many small particles with a crystallite size of 5 nm. The contents of Pt and Au species in energy dispersive spectrometer (EDS) results of the Pt-Au/MOx-CeO2 catalysts are close to the theoretical values (1 wt %). Chemical composition and textural properties of Pt-Au/MOx-CeO2 catalysts are seen in Table 1. Compared with the X-ray photoelectron spectroscopy (XPS) results presented in Table 1, contents of Pt and Au species in EDS results are higher, indicating that parts of Pt and Au species are distributed on the surface of the CeO2 nanoparticles. Brunauer-Emmett-Teller (BET) surface areas of Pt-Au/MOx-CeO2 catalysts decrease due to the introduction of MOx. The dispersions of metal on Pt-Au/MOx-CeO2 catalysts are very close due to the same preparation method.    Figure 4 shows the H2-TPR profiles of Pt-Au/MOx-CeO2 catalysts. The reduction temperatures of Pt species are 70-100 °C [42]; Au species reduction temperatures are usually 100-200 °C [43]; and pure CeO2 reduction temperature is around 553 °C [44]. The peak at 350 °C is attributed to the reduction of CeO2 surface oxygen [45]. Evidently, no reduction peaks ascribed to Pt species are observed for Pt-Au/CeO2 and Pt-Au/TiO2-CeO2 catalysts, suggesting that all the Pt species are metallic Pt species. For Pt-Au/MnO2-CeO2 and Pt-Au/Fe2O3-CeO2 catalysts, the reduction peak at 75 °C is attributed to Pt 2+ species. For Pt-Au/CeO2 catalyst, there are two reduction peaks at 160 and 553 °C, which are ascribed to the reduction peaks of Au species and CeO2, respectively [46]. Three reduction peaks at 146, 350 and 465 °C are observed for Pt-Au/TiO2-CeO2 catalyst. It can be observed that three reduction peaks are at 144, 341 and 450 °C for Pt-Au/MnO2-CeO2 catalyst. The reduction peaks are centered at 145, 350 and 464 °C for Pt-Au/Fe2O3-CeO2 catalyst. It is worth noting that the reduction temperature of CeO2 in Pt-Au/MOx-CeO2 is lower than that of Pt-Au/CeO2 nearly by 100 °C, which means that the oxidative performance of CeO2 in Pt-Au/MOx-CeO2 is higher than that of Pt-Au/CeO2. On the other hand, the introduction of MOx influences the reduction temperature of Au species. Lower reduction temperature of Au species indicates the active oxygen species formed on the Pt-Au/MOx-CeO2 catalysts are more active [43]. It is very interesting that, even though the Pt species should be metallic Pt due to the reduction of NaBH4 over the Pt-Au/MnO2-CeO2 and Pt-Au/Fe2O3-CeO2 catalysts, Pt 2+ species is observed. The presence of Pt 2+ species can be caused by the addition of MnO2 and Fe2O3, both of which improve the electron transfer from Pt sites to CeO2, thus leading to the oxidation of metallic Pt species to Pt 2+ species. Metallic Pt species are more active than Pt 2+ species for the oxidation reaction, which may clarify the poor activities of Pt-Au/MnO2-CeO2 catalyst and Pt-Au/Fe3O4-CeO2 catalyst for the H2 oxidation. H2 consumed amounts are 0.21, 0.31, 0.38 and 0.39 mmol −1 for the Pt-Au/CeO2, Pt-Au/TiO2-CeO2, Pt-Au/Fe2O3-CeO2, and Pt-Au/MnO2-CeO2 catalysts, respectively. It indicates that the addition of MOx can promote the redox property of Pt-Au/CeO2 catalyst.   [43]; and pure CeO 2 reduction temperature is around 553 • C [44]. The peak at 350 • C is attributed to the reduction of CeO 2 surface oxygen [45]. Evidently, no reduction peaks ascribed to Pt species are observed for Pt-Au/CeO 2 and Pt-Au/TiO 2 -CeO 2 catalysts, suggesting that all the Pt species are metallic Pt species. For Pt-Au/MnO 2 -CeO 2 and Pt-Au/Fe 2 O 3 -CeO 2 catalysts, the reduction peak at 75 • C is attributed to Pt 2+ species. For Pt-Au/CeO 2 catalyst, there are two reduction peaks at 160 and 553 • C, which are ascribed to the reduction peaks of Au species and CeO 2 , respectively [46]. Three reduction peaks at 146, 350 and 465 • C are observed for Pt-Au/TiO 2 -CeO 2 catalyst. It can be observed that three reduction peaks are at 144, 341 and 450 • C for Pt-Au/MnO 2 -CeO 2 catalyst. The reduction peaks are centered at 145, 350 and 464 • C for Pt-Au/Fe 2 O 3 -CeO 2 catalyst. It is worth noting that the reduction temperature of CeO 2 in Pt-Au/MO x -CeO 2 is lower than that of Pt-Au/CeO 2 nearly by 100 • C, which means that the oxidative performance of CeO 2 in Pt-Au/MO x -CeO 2 is higher than that of Pt-Au/CeO 2 . On the other hand, the introduction of MO x influences the reduction temperature of Au species. Lower reduction temperature of Au species indicates the active oxygen species formed on the Pt-Au/MO x -CeO 2 catalysts are more active [43]. It is very interesting that, even though the Pt species should be metallic Pt due to the reduction of NaBH 4 over the Pt-Au/MnO 2 -CeO 2 and Pt-Au/Fe 2 O 3 -CeO 2 catalysts, Pt 2+ species is observed. The presence of Pt 2+ species can be caused by the addition of MnO 2 and Fe 2 O 3 , both of which improve the electron transfer from Pt sites to CeO 2 , thus leading to the oxidation of metallic Pt species to Pt 2+ species. Metallic Pt species are more active than Pt 2+ species for the oxidation reaction, which may clarify the poor activities of Pt-Au/MnO 2 -CeO 2 catalyst and Pt-Au/Fe 3 O 4 -CeO 2 catalyst for the H 2 oxidation. H 2 consumed amounts are 0.21, 0.31, 0.38 and 0.39 mmol −1 for the Pt-Au/CeO 2 , Pt-Au/TiO 2 -CeO 2 , Pt-Au/Fe 2 O 3 -CeO 2 , and Pt-Au/MnO 2 -CeO 2 catalysts, respectively. It indicates that the addition of MO x can promote the redox property of Pt-Au/CeO 2 catalyst.   XPS measurements are conducted on the Pt-Au/MOx-CeO2 samples and the results are listed in Table 2. The peaks at 83.3-83.6 eV and 84.2-84.5 eV can be assigned to Au 0 species and Au + species, respectively [46,47]. The peaks at 70.2-70.8 eV and 72.4-72.6 eV are attributed to Pt 0 species and Pt 2+ species [48]. Figures 5 and 6 show that the addition of MOx influences the chemical states of Pt species and Au species due to the electron transfer from Au species and Pt species to CeO2 [49][50][51][52]. Cationic Au species possess higher activity than metallic Au species on the CO oxidation [42]. Compared with Pt-Au/CeO2 catalyst, the addition of MOx results in the presence of more cationic Au species over Pt-Au/MOx-CeO2 catalysts. The Ce 3d XPS peaks were fitted by searching for the optimum combination of Gaussian bands with the correlation coefficients (r 2 ) above 0.99. In Figure 7, the Ce 3d core level spectra of the catalyst can be divided into eight components and the content of Ce 3+ are listed in Table 2. The bands labeled u′ and v′ represent the 3d 10 4f 1 corresponding to Ce 3+ , and the bands labeled u, u′′, u′′′, v, v′′, and v′′′ represent the 3d 10 4f 0 corresponding to Ce 4+ [53]. Among Pt-Au/MOx-CeO2 catalysts, the content of Ce 3+ over Pt-Au/MnO2-CeO2 catalyst is the highest, indicating that more surface oxygen vacancies exist on Pt-Au/MnO2-CeO2 catalyst. Previous researches showed that the formation of Ce 3+ over Au/CeO2 catalysts was due to the charge transfer between Au sites and CeO2 [49][50][51]. Therefore, the introduction of MOx enhances the charge transfer from Au species and Pt species to CeO2 and leads to high content of cationic Au species. O1s XPS spectra of Pt-Au/MOx-CeO2 catalysts are shown in Figure 8 and two peaks at 529.1-529.4 and 531.1-531.4 eV, respectively, appear. The former is ascribed to lattice oxygen (OI) and the latter is attributed to chemisorbed oxygen (OII) [47]. OII ratios [OII/(OII + OI)] over Pt-Au/MOx-CeO2 catalysts are higher than that over Pt-Au/CeO2 catalysts due to the presence of higher Ce 3+ content.   Table 2. The peaks at 83.3-83.6 eV and 84.2-84.5 eV can be assigned to Au 0 species and Au + species, respectively [46,47]. The peaks at 70.2-70.8 eV and 72.4-72.6 eV are attributed to Pt 0 species and Pt 2+ species [48]. Figures 5 and 6 show that the addition of MO x influences the chemical states of Pt species and Au species due to the electron transfer from Au species and Pt species to CeO 2 [49][50][51][52]. Cationic Au species possess higher activity than metallic Au species on the CO oxidation [42]. Compared with Pt-Au/CeO 2 catalyst, the addition of MO x results in the presence of more cationic Au species over Pt-Au/MO x -CeO 2 catalysts. The Ce 3d XPS peaks were fitted by searching for the optimum combination of Gaussian bands with the correlation coefficients (r 2 ) above 0.99. In Figure 7, the Ce 3d core level spectra of the catalyst can be divided into eight components and the content of Ce 3+ are listed in Table 2. The bands labeled u and v represent the 3d 10 4f 1 corresponding to Ce 3+ , and the bands labeled u, u , u , v, v , and v represent the 3d 10 4f 0 corresponding to Ce 4+ [53]. Among Pt-Au/MO x -CeO 2 catalysts, the content of Ce 3+ over Pt-Au/MnO 2 -CeO 2 catalyst is the highest, indicating that more surface oxygen vacancies exist on Pt-Au/MnO 2 -CeO 2 catalyst. Previous researches showed that the formation of Ce 3+ over Au/CeO 2 catalysts was due to the charge transfer between Au sites and CeO 2 [49][50][51]. Therefore, the introduction of MO x enhances the charge transfer from Au species and Pt species to CeO 2 and leads to high content of cationic Au species. O1s XPS spectra of Pt-Au/MO x -CeO 2 catalysts are shown in Figure 8 and two peaks at 529.1-529.4 and 531.1-531.4 eV, respectively, appear. The former is ascribed to lattice oxygen (O I ) and the latter is attributed to chemisorbed oxygen (O II ) [47]. O II ratios [O II /(O II + O I )] over Pt-Au/MO x -CeO 2 catalysts are higher than that over Pt-Au/CeO 2 catalysts due to the presence of higher Ce 3+ content.     To further understand the relationship between catalyst activity and catalyst physicochemical property, In-situ diffuse reflectance infrared Fourier transform spectroscopy (In-situ DRIFT) spectra of the Pt-Au/MO x -CeO 2 catalysts obtained upon exposure to CO, H 2 and synthetic air at 25 • C are shown in Figure 9. Seven distinct bands are observed in the in-situ DRIFT spectra. The bands at 3324-3395, 1640, 3691-3701, 2169, 2083-2084, 1568-1575, and 1282-1298 cm −1 are ascribed to isolated hydroxyl groups υ(OH) [29], adsorbed water δ(H-O-H) [43], another hydroxyl groups υ(OH) [54][55][56][57][58], the adsorption of CO on Au sites [59][60][61], the adsorption of CO on Pt sites [42,62,63], the carbonate species [29], and carbonate species [29], respectively. It has been proposed that isolated hydroxyl groups originated from the decomposition of the OOH species, which were generated from the reaction between the associatively adsorbed oxygen and dissociative adsorbed hydrogen, and the isolated hydroxyl groups further react with dissociative adsorbed hydrogen to generate H 2 O [7,29].         To further understand the relationship between catalyst activity and catalyst physicochemical property, In-situ diffuse reflectance infrared Fourier transform spectroscopy (In-situ DRIFT) spectra of the Pt-Au/MOx-CeO2 catalysts obtained upon exposure to CO, H2 and synthetic air at 25 °C are shown in Figure 9. Seven distinct bands are observed in the in-situ DRIFT spectra. The bands at 3324-3395, 1640, 3691-3701, 2169, 2083-2084, 1568-1575, and 1282-1298 cm −1 are ascribed to isolated hydroxyl groups υ(OH) [29], adsorbed water δ(H-O-H) [43], another hydroxyl groups υ(OH) [54][55][56][57][58], the adsorption of CO  Intensity of reactant-related is due to reactant oxidation and reactant adsorption capability. To solve this problem, In-situ DRIFTS test results of Pt-Au/CeO 2 catalyst upon 3000 ppm CO + N 2 and 3000 ppm CO + 3000 pm H 2 + synthetic air can be observed in the Figure 10. The results indicate that the CO accumulation amount does not reach the CO saturated adsorption capability over Pt-Au/CeO 2 catalyst upon 3000 ppm CO + 3000 pm H 2 + synthetic air. CO saturated adsorption capability of Pt-Au/MO x -CeO 2 catalysts may be close to that of Pt-Au/CeO 2 catalyst due to the close amount of Pt and Au species over Pt-Au/CeO 2 catalyst and Pt-Au/MO x -CeO 2 catalysts. Little CO accumulation on Pt-Au/MO x -CeO 2 catalysts in Figure 9 is because of the enhanced CO oxidation instead of adsorption capability. For Pt-Au/CeO 2 catalyst, the intensity of bands at 2169 and 2084 cm −1 increases, which means that much CO accumulates on the Au and Pt active sites. The intensity of other bands are almost unchanged, which suggests little H 2 is oxidized because Pt active sites are occupied and poisoned by CO. Therefore, CO and H 2 cannot be simultaneously removed over Pt-Au/CeO 2 catalyst. For Pt-Au/MnO 2 -CeO 2 catalyst, no peaks can be observed at 2169 and 2084 cm −1 with the reaction proceeding, which suggests that the introduction of MnO 2 enhances the oxidation of CO. However, the intensity of bands at 3389 and 1640 cm −1 are seldom unchanged, which indicates that little H 2 is oxidized. Therefore, the introduction of MnO 2 improves the oxidation of CO but not the oxidation of H 2 . Over Pt-Au/Fe 2 O 3 -CeO 2 catalyst, the intensity of the peak at 2084 cm −1 suggests that little CO accumulates and the Fe 2 O 3 improves the activity of Pt-Au/CeO 2 catalyst for CO oxidation. The intensity of bands at 3386 and 1640 cm −1 suggest that little H 2 is oxidized to H 2 O. Consequently, Fe 2 O 3 improves the activity of Pt-Au/CeO 2 catalyst for the CO oxidation not for H 2 oxidation. Some CO accumulates on the Pt-Au/TiO 2 -CeO 2 catalyst, as confirmed by the presence of the band at 2084 cm −1 . The intensity change of peaks at 3691, 3395 and 1640 cm −1 suggests that many isolated -OH species and H 2 O are produced, indicating that much H 2 is oxidized into H 2 O. Based on the in-situ DRIFTS results of the Pt-Au/MO x -CeO 2 catalysts, Pt-Au/TiO 2 -CeO 2 catalyst presents the best catalytic activity for the co-oxidation of CO and H 2 .
oxidation. Some CO accumulates on the Pt-Au/TiO2-CeO2 catalyst, as confirmed by the presence of the band at 2084 cm −1 . The intensity change of peaks at 3691, 3395 and 1640 cm −1 suggests that many isolated -OH species and H2O are produced, indicating that much H2 is oxidized into H2O. Based on the in-situ DRIFTS results of the Pt-Au/MOx-CeO2 catalysts, Pt-Au/TiO2-CeO2 catalyst presents the best catalytic activity for the co-oxidation of CO and H2.   Figure 10. In-situ DRIFTS spectra of the Pt-Au/CeO2 catalysts after exposed upon: (a) 3000 ppm CO + 3000 ppm H2 + synthetic air; and (b) 3000 ppm CO + N2 for 60 min at 25 °C. Figure 11 shows the effect of CO concentration on the co-oxidation of CO and H2 through in-situ DRIFTS. The intensity of the bands at 3395 and 1640 cm −1 in Figure 11b are stronger than that shown in Figure 11a, indicating that more H2O are produced. Therefore, CO obviously hinders the oxidation of H2 in the co-oxidation of CO and H2, which is in accordance with previous reports [22]. Figure 11. In-situ DRIFTS spectra over Pt-Au/TiO2-CeO2 catalysts after exposed upon: (a) 5000 ppm CO + 3000 ppm H2 + synthetic air; and (b) 3000 ppm CO + 3000 pm H2 + synthetic air for 60 min at 25 °C.

Catalyst Preparation
The CeO2 nanospheres were prepared by hydrothermal method. Thirteen grams Ce(NO3)3·6H2O was dissolved in 13 mL ultra-pure water at room temperature. Then, 13 mL propionic acid and 390 mL ethylene glycol were added under stirring to form a uniform solution at room temperature. The uniform solution was transferred to a Teflon-sealed autoclave and heated at 180 °C for 7.5 h. After the hydrothermal treatment, the mixture was centrifuged and washed with ethanol for several times. The obtained solid was dried at 100 °C overnight, subsequently it was calcined in air at 400 °C for 4 h. Then CeO2 nanospheres were obtained.
MOx-CeO2 (M = Mn, Fe, Ti) supports, with a Ce/M molar ratio of 9, were obtained by precipitation method. CeO2 nanospheres were homogeneously dispersed in Mn(NO3)2 or Fe(NO3)3 aqueous solution, or tetra-n-butyl titanate ethanol solution, and the suspension was stirred for 2 h at room temperature. Then ammonia solution was added to the above solution under stirring until pH was 10 at room temperature. 0.05 Figure 10. In-situ DRIFTS spectra of the Pt-Au/CeO 2 catalysts after exposed upon: (a) 3000 ppm CO + 3000 ppm H 2 + synthetic air; and (b) 3000 ppm CO + N 2 for 60 min at 25 • C. Figure 11 shows the effect of CO concentration on the co-oxidation of CO and H 2 through in-situ DRIFTS. The intensity of the bands at 3395 and 1640 cm −1 in Figure 11b are stronger than that shown in Figure 11a, indicating that more H 2 O are produced. Therefore, CO obviously hinders the oxidation of H 2 in the co-oxidation of CO and H 2 , which is in accordance with previous reports [22]. . In-situ DRIFTS spectra of the Pt-Au/CeO2 catalysts after exposed upon: (a) 3000 ppm CO + 3000 ppm H2 + synthetic air; and (b) 3000 ppm CO + N2 for 60 min at 25 °C. Figure 11 shows the effect of CO concentration on the co-oxidation of CO and H2 through in-situ DRIFTS. The intensity of the bands at 3395 and 1640 cm −1 in Figure 11b are stronger than that shown in Figure 11a, indicating that more H2O are produced. Therefore, CO obviously hinders the oxidation of H2 in the co-oxidation of CO and H2, which is in accordance with previous reports [22]. Figure 11. In-situ DRIFTS spectra over Pt-Au/TiO2-CeO2 catalysts after exposed upon: (a) 5000 ppm CO + 3000 ppm H2 + synthetic air; and (b) 3000 ppm CO + 3000 pm H2 + synthetic air for 60 min at 25 °C.

Catalyst Preparation
The CeO2 nanospheres were prepared by hydrothermal method. Thirteen grams Ce(NO3)3·6H2O was dissolved in 13 mL ultra-pure water at room temperature. Then, 13 mL propionic acid and 390 mL ethylene glycol were added under stirring to form a uniform solution at room temperature. The uniform solution was transferred to a Teflon-sealed autoclave and heated at 180 °C for 7.5 h. After the hydrothermal treatment, the mixture was centrifuged and washed with ethanol for several times. The obtained solid was dried at 100 °C overnight, subsequently it was calcined in air at 400 °C for 4 h. Then CeO2 nanospheres were obtained.
MOx-CeO2 (M = Mn, Fe, Ti) supports, with a Ce/M molar ratio of 9, were obtained by precipitation method. CeO2 nanospheres were homogeneously dispersed in Mn(NO3)2 or Fe(NO3)3 aqueous solution, or tetra-n-butyl titanate ethanol solution, and the suspension was stirred for 2 h at room temperature. Then ammonia solution was added to the above solution under stirring until pH was 10 at room temperature. 0.05 Figure 11. In-situ DRIFTS spectra over Pt-Au/TiO 2 -CeO 2 catalysts after exposed upon: (a) 5000 ppm CO + 3000 ppm H 2 + synthetic air; and (b) 3000 ppm CO + 3000 pm H 2 + synthetic air for 60 min at 25 • C.

Catalyst Preparation
The CeO 2 nanospheres were prepared by hydrothermal method. Thirteen grams Ce(NO 3 ) 3 ·6H 2 O was dissolved in 13 mL ultra-pure water at room temperature. Then, 13 mL propionic acid and 390 mL ethylene glycol were added under stirring to form a uniform solution at room temperature. The uniform solution was transferred to a Teflon-sealed autoclave and heated at 180 • C for 7.5 h. After the hydrothermal treatment, the mixture was centrifuged and washed with ethanol for several times. The obtained solid was dried at 100 • C overnight, subsequently it was calcined in air at 400 • C for 4 h. Then CeO 2 nanospheres were obtained. MO x -CeO 2 (M = Mn, Fe, Ti) supports, with a Ce/M molar ratio of 9, were obtained by precipitation method. CeO 2 nanospheres were homogeneously dispersed in Mn(NO 3 ) 2 or Fe(NO 3 ) 3 aqueous solution, or tetra-n-butyl titanate ethanol solution, and the suspension was stirred for 2 h at room temperature. Then ammonia solution was added to the above solution under stirring until pH was 10 at room temperature. The suspension was filtered and washed with ultra-pure water. The obtained solid samples were first dried at 105 • C for 12 h and subsequently calcined in air at 400 • C for 4 h to obtain MO x -CeO 2 (M = Mn, Fe, Ti) supports.
Pt-Au/MO x -CeO 2 (M = Mn, Fe, Ti) catalysts were prepared by reduction-deposition precipitation method [49]. Four grams MO x -CeO 2 was uniformly dispersed into the H 2 PtCl 6 solution containing 0.04 g Pt at room temperature. After impregnation for 2 h, the pH value of suspension was adjusted to 10. NaBH 4 solution was quickly added into the suspension (NaBH 4 /Pt = 10, molar ratio) while being stirred for 2 h at room temperature. The suspension was filtered and washed with ultra-pure water, then dried under vacuum at 120 • C for 12 h to obtain Pt/MO x -CeO 2 . Then 2 g Pt/MO x -CeO 2 was uniformly dispersed into the HAuCl 4 solution containing 0.02 g Au at room temperature. Subsequently, urea was added as the precipitant. The mixture was stirred at 80 • C for 8 h and then aged for 12 h at room temperature. Then, the mixture was filtered and washed with ultra-pure water. The resulting powder was dried under vacuum at room temperature for 12 h to yield Pt-Au/MO x -CeO 2 catalysts.

Catalyst Characterization
XRD patterns were recorded with a Shimadzu (Tokyo, Japan) XRD-6000 diffractometer operated at 40 kV and 40 mA, using nickel-filtered Cu Kα (λ = 0.1542 nm) radiation. Surface areas of the catalysts were determined by the BET method by a Micromeritics ASAP 2000 instrument (Quantachrome, Boynton Beach, FL, USA). CO chemisorption measurements were measured an Autochem II 2920 (Micromeritics Instrument Corp, Atlanta, GA, USA) automated chemisorption analyzer. The surface chemical states of Pt-Au/MO x -CeO 2 catalysts were tested by XPS (PHI Quantro SXM ULVAC-PHI, Tokyo, Japan) using an Al Kα X-ray source (1486.7 eV) at 15 kV and 25 W with the binding energy calibrated by C 1s at 284.8 eV. HRTEM micrographs were obtained with a JEM-2100F (Jeol, Tokyo, Japan) microscope at 200 kV.
H 2 -TPR measurements equipped with a quadrupole mass spectrometer (Omnistar, Atlanta, GA, USA, GSD-301-O2) were carried out in a fixed bed microreactor. The 0.2-g sample was pretreated under Ar at 120 • C for 1 h. After cooled to 25 • C, the catalyst was reduced under 5% H 2 /N 2 gas flow (50 mL min −1 ) within the temperature of 30 to 700 • C at 10 • C min −1 . In-situ DRIFTS were recorded in a Nicolet 6700 FTIR spectrometer (Nicolet, Atlanta, GA, USA). Before characterization, all the catalysts were pretreated under Ar at a flow rate of 100 mL min −1 at 120 • C for 0.5 h. After cooled to 25 • C, the reactant gas mixture, comprised of 1000 ppm H 2 , 1000 ppm CO and synthetic air (50% relative humidity), was introduced into the DRIFT cell at a flow rate of 100 mL min −1 . All spectra were recorded by accumulating 32 scans with a resolution of 4 cm −1 .

Catalytic Activity Measurement
The activity evaluation for the co-oxidation of CO and H 2 was performed in a continuous flow fixed-bed quartz reactor (i.d. = 10 mm) by using 0.36 g catalyst at 25 • C. Before activity evaluation, the Pt-Au/MO x -CeO 2 catalysts were pretreated under Ar at 120 • C for 0.5 h at a flow rate of 100 mL·min −1 . The simultaneous reaction gas consisted of 100 ppm CO and 480 ppm H 2 , and air (50% relative humidity) as the balance gas, and the total flow rate was fixed at 3.6 L·min −1 , corresponding to a GHSV of 500,000 h −1 . H 2 , CO and CO 2 were measured by gas chromatograph (GC) equipped with TCD and FID detectors. CO and CO 2 were converted to CH 4 by Ni catalytic converter before the measurement.

Conclusions
A series of nanostructured Pt-Au/MO x -CeO 2 (M = Mn, Fe, Ti) catalysts were prepared and Pt-Au/TiO 2 -CeO 2 catalyst presented the best catalytic performance for the total co-oxidation of CO and H 2 at room temperature. The introduction of MO x into CeO 2 can enhance the charge transfer from Pt and Au sites to CeO 2 , which improves CO oxidation. The introduction of TiO 2 enhances the decomposition of OOH species into O species and OH species, while the introduction of MnO 2 and Fe 2 O 3 cannot. The addition of TiO 2 mainly accounts for the high activity for the co-oxidation of CO and H 2 over the Pt-Au/TiO 2 -CeO 2 catalyst.