Catalytic Performance of Gold Supported on Mn , Fe and Ni Doped Ceria in the Preferential Oxidation of CO in H 2-Rich Stream

Ceria supported metal catalysts often exhibit high activity in the preferential oxidation (PROX) of CO in H2-rich stream and doping the ceria support with other metals proves to be rather effective in further enhancing their catalytic performance. Therefore, in this work, a series of ceria materials doped with Mn, Fe and Ni (CeM, where M = Mn, Fe and Ni; M/Ce = 1/8) were synthesized by a modified hydrothermal method; with the doped ceria materials (CeM) as the support, various supported gold catalysts (Au/CeM) were prepared by the colloidal deposition method. The influence of metal dopant on the performance of these ceria materials supported with gold catalysts in CO PROX was then investigated in detail with the help of various characterization measures such as N2 sorption, XRD, TEM, Raman spectroscopy, H2-TPR, XPS and XAS. The results indicate that the incorporation of Mn, Fe and Ni metal ions into ceria can remarkably increase the amount of oxygen vacancies in the doped ceria support, which is beneficial for enhancing the reducibility of ceria, the metal-support interaction and the dispersion of gold species. Although the gold catalysts supported on various doped ceria are similar in the size and state of Au nanoparticles, the CO conversions for CO PROX over Au/CeMn, Au/CeFe and Au/CeNi catalysts are 65.6%, 93.0% and 48.2%, respectively, much higher than the value of 33.6% over the undoped Au/CeO2 catalyst at ambient temperature. For CO PROX over the Au/CeNi catalyst, the conversion of CO remains near 100% at 60–130 ◦C, with a PROX selectivity to CO2 of higher than 50%. The excellent performance of Au/CeNi catalyst can be ascribed to its large amount of oxygen vacancies and high reducibility on account of Ni incorporation. The insight shown in this work helps to clarify the doping effect of other metals on the physicochemical properties of ceria, which is then beneficial to building a structure-performance relation for ceria supported gold catalyst as well as developing a better catalyst for removing trace CO in the hydrogen stream and producing high purity hydrogen.


Introduction
The catalytic oxidation of CO at lower temperatures is widely used in the removal of trace content of CO from various atmospheres; in particular, the preferential oxidation (PROX) of CO in H 2 -rich stream has received extensive attention, as it is rather effective in getting CO-free hydrogen sources (CO < 10 ppm) to feed the polymer electrolyte membrane fuel cell (PEMFC) [1][2][3][4].An appropriate catalyst for CO PROX should be able to selectively oxidize CO with O 2 whereas have little impairment on the hydrogen stream such as catalyzing the oxidation of hydrogen to water; meanwhile, it should also have reasonable tolerance towards CO 2 (about 20-24%) and H 2 O (about 2-10%) in the feed [5].
Various catalysts have been reported in CO PROX, including supported platinum group metals (Pt, Ru, and Rh) [6][7][8][9], gold [10][11][12][13][14] and copper [15][16][17].The platinum group metal-based catalysts exhibited high activity, with a CO conversion of about 92-97% at 150-200 • C, but the PROX selectivity (about 40%) in the presence of CO 2 and H 2 O was relatively low [18].The Cu-based catalysts displayed high CO conversion (about 100%) and PROX stability (100%) at lower temperatures (about 90-170 • C); however, their performance decreased obviously at high space velocities and the resistance towards H 2 O was also relatively poor [15].In contrast, the gold-based catalysts were considered as a promising candidate for the selective removal of CO from reformate streams due to their high activity; over the gold-based catalysts, the conversion of CO reached almost 100% at low temperatures (<100 • C) [19,20].It is generally accepted that the performance of a gold catalyst is mainly dependent on the gold particle size and oxidation state as well as the gold-support interface, which is closely related to the nature of the support material and the preparation method.Exceptionally high activities for CO oxidation have been reported for the highly dispersed gold catalysts supported on reducible metal oxides, such as TiO 2 , Fe 2 O 3 , CeO 2 , Co 3 O 4 , and so on; the reducible metal oxides may not only act as a support for the nanosized gold particles but also provide a large amount of highly mobile oxygen species for the activation of CO [21][22][23].
In particular, the CeO 2 -based catalysts have demonstrated distinct advantages in the heterogeneous catalysis due to its remarkable redox properties (Ce 4+ /Ce 3+ ) [24,25].For the CO PROX, the ceria supported gold catalysts exhibited high activity and good preference for CO oxidation at 20-120 • C [26][27][28].Moreover, it was also found that the doping of ceria support with other elements of different ionic radii and oxidation states can improve the oxygen migration ability and enhance the noble metal dispersion.For example, doping ceria with smaller isovalent non-reducible cations like Zr 4+ and Hf 4+ could obviously enhance its oxygen storage capacity (OSC) by creating more intrinsic oxygen vacancies [29].Similarly, the incorporation of Ni 2+ into ceria could also greatly increase the oxygen vacancies and improve the oxygen diffusion in ceria [30].The performance of metal catalysts supported on doped ceria materials was then greatly enhanced.For CO PROX, Laguna and co-workers observed that the gold catalysts supported on Zr, Fe and Zn doped ceria were more active than undoped Au/CeO 2 at low temperatures, due to an enhancement in the reducibility and the oxygen exchange ability [11].Tabakova and co-workers prepared a series of Ce-Fe mixed oxides by varying the Ce/(Ce + Fe) ratio; they found that the ability of the supports to activate oxygen and to disperse the gold particles played a crucial role in determining the catalytic performance of Au/CeO 2 -Fe 2 O 3 in CO PROX [5].A series of rare earth metals (RE) (RE = La, Sm, Gd and Y) were used to dope ceria through co-precipitation (CP) and mechanochemical method (MA) by Ilieva and co-workers [13]; yttrium as a dopant gave a promising catalyst in CO PROX.Besides this, they also doped ceria with FeO x , MnO x and CoO x and found that the gold catalysts supported on FeO x and MnO x doped ceria were also highly active in CO PROX [23].Furthermore, the gold nanoparticles dispersed on other ceria supports with various dopants, such as CeO 1−x -ZrO x [31], CeO 2 -Al 2 O 3 [32], CuO-CeO 2 [33] and CeO 2 -Co 3 O 4 [10], have also attracted great attention due to their high activity in various reactions.In our previous works, we also found that Pd and CuO supported on Ti doped ceria (CeO 2 -TiO 2 ) exhibited excellent catalytic performance in CO oxidation and CO PROX, respectively [15,34].In addition, CO PROX over Au supported on CeO 2 -Co 3 O 4 mixed oxides has also been studied and a possible mechanism involving -OOH intermediate was proposed and used to explain the deactivation behavior in CO PROX [10,35].
To further enhance the performance of ceria supported gold catalyst in CO PROX and have a better understanding of the structure-performance relationship, in this work, a series of ceria materials doped with a second metal (CeM, M = Mn, Fe and Ni; M/Ce = 1/8) were synthesized by a modified hydrothermal method; with the doped ceria materials (CeM) as the support, various supported gold catalysts (Au/CeM) were prepared by the colloidal deposition method.The influence of metal dopant on the performance of these ceria materials supported gold catalysts in CO PROX was then investigated in detail with the help of various characterization measures.With the doped ceria materials having a defective fluorite structure as the support, the enhanced oxygen mobility, metal-support interaction, gold dispersion, and catalytic performance of Au/CeM in CO PROX were then demonstrated.This work should be helpful in clarifying the doping effect of other metals on the properties of ceria, which is then beneficial to building a structure-performance relation for ceria supported gold catalyst as well as for developing a better catalyst for CO oxidation.

Chemical Composition and Textural Properties
The chemical composition (M/Ce and Au loading) and textural properties (surface area, pore size and pore volume) of CeO 2 , CeM mixed oxides, and corresponding supported Au/CeM catalysts (M = Mn, Fe, and Ni) are given in Table 1.Obviously, all the doped ceria materials have a Ce/M molar ratio close to the nominal value of 1/8, indicating that the hydrothermal method used here is an appropriate approach to get the doped ceria materials with well controlled stoichiometry.Meanwhile, the composition of CeM mixed oxides remains unchanged upon subsequent treatment such as impregnation with chloroauric acid and calcination, suggesting the stable and homogeneous properties of the doped ceria materials.The Au loadings of gold catalysts are in the range of 0.7-0.96wt.%; although they are all close to the target value (1 wt.%), the relatively higher Au loadings for Au/CeM than that for Au/CeO 2 may suggest that the doped ceria materials as a catalyst support are more effective in dispersing and stabilizing the gold species.
Undoped CeO 2 exhibits the highest surface area (118 m 2 g −1 ); the surface area decreases after the addition of the second metal, especially for CeNi.After loading gold, the surface area of gold catalysts remains almost unchanged in comparison with the corresponding supports, although a slight decrease in the average pore size and pore volume is observed for the supported gold catalysts, as given in Table 1.

XRD Results
The XRD patterns of CeO 2 , CeM mixed oxides (M = Mn, Fe, and Ni), and corresponding supported Au/CeM catalysts are shown in Figure 1.All the CeM mixed oxides display only the diffraction peaks corresponding to the fluorite face-centered cubic (fcc) CeO 2 (JCPDS 34-0394); no diffraction lines for the dopant phases are observed, probably due to the relatively low content of dopants and the homogeneous incorporation of the metal dopants into the ceria lattice.As displayed in Figure 1, a slight shift of the diffraction lines to higher angles is observed for the doped CeM samples, in comparison with that for undoped CeO 2 , suggesting that the metal dopants have been incorporated into the CeO 2 lattice and a solid solution carrying the fluorite structure is formed, consistent with the previous observation [11,15,16].Meanwhile, as given in Table 2, the calculated cell parameter of cubic CeO 2 in the CeM mixed oxides is somewhat smaller than that in pure CeO 2 , further proving the formation of a solid solution, since the introduction of Mn, Fe and Ni components in the CeO 2 fluorite structure could lead to a contraction of the unit cell of CeM crystallites, due to the smaller ionic radii of the dopant elements than that of cerium [11,23,[36][37][38].In addition, the average crystallite sizes were also estimated by using the Scherrer equation, as provided in Table 2.The particle sizes of CeMn and CeFe composites are similar to that of CeO 2 ; however, the Ni doped CeNi shows a much larger crystallite size, consistent with its lower surface area (Table 1).a The CeO 2 cell parameter was calculated by the MDI Jade5 software; the average CeO 2 crystallite sizes were estimated from the broadening of CeO 2 (111), ( 200), (220), and (311) diffraction peaks by the Scherrer formula from the XRD patterns.b The O v /F 2g ratio is determined as the area ratio of the O v band to the F 2g band in the Raman spectra.c The surface Ce 3+ content was determined as A Ce(III) /(A Ce(III) + A Ce(IV) ), where A Ce(III) and A Ce(IV) were deconvoluted from the XPS spectra for the Ce(III) and Ce(IV) species, respectively.
After introducing the active gold component, the XRD patterns of gold catalysts remain almost unchanged in comparison with the corresponding supports and no distinct diffraction peaks for gold are detected, which is ascribed to the fact that the amount of gold is relatively small and/or the gold species is finely dispersed on the supports, with a particle size below 5 nm [39].

TEM and HRTEM Results
The TEM images of various Au catalysts shown in Figure 2 illustrate that all the Au/CeO 2 and Au/CeM catalysts take a well-defined rod-like morphology with the diameter of approximate 10 nm and length of 50 to 200 nm, although the as-obtained nanorods in Au/CeNi are relatively thicker and shorter [40].The HRTEM images shown in Figure 3 indicate that gold nanoparticles are highly dispersed on the supports and all the gold catalysts display a similar particle size distribution (3-5 nm); the lattice spacing of Au nanoparticle is 0.23 nm, which is indexed as the {111} planes.Similar phenomena were also reported for the Au catalysts supported on TiO 2 , ZnO, γ-Al 2 O 3 and ZrO 2 prepared by the same method [41]; by using the colloidal deposition method, the gold metal particles were generated before they were deposited on the support, which is a rather effective way to control the size of gold particles in preparing the supported gold catalysts.As a result, when considering the influence of dopant on the performance of the supported gold catalysts in detail, the effect of gold size can then be excluded.

Raman Spectroscopy
Figure 4 shows the Raman spectra of CeO 2 , CeM mixed oxides (M = Mn, Fe, and Ni), and corresponding supported Au catalysts.For pure CeO 2 , two main bands are observed: The intense one at 460 cm −1 is ascribed to the F 2g Raman active mode of CeO 2 fluorite structure, viewed as a symmetric breathing mode of the oxygen atoms around Ce 4+ ions, whereas the weak one at ca. 600 cm −1 is assigned to the oxygen vacancies (O v ) preferentially present on the surface of cubic CeO 2 [11,15,16,42,43].The appearance of oxygen vacancies is evidence for the presence of Ce 3+ ions.
In comparison with the undoped CeO 2 , an evident change is observed for the doped CeM materials in the position and width of these two bands, confirming the tight interaction between the dopants and parent CeO 2 , consistent with the XRD results.Meanwhile, the F 2g Raman band of CeO 2 is shifted to lower frequencies and becomes broader in the doped CeM materials, especially in the Mn-doped CeMn support [36,37].The observed shifting and widening of the F 2g band for the doped CeM materials may be ascribed to the fact that the surroundings of Ce cations were modified by the metal dopants added in CeO 2 [11,44], which should be also related to the formation of more oxygen vacancies upon the addition of the metal dopants [13].After loading the gold component, the F 2g band of CeO 2 for the supported gold catalysts is even broader than that of the corresponding support (Figure 4), reflecting the enhanced gold-support interaction.The surroundings of surface species on the support may be modified by loading gold through an epitaxial contact between Au and the oxide support [11].Besides, a decrease in the intensity of the weak Raman band (O v ) attributed to the oxygen vacancies is observed in Au/CeO 2 compared with that in CeO 2 , implying that the Au species may consume certain oxygen vacancies on CeO 2 .However, for the Au/CeM catalysts, the weak O v band is still rather intense, suggesting that there are still abundant oxygen vacancies on the doped CeM surface, which is helpful to disperse and stabilize the Au species.
Quantitatively, the ratio of O v band area to F 2g band area has been associated with the quantity of oxygen vacancies [44]; that is, the higher the O v /F 2g ratio, the larger the amount of oxygen vacancies is in the doped CeM materials.As shown in Table 2, the doped CeM composite oxides exhibit much higher O v /F 2g ratio than the undoped CeO 2 oxide, suggesting that the amount of oxygen vacancies can be enhanced greatly by doping ceria with the Mn, Fe and Ni species.The CeNi composite oxide doped with Ni shows the largest O v /F 2g ratio, implying the formation of abundant oxygen vacancies.After loading Au, the O v /F 2g ratio for Au/CeO 2 decreases markedly, compared with CeO 2 ; however, the O v /F 2g ratios for the Au/CeM catalysts, especially for Au/CeNi, remain at a much higher level, in comparison with those for the undoped Au/CeO 2 catalyst.In addition, as given in Table 2, the O v /F 2g ratios of three Au/CeM catalysts are similar to those of the corresponding CeM supports, which may be related to the enhanced reducibility of the supported Au catalysts, as discussed later in this article.
The oxygen vacancies on the CeO 2 surface help to disperse and stabilize the gold species and meanwhile are also of great importance in catalyzing CO oxidation [45].As explained by the Mars-van Krevelen-type mechanism, CO oxidation involves the alternant reduction and oxidation of the oxide surface; the formation of surface oxygen vacancies and their replenishment by gas-phase oxygen are crucial for the catalytic reaction cycles of CO oxidation [45].As a result, the abundant oxygen vacancies present on the doped ceria materials and corresponding supported gold catalysts may endow them with high activity in CO oxidation.

TPR Results
Figure 5 shows the H 2 -TPR profiles of CeO 2 , CeM mixed oxides (M = Mn, Fe, and Ni), and corresponding supported Au catalysts.For undoped CeO 2 (Figure 5a), two broad reduction peaks are observed; the low temperature one centered at 442 • C is attributed to the surface-capping oxygen of CeO 2 , whereas the high temperature one around 740 • C is related to the reduction of bulk CeO 2 [15,46].After doping with Mn, Fe and Ni, a significant decrease in the reduction temperature of the surface-capping oxygen in CeO 2 is observed, suggesting a great enhancement of the reducibility of CeO 2 by the dopants [11], which was related to the higher lattice oxygen mobility on account of the oxygen vacancies created by the dopants [13].In particular, the CeMn mixed oxide display three distinct reduction peaks (Figure 5b): the broad one at about 200 • C represents the reduction of MnO 2 to Mn 2 O 3 and the further reduction of Mn 2 O 3 to Mn 3 O 4 [47,48]; the peak around 330 • C corresponds to the combined reduction of Mn 3 O 4 to MnO and surface Ce 4+ to Ce 3+ species, which is shifted to lower temperatures in comparison with that of undoped CeO 2 [16]; lastly, the peak at 745 • C is then associated with the reduction of bulk CeO 2 .As for the CeFe composite (Figure 5c), the reduction of bulk CeO 2 appears around 754 • C; meanwhile, a small signal is detected around 533 • C and two overlapping reduction peaks centered at 334 and 402 • C are observed, with a broad shoulder at 228 • C. The reduction of Fe 2 O 3 goes through two steps, viz., the transition of hematite to magnetite (Fe 2 O 3 to Fe 3 O 4 ) around 370 • C and magnetite to metallic iron (Fe 3 O 4 to FeO to Fe) around 600 • C [23]; therefore, the reduction peak for CeFe at around 334 • C should be ascribed to the surface Fe 3+ species [49].However, an accurate differentiation of the specific reduction steps in the 400-600 • C region is especially difficult, because the reduction of surface Ce 4+ occurs simultaneously with that of Fe 3 O 4 to FeO.In contrast, the CeNi composite displays a similar TPR profile to CeFe (Figure 5d).As Ni 2+ and Ce 4+ cations are different in the size and valence state, the incorporation of Ni 2+ ions into ceria lattice may form a Ni-O-Ce solid solution and generate abundant oxygen vacancies, bringing on large amount of adsorbed oxygen species that can be easily reduced by H 2 [50].The reduction peak at 740 • C corresponds to bulk CeO 2 , whereas the overlapped signals at 110-450 • C (with a broad shoulder at 170 • C) can be attributed to the reduction of adsorbed oxygen.In particular, the reduction peak at 235 • C may correspond to free NiO, whilst the peak at 318 • C is assigned to the reduction of strongly interactive NiO species with CeO 2 [48,50,51].All of these prove that the introduction of metal dopants into CeO 2 is capable of facilitating the release of lattice oxygen and promoting the reduction of Ce 4+ to Ce 3+ [5,11,52]; the reducibility of four supports considered in this work follows the order of CeNi > CeFe > CeMn > CeO 2 .
After loading the active gold component, the reduction of bulk CeO 2 in the supported gold catalysts occurs at ca. 720 • C (Figure 5e-h), approximately 20 • C below that of the corresponding supports, suggesting that the reducibility of bulk CeO 2 can be enhanced by the deposition of gold species.Besides, a broad feature corresponding to the reduction of surface-capping oxygen of CeO 2 or dopant oxides appears at lower temperatures (220-560 • C) [11].In particular, an intense signal appears at very low temperatures (around 100 • C) in the H 2 -TPR profiles of the supported gold catalysts, which is mainly attributed to the reduction of CeO 2 strongly bound with the gold species; in addition, these low temperature peaks would overlap with those for the reduction of cationic gold species, if their presence was assumed [11].Moreover, it was reported that the easily reducible CeO 2 species, strongly bound with the gold species, are catalytically active for CO oxidation [53].Current results illustrate that the reducibility of CeO 2 can be greatly enhanced by loading Au, especially on the doped CeM supports, which may be ascribed the occurrence of spillover phenomena, involving either hydrogen activated on the metal phase or lattice CeO 2 oxygen induced by intimate metal-support interactions [11,23,46].Moreover, the enhanced reducibility of the supported Au/CeM catalysts may also contribute to their high content of oxygen vacancies expressed by the O v /F 2g ratios determined from Raman spectra (Table 2).

XPS and XAS Results
Figure 6 shows the Ce 3d XPS spectra of CeO 2 , CeM mixed oxides (M = Mn, Fe, and Ni), and corresponding supported Au catalysts.A deconvolution of these spectra suggests the presence of two types of cerium species, viz., Ce(III) and Ce(IV).The signals of U and V (916.5 and 898.4 eV), U and V (907.7 and 888.9 eV), and U and V peaks (901.3 and 882.7 eV) represent the Ce(IV) species, whereas the signals of U and V (903.5, 884.6 eV) and U 0 and V 0 (899.0, and 880.6 eV) correspond to the Ce(III) species [54]; as a result, the content of Ce(III) in the Ce-containing materials can be estimated by integrating these signals in the XPS spectra, as also shown in Table 2.The Ce(III) species is attributed to the interaction between ceria and the surrounding atoms and can be used as an indicator for the oxygen vacancies.Obviously, the contents of Ce(III) in the doped CeM supports and Au/CeM catalysts are much higher than those in the undoped CeO 2 support and Au/CeO 2 catalyst, consistent with the results of TPR and Raman spectroscopy.Figure 7 shows the Au 4f XPS spectra of the gold catalysts supported on CeO 2 and CeM mixed oxides.Obviously, all the gold catalysts are similar in their Au 4f XPS spectra, with a pair of distinct peaks around 83.7 and 87.5 eV that were characteristic for metallic Au [55,56].In contrast, the peaks located around 85.5 and 86.3 eV for the oxidized Au species cannot be detected [47], similar to that observed for the Au/TiO 2 catalyst prepared by the same method [41].It can then be inferred that the gold species here is still mainly in the metallic state, in good agreement with the previous report [57].It was known that the gold species in the catalyst prepared by deposition-precipitation and co-precipitation methods can be in the forms of metallic Au, Au 2 O 3 , Au (OH) 3 , AuOOH•nH 2 O, and so on; the performances of gold catalyst may be determined jointly by a mixture of these gold species with various nanostructures [58].However, by using the colloidal deposition method in this work, the colloidal gold metal particles are generated before they are deposited on the support and the formation of the gold particles is then well controlled; as a result, the effect of Au species can be excluded when considering the influence of dopant in the doped support on the performance of supported gold catalysts [41].In addition, the metallic gold species could activate hydrogen with subsequent spillover on the support and promote the reduction of ceria at lower temperature [56], as represented by the H 2 -TPR results.Figure 8 shows the O 1s XPS spectra of CeO 2 , CeM mixed oxides (M = Mn, Fe, and Ni), and corresponding supported Au catalysts.The main peak O' detected at about 529.1-529.3eV is typical for the surface lattice oxygen of the metal oxides, whereas the peak O" at 531.1 eV can be attributed to the loosely bonded surface absorbed oxygen species (hydroxide and adsorbed water) or the surface oxygen ions with low coordination numbers [57].Mover, the main peak O' of the CeM mixed oxides (M = Mn, Fe, and Ni) and all the supported Au catalysts is shifted slightly to higher binding energy in comparison with that of pure CeO 2 , which may indicate that the electron donating ability of oxygen species decreases somewhat after introducing Au and or dopant species.However, as the difference among all these samples in their O 1s XPS spectra is rather minor, the O 1s XPS spectra here disclose few messages concerning the redox and catalytic properties for the CeM mixed oxides and corresponding supported Au catalysts.
The contents of the bulk and surface gold component, determined by ICP-OES and XPS, respectively, are compared in Table 2. Obviously, the surface gold content is much higher than the bulk one for all the gold catalysts, meaning that the gold species is mainly located on the support surface.Moreover, it is noteworthy that the surface gold composition in the doped Au/CeM catalysts is higher than that in the undoped Au/CeO 2 catalyst.In comparison with undoped CeO 2 , the CeM composites are provided with abundant oxygen vacancies and could be better supports for anchoring and dispersing the gold nanoparticles, which may in turn give the doped Au/CeM catalysts higher reducibility and activity of CO oxidation.
Figure 9 shows the Au L III -edge X-ray absorption near-edge structure (XANES) spectra of the gold catalysts supported on CeO 2 and CeM mixed oxides, together with gold foil as a reference.Gold foil (Au 0 ) shows a shoulder at 11,933 eV as well as two intense peaks at 11,947 and 11,970 eV; the peaks at 11,933 and 11,947 eV are characteristic for pure gold f.c.c.structure ordered up to the third shell [59].Meanwhile, it was also reported that XAS was a very sensitive technique in detecting Au cations that gave a strong peak at 11,921.6 eV [60].Clearly, Figure 9 illustrates that all the gold catalysts are identical to Au foil in their Au L III -edge XANES spectra and almost no Au cations have been detected in the gold catalysts prepared by the colloidal deposition method in this work, in good agreement with the XPS results [57].In summary, the doping of ceria with Mn, Fe and Ni (especially Ni) can distort the cubic ceria structure and promote the creation of abundant oxygen vacancies.The oxygen vacancies not only play a crucial role in the dispersion and stabilization of gold species, but also enhance the lattice oxygen mobility and reducibility of CeO 2 , which are of great benefit in improving the catalytic performance of the gold catalysts supported on doped ceria (CeM) in the CO oxidation.

Catalytic Performance of CeM and Au/CeM in CO PROX
The catalytic performance of CeO 2 and CeM mixed oxides with different dopants in the CO PROX is presented in Figure 10.Obviously, undoped CeO 2 exhibits rather poor catalytic activity in the CO PROX; the conversion of CO is only 5.7% at 200 • C.However, after doping with other metals, the catalytic activity of the CeM composite oxides in CO PROX is greatly enhanced.Especially, for CO PROX in the H 2 -rich stream over CeNi, the conversion of CO reaches almost 100% at a temperature over 170 • C, with a PROX selectivity to CO 2 of higher than 50%.The catalytic activity of CeFe and CeMn composites is inferior to CeNi, but still much higher than that of undoped CeO 2 , illustrating the crucial influence of the metal dopant on the catalytic performance of the doped ceria materials, which should be related to their abundant oxygen vacancies and enhanced reducibility.As reported by Luo and co-workers, the catalytic activity of the modified ceria-related materials was directly related to their reducibility; high reducibility favored the activation of CO molecules over the catalyst surface [52].A comparison of various Au catalysts supported on CeO 2 and CeM mixed oxides in their performance in CO PROX is shown in Figure 11.As expected, the gold catalysts are much more active than the corresponding supports.Moreover, a significant promoting effect of the metal dopant in the CeM supports on the activity of Au/CeM catalysts is also observed.At ambient temperature, the conversions of CO are 65.6%, 93.0% and 48.2% for CO PROX over Au/CeMn, Au/CeFe and Au/CeNi catalysts, respectively, much higher than the value of 33.6% over the undoped Au/CeO 2 catalyst.Although Au/CeMn and Au/CeFe are somewhat more active than Au/CeNi at low temperature (<50 • C), the Au/CeNi catalyst may perform better in the temperature range of 50 to 200 • C. For CO PROX over the Au/CeNi catalyst, the conversion of CO remains near 100% at 60-130 • C, with a PROX selectivity to CO 2 of higher than 50%.This may be ascribed to the fact that the Au/CeNi catalyst is provided with large amount of oxygen vacancies and high reducibility, in comparison with other doped Au/CeM catalysts.
Long-term tests were also performed for CO PROX over the Au/CeFe and Au/CeNi catalysts at 80 • C, as shown in Figure 12.Similarly, the Au/CeNi catalyst displays a CO conversion of ca.92% and a PROX selectivity to CO 2 of ca.50%, a little higher than those (88% and 48%) over the Au/CeFe catalyst.Meanwhile, both the CO conversion and PROX selectivity to CO 2 over two catalysts remain almost unchanged even after 260 h on stream, suggesting the high stability of the gold catalysts supported on the Ni and Fe doped ceria materials.Moreover, as shown in Figures S1 and S2 in the Supplementary Information, there are no structural changes for the doped Au/CeFe and Au/CeNi catalysts after the long-term tests, in comparison with the fresh ones.As stated previously [57], the protective agent (PVA) used during the preparation of the gold catalysts cannot be completely removed through calcination at 250 • C and the residual protecting agent is capable of preventing the nano-particles of gold from sintering and aggregation during the thermal treatment and reaction at a relatively low temperature (<250 • C), which may contribute to the excellent stability of the Au/CeFe and Au/CeNi catalysts in CO PROX.The above results illustrate that the gold catalysts supported on the doped ceria (Au/CeM), especially Au/CeNi, exhibit much better performance in CO PROX than that supported on undoped ceria (Au/CeO 2 ).It was reported that the Au nanoparticles prepared by colloidal deposition method maintain in essence their metallic state and particle size distribution on different supports [41,57].In this work, various characterization techniques including XPS, XAS and TEM prove that all the gold catalysts are alike in the size and state of Au nanoparticles, as the gold component were loaded by the colloidal deposition under the same conditions.As a result, the difference between the doped Au/CeM and undoped Au/CeO 2 catalysts in their catalytic performance should be derived from their difference in the nature of supports.As suggested by the Raman spectroscopy, XAS, XPS, and TPR results, the incorporation of metal dopants (especially Ni) into CeO 2 can remarkably increase the amount of oxygen vacancies in the doped CeM materials, which is beneficial to strengthening the metal-support interaction, dispersing and stabilizing the gold species, and enhancing the reducibility of doped CeM composites and supported gold catalysts.All these may contribute to the high performance of the gold catalysts supported on the doped ceria (Au/CeM, especially Au/CeNi) in CO PROX.Actually, the catalytic performance of various gold catalysts follows the order of Au/CeNi > Au/CeFe > Au/CeMn > Au/CeO 2 , the same as that for the amount of oxygen vacancies in these catalysts.

Catalyst Preparation
A series of CeM (M = Mn, Fe and Ni) mixed oxides, with an M/Ce molar ratio of 1/8, were synthesized by a modified hydrothermal method [41,57].All chemical reagents were of analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd.(SCRC, Beijing, China) and used as received without further purification.Typically, Ce(NO 3 ) 3  O, 0.5 mmol) were dissolved in aqueous KOH solution (90 mL, 6 mol L −1 ) in a Teflon beaker, which was stirred continuously for 30 min at room temperature and then transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 110 • C for 24 h.After cooling down to room temperature, the obtained solid product was recovered by filtration and washed with deionized water.The final solid product was dried at 80 • C for 12 h and calcined at 400 • C in air for 4 h.For comparison, undoped CeO 2 was also prepared by the same procedure.
With the CeM composite oxides or CeO 2 as supports, gold catalysts (Au/CeM or Au/CeO 2 ) were then prepared through the colloidal deposition method, as described previously [41,57,58,61], with polyvinyl alcohol (PVA) as a protecting agent and NaBH 4 as a reducing agent.

Catalytic Tests and Analytical Procedure
The catalytic test for CO PROX in a H 2 -rich stream was carried out in a quartz tubular flow microreactor with an internal diameter of 6.0 mm at atmospheric pressure, as described previously [15].The catalyst was evaluated in the fresh state without any pretreatment.For each test, about 200 mg of catalyst sample (40-60 mesh) was used.The reaction stream consisted of 1.0% CO, 1.0% O 2 , 50.0%H 2 (by volume), and balanced N 2 , with a space velocity of 30,000 mL g −1 h −1 .O 2 , CO and CO 2 in the effluent gas were periodically analyzed online with a series of gas chromatographs.The CO and O 2 conversions (x CO and x O2 ) were determined with the CO and O 2 concentrations in the reactant and effluent streams, while the selectivity of CO PROX or CO 2 selectivity (S CO2 ) was defined as the fraction of O 2 consumption for the CO oxidation to CO 2 over the total O 2 consumption:

Conclusions
A series of ceria materials doped with Mn, Fe and Ni were synthesized by a modified hydrothermal method; with the doped ceria materials (CeM, M = Mn, Fe, and Ni; M/Ce = 1/8) as the support, gold catalysts (Au/CeM) were prepared by the colloidal deposition method.The influence of metal dopant on the performance of these ceria materials supported gold catalysts (Au/CeM) in CO PROX was investigated in detail with the help of various characterization measures.
The results indicate that a defective fluorite structure for the doped CeM materials was obtained by incorporating Mn, Fe and Ni metal ions into ceria.Due to the strong doping effect, the amount of oxygen vacancies in the doped ceria support (especially in CeNi) is remarkably increased, which is beneficial to strengthening the metal-support interaction, dispersing and stabilizing the gold species, and enhancing the reducibility of doped CeM composites and supported gold catalysts.
The gold catalysts supported on the doped ceria (Au/CeM), especially Au/CeNi, exhibits much better performance in CO PROX than that supported on undoped ceria (Au/CeO 2 ).For CO PROX over the Au/CeNi catalyst, the conversion of CO remains near 100% at 60-130 • C, with a PROX selectivity higher than 50%.Moreover, the Au/CeNi catalyst also displays high stability in a long-term test at 80 • C; the CO conversion (ca.92%) and PROX selectivity to CO 2 (ca.50%) remain almost unchanged even after 260 h on stream.
As the gold catalysts supported on various doped ceria materials are alike in the size and state of Au nanoparticles, the difference between the doped Au/CeM and undoped Au/CeO 2 catalysts in their catalytic performance should be mainly derived from their difference in the nature of supports; that is, the excellent performance of the doped Au/CeM catalysts (especially Au/CeNi) can be ascribed to their large amount of oxygen vacancies and high reducibility on account of second metal incorporation.The insight shown in this work helps to clarify the doping effect of other metals on the properties of ceria, which is then beneficial to building a structure-performance relation for ceria supported gold catalyst as well as developing a better catalyst for removing trace CO in the hydrogen stream and producing high purity hydrogen.

Table 1 .
Chemical composition and textural properties of CeO 2 , CeM mixed oxides with different dopants and corresponding supported Au/CeM catalysts (M = Mn,

Table 2 .
XRD and XPS results of CeO 2 , CeM mixed oxides with different dopants and corresponding supported Au/CeM catalysts (M = Mn, Fe and Ni).