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The water-gas shift reaction is studied using a series of Cu–Fe composite oxide catalysts, which were prepared by co-precipitation, deposition-precipitation, sol–gel, solid state reaction and mechanical mixing method, respectively. Distinct crystal structure and texture of the as-prepared catalysts were formed, and their reduction properties as well as surface basicities were modulated. The CO conversion in the water gas shift reaction is found to be correlation well with the Cu components in the temperature range of 200–350 °C, while the CO conversion is mainly controlled by Fe3O4 sites at temperature above 350 °C. The characterization results demonstrate that the formation of CuFe2O4 is ideal to obtain well dispersed Cu crystals; the weak basic sites are positive to the WGS reaction; and the strong interaction between copper and iron species can result in good Cu dispersion. The catalyst that was fabricated by the co-precipitation method, possesses the largest amount of CuFe2O4 and weak basic sites, as well as highly dispersed Cu crystals, thus showing the best catalytic activity and stability for the WGS reaction in our cases.

The precipitation processes of CuO–CeO2 catalysts preparation were modified, and their optimized copper content were also studied in detail. As indicated by the experimental results, the WGS catalytic activities of the CuO–CeO2 catalysts can be ranked as: stepwise precipitation (SP) > deposition–precipitation (DP) > co-precipitation (CP), suggesting that stepwise precipitation (i.e., a modified DP) is a convenient and effective method to prepare CuO–CeO2 WGS catalysts. The optimized copper contents of CuO–CeO2–SP and CuO–CeO2–CP catalysts are 20 wt.% and 25 wt.%, respectively. Their catalytic activities can be strongly correlated with the results from XRD, XPS, Raman, N2-physisorption, N2O chemisorption and H2-TPR. For DP and SP, a certain amount of copper has substitutively incorporated into ceria lattice with multiple Ce3+ and oxygen vacancies, which is considered as one relatively strong interaction between copper and ceria support. The substitutional incorporation of copper creates larger lattice distortion, lattice defect (embodying as larger lattice cell contraction of CeO2, microstrain and Raman shift) and stronger reducibility (i.e., lower reduction temperature of H2-TPR), as a consequence, higher surface energy and catalytic activity for WGS reaction. While for CP, copper nitrate and cerium nitrate were simultaneously precipitated, resulting in the burial of many copper species in ceria supports, i.e., occupancy incorporation of isolated copper into ceria lattice vacant site, which is considered as another weak interaction between copper and ceria support. Accordingly, combined with N2O chemisorption results, it is demonstrated that surface copper species of CuO–CeO2–CP are fewer and less active than those of CuO–CeO2–DP and CuO–CeO2–SP. Lastly, there is a direct relationship between the pore volume along with most probable pore size of the as-synthesized catalysts and their corresponding catalytic activities for WGS reaction.

A series of CuO/ZrO 2 catalysts with different Cu loadings (4.1, 6.1 and 8.4 wt.%) were synthesized by a deposition-precipitation method and evaluated with the water–gas shift (WGS) reaction. In order to distinguish the different supported copper oxide species, (NH 4 ) 2 CO 3 -leaching process was conducted on the three CuO/ZrO 2 catalysts. The parent and leached catalysts were characterized by ICP-OES, XPS, XRD, N 2 -physisorption, N 2 O titration, UV–Vis DRS, H 2 -TPR and CO-TPR. The results reveal that three types of copper oxide species are present on the parent CuO/ZrO 2 catalysts: (α) highly dispersed CuO that is weakly bound with ZrO 2 ; (β) strongly bound Cu-[O]-Zr species, which can not be leached by (NH 4 ) 2 CO 3 solution and is possibly associated with the surface oxygen vacancy of ZrO 2 ; (γ) crystalline CuO. The XRD and TEM results of the freshly reduced catalysts disclose that the three types of CuO species are transformed into their corresponding metallic states after the H 2 -pretreatment. It is found that the reaction rate correlates well with the amount of Cu-[O]-Zr species, indicating the metallic Cu derived from this species should be the catalytically active copper species for the WGS reaction. Moreover, CO-TPR results disclose that the Cu-[O]-Zr species play a significant role in promoting the reactivity of the surface hydroxyl groups, as is thought to be responsible for the high WGS activity of the CuO/ZrO 2 catalysts.

Three Cu/ZrO2 catalysts were synthesized utilizing co-precipitation (CP), deposition–precipitation (DP) and deposition–hydrothermal (DH) methods, respectively. The microstructure and texture of those catalysts are characterized by means of XRD, SEM, N2-physisorption, Raman and EPR characterizations. It is demonstrated that different morphologies and textures of ZrO2 are formed, and the micro- and crystal structure of Cu nanoparticles as well as the concentration of oxygen vacancies of ZrO2 are distinguish from each other. In addition, H2-TPR technique is employed to investigate the reducibility properties of the as-synthesized Cu/ZrO2 catalysts. It is found that the synergy interaction between Cu–ZrO2 obtained by the DH method is the strongest, owning to the possession of the largest amount of oxygen vacancies. Furthermore, their catalytic activities with respect to the water gas shift reaction are also performed, and the Cu/ZrO2-DH shows high catalytic activity, the reasons are the well dispersion and small crystallite size of Cu, the largest amount of oxygen vacancies, as well as the strongest interaction between Cu–ZrO2.

A series of CuO/CeO2 catalysts doping with Nb2O5 were fabricated by co-precipitation method. It is found that the introduction of Nb5+ will result in the substitution of Ce4+ with Nb5+, thus creating mobile electronic carriers in the as-prepared catalysts. The characterization results correlating with the catalytic activity evaluation disclose that the catalyst added with 1 wt. % Nb2O5 shows the most mobile electronic carries, certain amount of weak, medium basic sites and enhanced reducibility and chemical adsorption of CO, thus the best catalytic activity for water–gas shift reaction. However, excessive Nb2O5 addition prevents the incorporation of Cu2+ into CeO2 lattice and partially covers the surface of CuO and CeO2, resulting in weaken their reducibility and interaction between them, thus leading to inferior catalytic performance.

A series of Au/ZrO2 catalysts with low content of gold (

A series of CeO 2 supports were firstly prepared by precipitation method with NH 3 ⋅H 2 O (NH), (NH 4 ) 2 CO 3 (NC) and K 2 CO 3 (KC) as precipitant, respectively, and then CuO/CeO 2 catalysts were fabricated by depositing CuO on the as-obtained CeO 2 supports by deposition-precipitation method. The effect of CeO 2 supports prepared from different precipitants on the catalytic performance, physical and chemical properties of CuO/CeO 2 catalysts was investigated with the aid of XRD, N 2 -physisorption, N 2 O chemisorption, FT-IR, TG, H 2 -TPR, CO 2 -TPD and cyclic voltammetry (CV) characterizations. The CuO/CeO 2 catalysts were examined with respect to their catalytic performance for the water-gas shift reaction, and their catalytic activities and stabilities are ranked as: CuO/CeO 2 –NH > CuO/CeO 2 –NC > CuO/CeO 2 –KC. Correlating to the characteristic results, it is found that the CeO 2 support prepared by precipitation with NH 3 ⋅H 2 O as precipitant (i.e., CeO 2 –NH-300) has the best thermal stability and least surface “carbonate-like” species, which make the corresponding CuO/CeO 2 –NH catalyst presents the highest Cu-dispersion, the highest microstrain (i.e., the highest surface energy) of CuO, the strongest reducibility and the weakest basicity. While, the precipitants that contain CO 3 2- (e.g. (NH 4 ) 2 CO 3 and K 2 CO 3 ) result in more surface “carbonate-like” species of CeO 2 supports and CuO/CeO 2 catalysts. As a result, CuO/CeO 2 –NC and CuO/CeO 2 –KC catalysts present poor catalytic performance.

Water–gas shift reaction was carried out over a series of CuO/CeO2 catalysts doped with trivalent rare earth oxide (RE2O3, RE = Y, La, Nd and Sm), prepared via co-precipitation method. The effect of the dopants on the structure and catalytic properties of CuO/CeO2 catalysts was investigated with the aid of X-ray diffraction (XRD), Raman spectra, N2 physisorption, H2-TPR and selective N2O chemisorption characterizations. The results reveal the beneficial role of La2O3 and Nd2O3 doping in increasing the WGS catalytic activities and stabilities of CuO/CeO2 catalysts, while the addition of Y2O3 and Sm2O3 leads to the negative effect. Correlating to the characteristic results, it is found that the performance of CuO/CeO2–RE2O3 catalysts strongly depends on their surface copper dispersion, microstrain value and the amount of oxygen vacancies generated in ceria lattice. Besides, enough evidences suggest that, the most effective active site for WGS reaction is the moderate copper oxide (crystalline) interacted with surface oxygen vacancies of ceria in the CuO–CeO2 system.