CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the C=O bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).

On the basis of density functional theory calculations, the chemisorption of CO 2 on the transition metal surfaces was investigated to find out the key factors controlling its adsorption strength and activation degree. The interaction mechanism of CO 2 with the metal surfaces was discussed by analyzing the density of states. The adsorption strength of CO 2 is controlled by the d-band center of the metal surfaces and also affected by the charge transfer from the metal surfaces to the chemisorbed CO 2 . The degree of CdO bond activation depends on the transferred charge. Therefore, both d-band center of the metal surfaces and the charge transfer should control the chemisorption of CO 2 .

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CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.

Density functional theory calculations have been carried out to study CO adsorption on the (001), (100), and (110) surfaces of Fe5C2, which are considered as active catalysts in Fischer−Tropsch synthesis. It is found that CO prefers to adsorb at three 3-fold sites (three iron atoms) on the three surfaces at low coverage with maximum adsorption energies of −2.10, −2.21, and −2.34 eV, respectively. The adsorption energy of CO adsorbed around surface carbon atoms on (001) is higher than that on (110) by up to 0.94 eV. The difference is attributed to the C 2p−Fe 3d bands that mainly lie at the lower energy level on bare (110) than that on bare (001). This indicates that the surface Fe−C bonds on (110) are stronger than those on (001), which in turn hinders local CO adsorption on (110). In contrast to (001) and (110), a monolayer of the (100) surface only consists of iron atoms. The adsorption energy at low coverage on (100) is similar to those on (001) and (110), but the highest at high coverage among all surfaces. These results indicate the diversity of the atomic structure on the underlying Fe5C2 surface site. Apart from the (100) surface at high coverage, other surfaces may play dominant roles when practical working conditions permit.

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-FeO, FeO, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% "screened" Hartree-Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.

To develop the high-performance supported metal catalyst for industrial processes, it is highly desirable to elucidate and fully utilize the indispensable support part. Herein, the relationship between catalytic performance and the structure of support ZrO2 was elucidated by comprehensive analysis of the progressive calcination experiments, tests over model catalysts, and various characterizations of catalyst structures. We demonstrated that combination of Cu and tetragonal ZrO2 makes a highly active, selective, and especially stable catalyst for the hydrogenation of dimethyl oxalate to ethylene glycol. To obtain stable Cu particles, the catalyst was annealed at high temperatures (e.g., from 450 to 850 °C). The stable large Cu particles were formed, and the number of exposed Cu sites decreased. Fortunately, support ZrO2 was motivated into the tetragonal phase, compensating for and even improving the activity. Thus, the yield of ethylene glycol was greatly improved from ∼26 to 99%, and a stable performance was achieved (life span of >600 h). The strategy alleviated the dependence of hydrogenation on highly dispersed metal sites and provided an alternative way to enhance the catalytic stability. This simple way simultaneously improved the efficiency and reduced the level of irreversible deactivation due to sintering, which has great potential for industrial applications. Tetragonal ZrO2 also proved to be effective for a series of carbonyl hydrogenations (e.g., esters, aldehydes, ketones, and acids), indicating a general promotion of these reactions by ZrO2.

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