•Methanol can be synthesized from CO/CO2 hydrogenation which is dependent on Cu valence.•CO is the main carbon source when the surface is predominantly covered by Cu+ species.•CO2 is the primary carbon source when metallic Cu covers the surface.A systematic theoretical study was performed to investigate methanol synthesis from CO/CO2 hydrogenation and the water-gas-shift (WGS) reaction on Cu(111) and Cu2O(111) surfaces using density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations. Specifically, DFT was used to investigate methanol synthesis from CO/CO2 hydrogenation on these surfaces at P = 80 atm, T = 553 K and (CO+ CO2)/H2 = 20/80. The results show that methanol can be synthesized from CO or CO2 hydrogenation and is dependent on the catalyst’s preparation as well as the active site type. Further, CO is the main carbon source when the surface is predominantly covered by Cu+ species. However, CO2 is the primary carbon source when metallic Cu covers the surface. Under the reaction conditions investigated, H2 and CO easily reduce Cu2O to metallic Cu, and the Cu+ species are stabilized by the presence of H2O, CO2, carrier (such as MgO) or alkali metals. For this reason, the scale of methanol produced from CO or CO2 hydrogenation depends on the ratio of Cu+/Cu0.Download high-res image (349KB)Download full-size image

Benzoic acid (C6H5COOH) is selected as a coal-based model compound, and its catalytic pyrolysis mechanisms on ZnO, γ-Al2O3, CaO, and MgO catalysts are studied using density functional theory (DFT) compared to the non-catalytic pyrolysis mechanism. DFT calculation shows that the pyrolysis process of C6H5COOH in the gas phase occurs via the direct decarboxylation pathway (C6H5COOH → C6H6 + CO2) or the stepwise decarboxylation pathway (C6H5COOH → C6H6COO → C6H6 + CO2). For C6H5COOH catalytic pyrolysis on the ZnO (101̅0) surface, the preferred reaction pathway is C6H5COOH → C6H5COO + H → C6H6 + CO2, whereas the preferred reaction pathway on γ-Al2O3 (110), CaO (100), and MgO (100) surfaces is C6H5COOH → C6H5COO + H → C6H5 + CO2 + H → C6H6 + CO2, indicating that the presence of catalysts changed the pyrolysis mechanism of C6H5COOH. In addition, dissociative adsorption of C6H5COOH is observed on these surfaces. It is found that ZnO (101̅0), MgO (100), and CaO (100) are beneficial to C6H5COOH decomposition, but γ-Al2O3 (110) is disadvantageous to the C6H5COOH decomposition. At the same reaction temperature, the rate constants show the order: k(ZnO) > k(MgO) > k(CaO) > k(no catalyst) > k(γ-Al2O3).

•Methanol decomposition mechanism on the different surfaces has been investigated by DFT.•The solvation environment has been modeled by using four water molecules.•The presence of H2O molecules facilitate methanol decomposition.•The pre-adsorbed OH can alter the main reaction route.Methanol decomposition on the Cu(110) surface with and without the presence of H2O molecules has been systematically investigated by using density functional theory with the continuum solvation slab model. It is found that H2O molecules on the Cu(110) surface remarkably affect the adsorption configurations and adsorption energies. The results also show that the pre-adsorbed hydroxyl (OH) from H2O dissociation not only alters the reaction pathway of methanol decomposition but also influences the activation energy. By using the transition state theory, the rate constants of some mainly elementary steps under typical experimental temperature (T = 473–573 K) are calculated. In general, the methanol decomposition on the H2O/Cu(110) surface is more favorable than that on the clean Cu(110) surface, both thermodynamically and kinetically. Finally, the reaction network of methanol dehydrogenation is obtained and the rate-limiting steps as well as the most favorable reaction routes for methanol decomposition on the clean Cu(110), H2O/Cu(110) and OH pre-adsorbed H2O/Cu(110) surfaces are identified.

Density functional theory calculations with the continuum solvation slab model are performed to investigate the effect of metal dopants on the Cu(110) surface in the presence of H2O for the methanol decomposition. The sequential dehydrogenation of methanol (CH3OH → CH3O → CH2O → CHO → CO) is studied in the present work. The results show that the introduction of different metals (Pt, Pd, Ni, Mn) on the H2O/Cu(110) surface notably influence the adsorption configurations and adsorption energies of all adsorbates, and remarkably affect the reaction energies and activation energies of the elementary steps. The Pt, Pd and Ni doped H2O/Cu(110) surfaces are able to promote hydrogen production from methanol decomposition, but Mn doped H2O/Cu(110) surfaces are unfavorable for the reaction. The activity of methanol decomposition decreases as follows: Pd–H2O/Cu(110) > Pt–H2O/Cu(110) > Ni–H2O/Cu(110) > H2O/Cu(110) > Mn–H2O/Cu(110). Finally, the Brønsted–Evans–Polanyi plot for the main methanol dissociation steps on the metal doped and un-doped H2O/Cu(110) surfaces are identified, and a linear relationship between the reaction energies and transition state energies is obtained.

The sulfurized processes of H2S on dehydrated (100) and (110) as well as partially hydrated (110) surfaces of γ-Al2O3 were investigated using a periodic density functional theory method. The adsorption configurations of possible intermediates and the potential energy profiles of reaction are depicted. Our results show that H2S adsorbs preferentially on the Al site along with the S bond, and the adsorption energies are −32.52 and −114.38 kJ mol−1 on the dehydrated (100) and (110) surfaces, respectively. As the reaction temperature of the desulfurization changes, the (110) surface presents different levels of hydroxyl coverage, which affects the adsorption structures of species and reaction energies of dissociation processes. The bonding strengths of H2S on the partially hydrated (110) surfaces are weaker than on the dehydrated (110) surface. Compared with the 3.0 and 8.9 OH per nm2 surfaces, the H2S has the weakest adsorption energy (−39.85 kJ mol−1) and the highest activation energy (92.06 kJ mol−1) on the 5.9 OH per nm2 surface. On the 8.9 OH per nm2 surface, the activation energy of the second dissociation step (rate-determining step) for H2S dissociation is merely 38.32 kJ mol−1. On these involved surfaces, cleavage processes of the two H–S bonds present facile activation energies, which are facilitative to desulfurization.

Ethanol synthesis from syngas over CuZnAl catalyst without other promoters is studied using theoretical and experimental methods. The possible reaction paths of the ethanol synthesis in thermodynamic and dynamic over Cu cluster and Cu–O species adsorbed on ZnO surface are systematically identified at the molecular level. Three possible paths involving the formation of CH3 as the key intermediate are proposed, which are COH, CHOH, CH2OH, and CH3; CHO, CH, CH2, and CH3; and CH3OH and CH3. CO insertion into the CH3 intermediate produces CH3CO, which is further hydrogenated to yield CH3CHO and CH3CHOH and finally obtain ethanol. The CuZnAl catalyst, which is prepared by complete liquid-phase technology, has high ethanol selectivity and stability because of the strong interaction between Cu species and ZnO. In summary, the coexistence of both Cu0 and Cu+ is necessary for ethanol synthesis from syngas over CuZnAl catalyst without other promoters.