Abstract
Density functional theory (DFT) was used to calculate the step-by-step hydrogenation of carbon monoxide (CO) to form methanol over a Co4 cluster/Al2O3 surface. A three-dimensional Co4 tetrahedral structure was selected to explore its interaction with the supporting Al2O3 (104) surface. Co4 chemically reacted with Al2O3 to form a new chemical system. The calculated results show that Al2O3 support has strengthened the Co4 catalyst during the reaction since the formation of the Co–O bond. Loading Co4 on the Al2O3 surface increases CO adsorption ability but decreases the dissociation ability of C–O to produce hydrocarbons. As such, CH3OH formation becomes more favorable both kinetically and thermodynamically on Co4/Al2O3. In CO hydrogenation, methanol was synthesized through a CO reaction with hydrogen via either an Eley–Rideal or Langmuir–Hinshelwood pathway to form the intermediates C*-O-H, H-C*-OH, H2-C*-OH, and finally the hydrogenation of H2-C*-OH to methanol with both hydrogenation steps forming C*-OH and final product as rate-limiting. These results showed that the interaction between Co, Al2O3 and H2 pressure can change the pathway of CO hydrogenation on Co/Al2O3 and it may, therefore, influence distribution of the final products.
Highlights
carbon monoxide (CO) is more readily absorbed on Co4 /Al2 O3 than on alone a Co4 cluster
In the CO hydrogenation process, methanol was synthesized through the intermediates C*-OH, H-C*-OH, and H2 C*-OH: H2 → H* + H*
Our calculations show that the formation of C*-O-H induced a significant change in the geometry of Co4 and our calculated results show that the interaction between Al2 O3 support and Co catalyst plays a key role in the catalytic hydrogenation of CO to methanol
Summary
Methanol synthesis by carbon monoxide (CO) and carbon dioxide (CO2 ) hydrogenation has been widely studied both experimentally and theoretically [1,2,3,4,5,6] because methanol is an important raw material of crucial importance in the chemical and energy industries. DFT can simulate catalytic process at surfaces with the detail and accuracy required for calculated results to compare with experiments [31] This understanding allows theoretical optimization for better catalysts. Reimers et al [30] studied theoretically the catalytic activities of Zn, Ce and Ga oxides in the consecutive hydrogenation reactions of the CO molecule Their calculated results indicated that the methanol formation proceeds via formyl (HCO), followed by the formation of formaldehyde (H2 CO), methoxy (H3 CO) and, methanol. In the study of Studt et al [11] DFT results revealed that over Cu (211) the CO hydrogenation occurs at the carbon end of adsorbed CO and the formation of methanol is processed through HCO, H2 CO, H3 CO intermediates. Firstly we concentrate on the reactants adsorption and the configurations of the key intermediates involved in the process mechanism, followed by the energies
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