Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces

摘要

We report a periodic density functional theory (DFT) study of the methanol steam reforming (MSR) reaction on the Co(OOOl) and Co(111) surfaces. Thermochemistry and activation barriers for all elementary steps of two commonly accepted mechanisms, CH2O decomposition and H2COOH formation, were calculated along with the water gas shift (WGS) reaction. The adsorption energies on Co(OOOl) and Co(111) are within 0,05 eV for all the MSR intermediates examined, which suggests the same catalytic activity for both surfaces. On the basis of both the thermochemistry and barriers, CH2O decomposition into CHO and CO is favored over H2COOH formation on the Co(OOOl) surface. The strong CO binding on Co(OOOl) limits its WGS activity to convert CO into CO2. Our results of the MSR and WGS pathways suggest that Co will not show high selectivity toward CO2 for MSR, which matches the limited experimental data available. A simple Langmuir equilibrium model was applied to study the surface coverages on Co. The results show that O~* and OH~* coverages on Co are higher than on other transition metals such as Pt, Pd, and Cu due to the facile H2O activation on the surface, and reaction steps involving O-H bond breaking and forming may be facilitated by O~* and OH~*. The results also suggest that Co is more susceptible than other transition metals to oxide formation under steam reforming conditions, especially under high water to alcohol ratios.