Theoretical Investigation Of Methanol Decomposition On Pd, Pt, And Rh Surfaces

Investigation of methanol transiforming to hydrogen is paramount to the widely using of hydrogen energy. However, the catalysts selection is the critical point for the high efficient conversion of methanol to hydrogen. In this study, the catalyzed performaces of some metal for methanol decomposition to hydrogen are studied by using density functional theory; the catalyted decomposition mechanism is obtained, which is may used to guide the catalysts design in this reaction.Firstly, the degydrogenation of methanol on Pd(111) and Pd(100) is systematically studied. Because of the more openness of the (100) surface in face-centered cubic metal, Pd(100) accounts for stronger adsorptions than Pd(111) for most species except CHO, CO and H at hollow sites, which is due to the steric effects. Also, Pd(100) affords relatively low energy barriers and larger dehydrogenation rate constants for most elementary reactions as well as smaller desorption rates for the saturated adsorbates (methanol and formaldehyde). These facts suggest that the more open Pd surface really possesses the higher activity and selectivity for the complete dehydrogenation of methanol. On both facets, methanol desorption is preferred. For the methanol dehydrogenation, at both the typical UHV and MSR temperatures, Pd(111) predicts an unchanged dehydrogenation path, i.e., CH3OH→CH2OH→CH2O→CHO→CO; whereas Pd(100) predicts the same path as Pd(111) at the lower temperature but at the higher temperature (e.g., 500 K) the path is changed to CH3OH→CH3O (and/or CH2OH)→CH2O→CHO→CO.Then, methanol dehydrogenation into carbon monoxide on Rh(111) has been explored by DFT slab calculation and microkinetic modeling. Compared with Pd(111), we find that on Rh(111) the adsorption configurations of species are rather abundant, the most stable adsorptions afford the larger binding energy, and the PES of adsorbates is rather flat; for the reaction, it is found that the initial C?H and O?H activations are comparable, and the paths CH3OH→CH3O→CH2O→CHO→CO and CH3OH→CH2OH→CHOH→CHO→CO are the most possible pathways in energy barrier, which is different from other metal surfaces. The study indicated that the reason why oxidation does not take place at CH2O in methanol oxidized is partly because there is a very activated adsorption state (η1(C)?η1(O) ?η1(H)) for formaldehyde on Rh(111), and partly because formaldehyde is not involved in initial C?H activation. Furthermore, at all reaction conditions, CO and COH are the most abundant surface species, and coverages of other intermediates are rather low. Under UHV conditions, paths CH3OH→CH3O→CH2O→CHO→CO and CH3OH→CH2OH→CHOH→CHO→CO account for the dehydrogenation of methanol on Rh(111), and other paths are negligible. At high temperatures and pressures, these two paths are is still most favorable, whereas other paths are also significant. In addition, it is found that apparent activation energy decreases as a function of temperature at all reaction conditions; and reaction order of methanol decreases with the increment of methanol partial pressure.Finally, the decomposition network of methanol on Pt(111) is explored. The results indicate all of species involved in methanol decomposition are inclined to dehydrogenation, and the C?O bond scission takes place very slowly. By calculating the rate constants of every elementary reaction, the most favorable decomposition path is obtained: CH3OH→CH3O→CH2O→CHO→CO. Furthermore, it is found that the activation energies of C?O increase with its bond order fortifyin; and CHOH has the lowest energy barrier for C?O bond cleavage.