Mechanistic insight into the hydrazine decomposition on Rh(111): effect of reaction intermediate on catalytic activity.

Periodic density functional theory (DFT) calculations have been performed to systematically investigate the effect of reaction intermediate on catalytic activity for hydrazine (N2H4) decomposition on Rh(111). Reaction mechanisms via intramolecular and NH2-assisted N2H4 decompositions are comparatively analyzed, including adsorption configuration, reaction energy and barrier of elementary step, and reaction network. Our results show that the most favorable N2H4 decomposition pathway starts with the initial N-N bond scission to the NH2 intermediate, followed by stepwise H stripping from adsorbed N2Hx (x = 1-4) species, and finally forms the N2 and NH3 products. Comparatively, the stepwise intramolecular dehydrogenation via N2H4→ N2H3→ N2H2→ N2H → N2, and N2H4→ NH2→ NH → N with or without NH2 promotion effect, are unfavorable due to higher energy barriers encountered. Energy barrier analysis, reaction rate constants, and electronic structures are used to identify the crucial competitive route. The promotion effect of the NH2 intermediate is structurally reflected in the weakening of the N-H bond and strengthening of the N-N bond in N2Hx in the coadsorption system; it results intrinsically from the less structural deformation of the adsorbate, and weakening of the interaction between dehydrogenated fragment and departing H in transition state. Our results highlight the crucial effect of reaction intermediate on catalytic activity and provide a theoretical approach to analyze the effect.

Initial hydrogenations of pyridine on MoP(001): a density functional study.

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(ß),C(ß),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(ß) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.

Decomposition of methanthiol on Pt(111): a density functional investigation.

Decomposition of methanthiol on Pt(111) is systematically investigated using self-consistent periodic density functional theory (DFT), and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to form the sp(3) hybridized configuration of both C and S atoms, in which C is almost tetrahedral and S has the tendency to bond to three atoms. Spontaneous dissociation rather than desorption is preferred for adsorbed methanthiol. Based on the harmonic transition state theory calculations, the decomposition rate constants of the thiolmethoxy and thioformaldehyde intermediates are found to be much lower than those for their formation, leading to long lifetimes of the intermediates for observation. Under the ultrahigh vacuum (UHV) condition, the most possible decomposition pathway for methanthiol on Pt(111) is found as CH(3)SH --> CH(3)S --> CH(2)S --> CHS --> CH + S --> C + S, in which the C-S bond cleavage mainly occurs at the CHS species. However, the decomposition pathway is CH(3)SH --> CH(3)S --> CH(3) + S under the hydrogenation condition; the C-S bond scission mainly occurs at CH(3)S. The Brønsted-Evans-Polanyi relation holds for each of the S-H, C-H, and C-S bond scission reactions.