Reaction mechanism of methylamine decomposition on Ru(0001): a density functional theory study.

The reaction mechanism of methylamine decomposition on Ru(0001) has been systematically investigated by density functional theory slab calculations. The decomposition network has also been described. The adsorption energies under the most stable configuration of the possible species and the energy barriers of the possible elementary reactions involved are obtained. Desorption is preferred for adsorbing methylamine and hydrogen, whereas for the other species, decomposition is more favorable. Our calculated results show that methylamine decomposition on Ru(0001) starts with H2CNH2 formation from methyl dehydrogenation, followed by subsequent methylene dehydrogenation, bond breaking of N--H and C--N in HCNH2, CH dehydrogenation and C-N bond cleavage in HCNH, and dehydrogenation of NH2, NH, and CH.

Structure sensitivity of CO oxidation on Co3O4: a DFT study.

The reaction mechanism of CO oxidation on the Co(3)O(4) (110) and Co(3)O(4) (111) surfaces is investigated by means of spin-polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O(2f)/O(3f)) to form CO(2) via the Mars-van Krevelen reaction mechanism on both (110)-B and (111)-B; 3) pre-adsorbed molecular O(2) can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO(2) [O(2)(a)+CO(g)→CO(2)(g)+O(a)] on (110)-A/(111)-A; 4) CO oxidation is a structure-sensitive reaction, and the activation energy of CO oxidation follows the order of Co(3)O(4) (111)-A(0.78 eV)>Co(3)O(4) (111)-B (0.68 eV)>Co(3)O(4) (110)-A (0.51 eV)>Co(3)O(4) (110)-B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O(2f), it was also found that Co(3+) is more active than Co(2+), so both O(2f) and Co(3+) control the catalytic activity of CO oxidation on Co(3)O(4), as opposed to a previous DFT study which concluded that either Co(3+) or O(2f) is the active site.

Decomposition of methylamine on nitrogen atom modified Mo(100): a density functional theory study.

Three possible pathways for C-N bond breaking in methylamine have been investigated over clean Mo(100) and nitrogen atom-modified Mo(100) surfaces with a nitrogen coverage of 0.25 monolayer (ML) (N-Mo(100)) firstly, and the C-N bond breaking following the intramolecular hydrogen transfer from the CH3 to NH2 is excluded owing to the high barriers. Then methylamine decomposition starting with C-H, N-H, and C-N scission over the nitrogen atom-modified Mo(100) surface with a nitrogen coverage of 0.5 ML (2N-Mo(100)) has been systematically investigated, and the decomposition network has been mapped out. The thermochemistry and energy barriers for all the elementary steps, starting with C-H, N-H, and C-N scission, and sequential reactions from the resulting intermediates, are presented here. The most likely decomposition path is H3CNH2→ H2CNH2 + H → HCNH2 + H + H → CNH2 + H + H + H → C + NH2 + H + H + H → C + NH3 + H2→ C + NH3(g) + H2(g). For the decomposition reactions involved in the likely decomposition path, there is a linear relationship between the energy of transition state and the energy of final state. For the reverse processes of the dehydrogenation of CH, NH, NH2, it is found that there is a linear relationship between the barrier and the valency of A (A[double bond, length as m-dash]C, N, and NH).

A DFT+U study of acetylene selective hydrogenation on oxygen defective anatase (101) and rutile (110) TiO2 supported Pd4 cluster.

The reaction mechanisms for selective acetylene hydrogenation on three different supports, Pd(4) cluster, oxygen defective anatase (101), and rutile (110) titania supported Pd(4), cluster are studied using the density functional theory calculations with a Hubbard U correction (DFT+U). The present calculations show that the defect anatase support binds Pd(4) cluster more strongly than that of rutile titania due to the existence of Ti(3+) in anatase titania. Consequently, the binding energies of adsorbed species such as acetylene and ethylene on Pd(4) cluster become weaker on anatase supported catalysts compared to the rutile supported Pd(4) cluster. Anatase catalyst has higher selectivity of acetylene hydrogenation than rutile catalyst. On the one hand, the activation energies of ethylene formation are similar on the two catalysts, while they vary a lot on ethyl formation. The rutile supported Pd catalyst with lower activation energy is preferable for further hydrogenation. On the other hand, the relatively weak adsorption energy of ethylene is gained on anatase surface, which means it is easier for ethylene desorption, hence getting higher selectivity. For further understanding, the energy decomposition method and micro-kinetic analysis are also introduced.

First-principles analysis of the C-N bond scission of methylamine on Mo-based model catalysts.

The C-N bond breaking of methylamine on clean, carbon (nitrogen, oxygen)-modified Mo(100) [denoted as Mo(100) and Mo(100)-C(N,O), respectively], Mo(2)C(100), MoN(100), and Pt(100) surfaces has been investigated by the first-principles density functional theory (DFT) calculations. The results show that the reaction barriers of the C-N bond breaking in CH(3)NH(2) on Mo(100)-C(N,O) are higher than that on clean Mo(100). The calculated energy barrier can be correlated linearly with the density of Mo 4d states at the Fermi level after the adsorption of CH(3)NH(2) for those surfaces. Moreover, the DFT results show that the subsurface atom, e.g., carbon, can reduce the reaction barrier. In addition, We noticed that the activation energies for the C-N bond breaking on Mo(2)C(100) and MoN(100) are similar to that on Pt(100), suggesting that the catalytic properties of the transition metal carbides and nitrides for C-N bond scission of CH(3)NH(2) might be very similar to the expensive Pt-group metals.

Methane combustion on Pd-based model catalysts: Structure sensitive or insensitive?

The C-H breaking of methane on the clean and the oxygen precovered palladium single crystal surfaces with the simplest orientations, namely, the dense (111), (100), the more open (110), and the stepped (111) surfaces, the corresponding O/Pd surfaces with different coverage of oxygen, as well as the palladium oxide PdO(100) and PdO(110) surfaces, has been studied with the density functional theory-generalized gradient approximation method using the repeated slab models. The adsorption energies under the most stable configuration of the possible species and the activation energy barriers of the reaction are obtained in the present work. Through systematic calculations for the C-H breaking of methane CH(4)-->CH(3)+H on these surfaces, it is found that such a reaction is structure sensitive on clean palladium and oxygen precovered palladium surfaces with lower oxygen coverage, but it is insensitive on oxygen precovered palladium surfaces with higher oxygen coverage and on palladium oxides. These results are in general agreement with the experimental observations.