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.
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.
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.
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