Water dissociation is the rate-determining step (RDS) in the industrially important water gas shift (WGS) reaction. Low temperature Cu catalysts are limited by a higher barrier to dissociation whereas Ni surfaces with lower barriers for this reaction are deactivated by carbon deposition due to CO dissociation. Density functional theory (DFT) calculations are performed on a series of overlayer and subsurface bimetallics starting with Ni(111) and Cu(111) to understand the synergistic catalytic activity of Cu/Ni bimetallics toward H 2 O dissociation which is the RDS. Surface parameters like surface energy, work function and density of states were calculated and were correlated with the change in reactivity. Transition state (TS) calculations showed that addition of Ni to Cu(111) surfaces decreased dissociation barriers while the scenario is reversed when Cu atoms replace Ni in Ni(111) surface with no linear relation with any calculated surface properties in both cases. Linear relations were found to correlate well the reaction energies with the activation energy barriers. Effects of surface temperature were included by determining the change in the barrier heights and barrier locations with lattice atom motion calculated from TS calculations. Dissociation probabilities calculated at different surface temperatures using semiclassical methods showed that increase in surface temperature increases dissociation probabilities where the extent of increase is strongly dependent on the change in barrier heights. Overall, Ni addition to Cu(111) surface proved beneficial while the Cu addition to Ni(111) surface proved detrimental to H 2 O dissociation.

Several full-dimensional potential energy surfaces (PESs) are reported for vibrating CO adsorbates at two coverages on a rigid NaCl(100) surface based on first-principles calculations. These PESs reveal a rather flat energy landscape for physisorption of vibrationless CO on NaCl(100), evidenced by various C-down adsorption patterns within a small energy range. Agreement with available experimental results is satisfactory, although quantitative differences exist. These PESs are used to explore isomerization pathways between the C-down and higher-energy O-down configurations, which reveal a significant isomerization barrier. As the CO vibration is excited, however, the energy order of the two isomer changes, which helps to explain the experimental observed flipping of vibrationally excited CO adsorbates.

Methane-dissociative chemisorption is the rate-determining step in the industrially important steam reforming and dry reforming reactions of methane. Widely used industrial catalysts containing Ni as the active metal face the problems of carbon deposition and deactivation, whereas Pt surfaces with lower barrier are expensive to be used in the industrial scale. Using density functional theory calculations, a series of surface and subsurface Ni–Pt bimetallic surfaces were studied to understand the synergistic catalytic activity of alloying elements toward facilitating methane dissociation and in resisting carbon formation. Addition of Ni to Pt(111) decreased activation energy barriers, whereas a linear increase in barrier was found when Pt is added to Ni(111) surface. The observed reactivity trends were explained using surface-based descriptors like work function, surface energy, and d-band center and also using energy-based descriptors, namely, Bronsted–Evans–Polanyi and transition-state scaling relationships. Changes in barrier heights and locations of the barrier with lattice atom motion were calculated to include the effect of surface temperature on dissociation probabilities. Dissociation probabilities thus calculated at different surface temperatures using semiclassical methods showed that reactivity increased with surface temperature on all surface alloys. Overall, two surfaces, viz., Ni9/Pt(111) and sub-Pt9/Ni(111), showed improved behavior toward CH4 dissociation, irrespective of the composition of underlying layers. C2 formation on these two alloys also showed higher barriers compared to pure Ni(111) surface. However, considering all aspects like energy barriers to CH4 dissociation and CH dissociation, carbon adsorption energy, and cost, the subsurface alloy, sub-Pt9/Ni(111), showed an enhanced overall performance as a reforming catalyst.

Direct chemical dynamics simulations at high temperatures of reaction between 3 O 2 and graphene containing varied number of defects were performed using the VENUS-MOPAC code. Graphene was modeled using (5a,6z)-periacene, a poly aromatic hydrocarbon with 5 and 6 benzene rings in the armchair and zigzag directions, respectively. Up to six defects were introduced by removing carbon atoms from the basal plane. Usage of the PM7/ unrestricted Hartree−Fock (UHF) method, for the simulations, was validated by benchmarking singlet-triplet gaps of n-acenes and (5a,nz) periacenes with high-level theoretical calculations. PM7/UHF calculations showed that graphene with different number of vacancies has different ground electronic states. Dynamics simulations were performed for two 3 O 2 collision energies E i of 0.4 and 0.7 eV, with the incident angle normal to the graphene plane at 1375 K. Collisions on graphene with one, two, three, and four vacancies (1C-, 2C-, 3C-, and 4C-vacant graphene) showed no reactive trajectories, mainly due to the nonavailability of reactive sites resulting from nascent site deactivation, a dynamical phenomenon. On the other hand, 3 O 2 dissociative chemisorption was observed for collisions on four-(with a different morphology), fiveand six-vacant graphene (4C-2-, 5C-and 6C-vacant graphene). A strong morphology dependence was observed for the reaction conditions. On all reactive surfaces, larger reaction probabilities were observed for collisions at E i = 0.7 eV. This is in agreement with the nucleation time measured by supersonic molecular beam experiments wherein about 2.5 times longer nucleation time for O 2 impinging at 0.4 eV compared with 0.7 eV was observed. Reactivity at both collision energies, viz., 0.4 and 0.7 eV, showed the following trend: 5C-< 6C-< 4C-vacant graphene. Formation of carboxyl/semiquinone (CO)-and ether (−C−O−C−)-type dissociation products was observed on all reactive surfaces, whereas a higher probability of formation of the ether (−C−O−C−) group was found on 4C-vacant graphene on which dangling carbon atoms are present in close proximity. However, no gaseous CO/CO 2 formation was observed on any of the graphene vacancies even for simulations that were run up to 10 ps. This is apparently the result of the absence of excess oxygen atoms that can aid the formation of larger groups, the precursors for CO/CO 2 formation. Although the results of this study do not provide a conclusive understanding of the mechanism of graphene/graphite oxidation, this work serves as an initial study attempting to understand the 3 O 2 dissociative chemisorption dynamical mechanism on defective-graphene/graphite surfaces at high temperatures.

Several full-dimensional potential energy surfaces (PESs) are reported for vibrating CO adsorbates at two coverages on a rigid NaCl(100) surface based on first principles calculations. These PESs reveal a rather flat energy landscape for physisorption of vibrationless CO on NaCl(100), evidenced by various C-down adsorption patterns within a small energy range. Agreement with available experimental results is satisfactory, although quantitative differences exist. These PESs are used to explore isomerization pathways between the C-down and higher energy O-down configurations, which reveal a significant isomerization barrier. As CO vibration is excited, however, the energy order of the two isomer changes, which helps to explain the experimental observed flipping of vibrationally excited CO adsorbates.