Methanol synthesis from CO2 hydrogenation on the defective In2O3(110) surface with surface oxygen vacancies has been investigated using periodic density functional theory calculations. The relative stabilities of six possible surface oxygen vacancies numbered from Ov1 to Ov6 on the perfect In2O3(110) surface were examined. The calculated oxygen vacancy formation energies show that the D1 surface with the Ov1 defective site is the most thermodynamically favorable while the D4 surface with the Ov4 defective site is the least stable. Two different methanol synthesis routes from CO2 hydrogenation over both D1 and D4 surfaces were studied, and the D4 surface was found to be more favorable for CO2 activation and hydrogenation. On the D4 surface, one of the O atoms of the CO2 molecule fills in the Ov4 site upon adsorption. Hydrogenation of CO2 to HCOO on the D4 surface is both thermodynamically and kinetically favorable. Further hydrogenation of HCOO involves both forming the C–H bond and breaking the C–O bond, resulting in H2CO and hydroxyl. The HCOO hydrogenation is slightly endothermic with an activation barrier of 0.57 eV. A high barrier of 1.14 eV for the hydrogenation of H2CO to H3CO indicates that this step is the rate-limiting step in the methanol synthesis on the defective In2O3(110) surface.

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Catalytic conversion of CO2 to liquid fuels or valuable chemicals is an attractive alternative to geological sequestration. In the present study, we applied density functional theory slab calculations in the investigation of the adsorption and hydrogenation of CO2 on the (110) surface of In2O3. Our results indicate that the adsorbed CO2 is activated, forming a surface carbonate species by combining with surface oxygen, and has an adsorption energy of −1.25 eV. Heterolytic dissociative adsorption of H2 results in a surface hydroxyl from H binding the surface O site and a hydride from H binding the In site. The migration of H from the In site to the neighboring O site is energetically favorable but has a significant activation barrier of 1.32 eV. Water may adsorb on the surface either molecularly or dissociatively, with adsorption energy of −0.83 eV and −1.19 eV, respectively. Starting from CO2 coadsorbed with the H adatoms on the In2O3 surface, we examined two possible conversion pathways for CO2: (a) CO2 is hydrogenated by the H adatom on the In site to form a surface formate species (HCOO); (b) CO2 is protonated by the H adatom on the O site to form a surface bicarbonate species (COOH). Reaction a is endothermic by +0.33 eV, whereas b is exothermic by −0.78 eV. Although the formation of the bicarbonate species is energetically favorable, the subsequent step to form CO and OH is highly endothermic, with a reaction energy of +1.07 eV. Furthermore, the bicarbonate species can react with a surface hydroxyl easily, resulting in coadsorbed H2O and CO2. These results indicate that hydrogenation of CO2 to the formate species is favorable over protonation to the bicarbonate species on the In2O3 surface. These results are consistent with the experimental observations that the indium oxide based catalyst has a high CO2 selectivity and H2O resistance.

The adsorption and activation of CO over flat Co{0001}, corrugated Co{1120}, and stepped Co{1012} and Co{1124} surfaces have been analyzed using periodic density functional theory calculations. CO strongly chemisorbs on all these surfaces but does not show a strong dependence on the surface structure. The calculated structure of adsorbed CO on Co{0001} at 1/3 monolayer (ML) of coverage was found to be in good agreement with the experiment. The barrier for CO dissociation over Co{0001} was found to decrease with decreasing CO coverage, taking on a value of 232 kJ/mol at 1/4 ML and 218 kJ/mol at 1/9 ML. The presence of the "zigzag" channel on Co{1120} enhances the reactivity slightly by reducing the barrier for CO dissociation to 195 kJ/mol. In contrast, the stepped Co{1012} and Co{1124} surfaces are much more active than the flat and corrugated surfaces. Both stepped surfaces provide direct channels for CO dissociation that do not have barriers with respect to gas-phase CO. In general the activation barriers lower as the reaction energies become more exothermic. Reconstruction of the step edges that occur in the product state, however, prevents a linear correlation between the reaction energy and the activation energy.

Methanol synthesis from CO 2 hydrogenation on a model Pd/In 2 O 3 catalyst, i.e. Pd 4 /In 2 O 3 , has been investigated using density functional theory (DFT) and microkinetic modeling. Three possible routes in the reaction network of CO 2 + H 2 → CH 3 OH + H 2 O have been examined. Our DFT results show that the HCOO route competes with the RWGS route whereas a high activation barrier blocked the HCOOH route kinetically. The DFT results also suggest that H 2 COO* + H* ↔ H 2 CO* +OH* and cis-COOH* + H* ↔CO* + H 2 O* are the rate limiting steps in the HCOO route and the RWGS route, respectively. Microkinetic modeling results demonstrate that the HCOO route is the dominant pathway for forming methanol from CO 2 hydrogenation. Furthermore, the activation of the H adatom on the Pd cluster and the presence of H 2 O on the In 2 O 3 substrate play important roles in promoting methanol production. The hydroxyl adsorbed at the interface of Pd 4 /In 2 O 3 induces structural transformation of the supported Pd 4 cluster from a butterfly shape into a tetrahedron one. This structural change not only indicates the dynamical nature of the supported nanocatalysts during the reaction but also causes the final hydrogenation step to change from CH 3 O to H 2 COH.

Understanding the effect of metal particle size on the reactions during hydrodeoxygenation of phenolics is of great importance for rational design of a catalyst for selective control of a desirable reaction. To this end, vapor phase hydrodeoxygenation of m-cresol was studied over 5% Ni/SiO 2 catalysts with varying Ni particle sizes (2−22 nm) at 300 °C and 1 atm H 2 . The Ni particle sizes were confirmed by several characterization techniques, and the varying surface concentration of terrace, step, and corner sites with Ni particle sizes was verified by H 2 temperature-programmed desorption. Decreasing the Ni particle size from 22 to 2 nm improves the intrinsic reaction rate by 24 times and the turnover frequency (TOF) by 3 times. The TOFs for toluene and methylcyclohexanone/methylcyclohexanol formation increase by 6 and 4 times, respectively, while the TOF for CH 4 formation decreases by 3/4, indicating that smaller particles with more defect sites (step and corner) favor deoxygenation and hydrogenation while larger particles with more terrace sites favor C−C hydrogenolysis. Density functional theory study shows that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211), and defected Ni(211) decreases from 175.6 to 145.6 and then to 120.5 kJ/mol. The results indicate that a highly coordinatively unsaturated surface Ni site is responsible for C−O cleavage through facile adsorption and stabilization of −OH in the transition state, thus facilitating deoxygenation toward toluene. Our results indicate that tuning the metal particle size is an effective approach to control reactions during hydrodeoxygenation.

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The adsorption, dissociation and hydrogenation of phenol on the Pt(111) and Pd (111) surfaces have been studied using density functional theory slab calculations. The results show that phenol favors adsorption through a mixed σ-π interaction on both surfaces through its phenyl ring, with the hydrogen atoms and hydroxyl tilted away from the surface. The dissociation of phenol to phenoxy is both thermodynamically and kinetically favored on Pd but not on Pt. The phenoxy adsorbs on Pd through both the phenyl ring and the oxygen atom whereas the O atom points away from the surface on Pt. On Pt, the barrier for adding one hydrogen atom to the adsorbed phenol is 0.49 eV lower than the overall barrier for phenol dissociation to phenoxy followed by adding the hydrogen atom to its phenyl ring, resulting in direct hydrogenation of the adsorbed phenol to cyclohexanol as the dominant reaction pathway. In contrast, on Pd, the barrier for direct hydrogenation (1.22 eV) is higher than the overall barrier of dissociation followed by hydrogenation process (0.85 eV), resulting in hydrogenation of the adsorbed phenoxy to cyclohexanone as the major reaction pathway. Microkinetics analysis confirms that hydrogenation of the adsorbed phenol is the dominant pathway on Pt whereas phenoxy hydrogenation drives the turn-over on Pd. These results are consistent with the experimentally observed selectivity of phenol hydrogenation on Pd and Pt catalysts.