We use density functional theory (DFT) and ab-inito theromodynamics to investigate the dissociative adsorption of H2 on In2O3(111) and In2O3(110). Aditionally, O 1s core level shifts are calculated under hydrogen coverage conditions for both surfaces. At typical methanol-synthesis conditions (200-300 °C, 2-5 MPa), we find that the thermodynamically stable phase for (111) and (110) orientations correspond to moderate (33-50%) and heavy (83%) surface hydroxylation, respectively. The calculated O 1s shifts show that HOads, adsorbed on an In-ion, and HOsurf, formed on the oxygen lattice possition give rise to two distinctly different features. The relative core-level shifts for the two kinds of hydroxyl groups are at low coverage calculated to be 1.25 and 2.12 eV for In2O3(111) and 1.31 and 2.47 eV for In2O3(110), for HOads and HOsurf. We find that the presence of surface oxygen vacancies is not visible on the O 1s signature of other surface atoms. We find that the barrier for H2 dissociative adsorption on the pristine surface is 0.48 and 0.31 eV for (111) and (110), respectively. A heterolytic dissociation mechanims is proposed for both surfaces. Moreover, oxygen vacancies on (111) only facilitate slightly the dissociation process. Hydrogen difussion from the heterolytic configuration to a homolytic adsorption mode is associated with barriers of 0.78 and 1.02 eV for (111) and (110), respectively. This step is found to be strongly exothermic. Finally, we find that there is no thermodynamic driving force for oxygen vacancy formation at low hydrogen coverages. Oxygen vacancies can instead be formed at high hydrogen coverages. The work suggests surface hydroxylation to be present during typical methanol-synthesis conditions. Moreover, hydroxylation of the surface results in change of oxidation state of some In-ions. Hydrogen dissociation appears to be more facile than hydrogen difussion. Finally, oxygen vacancy creation appears to be an activated process relevant at high hydrogen coverages.

Hydrogen adsorption on In2O3(111) and In2O3(110)

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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