Initial Reduction of the PdO(101) Surface: Role of Oxygen Vacancy Formation Kinetics

Abstract

We investigated the initial thermal reduction of the PdO(101) surface using density functional theory (DFT) calculations. The defect-free PdO(101) surface consists of parallel rows of undercoordinated Pd and O atoms (Pdcus and Ocus, respectively). DFT was used to map out the elementary processes of Ocus vacancy formation, oxygen surface atom diffusion along the Pdcus rows, and various O2 formation pathways. Because oxygen vacancies form during the reduction of the surface, the elementary processes were examined with and without the presence of adjacent Ocus vacancies. DFT calculations show that the presence of oxygen vacancies strongly affects the barriers of adjacent surface processes. Barriers for O vacancy formation are reduced by nearly 50% with an adjacent O vacancy. The barrier for O2 formation along the Pdcus row drops from 1.54 eV on the defect-free PdO(101) surface to 0.81 eV in the presence of an Ocus vacancy dimer. However, the presence of Ocus vacancies also leads to more strongly bound O2 on the Pdcus row. The strong neighbor effect of O vacancies on the PdO(101) surface will favor the growth of O vacancy chains and increase the rate of thermal reduction of the surface, both features that have been observed in recent ultrahigh vacuum isothermal reduction experiments of the PdO(101) surface.

Enhancement of Desulfurization Capacity with Cu-Based Macro-Porous Sorbents for Hydrogen Production by Gasification of Petroleum Cokes

“…The adjacent oxygen vacancy affects the electronic structures of the adjacent Cu cus site significantly. It would trigger stronger back donation from Cu cus to the adsorbed CO* molecule, thereby providing the high stability of adsorbed CO* [37]. Such effects of oxygen vacancies have been reported in other reactions on CuO surfaces and other transition metal oxides [37][38][39].…”

“…It would trigger stronger back donation from Cu cus to the adsorbed CO* molecule, thereby providing the high stability of adsorbed CO* [37]. Such effects of oxygen vacancies have been reported in other reactions on CuO surfaces and other transition metal oxides [37][38][39]. In terms of the sulfurized surface modeled by replacing surface O atom with S atom, the relaxed adsorbate images are provided in Figure 11c,g,k. A black dashed circle indicates the S surface insulted sites.…”

CO2 Hydrogenation on NixMg1−xAl2O4: A Comparative Study of MgAl2O4 and NiAl2O4

“…(100) was predicted to be exothermic (207.4 kJ/mol), but that on reduced NiAl 2 O 4 (100) was predicted to be endothermic (693.6 kJ/mol). Kinetic enhancement by adjacent oxygen vacancies has been reported for transition metal oxides [39,40,43]. In particular, on PdO(101), oxygen vacancies were found to impact CO oxidation and thermal reduction kinetics significantly.…”

“…Overall, our computational results confirm that the experimentally observed higher selectivity toward CO for CO 2 methanation on MgAl 2 O 4 stems from the low stability of adsorbed CO, which leads to facile CO desorption during the methanation reaction During catalytic reactions, surfaces are not pristine because of on-going oxidation and reduction reactions. Thus, catalytic surfaces have edges, kinks, and some reduced sites, and these features have been proposed to be highly active because they are coordinatively more saturated [39][40][41][42]. To consider these effects during CO 2 methanation, we assume that the effects of oxygen vacancies (Ov) can be representative of the effects of reduced sites, which would provide initial insights for the effects of reduced sites.…”

“…To consider these effects during CO 2 methanation, we assume that the effects of oxygen vacancies (Ov) can be representative of the effects of reduced sites, which would provide initial insights for the effects of reduced sites. We first checked the Ov formation energy to evaluate the thermodynamic feasibility of Ov formation on the (100) surfaces [39]. The predicted Ov formation energies were 589 kJ/mol for MgAl 2 O 4 (100) and 414 kJ/mol for NiAl 2 O 4 (100).…”