H₂ Adsorption on Wurtzite ZnO and on ZnO/M(111) (M=Cu, Ag and Au) Bilayer Films

Abstract

The dissociative adsorption of H2 on the wurtzite ZnO (10true1‾ 0) surface and on ZnO/M(111) (M=Cu, Ag and Au) bilayer films has been investigated by mean of DFT+U calculations. On ZnO wurtzite and free‐standing ZnO bilayer, H2 prefers to dissociate heterolytically, forming a proton adsorbed on an O atom and an hydride atom sitting on a Zn cation. While this adsorption mechanism is exothermic on the wurtzite ZnO (10true1‾ 0) surface, an endothermic process is observed on a free‐standing, unsupported ZnO bilayer. The trend is completely reversed when H2 is adsorbed and dissociated on a metal supported ZnO bilayer. Here the homolytic dissociation mechanism is exothermic and preferred over the heterolytic dissociation. This is due to the effect of the metal support which acts as an electron reservoir to accept extra electrons accompanied to homolytic dissociation of H2 molecule (two electrons are transferred to the metal support with formation of two H+ ions). The study of the reaction pathway for dissociative adsorption of H2 on ZnO/Cu(111) shows that the reaction occurs via (a) heterolytic splitting and (b) the displacement of an H atom from a Zn site to the neighboring O site, forming the final product of homolytic dissociation.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces

“…The chemical potential of oxygen (Δμ O ) was fixed at −1.01 eV, which corresponds to 5 × 10 −7 mbar O 2 at 550 K, and the chemical potential of hydrogen (Δμ H ) was fixed at −0.91 eV, corresponding to 1 × 10 −10 mbar H 2 ; these conditions closely mimic the experimental gas-phase conditions. The results demonstrate that hydrogen can adsorb on the ZnO layers, 46 and the optimal hydrogen coverage on the topmost ZnO layer is 0.33 ML or one hydrogen per three surface oxygen atoms (Figure 8d). This structure may contribute to the experimentally observed v(O−H) frequency under these conditions.…”

A DFT+U Study of CO and H2 Adsorption Properties on Ga, Li, and Cu Doped ZnO Surfaces

“…To further evaluate the effect of doped Ga, Li, and Cu on the electronic and adsorption properties of ZnO$${\left(10\bar{1}0\right)}$$ , the dissociative adsorption of H 2 has been considered via the homolytic mechanism, forming two OH groups, and the heterolytic mechanism, forming OH and MH groups [33] . This is due to the fact that the dissociation of H 2 is the first step and plays an important role during the syngas conversion [28,49] .…”

“…Adsorption of CO and H 2 has been mostly considered on pristine ZnO surface. While CO is molecularly adsorbed on Zn 2+ sites, [32] H 2 is preferentially heterolytically dissociated forming Zn−H and O−H groups on the pristine ZnO$${\left(10\bar{1}0\right)}$$ surface [33] …”

“…[30] Adsorption of CO and H 2 has been mostly considered on pristine ZnO surface. While CO is molecularly adsorbed on Zn 2 + sites, [32] H 2 is preferentially heterolytically dissociated forming ZnÀ H and OÀ H groups on the pristine ZnO 10 � 10 ð Þ surface. [33] However, the doping of different types of heteroatoms in the range of n-type, p-type, and transition metals (equivalent valence metals) has not yet been investigated comparatively to figure out the comprehensive aspects of structural and electronic properties of ZnO materials for particular applications, such as CO and H 2 adsorption and further steps to convert CO and H 2 to useful chemicals.…”

Tunable properties and composition of ZnO films supported on metal surfaces

“…Dispersive interactions are expected to be important for the accurate description of lms on metal surfaces. 43,69 Here, to improve the description of dispersive interactions, we employed the pairwise additive zero-damping D3 van der Waals corrections with the default parameters available in VASP. 70 The dispersive corrections decreased the corrugation of metal-supported ZnO lms by shortening metal-oxide distances in the regions of weak binding and had a moderate effect on the reactivity of the lms (Table S1 †).…”