Water Adsorption on MO₂ (M = Ti, Ru, and Ir) Surfaces. Importance of Octahedral Distortion and Cooperative Effects

Abstract

Understanding metal oxide MO 2 (M = Ti, Ru, and Ir)–water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H 2 O–MO 2 interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew–Burke–Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO 2 and in its dissociative form on (110)-RuO 2 and (110)-IrO 2 . The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO 2 and tetragonal compression for RuO 2 and IrO 2 ) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO 2 , AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO 2 , water monolayer exhibits a 50% dissociation, the formation of H 3 O 2 – motifs being essential. Finally, on (110)-IrO 2 , the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO 2 , it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H 2 O–MO 2 intrinsic interaction and cooperative hydrogen bonding and adsorption effects.

Metal Coordination Determines the Catalytic Activity of IrO2 Nanoparticles for the Oxygen Evolution Reaction

“…The six centers differ on the coordination number and environment. Taking into account that iridium in rutile shows two short Ir-O axial distances and four long equatorial ones, [60] the six centers can be divided in five different types: i) pentacoordinated atoms with an axial vacant site (A5ax and B5ax); ii) pentacoordinated sites with an equatorial vacancy (C5eq); iii) tetracoordinated centers with one axial and one equatorial vacant site (D4ax/eq); iv) tetracoordinated sites with two equatorial vacant sites (E4eq/eq); and v) iridium centers presenting only three bonds with lattice oxygens (F3ax/eq/eq). While other sites exist in larger Wulff-like nanoparticle models, they can all be classified in one of these five types of centers.…”

“…Wulff-like stoichiometric nanoparticle models of 1.2 nm size were constructed with the BCN-M computational tool (Figure 1). [59] The surface energies provided to construct the model are 0.150, 0.158, 0.185 and 0.206 eV A -2 for the (110), (011), (100) and (001) crystallographic surfaces, [60] respectively. The final model size is close to some of the experimentally reported active nanoparticles, [21,23,26,29,30] and it is computationally affordable.…”

pH Sensitivity Estimation in Potentiometric Metal Oxide pH Sensors Using the Principle of Invariance

“…Since the fabrication temperature governs the number of surface hydroxyl sites present on a metal oxide electrode [20], it was imperative to vary the hydroxyl site density values, N s , for metal oxides wherever applicable. RuO 2 , low band-gap metal oxide known for excellent pH sensitivity [58], has a tendency to have higher N s values since water dissociation on RuO 2 rutile surfaces tends to form greater amounts of hydroxyl groups than other rutile surfaces such as TiO 2 , for instance [59].…”

“…To estimate the site density of a hydrated form of RuO 2 , the contribution to the site density from water dissociation reactions was added to the site density of unhydrated metal oxide. To do this, the amount of water molecules that typically adsorb on rutile surfaces (0.05 water/ A°) [60] and the percentage of water dissociation that occurs on the surface of rutile RuO 2 (50%) [59] were extracted from the literature. e product of these values is multiplied by 2 to represent the fact that two hydroxyl groups are formed for every water molecule that gets dissociated.…”

“…RuO 2 displays a similar behaviour to its counterparts in the low-temperature-annealed phase in that there is a greater decrease in sensitivity with increasing capacitance value choice for a lower electrolyte ionic strength and a Nernstian pH sensitivity invariance point of 59.1094 mV/pH for a low C 1 capacitance. is is expected due to its high site densities [42,66] even in its high-temperature-annealed rutile phase as compared to other metal oxides such as TiO 2 [59]. Its exceptional pH sensitivity, as compared to other metal oxides, has been cited by many experimenters [10,11].…”