Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation.

Baohuan Wei,Frederik Tielens,Monica Calatayud

doi:10.3390/nano9091199

Abstract

Titanium oxide (TiO2) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H2 with TiO2 surface plays an important role. However, the activation of hydrogen over rutile TiO2 surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H2 as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO2 orientations.

Highlights

Titanium oxide (TiO2) has been widely used in numerous fields, from everyday applications to technological devices, such as dye-sensitized solar cells (DSSCs) [1,2], photoelectrochemical cells [3], photocatalysts [4], catalysis [5,6], sensors [7,8], biomedical treatments [9], lithium ion batteries [10], or photovoltaics [11,12]
Despite the high interest generated by hydrogen-titania interfaces, the nature of the species involved is still poorly understood—protons are generally reported as being stable in hydrogenated rutile (110) [28], atomic surface hydrogen has been found to prevent electron-hole recombination on an Au-TiO2 photocatalyst [14], and very recently hydride species have been characterized as being stable on its surface [29,30]
García-Melchor et al and Fernandez-Torre et al reported that H2 dissociation on CeO2 (111) follows a heterolytic path, with H being transferred from Ce to a neighboring O, generating the homolytic product [34,40]

Summary

Introduction

Titanium oxide (TiO2) has been widely used in numerous fields, from everyday applications (paint, inks, toothpaste, makeup) to technological devices, such as dye-sensitized solar cells (DSSCs) [1,2], photoelectrochemical cells [3], photocatalysts [4], catalysis [5,6], sensors [7,8], biomedical treatments [9], lithium ion batteries [10], or photovoltaics [11,12]. Despite the high interest generated by hydrogen-titania interfaces, the nature of the species involved is still poorly understood—protons are generally reported as being stable in hydrogenated rutile (110) [28], atomic surface hydrogen has been found to prevent electron-hole recombination on an Au-TiO2 photocatalyst [14], and very recently hydride species have been characterized as being stable on its surface [29,30]. Two main mechanisms are proposed: homolytic and heterolytic dissociation [39]. Liu et al reported the surface characteristics of anatase TiO2 after reduction with H2 In their study, they proposed that H2 can dissociate on oxygen vacancies—one H atom binds with a Ti to form the Ti-H bond, whereas the other one bonds with O to form Ti–OH [41]. Vibrational frequencies for TiH and OH are reported as a guide to identify relevant species on the different terminations

Materials and Methods

Results and Discussion

Electronic Structure

Effect of the Hubbard Correction U