Unraveling the Diffusion of Bulk Ti Interstitials in Rutile TiO₂(110) by Monitoring Their Reaction with O Adatoms

Abstract

in rutile TiO 2 (110) from the bulk to the surface has been studied utilizing two experimental techniques. Electron-stimulated desorption of O + ions was employed to kinetically monitor the reaction between oxygen adatoms with Ti i 3+ species at temperatures between 360 and 400 K. Scanning tunneling microscopy was also used to measure the Ti i 3+ diffusion rate. Both methods yield a rate constant k Ti i 3+ ) 5 × 10 -4 s -1 at 393 K. The activation energy as measured by the rate dependence on temperature is ∼1.0 eV. Titania (TiO 2 ) has wide applications in the fields of heterogeneous catalysis, photocatalysis, photovoltaic cells, and gas sensors. 1-4 To better understand the relationship between atomic and electronic structure and reactivity, the rutile TiO 2 (110)-(1 × 1) surface ( Figure 1a) has been extensively studied as a model oxide surface. 1-3,5,6 Surface as well as bulk defects have been postulated as active sites for the chemical and photochemical reactivity of the TiO 2 surface. 1,2,7,8 Since reduced TiO 2 crystals contain interstitial Ti i 3+ ions, 8-15 the diffusion of these ions to the TiO 2 surface may well be involved in a variety of surface processes. Noteworthy is the SMSI (strong metal-support interaction) effect, where TiO x layers, produced by Ti i 3+ diffusion to the surface, cover the surface of metal particles deposited on the TiO 2 surface, strongly affecting the rates of surface reaction on the particles. [16][17][18][19] Therefore, accurate measurements of the rate and activation energy for Ti i 3+ bulk diffusion are crucial for an improved understanding of surface processes driven by Ti i 3+ interstitial ions in the TiO 2 bulk. Because of a lack of sensitivity to buried species the study of interstitial species in the bulk and near surface region is a challenge to conventional surface science methods. Recently, however, it has been found from electron-stimulated desorption (ESD) studies that oxygen adatoms (O t ) on the TiO 2 (110)-(1 × 1) surface (Figure 1a), produced by dissociation of O 2 and chemisorption on the 5-fold-coordinated Ti (Ti 5c ) sites, 6,20-22 have a very high ionic cross section for O + production. 23 The high sensitivity of ESD to the O t adatoms therefore provides an excellent tool to measure the kinetics of the reaction between O t adatoms and out-diffusing Ti i 3+ interstitial species. In this letter we study the kinetics of interstitial Ti i 3+ species diffusion through rutile TiO 2 (110) crystals using both ESD and high-resolution scanning tunneling microscopy (STM). Monitoring the reaction between chemisorbed oxygen atoms with Ti i 3+ species at temperatures between 360 and 400 K by means of ESD allows us to deduce the rate and activation energy for the diffusion process. Excellent agreement is found for the Ti i 3+ diffusion rate constant at 393 K, which is ∼5 × 10 -4 s -1 as extracted independently from ESD and STM studies. In addition, an energy barrier of ∼1 eV was estimated for the diffusion of Ti i 3+ interstitials toward the surface.The ESD and STM expe...

An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

“…The O vac density estimated from Figure 4a is 5.4% monolayer (ML), where 1 ML is defined as the density of unit cells of the TiO 2 (110)-(1×1) surface, i.e., 5.2 × 10 14 cm −2 . This O vac concentration is comparable to those found in former studies for reduced crystals [41][42][43][44][45][46][47]. Pt + 1 has been deposited with a kinetic energy of 7 eV by means of the mass-selected cluster source on the clean TiO 2 (110)-(1×1) surface kept at RT.…”

Effect of the TiO2 Reduction State on the Catalytic CO Oxidation on Deposited Size-Selected Pt Clusters

“…Exposure of this surface to 50 L of O 2 at 100 K and subsequent annealing it to 300 K heals the surface O vac . 42 (For an atomic scale image of the resulting surface, see Figure S3 of the SI.) Figure 4 shows this surface after Pt 7 deposition at 300 K and reveals that it is still bare of O vac .…”

Overcoming the Strong Metal−Support Interaction State: CO Oxidation on TiO2(110)-Supported Pt Nanoclusters

“…The O vac surface density in the cluster-free surface areas is estimated to 12.5% ML (with respect to the density of the TiO 2 (110)-(1 Â 1) unit cells of 5.2 Â 10 14 cm -2 ), which means that the surface of the TiO 2 crystal is strongly reduced. [53][54][55][56] The STM contrast on the TiO 2 (110) terraces is related to the empty states and well-known to be dominated by electronic effects. 53 Figure 2a displays the CO 2 production on the surface shown in Figure 1 as a function of sample temperature when CO and O 2 reactants are pulsed alternatingly on the sample.…”

“…3,4 For instance, the surface of bulk Au is well-known to be catalytically inactive, whereas dispersed Au nanoparticles on oxides exhibit a high catalytic activity for various chemical reactions, for example, for low-temperature CO oxidation. [5][6][7][8][9][10][11][12][13] Electron charging of Au clusters bound to MgO(001) surface defects strongly influences CO adsorption 14 and CO oxidation. [15][16][17][18] In contrast to Au, bulk Pt is known to be an excellent catalyst for a number of reactions, including CO oxidation, 19,20 which justifies its wide use in current industrial catalysts.…”