This journal is © the Owner Societies 2017 Phys. Chem. Chem. Phys., 2017, 19, 4541--4552 | 4541 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 4541 Adatom surface diffusion of catalytic metals on the anatase TiO 2 (101) surface† Afnan Alghannam, a Christopher L. Muhich‡* a and Charles B. Musgrave ab Titanium oxide is often decorated with metal nano-particles and either serves as a catalyst support or enables photocatalytic activity. The activity of these systems degrades over time due to catalytic particle agglomeration and growth by Ostwald ripening where adatoms dissociate from metal particles, diffuse across the surface and add to other metal particles. In this work, we use density functional theory calculations to study the diffusion mechanisms of select group VIII and 1B late-transition metal adatoms commonly used in catalysis and photocatalysis (Au, Ag, Cu, Pt, Rh, Ni, Co and Fe) on the anatase TiO 2 (101) surface. All metal adatoms preferentially occupy the bridge site between two 2-fold- coordinated oxygen anions (O 2c ). Surface migration was investigated by calculating the minimum energy pathway from one bridge site to another along three pathways: two in the [010] direction along a row of surface O 2c anions and one in the [10 % 1] direction between two rows of surface O 2c anions. For all adatoms, migration along the [010] direction is favored over migration along the [10 % 1] direction due to closer packing of the atoms in the [010] direction and therefore stronger adatom–surface interactions. As the adatom hops along the [010] direction, it preferentially moves through a metastable OTiO structure in which the adatom partially embeds itself within the surface, with the exception of Au, which remains above the surface. The adatoms migrate with relative activation energies of: Au (0.24 eV) o Ag (0.48 eV) o Rh (0.60 eV) o Co (0.78 eV) o Pt (0.84 eV) o Ni (0.86 eV) o Cu (1.23 eV) o Fe (1.79 eV) along the favored pathway. This preference arises from the strength of adatom–surface bonding and the electronegativity difference between the metal adatom and the TiO 2 surface. We found a linear correlation between the binding energy/electronegativity and the activation energy for hopping where stronger binding energies and more oxidized adatoms have higher activation energies for adatom migration. The linear correlation developed in this work enables rapid estimations of the hopping rates of other transition metal adatoms across the TiO 2 surface. Introduction Titanium dioxide (TiO 2 ) is one of the most widely studied materials as an active photocatalyst and as a catalytic support because it is chemically stable in aqueous and steam environments, photocatalytically active, relatively inexpensive, and composed of earth abundant elements. In its role as a photocatalyst, it is often employed in reactions such as water splitting, 1–5 pollution degradation, 5–12 and water purification. 5,13–15 The performance of TiO 2 for such applications, however, depends on the TiO 2 phase, which includes rutile, anatase, or brookite. While TiO 2 in the rutile form is the most thermodynamically stable bulk phase, 7 anatase is the predominate phase used for photocatalysis applications due to its higher performance. 16,17 Although TiO 2 is extensively used as a catalyst support or as a photocatalyst, it is not itself an efficient catalyst. Several studies have shown that the deposition of transition metal nanoparticles, including Au, Ag, Cu, Pt, Pd, Ru, and Co, onto TiO 2 substrates significantly enhances the catalytic activity, or photocatalytic reaction rates. 18–52 Pt supported on TiO 2 (Pt–TiO 2 ) has gained the most attention. In addition to TiO 2 ’s use as an inactive support for Pt catalysts, Pt is used to accelerate the photocatalytic decomposition of many organic pollutants and to enhance the rate of photocatalytic water splitting. 18–25 Other TiO 2 supported transition metal catalysts are also of interest. For instance, Au–TiO 2 has been shown to catalyze CO oxidation, 26 Ag–TiO 2 catalyzes both hydrogenation 27 and photodecomposition of organic materials, 28 Cu–TiO 2 catalyzes H 2 and CH 4 photo- generation from mixtures of methanol/ethanol and water, 29,30 Pd–TiO 2 enhances acetylene hydration, 31 Ru–TiO 2 catalyzes CO 2 a Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA. E-mail: christopher.muhich@colorado.edu b Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA † Electronic supplementary information (ESI) available: Key geometries of adatom diffusion. See DOI: 10.1039/c6cp08789b ‡ Present address: Department of Mechanical and Process Engineering, ETH, 8092 Zurich, Switzerland. Received 23rd December 2016, Accepted 14th January 2017 DOI: 10.1039/c6cp08789b rsc.li/pccp PCCP PAPER