The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the "melt-and-quench" technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the "leaky" TiO2 recently discovered for the photochemical water-splitting applications.

Electrocatalytic reduction of CO 2 to energy-rich hydrocarbons such as alkanes, alkenes, and alcohols is a very challenging task. So far, only copper has proven to be capable of such a conversion. We report density functional theory (DFT) calculations combined with the Poisson−Boltzmann implicit solvation model to show that single-atom alloys (SAAs) are promising electrocatalysts for CO 2 reduction to C 1 hydrocarbons in aqueous solution. The majority component of the SAAs studied is either gold or silver, in combination with isolated single atoms, M (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir), replacing surface atoms. We envision that the SAA behaves as a one-pot tandem catalyst: first gold (or silver) reduces CO 2 to CO, and the newly formed CO is then captured by M and is further reduced to C 1 hydrocarbons such as methane or methanol. We studied 28 SAAs, and found about half of them selectively favor the CO 2 reduction reaction over the competing hydrogen evolution reaction. Most of those promising SAAs contain M = Co, Rh, or Ir. The reaction mechanism of two SAAs, Rh@Au(100) and Rh@Ag(100), is explored in detail. Both of them reduce CO 2 to methane but via different pathways. For Rh@Au(100), reduction occurs through the pathway *CO → *CHO → *CHOH → *CH + H 2 O (l) → *CH 2 + H 2 O (l) → *CH 3 + H 2 O (l) → * + H 2 O (l) + CH 4(g) ; whereas, for Rh@Ag(100), the pathway is *CO → *CHO → *CH 2 O→ *OCH 3 → *O + CH 4(g) → *OH + CH 4(g) → * + H 2 O (l) + CH 4(g). The minimum applied voltages to drive the two electrocatalytic systems are −1.01 and −1.12 V RHE for Rh@Au(100) and Rh@Ag(100), respectively, at which the Faradaic efficiencies for CO 2 reduction to CO are 60% for gold and 90% for silver. This suggests that SAA can efficiently reduce CO 2 to methane with as small as 40% loss to the hydrogen evolution reaction for Rh@Au(100) and as small as 10% for Rh@Ag(100). We hope these computational results can stimulate experimental efforts to explore the use of SAA to catalyze CO 2 electrochemical reduction to hydrocarbons.

We propose a mechanism of water splitting on cobalt oxide surface with atomistic thermodynamic and kinetic details. The density-functional theory studies suggest that the oxidation process could proceed with several nonelectrochemical (spontaneous) intermediate steps, following the initial electrochemical hydroxyl-to-oxo conversion. More specifically, the single oxo sites CoIVO can hop (via surface proton/electron hopping) to form oxo pair CoIV(O)-O-CoIVO, which will undergo nucleophilic attack by a water molecule and form the hydroperoxide CoIII–OOH. Encounter with another oxo would generate a superoxo CoIII–OO, followed by the O2 release. Finally the addition and deprotonation of a fresh water molecule will restart the catalytic cycle by forming the hydroxyl CoIII–OH at this active site. Our theoretical investigations indicate that all nonelectrochemical reactions are kinetically fast and thermodynamically downhill. This hypothesis is supported by recent in situ spectroscopic observations of surface superoxo that is stabilized by hydrogen bonding to adjacent hydroxyl group as an intermediate on fast-kinetics Co catalytic site.