Molecular Understanding of Reactivity and Selectivity for Methanol Oxidation at the Au/TiO₂ Interface

Abstract

Goldkatalyse: Experimentell und theoretisch wurde bestätigt, dass die Grenzflächenatome eines Au/TiO2‐Katalysators sowohl hoch reaktiv als auch hoch selektiv in der Oxidation von Methanol zu Formaldehyd mit O2 bei niedrigen Temperaturen sind. Der mikroskopische Mechanismus dieser komplexen Reaktion wurde im Detail auf molekularer Ebene aufgeklärt (siehe Darstellung eines Goldclusters auf einer TiO2‐Oberfläche).

Selective Aerobic Oxidation of 5‐(Hydroxymethyl)furfural over Heterogeneous Silver‐Gold Nanoparticle Catalysts

“…The rate‐limiting alcohol oxidation, on the other hand, has a higher energy barrier and only occurs on Au catalysts, as Ag is inactive in FDCA synthesis [19a] . Besides activation barriers, oxygen activation is a crucial factor, which is limited on Au [61] or limited to active Au sites possibly forming a perimeter with the support similar to literature on selective alcohol oxidation with Au/TiO 2 [62] . Since hydroxide ions form a geminal diol in solution by nucleophilic attack on the aldehyde moiety of HMF, which is then dehydrogenated, oxygen is not incorporated in the oxidized molecule and thus has an indirect role in the mechanism by removal of electrons and adsorbed H‐species [60,63] …”

“…[19a] Besides activation barriers, oxygen activation is a crucial factor, which is limited on Au [61] or limited to active Au sites possibly forming a perimeter with the support similar to literature on selective alcohol oxidation with Au/TiO 2 . [62] Since hydroxide ions form a geminal diol in solution by nucleophilic attack on the aldehyde moiety of HMF, which is then dehydrogenated, oxygen is not incorporated in the oxidized molecule and thus has an indirect role in the mechanism by removal of electrons and adsorbed Hspecies. [60,63] During oxygen activation, hydrogen peroxide is formed, whose decomposition has a high energy barrier on Au but proceeds readily on Ag or AgÀ Au alloys.…”

Atomic-Scale Explanation of O2Activation at the Au–TiO2Interface

“…76−78 The spin-polarized Kohn−Sham equations were solved in the plane-wave/ pseudopotential framework using Vanderbilt's ultrasoft pseudopotentials 79 with a cutoff of 25 Ry, where k-point sampling was restricted to the Γ-point. In line with our previous work, 22,71,72,80,81 we again made use of our GGA+U implementation 82 based on a self-consistent U calculation, 83 yielding without any adjustment a value of U = 4.2 eV for the present case. 80 For the static electronic structure calculations, the energy threshold for the convergence of the Kohn−Sham orbitals was set 5 × 10 −7 Ry.…”

“…In short, the supported nanocatalyst model consists of a Au 11 cluster on top of an oxygen surface vacancy of the TiO 2 support, which effectively pins the metal nanoparticle in space. 22,71,72 The supporting titania is modeled by four O−Ti 2 O 2 −O trilayers forming a large (6 × 2) supercell slab with the orthorhombic cell parameters a = 13.15 Å and b = 17.80 Å. The two trilayers at the bottom of the slab are constrained at their equilibrium positions.…”

“…To capture this key phenomenon, we carried out finite-temperature ab initio simulations using a validated model of the nanocatalyst consisting of a gold cluster consisting of 11 atoms, Au 11 , pinned to an oxygen vacancy of the rutile TiO 2 (110) surface (see Computational Details). 22,72 An oxygen molecule was adsorbed either at a perimeter site of the Au/TiO 2 nanocatalyst or at a free surface position (Figure 3). This reduced surface hosts thermalized excess electrons located at Ti 3+ sites, which is the same situation as encountered in the EPR experiments after UV irradiation and subsequent ultrafast relaxation of the photogenerated electrons to the electronic ground state.…”