Advanced Photocatalysts: Pinning Single Atom Co‐Catalysts on Titania Nanotubes

Abstract

Single atom (SA) catalysis, over the last 10 years, has become a forefront in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. Most crucial when engineering a SA catalyst/support system is the creation of defined anchoring points on the support surface to stabilize reactive SA sites. Here, a so far unexplored but evidently very effective approach to trap and stabilize SAs on a broadly used photocatalyst platform is introduced. In self-organized anodic TiO 2 nanotubes, a high degree of stress is incorporated in the amorphous oxide during nanotube growth. During crystallization (by thermal annealing), this leads to a high density of Ti 3+ -O v surface defects that are hardly present in other common titania nanostructures (as nanoparticles). These defects are highly effective for SA iridium trapping. Thus a SA-Ir photocatalyst with a higher photocatalytic activity than for any classic co-catalyst arrangement on the semiconductive substrate is obtained. Hence, a tool for SA trapping on titania-based back-contacted platforms is provided for wide application in electrochemistry and photoelectrochemistry. Moreover, it is shown that stably trapped SAs provide virtually all photocatalytic reactivity, with turnover frequencies in the order of 4 × 10 6 h −1 in spite of representing only a small fraction of the initially loaded SAs.

A Few Pt Single Atoms Are Responsible for the Overall Co‐Catalytic Activity in Pt/TiO2 Photocatalytic H2 Generation

“…Note that the above definition [35,37] deviates from methods used in photocatalysis. [46] The Pt SA density was directly determined from the HAADF-TEM images, while also considering the reactive area to be represented by the illumination area (%1 cm 2 ).…”

“…[29,35,36] Desired defects may either be intrinsically present in some substrates as a result of the synthesis procedure or can be introduced by defect engineering. [33,37,38] More recently, we reported that SAs can be attached to TiO 2 surfaces by trapping at suitable substrate defects. [29] Intrinsic defects of Ti 3þ -O v on TiO 2 nanotubes favor the stabilization of the isolated Pt atoms.…”

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

“…For example, Zhou et al showed that anodic growth of TiO 2 nanotubes poses a considerable strain on tube walls ensuing the formation of the high density of Ti 3+ -O v surface defects. 404 Taking advantage of defect-rich surface they pinned Ir SA sites by a simple impregnation approach. The resulting 0.58% IrSA TiO 2 catalyst displayed a turnover frequency of 4 × 10 6 h −1 .…”

Pt Single Atoms on TiO2 Polymorphs—Minimum Loading with a Maximized Photocatalytic Efficiency

“…[ 32,33 ] In contrast to these approaches, we recently reported that SA trapping can be achieved on defect‐engineered TiO 2 layers [ 34–36 ] or defects of TiO 2 nanotubes grown by anodization on Ti metal. [ 37–39 ]…”