Insight into the transient inactivation effect on Au/TiO2 catalyst by in-situ DRIFT and UV–vis spectroscopy

Stable and Highly Active Single Atom Configurations for Photocatalytic H2 Generation

The employment of single atoms (SAs), especially Pt SAs, as co-catalysts in photocatalytic H2 generation has gained significant attention due to their exceptional efficiency. However, a major challenge in their application is the light-induced agglomeration of these SAs into less active nanosized particles under photocatalytic conditions. This study addresses the stability and reactivity of Pt SAs on TiO2 surfaces by investigating various post-deposition annealing treatments in air, Ar, and Ar-H2 environments at different temperatures. It is described that annealing in an Ar-H2 atmosphere optimally stabilizes SA configurations, forming stable 2D rafts of assembled SAs ≈0.5-1 nm in diameter. These rafts not only resist light-induced agglomeration but also exhibit significantly enhanced H2 production efficiency. The findings reveal a promising approach to maintaining the high reactivity of Pt SAs while overcoming the critical challenge of their stability under photocatalytic conditions.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Very Low Temperature CO Oxidation over Atomically Precise Au25 Nanoclusters on MnO2

Atomically precise Au25 nanoclusters have garnered significant interest in the field of heterogeneous catalysis due to their remarkable activity and selectivity. However, for the extensively studied reaction of low-temperature CO oxidation, their performance has not been competitive compared to other known gold nanocatalysts. To address this, we deposited Au25(SR)18 (R = CH2CH2Ph) nanoclusters onto a manganese oxide support (Au25/MnO2), resulting in a very stable and highly active catalyst. By optimizing the pretreatment temperature, we were able to significantly enhance the performance of the Au25/MnO2 catalyst, which outperformed most other gold catalysts. Impressively, 100% conversion of CO was achieved at temperatures as low as -50 °C, with 50% conversion being reached below -70 °C. Furthermore, the existence of ligands could also influence the negative apparent activation energy observed at intermediate temperatures. Analysis using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD) techniques indicated that the Au25 nanoclusters remained stable on the catalyst surface even after pretreatment at high temperatures. In-situ modulation excitation spectroscopy (MES) spectra also confirmed that the Au cluster was the active site for CO oxidation, highlighting the potential of atomically precise Au25 nanoclusters as primary active sites at very low temperatures.

Revealing the Selective Bifunctional Electrocatalytic Sites via In Situ Irradiated X-Ray Photoelectron Spectroscopy for Lithium-Sulfur Battery

The electrocatalysts are widely applied in lithium-sulfur (Li-S) batteries to selectively accelerate the redox kinetics behavior of Li2 S, in which bifunctional active sites are established, thereby improving the electrochemical performance of the battery. Considering that the Li-S battery is a complex closed "black box" system, the internal redox reaction routes and active sites cannot be directly observed and monitored especially due to the distribution of potential active-site structures and their dynamic reconstruction. Empirical evidence demonstrates that traditional electrochemical test methods and theoretical calculations only probe the net result of multi-factors on an average and whole scale. Herein, based on the amorphous TiO2- x @Ni selective bifunctional model catalyst, these limitations are overcome by developing a system that couples the light field and in situ irradiated X-ray photoelectron spectroscopy to synergistically convert the "black box" battery into a "see-through" battery for direct observation of the charge transportation, thus revealing that amorphous TiO2- x and Ni nanoparticle as the oxidation and reduction sites selectively promote the decomposition and nucleation of Li2 S, respectively. This work provides a universal method to achieve a deeper mechanistic understanding of bidirectional sulfur electrochemistry.