Catalytic

Understanding how nanomaterials catalyze chemical reactions is important for developing highly efficient catalytic materials for use in a variety of energy and environmental technologies. These nanoscale materials typically expose different surface sites and have different reactivity to the turnover of reactants. Therefore, the identification and characterization of surface-active sites or site-specific reactivity has attracted much attention in recent years. However, nanoscale catalysts often produce a dynamic response to the reaction environment or stimulus, because changes in the reaction environment affect the free energy of exposed surface sites. Therefore, the surface structure, composition and reactivity are dependent on the reaction conditions, so it is extremely important to identify the active site and its properties in situ during catalysis. Although there are many in situ and manipulation techniques available, obtaining the structure and composition of active sites in nanomaterials under realistic reaction environments and atomic-level stimuli remains a challenge.

Heterogeneous catalysts are widely used in energy, environment and chemical industries. The study of active sites is one of the focuses of related research. Based on a precise molecular understanding of active sites, the selectivity and activity of these complex catalyst systems can be enhanced through a robust structural-reactive approach. In these changes in the catalyst, the migration, agglomeration, and reconstitution of the active site also change accordingly, which creates obstacles to the characterization of the active site.

In this study, the authors achieved direct TEM observation of photocatalytic reactions on unstableCu2Ocatalysts for the first time using homemade in-situ TEM scaffolds and liquid cells. The in situ structural changes of Cu2Ocatalysts during light-induced hydrogen production were captured. It was proved that the surface layer of theCu2Ocube is self-reduced to nanoCu. Further quantitative analysis showed that the self-reduction degree ofCu2Owas highly matched with the photocatalytic hydrogen production rate. This work is the first to identify and study the role of active sites on nanocatalysts in situ formation in the process of photocatalytic hydrogen production through in situ TEM, which provides a new method for further understanding the catalytic mechanism of catalysts.

Image:

HRTEM diagram and structural change diagram ofCu2Ounder different irradiation times

Ag nanoparticles are formed sequentially at the vertices, edges, and surfaces of theCu2Ocube

Links to papers: Shuohan Yu, Youhong Jiang, Yue Sun, Fei Gao, Weixin Zou, Honggang Liao, Lin Dong. Real time imaging of photocatalytic active site formation during H2evolution by in-situ TEM. Applied Catalysis B: Environmental 284 (2021) 119743.

Ultra-small gold (Au) clusters are considered to be one of the prototype materials for solar energy conversion due to their unique strong molecular light absorption properties. However, the aggregation of light-induced gold clusters into nanoparticles is one of the most important factors limiting its application in photocatalysis. Although Au cluster aggregation has been widely demonstrated, the underlying mechanism of cluster fusion remains unclear due to lack of experimental evidence. Here, we report direct observation of Au clusters clustered on TiO2 nanosheets when used as a visible photocatalyst for the reduction of nitroaromatic hydrocarbons. By in situ high-resolution transmission electron microscopy (TEM), two fusion mechanisms for Au clusters on TiO2 under ultraviolet-visible (UV-Vis) light irradiation in air were identified, namely migration and coalescence (MC) and Ostwald maturation (OR). In addition, the correlation between the photostability of gold clusters and the reaction atmosphere has been studied, where gold clusters are more stable in an inert N2 atmosphere or vacuum than oxidizing atmospheres (i.e., air and O2). These results indicate that Au clusters are inherently stable during photocatalysis, while instability comes from ligand layer depletion. This work not only reveals the potential mechanism of gold cluster sintering, but also provides guidance for improving the stability of metal cluster-based photocatalysts.

Image:

In situ STEM images before and after UV-Vis light irradiation under ambient conditions in air show the sintering behavior of gold clusters in TBA composites and a schematic of the gold cluster sintering mechanism observed under illumination.

Links to papers: Weng B, Jiang Y, Liao H G, et al. Visualizing light-induced dynamic structural transformations of Au clusters-based photocatalyst via in situ TEM[J]. Nano Research, 2021, 14(8): 2805-2809.