In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide–supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.

Insight into the location, state, and function of a promoter in heterogeneous catalysis was obtained through atomic-resolution in situ transmission electron microscopy. In the most active ruthenium catalyst for ammonia synthesis known so far, the barium promoter is shown to be located in two different phases in the catalyst. The increased activity is suggested to be related to a two-dimensional barium-oxygen overlayer on the ruthenium crystals. The possibility for conducting such studies for other reactions could add substantially to our current understanding of heterogeneous catalysis. Heterogeneous catalysis plays an increasingly important role in environmental protection processes, in fuel upgrading, and in providing the majority of the chemical building blocks required by contemporary society. Most heterogeneous catalysts of industrial importance are multicomponent materials that are designed by trial-and-error experimentation. Application of even the most sophisticated physical-chemical characterization techniques is usually not sufficient to obtain a complete understanding of the structure of the active site, the reaction mechanism and kinetics, the structural dynamics, and the specific roles of all catalyst components.

Rotation during reaction: Determining changes in heterogeneous catalysts under reaction conditions can provide insight into mechanisms. Under reaction conditions, not only can metal nanoparticles change shape but their interaction with the oxide support could also be affected. Yuan et al. used aberration-corrected environmental transmission electron microscopy to study gold nanoparticles on titanium surfaces at low electron beam doses. During carbon monoxide (CO) oxidation at total pressures of a few millibars and 500°C, they observed that gold nanoparticles rotated by about 10° but returned to their original position when CO was removed. Density function theory calculations indicated that rotation was induced by changes in the coverage of adsorbed molecular oxygen at the interface. Science , this issue p. 517