Atomic force phase imaging for dynamic detection of adsorbed hydrogen on a catalytic palladium surface under liquid.

Dynamic observation of hydrogen on catalytic metal surfaces is a challenging aspect of studying liquid-phase heterogeneous catalysis. Current methods suffer from one or more of the following limitations: the requirement to observe the surface in high vacuum, the inability to provide nanometer-level spatial resolution, the inability to deal with opaque catalysts and/or liquid immersion phase, the lack of real-time scanning of the surface area, and the inability to assess pronounced topographies or mixed materials. Atomic force microscopy (AFM) phase-shift imaging remedies these issues and provides an opportunity for dynamic direct observation of catalyst surfaces at or near actual reaction conditions immersed in liquid. Hydrogen was delivered to a palladium surface immersed in water by diffusion through a support film of dense polycarbonate. The palladium surface was continuously probed by tapping-mode AFM. The theoretically predicted time-dependent appearance of hydrogen on the water-covered palladium surface matched the experimental observation reasonably well. The technique demonstrated here is unique in that the appearance of hydrogen is dynamically detected in real time on a catalyst surface immersed in water with nanometer-scale spatial resolution. The results presented here supply a new level of information for heterogeneous catalysis that is not available with existing techniques. This work opens new avenues in the study of heterogeneous catalysis, a field with tremendous practical importance and serious analytical challenges.