Energetics of Surface Processes by Single Crystal Adsorption Calorimetry

Establishing the correlation between the energetics of adsorbate-surface interaction and the structural properties of a catalyst is an important fundamental issue and an essential prerequisite for understanding the realistic catalytic processes. We recently developed and set up a new Single Crystal Adsorption Calorimetry (SCAC) apparatus based on the molecular beam techniques and operated under UHV conditions. This method is applied for quantitative measurements of the adsorption and reaction enthalpies of gaseous molecules on model supported catalysts. The major advantage of using SCAC consists in the ability to directly measure the interaction strength of gaseous molecules with the surface of interest. The traditionally used experimental techniques for probing the energetics of adsorption - temperature-programmed desorption (TPD) and equilibrium adsorption isotherm measurements - provide reliable results only for systems with fully reversible adsorption i.e. most of the catalytically-relevant processes, involving dissociation, reaction with co-adsorbates, clustering or diffusion into bulk cannot be probed by these methods correctly. In our experimental setup, we for the first time combined the direct measurement of adsorption and reaction energies by SCAC with the tools for preparation of model catalysts. This unique combination allows us to perform direct calorimetric measurements on surfaces with different levels of complexity ranging from the extended single crystal metal surfaces to the complex nano-structured supported catalysts, providing an experimental possibility to controllably vary certain structural features and correlate them with the adsorption strength of adsorbed species. Such data provide highly important benchmarks for theoretical calculations and are generally not available at the moment.

We apply SCAC to determine the adsorption heats of gas phase molecules (such as carbon monoxide and oxygen) and surface reaction (e.g. CO oxidation) on Pd nanoparticles supported on a well-define Fe3O4/Pt(111) film. Particularly, we systematically vary the Pd cluster size in the range of ~ 100 to 5000 Pd atoms to address the energetics of CO and O interaction with the nanoparticles of different dimensions. As a reference for interaction with an extended surface, the adsorption heats are also determined on Pd(111). First results showed strong and unexpected dependence of adsorption heats on the particle size, which can be traced back to size-dependent electronic properties of Pd nanoparticles. (Peter et. al., Angew. Chem. Int. Ed. 52 (2013) 5175-5179)