Adsorption Energetics on Pd Model Catalysts by Microcalorimetry

The efficient use of the limited resources on earth is a critical factor to sustainable life. The development of better catalysts can make a significant contribution. Complete understanding of the catalytic activity would facilitate the design and control of specific catalytic processes. Consequently, by tuning of the catalytic reactivity, the amount of energy to run a reaction and the amount of waste products could be reduced significantly. In this work, the correlation of the catalyst structure and the heats of adsorption of gasphase particles was investigated. In particular, the heat of adsorption for CO on Pd particles was determined as a function of particle size, using a well-characterized model catalyst system, Pd particles supported on an iron oxide film, and UHV single crystal adsorption microcalorimetry. The presented thesis is divided into two distinct parts. The subject of the first part is the construction and calibration of a new UHV adsorption microcalorimeter experiment. The second part presents results on adsorption heats on single crystals as well as oxide supported metal nanoparticles. The completed microcalorimeter experiment comprises a preparation chamber and a calorimetry chamber, providing all means to prepare and characterize oxide supported metal nanoparticles and to perform adsorption energy measurements. The calorimeter is based on the design of Campbell et al., using a pyroelectric β -PVDF ribbon as a detector. Improvements with respect to alignment, temperature stability, and vibration isolation were implemented. A pulsed molecular beam is used to expose the surface to a stable and homogeneous flux of gas-phase molecules. Further, a dedicated in situ reflectivity measurement setup allows optical characterization of the model catalyst surfaces, which is crucial for an accurate energy calibration of the calorimeter. The calorimeter shows a very good sensitivity and is capable to resolve deposited energy densities of about 120nJ/cm2, which corresponds to an equivalent amount of 4 ·10−4 monolayers of CO on Pt(111) (with a heat of adsorption of 130kJ/mol). A monolayer is defined by the number of Pt(111) surface atoms, being 1.5 · 1015 cm−2. The estimated heat of adsorption has a precision of 6.3 to 32.0% and an accuracy of about 5%. The heats of adsorption for CO and benzene on Pt(111) were determined as a function of coverage, being initially (130± 3)kJ/mol and (119± 3)kJ/mol at room temperature, respectively. For the first time, the heat of adsorption of CO was measured also at 120K, coinciding mainly with the room temperature data. The initial heat of adsorption was (124± 3)kJ/mol. The obtained data are compared to previous studies, verifying the proper functioning of the calorimeter experiment by the good agreement. By the unique combination of single crystal adsorption calorimetry, molecular beam techniques and supported model catalysts, it was possible to resolve the longstanding controversy, how the heat of adsorption of CO on Pd particles changes with particle size. The heat of adsorption for CO on Pd particles was studied on Fe3O4 supported Pd particles with a mean diameter of 1.8 to 8nm and Pd(111). The initial heat of adsorption was found to decrease with decreasing particle size. Presumably, this observation can be rationalized as a reduction of the chemisorption strength and the Van der Waals interaction.