Adsorption and Selective Hydrogenation of α, β-unsaturated Aldehydes on Pt(111) and Pt-Sn Model Catalysts

Abstract

Studies of complex interacting systems such as the adsorption of α , β-unsaturated aldehydes on model catalyst surfaces are a present day challenge. Although numerous studies have been published on the catalytical properties of systems like crotonaldehyde and prenal on Pt(111) single crystals or ordered Pt-Sn surface alloys in recent years, these systems still remain under debate. Gaining insight into the molecule-surface bonding is a prerequisite to understand the different hydrogenation activities and selectivities on various catalysts. In addition to the already complex adsorption behavior on monometallic surfaces, alloying effects encountered on bimetallic catalysts lead to new modifications of the molecule-substrate interactions and can change the properties of the catalysts dramatically. <br /> The adsorption and the selective hydrogenation of crotonaldehyde and prenal on a Pt(111) single crystal surface and two ultra-thin Sn-Pt(111) surface alloys are studied in this work with two complementary approaches: The High Resolution Electron Energy Loss Spectroscopy (HREELS) and Density Functional Theory (DFT) calculations. On the one hand, HREELS has proven to be a powerful tool to characterize molecule-surface interactions and surface processes experimentally. On the other hand, using DFT it is possible to study adsorption energies, adsorption structures and vibrational spectra from a theoretical point. The properties and stability of various possible adsorption structures can be explored as a function of coverage. Also experimentally short-lived intermediates become accessible. Combining both powerful approaches, it is possible to gain new insights into molecule-surface interactions, modifications by alloying effects and the surface processes.<br /> First, a brief overview on the importance of surface studies in the field of catalysis is presented in Chap. 1. With a focus on the adsorption, interaction and reactivity of the molecules prenal and crotonaldehyde on various catalysts and single crystal surfaces, a review of previous results of both high pressure and also vacuum studies is given in Chap. 2. After a general introduction to the experimental techniques employed throughout this thesis (Chap. 3), the basics of the Density Functional Theory (DFT) will be discussed in Chap. 4. Additionally, details on the computation of vibrational normal modes are given in Chap. 4, too. The characterization and preparation of the Pt and Pt-Sn model catalysts used in this work is described in Chap. 5. <br /> New TPD (Temperature Programmed Desorption) Spectroscopy measurements concerning crotonaldehyde (Chap. 6) on Pt(111) show the desorption of intact molecules at low temperatures. Between 200 K and 300 K highly complex HREELS spectra are measured. Since crotonaldehyde exists in the gas phase in four rota-isomers, the DFT calculations of potential adsorption modes on the pure Pt surface result in a large set of 19 stable structures. Comparison of the computed HREELS spectra to the experimental data suggests a mixture of various adsorption modes coexisting on the surface. Several adsorption modes of η² , η³ and η⁴ hapticity (see Chapts. 6 and 7) have to be taken into account in order to interpret these spectra successfully. The decomposition pathway of crotonaldehyde observed with HREELS and TPD leads to many vibrational loss signals, which are correlated with five specific surface intermediates. On the Pt-Sn surface alloys much larger sets of conceiveable adsorption structures have to be treated in the DFT calculations, but most of them turn out to be un- or weakly stable. Due to the still relatively high adsorption energies obtained by DFT for the remaining structures on both Pt-Sn alloys, crotonaldehyde is still chemisorbed here. On the Pt₂Sn(111), the reversibly chemisorbed species at low temperatures are mainly interpreted as η² species. <br /> The studies of prenal (Chap. 7) adsorbed on Pt(111) show several different desorption states from the surface. While desorption of intact prenal is observed below 200 K, also strongly adsorbed species are found on the surface, which start to decompose at higher temperatures. The HREELS spectra obtained at low temperatures are interpreted as a combination of flat η⁴ and η³ adsorption modes as well as lower coordinated η² species. As for crotonaldehyde, several fragments stemming from the decomposition have been identified by the combination of HREELS and DFT. This allows to propose a decomposition pathway for prenal on Pt(111). On the Pt₃Sn/Pt(111) surface alloy, experiments and DFT total energy calculations clearly lead to the conclusion that a change in the coordination type compared to Pt(111) occurs. Here prenal is chemisorbed in two vertical η¹ top-adsorption geometries, bonded via the aldehydic oxygen to the surface Sn. In contrast, on the Pt₂Sn/Pt(111) surface alloy prenal is found to be only physisorbed. Both, theory and experiment have difficulties to reach a doubtless conclusion when considered on their own. Yet the combination of the results from both approaches allows to conclude that prenal is physisorbed in this case. <br /> Generally the DFT analysis of the experimental HREELS data provides not only the identification of certain adsorption structures and the characterization of their vibrational properties, but furthermore gives deeper insights in the specific interactions to the surfaces. By an analysis of the energetic contributions to the adsorption energy of the various possible configurations, the interaction strength of the surface species with the substrate can be estimated and their vibrational properties understood in more detail. Finally the conclusions drawn from the combined approach of experiments and theoretical calculations presented in this thesis are summarized in Chap. 8.