Abstract
Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved (13)C nuclear magnetic resonance (NMR) T1 relaxation times. The ratio of relaxation times for the adsorbed atoms T1,ads to the bulk liquid relaxation time T1,bulk provides an indication of the mobility of the atom. Hence a low T1,ads/T1,bulk ratio indicates a stronger surface interaction. The carbon atoms associated with unsaturated bonds in the molecules are seen to exhibit a larger reduction in T1 on adsorption relative to the aliphatic carbons, consistent with adsorption occurring through the carbon-carbon multiple bonds. The relaxation data are interpreted in terms of proximity of individual carbon atoms to the alumina surface and adsorption conformations are inferred. Furthermore, variations of interaction strength and molecular configuration have been explored as a function of adsorbate coverage, temperature, surface pre-treatment, and in the presence of co-adsorbates. This relaxation time analysis is appropriate for studying the behaviour of hydrocarbons adsorbed on a wide range of catalyst support and supported-metal catalyst surfaces, and offers the potential to explore such systems under realistic operating conditions when multiple chemical components are present at the surface.
Highlights
The adsorption of molecules onto active surface sites is a fundamental step in heterogeneous catalysis
Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved 13C nuclear magnetic resonance (NMR) T1 relaxation times
Previous studies have observed the existence of a di-s bond between alkenes and metal surfaces,[22,23] only a weak molecular interaction was observed between the adsorbate and the alumina surface for the systems studied here, as confirmed by Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) studies, see Fig. 3 and discussion below
Summary
The adsorption of molecules onto active surface sites is a fundamental step in heterogeneous catalysis. To develop an improved understanding of catalytic reaction mechanisms on a molecular level it is necessary to identify the configuration adopted by molecules upon adsorption, the nature of the surface interaction, and the strength of this interaction. The geometry occupied by reactant molecules is critical in dictating both their activity and reaction kinetics, while the efficacy of chiral modifiers is known to be dependent upon their conformation on a catalyst surface.[1] Many techniques applied both under high vacuum and at realistic process conditions have been employed previously to probe the interaction of adsorbates with catalyst surfaces.[2,3] There remains, a Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK a lack of information on the molecular configuration of molecular species at interfaces.[4]. This method has been used to study the chemisorption of hydrogen on Pt and Pd surfaces using 1H NMR,[7] and of fluorine-containing adsorbates on a variety of surfaces, including aluminas, using 19F NMR.8 13C
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