Abstract
Atomistic-level understanding of the interaction of α,β-unsaturated aldehydes and their derivatives with late transition metals is of fundamental importance for the rational design of new catalytic materials with the desired selectivity towards C[double bond, length as m-dash]C vs. C[double bond, length as m-dash]O bond partial hydrogenation. In this study, we investigate the interaction of acrolein, and its partial hydrogenation products propanal and allyl alcohol, with Pd(111) as a prototypical system. A combination of infrared reflection-absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) experiments was applied under well-defined ultrahigh vacuum (UHV) conditions to obtain detailed information on the adsorption geometries of acrolein, propanal, and allyl alcohol as a function of coverage. We compare the IR spectra obtained for multilayer coverages, reflecting the molecular structure of unperturbed molecules, with the spectra acquired for sub-monolayer coverages, at which the chemical bonds of the molecules are strongly distorted. Coverage-dependent IR spectra of acrolein on Pd(111) point to the strong changes in the adsorption geometry with increasing acrolein coverage. Acrolein adsorbs with the C[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds lying parallel to the surface in the low coverage regime and changes its geometry to a more upright orientation with increasing coverage. TPD studies indicate decomposition of the species adsorbed in the sub-monolayer regime upon heating. Similar strong coverage dependence of the IR spectra were found for propanal and allyl alcohol. For all investigated molecules a detailed assignment of vibrational bands is reported.
Highlights
The atomistic-level understanding of factors governing the selectivity in partial selective hydrogenation of a,b-unsaturated aldehydes and ketones over transition metal surfaces is of fundamental importance for numerous industrial processes.[1]
The adsorption of acrolein, propanal, and allyl alcohol was investigated on Pd(111) at 120 K under well-defined ultrahigh vacuum (UHV) conditions by infrared reflection–absorption spectroscopy (IRAS)
The molecular structures and the adsorption geometry of the adsorbates were studied as a detailed function of coverage
Summary
The atomistic-level understanding of factors governing the selectivity in partial selective hydrogenation of a,b-unsaturated aldehydes and ketones over transition metal surfaces is of fundamental importance for numerous industrial processes.[1] The primary hydrogenation products of this class of reactions are either a saturated aldehyde or an unsaturated alcohol. Our recent work on acrolein hydrogenation over a Pd(111) surface under well-defined ultrahigh vacuum (UHV) conditions provided new insight into the relationship between the catalyst structure and its selectivity in partial hydrogenation of acrolein. For the first time we showed that a near 100% selectivity towards hydrogenation of CQO in acrolein is possible over Pd(111), while no unsaturated alcohol was formed over Pd nanoparticles.[9] This was a unexpected result, since numerous studies on powdered Pd catalysts have previously shown near 100% selectivity towards CQC bond hydrogenation
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