Determining adsorbate configuration on alumina surfaces with ¹³C nuclear magnetic resonance relaxation time analysis

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.

Effect of carbon oxygen functionalization on the activity of Pd/C catalysts in hydrogenation reactions

“…In particular, the NMR observable of interest is the spin relaxation, that can be used as a sensitive probe for molecular dynamics. Relaxation times are the longitudinal relaxation time (T1) and the transverse relaxation time (T2), and they may be used to identify molecular dynamics processes since they both depend on the rotational correlation time of the reactant [30] . Indeed, reduced T1 and T2 relaxation times are observed when liquid molecules adsorb on a solid surface due to a decrease in molecular mobility [31] .…”

“…Relaxation times are the longitudinal relaxation time (T1) and the transverse relaxation time (T2), and they may be used to identify molecular dynamics processes since they both depend on the rotational correlation time of the reactant. [30] Indeed, reduced T1 and T2 relaxation times are observed when liquid molecules adsorb on a solid surface due to a decrease in molecular mobility. [31] Using in particular 13 C NMR, T1 by its own can be a suitable probe for surface interactions, because the relatively slow molecular motion of the adsorbate on the adsorbent surface causes a decrease in T1 compared to its bulk phase.…”

An Insight into the Role of Reactant Structure Effect in Pd/C Catalysed Aldehyde Hydrogenation

“…As a lower T1 ads /T1 bulk ratio indicates a stronger benzaldehyde adsorption [ 15 ], the mean adsorption energy is mostly higher on the Pd-supported catalyst than on bare carbon ( Table 4 ), due to the stronger adsorption of benzaldehyde, possibly due to the presence of Pd NPs acting as binding sites. Moreover, looking at the T1 values of the individual carbon atoms on Pd/GNP, we were able to identify the specific adsorption mode of benzaldehyde.…”

“…Moreover, as reported by Gladden et al [ 15 ], in 13 C NMR the longitudinal relaxation time T1 by its own can be a suitable probe for surface interactions, omitting the use of T2 which is more likely to suffer from resolution issues. The relatively slow molecular motion of the adsorbate on the adsorbent surface causes a decrease in T1 compared to its bulk phase.…”

“…In the present paper, an alternative approach based on 13 C NMR is presented showing some practical advantages. The larger chemical shift range of 13 C allows to discriminate the individual resonances [ 15 ], thus the surface interaction of each individual carbon atom can be analysed. Additionally, concerning the coupling constant effect, proton decoupled 13 C NMR does not suffer from spin coupling due to the very low natural abundance of 13 C (1.1%).…”