Observation of Substituent Effects in the Electrochemical Adsorption and Hydrogenation of Alkynes on Pt{hkl} Using SHINERS.

Shaoliang Guan,Gary A Attard,Andrew J Wain

doi:10.1021/acscatal.0c02967

Abstract

By combining cyclic voltammetry (CV) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), the adsorption behavior of two alkynes, propargyl alcohol (PA) and 2-methyl-3-butyn-2-ol (MeByOH), undergoing hydrogenation on Pt basal plane single-crystal electrodes is investigated. It is found that PA and MeByOH give rise to strong surface sensitivities in relation to both hydrogenation activity and molecular fragmentation into adsorbed species such as CO. For PA, irreversible adsorption is strongly favored for Pt{100} and Pt{110} but is weak in the case of Pt{111}. It is suggested that the presence of the primary alcohol substituent is key to this behavior, with the order of surface reactivity being Pt{100} > Pt{110} > Pt{111}. In contrast, for MeByOH, strong irreversible adsorption is observed on all three basal plane Pt surfaces and we propose that this reflects the enhanced activity of the alkyne moiety arising from the inductive effect of the two methyl groups, coupled with the decreased activity of the tertiary alcohol substituent toward fragmentation. Pt{111} also exhibits singular behavior in relation to MeByOH hydrogenation in that a sharp Raman band at 1590 cm–1 is observed corresponding to the formation of a di-σ/π-bonded surface complex as the alkyne adsorbs. This band frequency is some 20 cm–1 higher than the analogous broadband observed for PA and MeByOH adsorbed on all other basal plane Pt surfaces and may be viewed as a fingerprint of Pt{111} terraces being present at a catalyst surface undergoing hydrogenation. Insights into the hydrogenation activity of different Pt{hkl} surfaces are obtained using quantitative comparisons between Raman bands at hydrogenation potentials and at 0.4 V vs Pd/H, the beginning of the double-layer potential region, and it is asserted (with support from CV) that Pt{110} is the most active plane for hydrogenation due to the presence of surface defects generated via the lifting of the (1 × 2) to (1 × 1) clean surface reconstruction following flame annealing and hydrogen cooling. Our findings are also consistent with the hypothesis that Pt{111} planes are most likely to provide semihydrogenation selectivity of alkynes to alkenes, as reported previously.

Highlights

Unsaturated alcohols are important reagents for the synthesis of fine chemicals and their selective hydrogenation plays a critical role in the pharmacology, perfumery, and food industries
Propargyl alcohol (PA, or 2-propyn-1-ol) (Figure 1a) is one such compound that has found considerable attention, and whose adsorption behavior on Pt has been shown to resemble that of acetylene, which adsorbs through a nondissociative process involving a partial rupture of the C C bond.[4,5]
propargyl alcohol (PA) adsorption on the three basal plane Pt single-crystal electrodes was investigated in 0.1 M H2SO4 using cyclic voltammetry

Summary

Introduction

Unsaturated alcohols are important reagents for the synthesis of fine chemicals and their selective hydrogenation plays a critical role in the pharmacology, perfumery, and food industries. Electron energy loss spectroscopy (EELS) and Auger spectroscopy indicate that PA is adsorbed with the unsaturated carbon−carbon bond parallel to the Pt surface.[5] electrochemical methods have been used to study the adsorption and redox behavior of PA in acid solutions, and a nondissociative adsorption on Pt was postulated based on voltammetric measurements.[6] Different products of the electrochemical reduction of PA on Pt have been reported, with propane, propylene, and ethane being observed previously,[7,8] while electro-oxidation of PA was shown to produce the corresponding aldehyde.[9−13] PA strongly adsorbs on Pd in acid solutions,[14−16] generating similar products to Pt during electroreduction but CO2 as the sole electro-oxidation product.[14] The adsorption of PA on Ru16 and Au17,18 has been studied

Results

Discussion

Conclusion