Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations

Abstract

Conspectus Rhenium and manganese bipyridyl tricarbonyl complexes have attracted intense interest for their promising applications in photocatalytic and electrocatalytic CO2 reduction in both homogeneous and heterogenized systems. To date, there have been extensive studies on immobilizing Re catalysts on solid surfaces for higher catalytic efficiency, reduced catalyst loading, and convenient product separation. However, in order for the heterogenized molecular catalysts to achieve the combination of the best aspects of homogeneous and heterogeneous catalysts, it is essential to understand the fundamental physicochemical properties of such heterogeneous systems, such as surface-bound structures of Re/Mn catalysts, substrate–adsorbate interactions, and photoinduced or electric-field-induced effects on Re/Mn catalysts. For example, the surface may act to (un)­block substrates, (un)­trap charges, (de)­stabilize particular intermediates (and thus affect scaling relations), and shift potentials in different directions, just as protein environments do. The close collaboration between the Lian, Batista, and Kubiak groups has resulted in an integrated approach to investigate how the semiconductor or metal surface affects the properties of the attached catalyst. Synthetic strategies to achieve stable and controlled attachment of Re/Mn molecular catalysts have been developed. Steady-state, time-resolved, and electrochemical vibrational sum-frequency generation (SFG) spectroscopic studies have provided insight into the effects of interfacial structures, ultrafast vibrational energy relaxation, and electric field on the Re/Mn catalysts, respectively. Various computational methods utilizing density functional theory (DFT) have been developed and applied to determine the molecular orientation by direct comparison to spectroscopy, unravel vibrational energy relaxation mechanisms, and quantify the interfacial electric field strength of the Re/Mn catalyst systems. This Account starts with a discussion of the recent progress in determining the surface-bound structures of Re catalysts on semiconductor and Au surfaces by a combined vibrational SFG and DFT study. The effects of crystal facet, length of anchoring ligands, and doping of the semiconductor on the bound structures of Re catalysts and of the substrate itself are discussed. This is followed by a summary of the progress in understanding the vibrational relaxation (VR) dynamics of Re catalysts covalently adsorbed on semiconductor and metal surfaces. The VR processes of Re catalysts on TiO2 films and TiO2 single crystals and a Re catalyst tethered on Au, particularly the role of electron–hole pair (EHP)-induced coupling on the VR of the Re catalyst bound on Au, are discussed. The Account also summarizes recent studies in quantifying the electric field strength experienced by the catalytically active site of the Re/Mn catalyst bound on a Au electrode based on a combined electrochemical SFG and DFT study of the Stark tuning of the CO stretching modes of these catalysts. Finally, f...

Vibrational Stark shift spectroscopy of catalysts under the influence of electric fields at electrode–solution interfaces

“…(b) A representative VSFG spectra of the nitrile-stretch of 4-mercaptobenzonitrile (4-MBN) on gold as a function of potential, adapted from a publication by Sarkar et al 63 (c) The electrochemical VSFG set-up used in the Lian lab. Figures (a) and (c) are adapted from a publication by Ge et al 64 Stark shift measurements have been successfully reported for a range of catalysts attached to various metal and semiconductor surfaces with suitable anchoring groups. Synthetic efforts have contributed significantly by functionalizing the molecules with anchoring groups that attach to Au electrode surfaces with thiolate, nitrile, or isocyanide groups, or to metal-oxide surfaces with carboxylate, or phosphate anchoring groups.…”

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes

“…Self-assembled monolayers (SAM) of ReS and ReSS on Au films were probed by SFG to characterize the synthesized molecular catalyst as heterogenized on gold surfaces, with an emphasis on characterization of the binding motif of the complexes on the surface. SFG is a surface-selective technique with submonolayer sensitivity (Nihonyanagi et al, 2013;Shen, 2013;Ge et al, 2019). When combined with computational modeling, it can provide information on molecular orientation at the interface.…”

“…The orientation and properties of molecular catalysts on electrode surfaces can be analyzed by combining vibrational sum-frequency generation (SFG) spectroscopy and computational modeling (Anfuso et al, 2011(Anfuso et al, , 2012Clark et al, 2016Clark et al, , 2018bGe et al, 2016Ge et al, , 2017. The resulting insights should be particularly informative for the rational design of catalytic materials (Ge et al, 2019).…”

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…[25][26][27] In parallel, the increasing interest for in situ and operando microscopic and spectroelectrochemical (SEC) techniques applied to both, the homogeneous and heterogeneous systems has led to a strong improvement in the fundamental understanding of the factors governing the CO 2 RR process. [28][29][30][31][32] In spite of this, further improvement of the catalyst performance in terms of activity, selectivity and stability is highly desired to match the requirements for practical applications.…”