Solvent Water Controls Photocatalytic Methanol Reforming

Abstract

Understanding the role of different solvent molecules for practical solid–liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reaction. In this study, the operando nuclear magnetic resonance (NMR) method, combined with density functional theory (DFT) calculations, was employed to evaluate the control effect of solvent water in the photocatalytic reforming mechanism of methanol with a Pt-TiO2 catalyst. Results indicate that the presence of water effectively promotes the formation of the HCHO intermediate but inhibits the H2 evolution originating from the switch of the hydrogen source of the H2 formation from CH3OH to H2O. More interestingly, as detected directly in the ab initio molecular dynamics simulation, a small amount of H2O can dissociate, and the evolved −OH species at Ti5c site can greatly reduce the C–H activation barrier of −CH3O, contributing to the formation of oxidation products (e.g., HOCH2OH and CH3OCH2OH) on the Pt-TiO2 surface.

Operando NMR Spectroscopic Analysis of the Effects of Pt Nanoparticle Size and Crystal Facet Structure on the Alcohol Reforming Reactions

“…The 1 H NMR spectra corresponding to the methanol reforming products obtained after 8 h of visible light irradiation for Pt/g-C 3 N 4 photocatalysts with Pt concentrations of 0.5, 1.5, and 3.0 wt % are presented in Figure . These 1 H NMR spectra represent three reforming products based on the CH 2 group of hemiacetal (CH 3 OCH 2 OH) at a chemical shift δ of 4.64 ppm, the CH 2 group of acetal (CH 2 (OCH 3 ) 2 ) at δ = 4.54 ppm, and the CH 3 group of ether (CH 3 OCH 3 ) at δ = 3.31 ppm . The concentrations of these products corresponding to the individual signal intensities were evaluated quantitatively based on the signal intensity of the known concentration of TMS in the solution, and the results are plotted in the inset of Figure .…”

“…These 1 H NMR spectra represent three reforming products based on the CH 2 group of hemiacetal (CH 3 OCH 2 OH) at a chemical shift δ of 4.64 ppm, the CH 2 group of acetal (CH 2 (OCH 3 ) 2 ) at δ = 4.54 ppm, and the CH 3 group of ether (CH 3 OCH 3 ) at δ = 3.31 ppm. 14 The concentrations of these products corresponding to the individual signal intensities were evaluated quantitatively based on the signal intensity of the known concentration of TMS in the solution, and the results are plotted in the inset of Figure 1. We note from these results that hemiacetal is typically the main oxidation product of methanol observed in solution, which owes its formation to a condensation reaction between an HCHO intermediate and a nearby CH 3 OH molecule and methoxy group (−CH 3 O).…”

“…The present study addresses the above-discussed issue by applying an operando 1 H nuclear magnetic resonance (NMR) spectroscopy method to investigate the detailed relationships between the NP size and crystal facet structure of supported Pt graphitic carbon nitride (Pt/g-C 3 N 4 ) photocatalysts and the selectivity and reaction rates of methanol (CH 3 OH) and ethanol (CH 3 CH 2 OH) reforming products. Here, operando 1 H NMR spectroscopy technology has been proven to be a very effective technique for probing fundamental mechanisms operative in photocatalytic reactions. , The work employs three types of Pt/g-C 3 N 4 photocatalysts with different Pt NP sizes and crystal facet structures obtained by simply adjusting the level of Pt loading from 1.5 to 3.0 wt % during the synthesis process, and the dynamic changes in the methanol and ethanol reforming products and yields in each system were evaluated in situ by operando NMR spectroscopy. The results demonstrate that relatively small Pt NPs without Pt(111) crystal surfaces have high selectivity and efficiency for producing reforming products with a low degree of polymerization (e.g., aldehydes, hemiacetals, and acids), while relatively large NPs with Pt(111) crystal surfaces tend to produce reforming products with an increased degree of polymerization (e.g., acetals, ethers, and esters).…”

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

“…Understanding the role of different solvent molecules for practical solid‐liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reactions in the presence of a liquid phase. As an example, the influence of the solvating water molecules was demonstrated by Xu et al [167] …”

“…Understanding the Chemistry-A European Journal role of different solvent molecules for practical solid-liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reactions in the presence of a liquid phase. As an example, the influence of the solvating water molecules was demonstrated by Xu et al [167] A different approach is to include the photocatalyst and all processes regarding charge carrier generation and recombination into a combined experimental and theoretical approach. Xiong et al [148d] describe the different reactive oxygen species and challenges regarding the selectivity to desired products in the photocatalytic oxidation of organic substrates.…”