Why Is Water More Reactive Than Hydrogen in Photocatalytic CO2 Conversion at Higher Pressures? Elucidation by Means of X-Ray Absorption Fine Structure and Gas Chromatography–Mass Spectrometry

Abstract

Photocatalytic conversion of CO2 into mainly methane using Pd/TiO2 photocatalyst proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant. The former reaction (CO2 + water) consists of two steps: first, water photosplitting and second, the latter reaction (CO2 + hydrogen). It was paradoxical that total steps proceeded faster than each step based on simple kinetics. To elucidate the reason, Pd and Ti K-edge X-ray absorption fine structure (XAFS) was monitored during CO2 photoconversion using H2 or moisture and the exchange reaction of 13CO2 at Pd/TiO2 surface was also monitored. As a result, the coordination number, N(Ti–O) and N[Ti(–O–)Ti] values, decreased from original values for TiO2 crystalline (6 and 12) to 4.9–5.7 and 9.7–10.6 under CO2 and moisture, respectively, in contrast to significantly smaller decreases under CO2 and H2 and under Ar. The exchange of gas-phase 13CO2 with preadsorbed 12CO2 reached the equilibrium in ~20 h with a rate constant of 0.20 h−1. The reason of the higher activity using water rather than H2 could be explained owing to the oxygen vacancy (Ov) sites as confirmed by XAFS. The reaction of TiO2 surface with water formed Ov sites responsible for water oxidation, specially separated from Pd nanoparticle sites for CO2 reduction. In contrast, Pd nanoparticle sites were competed by CO2 and H species, and the photoconversion of CO2 was suppressed at the elevated pressure of CO2 + H2.

Dual Photocatalytic Roles of Light: Charge Separation at the Band Gap and Heat via Localized Surface Plasmon Resonance To Convert CO2 into CO over Silver–Zirconium Oxide

“…A packed column of 13X-S molecular sieves (3 m length, 3 mm internal diameter; GL Sciences, Inc., Japan) was employed for online gas chromatography–mass spectrometry analyses (GC–MS; Model JMS-Q1050GC, JEOL, Japan). − Helium (purity >99.9999%) was used as the carrier gas at 0.40 MPa. The sampling loop comprised a Pyrex glass system kept under vacuum using rotary and diffusion pumps (10 –6 Pa) connected to the GC–MS via 1.5 m deactivated fused silica tubes (No.…”

Adsorbed CO2-Mediated CO2 Photoconversion Cycle into Solar Fuel at the O Vacancy Site of Zirconium Oxide

“…Alternatively, the E act value for water formation from species n to o (Scheme 2A) drastically decreased to 1. 6 The energy levels during the following reaction steps to hydroxycarbonyl and OCH species (Scheme 4A(a′−d′) and Chart S3a′−d′) were essentially in parallel separated by the difference of E ads value (2.0 eV, Scheme 4B(a−d),(a′−d′) and Chart S3(a′−d′)); however, the E act value from hydroxycarbonyl and OCH species (Scheme 4(d′,s) and Chart S3(d′,s)) was 0.32 eV at the interface between ZrO 2 and Ni nanoparticle (Scheme 4B(d′,s)), significantly smaller compared to 1.37 eV in the associate route, 1.30 eV in the hopping route, and 1.18 eV in the associative, shortcut route (Scheme 2A), because COH species was significantly more stable on the Ni(111) surface [Scheme 4A(s) and Chart S3-TS from d′−s,s] compared to that on the ZrO 2 (111) surface (Scheme 2A(k)), and the intersection of potential curves for OCOH and COH species in the reaction coordinate was lowered more on the Ni (111) surface. The above discussion strongly suggested the critical photocatalytic role of the interface site between the ZrO 2 surface and Ni for the CO 2 photoreduction.…”

“…The other possibility is the carbene pathway that is via dissociated C and carbene (CH 2 ) . However, an essential question remains as to why ZrO 2 can photoconvert 13 CO 2 into 13 CO, while the other semiconductors, e.g., TiO 2 , ZnO, and Cu x O ( x = 1, 2), often significantly form 12 CO derived from adventitious C . To achieve sustainable energy and create a new carbon neutral cycle, the reason why ZrO 2 exclusively forms 13 C-labeled products using 13 CO 2 , not affected by adventitious C, ,,− except for CO 2 strongly adsorbed from the air was investigated. , This paper reports the unexpected catalytic role of V O •• sites on ZrO 2 and the detailed photocatalytic reaction routes to CO and/or COH and further to CH 4 with the aid of Ni nanoparticles.…”