Photocatalyst: To Be Dispersed or To Be Immobilized? The Crucial Role of Electron Transport in Photocatalytic Fixed Bed Reaction

Time Constant Estimation and Alleviation of Interior Charge Recombination for Photocatalytic Reaction Guided by Correlation with Photoelectrochemical Behaviors

Interior charge recombination that inherently competes with the separation of photogenerated charges is crucial to the photocatalytic utilization of incident photons. We here propose to simultaneously promote both the desired hole extraction and semiconductor-cocatalyst interfacial electron transfer and suppress the undesired interior charge recombination by shifting the equilibrium potential of the semiconductor in an actual photocatalytic reaction. By correlating with these interfacial electronic processes, we estimate the time constants for the occurrence of interior charge recombination, which range from several milliseconds to several deciseconds in the actual photocatalytic reaction. This time scale estimated from photoelectrochemical behaviors not only provides a substantial guide to photocatalytic reaction design but also avoids relying on a very dense photon beam that greatly deviates from actual working conditions to generate discernible optical signals in certain common methods.

Evaluating a Second Time Scale for Hole Extraction in an Actual Photocatalytic Reaction: The Method

Photoinduced hole extraction, which is widely believed to occur on the picosecond-nanosecond time scale, exhibits sensitive correlation with semiconductor-cocatalyst interfacial electron transfer that occurs on a second time scale in a photocatalytic reaction. This inherent correlation means that the required high density of electrons for overcoming the potential barrier severely suppresses the survival and interfacial extraction of photogenerated holes. We here propose to evaluate the time constant for the occurring hole extraction by separately monitoring the rise and decay of the photoinduced potential. The evaluated second time scale indicates that hole extraction crucially influences the actual photocatalytic reaction. To improve photocatalytic photon utilization, we facilitate hole extraction by lowering the semiconductor-cocatalyst contact barrier.

Three-Channel Electron Transfer for Estimating Time Constants Correlated with Photocatalytic Photon Utilization

Although the potential barrier for semiconductor-cocatalyst interfacial electron transfer can be reduced by intensifying the irradiation, the photocatalytic reaction still suffers from a low photon utilization. We here propose a trichannel electron transfer model to demonstrate that the photogenerated oxidative intermediates deprive electrons from the photocatalyst and compete with the target reaction. This model can evaluate the time constant for each electronic process involved in the target reaction and predict photocatalytic photon utilization, which is closely related to the evolution of the oxidative intermediates.

Self-Promoted H2 Formation: The Feasibility of Photoinduced CO Removal for Lossless Hydrogen Purification

A photoinduced radical reaction operated at low temperature can be used to remove trace CO from a H₂ stream by minimizing the reverse water-gas shift. However, H₂ consumption resulting from nonselective oxidation by hydroxyl radicals becomes an obstacle to practical hydrogen purification. Inspired by hydrogen exchange transfer, we demonstrate here that molecular hydrogen can promote H₂ formation from hydrogen radicals, which are generated from the reaction of CO and H₂ with hydroxyl radicals. The slight increment in H₂ along with the radical reaction encouraged us to configure a photocatalytic hydrogen purification fixed-bed reactor, which can reduce CO to ≤1 ppm in the H₂ stream.