Abstract
The intrinsic behavior of photogenerated charges and reactions with chemicals are key for a photocatalytic process. To observe these basic steps is of great importance. Here we present a reliable and robust system to monitor these basic steps in powder photocatalysts, and more importantly to elucidate the key issue in photocatalytic methane conversion over the benchmark catalyst TiO2. Under constant excitation, the absorption signal across the NIR region was demonstrated to be dominated by photoexcited electrons, the absorption of photoexcited holes increases toward shorter wavelengths in the visible region, and the overall shapes of the photoinduced absorption spectra obtained using the system demonstrated in the present work are consistent with widely accepted transient absorption results. Next, in situ measurements provide direct experimental evidence that the initial step of methane activation over TiO2 involves oxidation by photoexcited holes. It is calculated that 90 ± 6% of photoexcited electrons are scavenged by O2 (in dry air), 61 ± 9% of photoexcited holes are scavenged by methane (10% in argon), and a similar amount of photoexcited electrons can be scavenged by O2 even when the O2 concentration is reduced by a factor of 10. The present results suggest that O2 is much more easily activated in comparison to methane over anatase TiO2, which rationalizes the much higher methane/O2 ratio frequently used in practice in comparison to that required stoichiometrically for photocatalytic production of value-added chemicals via methane oxidation with oxygen. In addition, methanol (a preferable product of methane oxidation) is much more readily oxidized than methane over anatase TiO2.
Highlights
INTRODUCTION AND BACKGROUNDPhotocatalysis is a promising sustainable and green technology with a wide range of applications, including solar fuel production,[1−5] organics conversion,[3,4,6−8] and air and water purification.[9,10] the current efficiency in these photochemical processes is very moderate
The intensity of the excitation light attenuates more quickly than the intensity of the measurement light; the concentration of absorbers within the sample is inhomogeneous and nonisotropic within distances traveled by the measurement light
Diffuse-reflectance measurements of anatase TiO2 powder under different conditions have demonstrated the capabilities of the present setup to measure the in situ absorption characteristics of photoexcited charge carriers in powder samples
Summary
Photocatalysis is a promising sustainable and green technology with a wide range of applications, including solar fuel production,[1−5] organics conversion,[3,4,6−8] and air and water purification.[9,10] the current efficiency in these photochemical processes is very moderate. The monitoring of charge carriers is the key to provide a fundamental understanding of the performance of different photocatalytic materials and to aid in the rational design of efficient photocatalysts. Among these photocatalytic processes, the photocatalytic conversion of methane, the main component of natural gas and shale gas, to high-value chemicals has recently attracted much attention in the literature. The photocatalytic processes were able to drive the reaction more efficiently with much lower energy consumptions and CO2 emissions in Received: May 3, 2021 Revised: June 1, 2021 Published: June 20, 2021
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