In Situ Investigation of Charge Performance in Anatase TiO₂ Powder for Methane Conversion by Vis–NIR Spectroscopy

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 TiO 2 . 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 TiO 2 involves oxidation by photoexcited holes. It is calculated that 90 ± 6% of photoexcited electrons are scavenged by O 2 (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 O 2 even when the O 2 concentration is reduced by a factor of 10. The present results suggest that O 2 is much more easily activated in comparison to methane over anatase TiO 2 , which rationalizes the much higher methane/O 2 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 TiO 2 .

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

“…This case of study demonstrated that TRIR is capable of systematically characterizing midgap energy levels in semiconductor photocatalysts. In addition, the near IR spectroscopy can also provide complementary information to TRIR, 103 in which we found that the free or shallow trapped photoelectrons in TiO 2 could be monitored readily using near IR spectroscopies.…”

Insights on Carbon Neutrality by Photocatalytic Conversion of Small Molecules into Value-Added Chemicals or Fuels

“…Most recently, we constructed an in situ vis–NIR spectroscopy system to elucidate the initial steps of methane photoactivation over anatase TiO 2 ( Figure 8 a). 52 From the in situ experimental study, under constant light irradiation, the intensity of photogenerated electrons on anatase TiO 2 increased in the NIR region under methane atmosphere as compared to that under argon gas. Combined with the significantly decreased signal intensity of electrons under air condition ( Figure 8 b), it was strongly evidenced that methane was a hole scavenger in photocatalytic methane conversion ( Figure 8 c).…”

“…Further experimental and DFT results demonstrated the inverse correlation between the yield of deep-trapped long-live inactive electrons over g-C 3 N 4 and the related H 2 production. Most recently, we constructed an in situ vis–NIR spectroscopy system to elucidate the initial steps of methane photoactivation over anatase TiO 2 (Figure a) . From the in situ experimental study, under constant light irradiation, the intensity of photogenerated electrons on anatase TiO 2 increased in the NIR region under methane atmosphere as compared to that under argon gas.…”

“…Photoinduced adsorption spectra (PIA) on anatase TiO 2 at 365 nm irradiation in the presence of (b) air and (c) 10% methane. Reproduced with permission from ref ( 52 ). Copyright 2021 American Chemical Society.…”

Photocatalytic Methane Conversion to C1 Oxygenates over Palladium and Oxygen Vacancies Co‐Decorated TiO2

“…As the oxygen source for CH 3 OH came from both H 2 O and O 2 , it was accordingly concluded that •CH 3 radicals reacted with both •OH radicals and •OOH radicals (Equation 6 and 7), which were generated from H 2 O oxidation and O 2 www.advancedsciencenews.com www.solar-rrl.com reduction, respectively. The generation of HCHO [30] was derived from further oxidation of the as-produced CH 3 OOH and CH 3 OH. [7] The overoxidation of C1 oxygenates to CO 2 might be caused by •OH radicals.…”