Photocatalytic Gas Phase Reactions

Abstract

Photocatalytic gas phase reactions represent a less explored yet promising direction within the highly active research area of photocatalysis. While photocatalytic liquid phase reactions are typically performed in water, photocatalytic gas phase reactions focus on gaseous and vaporized compounds. With this change in reaction environment, it is possible to address completely new questions and topics. Examples lie in the field of solar fuels, air pollution, global warming, and green chemistry. For this review, we selected five photocatalytic gas phase reactions, namely the reduction of CO2, water splitting, the oxidation of volatile organic compounds, the degradation of nitrogen oxides, and the synthesis of ammonia, which we discuss in the context of why they are important from a scientific, technological, and also societal point of view. We present the chemical mechanisms behind these photocatalytic processes and propose ideas and strategies for how these processes can be made more efficient. Our literature survey results in a list of 11 points regarding how the selectivity and yield of photocatalytic gas phase reactions can be increased by optimizing the composition of the photocatalysts, their surface chemistry, and experimental parameters such as temperature, gas flow, and gas composition.

Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide

“…Gas phase photocatalytic reactions have been poorly explored in comparison to those performed in aqueous phase, and they constitute a promising alternative to the latter [ 66 ]. In some cases, the major economical drawback is the high temperature required for hydrogen production [ 67 ].…”

Electrochemically Synthesized Nano-photocatalysts for Photodegradation of Organic Compounds

“…e − CB + h + VB → heat (5) where hν is the UV irradiation, h + VB is valence-band holes, and e − CB is the conduction-band electrons. It is known that active oxygen and radical species existing in the presence of oxygen and water take part in the oxidation-reduction reaction and destroy decompose organic molecules and bacteria [5].…”

“…e − CB + h + VB → heat (5) where hν is the UV irradiation, h + VB is valence-band holes, and e − CB is the conduction-band electrons. It is known that active oxygen and radical species existing in the presence of oxygen and water take part in the oxidation-reduction reaction and destroy decompose organic molecules and bacteria [5]. The nano-photocatalysts must be harmless; their concentration in water must be lower than permissible level; and nanoparticles must form agglomerates during required time, coagulate and precipitate, i.e., must form safe ordinary particles.…”

The Active Sites Engineering of Catalysts for CO2 Activation and Conversion

“…The valence band (VB) maximum of photocatalyst needs to be higher than the oxidation potential of H 2 O (0.82 V vs NHE) and the conduction band (CB) minimum must be more negative than the reduction potential of target products, e.g., −0.53 V (CO/CO 2 ), −0.24 V (CH 4 /CO 2 ), and −0.38 V (CH 3 OH/CO 2 ). [ 35 ] On the other hand, the efficiency and selectivity of CO 2 conversion are closely connected with the dynamics of whole redox reactions. The reduction of H + is a competitive reaction to consume electrons on the surface of photocatalyst, resulting in the decreased CO 2 conversion efficiency.…”