A high-purity gas-solid photoreactor for reliable and reproducible photocatalytic CO2 reduction measurements.

Reactions between a gas phase and a solid material are of high importance in the study of alternative ways for energy conversion utilizing otherwise useless carbon dioxide (CO2). The photocatalytic CO2 reduction to hydrocarbon fuels like e.g., methane (CH4) is such a potential candidate process converting solar light into molecular bonds. In this work, the design, construction, and operation of a high-purity gas-solid photoreactor is described. The design aims at eliminating any unwanted carbon-containing impurities and leak points, ensuring the collection of reliable and reproducible data in photocatalytic CO2 reduction measurements. Apart from the hardware design, a detailed experimental procedure including gas analysis is presented, allowing newcomers in the field of gas-solid CO2 reduction to learn the essential basics and valuable tricks. By performing extensive blank measurements (with/without sample and/or light) the true performance of photocatalytic materials can be monitored, leading to the identification of trends and the proposal of possible mechanisms in CO2 photoreduction. The reproducibility of measurements between different versions of the here presented reactor on the ppm level is evidenced.

Pivotal Role of Holes in Photocatalytic CO2 Reduction on TiO2.

Evidence is provided that in a gas-solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO2 ) photocatalyst is always detrimental for photocatalytic CO2 reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH4 as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen-evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon-containing products from CO2 reduction are no longer observed. H2 addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO2 . The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO2 reduction. The formation of CH4 from CO2 is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.

Judging the feasibility of TiO2 as photocatalyst for chemical energy conversion by quantitative reactivity determinants.

In this study we assess the general applicability of the widely used P25-TiO2 in gas-phase photocatalytic CO2 reduction based on experimentally determined reactivity descriptors from classical heterogeneous catalysis (productivity) and photochemistry (apparent quantum yield/AQY). A comparison of the results with reports on the use of P25 for thermodynamically more feasible reactions and our own previous studies on P25-TiO2 as photocatalyst imply that the catalytic functionality of this material, rather than its properties as photoabsorber, limits its applicability in the heterogeneous photocatalytic CO2 reduction in the gas phase. The AQY of IrOx/TiO2 in overall water splitting in a similar high-purity gas-solid process was four times as high, but still far from commercial viability.

The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity.

Although the photocatalytic reduction of CO2 to CH4 by using H2O as the oxidant presupposes the formation of O2, it is often not included in the product analysis of most of the studies dealing with photocatalytic CO2 reduction or it is reported to be not formed at all. The present study aims to clarify the absence of O2 in the photocatalytic gas phase CO2 reduction on TiO2. By modifying P25-TiO2 with IrOx co-catalysts it was possible to observe photocatalytic water splitting, i.e. the formation of gaseous O2 and H2 in almost stoichiometric amounts, without the use of sacrificial agents, while bare P25-TiO2 showed no activity in H2 and O2 formation under similar reaction conditions. Investigating the effect of improved H2O oxidation properties on the photocatalytic CO2 reduction revealed that the CH4 formation on P25 from CO2 was completely inhibited as long as the H2O splitting reaction proceeded. Furthermore, we found that a certain amount of O2 is consumed under conditions of photocatalytic water oxidation. A quantification showed it to be in the same order of magnitude as the oxygen which is missing as a byproduct from photocatalytic CO2 conversion. A detailed interpretation of the results in the context of the general understanding of the photocatalytic CO2 reduction with H2O on TiO2 allows the hypothesis that P25-TiO2 undergoes a stoichiometric reaction, meaning that the CH4 formation is not based on a true catalytic cycle and runs only as long as TiO2 can consume oxygen.

Development of a tubular continuous flow reactor for the investigation of improved gas-solid interaction in photocatalytic CO2 reduction on TiO2.

A self-made, low-cost tubular reactor for the gas-phase photocatalytic CO2 reduction was developed. The resulting flow conditions cause an intensive interaction between the reactants in the gas-phase and the fixed bed photocatalyst. This approach is used to test the scalability of tubular reactors for the photocatalytic CO2 reduction.

Au@TiO2 Core-Shell Composites for the Photocatalytic Reduction of CO2.

Au/TiO2 catalysts in different geometrical arrangements were designed to explore the role of morphology and structural properties for the photocatalytic reduction of CO2 with H2 O in the gas-phase. The most active sample was a Au@TiO2 core-shell catalyst with additional Au nanoparticles (NPs) deposited on the outer surface of the TiO2 shell. CH4 and CO are the primary carbon-containing products. Large amounts of H2 are additionally formed by photocatalytic H2 O splitting. Shell thickness plays a critical role. The highest yields were observed with the thickest layer of TiO2 , stressing the importance of the semiconductor for the reaction. Commercial TiO2 with and without Au NPs was less active in the production of CH4 and CO. The enhanced activation of CO2 on the core-shell system is concluded to result from electronic interaction between the gold core, the titania shell, and the Au NPs on the outer surface. The improved exposure of Au-TiO2 interface contributes to the beneficial effect.

Identification and exclusion of intermediates of photocatalytic CO2 reduction on TiO2 under conditions of highest purity.

Using a high-purity gas phase photoreactor and highly sensitive trace gas analysis, new insights into the mechanism of photocatalytic CO2 reduction on TiO2 P25 have been obtained. The reactor design and sample pretreatment excludes product formation from intermediates. Apart from CO2, the only other reactant offered to the catalyst is water. The main products found on this prominent photocatalyst are methane and carbon monoxide. To distinguish between the three possible mechanisms reported in previous studies, likely intermediates of the reaction were added to the TiO2 photocatalyst and their reactivity was followed by gas chromatographic analysis. Based on the results, we can clearly rule out CO as intermediate of any photocatalytic reaction pathway on TiO2, because CO was not converted at all within a course of six hours. An improvement of carbonate formation on TiO2 brought about by surface-doping with sodium decreased product yields, so carbonates are unlikely intermediates as well. Methanol, formaldehyde and formic acid were exclusively oxidized back to CO2. We thus support a mechanism running over C2-intermediates, and we tested our hypothesis by reacting glyoxal, glyoxylic acid, acetic acid and acetaldehyde on TiO2. The reactions of acetaldehyde and acetic acid led to product distributions very similar to those obtained from CO2 under the standard reaction conditions, strongly supporting the C2 mechanism. This mechanism can also explain the small amounts of ethane usually found in the product mixture.