Liquid vs. Gas Phase CO2 Photoreduction Process: Which Is the Effect of the Reaction Medium?

Abstract

Abstract:The use of carbon dioxide, the most concerning environmental issue of the 21st century, as a feedstock for fuels productions still represents an innovative, yet challenging, task for the scientific community. CO 2 photoreduction processes have the potential to transform this hazardous pollutant into important products for the energy industry (e.g., methane and methanol) employing a photocatalyst and light as the only energy input. In order to design an effective process, the high sustainability of this reaction should be matched with the perfect reaction conditions to allow the reactant, photocatalyst, and light source to come together: therefore, the choice of reaction conditions, and in particular its medium, is a crucial issue that needs to be investigated. Throughout this paper, a careful study of carbon dioxide photoreduction in liquid and vapour phases are reported, focusing on their effect on catalyst performances in terms of light harvesting, productivity, and selectivity. Different from most papers in the literature, catalytic tests were performed under extremely low light irradiance, in order to minimise the primary energy input, highlighting that this experimental variable has a great effect on the reaction pathway and, thus, product distribution.

Sustainable Carbon Dioxide Photoreduction by a Cooperative Effect of Reactor Design and Titania Metal Promotion

“…Finally, bands at 1689, 1405, and 1202 cm −1 , with almost the same intensity for both samples, due to bicarbonate species produced by reaction of CO 2 with some basic -OH groups, are observed [80,94]. A component at about 1730 cm −1 , more evident in the case of Au-TiO 2 and tentatively assigned to carboxylate species, is also detected [95].…”

“…Moreover, on the CuO-TiO 2 catalyst, the produced species are slightly more stable to the outgassing at r.t. than on Au-TiO 2 , as revealed by the comparison between the initial intensity (bold curves) and final intensity (fine curves) of the bands related to each sample. Finally, bands at 1689, 1405, and 1202 cm −1 , with almost the same intensity for both samples, due to bicarbonate species produced by reaction of CO2 with some basic -OH groups, are observed [80,94]. A component at about 1730 cm −1 , more evident in the case of Au-TiO2 and tentatively assigned to carboxylate species, is also detected [95].…”

“…Moreover, the product distribution is definitely shifted towards methane; as a matter of fact, selectivity to methane is 95% for both samples. This can be explained by remembering proposed reaction mechanism in gas phase in similar conditions [80]; by combining experimental data with spectroscopic evidences from literature, it was hypothesised that fast deoxygenation drives selectivity towards methane, thus avoiding the production of oxygentated compounds. However, in literature several reaction mechanisms for methane and oxygen formation from carbon dioxide and water are reported, which means that stoichiometry for this process has not been unanimously established yet and non-stoichiometric reaction cannot be excluded.…”

Photoreduction of a Pd-Doped Mesoporous TiO2 Photocatalyst for Hydrogen Production under Visible Light

“…Until today, synthesized titanium dioxide (TiO 2 ) has been the most frequently used semiconductor due to its oxidation and chemical resistance properties, accessibility, and affordability [21]. TiO 2 contains three allotropic phases: anatase, rutile, and brookite [22]. However, anatase appears to be the most desirable phase for photocatalytic hydrogen production [23].…”

High Pressure Photoreduction of CO₂: Effect of Catalyst Formulation, Hole Scavenger Addition and Operating Conditions

“…The photoreduction reaction involves multiple protoncoupled electron transfer reactions and can lead to the formation of many different products, either in liquid phase: HCOOH, HCHO, CH3OH or gas phase: H2, CO, CH4, depending on the reaction pathways, which makes this process rather complex (Scheme 1). A comparative study has been also carried out between the reaction in gas or liquid phase, the latter being the most promising [26] and leading to a promising rout for the storage of solar energy in form of organic molecules [27].…”