CO₂ photoconversion to fuels under high pressure: effect of TiO₂ phase and of unconventional reaction conditions

Abstract

Artificial photosynthesis is an up to date topic for the fixation of CO2 in form of valuable fuels. Recently, we set up an innovative concept of photoreactor to be applied to CO2 photoreduction, enabling operation up to 20 bar. This allows to improve CO2 solubility and to explore unconventional operating conditions for this very challenging reaction.Following a preliminary communication, to demonstrate the validity of the approach systematic testing has been here carried out on different TiO2 photocatalysts (anatase, rutile and anatase+rutile) doped with 0.1 wt% Au, by tuning pressure, temperature and pH.The operating conditions have been selected to maximise the production of gas phase products, i.e. CH4 + H2, with respect to liquid phase organic compounds. The present results overperform many of the recent achievements reported in the literature and throw the basis for a further optimisation of the photocatalytic material and of the photoreactor.

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

“…The productivity of the main products, HCOOH and H2, increased in basic pH in fair agreement with previous observations [24,32]. Increasing the pH, improves the CO2 solubility by forming CO or HCO , which further reduced to HCOOH or HCHO in a series of subsequent reactions (Scheme 2).…”

“…The photoreduction of CO2 may lead to a broad spectrum of products depending on photocatalyst formulation and reaction conditions, due to occurrence of many parallel and consecutive reaction steps [24,25,42].…”

“…The internal capacity of the reactor is ca. saturation pressure of 7 bar and temperature of 80 ˚C, based on previous studies [24,32]. Testing was carried out under the same conditions, if not else specified.…”

“…We have already reported an innovative high pressure photoreactor, operating up to 20 bar [24,25] to successfully improve CO2 solubility. 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).…”

Chemical reaction engineering, process design and scale-up issues at the frontier of synthesis: Flow chemistry

“…The use of transparent materials is crucial to fabricate suitable photoreactors, which is not a trivial task, especially when working under unconventional conditions (e.g. under high pressure) [131,132]. The reactor material should allow for efficient photon transport from the light source to the reaction medium.…”