Effect of Metal Cocatalysts and Operating Conditions on the Product Distribution and the Productivity of the CO₂ Photoreduction

Abstract

The CO 2 photoreduction is a promising way to convert one of the most abundant greenhouse gases to valuable chemicals. The photoreduction in the liquid phase is limited by the low solubility of CO 2 in water, but this point is overcome here by using an innovative photoreactor, which allows one to work up to pressures of 20 bar, improving the overall productivity. The photoreduction was performed in the presence of Na 2 SO 3 and using in primis commercial titanium dioxide (P25) and a set of titania catalysts functionalized by surface deposition of either monometallic or bimetallic cocatalysts. The gaseous products were hydrogen and traces of CO, while, in the liquid phase, formic acid/formate, formaldehyde and methanol were quantitatively detected. The pH was observed to shift the products distribution. A neutral environment led mainly to hydrogen and methanol, while, at pH 14, formate was the most abundant compound. The trend for monometallic cocatalysts showed enhanced productivity when using noble metals (i.e., gold and platinum). In order to limit the cost of the catalytic material, bimetallic cocatalysts were explored, adding titania with Au+Ag or Au+Pt. This may open to the possibility of performing the reaction with a smaller amount of the most expensive metals. In the end, we have expressed some conclusions on the cost of the photocatalysts here employed, to support the overall feasibility assessment of the process.

Low Metal Loading (Au, Ag, Pt, Pd) Photo-Catalysts Supported on TiO2 for Renewable Processes

“…3.6.1. Photo-Reduction of CO 2 Figure 6 illustrates some of the most promising photo-catalysts employed for the conversion and valorization of carbon dioxide, which have also been reported elsewhere [41,71,72]. The most active material, at least in terms of formic acid productivity, is the bi-metallic photo-catalyst 1 wt% (Au 2 Ag 8 ), leading to 9.5 mol/kg cat h (±5%) and outperforming the non-promoted titania P25 (3.2 mol/kg cat h (±5%)).…”

Reactor and Plant Designs for the Solar Photosynthesis of Fuels

“…The selectivity can be tuned by changing photocatalyst formulation, e.g., adding a co-catalyst (for instance, Cu 2+ favours the production of methanol, while Au is often reported to favour selectivity to CH4); changing temperature, pressure or pH; creating oxygen vacancies; or doping with non-metals. By optimising these factors, it is possible to tune the selectivity to favour the production of specific desired products [38]. An efficient way to compare the performances of the photocatalysts can be achieved by calculating the total stored energy [39,40].…”

“…The addition of NaOH increases the solubility of CO2 with respect to pure H2O because OH − ions react with the CO2 to produce CO3 2− and HCO3 − . It has been proposed that a high concentration of HCO3 − in the system can accelerate the photoreduction reaction, improving its performance [38,72]. 3.…”

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

“…To overcome these limitations, metal nanoparticles (NPs) are often loaded on the surface of the semiconductor photocatalyst as cocatalysts. Previous studies have reported that metal NPs or metal oxide NPs consisting of platinum, [26] palladium,[ 26l , 26u , 26ab , 26af , 27 ] gold,[ 26l , 26aa , 26ab , 26ac , 26ai , 26aj , 27k , 28 ] rhodium,[ 26z , 26al , 29 ] silver,[ 26l , 26ac , 26ai , 26aj , 26am , 27k , 30 ] ruthenium,[ 26aj , 26ak , 28b , 31 ] copper, –[ 26aj , 27g , 27k , 32 ] nickel,[ 27h , 32n , 33 ] molybdenum, [34] titanium,[ 32x , 35 ] indium, [36] or iridium, [37] can function as CRR cocatalysts. In particular, Pt and PdNPs are used for the hydrogenation reactions because they are favorable for the adsorption and desorption of protons and thereby easy to produce CH 4 .…”