Photoreduction of CO2 at high pressure: effect of co-catalysts and conditions

PHOTOREDUCTION OF CO2 AT HIGH PRESSURE: EFFECT OF CO-CATALYST AND CONDITIONS Gianguido Ramis1*, Francesco Conte2 and Ilenia Rossetti2 1 DICCA, Università degli Studi di Genova, via all’Opera Pia 15A, 16100 Genova, Italy 2 Chemical Plants and Industrial Chemistry Group, Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano (MI), Italy *gianguidoramis@unige.it Introduction Several studies on CO2 photoreduction have already been carried out [1–3]. They usually consist of the use of a photocatalyst that allows CO2 to react under milder conditions than those needed through thermo-catalytic activation. Unfortunately, the reaction has some criticisms, which limit its applicability at the point that currently no fully feasible solution exists. In particular, the need to reduce charge recombination rate and the limited solubility of CO2 in water are the main issues. In this work we addressed the former issue by deposition of metallic (Cu, Ag, Au, Pt) co-catalysts over TiO2, while the latter was improved through the increase of the operating pressure up to 20 bar and tuning the pH. Materials and Methods P25 commercial TiO2 by Evonik was used and added with different co-catalysts either by impregnation or by deposition-precipitation with loading up to 1 wt%. Bimetallic alloys were also used as co-catalysts (AuxAgy 1%wt/P25 and AuxPty 1%wt/P25), prepared by colloidal-immobilization synthesis. The materials were characterised by XRD, N2 adsorption and desorption, DR-UV-Vis spectra. Photocatalytic activity tests were conducted using an innovative pressurized batch photo-reactor [4] , 1.3 L capacity, operating up to 20 bar and 90 °C. Irradiation was done with a medium pressure 125 W Hg vapour lamp emitting in the range of 254-364 nm with measured irradiance 120 W/m2 in the UVA range. Na2SO3 was used as hole scavenger (HS) and negligible CO2 photoreduction has been observed without its addition. After its complete conversion H2 started forming due to photoreforming of the produced organic products. Testing was accomplished for 3-24 h, analysing the liquid and gas products by HPLC and GC. HS conversion was determined by means of iodometric titration. Results and Discussion The overall CO2 conversion increased at basic pH, with a shift from gaseous towards liquid products (mainly formate). A possible explanation of the enhanced productivity, independently from the kind of products obtained, is that a basic solvent improves CO2 solubility and leads to formation of CO32- and HCO3-, which may be subsequently reduced to formaldehyde or formate. The back oxidation of HCHO to HCOOH is favored ad basic pH, while the reduction of HCHO to methanol is easier at lower pH. In addition, the liquid organic products formed can act themselves as hole scavengers with production of CO2 and H2 when the sulphite is fully consumed.At pH=14 the energy content of the products, based on the heating value (HV), is greatly improved with respect to pH=7, since the formic acid has a lower HV than methanol, but this is counterbalanced by far by the higher productivity. These results corresponded to 0.22 or 0.40% at pH = 7 and 14 based on the measured irradiance. The pressure of CO2 manly impacts on the overall productivity when working at basic pH (=14). Operating at higher pressure allows to increase the storing energy efficiency at pH 14 increasing from 0.05% to 0.4% when passing from 8 to 18 bar. As for co-catalyst addition, the best results were achieved with bi-metallic formulations, in particular with 1% Au2Ag8/P25 (H2 productivity was 20 mol/h kgcat and for HCOOH 26 mol/h kgcat). Significance The photoreduction of CO2 has been studied operating under different conditions and investigating the effects of catalyst formulation. A comparison between different techniques for loading the co-catalyst (wet impregnation, deposition-precipitation) was done as well. Significantly higher productivity than the state of the art was achieved by operating at pressure up to 20 bar. pH was found to play a major role in the product distribution. On one hand, neutral pH seems to favour the production of hydrogen and methanol, while basic pH enhances the conversion and the productivity of formic acid, besides hydrogen. The latter is due to the full consumption of the sulphite used as hole scavenger. Indeed, tests with limited HS conversion overperformed the results at each pH tested, preventing the consumption of the organics formed in liquid phase and leading to negligible gas phase products. Moreover, the productivity increases along with pressure, especially at basic pH, due to the increase of reactants concentration in the liquid phase. Among the mono-metallic promoted catalysts, the ones loaded with Au and Pt gave larger improvement in terms of productivity with respect to bare P25, due to the efficient electron drainage, which prevents the electron-hole recombination. In contrast, Ag and Cu deposition produced smaller benefits even if Cu may be interesting for photoreduction under solar light as its band gap is only 2.71 eV and its cost is negligible in catalyst formulation. Bi-metallic catalysts led to very promising results, in particular Au2Ag8 1%wt/P25, and may help with reducing the costs of catalyst combining valuable metals with less expensive ones through a considerable improvement of productivity. References 1. Indrakanti, V.P., Kubicki, J.D., Schobert, H.H. Energy Environ. Sci. 2, 745 (2009). 2. Liu, L., Li, Y. Aerosol Air Qual. Res. 14, 453 (2014). 3. Schneider, J., Matsuoka, M., Takeuchi, M., Zhang, J., Horiuchi, Y., Anpo, M. Chem. Rev. 114, 9919 (2014). 4. Galli, F., Compagnoni, M., Vitali, D., Pirola, C., Bianchi, C.L., Villa, A., Appl. Catal. B  200, 386 (2017).