Carbon dioxide reduction to fuels using modified titanium nanocatalysts in monolith photoreactor

Abstract

Carbon dioxide (CO2) is the largest contributor to global warming and its conversion to renewable fuels has stirred interest for greenhouse gas mitigation and energy crises alleviations. The photocatalytic CO2 reduction to fuels is promising, yet existing technologies registered low CO2 reduction efficiency. The main objective of this study was to develop a microchannel system for selective CO2 reduction to fuels. Initially, nanocatalysts were investigated using cell type reactor with methane (CH4) and carbon monoxide (CO) as the main products during CO2 reduction with water vapour (H2O) over indium (In)/TiO2 and montmorillonite (MMT)/TiO2 catalysts. Yield of CH4 over TiO2 was 31.25, enhanced to 244 μmole g-catal.-1 h-1 using 10% In-doped TiO2. Loading MMT evidently enhanced TiO2 performance with CH4 yield rate 441.5 μmole g-catal.-1 h-1. Next, microchannel monolith photoreactor was explored for selective CO2 reduction using H2O and hydrogen (H2) as reducing agents. Yield rate of CO attained was 962 μmole g-catal.-1 h-1 and selectivity 95%. Performance comparison revealed 183 fold higher yield rate in monolith compared to cell type reactor. Significantly higher monolith reactor performance reached using H2 reducing agent and co-metal-doped TiO2 nanocatalysts. Yield rate of CO over nickel (Ni) and In-co-doped TiO2 reached to 12028 μmole g-catal.-1 h-1, higher in order of 1.8 times than copper (Cu)- In/TiO2, 5.93 times than In/TiO2, 207.4 times than TiO2 with performance 902 fold higher than the cell type reactor. Besides, monolith geometry, reaction temperature, feed ratios, metals-content and irradiation time contributed significantly to enhance reactor performances. Quantum efficiency of CO production was 1.04 %, 87 fold higher than reported in literature. Finally, Langmuir-Hinshelwood and kinetic model were developed to investigate adsorption behaviors and photocatalytic oxidation and reduction process, fitted well with experimental data, and assured efficient adsorption-desorption inside microchannels. In conclusion, microchannel monolith photoreactor with modified TiO2 nanocatalysts could make possible markedly higher CO2 reduction to fuels with higher selectivity.