Hydrogenation Of Carbon Dioxide To Carbon Monoxide With Au/CeO₂ Under Photothermal Catalysis

With the development of industry,fossil energy consumption accounts for 70%of total energy consumption,followed by rising global carbon dioxide levels,climate change and ocean acidification.In order to reduce carbon dioxide emissions and cope with energy consumption crisis,how to convert carbon dioxide into valuable chemicals has been becoming one of research hotspots.Hydrogenation of carbon dioxide is one of most useful methods which not only reduces carbon dioxide emissions,but also converts carbon dioxide into value-added products.Conventional thermal catalysis of CO2 reduction generally requires high temperature and high pressure to achieve high conversion.Recently,photothermal catalysis is used as one new idea applied to the reduction of carbon dioxide,in which process light can be transformed into heat activating carbon dioxide,and providing energy for subsequent conversion.However,current works reveal that almost all of the CO2 reduction products are methane.In industrial applications,CO has more application prospects than methane.At present,the conversion and the selectivity of CO converted from CO2 is low at normal temperature and atmosphere pressure.Therefore,how to produce CO with high activity and high selectivity becomes a major problem in the hydrogenation of carbon dioxide.Au catalyst can convert CO2 to CO with high selectivity in the thermal catalysis process,because of the difficulty of hydrogen activation on the surface of gold,the CO2 conversion rate is limited.Here Au/CeO2 was selected as the catalyst to hydrogenate CO2 under photothermal process.In the experiment,due to its highest activity of reducing CO2,0.9%Au/CeO2 was chosen as the optimal loading ratio catalyst to carry out the catalytic reaction.At the same reaction temperature(673 K),the CO2 conversion in the photothermal(λ≥420 nm)process(40%)is 13 times than the thermal process(3.2%),and CO selectivity in both process are nearly 100%,the apparent activation energy in the photothermal process is similar to the thermal process.By comparing the catalytic activity of CeO2 and Au/CeO2 in the photothermal and thermal processes,it was found that Au loading promotes the reduction of CO2,and light can enhance the reduction of CO2 to CO on Au/CeO2.The relation between light and CO2 conversion was measured through controlling the temperature same at 673 K and changing the light intensity,results indicates that with light intensity promoting,CO2 conversion enhanced,and the hydrogen consumption rate was linearly related to light intensity,which showed that the light has a promoting effect on the activation of hydrogen.In the photothermal process,the CO2 conversion depends on the H2 activation at 673 K.H2-TPR results show that Au/CeO2 and CeO2 have weak H2 activation at 673 K,which is in response to the low activity of Au/CeO2 and CeO2 under thermal process.By measuring CO2 and H2 orders of reaction in photothermal and thermal processes,it was found that the photothermal process only changed the H2 reaction order,while the CO2 reaction order was almost constant comparing to thermal process,indicating that photothermal heat promotes the activation of hydrogen then enhance CO2 conversion.The diffuse reflectance Fourier transform infrared spectroscopy results show that Au is conducive to hydrogen activation and the formation of Au-H under photothermal conditions,and CeO2 cannot be excited by visible light(λ≥420 nm)to produce electrons depending on its energy band gap.The results show that hydrogen activation under photothermal process is mainly attributed to gold.Using alumina as a carrier,the ability of Au/Al2O3 activating hydrogen under photothermal conditions also indicates that hydrogen activation is from gold.The formation of intermediates formic species were also monitored in the in-situ IR data pattern.Compared to thermal process,FTIR spectrums showed no changes in the species produced by the photothermal process,and there was no change in the peak species of the C species in the XPS data,indicating thatphotothermal process had not changed the reaction pathway comparing to thermal process,but only accelerated hydrogen activation,which promotes the reduction ofcarbon dioxide,these results are consistent with similar apparent activation energies under photothermal process and thermal process.At present,almost all research papers on photothermal catalysis considered that the photothermal effect is heat that promoting the reduction of CO2.However,in this work,it was found that the hydrogenation of CO2 under photothermal conditions is obviously better than theermal process,in which the analysis of the mechanism and desigh of the catalyst had a certain degree of innovation,providing new ideas for the application of photothermal in the field of CO2 capture and utilization in the future.