Photo-thermal Catalytic Performances Of Supported Pt-based Nanocatalysts

At the "Nobel Laureates Beijing Forum 2007",several Nobel laureates and scientists in the world considered that energy and environmental issues are not only maj or issues in China’s current economic development,but also a common challenge for all countries around the world.The rapid consumption of fossil fuels has caused a global energy crisis,while leading to a sharp rise in atmospheric CO2 concentration,resulting in the phenomenon of global warming,which has brought about ecological and environmental problems such as melting glaciers,rising sea levels,and northern shift of climate zones.In addition,volatile organic compounds(VOCs)in office supplies,decoration materials and other indoor environments have also pose huge threats to human health and the environment.Photo-thermal catalysis has been widely studied due to its low activation energy,mild conditions and energy saving.Otherwise,the choice of suitable catalysts is pivotal for photo-thermal catalysis.Pt-based catalysts are widely performed in CO2 reduction and VOCs degradation due to their high catalytic activity at low temperatures.Pt nanoparticles are often loaded on semiconductor materials and become electron capture centers in the photo-thermal catalytic process due to their low Fermi level,thus promoting the separation efficiency of photo-generated electron-hole pairs and providing conditions for the subsequent redox reactions.In addition,Pt nanoparticles have excellent photothermal effect,which can convert light energy into heat energy under light illumination,thus enhancing the local temperature and promoting the catalytic reaction.However,economically unprofitable cost and scarcity of noble metals greatly limit their practical applications,so exploring the reduced use of noble metals has arisen as an effective approach for solving this problem.Based on Pt-based catalysts,this paper explores the application value of Pt in CO2 reduction,product selectivity of CO2 hydrogenation and toluene degradation by adjusting the loading and size of Pt and compounding transition metals,which promotes the decrement application of Pt.Besides,we further investigate the catalytic mechanism.The main research contents are as follows:(1)Pt nanoparticles with different mass fractions loaded on LaCoO3 were prepared by sodium borohydride reduction method.The effect of Pt loading of Pt/LaCoO3 catalyst on the performance of photo-thermal catalytic reduction of CO2 was investigated in the flow system,and the CO2 hydrogenation mechanism was revealed.The characterization results exhibited that Pt nanoparticles were in the size of 2-4 nm and uniformly dispersed on LaCoO3 which could broaden the light absorption range of the catalysts thus effectively utilize visible light.The highest conversion rate of CO2 with 61%under visible light irradiation at 250℃ was achieved when the Pt loading was 0.6 wt.%,and the selectivity of methane was at 87%with a methane yield of 119.8 mmol gcat-1 h-1.We found that either too high or too low loading of Pt was not conducive to the increase of CO2 conversion rate,probably because catalysts with too low loading have fewer active sites,while higher loading has a certain degree of agglomeration of nanoparticles and a decrease of acive sites.Therefore,it is important to optimize the Pt loading to improve the catalytic reaction efficiency.In addition,we investigated the mechanism of the reaction by in situ DRIFT test and we found that both elevated temperature and the introduction of light during CO2 hydrogenation all favored the formation of CH4,while the conversion of intermediates(*HCOO and*CO)to CH4 could be further accelerated after the introduction of light.(2)Pt nanoparticles with different sizes(4.7 nm,5.5 nm,6.1 nm,8.4 nm,9.9 nm)were modulated by formamide system and loaded on TiO2 to form the nanocomposites and we investigated the effect of different sizes of Pt nanoparticles on the performance of photothermal catalytic CO2 reduction.The regular size and activity of Pt nanoparticles were ascribed to volcano-type law.Pt/TiO2 catalyst with Pt nanoparticles of 5.5 nm showed the best performance under full spectrum light at 300℃ with a CO2 conversion rate of 49.6%.CO and CH4 yields was 95.1 mmol gcat-1 h-1 and 21.3 mmol gcat-1 h-1,respectively,indicating the size effect of Pt nanoparticles.The UV-Vis diffuse reflectance spectra and photo-to-thermal characterization results revealed that Pt/TiO2 catalyst with Pt nanoparticles of 5.5 nm could utilize the light source more effectively and had a relatively high local elevated temperature under light irradiation,indicating its higher photo-thermal catalytic CO2 hydrogenation performance.In addition,it was demonstrated by in situ DRIFTS test that*HCOO and*CO was the main intermediate product of Pt/TiO2 for the generation of CO or CH4 in the thermal catalytic process at 200℃ and*HCOO was the main intermediate product after the introduction of light.In addition,the introduction of light could accelerate the conversion rate of the intermediate product*HCOO to CO.(3)Pt-Cu/TiO2 catalysts with different ratios were prepared by sodium borohydride reduction,and the presence of PtCu solid solution alloy was demonstrated by HAADF-STEM,XPS and EDS tests.Compared with Pt0.5wt.%-TiO2 catalyst(T80=100℃),Pt0.4wt.%Cu0.1wt.%TiO2 catalyst was more efficient in the photo-thermal catalytic degradation of toluene(T80=77℃),and Pt0.4wt.%Cu0.1 wt.%-TiO2 catalyst could completely degrade toluene at 110 ℃.The greatly enhanced activity could be ascribed to the synergistic effects arising from photothermal effects of the high dispersion of PtCu alloy nanoparticles,intimate interaction between the metal elements and the effective separation of photo-generated carriers in TiO2.In addition,the thermocatalytic conversion of Pt0.4wt.%Cu0.1wt%-TiO2 catalyst at 110℃ was only 0.38%,while the photocatalytic toluene conversion rate under light alone was 55%.The degradation performance of toluene under photo-thermal catalytic conditions was much greater than the sum of photocatalysis and thermal catalysis,indicating that light and heat had synergistic effects,which further illustrated the promising application of photo-thermal catalytic system.