On Photothermocatalysis With Typical Oxide Photocatalysts

Highly efficient catalysis over gas molecules such as carbon dioxide（CO₂）and volatile organic compounds（VOCs）is an important topic in the field of energy conversion and environmental purification.Photocatalytic technology is promising in a wide range of applications due to its unique advantages such as eco-friendlyness,high-efficiency and energy-saving.The low efficiency of single photocatalytic system has become the bottleneck for further development of photocatalytic technology.Photothermocatalysis is the research frontier in the field of solar photochemistry,which could integrate the benefits of both photocatalysis and thermocatalysis.Analysing the mechanism and designing highly efficient photothermocatalysts are two key areas in photothermocatslysis.In this dissertation,TiO₂ and other typical oxide photocatalysts were used as model catalysts.We explored their applications in photothermocatalytic degradation of VOCs and CO₂ reduction.In-situ photoconductivity measurement was designed to thoroughly analyze the mechanism of photothermocatalysis.Based on the established mechanism,some highly efficient photothermocatalysts were designed.Photothermocatalysis enhances the deep oxidation effect of VOCs.The process of photocatalytic degradation of VOCs tends to be deactivated due to the accumulation of intermediate products and deposition of carbon,which leads to a low efficiency.The first part of this thesis took typical oxide photocatalysts such as TiO₂,WO₃,ZnO as the model catalysts and explored the effective ways to improve their photothermocatalytic efficiency.For TiO₂（ST-01）,the degradation rate and mineralization rate with UV light under 343 K were found to be 2.7 times and 1.4 times higher than that under room temperature（RT）,leading to an increase in the apparent quantum efficiency by 2.67 times.Comprehensively analyses of the photothermocatalytic performance over six kinds of TiO₂ were conducted.It showed that as the specific surface area increases or the particle size decrease,the ratio of mineralization rate of photothermocatslysis increases relative to photocatalysis.For WO₃ and ZnO,the photothermocatalytic mineralization rate at 333 K,in comparison with RT,is increased by 1.8 times and 1.2 times,respectively.With IPA and acetone as model VOCs,the catalytic activity in photothermocatalysis was higher than that in photocatalysis.The above research results indicated that photothermocatalysis is an effective strategy to improve the catalytic deep oxidation of VOCs.The increased activity of surface lattice oxygen under photothermocatalysis is a possible reason for enhanced mineralization properties of acetaldehyde.Furthermore,to improve the reactivity of the surface lattice oxygen,a surface oxygen deficient WO_3-xcatalyst was prepared and showed significantly enhanced photothermocatalytic efficiency.It provides a useful strategy for designing highly efficient photothermocatalyst by surface defect engineering.In the second part,in-situ photoconductive method was developed to explore the photothermocatalytic reaction mechanism.To reveal the intrinsic mechanism of synergistic effect of photothermocatalysis,ST-01 was used as a model catalyst,and the in-situ photoconductivity coupled with a heating field was used to analyze the influence of the reaction atmosphere and temperature on the conductivity.The maximum value of current of ST-01 increased with elevating temperature in nitrogen with ethanol.It indicated that more lattice oxygen reacted with ethanol.In air with ethanol,the maximum value of current decreased under high temperature,which indicated more photogenerated electrons were captured by oxygen.Furthermore,it was confirmed that acceleration of photogenerated electrons transfer to oxygen and enhancement of lattice oxygen activity should be the reason for improved photothermocatalytic degradation of CH₃CHO.The measurement was further used to explore photothermocatalytic reduction of CO₂ process.The results showed the photoconductivity decay at 393 K was faster than RT in CO₂.It indicated that more electrons were involved in reaction under photothermal condition.For oxygen deficient TiO₂,the decay rate at 393 K was slower than RT in nitrogen and faster in CO₂.It indicated that more electrons reacted with CO₂andthe electrons trapped by oxygen vacancies could be excited to conduction band by thermal activation to further react with CO₂,thereby improving photothermocatalytic CO₂ reduction efficiency.Finally,the strategy for designing highly efficient photothermocatalyst for CO₂ reduction was explored.To design a highly efficient photothermocatalyst,TiO₂ nanotube with large specific surface area was used as a model catalyst.A photothermocatalyst modified with oxygen vacancies and co-catalyst was designed.The CO-yield under 393 K was 8.3 times higher than that at RT of pristine TiO₂.The CH₄-yield of modified TiO_2-x/CoO_x was 175.1 times that of pristine sample in photothermocatalysis.High-resolution transmission electron microscopy,electron paramagnetic resonance and photovoltage measurements showed that the synergistic effect of oxygen vacancy and CoO_x lead to the increase of activity.The oxygen vacancy reduced the activation energy of CO₂ and was beneficial for ultra-dispersion of CoO_x.Ultra-dispered CoO_x accelerated the water oxidation rate by capturing holes.The synergistic effect of oxygen vacancy and CoO_x significantly promoted the reduction of CO₂ to methane under photothermocatalysis.