| There is an urgent need to adjust the energy structure and develop clean and renewable energy in the context of resource scarcity and environmental degradation.Photocatalysis is an effective way to alleviate the above problems by enabling the production of clean fuels and the degradation of pollutants under the irradiation of abundant renewable solar energy.Cellulose is the main components of forest biomass,its hydrolysate,glucose,is a potential substrate employed in photoreforming to co-produce clean energy and high value-added chemicals.To realize the high-value utilization of forest biomass derivatives,a series of TiO2-based heterogeneous composites were constructed.With glucose as model substrate,the photoreforming activity and mechanisms of TiO2-based heterogeneous composites for the co-production of H2 and value-added chemicals（arabinose,formic acid,and acetic acid）were investigated.This work shed light on the in-depth development and application of TiO2-based heterojunction composites in the field of biomass photoreforming.To achieve sufficient contact between the catalyst and the biomass macromolecules,3D hierarchical urchin-like TiO2 microspheres（THM）were prepared by solvothermal method.The formation process of THM was studied by various Ti Cl4 concentration-,reaction time-,and temperature-dependent reactions,which may follow the process of"nucleation-dissolution and recrystallization-self-assembly".The selective adsorption of Cl-on rutile（110）plane promoted the anisotropic growth of nanorods along the[001]direction.This was confirmed by control experiment with other titanium source as precursor.Compared with commercial P25,the glucose adsorption and photoreforming activity of THM was significantly improved.The H2 evolution efficiency of THM in glucose solution was 2.64 mmol g-1 h-1,which was 26.40 times that of P25(0.10 mmol g-1 h-1),with Pt as cocatalyst.The advantages of 3D hierarchical urchin-like microspheres in promoting mass transfer were further confirmed by glucose and THM concentration-dependent experiments.To further improve the photoabsorption and the separation of photogenerated carriers of THM,THM modified by Au NPs of different sizes were synthesized by solvothermal and impregnation reduction methods（The samples were labeled as LAu/THM and SAu/THM,respectively,when the diameter of Au NPs was 59～82 nm and 2～5 nm）.Photochemical and simulation analyses illustrated that the photoabsorption and separation of photogenerated carrier of Au/THM were significantly improved compared with that of THM.Moreover,the intimate contact between SAu and THM promoted the light absorption and electron transfer of SAu/THM,which was obviously better than that of LAu/THM.SAu/THM demonstrated good H2 production,glucose conversion,formic acid and arabinose selectivity of 6.03 mmol g-1 h-1,93.03%,12.55%,and 79.62%,which were 1.26,1.03,1.12,and 1.17 times than that of LAu/THM,respectively.Mechanism analysis showed that·O2-was the main active oxygen species in glucose photorefinery.Glucose was first converted to gluconic acid by·O2-.Gluconic acid was then decarboxylated by C1-C2α-cleavage to form arabinose and formic acid.In response to the poor selectivity of arabinose in SAu/THM,THM modified with bimetal Ni NPs and Au NPs（Ni Au/THM）were prepared by impregnation reduction method.The diameter of Au NPs and Ni NPs were about 5 nm,and both were deposited on THM nanorods.The UV-Vis diffuse reflectance spectrum of Ni Au/THM was relatively consistent with that of Au/THM,indicating that no alloy formed between Au and Ni NPs,and they are relatively close but independent to each other.Based on the potential of Ni NPs in cleaving C-C bonds and the LSPR effect of Au NPs,the H2 production rate,glucose conversion,arabinose and formic acid selectivity of Ni Au/THM from glucose photoreforming were 6.39 mmol g-1 h-1,95.08%,36.54%,and 34.65%,respectively.Ni Au/THM also showed good photoreforming activity against other biomass-based monosaccharides,disaccharides,and polysaccharides.For purpose of reducing the agglomeration of TiO2 nanoparticles（NPs）and the recombination of photogenerated charge carriers in it,Cd S/TiO2/biochar（BC）heterojunction（Cd S/TiO2/BC）were fabricated by combining solvothermal and calcination.Cd S nanorods and TiO2 NPs were uniformly loaded on the surface of lamellar BC,providing sufficient reactive sites for glucose photoreforming.Cd S/TiO2/BC showed excellent photoreforming activity,with12.77 mmol g-1 h-1 of H2 generation rate,96.59%of glucose conversion rate,and 63.94%of acetic acid selectivity,respectively,with Pt as the cocatalyst.The samples displayed good cycling stability and structure stability.Radical capture experiments exhibited that·O2-and·OH were the main reactive oxygen species in glucose photooxidation.In an attempt to regulate the species of active free radicals in glucose photoreforming,Cd S/（001）plane exposed TiO2 nanosheets direct Z-Scheme heterojunction（Cd S/TiO2）was constructed by solvothermal method.Fluorescence spectroscopy confirmed that the·OH concentration produced by（001）crystal-exposed TiO2 nanosheets was much higher than that of TiO2 with（101）-exposed.Cd S/TiO2 showed excellent H2 production activity and acetic acid selectivity.The H2 generation rate,glucose conversion,acetic acid yield and selectivity over Cd S/TiO2 reached 7.08 mmol g-1 h-1,76.81%,381.50 mg L-1 and 74.58%,respectively.The generation of·OH radical with mild oxidizing ability in Cd S/TiO2 promoted the improvement of acetic acid selectivity.The photoconversion path of glucose to acetic acid could be summarized as follows:glucose was first isomerized to fructose under alkaline conditions.Fructose was then converted to glyceraldehyde and 1,3-dihydroxyacetone by retro-aldol reaction under the oxidation of·OH and·O2-.Finally,the generated glyceraldehyde and 1,3-dihydroxyacetone were dehydrated to form pyruvaldehyde,which was then oxidized to lactic acid and acetic acid.This study provides a novel insight for the design of photocatalysts with high liquid phase product selectivity. |
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