Photocatalytic Reforming Of Glycerol To Syngas And Formic Acid Over TiO₂ Catalysts

Fossil energy utilization with high carbon emissions aroused people’s concern and attention,cleaner and more sustainable development of biomass resources to replace fossil energy become a consensus.Photocatalytic reforming technology is a mild method which can be used to reform biomass to high value-added fuels and carbon-based chemicals,which provides a new idea for energy storage while relieving environmental and energy pressure.TiO₂is widely investigate in the field of biomass photocatalytic reforming for its advantages of non-toxicity,chemical stability,light corrosion resistance,favorable band position and low price.However,the photogenerated electrons and holes recombine quickly and cannot be efficiently transferred,which seriously inhibits its photocatalytic activity.In this study,glycerol was used as a biomass model molecule and surface-modified TiO₂ was used as catalyst.We enhanced the activity and selectivity of photocatalytic reforming of glycerol to prepare syngas and formic acid by constructing active sites and enhancing the interfacial charge separation.The specific research content is as follows:（1）Cu₂O-TiO₂ nanorods were prepared by alkaline hydrothermal-ion exchange method,and the performance of photocatalytic reforming glycerol to hydrogen was studied.The alkaline hydrothermal method introduced Ti³⁺and V_O on the TiO₂ surface,TiO₂ with the longer hydrothermal time had more defects and more stable tubular structure.With the increase of Cu doping,Cu on the TiO₂ surface mainly existed as Cu²⁺,Cu₂O and Cu O.Cu₂O-TiO₂nanorods were prepared by 6 h hydrothermal time and 0.5 wt%Cu doping amount and had the highest hydrogen precipitation activity.After 300～400 nm UV light irradiation for 6 h,hydrogen precipitation rate of Cu₂O-TiO₂ nanorods were 622.32μmol·g^-1·h^-1,which was 41.5 times of unmodified TiO₂.Cu doping achieved in situ loading of Cu₂O on the surface of TiO₂ and successfully constructed Cu₂O-TiO₂ nanorod heterojunctions,which can broaden the light absorption range of TiO₂ and inhibit the recombination of photogenerated carriers,thus improving the performance of photocatalytic reforming of glycerol for hydrogen precipitation.（2）Cu and N co-doped TiO₂ nanotubes（Cu/N-TNT）were prepared by alkaline hydrothermal-ion exchange methods,and the performance of photocatalytic reforming glycerol solution to syngas（H₂ and CO）was studied.Cu/N-TNT had a tubular structure rich in oxygen vacancies（V_O）.N formed the impurity energy level by substituting the partial O in the form of Ti-N.Cu was introduced into the lattice gap of the catalyst in the form of Cu²⁺.Cu,N co-doping promoted the effective separation of the surface charge,and hence improved the selectivity and activity of photocatalytic reforming glycerol to syngas.CO and H₂ yield on Cu/N-TNT catalyst doped with 0.1%Cu was 7.3 and 8.5 mmol·g^-1,respectively,which was 9.1and 70.8 times of the original TiO₂.The ratio of H₂/CO was increased from 0.52 to 1.18,and the ratio of CO/CO₂ was raised from 0.21 to 0.42 after 8 h of UV irradiation.N and V_O on TiO₂surface provided the active sites to produce CO from decarbonylation of aldehydes and dehydration of formic acid.Cu²⁺in the TiO₂ lattice gap acted as the shallow potential trap to inhibit electron-hole recombination.The mechanism analysis showed that photogenerated holes（h⁺）played a primary role on CO generation,and excessive·OH and·O₂^-favored for the overoxidation of glycerol to CO₂ and decreased the selectivity of CO.（3）TiO₂ with different facets were prepared by hydrothermal method for photocatalytic reforming glycerol to formic acid.The structural composition and optical properties of TiO₂were characterized by SEM,TEM,XRD,XPS,EPR,Raman,UV-vis DRS and PL.The results showed that TiO₂ synthesized via hydrothermal method using titanium isopropoxide as Ti source and hydrofluoric acid as morphology control agent exposed（001）crystalline facets.TiO₂-2F with 2 m L hydrofluoric acid addition coexposed anatase（001）and（101）crystalline facets and demonstrated the highest catalytic activity and selectivity.With UV light irradiation for 4h,the conversion of glycerol over the TiO₂-2F catalyst was 49.0%and the formic acid selectivity was 55.6%.The interface-contacted（001）and（101）facets formed surface heterojunctions improved the active of photocatalytic reforming glycerol to formic acid.（001）facet with oxygen vacancy and coordination unsaturated sites O_2c–Ti_5c–O_2c caused the deep oxidation of glycerol and its intermediates which improved the selectivity of formic acid.Mechanistic analysis showed that·OH and·O₂^-were the key active species for the selective oxidation of glycerol to formic acid.