Study On Preparation And Performance Of Non-noble Metal Modified Layered Perovskite-based Photocatalysts

It’s a perfect route to produce hydrogen from photocatalytic water splitting withphotocatalysts under solar irradiation. In the process, no other extra energy is needed forhydrogen prouduction, and the resultant of the energy supply course of hydrogen combustionis clean water. It can resolve both problems about energy shortage and environmentalpollution, moreover, the natural solar energy is used effectively. For implement of the processsuccessfully, the photocatalyst is the key question, a visible light responsed and high efficientphotocatalyst is necessary for hydrogen generation from water splitting under solar irradiation.At present, the emphases of studies are the response to visible light and high efficiency ofphotocatalysts. In this paper, modified layer perovskite K₂La₂Ti₃O₁₀-based photocatalystswere synthesized, and the photocatalysts were used in the reactions of hydrogen productionfrom water splitting. The sol-gel and hydrothermal preparation methods, the influence ofcalcination means, calcination temperature and calcination time in sol-gel method for catalyticactivity were studied. The photocatalyst was modified with ion doping and cocatalyst loading,the effect of doped ion species, the concentration of doped ion and loaded cocatalyst onphotocatalytic activity was investigated. The sacrificial agent systems were selected also. Therelationship between the band structure of photocatalyst and its photocatalytic activity wasdiscussed, and then the mechanism of the photocatalytic process was proposed. The mainresults are summarized as follows:1. The sample prepared with sol-gel method possess Ruddlesden-Popper typemesoporous layer perovskite structure, the sample has larger porosity and better activity thanthe samples made by other methods. The photocatalyst calcinated in950℃for2hours hascomplete perovskite structure of K₂La₂Ti₃O₁₀, and its photocatalytic activity for watersplitting is highest. It can’t form complete perovskite structure of K₂La₂Ti₃O₁₀whether whenthe calcination temperture is lower than950℃or the calcination time is shorter than2hours,however, too long calcination time leads to sintering of crystal and reduced porosity ofsamples, and then the photocatalytic activity is decreased.2. There were many lattice defects in the sample calcinated with cold plasma, thedefects provided active sites for the recombination of photoelectrons and holes, the photocatalytic activity of sample was lower than the sample treated with conventional hightemperture calcination.3. The doping of transition metal cation could introduce impurity level in the energygap of semiconductor photocatalyst and reduce the needed energy for photoexcitation, thedoped catalysts could response to visible light, and then it was benefit for the enhancement ofphotocatalytic activity. The sample doped with Fe³⁺ion had the highest photocatalytic activity.In the photocatalyst, Fe³⁺ion entered into the lattice and substituted Ti⁴⁺ion partly, pointdefects were emerged, whereas the perovskite crystal structure of matrix had no change at all.4. The doping of Fe³⁺ion was benefit for the completion of （110） crystal face, theintensity of crystalline phase was reduced with the increase of Fe³⁺ion concentration, there’san optimum value of doped Fe³⁺ion, that was Fe:La=0.05:1（molecular ratio）. The excessdoped Fe³⁺ion would form a great quantity of point defect in the crystal, which providedactive sites for the recombination of photo-induced electrons and holes pair and then lead tothe decrease of catalytic activity. After the doping of Fe³⁺ion, the edge of light absorption hada red shift and extended to visible light region, and with the increase of doped Fe³⁺ionconcentration, the degree of light absorption rose.5. The loading of noble metal Pt cocatalyst promoted the separation of charges and thephotocatalytic activity was increased greatly. The photocatalytic activity of samplesynthetized with traditional impregnation followed with H2reduction method was superior tothat of impregnation followed with H2plasma reduction and in situ photodeposition samples.The loading of Pt led to a remarkable reduction on the diffraction peak intensity of sample,which would be created by the cover of Pt on the crystal surface.6. The highest photocatalytic activity of the layer perovskite K₂La₂Ti₃O₁₀-basedphotocatalyst could be obtained in the methanol sacrificial agent system.7. The loading of cocatalyst can promote the separation of photo charges and thenincrease the photocatalytic activity. It was benefit for the transfer and separation of photocharges by loading p-type core-shell structure Ni/NiO（NiO_x） on the surface of matrix andforming p-n junction with n-type K₂La₂Ti₃O₁₀semiconductor, and the photocatalytic activitywas promoted.3wt%NiO_xloaded Fe-K₂La₂Ti₃O₁₀photocatalyst had the highest catalyticactivity for water splitting, the accumulated hydrogen production after6hours reaction could reach to about600μmol/g, a masking effect would take place when excess NiO_xwas loadedon the surface of matrix，the light absorption efficiency decreased and the photocatalyticactivity reduced.8. Two states of Ni0and Ni2+existed in the photocatalyst after the reduction-reoxidationtreatment of NiO loaded photocatalyst, which promoted the seperation of electrons and holesand made for the reduction of H+.9. Both photocatalysts doping with Fe³⁺ion and loading with NiO_xpossessed directband gap, the absorption edge of photocatalysts were extended to visible light region. Theloading of NiO_xintroduced acceptor impurity into the semiconductor matrix, which was infavour of transport of photo-induced electrons and promoted the reduction of H+on thesurface active sites of photocatalyst, the recombination probability of electrons and holes wasreduced.10. The results of XPS characterization showed that the loaded Ni0on the surface wasthe active site for the reduction of H+, NiO may took part in the electrons supplement in thereaction of hydrogen production reduced from H+.11. Two states of surface adsorption oxygen（acidic hydroxyl） and lattice oxygen existedin the photocatalysts, the quantity of lattice oxygen in the photocatalyst increased afterreaction, suggested that the surface adsorption oxygen converted to lattice oxygen possiblyduring the reaction.12. Associated with in-situ FTIR characterization results, in the layer perovskiteK₂La₂Ti₃O₁₀-based photocatalyst, the Ti on the laminate was the active adsorption sites formethanol, the saturation adsorption state of methanol on the photocatalys was free hydroxideradical, water adsorbed on the photocatalyst with surface adsorbed water state, there’sscarcely no strong hydrogen bond exist. After UV irradiation, the photocatalytic reactionhappened, the hydroxide radical of methanol were consumed and oxidated to formaldehydeand up to formic acid, the mechanism of photocatalytic hydrogen generation with methanolsacrificial agent was proposed therefrom. The distinguishing features of this dissertation are as follows:1. The visible light response and effective non-noble metal modified layer perovskiteNiO_x/Fe-K₂La₂Ti₃O₁₀photocatalyst was prepared, the accumulated hydrogen production after6hours reaction could reach to about600μmol/g with the photocatalyst from water splitting.2. The influence of modification with transition metal doping and the forming of p-nconjunction between loaded non-noble metal NiO_xand matrix on the photocatalytic activitywere investigated, the relationship between the modification and the improvement of materialband structure and photophysical property were analyzed, the relationship between the bandstructure of photocatalyst and the catalytic activity was discussed also.3. The adsorbed species and the change of these adsorbed species were studied within-situ FTIR characterization, the adsorbed states of methanol and water on photocatalystsurface and the mechanism of photocatalytic hydrogen generation reaction with methanolsacrificial agent were brought forward.