【作者】 吴俊荣；

【导师】 林维明；

【作者基本信息】 华南理工大学 ， 工业催化， 2013， 博士

【摘要】 借助光催化剂，利用太阳能光催化分解水产氢可以同时解决能源短缺与环境污染两大问题，氢能产生过程无需外加能量，氢燃烧供能产物是水，不会造成环境污染，并能利用自然的太阳能，是非常理想的产能途径。要实现该过程，光催化剂是问题的关键，研制出能响应可见光，具有高效光催化分解水制氢催化剂才能有效将太阳能转换为氢能。目前光催化剂研究的重点在于催化剂的可见光响应及高效，本论文制备了改性层状钙钛矿型K₂La₂Ti₃O₁₀基光催化剂，用于光催化分解水制氢反应。考察了催化剂的溶胶-凝胶和水热制备法，溶胶-凝胶法中的焙烧温度，焙烧时间及焙烧方法对催化剂活性的影响；对催化剂进行离子掺杂和助剂负载，考察了掺杂离子种类和浓度以及负载助剂浓度对催化活性的影响；对牺牲剂体系进行筛选；探讨了催化剂的能带结构与催化剂活性间的关系，并研究了光催化过程的机理。1.采用溶胶-凝胶法制备的样品具有介孔Ruddlesden-Popper型层状钙钛矿结构，孔隙度较大，是较优良的光解水制氢催化剂。950℃下焙烧2小时得到的样品具有完整的K₂La₂Ti₃O₁₀钙钛矿晶体结构，光催化分解水活性较高。焙烧温度低于950℃和焙烧时间较短，都不能形成完整的K₂La₂Ti₃O₁₀晶体结构，焙烧时间过长会使样品中晶体烧结，孔隙度减小，催化活性降低。2.冷等离子体焙烧处理，在样品中形成了较多的晶格缺陷，为电子-空穴的复合提供了活性位，样品的催化活性低于传统高温焙烧样品。3.过渡金属阳离子的掺杂，可以在半导体带隙中引入杂质能级，降低产生光生电子所需能量，使催化剂对可见光响应，有利于提高催化剂活性。Fe³⁺离子掺杂的光催化剂活性最高，Fe³⁺离子进入晶格中取代部分Ti⁴⁺离子，在催化剂晶格中产生点缺陷，但不改变基质的钙钛矿晶型结构。4. Fe³⁺离子的掺杂有利于样品（110）晶面的完整，随着Fe³⁺离子浓度的增加，晶相强度逐渐降低，掺杂浓度有一个最佳值Fe∶La＝0.05∶1（分子比），过量的Fe³⁺离子会在晶体中形成大量点缺陷，为电子-空穴的复合提供活性位，导致催化剂活性下降。掺杂Fe³⁺离子后，催化剂的光吸收边红移至可见光区，且随着Fe³⁺离子浓度的增加，对光的吸收程度升高。5.贵金属Pt助剂的负载能促进电荷分离，大大提高催化剂的活性，用传统浸渍-H2还原法负载Pt的催化剂样品活性比浸渍-H2等离子体还原法和在线光沉积法制备样品的活性高。Pt的负载导致样品衍射峰强度显著降低，可能是Pt在晶体表面覆盖造成。6.层状钙钛矿型K₂La₂Ti₃O₁₀基催化剂在甲醇牺牲剂体系中光催化活性最高。7.助剂负载能促进电荷分离，提高催化剂活性。用浸渍法在基质表面负载p-型Ni/NiO核/壳结构（NiO_x），有利于电子的快速传递；与n-型K₂La₂Ti₃O₁₀半导体基质形成p-n异质结，能促进电荷的分离，提高催化活性。NiO_x的负载量有最佳值，3wt%NiO_x负载的Fe-K₂La₂Ti₃O₁₀催化剂光解水活性最高，6h累积产氢量可达约600μmol/g，过多NiO_x的负载会导致催化剂基质表面被过多遮蔽，光吸收表面减少，对光的吸收利用率减小，催化活性降低。8. NiO负载后进行的还原-重氧化处理，使催化剂中存在Ni0和Ni2+两种状态，促进电子-空穴分离，同时有利于H+的还原。9. Fe³⁺离子掺杂与NiO_x负载催化剂具有直接禁带，催化剂的吸收边拓展至可见光区，Fe-K₂La₂Ti₃O₁₀的吸收带边为458nm，NiO_x/Fe-K₂La₂Ti₃O₁₀的带边为434nm。NiO_x的负载在半导体基质上引入受主杂质，有利于光生电子向催化剂表面的迁移，促进H+在表面活性位的还原，减少电子-空穴的复合几率。10. XPS表征结果显示，催化剂表面负载的Ni0是H+还原为H2的表面活性反应位，NiO也可能参与了H+还原产氢反应的给电子过程。11.催化剂中存在表面吸附氧（酸性羟基）和晶格氧两种形态，反应后催化剂中晶格氧数量增加，表明在反应过程中，表面的吸附氧可能转变成了晶格氧。12.原位红外实验显示，层状钙钛矿型K₂La₂Ti₃O₁₀基催化剂中，层板上的Ti是甲醇的活性吸附位，甲醇以自由羟基形式饱和吸附在催化剂上，水在催化剂上以表面吸附水形式存在，基本不产生强烈的氢键。紫外光照射反应后，甲醇羟基被消耗，氧化为甲醛直至甲酸，由此推测甲醇牺牲剂存在时光催化分解水制氢机理。本论文创新之处：1.研制出可见光响应、较高效的非贵金属改性层状钙钛矿型光催化剂NiO_x/Fe-K₂La₂Ti₃O₁₀，用于光催化分解水反应，能有效产氢，6h累积产氢量可达600μmol/g。2.考察了对催化剂进行过渡金属阳离子掺杂及非贵金属NiO_x负载与基质形成p-n结等改性过程对催化剂活性的影响，并分析了改性与催化剂能带结构及光物理性质的改善之间的关系，探讨催化剂的能带结构与催化剂活性间的关系。3.采用原位红外技术在线考察了催化剂上吸附物种的变化，研究了甲醇及水在催化剂表面的吸附情况，并提出甲醇牺牲剂存在时光催化制氢反应的机理。更多还原

【Abstract】 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.更多还原