过渡金属催化剂的X射线吸收和衍射谱学研究

【作者】 韦正；

【导师】 韦世强；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2007， 博士

【摘要】 燃油加氢精制和汽车尾气处理是现代催化研究的重要领域，是控制环境污染和实现绿色环保的重要措施之一。本文重点利用XAFS技术系统研究了NiB纳米非晶态合金和Pd-Pt／Al₂O₃加氢催化剂以及CoxO／SiO₂的CO催化氧化催化剂的局域结构和界面效应受制备条件、组成和助剂等的影响，并建立其制备、结构和性能三者之间的关系。XRD、EXAFS和XANES等方法研究NiB纳米非晶态合金的原子结构、电子结构和退火晶化行为，确定了在不同制备条件下形成的两种非晶结构。根据Pd-Pt／Al₂O₃催化剂有无氟修饰和H₂S毒物存在时的EXAFS、XANES结果，对Pt L₃边吸收谱进行分析，从Pt的5d轨道空穴数来描述Pt第一近邻壳层的配位情况，研究了负载Pt-Pd二元合金相的原子结构和电子结构。利用XRD、Raman、XPS和XAFS联合研究了不同温度煅烧的CoxO／SiO₂催化剂的局域结构和电子结构特点，解释了较高CO催化氧化活性的来源。具体的研究结果分为如下三个方面：1．NiB纳米非晶态合金采用化学还原沉积法制备NiB纳米非晶态合金。在313K油浴和1 MPa H₂压力下，选用苯加氢反应测试样品的催化性能。改变样品制备温度、反应物pH值、反应物浓度等条件，制备了一系列NiB纳米非晶态合金，虽然ICP表明样品表观元素组成都为Ni₇₀B₃₀，但是它们的苯加氢性能存在显著差异。在273K、293K、313K和333K的不同水浴温度制备的样品中，NiB（313K）样品的苯加氢转化率最高，为33.6％，高于其它样品1倍以上；Ni（CH₃COO）₂溶液pH值为6.1时，苯加氢转化率为33.5％，明显高于低pH值样品；反应物浓度对催化活性没有明显的影响。这些结果表明Ni（CH₃COO）₂溶液pH值为弱酸性、在313K温度下制备出的NiB非晶具有较好的苯加氢活性。SEM和XRD结果表明，NiB初始样品粒径都在20-30nm左右。经退火处理后，XRD结果表明虽然所有样品在673K退火后都表现出同样的晶态Ni特征，但是在573K左右退火时，却存在是否具有Ni₃B中间物相的差别：较低水浴温度（273K、293K）和较低Ni（CH₃COO）₂溶液pH值（pH＜4）制得的NiB非晶态合金样品在退火过程中没有见到明显的晶态Ni₃B中间物相；而较高温度（313K、333K）和较高Ni（CH₃COO）₂溶液pH值（pH=6.1）制备的NiB样品在573K退火后有明显的晶态Ni₃B中间物相生成。这一结果表明制备条件能影响NiB非晶的初始结构。利用原位XAFS进一步研究了NiB（273K）和NiB（313K）样品在不同温度处理后局域结构的演变，发现NiB（273K）样品在498K开始晶化，随着温度的升高，有序性缓慢增强，在598K时晶化基本完成，产物为晶态Ni；而NiB（313K）样品在523K时出现了微弱的晶化特征，温度升高到548K，晶化为晶态Ni₃B，在573K时，晶化产物并没有明显的增多，仍然保持晶态Ni₃B结构，温度升高到598K时，大部分Ni₃B分解生成晶态Ni，623K时，晶化基本完成，产物为晶态Ni。原位XAFS研究结果表明，NiB（273K）和NiB（313K）样品具有不同的热稳定性，其晶化产物的不同直接反应了非晶样品的内部结构的不均匀性及构型差异。XANES计算表明金属Ni在只具有短程序结构并且无序度较大时，Ni K边XANES具有与非晶NiB相似的白线峰特征。NiB（313K）样品边后40eV附近的隆起特征比NiB（273K）样品强，表明其Ni第一近邻壳层中的B原子较多。EXAFS拟合结果也表明，NiB（313K）样品第一壳层Ni-B平均配位为3.0，Ni-Ni配位结构无序为0.20（?），而NiB（273K）样品第一壳层Ni-B平均配位为0.7左右，Ni-Ni配位结构无序为0.24（?），表明NiB（273K）样品中存在Ni和B原子各自的局部富集，元素分布很不均匀，而NiB（313K）样品中Ni和B原子的相互作用比例接近3∶1。以上研究结果表明，较低的反应温度273K和Ni（CH₃COO）₂溶液pH＜4时制得的NiB非晶中Ni和B分别发生明显的富集，Ni的局域结构和非晶态金属Ni相似，其催化活性相对较差；而较高温度（313K）制得的NiB非晶中Ni和B充分混合，Ni的局域结构类似于Ni₃B，这种结构具有较好的苯加氢催化活性。2．Pd-Pt／γ-Al₂O₃催化剂浸渍法制备Pd_0.5Pt_0.6／（Al₂O₃）_98.9及氟作为助剂的Pt_0.5Pt_0.6／F_3.0-（Al₂O₃）_95.9催化剂样品，在573K下用H₂S进行中毒处理，发现加氟后酸性增强的分子筛载体可以提升Pd-Pt催化剂的抗硫性能。通过比较XAFS结果，研究有无加氟Al₂O₃载体上负载的Pt-Pd二元合金催化剂的原子结构和电子结构，以及其与H₂S毒物作用后的结构变化：Pt和Pd原子第一近邻壳层的拟合结果表明，未经氟修饰的催化剂样品，Pt-Pd合金相的结构类似于Pd取代了fcc-Pt中部分Pt位的固溶体，颗粒平均直径为1.9nm，硫中毒后，Pt-Pd的配位峰消失，Pt-S的配位峰出现；经氟修饰的催化剂样品，其Pt-Pt和Pd-Pd的配位数增加，颗粒平均直径为2.7nm，Pd除了和Pt形成固溶体外，还形成了Pd在颗粒表面的局部富集，这种结构的催化剂在有H₂S存在时没有形成可见的Pt-S配位，具有很好的抗硫中毒性能。XANES比较样品的电子结构，表明氟的修饰使得载体酸性增强，Pt与酸性载体的作用较强，呈现缺电子状态，不容易与得电子能力较强的S原子作用，保持了较好的抗硫性能。3．CoOx／SiO₂催化剂浸渍法制备6wt％负载量的CoOx／SiO₂催化剂，并在200-700℃温度下煅烧。连续流动管状微反应器上常压升温测试CO催化氧化活性，结果表明，提高煅烧温度会使得样品催化活性下降，CoOx（200）／SiO₂样品在200℃即能达到90％的转化率，明显好于其它样品是（250-350℃）。XRD、Raman、XPS和XANES结果表明，CoOx（200）／SiO₂催化剂样品表面存在分散性很好的Co²⁺，升高煅烧温度以后，Co²⁺被氧化成Co₃O₄，并伴随发生了表面聚集，导致了催化活性的下降。EXAFS分析发现CoOx（200）／SiO₂样品中存在晶态Co₃O₄和受载体SiO₂作用时Co-O键长不同于晶态CoO的高分散CoO／SiO₂相。研究发现，煅烧温度可以影响Co物种在CoOx／SiO₂催化剂表面的价态和分散性，载体与Co原子之间的较强作用能够使得Co²⁺稳定存在于200℃煅烧的催化剂样品中，形成了高分散的局域结构类似于Co₂SiO₄的CoO／SiO₂，这一物相具有比晶态Co₃O₄更好的催化氧化CO性能。更多还原

【Abstract】 In this thesis, XAFS and XRD are the main tools used to study the local and interface structures of NiB nano-amorphous alloys, Pd-Pt/Al₂O₃ hydrogen catalyst and CoxO/SiO₂ catalyst of CO oxidation. The relationships among preparing conditions, structures and catalysis properties have been established. The annealed behavior, atom structures and electronic structures, studied by XRD, EXAFS and XANES, revealed that two different amorphous structures can be formed under different preparation conditions. The first nearest coordination structures of Pd-Pt/Al₂O₃ and its fluorin promoted state are studied by EXAFS and XANES. The hole number of Pt 5d orbits described the Pt-Pd bi-metallic electronic structure. XRD, Raman, XPS and XAFS are used to study the characters of local structure of CoxO/SiO₂ catalysts in different calcination temperatures and establish the relationship between surface species and catalyst ability.1. NiB nano-amorphous alloyThe series NiB nano-amorphous alloys were synthesized by chemical reduction method. Their catalytic properties are characterized by the reaction of benzene hydrogenation with 10 MPa H₂ at 313K. The results of ICP show the same atomic composition of NiB samples prepared at different temperature, pH and concentration of precursor salt but with different catalyst activity. The NiB（313K） has the highest benzene hydrogenation conversion of 33.6% among all the samples prepared at different temperatures （273 K, 293 K, 313 K, and 333 K）, which is one times higher than others. The NiB with the 6.1 pH value of Ni（CH₃COO）₂ solutions also has the 33.5% conversion, higher than the low pH value ones.The SEM results revealed that all the NiB samples have the similar particle sizes with 20-30nm. Upon annealing at 573K, the XRD results indicated that the NiB（313K） together with NiB（333K） have Ni₃B intermediate product but the NiB（313K） and NiB（333K） samples have not. The NiB sample with 6.1 pH value of Ni（CH₃COO）₂ solution also has the Ni₃B intermediate product. We considered that there may be two kinds of amorphous structures of NiB nano-amorphous alloy.The in-situ XAFS shows that the crystallization of the NiB（313K） is completed in two steps: the generation of intermediate product Ni₃B starting from 548 K and finishing at about 573 K; the unique formation of crystalline Ni from 598 K till 623 K. However, the NiB（273K） crystallizes only in one step with the formation of fcc-Ni in a wide crystallization temperature range beginning from ca. 498 K and ending at 598 K. These indicate that the NiB（273K） and NiB（313K） samples have different initial amorphous structure with different thermal stability. The NiB（313K） is more symmetrical than the NiB（273K） which has broad crystallization temperature range.The XANES spectra of metal Ni considering Debye-Waller factor and 1-8 shells are calculated based on FEFF8.0. While the metal Ni with only one or two shells nearest atoms and higherσ², the Ni K edge XANES spectra is like the NiB amorphous. The hump at 40 eV in the XANES of NiB is attributed to the boron numbers in the first nearest shell of Ni atoms, the higher with the more. The fitting results of EXAFS shown that the Ni-B coordinate number of first nearest shell with NiB（313K） is 3.0, and theσ_s of Ni-Ni shell is about 2.20 A. But the Ni-B coordinate number is only 0.7 with NiB（273K） and theσ_s of Ni-Ni is about 0.24A. As the atomic composition is Ni₇₅B₂₅ It conclude that the NiB（313K） has the Ni₃B like symmetrical atom structure and The NiB（273K） has amorphous Ni like atom structure with poor symmetry accompanied with Ni and B enrichment respectively.In summary, the low preparing temperature of 273K and low pH value （pH<4） is favorable to form the local structure of amorphous Ni with Ni-rich and B-rich regions which have low thermal stability and poor benzene hydrogenation ability. When preparing the NiB nano-amorphous alloy at 313K and normal pH value （pH=6）, the NiB catalyst would be good to benzene hydrogenation.2. Pd-PtAy-Al₂O₃ catalystA series of bi-metallic Pt and Pd catalysts supported on both pristine and fluorine-promotedγ-alumina supports were investigated with x-ray absorption spectroscopy. A combination of the XAFS analysis at both Pt and Pd-edges leads to a model which explains the metal-support interaction as a change in the valence states of the Pt atoms: increase of Pt ionization potential with increasing acidity. This model is based upon an electrostatic interaction induced by changes in charge density of the support oxygen ions. Both structure and electronic properties of bimetallic Pt-Pd catalysts were modified by the change in metal-support interaction caused by the fluorine pretreatment of alumina support. The EXAFS analysis indicated that the formation of a new bimetallic phase favored by the addition of fluorine might be an important factor for this increase in sulfur resistance. The high sulfur tolerance of noble metals supported on acidic supports should be attributed to the electron-deficient characteristic of noble metals resulting from the interaction with acidic support （electron acceptor）.3. CoOx/SiO₂ catalystCoOx/SiO₂ catalysts with 6 wt% loading were prepared by conventional incipient wetness impregnation method on the silica support and subsequent calcination at elevated temperatures. The catalysts were characterized with X-ray diffraction, laser Raman spectroscopy, Brunauer-Emmett-Teller method, XPS, XAFS and their catalytic activities towards CO oxidation were also tested. The catalysts behave a significant activity for CO combustion reaction, and it was found that the activity decreases with increasing calcination temperature. The CO oxidation achieves an 100% conversion at 200℃over the CoOx/SiO₂ catalyst calcined at 200℃（CoOx（200）/SiO₂）. XPS and XAFS results clearly demonstrate the dominance of Co（II） interface with SiO₂ in CoOx（200）/SiO₂. With increasing calcination temperature, Co（II） is oxidized to Co₃O₄, accompanied by the agglomeration, resulting in the decrease of the catalytic activity. This can be explained by the interaction between the cobalt oxide and the silica, together with the dispersion and sintering of the cobalt active species varying with the calcination temperature.更多还原