Rayner Matthew K, Billing David G, Coville Neil J
DST/NRF Centre of Excellence in Catalysis and Molecular Science Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, PO Wits, 2050, South Africa.
Molecular Science Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, PO Wits, 2050, South Africa.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2014 Jun;70(Pt 3):498-509. doi: 10.1107/S2052520614011238. Epub 2014 May 31.
This study focuses on the use of in situ powder X-ray diffraction (PXRD) and quantitative phase analysis using the Rietveld method to monitor the structural properties of a titania-supported iron (10% Fe/TiO2) pre-catalyst during calcination (oxidation) and activation (reduction) in the temperature range 25-900°C. The TiO2 oxidation study revealed an increase in anatase particle size before the anatase to rutile phase transformation, lending credibility to the bridging mechanism proposed by Kim et al. [(2007), Mater. Sci. Forum, 534-536, 65-68]. Pre-catalyst oxidation experiments allowed for the determination of a suitable calcination temperature (450°C) of the pre-catalyst in terms of maximum hematite concentration and appropriate particle size. These experiments also confirmed that the anatase to rutile phase transformation occurred at higher temperatures after Fe addition and that anatase was the sole donor of Ti(4+) ions, which are known to migrate into hematite (Gennari et al., 1998), during the formation of pseudobrookite (Fe2TiO5) at temperatures above 690°C. Using the results from the oxidation experiments, two pre-catalyst samples were calcined at different temperatures; one to represent the preferred case and one to represent a case where the pre-catalyst had been excessively heated. Samples of the excessively heated catalysts were exposed to different reducing gas atmospheres (5, 10 and 100% H2/N2) and heated in the in situ PXRD reactor, so that diffraction data could be collected during the activation process. The results show that reduction with gases containing low concentrations of H2 (5 and 10%) led to the formation of ilmenite (FeTiO3) and we were able to show that both anatase and rutile are consumed in the reaction. Higher concentrations of H2 led to the formation of magnetite (Fe3O4) and metallic iron (Fe(0)). We also noted a decrease in the anatase to rutile transformation temperature under reducing atmospheres when compared with the pre-catalyst heat-treatment experiment. A reduced calcination temperature prior to reduction allowed more facile Fe reduction.
本研究聚焦于使用原位粉末X射线衍射（PXRD）以及采用Rietveld方法进行定量相分析，以监测二氧化钛负载铁（10% Fe/TiO₂）预催化剂在25至900°C温度范围内煅烧（氧化）和活化（还原）过程中的结构性质。二氧化钛氧化研究表明，在锐钛矿向金红石相转变之前，锐钛矿粒径增大，这为Kim等人[（2007年），《材料科学论坛》，534 - 536，65 - 68]提出的桥连机制提供了可信度。预催化剂氧化实验确定了就最大赤铁矿浓度和合适粒径而言预催化剂的适宜煅烧温度（450°C）。这些实验还证实，添加铁后，锐钛矿向金红石相转变发生在更高温度，并且在690°C以上温度形成假板钛矿（Fe₂TiO₅）期间，锐钛矿是Ti(4+)离子的唯一供体，已知Ti(4+)离子会迁移到赤铁矿中（Gennari等人，1998年）。利用氧化实验结果，将两个预催化剂样品在不同温度下煅烧；一个代表优选情况，一个代表预催化剂过度加热的情况。将过度加热催化剂的样品暴露于不同的还原气体气氛（5%、10%和100% H₂/N₂）中，并在原位PXRD反应器中加热，以便在活化过程中收集衍射数据。结果表明，用低浓度H₂（5%和10%）的气体还原会导致钛铁矿（FeTiO₃）的形成，并且我们能够证明在反应中锐钛矿和金红石都会被消耗。更高浓度的H₂会导致磁铁矿（Fe₃O₄）和金属铁（Fe(0)）的形成。我们还注意到，与预催化剂热处理实验相比，在还原气氛下锐钛矿向金红石的转变温度降低。还原前较低的煅烧温度使铁的还原更容易。
