Davies Catherine J, Mayer Alexander, Gabb Jess, Walls Jake M, Degirmenci Volkan, Thompson Paul B J, Cibin Giannantonio, Golunski Stan, Kondrat Simon A
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK.
Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE113TU, UK.
Phys Chem Chem Phys. 2020 Sep 8;22(34):18976-18988. doi: 10.1039/d0cp01227k.
The chemical and structural nature of potassium compounds involved in catalytic soot oxidation have been studied by a combination of temperature programmed oxidation and operando potassium K-edge X-ray absorption spectroscopy experiments. These experiments are the first known operando studies using tender X-rays (∼3.6 keV) under high temperature oxidation reaction conditions. X-ray absorption near edge structure analysis of K2CO3/Al2O3 catalysts during heating shows that, at temperatures between 100 and 200 °C, potassium species undergo a structural change from an initial hydrated K2CO3·xH2O and KHCO3 mixture to well-defined K2CO3. As the catalyst is heated from 200 °C to 600 °C, a feature associated with multiple scattering shifts to lower energy, indicating increased K2CO3 dispersion, due to its mobility at high reaction temperature. This shift was noted to be greater in samples containing soot than in control experiments without soot and can be attributed to enhanced mobility of the K2CO3, due to the interaction between soot and potassium species. No potassium species except K2CO3 could be defined during reactions, which excludes a potential reaction mechanism in which carbonate ions are the active soot-oxidising species. Simulations of K-edge absorption near edge structures were performed to rationalise the observed changes seen. Findings showed that cluster size, unit cell distortions and variation in the distribution of potassium crystallographic sites influenced the simulated spectra of K2CO3. While further simulation studies are required for a more complete understanding, the current results support the hypothesis that changes in the local structure on dispersion can influence the observed spectra. Ex situ characterisation was carried out on the fresh and used catalyst, by X-ray diffraction and X-ray photoelectron spectroscopy, which indicated changes to the carbonate species, in line with the X-ray absorption spectroscopy experiments.
通过程序升温氧化和原位钾K边X射线吸收光谱实验相结合的方法，研究了参与催化碳烟氧化的钾化合物的化学和结构性质。这些实验是首次在高温氧化反应条件下使用软X射线（约3.6 keV）进行的原位研究。对加热过程中K2CO3/Al2O3催化剂的X射线吸收近边结构分析表明，在100至200°C的温度范围内，钾物种经历了从初始的水合K2CO3·xH2O和KHCO3混合物到明确的K2CO3的结构变化。当催化剂从200°C加热到600°C时，与多次散射相关的特征向较低能量移动，这表明由于K2CO3在高反应温度下的迁移率增加，其分散性增强。在含有碳烟的样品中，这种移动比没有碳烟的对照实验中更大，这可归因于碳烟与钾物种之间的相互作用增强了K2CO3的迁移率。在反应过程中，除了K2CO3外，无法确定其他钾物种，这排除了碳酸根离子是活性碳烟氧化物种的潜在反应机制。对K边吸收近边结构进行了模拟，以解释观察到的变化。研究结果表明，团簇尺寸、晶胞畸变和钾晶体学位置分布的变化影响了K2CO3的模拟光谱。虽然需要进一步的模拟研究来更全面地理解，但目前的结果支持了分散时局部结构的变化会影响观察到的光谱这一假设。通过X射线衍射和X射线光电子能谱对新鲜和使用过的催化剂进行了非原位表征，结果表明碳酸盐物种发生了变化，这与X射线吸收光谱实验一致。
