Operando potassium K-edge X-ray absorption spectroscopy: investigating potassium catalysts during soot oxidation.

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.

Enhanced Activity and Stability of Gold/Ceria-Titania for the Low-Temperature Water-Gas Shift Reaction.

Gold supported on ceria-zirconia is one of the most active low temperature water-gas shift catalysts reported to date but rapid deactivation occurs under reaction conditions. In this study, ceria-titania was evaluated as an alternative catalyst support. Materials of different Ce:Ti compositions were synthesized using a sol-gel methodology and gold was supported onto these using a deposition-precipitation method. They were then investigated as catalysts for the low-temperature water-gas shift reaction. Au/Ce0.2Ti0.8O2 exhibited superior activity and stability to a highly active, previously reported gold catalyst supported on ceria-zirconia. High activity and stability was found to be related to the support comprising a high number of oxygen defect sites and a high specific surface area. These properties were conducive to forming a highly active catalyst with well-dispersed Au species.

A practical demonstration of electronic promotion in the reduction of ceria coated PGM catalysts.

When ceria is deposited over supported PGM catalysts its reducibility is dependent on the work function of the underlying metal.

Non-lattice surface oxygen species implicated in the catalytic partial oxidation of decane to oxygenated aromatics.

The one-step transformation of C(7)-C(12) linear alkanes into more valuable oxygenates provides heterogeneous catalysis with a major challenge. In evaluating the potential of a classic mixed-metal-oxide catalyst, we demonstrate new insights into the reactivity of adsorbed oxygen species. During the aerobic gas-phase conversion of n-decane over iron molybdate, the product distribution correlates with the condition of the catalyst. Selectivity to oxygenated aromatics peaks at 350 °C while the catalyst is in a fully oxidized state, whereas decene and aromatic hydrocarbons dominate at higher temperatures. The high-temperature performance is consistent with an underlying redox mechanism in which lattice oxide ions abstract hydrogen from decane. At lower temperatures, the formation of oxygenated aromatics competes with the formation of CO(2), implying that electrophilic adsorbed oxygen is involved in both reactions. We suggest, therefore, that so-called non-selective oxygen is capable of insertion into carbon-rich surface intermediates to generate aromatic partial oxidation products.

Enhanced para-xylene selectivity in the toluene alkylation reaction at ultralow contact time.

Dramatic improvements in the para-xylene selectivity of the toluene alkylation reaction can be effected by operating the catalytic reaction at ultralow contact time. Unexpectedly, the rate of alkylation is sustained, while unwanted side reactions are suppressed. By demonstrating that contact time directly influences the fate of para-xylene, which is known to form and diffuse preferentially within the zeolite catalyst, we conclude that external mass transfer is a key parameter in controlling selectivity. Even non-optimized catalysts can be made to achieve near-perfect selectivity, without sacrificing conversion.

Active-site coating for molecular discrimination in heterogeneous catalysis.

A coherent layer of zeolite A has been applied to a SiO2-supported Pt-Fe oxidation catalyst, using total surface charge-reversal. The zeolite has tracked the metals into the pore structure of the support, covering the active sites. The zeolite channel size is large enough to allow access of CO and O2 to the active sites, and to allow CO2 to emerge, but it excludes larger molecules. The presence of the zeolite membrane transforms the supported Pt-Fe into a highly specific catalyst, which can discriminate between CO and butane, even after the macroscopic catalyst particles are crushed.