Stan Golunski

It has previously been shown that an Ag/CZA catalyst can simultaneously remove NOx and soot from an oxygen-rich exhaust gas at low temperatures, by utilising the N₂O generated preferentially during incomplete NOx reduction. Here, we examine the effect of reformulating the catalyst to include potassium, which is a known promoter of soot combustion. On including 2 wt% K, NOx-reduction occurs both in the absence and presence of soot, but the N₂O formed does not play a part in the oxidation of soot. At higher K loadings (5, 10 and 15 wt%), NOx reduction is almost completely disabled, and only contributes to the activity of the catalyst containing 5 wt% K when tested in the presence of soot. At a loading of 20 wt% K, the potassium phase segregates, leaving NO and NH₃ adsorption sites exposed. In the absence of soot, this catalyst can remove NOx by reduction on the Ag/CZA component and through nitration of the potassium phase. Although the presence of potassium lowers the onset temperature for soot oxidation to within the range of NOx reduction over Ag/CZA, the mobile K species prevents the desirable C+N₂O reaction.

The NO and propane oxidation activities of a series of 1%Pt/TiO₂–SiO₂ catalysts show different underlying trends as the support composition changes. Surface characterisation of the catalysts indicates that the trend for NO conversion is consistent with the oxidation rate being dependent on the degree of metallic character of the Pt nanoparticles, rather than their morphology. Although a similar correlation is expected for the total oxidation of propane, it is masked by the effects of adventitious ions originating during manufacture of the support materials. When residual chloride is present in the support, most of the exposed Pt is stabilised in its low-activity ionic form; while support materials containing W or oxidised-S ions give rise to catalysts with much higher activity than expected from their measured Pt⁰ content. When a Cl-containing, but SiO₂-free, TiO₂ support material is pre-treated hydrothermally, the propane-oxidation activity of the resultant Pt/TiO₂ catalyst is substantially improved, so that it matches the performance of highly-metallic Pt supported on TiO₂ containing 16 wt% SiO₂. The hydrothermal pre-treatment removes residual chloride from the support material, but it also leaves the catalyst in a hydrated state. We show that, by controlling the metallic content of Pt nanoparticles, understanding the promoting and inhibiting effects of adventitious ions, and optimising the degree of catalyst hydration, the activity of 1%Pt/TiO₂–SiO₂ catalysts can be made to exceed that of a benchmark 2%Pt/γ-Al₂O₃ formulation for both NO and propane oxidation.

Operando X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) provide complementary information on the catalyst structure, surface reaction mechanisms and activity relationships. The powerful combination of the techniques has been the driving force to design and engineer suitable spectroscopic operando reactors that can mitigate limitations inherent to conventional reaction cells and facilitate experiments under kinetic regimes. Microreactors have recently emerged as effective spectroscopic operando cells due to their plug-flow type operation with no dead volume and negligible mass and heat transfer resistances. Here we present a novel microfabricated reactor that can be used for both operando XAS and DRIFTS studies. The reactor has a glass–silicon–glass sandwich-like structure with a reaction channel (3000 μm × 600 μm; width × depth) packed with a catalyst bed (ca. 25 mg) and placed sideways to the X-ray beam, while the infrared beam illuminates the catalyst bed from the top. The outlet of the reactor is connected to MS for continuous monitoring of the reactor effluent. The feasibility of the microreactor is demonstrated by conducting two reactions: i) combustion of methane over 2 wt% Pd/Al₂O₃ studied by operando XAS at the Pd K-edge and ii) CO oxidation over 1 wt% Pt/Al₂O₃ catalyst studied by operando DRIFTS. The former shows that palladium is in an oxidised state at all studied temperatures, 250, 300, 350, 400 °C and the latter shows the presence of linearly adsorbed CO on the platinum surface. Furthermore, temperature-resolved reduction of palladium catalyst with methane and CO oxidation over platinum catalyst are also studied. Based on these results, the catalyst structure and surface reaction dynamics are discussed, which demonstrate not only the applicability and versatility of the microreactor for combined operando XAS and DRIFTS studies, but also illustrate the unique advantages of the microreactor for high space velocity and transient response experiments.

A range of Cs-doped Fe/Al₂O₃ catalysts were prepared for the ammonia decomposition reaction. Through time on-line studies it was shown that at all loadings of Cs investigated the activity of the Fe/Al₂O₃ catalysts was enhanced, with the optimum Cs:Fe being ca. 1. Initially, the rate of NH₃ decomposition was low, typically < 10% equilibrium conversion (99.7%@500°C) recorded after 1 h. All catalysts exhibited an induction period (typically ca. 10 h) with the conversion reaching a high of 67% equilibrium conversion for Cs:Fe = 0.5 and 1. The highest rate of decomposition observed was attributed to the balance between increasing the concentration of Cs without blocking the active site. Analysis of H₂-TPR and XPS measurements indicated that Cs acts as an electronic promoter. Previously, Cs has been shown to act as a promoter for Ru, where Cs alters the electron density of the active site, thereby facilitating the recombination of N₂ which is considered the rate determining step. In addition, XRD and N₂ adsorption measurements suggest that with higher Cs loadings deactivation of the catalytic activity is due to a layer of CsOH that forms on the surface and blocks active sites. Graphic

A series of layered birnessite (AMn₄O₈) catalysts containing different alkali cations (A = H⁺, Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺) was synthesized. The materials were thoroughly characterized using X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, Raman spectroscopy, specific surface area analysis, work function, thermogravimetry/differential scanning calorimetry, and transmission electron microscopy. The catalytic activity in soot combustion in different reaction modes was investigated (tight contact, loose contact, loose contact with NO addition). The activity in the oxidation of light hydrocarbons was evaluated by tests with methane and propane. The obtained results revealed that alkali-promoted manganese oxides are highly catalytically active in oxidative reactions. In soot combustion, the reaction temperature window was shifted by 195 °C, 205 °C, and 90 °C in tight, loose + NO, and loose contact conditions against uncatalyzed oxidation, respectively. The catalysts were similarly active in hydrocarbon combustion, achieving a 40% methane conversion at 600 °C and a total propane conversion at ~450 °C. It was illustrated that the difference in activity between tight and loose contacts can be successfully bridged in the presence of NO due to its facile transformation into NO₂ over birnessite. The particular activity of birnessite with H⁺ cations paves the road for the further development of the active phase, aiming at alternative catalytic systems for efficient soot, light hydrocarbons, and volatile organic compounds removal in the conditions present in combustion engine exhaust gases.

Ag/CeO₂-ZrO₂-Al₂O₃, a known catalyst for the simultaneous removal of NOx and soot, was modified by the addition of K, and was prepared using various techniques: wet impregnation, incipient wetness, and chemical vapor impregnation at different temperatures. The effect of the preparation method on catalyst activity was studied. It was found that catalysts prepared via wet impregnation, incipient wetness, and chemical vapor impregnation at 80 °C were able to utilize in situ formed N₂O at low temperatures, to simultaneously remove NOx and soot. The difference in preparation method affected the catalyst’s ability to produce and use N₂O as an oxidant for soot. The temperature at which chemical vapor impregnation was performed greatly influenced the catalyst’s ability to oxidize soot. The introduction of K to the Ag/CeO₂-ZrO₂-Al₂O₃ vastly improved the soot oxidation activity, particularly for the catalyst prepared via wet impregnation. However, the incorporation of K had an adverse effect on the reduction of NOx.