Sven Kureti

Hydrothermal aging significantly affects the performance of Pd-based oxidation catalysts employed in lean-burn exhaust gas aftertreatment systems. In this study, powder catalysts with a Pd load of 2 wt.% were hydrothermally exposed to 650, 750, 850 and 950 °C, followed by comprehensive physicochemical characterization and evaluation of lean CO oxidation activity under steady-state conditions in the presence of H₂O. The catalytic performance was correlated with the number of accessible Pd surface sites quantified by temperature-programmed desorption of H₂ and scanning transmission electron microscopy. A marked loss of accessible Pd surface sites was observed after hydrothermal aging above 750 °C, accompanied by a corresponding decline in CO oxidation efficiency. Steady-state kinetic analysis revealed a progressive increase in the apparent activation energy from 36 kJ mol⁻¹ for the fresh catalyst to 74, 82 and 135 kJ mol⁻¹ after aging at successively higher temperatures. X-ray diffraction and X-ray photoelectron spectroscopy indicated dynamic changes in the Pd oxidation state during CO conversion to CO₂. The increase in activation energy was attributed to the combined effects of an increasing fraction of metallic Pd, Pd particle growth and phase transformations of the alumina support with increasing aging temperature. These findings demonstrate the pronounced structure sensitivity of lean CO oxidation on Pd/Al₂O₃ catalysts and highlight the critical impact of hydrothermal aging at temperatures of 750 °C and above on the physicochemical properties driving catalytic activity.

The gas‐phase reaction of NO and NH ₃ according to the selective non‐catalytic reduction (SNCR) was investigated for the removal of NO _x from the exhaust gas of small‐scale wood gasification boilers. The SNCR process was studied in a specially designed laboratory‐scale flow reactor over a temperature range of 700°C–1050°C. The results demonstrated the practical potential of SNCR for application in wood gasification boilers, achieving a NO _x conversion of 51% at 900°C and a residence time of 0.465 s, while maintaining an NH ₃ slip below 10 ppmv and N ₂ O yields below 3 ppmv. Additionally, a simplified global kinetic model of the SNCR reaction was developed to estimate achievable NO _x conversions and the required reducing agent demand with limited computational effort.

Platinum‐coated cordierite monoliths were evaluated for catalytic oxidation of carbon monoxide and volatile organic compounds (VOCs) in coffee roasting exhaust. Catalysts (3 and 7 g/L Pt) were tested in synthetic flue gas at gas hourly space velocity up to 200 000 1/h. Higher Pt loading lowered light‐off temperature, but above 220°C, both samples showed similar conversions due to external mass transfer limitations. The 3 g/L catalyst was scaled up and installed in a 5 kg drum roaster. At 350°C inlet temperature, conversions reached 96% for carbon monoxide and about 78% for major VOCs such as acetaldehyde, 1,3‐butadiene, and ethane. Stable catalyst operation requires flue gas temperatures of 350°C and above.

The hydrogenation of CO ₂ to CH ₄ was studied over a series of Mg-promoted Fe catalysts containing 2 wt% Mg. The catalysts were synthesized using different preparation methods and Mg precursors to systematically assess the impact of bulk and surface properties on catalytic behavior. Comprehensive characterization was performed using X-ray diffraction, N ₂ physisorption, ⁵⁷ Fe Moessbauer spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H ₂ , temperature-programmed desorption of NH ₃ and temperature-programmed hydrogenation. Structure–activity relations indicate that a homogeneous Mg distribution in the bulk enhances the formation of surface defects and amorphous phases, while improving the reducibility of the fresh catalysts. These characteristics promote the formation of iron carbide and carbonaceous surface species, which are identified as the active phases responsible for CH ₄ formation during CO ₂ hydrogenation. The reverse water–gas shift reaction was found to be a key step driving CO ₂ activation. The catalyst prepared by flame spray pyrolysis using magnesium acetate as precursor exhibited the highest performance. Under industrially relevant conditions (400 °C, 14 bar, 8500 h⁻ ¹ ), a CO ₂ conversion of 68% and a CH ₄ selectivity of 78% were achieved, representing the highest efficiency reported to date for Fe-based catalysts.

The present study deals with the production of sustainable fuels by the methanol to gasoline (MtG) process. As the conversion suffers from catalyst coking, this work investigated the effect of local coke deposits on the MtG performance using H‐ZSM‐5 zeolite extrudates. Spent catalysts were taken from defined sections of an MtG pilot plant after an operation time of 1000 h. The samples were evaluated toward MtG performance and physicochemical properties employing N₂ physisorption, X‐ray diffraction, temperature‐programmed desorption of ammonia and isopropylamine, thermogravimetry, UV/Vis spectroscopy, Raman spectroscopy, and laser desorption and ionization coupled with mass spectrometry. The catalyst characterization showed substantial deposition of polyaromatic hard coke decreasing from 0.129 to 0.046 g/g along the catalyst bed. The strong coking at the inlet was associated with the initially high MtG rate and the recycling of short‐chain olefins, enhancing the presence of coke precursors. The coke deposits blocked active Brønsted acid sites of the zeolite, which decreased from 43 to 15 µmol/g toward the inlet. Consequently, the MtG activity and selectivity were clearly affected by the axial position, with the lowest performance in the inlet section due to loss of hydrocarbon coupling activity of the zeolite.

This study deals with the effect of SO₂ on the soot oxidation activity of flame spray pyrolysis-prepared manganese oxide in gasoline model exhaust. The catalyst was exposed to 15 and 30 ppm SO₂ at 250 °C and was characterized by N₂ physisorption, SO₂-TPD, O₂-TPD, DRIFTS, XPS and PXRD. It was shown that the SO₂ adsorption results in the formation of surface sulfate, while the uptake increased from 26 to 45 μmol/g with growing sulfur content of the model exhaust. The sulfur adsorption reduces the mobility and availability of oxygen on the catalyst thus inhibiting the oxygen transport from gas phase over the catalyst to the contact points of the soot. Consequently, the soot oxidation activity, investigated with tight contact blends of catalyst and soot, decreases with inclining amount of sulfate. Finally, the sulfate species were mostly removed by thermal treatment at 705 °C, which additionally provoked catalyst sintering. As a result, the catalytic performance of the de-sulfated catalyst was slightly lower compared to the sulfated sample.

For NO_xremoval from diesel exhaust, the selective catalytic reduction (SCR) is the most common abatement technology. However, low engine load scenarios such as city driving and cold start phases demand efficient NO_xreduction clearly below 200 °C, which is difficult to achieve with SCR. Hence, this work investigates the potential of the low-temperature NO_xreduction with H₂in diesel exhaust. A monolithic Pt/WO_x/ZrO₂catalyst, recently reported as highly active, was evaluated in synthetic and real diesel exhaust. The monolith demonstrated high deNO_xactivity between 130 and 215 °C in the synthetic exhaust including peak conversion of 90% with N₂selectivity up to 85%. CO/HC components were shown to inhibit the H₂-deNO_xconversion thus requiring a pre-oxidation catalyst in practice. Furthermore, studies performed in an optical reactor indicated strong heat evolution along the monolith. As a result, the reaction kinetics was accelerated with an enhanced consumption of H₂limiting the H₂-deNO_xefficiency above 200 °C. Stationary tests in diesel engine exhaust substantiated the low-temperature H₂-deNO_xperformance of the monolith including NO_xconversions up to 80% at temperatures as low as 135 °C.