Freisetzung von toxischem HCN bei der Stickoxidreduktion mittels NH₃‐SCR in mager betriebenen Erdgasmotoren

Die Reduzierung der Emissionen von Treibhausgasen und gesundheitsgefährdenden Schadstoffen ist eines der wichtigsten Ziele unserer Industriegesellschaft. Erdgasmotoren sind insbesondere bei magerem Betrieb attraktiv, da sie weniger CO$_{2}$ ausstoßen als Verbrennungsmotoren, die mit flüssigen Kraftstoffen betrieben werden. Sie benötigen allerdings ein katalytisches Kontrollsystem, um Stickoxide (NO$_{x}$) aus dem Abgas zu entfernen. Hier haben wir das Zusammenspiel der aktuellen Technologie zur Stickoxidreduktion, die selektive katalytische Reduktion (SCR) mittels NH$_{3}$, und Formaldehyd, einer schädlichen Emission in Erdgasmotoren, untersucht. Bei der NO$_{x}$ Entfernung werden beachtliche Mengen an toxischem Cyanwasserstoff (HCN) gebildet. Alle getesteten Katalysatoren wandeln Formaldehyd teilweise in HCOOH und CO um. Zusätzlich werden Sekundäremissionen von HCN über die katalytische Reaktion von Formaldehyd und dessen Oxidationsintermediaten mit NH$_{3}$ beobachtet. Da die HCN Emissionen mit den derzeitigen Komponenten nicht effizient in schadstofffreie Gase umgewandelt werden können, ist die Entwicklung von Katalysatoren mit einer gesteigerten Oxidationsaktivität nötig, um dieses kritische Problem zu lösen

Chemical gradients in automotive Cu-SSZ-13 catalysts for NOx_{x} removal revealed by operando X-ray spectrotomography

NOx emissions are a major source of pollution, demanding ever improving performance from catalytic aftertreatment systems. However, catalyst development is often hindered by limited understanding of the catalyst at work, exacerbated by widespread use of model rather than technical catalysts, and global rather than spatially-resolved characterisation tools. Here we combine operando X-ray absorption spectroscopy with microtomography to perform 3D chemical imaging of the chemical state of copper species in a Cu-SSZ-13 washcoated monolith catalyst during NO$_{x}$ reduction. Gradients in copper oxidation state and coordination environment, resulting from an interplay of NOx reduction with adsorption-desorption of NH$_{3}$ and mass transport phenomena, were revealed with micrometre spatial resolution while simultaneously determining catalytic performance. Crucially, direct 3D visualisation of complex reactions on nonmodel catalysts is only feasible using operando X-ray spectrotomography, which can improve our understanding of structure-activity relationships including the observation of mass and heat transport effects

The Impact of Pressure and Hydrocarbons on NOx Abatement over Cu- and Fe-Zeolites at Pre-Turbocharger Position

Positioning the catalysts in front of the turbocharger has gained interest over recent years due to the earlier onset temperature and positive effect of elevated pressure. However, several challenges must be overcome, like presence of higher pollutant concentrations due to the absence or insufficient diesel oxidation catalyst volume at this location. In this context, our study reports a systematic investigation on the effect of pressure and various hydrocarbons during selective catalytic reduction (SCR) of NOx with NH3 over the zeolite-based catalysts Fe-ZSM-5 and Cu-SSZ-13. Using a high-pressure catalyst test bench, the catalytic activity of both zeolite catalysts was measured in the presence and absence of a variety of hydrocarbons under pressures and temperatures resembling the conditions upstream of the turbocharger. The results obtained showed that the hydrocarbons are incompletely converted over both catalysts, resulting in numerous byproducts. The emission of hydrogen cyanide seems to be particularly problematic. Although the increase in pressure was able to improve the oxidation of hydrocarbons and significantly reduce the formation of HCN, sufficiently low emissions could only be achieved at high temperatures. Regarding the NOx conversion, a boost in activity was obtained by increasing the pressure compared to atmospheric reaction conditions, which compensated the negative effect of hydrocarbons on the SCR activity

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein we simulate the anharmonic infrared spectrum of adsorbed CO using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. We systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5 nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, we isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction

Direct Observation of Reactant, Intermediate, and Product Species for Nitrogen Oxide-Selective Catalytic Reduction on Cu-SSZ-13 Using <i>In Situ</i> Soft X‑ray Spectroscopy

Catalytic processes have supported the development of myriad beneficial technologies, yet our fundamental understanding of the complex interactions between reaction intermediates and catalyst surfaces is still largely undefined for many reactions. Experimental analyses have generally been limited to investigation of catalyst materials or a subset of functional groups as indirect probes of the critical surface-bound intermediate species and reaction mechanisms. A more direct approach is to probe the intermediate species themselves, but this requires direct study of the local chemical environment of light elements. In this work, we use soft X-ray emission spectroscopy (XES) and a custom-designed in situ reactor cell to directly observe and characterize the electronic structure of reactant, intermediate, and product species under reaction conditions. Specifically, we employ N K XES to probe the interaction of various nitrogen species with a Cu-SSZ-13 catalyst during selective catalytic reduction of nitrogen oxides (NO and NO2) by ammonia (NH3-SCR), a reaction that is critical for the removal of NOx pollutants formed in combustion reactions. This work reveals a novel spectral feature for all spectra measured with flowing NO gas present, which we attribute to the interaction of NO with the catalyst. We find that introducing both NO and O2 gases (compared to only NO) increases the interaction of NO with Cu-SSZ-13. Adsorption of NH3 leads to a more pronounced spectral signal compared to NO adsorption. For the standard NH3-SCR reaction, we observe a strong N2 signal, comprising 30% of the total spectral intensity. These results demonstrate the vast potential of this technique to provide direct, novel insights into the complex interactions between reaction intermediates and the active sites of catalysts, which may guide advanced knowledge-based optimization of these processes