Thermal ageing phenomena and strategies towards reactivation of NO x - storage catalysts

The thermal ageing and reactivation of Ba/CeO2 and Ba/Al2O3 based NO x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl2O4 and BaCeO3, which lower the storage activity, are formed during ageing above 850°C and 900°C, respectively. Interestingly, the decomposition of BaCeO3 in an atmosphere containing H2O/NO2 leads again to NO x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO2 based model catalysts. This can be technically exploited, particularly for the Ba/CeO2 catalysts, as reactivation studies on thermally aged Ba/CeO2 and Ba/Al2O3 based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantl

Emission of Toxic HCN During NOx Removal by Ammonia SCR in the Exhaust of Lean-Burn Natural Gas Engines

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment. Natural gas (NG) engines, in particular at lean conditions, emit less CO$_{2}$ in comparison to combustion engines operated with liquid fuels but NG engines still require emission control devices for NO$_{x}$ removal. Using state‐of‐the‐art technologies for selective catalytic reduction (SCR) of NO$_{x}$ with NH$_{3}$, we evaluated the interplay of the reducing agent NH$_{3}$ and formaldehyde, which is always present in the exhaust of NG engines. Our results show that a significant amount of highly toxic hydrogen cyanide (HCN) is formed. All catalysts tested partially convert formaldehyde to HCOOH and CO. Additionally, they form secondary emissions of HCN due to catalytic reactions of formaldehyde and its oxidation intermediates with NH$_{3}$. With the present components of the exhaust gas aftertreatment system the HCN emissions are not efficiently converted to non‐polluting gases. The development of more advanced catalyst formulations with improved oxidation activity is mandatory to solve this novel critical issue

Catalytic co oxidation and H2_{2}O2_{2} direct synthesis over pd and pt-impregnated titania nanotubes

Titania nanotubes (TNTs) impregnated with Pd and Pt nanoparticles are evaluated as heterogeneous catalysts in different conditions in two reactions: catalytic CO oxidation (gas phase, up to 500 °C) and H$_{2}$O$_{2}$ direct synthesis (liquid phase, 30 °C). The TNTs are obtained via oxidation of titanium metal and the intermediate layer-type sodium titanate Na$_{2}$Ti$_{3}$O$_{7}$. Thereafter, the titanate layers are exfoliated and show self-rolling to TNTs, which, finally, are impregnated with Pd or Pt nanoparticles at room temperature by using Pd(ac)$_{2}$ and Pt(ac)$_{2}$. The resulting crystalline Pd/TNTs and Pt/TNTs are realized with different lengths (long TNTs: 2.0–2.5 µm, short TNTs: 0.23–0.27 µm) and a specific surface area up to 390 m$^{2}$/g. The deposited Pd and Pt particles are 2–5 nm in diameter. The TNT-derived catalysts show good thermal (up to 500 °C) and chemical stability (in liquid-phase and gas-phase reactions). The catalytic evaluation results in a low CO oxidation light-out temperature of 150 °C for Pt/TNTs (1 wt-%) and promising H$_{2}$O$_{2}$ generation with a productivity of 3240 mol$_{H2O2}$ kg$_{Pd}$$^{-1}$h$^{-1}$ (Pd/TNTs, 5 wt-%, 30 °C). Despite their smaller surface area, long TNTs outperform short TNTs with regard to both CO oxidation and H$_{2}$O$_{2}$ formation

The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH3-SCR

Cu-Zeolites with chabazite structure show a peculiar dual-maxima NO conversion profile, also known as a seagull profile, during the selective catalytic reduction by ammonia. In order to understand the origin of this behavior, systematic catalytic tests and operando spectroscopy were applied to derive structure–performance relationships for Cu-SSZ-13 catalysts with low and high Cu loading. Operando X-ray absorption, X-ray emission and in situ electron paramagnetic resonance spectroscopy measurements, including novel photon-in/photon-out techniques, demonstrated the interconversion of isolated Cu sites and dimeric bis(μ-oxo) Cu species, the former occurring via formation of ammonia Cu2+/Cu+ complexes and the latter in an oxidizing gas mixture. The formation of dimeric Cu+–O2–Cu+ species by involving Cu sites in close vicinity was linked to the high activity at low temperatures of the highly loaded Cu-SSZ-13 sample. In contrast, the isolated Cu sites present at very low Cu loadings are strongly poisoned by adsorbed NH3. The activity decrease around 350 °C that gives rise to the seagull shaped NO conversion profile could be attributed to a more localized structure of mono(μ-oxo)dicopper complexes. Above this temperature, which corresponds to partial NH3 desorption from Cu sites, the isolated Cu sites migrate to form additional dimeric entities thus recovering the SCR activity

NH3_{3}-SCR over V-W/TiO2_{2} Investigated by Operando X-ray Absorption and Emission Spectroscopy

V–W/TiO$_{2}$-based catalysts, which are used for the removal of NO$_{x}$ from the exhaust of diesel engines and stationary sources via selective catalytic reduction with NH$_{3}$ (NH$_{3}$-SCR), were studied by operando X-ray absorption spectroscopy (XAS) and emerging photon-in/photon-out techniques. In order to minimize the influence of highly X-ray absorbing tungsten and the fluorescence of titanium, we used a high-energy-resolution fluorescence setup that is able to separate efficiently the V Kβ$_{1,3}$ emission lines and additionally allows to record valence-to-core (vtc) X-ray emission lines. High-energy resolution fluorescence-detected XAS (HERFD-XAS) and vtc X-ray emission spectroscopy (vtc-XES) proved to be the only way to perform an operando V K edge X-ray spectroscopic study on industrially relevant V–W/TiO$_{2}$ catalysts so far. The V–W/TiO$_{2}$ and V/TiO$_{2}$ samples synthesized by incipient wetness impregnation and grafting exhibited high activity toward NH$_{3}$-SCR. Raman spectroscopy showed that they mainly contained highly dispersed, isolated, and polymeric V-oxo species. HERFD-XAS and XES identified redox cycling of vanadium species between V$^{4+}$ and V$^{5+}$. With respect to most of the potential NH$_{3}$ adsorption complexes, density functional theory calculations further showed that vtc-XES is more limited than surface-sensitive techniques such as infrared spectroscopy; hence, a combination of X-ray techniques with IR or similar spectroscopies is required to unequivocally identify the mechanism of NH$_{3}$-SCR over vanadia-based catalysts

The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH₃-SCR

Cu-Zeolites with chabazite structure show a peculiar dual-maxima NO conversion profile, also known as a seagull profile, during the selective catalytic reduction by ammonia. In order to understand the origin of this behavior, systematic catalytic tests and operando spectroscopy were applied to derive structure–performance relationships for Cu-SSZ-13 catalysts with low and high Cu loading. Operando X-ray absorption, X-ray emission and in situ electron paramagnetic resonance spectroscopy measurements, including novel photon-in/photon-out techniques, demonstrated the interconversion of isolated Cu sites and dimeric bis(μ-oxo) Cu species, the former occurring via formation of ammonia Cu2+/Cu+ complexes and the latter in an oxidizing gas mixture. The formation of dimeric Cu+–O2–Cu+ species by involving Cu sites in close vicinity was linked to the high activity at low temperatures of the highly loaded Cu-SSZ-13 sample. In contrast, the isolated Cu sites present at very low Cu loadings are strongly poisoned by adsorbed NH3. The activity decrease around 350 °C that gives rise to the seagull shaped NO conversion profile could be attributed to a more localized structure of mono(μ-oxo)dicopper complexes. Above this temperature, which corresponds to partial NH3 desorption from Cu sites, the isolated Cu sites migrate to form additional dimeric entities thus recovering the SCR activity

HERFD-XANES and XES as complementary operando tools for monitoring the structure of Cu-based zeolite catalysts during NOx-removal by ammonia SCR

In this article, we demonstrate the potential of hard X-ray techniques to characterize catalysts under working conditions. Operando high energy resolution fluorescence detected (HERFD) XANES and valence to core (vtc) X-ray emission spectroscopy (XES) have been used in a spatially-resolved manner to study Cu-zeolite catalysts during the standard-SCR reaction and related model conditions. The results show a gradient in Cu oxidation state and coordination along the catalyst bed as the reactants are consumed. Vtc-XES gives complementary information on the direct adsorption of ammonia at the Cu sites. The structural information on the catalyst shows the suitability of X-ray techniques to understand catalytic reactions and to facilitate catalyst optimization

Using combined XAS/DRIFTS to study CO/NO Oxidation over Pt/Al2O3 catalysts

Combined X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were applied to investigate the interaction between reaction atmosphere, adsorbates and Pt oxidation state of Pt/Al$_{2}$O$_{3}$ model diesel oxidation catalysts under CO/NO oxidation conditions. The Pt oxidation state was correlated to the adsorbates on the catalyst’s surface. Even at low temperature the reaction atmosphere had a strong impact on the oxidation state of the catalyst, and the oxidation state in turn strongly affected CO adsorption on the Pt particles