Cu-exchanged small-pore SSZ-13 catalysts have found wide use for the selective catalytic reduction (SCR) of nitrogen oxides from automotive exhaust gases. The transient working environment of the Cu-SSZ-13 catalyst during NH3-SCR requires studying the rate limiting steps under the different operation conditions this catalyst is exposed to. By exploiting time-resolved operando X-ray absorption spectroscopy in combination with multivariate analysis we followed the transient speciation of Cu during unsteady state conditions. The results reveal that depending on operating temperature two different rate limiting behaviours inhibit the reduction of NO. At temperatures below 283 degrees C, ammonia hinders reoxidation of solvated Cu-I species thereby inhibiting reduction of NO. Whilst at temperatures of 283 degrees C and above, the reduction of zeolite bound Cu-II(OH-) is the rate limiting step in the SCR reaction. The results also reveal the presence of two detrimental side reactions occurring, the direct oxidation of zeolite bound Cu-I at low temperatures and the oxidation of ammonia over Cu at temperatures in excess of 283 degrees C. Between 250 degrees C and 350 degrees C, both side reactions may be present and could explain the dip in the SCR activity typically denoted by the seagull shape.

Perovskite-derived Ni catalysts offer the remarkable benefit of redox stability that allows their regeneration after deactivation through poisoning or Ni particle growth. Here, the catalytic activity of LaFe0.8Ni0.2O3 towards CO2 methanation was improved by increasing Ni reducibility and segregation to the perovskite surface through partial substitution of La by Sr (La1-xSrxFe0.8Ni0.2O3-delta, 0 <= x <= 0.1). Temperature programmed reduction, X-ray diffraction, scanning electron microscopy and X-ray absorption spectroscopy were used to characterize the materials, their stability against severe reduction at high temperatures and to quantify Ni segregation. It is shown that Ni reducibility was significantly increased (up to 50%) upon introduction of Sr, because more Ni was exsoluted during catalyst pre-reduction. Nickel reincorporation into the perovskite-type oxide lattice during reoxidation was not affected at these Sr levels and complete redox stability could be demonstrated for all compositions investigated in this work.

Time-resolved X-ray absorption spectroscopy (XAS) offers the possibility to monitor the state of materials during chemical reactions. While this technique has been established for transmission measurements for a number of years, XAS measurements in fluorescence mode are challenging because of limitations in signal collection as well as detectors. Nevertheless, measurements in fluorescence mode are often the only option to study complex materials containing heavy matrices or in samples where the element of interest is in low concentration. Here, it has been demonstrated that high-quality quick-scanning full extended X-ray absorption fine-structure data can be readily obtained with sub-second time resolution in fluorescence mode, even for highly diluted samples. It has also been demonstrated that in challenging samples, where transmission measurements are not feasible, quick fluorescence can yield significant insight in reaction kinetics. By studying the fast high-temperature oxidation of a reduced LaFe 0.8 Ni 0.8 O 3 perovskite type, an example where the perovskite matrix elements prevent measurements in fluorescence, it is shown that it is now possible to follow the state of Ni in situ at a 3 s time resolution.

The perovskite-type mixed oxide La0.3Sr0.55Ti0.95Ni0.05O3-delta (LSTN) is demonstrated to exhibit the remarkable property of structural regeneration, where Ni can be reversibly exsoluted from the host perovskite lattice resulting in a regenerable Ni catalyst for solid oxide fuel cell anode applications. Results of catalytic tests for the water gas shift reaction and electrochemical investigations on a button sized fuel cell demonstrate the redox stability of LSTN, its potential application in solid oxide fuel cells, and its ability to recover catalytic activity completely after sulfur poisoning: Nickel segregation was characterized and quantified on powder samples by means of electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, and temperature-programmed reduction-reoxidation cycles. Catalyst stability was much improved compared to impregnated Ni/La0.3Sr0.55TiO3-delta and Ni/Y0.08Zr0.92O2 anode materials. A full cell was tested under both open circuit voltage and polarized conditions, showing a stable cell voltage over redox cycles as well as periods of reverse potential and current overload. The area-specific resistance of the anode layer was as low as 0.58 Omega cm(2) at 850 degrees C. This allows LSTN to be applied in redox-stable solid oxide fuel cell anodes and reversible segregation of Ni to be exploited for fast recovery from sulfur poisoning.

Co3O4 with spinel structure shows CO oxidation activity at very low temperature under dry conditions. This study aims at finding the origin of the unique catalytic activity of Co species in Co3O4 based oxides. Although, octahedral site Co3+ species have been reported to be active in Co3O4 based catalysts, there is no solid explanation as to why Co is so special as compared with other metals like Fe having similar redox states. In this study, mainly, three model spinel catalysts including MnCo2O4, MnFe2O4, and CoCr2O4 have been chosen. A detailed analysis of bulk and crystal surface structure, surface properties of the catalysts, and redox properties of the active metals has been performed to understand the unusual catalytic activity. Low-temperature CO oxidation activity decreases in the following order: MnCo2O4 ≫ MnFe2O4 > CoCr2O4. It indicates that the Co2+ species in a tetrahedral site (in CoCr2O4) remains inactive for low-temperature catalytic activity, while Co3+ in an octahedral site (in MnCo2O4) is...

Perovskite derived Ni catalysts offer the remarkable benefit of regeneration after catalyst poisoning or Ni particle growth through the reversible segregation of Ni from the perovskite-type oxide host. Although this property allows for repeated catalyst regeneration, improving Ni catalyst stability towards sulfur poisoning by H2S is highly critical in solid oxide fuel cells. In this work Mn, Mo, Cr and Fe were combined with Ni at the B-site of La0.3Sr0.55TiO3±δ to explore possible benefits of segregation of two transition metals towards sulfur tolerance. Catalytic activity tests towards the water gas shift reaction were carried out to evaluate the effect of the additional metal on the catalytic activity and sulfur stability of the Ni catalyst. The addition of Fe to the Ni perovskite catalyst was found to increase sulfur tolerance. The simultaneous segregation of Fe and Ni from La0.3Sr0.55Ti0.95-xNi0.05FexO3±δ (x ≤ 0.05) was investigated by temperature programmed reduction, X-ray diffraction and X-ray absorption spectroscopy and catalytic tests after multiple redox cycles. It is shown that catalytic properties of the active phase were affected likely by the segregation of Ni/Fe alloy particles and that the reversible segregation of Ni persisted, while it was limited in the case of Fe under the same conditions.

Practical catalysts often operate under dynamic conditions of temperature variations and sudden changes of feed composition that call for understanding of operation and catalyst structure under analogous experimental conditions. For instance, the copper-exchanged small-pore SSZ-13 catalyst used currently in the selective catalytic reduction of harmful nitrogen oxides from the exhaust gas of diesel-fuelled vehicles operates under recurrent ammonia dosage. Here, we report the design of unsteady state experiments that mimic such a dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation, we were able to capture an ammonia inhibition effect on the rate-limiting copper re-oxidation at low temperature. The practical relevance of this observation was demonstrated by optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and an increase in nitrogen oxide conversion at low temperature.

The deactivation of supported nickel catalysts by coking is an important technological subject for many chemical processes, especially when high concentrations of unsaturated hydrocarbons are present in the feed gas. Here, the reversible segregation of Ni from a LaFeO 3± δ perovskite‐type host lattice was exploited to completely recover a LaFe 0.8 Ni 0.2 O 3± δ catalyst after it had been deliberately subjected to severe carbon deposition during CO 2 methanation in ethylene rich feed gas for several hours. Temperature programmed reduction, X‐ray diffraction, electron microscopy, X‐ray absorption spectroscopy and catalytic activity tests were used to follow the catalyst structure along the various steps of reduction, reaction, coking and subsequent regeneration, while Raman spectroscopy and electron microscopy were used to characterize the nature of the carbon deposits. It is shown that upon reduction Ni atoms segregate to the surface of the perovskite to form catalytic active Ni particles. Oxidation stimulates Ni atoms to readopt the coordination environment of Fe in the perovskite matrix. This property persisted after severe catalyst deactivation by filamentous, partially graphitic carbon. It is demonstrated that simple catalyst reoxidation can be applied to oxidize all carbon deposits while additionally reverting segregated Ni back into the host lattice, thus protecting Ni from particle growth and resultant long‐term loss of catalyst activity over multiple regeneration cycles.

Co3O4 with a spinel structure shows unique activity for CO oxidation at low temperature under dry conditions; however the active surface is not very stable. In this study, two series of Fe- and Cr-doped Co3O4 catalysts were prepared by a single-step solution combustion technique. Fe was chosen because of its redox activity corresponding to the Fe2+/Fe3+ redox couple and compared to Cr, which is mainly stable in the Cr3+ state. The catalytic activity of new materials for low-temperature CO oxidation was correlated to the nature of the dopant. As a function of dopant concentration, the temperature corresponding to the 50% CO conversion (T-50) demonstrated significant differences. The maximal activity was achieved for 15% Fe-doped Co3O4 with T-50 of -85 degrees C and remained almost constant up to 25% Fe. In the case of Cr, the activity was observed to be maximum 100 for 7% of Cr with T50 of -42 degrees C and significantly decreased for higher Cr loadings. Similarly, there was a contrasting behavior in catalyst stability too. 100% CO conversion was achieved below -60 degrees C for 15% Fe/Co3O4 catalyst and remained unchanged even after calcination at 600 degrees C. In contrast, Co3O4 or 15% Cr/Co3O4 catalysts strongly deactivated after the same treatment. These differences were correlated to the oxidation states, coordination numbers, the nature of surface planes, and the redox properties. We observed that both Cr and Fe were typically present in the +3 oxidation state, occupying octahedral sites in the spinel structure. The catalysts were mainly exposed to (111) and (220) planes on the surface. H-2-TPR indicated clear differences in the redox activity of materials due to Fe and Cr substitutions. The reducibility of surface Co3+ species remained similar in all Fe-doped Co3O4 catalysts in contrast to nonreducible Cr-doped analogs, which shifted the reduction temperature to the higher values. As the Fe3+/Fe2+ redox couple partly substituted the Co3+/Co2+ redox couple in the spinel structure, similar bond strength of Fe-O keep redox activity of Co3+ species almost unchanged leading to higher activity and stability of Fe/Co3O4 catalysts for low-temperature CO oxidation. In contrast, nonreducible Cr3+ species characterized by strong Cr-O bond substituting active Co3+ sites can make the Cr/Co3O4 surface less active for CO oxidation.

Perovskite-type oxides have shown the ability to reversibly segregate precious metals from their structure. This reversible segregation behavior was explored for a commonly used catalyst metal, Ni, to prevent Ni sintering, which is observed on most catalyst support materials. Temperature-programmed reduction, X-ray diffraction, X-ray absorption spectroscopy, electron microscopy, and catalytic activity tests were used to follow the extent of reversible Ni segregation. LaFe1-xNixO3-delta (0 <= x <= 0.2) was synthesized using a citrate-based solution pro-cess. After reduction at 600 degrees C, metallic Ni particles were displayed on the perovskite surfaces, which were active towards the hydrogenation of CO2. The overall Ni reducibility was proportional to the Ni content and increased from 35% for x=0.05 to 50% for x=0.2. Furthermore, Ni could be reincorporated reversibly into the perovskite lattice during reoxidation at 650 degrees C. This could be exploited for catalyst regeneration under conditions under which impregnated materials such as Ni/ LaFeO3-delta and Ni/Al2O3 suffer from sintering.

This paper presents a proof-of-concept study and demonstrates the next generation of a “smart” catalyst material, applicable to high temperature catalysis and electro-catalysis such as gas processing and as a catalyst for solid oxide cells.