Infrared spectroscopy is typically not used to establish the oxidation state of metal-based catalysts. In this work, we show that the baseline of spectra collected in diffuse reflectance mode of a series of Pd/Al2O3 samples of increasing Pd content varies significantly and reversibly under alternate pulses of CO or H2 and O2. Moreover, these baseline changes are proportional to the Pd content in Pd/Al2O3 samples exhibiting comparable Pd particle size. Similar measurements by X-ray absorption spectroscopy on a different 2 wt.% Pd/Al2O3 confirm that the baseline changes reflect the reversible reduction-oxidation of Pd. Hence, we demonstrate that changes in oxidation state of metal-based catalysts can be determined using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and that this behavior is part of the spectral changes that are returned by experiments under operando conditions.

WO3/CeO2/TiO2, CeO2/TiO2 and WO3/TiO2 catalysts were prepared by wet impregnation. CeO2/TiO2 and WO3/TiO2 showed activity towards the selective catalytic reduction (SCR) of NOx by NH3, which was significantly improved by subsequent impregnation of CeO2/TiO2 with WO3. Catalytic performance, NH3 oxidation and NH3 temperature programmed desorption of wet-impregnated WO3/CeO2/TiO2 were compared to those of a flame-made counterpart. The flame-made catalyst exhibits a peculiar arrangement of W-Ce-Ti-oxides that makes it very active for NH3-SCR. Catalysts prepared by wet impregnation with the aim to mimic the structure of the flame-made catalyst were not able to fully reproduce its activity. The differences in the catalytic performance between the investigated catalysts were related to their structural properties and the different interaction of the catalyst components.

The interactions between a platinum-group metal (PGM) and a perovskite-type oxide are complex since the latter can accommodate the former in its structure, simply act as a support or, in specific cases, reversibly switch between these two behaviours, depending on the redox environment. Despite promising performances as oxidation catalysts, Y-based perovskite-type oxides are far less studied than their La-based counterparts and their interactions with PGM need to be better understood. The morphology, coordination and oxidation state of Pd species in Pd-doped YFeO3±δ catalysts prepared by flame spray synthesis were investigated in dependence on Pd loading in the range of 0–2.5 wt%. Their thermal stability was assessed by calcination of the flame-made materials at 700 °C. Fresh and calcined samples were thoroughly characterized by STEM, N2-physisorption, XRD, XPS, DRIFTS and OSCC. Pd species were predominantly in the form of metallic nano-particles supported on YFeO3±δ. The size of these nano-particles increased with increasing loading as evidenced by DRIFTS. XPS facilitated the identification of Pd2+ species in strong interaction with the hexagonal YFeO3 lattice, suggesting the partial incorporation of noble metal ions in the perovskite-type structure. After calcination at 700 °C, this contribution vanished in the catalysts containing at least 2 wt % Pd. The catalysts were tested for methane oxidation under stoichiometric conditions up to 850 °C. The catalyst with 2 wt% Pd exhibited the highest CH4 oxidation activity. Reduction of the Pd content to 0.5 wt% resulted in the shift of the 50 % CH4 conversion by only ca. 40 °C. Hence, flame-made Pd/YFeO3±δ demonstrated to be a suitable material to maintain CH4 conversion with reduced noble metal content.

Au, Pd, and AuPd supported on TiO2 were prepared by a sol immobilization route. Operando attenuated total reflectance (ATR) IR spectroscopy and catalytic batch reactor experiments were performed in parallel to elucidate the different catalytic performance of the catalysts in the liquid‐phase oxidation of benzyl alcohol. Pd/TiO2 exhibited a higher activity than AuPd/TiO2 and Au/TiO2, but the modification of Pd with Au demonstrated a significant stability enhancement. ATR‐IR spectroscopy evidenced that the presence of Au facilitates the desorption of byproducts, which thus reduces the extent of deactivation of the active sites caused by the irreversible adsorption of benzoate species. Although benzaldehyde was the main product in both catalysts, the nature of the byproducts differs. Pd/TiO2 favored the deoxygenation of benzyl alcohol to produce toluene as the main byproduct. Conversely, AuPd/TiO2 promoted the transformation of benzaldehyde to benzoic acid.

Materials based on a combination of cerium-tungsten-titanium are potentially durable catalysts for selective catalytic NOx reduction using NH3 (NH3-SCR). Flame-spray synthesis is used here to produce WO3/CeOx-TiO2 nanoparticles, which are characterized with respect to their phase composition, morphology, and acidic properties and are evaluated by NH3-SCR. HR-TEM and XRD revealed that flame-made WO3/CeOx-TiO2 consists of mainly rutile TiO2, brannerite CeTi2O6, cubic CeO2, and a minor fraction of anatase TiO2. These phases coexist with a large portion of amorphous mixed Ce-Ti phase. The lack of crystallinity and the presence of brannerite together with the evident high fraction of Ce3+ are taken as evidence that cerium is also present as a dopant in TiO2 and is well dispersed on the surface of the nanoparticles. Clusters of amorphous WO3 homogeneously cover all particles as observed by STEM. Such morphology and phase composition guarantee short-range Ce-O-Ti and Ce-O-W interactions and thus the high surface concentration of CO3+. The presence of the WO3 layer and the close Ce-O-W interaction further increased the Ce3+ content compared to binary Ce-Ti materials, as shown by XPS and XANES. The acidity of the materials and the nature of the acid sites were determined by NH3 temperature-programmed desorption (NH3-TPD) and DRIFT spectroscopy, respectively. TiO2 possesses mainly strong Lewis acidity; addition of cerium, especially the presence of surface CO3+ in close contact with titanium and tungsten, induces Bronsted acid sites that are considerably increased by the amorphous WO3 clusters. As a result of this peculiar element arrangement and phase composition, 10 wt % WO3/10 mol % CeOx-90 mol % TiO2 exhibits the highest NO reduction efficiency, which matches that of a V2O5-WO3/TiO2 catalyst. Preliminary activity data indicate that the flame-made catalyst demonstrates much higher performance after thermal and hydrothermal aging at 700 degrees C than the V-based analogue despite the presence of the rutile phase. Ce3+ remains the dominating surface cerium species after both aging treatments, thus confirming its crucial role in NH3-SCR by Ce-W-Ti-based catalysts.

Carbon deposition during CO methanation from biomass-derived gas is a significant challenge in terms of catalyst lifetime. It results from the severe reaction conditions imposed by the presence of unsaturated hydrocarbons in the gasified feedstock. This work investigated the structure of boron-modified Ni/Al2O3 catalysts exhibiting enhanced carbon resistance. As a consequence of B promoting the growth of Ni crystallites, the structure of the B-modified catalyst was demonstrated to be different at the nanoscale, especially after calcination. The modified catalyst possesses larger Ni particles with porous regions in which B is present. The absence of carbidic and amorphous carbon species, which are considered critical for catalyst deactivation in low-temperature processes, confirms that B effectively prevents carbon diffusion into Ni, which thus enhances the durability of the catalyst for CO methanation. These results may reveal a strategy of wider significance for developing catalysts with improved stability.

X-ray diffraction (XRD) is typically silent towards information on low loadings of precious metals on solid catalysts because of their finely dispersed nature. When combined with a concentration modulation approach, time-resolved high-energy XRD is able to provide the detailed redox dynamics of palladium nanoparticles with a diameter of 2 nm in 2 wt % Pd/CZ (CZ = ceria-zirconia), which is a difficult sample for extended X-ray absorption fine structure (EXAFS) measurements because of the cerium component. The temporal evolution of the Pd(111) and Ce(111) reflections together with surface information from synchronous diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements reveals that Ce maintains Pd oxidized in the CO pulse, whereas reduction is detected at the beginning of the O2 pulse. Oxygen is likely transferred from Pd to Ce(3+) before the onset of Pd re-oxidation. In this context, adsorbed carbonates appear to be the rate-limiting species for re-oxidation.

The ageing characteristics of flame-made 2 wt% Pd supported on YFeO3 were analysed in comparison with a Pd/Al2O3–CeO2–ZrO2 three-way catalyst (TWC) with respect to structural changes and catalytic performance for methane oxidation under stoichiometric reaction conditions. Thermal treatment under lean conditions (air, 900 °C) resulted in slight decrease in the methane oxidation activity of the TWC. In marked contrast, YFeO3-supported Pd catalysts exhibit an increase in activity after such treatment. Activity enhancement is even higher when the treatment was performed under stoichiometric conditions (air–fuel equivalence ratio, λ = 1, 900 °C). To explain this observation, in-depth characterization (BET, STEM, OSCC, XAS, and CO chemisorption) of fresh and aged catalysts was performed. Both thermal and stoichiometric ageing cause a severe sintering of the support particles and the phase transformation from hexagonal to orthorhombic YFeO3. Despite the absence of a mixed Pd–YFeO3 phase, the growth of Pd particles appears to be limited under the λ = 1 atmosphere. In contrast to thermally aged catalysts where large PdO particles are formed, well-defined metallic Pd nanoparticles of 10–20 nm are present after stoichiometric ageing along with higher methane oxidation activity. Although it is tempting to conclude that metallic Pd is active for methane oxidation under the given conditions, reversible and periodic partial oxidation of the large metallic particles is observed in modulation excitation high energy X-ray diffraction (HXRD) experiments designed to simulate the oscillating redox conditions experienced during operation. These results indicate that large Pd particles exhibit improved methane oxidation activity but equally confirm that activity under stoichiometric conditions is the result of a delicate equilibrium dictated by the bulk-Pd/surface-PdO pair.

A model Pd-only three-way catalyst has been subjected to simulated driving conditions of natural gas and gasoline operation in an operando reactor cell for X-ray absorption spectroscopy that included alternated, but longer than real oscillations, rich and lean periods and a high temperature surge (850–900°C). The X-ray absorption near edge structure (XANES) spectra indicated that metallic palladium is observed in the whole temperature range investigated (up to 900°C) and irrespective of the air/fuel ratio. In both natural gas and gasoline cycles, the XANES data show that the PdO reduced in the rich periods cannot be restored in the lean periods. With this background, activity for methane abatement in the high temperature regime is greatly affected by the oxidation state of palladium rather than by the change of air/fuel ratio. In the case of propene oxidation, while Pd also remains predominantly in the reduced state, activity is dictated by the oxygen concentration in the feedstock. Comparison between the two hydrocarbons demonstrates that the oxidation state of Pd may be responsible for observed methane emissions under realistic operating circumstances. Moreover, the experiments demonstrate that reduced Pd may be continuously present during operation in agreement with observations on real catalytic converters. Although this may be the average oxidation state of Pd, more advanced probes are certainly necessary to capture variations of oxidation state under the fast oscillatory conditions needed to imitate real operation.

Three-way catalysts (TWC) are the key technology to reduce emissions of pollutants from stoichiometric engines. Perovskite-type catalysts of general formula ABO(3 +/-delta) (A = La, Y; B = Mn, Fe) containing 2 wt% Pd were produced by flame spray synthesis (FSS) using metal nitrate precursors. The structural properties of the catalysts were characterized by X-ray diffraction (XRD), surface area determination (BET) and transmission electron microscopy (TEM). Crystalline metal oxide nano-particles of 20 nm average size were accompanied by minority La2O3 and Y2O3 phases. The state of Pd in the catalysts was characterized using X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and CO adsorption by infrared spectroscopy. Metallic Pd coexisted with Pd in oxidation state +2 and higher on all fresh samples. TEM confirmed the presence of dispersed Pd particles 2-5 nm in diameter. Therefore, under the chosen synthesis conditions, FSS provides supported palladium nano-particles rather than a solid solution. PdO was the dominant Pd species after calcination at 700 degrees C. The TWC activity was tested in a simulated stoichiometric gas mixture comprising CH4, CO, NOx, and O-2. PdO in combination with YFeO3 +/-delta exhibited the lowest temperature for CH4 oxidation (T-50 = 450 degrees C), which was ca. 100 degrees C lower than that of the sample obtained by the conventional wet-chemical method. After cycling under reaction conditions up to 850 degrees C, a large improvement of catalytic activity for CH4 oxidation was observed which associated with the formation of metallic Pd particles (ca. 20 nm) and the hexagonal -> orthorhombic phase transition of YFeO3 +/-delta. (C) 2013 Elsevier B.V. All rights reserved.

A commercially available honeycomb CO oxidation catalyst used to control the exhaust of a solid oxide fuel cell (SOFC) based power system has been characterized after prolonged use. X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, N-2 physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were employed to determine the phase composition, the morphology and the chemical state of the various components. Besides sintering of the active phase, deactivation was found to occur mainly as the result of the deposition of chromium-containing species on the catalyst washcoat. These fouling species mainly appeared as highly crystalline Cr2O3 particles and could still maintain acceptable CO oxidation activity under dry atmosphere. However, the formation of surface chromium oxyhydroxide species was found to occur in the presence of water vapor, leading to significant catalyst deactivation. (C) 2013 Elsevier B.V. All rights reserved.

Flame spray synthesis (FSS), ultrasonic spray combustion (USC) and solution combustion synthesis (SCS) were selected for the preparation of Pd-substituted perovskite-type oxides and were compared with the citric acid method (CM). La(Fe,Pd)O3 and La(Mn,Pd)O3 catalysts were characterized using N2-adsorption (BET method), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The structure of Pd was characterized using X-ray absorption near edge structure (XANES) spectroscopy: Pd in octahedral coordination can be clearly distinguished from PdO-like species from the shape of the whiteline. Except for SCS samples, the BET surface area of all samples varied between 12 and 39m2/g. In the case of LaFeO3±δ, only CM and USC methods provided Pd in octahedral coordination, whereas a mixed Pd2+/Pd0 oxidation state was obtained from FSS and SCS methods. On the contrary, Pd supported on LaMnO3±δ was obtained irrespective of the synthesis method. As a consequence, the peculiar structure of Pd obtained from each synthesis method influenced the catalytic activity for methane oxidation. The characterization and catalytic data confirm that the coordination of Pd depends on perovskite composition and suggest that it is also dependent on the synthesis method.

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850 °C) and cooling (to 100 °C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500 °C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd2+/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria–zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperatures.

Thermoelectric CaMn0.98Nb0.02O3-delta single crystals were grown from sintered polycrystalline material using the traveling-solvent floating zone (TSFZ) method. The floating-zone furnace was operated at over-pressure using an Ar/O-2 mixture to prevent evaporation during the growth process. Six twin-domain variants were detected with single-crystal X-ray diffraction (XRD) and confirmed by electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM). The Seebeck coefficient (S) of the single-crystalline material indicates n-type semiconducting behavior. Within 10 K < T < 125 K a negative peak in S is observed, known to be characteristic of antiferromagnetic ordering. The ferromagnetic long-range ordering, expected on the basis of double exchange between Mn4+ and doped Mn3+ species, thus appears to remain suppressed in the single-crystalline material. (c) 2013 Elsevier B.V. All rights reserved.

Palladium is the precious metal of choice for methane oxidation and perovskite-type oxides offer the possibility to stabilize it as PdO, considered crucial for catalytic activity. Pd can adopt different oxidation and coordination states when associated with perovskite-type oxides. Here, we review our work on the effect of perovskite composition on the oxidation and coordination states of Pd and its influence on catalytic activity for methane oxidation in the case of typical Mn, Fe and Co perovskite-based oxidation catalysts. Especially X-ray absorption near edge structure (XANES) spectroscopy is shown to be crucial to fingerprint the different coordination states of Pd. Pd substitutes Fe and Co in the octahedral sites but without modifying catalytic activity with respect to the Pd-free perovskite. On LaMnO3 palladium is predominantly exposed at the surface thus bestowing catalytic activity for methane oxidation. However, the occupancy of B-cation sites of the perovskite structure by Pd can be exploited to cyclically activate Pd and to protect it from particle growth. This is explicitly demonstrated for La(Fe, Pd)O3, where catalytic activity for methane oxidation is enhanced under oscillating redox conditions at 500 °C, therefore paving the way to the practical application in three-way catalysts for stoichiometric natural gas engines.

High-performance thermoelectric half-Heusler alloys with nominal compositions of Ti0.37Zr0.37Hf0.26NiSn were investigated. Scanning electron microscopy compositional mapping showed a remarkable separation of Ti and Zr + Hf rich phases, while Ni and Sn are homogeneously distributed. Additionally, a nanoscale phase separation of different half-Heusler phases was observed. The thermoelectric properties of the compounds were measured at 2 K < T < 900 K and showed a high figure of merit, reaching a value of ZT = 1 at 725 K. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The catalytic hydrogenation of CO(2) at the surface of a metal hydride and the corresponding surface segregation were investigated. The surface processes on Mg(2)NiH(4) were analyzed by in situ X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS) and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). CO(2) hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that for the CO(2) methanation reaction, the dissociation of H(2) molecules at the surface is not the rate controlling step but the dissociative adsorption of CO(2) molecules on the hydride surface.

Oxynitrides with the nominal composition LaTiO2N were prepared from a La2Ti2O7 precursor by thermal ammonolysis of the oxide under an NH3 flow for different durations t (4 – 30 h). X-ray diffraction (XRD) indicated that phase-pure LaTiO2N samples were obtained when t ≥ 13 h. The material was further characterised by scanning electron microscopy (SEM), surface area measurements based on the BET method, thermogravimetric analysis (TGA) and UV-visible diffuse reflectance spectroscopy (DRS). The powders displayed an increased crystallite size and a decreased surface area with increasing t. The ratio of N/(N+O) increased with t from 0.22 – 0.27. Correlated to this nitrogen increase, a small variation of the bandgap energy was observed from 2.19 to 2.12 eV with t. All LaTiO2N samples oxidised H2O into O2 in the presence of an electron acceptor (Ag+). The O2 evolution was increased from 12 μmol/h (t = 13 h) to 22 – 24 μmol/h (16 h ≤ t ≤ 30 h). It was found that the nitrogen content and the amount of defects played a key role in the photocatalytic O2 evolution.