Ir-Catalysed Nitrous Oxide (N2O) Decomposition:Effect of Ir Particle Size and Metal–Support Interactions

The effect of the morphology of Ir particles supported on γ-Al2O3, 8 mol%Y2O3-stabilized ZrO2 (YSZ), 10 mol%Gd2O3-doped CeO2 (GDC) and 80 wt%Al2O3–10 wt%CeO2–10 wt%ZrO2 (ACZ) on their stability on oxidative conditions, the associated metal–support interactions and activity for catalytic decomposition of N2O has been studied. Supports with intermediate or high oxygen ion lability (GDC and ACZ) effectively stabilized Ir nanoparticles against sintering, in striking contrast to supports offering negligible or low oxygen ion lability (γ-Al2O3 and YSZ). Turnover frequency studies using size-controlled Ir particles showed strong structure sensitivity, de-N2O catalysis being favoured on large catalyst particles. Although metallic Ir showed some de-N2O activity, IrO2 was more active, possibly present as a superficial overlayer on the iridium particles under reaction conditions. Support-induced turnover rate modifications, resulted from an effective double layer [Oδ−–δ+](Ir) on the surface of iridium nanoparticles, via O2− backspillover from the support, were significant in the case of GDC and ACZ

Characterization of deactivated and regenerated zeolite ZSM-5-based catalyst extrudates used in catalytic pyrolysis of biomass

A major issue in the catalytic fast pyrolysis (CFP) of biomass is the rapid deactivation of the typically employed zeolite-based catalysts. Detailed understanding of the deactivation pathways and the type and location of coke deposits are essential for the further development of improved or new catalyst materials, including appropriate regeneration protocols. Such deactivation and regeneration studies focus almost invariably on small-scale CFP reactor units employing catalyst materials in powder form. In this study, we report the in-depth characterization of deactivated and regenerated ZrO2-promoted zeolite ZSM-5 catalyst extrudates after ex-situ CFP tests carried out in a bench scale reactor. The findings support that coking is the main reason for catalyst deactivation, i.e. for the observed decreased activity in cracking and deoxygenation. Post-mortem characterization by confocal fluorescence microscopy reveals an egg-shell spatial distribution of the coke deposits within the catalyst extrudates. These deposits are heavily poly-aromatic in nature. The majority of the coke build-up occurs in the first 20 min of the reaction and is formed on the strong Brønsted acid sites, which promote deep deoxygenation and cracking. With increasing time-on-stream, the coke deposition slows down, occurring now mainly on the external surface of the zeolite to generate a softer, i.e. more hydrogen-rich, coke on the ZrO2 domains. The catalyst is readily regenerated via thermal oxidation in air, with optimal regeneration at 500 °C. This temperature removes all coke deposits, with no detrimental effect on the catalyst's structural, textural and acid (type and strength) properties

Characterization of deactivated and regenerated zeolite ZSM-5-based catalyst extrudates used in catalytic pyrolysis of biomass

A major issue in the catalytic fast pyrolysis (CFP) of biomass is the rapid deactivation of the typically employed zeolite-based catalysts. Detailed understanding of the deactivation pathways and the type and location of coke deposits are essential for the further development of improved or new catalyst materials, including appropriate regeneration protocols. Such deactivation and regeneration studies focus almost invariably on small-scale CFP reactor units employing catalyst materials in powder form. In this study, we report the in-depth characterization of deactivated and regenerated ZrO2-promoted zeolite ZSM-5 catalyst extrudates after ex-situ CFP tests carried out in a bench scale reactor. The findings support that coking is the main reason for catalyst deactivation, i.e. for the observed decreased activity in cracking and deoxygenation. Post-mortem characterization by confocal fluorescence microscopy reveals an egg-shell spatial distribution of the coke deposits within the catalyst extrudates. These deposits are heavily poly-aromatic in nature. The majority of the coke build-up occurs in the first 20 min of the reaction and is formed on the strong Brønsted acid sites, which promote deep deoxygenation and cracking. With increasing time-on-stream, the coke deposition slows down, occurring now mainly on the external surface of the zeolite to generate a softer, i.e. more hydrogen-rich, coke on the ZrO2 domains. The catalyst is readily regenerated via thermal oxidation in air, with optimal regeneration at 500 °C. This temperature removes all coke deposits, with no detrimental effect on the catalyst's structural, textural and acid (type and strength) properties

Nitrous oxide decomposition over Al2O3 supported noble metals (Pt, Pd, Ir): Effect of metal loading and feed composition

Δημοσίευση σε επιστημονικό περιοδικόSummarization: The N2O decomposition (de-N2O) performance of Al2O3 supported, low content (0.25, 0.5 and 1.0 wt.%) noble metal (Pt, Pd, Ir) catalysts, is comparatively explored in the present study. The effect of metal content, operation temperature and feed composition on de-N2O performance is investigated. Characterization studies involving BET, XRD, TEM and H2-TPR were also carried out to reveal the impact of metal entity and content on the structural, morphological and redox characteristics of the catalysts. The catalytic results imply that the de-N2O performance is in general increased upon increasing metal loading, a fact being more intense over Ir-based catalysts. Under oxygen deficient conditions, N2O conversions as high as ∼100% and ∼80% are reached at 600 °C over Ir- and Pd-based catalysts, respectively, instead of only ∼30%, achieved over Pt-based catalysts. A moderate degradation in oxygen excess conditions is observed with Ir and Pd catalysts, while Pt-based catalysts are almost fully depressed. The superior de-N2O performance of Ir-, Pd-based catalysts can be mainly interpreted by taking into account the formation of metal oxide phases, not easily susceptible to oxygen poisoning. For Ir-based catalysts the active phase seems to be mainly the metal oxide phase (IrO2), as revealed by H2-TPR, XRD and TEM experiments. In the case of palladium catalysts two different metal phases, i.e. PdO and metallic Pd0 were detected. On the other hand, platinum catalysts presented only metallic Pt0 species, which are prone to poisoning by strongly adsorbed oxygen atoms.Παρουσιάστηκε στο: Journal of Environmental Chemical Engineerin

Effect of alkali promoters (K) on nitrous oxide abatement over Ir/Al2O3 catalysts

Summarization: The promoting impact of potassium (0–1 wt% K) on nitrous oxide (N2O) catalytic decomposition over Ir/Al2O3 is investigated under both oxygen deficient and oxygen excess conditions. All samples were characterized by means of X-ray powder diffraction (XRD), temperature-programmed reduction (H2-TPR), ammonia desorption (NH3-TPD) and Fourier Transform Infrared Spectroscopy of pyridine adsorption (FTIR-Pyridine). The results reveal that the K-free Ir/Al2O3 catalyst consists mainly of the IrO2 phase, exhibiting also significant Lewis acidity, which is gradually eliminated by the addition of K. Catalytic performance results showed that the deN2O performance in the absence of O2 in the feed mixture is negatively affected upon increasing potassium loading. However, under oxygen excess conditions, a pronounced effect of K is observed. Although the catalytic performance of the un-doped catalyst is drastically hindered by the presence of O2, the K-promotion notably prohibits the oxygen poisoning. The optimum deN2O performance under oxygen excess conditions is obtained with potassium loading of 0.5 wt% K, which offers complete conversion of N2O at 580 °C, instead of the corresponding 50 % N2O conversion achieved with the un-modified sample. On the basis of characterization results, it was concluded that alkali-doping in combination with oxygen excess conditions are required towards the formation of active Ir entities.Presented on: Topics in Catalysi

A comparative study of the H2-assisted selective catalytic reduction of nitric oxide by propene over noble metal (Pt, Pd, Ir)/γ-Al2O3 catalysts

Summarization: The impact of H2 as additional reducing agent on the SCR of NO with C3H6 in excess oxygen, was comparatively explored over low noble metal loading (0.5 wt%), Pt/γ-Al2O3, Pd/γ-Al2O3, Ir/γ-Al2O3 catalysts. To gain insight into the role of H2, the reactions NO + C3H6 + O2 (R#1), NO + C3H6 + O2 + H2 (R#2), NO + H2 + O2 (R#3) were employed. In respect to propene oxidation, the Pd > Pt > Ir sequence was obtained under R#1, since they exhibit complete conversion at 220, 250, 325 °C, respectively; all metals exhibit moderate deNOx performances (XNO, <40%). H2 co-presence (R#2) promotes both the NO and C3H6 conversions, which is valid in the whole temperature interval investigated (50-400 °C), being more substantial for Pt/γ-Al2O3 and Ir/γ-Al2O3, less beneficial for Pd/γ-Al2O3. A two-maxima feature is obtained on XNO pattern (at ∼100 and ∼230 °C) of Pt and Pd during R#2. The low temperature maximum-attributed to NO reduction by H2-is substantially more pronounced on Pt than Pd, offering XNO ∼90% and SN2 ∼85%; the high temperature maximum-attributed to NO reduction by C3H6-is higher by ∼15% on both Pt and Pd, in respect to the values obtained during R#1, while SN2 remained unaffected. Different XNO pattern with one maximum is obtained over Ir, implying a synergistic interaction between H2 and C3H6. This synergy is accompanied by a substantial widening of the NO reduction window toward lower temperatures and a considerable increase on both XNO,max and SN2 (from XNO ∼30% with SN2 ∼55% during R#1 to XNO ∼70% with SN2 ∼95% during R#2). The specific features of all reactions and metals employed are comparatively discussed.Presented on: Journal of Environmental Chemical Engineerin

N2O decomposition over ceria-promoted Ir/Al2O3 catalysts: the role of ceria

Περίληψη: The impact of CeO2 in the Al2O3-20wt% CeO2 support prepared by the co-precipitation method on the Ir particle size, morphology and oxidation state, and in turn on the deN2O catalytic activity (1000ppmN2O) of supported Ir catalysts were investigated in the absence and presence of excess O2 (2vol%) conditions. It was demonstrated that the deN2O activity of Ir/Al2O3 is notably suppressed by the presence of oxygen in the feed stream, namely, the N2O conversion at 600°C is declined to 65% in the presence of oxygen as compared to 100% in the absence of oxygen. A similar detrimental catalytic effect was also observed for the Ir/CeO2 solid. On the contrary, the deN2O performance of CeO2-modified Ir/Al2O3 catalyst is only slightly affected by the presence of oxygen. An extensive characterization study involving surface texture analysis (N2 adsorption-desorption at -196°C), temperature-programmed reduction in H2 (H2-TPR), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption and desorption (CO-DRIFTS) was carried out to gain insight into the origin of the CeO2-induced promotional effect. The characterization results revealed the existence of IrO2 phase (H2-TPR, XRD, HRTEM, EELS and CO-DRIFTS) as well as of very small isolated particles of Ir on the Al2O3, CeO2 and CeO2-Al2O3 supports (STEM) but to a notably different extent. The coexistence of large IrO2 particles of perfect crystallite structure and very small Ir particles located at the Ir-ceria interface was revealed only in Ir/AlCe. The establishment of a certain Irδ+/Ιr0 ratio and oxygen vacant sites (VO) concentration in ceria around very small Ir particles under oxidative reaction conditions seem to largely promote N2O adsorption and subsequent decomposition into N2 and O2 over the CeO2-promoted Ir/Al catalyst. In the case of Ir/Al, a different deN2O decomposition mechanism occurs, where the site reactivity of Irδ+/Ιr0 established under oxidizing conditions is reduced significantly.Presented on: Applied Catalysis B: Environmenta