The nature of metal oxide on adsorptive and catalytic properties of Pd/MeOx/Al2O3 catalysts (original) (raw)
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Inorganica Chimica Acta, 2002
Adsorption of CO-, NO-and CO/NO-containing mixtures was investigated by in situ IR spectroscopy under flow conditions over Pd/d-Al 2 O 3 and Pd/Ce 0.6 Zr 0.4 O 2 /d-Al 2 O 3. The attention is focused on the effects of Ce 0.6 Zr 0.4 O 2 addition to Pd/Al 2 O 3 on the catalytic properties and reaction intermediates. Linearly bonded and bridged CO species adsorbed on palladium were clearly identified on Pd/Al 2 O 3 while CO adsorption is inhibited under reaction conditions in the presence of Ce 0.6 Zr 0.4 O 2. Addition of Ce 0.6 Zr 0.4 O 2 affects the nature of the Pd sites by stabilizing oxidized dispersed Pd species. Evidence is obtained that the presence of the Ce 0.6 Zr 0.4 O 2 solid solution modifies the adsorption properties of the supported Pd species and promotes the reduction of NO below 500 K via an alternative mechanism. This presumably involves reactive oxygen species at the Pd Á/Ce 0.6 Zr 0.4 O 2 interface generated by the redox Ce 4' /Ce 3' couple.
Journal of Catalysis, 1996
were prepared to contain different amounts of PdO. The effect of the chemical state of Pd on adsorption and thermal properties of CO and NO were investigated by temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FT-IR). CO adsorbed molecularly on all Pd catalysts forming linear and bridged CO species. The presence of PdO was found to strongly decrease the CO binding energy on Al 3 O 3 -supported Pd catalysts, as indicated by the subsequent lowering of the temperature of the TPD peak maximum. The destabilization of CO bonding was even further enhanced on Pd catalysts supported on LaAlO 3 and La 2 O 3 -Al 2 O 3 . The interaction of CO with the Al 2 O 3 , La 2 O 3 -Al 2 O 3 , and LaAlO 3 supports was negligible. In contrast to the behavior of CO, NO was observed to adsorb molecularly on Al 2 O 3 , La 2 O 3 -Al 2 O 3 , and LaAlO 3 supports with high efficiency. Strong absorption bands of NO in the range of 1600-1200 cm −1 were detected. The dissociation of NO, followed by the formation of N 2 and N 2 O during the temperature programmed reaction, was seen on all the Pd catalysts. The importance of the chemical state of Pd and the La-induced effects on the reactivity of CO and NO are discussed. c 1996 Academic Press, Inc.
Applied Catalysis B: Environmental, 2000
We have examined the adsorption of CO and NO on powder Pd/Al 2 O 3 , Pd-Ce/Al 2 O 3 and CeO 2 /Al 2 O 3 catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd-Ce/Al 2 O 3 TPD profiles are identical to those observed for Pd/Al 2 O 3 , suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al 2 O 3 and Pd-Ce/Al 2 O 3. On oxidized catalysts, Pd/Al 2 O 3 is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd-Ce/Al 2 O 3 and react to N 2 O and N 2. In comparison with Pd/Al 2 O 3 , reduced Pd-Ce/Al 2 O 3 catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N 2 over N 2 O.
Reduction of NO by CO using Pd–CeTb and Pd–CeZr catalysts supported on SiO2 and La2O3–Al2O3
Journal of Environmental Sciences, 2015
was studied using the reduction of NO by CO. The catalysts were characterized by X-ray fluorescence, surface area, X-ray diffraction, temperature-programmed reduction, CO chemisorption and oxygen storage capacity. Temperature-programmed reduction results indicated that Tb or Zr incorporation improves the reducibility and oxygen storage capacity. CO chemisorption data suggested the presence of large PdO particles due to the low CO/Pd ratio. No significant differences were obtained in light off temperatures (T Light off) for all Pd catalysts and the most active was 1.5%Pd/Ce 0.6 Zr 0.4 O x /SiO 2 .
Journal of Catalysis, 2005
Pd-Ni catalysts supported on Al 2 O 3 , (Ce,Zr)O x /Al 2 O 3 , and (Ce,Zr)O x were examined with the principle objective of determining the effects of Ni on catalytic activity for CO oxidation and NO reduction reactions under stoichiometric conditions. Catalytic activity findings for the CO + O 2 and CO + O 2 + NO reactions were analyzed in conjunction with in situ DRIFTS and XANES results to obtain information on the processes occurring in the catalysts during the course of the reactions. The results reveal a significant dependence on the nature of the support in terms of the catalytic changes produced by nickel. In the absence of significant nickel-induced electronic perturbations of palladium, these are related to indirect effects on palladium distribution over the catalysts or to a certain impediment of the interactions between active palladium and Ce-Zr mixed-oxide components. Significant promotion of CO oxidation was observed for the (Ce,Zr)O x /Al 2 O 3 -supported catalyst, which reveals a relevant role for the particle size of the nanostructured Ce-Zr mixed oxide in this reaction. 2005 Elsevier Inc. All rights reserved. .es (A. Martínez-Arias).
Reactivity study of CO+NO reaction over Pd/Al2O3 and Pd/CeZrO2 catalysts
Catalysis Today, 2019
To elucidate the reactions involved in the three-way catalyst under stoichiometric conditions, the reactivity of CO + NO has been studied by continuous flow and surface reactions over Pd supported on Ce 0.75 Zr 0.25 O 2 (CZO) and Al 2 O 3 model catalysts with different Pd particles sizes and oxidation states. Pd/CZO showed higher activity with complete NO removal at 125°C than Pd/Al 2 O 3 at 300°C As a primary by-product, N 2 O was produced on both catalysts prior to reaching complete CO conversion in a 1:1 NO/CO feed ratio. NO inhibition through N 2 O formation was intensified when 2:1 NO/CO feed ratio was used, where the complete CO conversion can't be achieved during the course of the reaction. When 1:1 NO/CO feed ratio was used over Pd/Al 2 O 3 , a concave feature in the CO light-off curve was observed, where the CO light-off curve showed a downward inflections after complete NO conversion had been reached, and then started to increase again with further increase in temperature. The inflection of CO conversion coincided with the maximum yield of N 2 O vs. reaction temperature. It was also found that the CO + NO reactivity was affected by the Pd oxidation state and particles size. in situ DRIFT experiments showed that the formation of NCO and NCO-derived N 2 O intermediate species is closely related to the inflection of CO conversion. A pathway of N 2 O formation via the NCO intermediate species has been proposed to be responsible for the fallback in CO conversion when the reaction temperature increased, as both interactions of "NCO ad + N ad " and "NCO ad + NO ad ," which produce N 2 and N 2 O, respectively, consume NO but generate CO.
Influence of Supports on Pd Catalysts for the Selective Catalytic Reduction of NOx with H2 and CO
Emission Control Science and Technology, 2015
A major concern of current commercial catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NO x) (DeNO x) is the lack of sufficient catalytic activity at low temperatures (below 200°C) that typically occur during cold start and urban driving conditions. To address this issue, a study was undertaken to develop an (H 2 + CO)-SCR Pd-based catalyst to obtain high DeNO x activity in this low-temperature range. A series of catalysts with Pd loaded on supports containing various amounts of γ-alumina (γ-Al 2 O 3) (0-65 wt%), CBV-2314 (15-100 wt%), and TiO 2 (0-20 wt%) were tested for DeNO x activity and aging in a catalyst testing unit (CTU). The results showed that each component in the support played a role in the NO x conversion, depending on the temperature. Catalysts with Pd supported on {[γ-Al 2 O 3 (60 wt%)+CBV-2314 (20 wt%)]-TiO 2 (20 wt%)} achieved the best overall DeNO x activity. Catalysts with various Pd loadings (0.05 wt% to 0.5 wt%) were prepared with a {[γ-Al 2 O 3 (60 wt%)+CBV-2314 (20 wt%)]−TiO 2 (20 wt%)} support. The catalysts loaded with 0.05 wt% and 0.2 wt% Pd demonstrated the best overall DeNO x activity (up to 90 % NO x conversion starting at 150°C). These catalysts were subjected to an accelerated aging regime. Based on the aging studies, the 0.2 wt% Pd/{[γ-Al 2 O 3 (60 wt%)+CBV-2314 (20 wt%)]− TiO 2 (20 wt%)} composition was the most active and stable catalyst. Keywords NO x /H 2 /CO. Palladium catalyst. Promoter. γ-Al 2 O 3-CBV-2314-TiO 2. Catalyst aging * Gianni Caravaggio
Catalysis Today, 2001
The role of palladium precursors (e.g. chloride -PdCl 2 ; acetylacetonate -Pd(acac) 2 ; nitrate -Pd(NO 3 ) 2 ) in the catalytic properties of Pd/Al 2 O 3 and Pd/CeO 2 /Al 2 O 3 catalysts toward CO oxidation was herein investigated. The characterization techniques used for mapping the Pd sites were H 2 and CO chemisorption, infrared spectroscopy (FTIR) of CO adsorbed, and temperature-programmed desorption (TPD). Unsteady-state CO oxidation was carried out by temperature-programmed surface reaction (TPSR). The nature of palladium precursors and their interaction with CeO 2 affected the metallic dispersion and the site morphologies. Highly dispersed metal particles (d > 50%) were obtained by using palladium chloride and acetylacetonate precursors on Pd/Al 2 O 3 catalysts. Pd(1 0 0) and Pd(1 1 1) were the major palladium crystallite orientations in these samples, but a larger amount of low coordination sites located on Pd(1 0 0) faces was observed for the ex-chloride sample. These sites accounted for the oxidation of CO at very low temperatures on Pd-Cl catalyst. In the presence of ceria, the Pd dispersion was a function of the way in which each Pd precursors interacted with CeO 2 . A two-fold decrease of dispersion was obtained to ex-chloride sample, while a two-fold increase to ex-nitrate and the same dispersion to ex-acetylacetonate samples were otherwise observed. The metallic redispersion may be the result of the occupancy of ceria oxygen vacancies by the palladium crystallites. Ultimately, the interaction with ceria redispersed Pd crystallites in a more organized bi-dimensional structure with the predominance of (1 1 1) orientation. Due to the transient conditions of the reaction, ceria did not promote, as expected, the oxidation of CO. Ceria reduced species (Ce 3+ ) were not able to help CO oxidize at lower temperatures due to lack of oxygen into its lattice. Thus, the highest rates for CO oxidation were only observed at higher temperatures for the Pd/CeO 2 /Al 2 O 3 catalysts, a result of a combination of strong competition for oxygen molecules, which have replenished the ceria lattice, and the low activity of Pd(1 1 1) sites.