P. Gelin

Infrared study of rhodium clusters entrapped within zeolites

Samples containing 0.5 and 1.5% by wt rhodium were prepared by sublimating Rh/sub 6/(CO)/sub 16/, onto Y zeolite, subjected to various outgassing, oxidizing, and reducing treatments, and studied by IR spectroscopy. The IR spectra were compared with spectra of zeolite-supported (Rh(CO)/sub 2/ Cl)/sub 2/, alumina-supported Rh/sub 6/(C0)/sub 16/, and CO adsorbed on metallic rhodium. At sufficiently low rhodium content, the zeolite-supported Rh/sub 6/(CO)/sub 16/ species could be decarbonylated reversibly by oxygen or evacuation without losing the cluster structure. At higher rhodium loadings, a rhodium(I) dicarbonyl species formed as well as the decarbonylated zero-valent rhodium.

NOx adsorption onto dehydroxylated or hydroxylated tin dioxide surface. Application to SnO2-based sensors

Abstract SnO2-based sensors present strong interactions with NOx-containing atmospheres. For this reason, it is fundamental to identify the nature of the adsorbed species resulting from the interaction of NO and NO2 with the tin oxide surface. This aspect has been studied in literature by Temperature Programmed Desorption (TPD). To state more precisely structures of adsorbed species, we link TPD with Diffuse Reflectance Fourier Transform Spectroscopy (DRIFTS). Concerning NO2 adsorption, DRIFTS experiments indicate the formation of monodentate nitrato species at room temperature. By increasing the temperature, a conversion of adsorbed species is observed. The nature of surface groups is discussed. In the presence of hydroxyl groups (two different OH groups are evidenced), the formation of hydrogeno nitrates is detected.

Infrared spectroscopic investigations of carbon monoxide physically adsorbed on decationated Y-type zeolite

The physical adsorption of CO at low temperature on HY zeolite leads to the formation of three types of physisorbed CO, depending on the CO pressure. At low CO pressure, CO primarily interacts with HF hydroxyls through hydrogen bonding and gives rise to νCO at 2176 cm–1(species A). Simultaneously, the νOH vibration of the perturbed hydroxyls is shifted by 298 cm–1 toward lower wavenumbers, which provides a measurement of the acid strength of the protons. The saturation of hydroxyl sites is followed by the formation of more weakly physisorbed CO (species C) exhibiting νCO at 2160 cm–1, tentatively ascribed to CO adsorbed on framework oxygens acting as basic sites. At higher CO pressures, a liquid-like CO phase (species B, νCO= 2140 cm–1) forms extensively, exhibiting hindered rotational behaviour. The progressive formation of OH–CO complexes is accompanied by frequency shifts of all νOH and νco vibrations, whether the corresponding species are directly involved or not in the CO adsorption. In particular, LF hydroxyls, although not interacting with physisorbed CO, experience coverage-dependent downward shifts. These phenomena are demonstrated to correlate better with CO-induced effects than with intrinsic acid strength heterogeneity.

Formation of iridium carbonyl complex in NaY zeolite

Abstract The structure of iridium carbonyl complexes trapped within the zeolite cavities and the reactivity of these species towards the methanol carbonylation have been investigated. Ir(NH3)5Cl2+-exchanged NaY zeolite upon thermal decomposition is converted into an Ir(OH)x species. The carbonylation of iridium was followed by quantitative measurement of the 12CO uptake, infrared spectra of the reaction with 12CO, 13CO, and a mixture of 12COue5f813CO. Ir(I)(CO)3 was found to be very active and selective for the vapor phase carbonylation of methanol at atmospheric pressure in the presence of methyl iodide.

Palladium-exchanged MFI-type zeolites in the catalytic reduction of nitrogen monoxide by methane. Influence of the Si/Al ratio on the activity and the hydrothermal stability

Abstract Pd-exchanged MFI-type zeolites containing 3.7 and 0.7 framework aluminium atoms per unit cell (corresponding to Si/Al ratios of 25 and 131) were found active in the selective reduction of nitrogen monoxide in the presence of excess oxygen. Upon steaming at 800°C, both catalysts exhibited the total loss of their catalytic activity in the reduction of NO. Such a behaviour was ascribed to the complete aggregation of Pd ions into large metal particles on the external surface of the zeolite crystals. Both supports, although maintaining their crystallinity, are shown to experience extended dealumination upon steaming. Although the loss of Pd exchange capacity could partially explain the Pd migration and sintering, a mechanism involving the formation of mobile Pd hydroxyl entities condensing into PdO particles outside the zeolite crystallites is preferred.

Coordination chemistry of rhodium and iridium in constrained zeolite cavities: methanol carbonylation

Faujasites - like structure zeolites, possessing in their three dimensional framework, void cavities of 12-13 A diameter, were found effective for anchoring soluble rhodium and iridium carbonyl compounds. Rh (CO)2 and Ir (CO)2 were in-situ synthesized by reacting Rh and Ir exchanged NaY zeolite with CO. The complexes were identified by infrared spectroscopy and other techniques. Reaction at high temperature with CO-H2, CO-H 0 mixture led to the formation of Rh6(C0Il6 and Ir4(C0)12 metal carbonyl clusters. The zeolite framework not only allows the heterogenization of these soluble compounds but also allows excess of charge on the Rh or Ir metal atoms. The mononuclear monovalent Rh and Ir dicarbonyls entrapped in the zeolite cavities were active for the vapor phase carbonylation of methanol in the presence of methyl iodide. The interesting feature of these systems was that methyl chloride can be used as promoter. I I I11

Role of the amorphous matrix in the hydrothermal ageing of fluid catalytic cracking catalysts

Abstract The influence of hydrothermal ageing on a matrix-embedded lanthanum-exchanged zeolite Y has been reinvestigated using several physical techniques (IR, XRD, NMR and STEM). It was found that the crystallinity of LaY zeolite was totally preserved when the zeolite was incorporated into the amorphous matrix, while the same zeolite, if not embedded, lost 55% of its initial crystallinity upon steaming. In addition, a significant dealumination of the zeolite structure was observed. Direct local measurements of the chemical composition of the steamed catalyst using a STEM analytical microprobe showed that severe steaming induced material transportation between the zeolite and the matrix. Isotopic29Si labelling of the matrix combined with29Si MAS-NMR provided the decisive proof for asilicon transportation from the matrix toward the zeolite component and its subsequentreincorporation into the zeolite framework in the place of the expelled aluminium atoms. This phenomenon is thought to be responsible for the “healing” of matrix-embedded zeolites and therefore to explain their increased resistance against severe steaming conditions.

Infrared and calorimetric studies of the adsorption of carbon monoxide on zeolite-supported iridium catalysts

The adsorption of carbon monoxide on the surface of small iridium particles embedded in NaY zeolites was investigated by combined infrared spectroscopy and calorimetry. At low surface coverage (< 0.5), carbon monoxide is readily adsorbed on the metallic surface to form mainly linear species characterized by a single carbon monoxide band around 2050 cm−1. The heat of formation of these species (around 130 kJ mol−1) is consistent with that reported for carbon monoxide adsorbed on supported iridium metal particles. Slight variations in the heat of adsorption are tentatively ascribed to differences in the average particle size. Further addition of carbon monoxide molecules induces the slow formation of monovalent iridium dicarbonyl species, the heat of formation of which is the same as that evolved for carbon monoxide linearly bonding to the metallic surface. A mechanism for the formation of monovalent dicarbonyl complexes from zeolite-supported iridium particles is discussed.

Influence of the pre-treatment on the structure and reactivity of Ir/γ-Al2O3 catalysts in the selective reduction of nitric oxide by propene

Abstract The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/Al 2 O 3 under lean-burn conditions (1000xa0vpm NO, 2000xa0vpm C 3 H 6 , 500xa0vpm CO, 10xa0vol.% O 2 ) was studied. The activity was shown to be strongly enhanced after exposure of the catalyst at 600°C under the reaction mixture, irrespective of the oxidising or reducing pre-treatment. Simultaneously, the Ir dispersion decreased from 78 to 10%. The influence of each component of the reaction mixture on the activation process was examined. The presence of both CO and O 2 was found to be necessary to activate Ir/Al 2 O 3 while NO would not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir 0 reduced surface sites ( ν CO at 2060xa0cm −1 ) which adsorbed CO up to 350–400°C. The activation under reactants was related to the formation of these sites. The presence of reduced (or partially reduced) Ir sites, possibly siting at the surface of IrO 2 particles and stabilised by CO adsorption, was proposed to be responsible for the SCR activity.

Study of the interactions between carbon monoxide and high specific surface area tin dioxideThermogravimetric analysis and FTIR spectroscopy

The interactions of CO with a high specific surface area tin dioxide was investigated by FTIR spectroscopy and thermogravimetric analysis. FTIR study of CO interactions have shown that CO can adsorb on cus (coordinatively unsaturated sites) Sn4+ cation sites (band at 2201 cm-1). In addition, CO reacts with surface oxygen atoms. This leads to the partial reduction of SnO2 surface and to the formation of ionised oxygen vacancies together with the release of free electrons, which are responsible for the loss of transmission. Formed CO2 can chemisorb on specific surface sites: on basic sites to form carbonates species and on acidic sites (Sn4+-CO2 species) which is in competition with the formation of Sn4+-CO species. TG experiment have shown that the reduction of SnO2 by CO at 400°C occurs in two steps. First, the reduction of SnO2 surface, which is a quick phenomenon. This has allowed to evaluate that more than 12% of reducible surface oxygens can react with CO, essentially because of the presence of a large amount of surface hydroxyl groups. The second step of the reduction of SnO2 would be the progressive reduction of SnO2 bulk by the slow diffusion of oxygen atoms from the bulk to the surface.