Carlo Dossi

Carbon tetrachloride hydrodechlorination with organometallics-based platinum and palladium catalysts on MgO

Hydrodechlorination of carbon tetrachloride was performed at atmospheric pressure over Pd and Pt catalysts supported on various inorganic oxides (Al2O3, MgO, CeO2) and obtained from different organometallic precursors (Pd(acac)2, [Pd(C3H5)Cl]2, Pt(acac)2 and Pt(CH3CN)2Cl2. Generally, Pd-based catalysts show low conversion levels and high selectivity to C1–C7+ hydrocarbons with a distribution that fits a typical Schulz–Flory mechanism for alkyl polymerization. On the other hand, Pt catalysts show high and stable activity, the major products are CHCl3 and in lesser extent methane. Catalytic performances depend on the stability, under reaction conditions, of the metal phase, that could be related to the nature of chlorine species pre-adsorbed on catalysts. Owning to their superior catalytic performances, the structural modifications of Pt catalysts under reaction conditions were characterized by EXAFS spectroscopy.

Fast transient infrared studies in material science: development of a novel low dead-volume, high temperature DRIFTS cell.

A prototype DRIFTS flow reaction chamber was designed and developed in order to find analytical application in the study of heterogeneous catalysts operating at high temperatures under fast transient gas feed conditions. Minimisation of dead-volumes allows gas replacement in 8-10s at 10mLmin(-1) total flow. To overcome problems related to the reactivity of the cell walls under alternating oxidizing/reducing gases, the cell was built with Inconel 600trade mark, which was tested to be very inert even at high temperatures. The sample holder, which was developed to closely resemble a micro plug-flow reactor, poses some problems in terms of heat transfer to the outer body of the cell (limiting then the maximum reachable temperature) and of the correct measurement of the actual sample temperature. These problems were solved with a careful re-design of the upper part of the cell. The second prototype thus derived is able to reach temperatures up to 803K and allows gas replacement in less than 4s at 10mLmin(-1). The cell is inserted in a MCT-FT-IR, which allows to collect high quality spectra with a 1s time-resolution. The downstream flow can be analysed by a quadrupole mass spectrometer equipped with an enclosed source and by a commercial GC. The performances of this prototype cell are presented showing some tests carried out with ceria-zirconia (Ce(x)Zr(1-x)O(2)) catalysts for CO abatement under real operando conditions.

Molecular metal carbonyl clusters and volatile organometallic compounds for tailored mono and bimetallic heterogeneous catalysts

Abstract Specific studies carried in Milan laboratory in the application of metal carbonyl clusters to preparation of supported catalysts are reviewed. Examples are reported of catalysis by heterogeneous organometallic surface species, such as 1-butene isomerisation catalysed by silica-anchored osmium carbonyl species, or of catalysis by very small metal particles produced under catalytic conditions, such as ethylene hydroformylation using silica supported [Rh12(CO)30]2− as precursor. Highly active bimetallic FeIr MgO catalysts for methanol synthesis, have been pr prepared by controlled reductive decomposition of physisorbed [Et4N]2[Fe2Ir4(CO)16] and [Et4N]2[Fe2Ir2(CO)12] clusters. In these examples, evidence of the nature of the catalytic entities is reached by in situ DRIFTS characterisation under catalytic conditions or using EXAFS measurements. Metal particles deposited inside the zeolitic cavities have been obtained by mild reduction conditions of entrapped organometallic compounds introduced by chemical vapour deposition (CVD). In this way, non-acidic Pt KL , Pd NaY catalysts for hydrocarbon reforming have been prepared. Bimetallic Pt Re and Rh Mn particles, entrapped inside a NaY zeolite, have been easily obtained by CVD of M2(CO)10 carbonyls (M  Re, Mn) on pre-reduced metal particles.

Problems in the application of the three-step BCR sequential extraction to low amounts of sediments: an alternative validated route.

Poor recoveries are obtained if the BCR three-step sequential extraction is applied to 100 mg specimens rather than to 1 g. It is observed that analytes are lost during each phase separation which is carried out via centrifugation and can be hardly quantitatively performed on 100 mg sediment specimens. An alternative procedure, which is carried out on a single empty SPE column and involves separation by filtration, is developed to solve this problem. The proposed method is validated on 100 mg samples of certified sediment (BCR-701), but could be potentially used for even lower sediment specimens. Problems related to pH stability during step 2 and its influence on recoveries is also reported.

On the role of carbonaceous material in the reduction of Cu2+ to Cu+ in Cu-ZSM-5 catalysts

Abstract A multi-technique analytical approach has been used to show that carbonaceous material entrapped in the channels system of Cu- ZSM-5 catalysts may contribute to maintain the copper ions in the oxidation state +1. This suggests that the reduction of Cu 2+ to Cu + does not only occur via water elimination from CuOH + species or via oxygen evolution from [Cu–O–Cu] 2+ species, but also via oxidation of the carbonaceous materials to give CO and CO 2 . This mechanism occurs under high temperature (>300°C) treatments in vacuo or He of Cu-ZSM-5 catalysts containing carbonaceous species and strongly adsorbed water molecules in the first coordination shell of the Cu 2+ . Our results may support the hypothesis about the roles of O 2 and hydrocarbons in the SCR of NO x ; O 2 maintains together with hydrocarbons the optimal valence state of copper (Cu + ) necessary to have the maximum conversion of NO x and hydrocarbons into N 2 and CO 2 .

DRIFT study of CO chemisorption on organometallics-derived Pd/MgO catalysts : the effect of chlorine

Magnesia-supported palladium catalysts were prepared from chemical vapour deposition (CVD) of [Pd(C3H5)(C5H5)] and incipient wetness impregnation of [Pd(C3H5)Cl]2 and [Pd(acac)2]. DRIFT spectroscopy of adsorbed CO on prereduced catalysts indicates that the electronic state of metal particles depends on the preparation methodology and markedly on the organometallic precursor. Inn-heptane reforming at 500°C, the highest activity and selectivity were shown by the CVD-based system. Chloride ions deriving from the impregnation solvent exchange with surface hydroxyls. Acidic Mg-Cl sites are thus formed, which induce a beneficial effect on the catalytic properties. The reforming activity collapsed when a chlorine-containing precursor was used, due to a partial coverage of the palladium surface with chemisorbed chlorine atoms.

Validation of an isotope dilution, ICP-MS method based on internal mass bias correction for the determination of trace concentrations of Hg in sediment cores.

The development and validation of a method for the determination of mercury in sediments using a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) for detection is described. The utilization of isotope dilution (ID) calibration is shown to solve analytical problems related to matrix composition. Mass bias is corrected using an internal mass bias correction technique, validated against the traditional standard bracketing method. The overall analytical protocol is validated against NRCC PACS-2 marine sediment CRM. The estimated limit of detection is 12ng/g. The proposed procedure was applied to the analysis of a real sediment core sampled to a depth of 160m in Lake Como, where Hg concentrations ranged from 66 to 750ng/g.

Manganese-promoted rhodium/NaY zeolite catalysts. An IR spectroscopic study

Manganese-promoted rhodium/NaY catalysts have been prepared by ion exchange and subsequent H2 reduction and by chemical vapour deposition (CVD) of [Rh(CO)2(acac)] and Mn2(CO)10 followed by thermal decomposition. Carbonyl FTIR spectra reveal significant differences between monometallic Rh/NaY and Mn/NaY and their bimetallic counterparts and they demonstrate profound effects of the synthetic methodology.The H2-reduced Rh/NaY samples prepared by ion exchange contain an appreciable amount of protons. As a consequence Rh+(CO)2 complexes are formed in the presence of CO. In contrast, samples prepared by CVD contain only Rh4(CO)12 and Rh6(CO)16 which can easily interconvert.The samples prepared by ion exchange and containing both Rh and Mn, exhibit bands at 1800 and 1830 cm–1 characteristic of bridging CO ligands which are always accompanied by low-frequency bands at 1684 and 1700 cm–1. These bands are attributed to an Rh2—CO—Mn2+ complex. It is suggested that these η2-CO species are formed by interaction between bridging CO ligands and Rhn–carbonyl clusters (n= 4 and 6) with Mn2+ ions via the oxygen end.This complex is totally absent in the bimetallic samples prepared by CVD of neutral organometallic precursors. These materials do not contain Mn2+ ions since the Mn2(CO)10 precursor is decomposed on previously deposited Rh° particles, thus forming bimetallic particles, the surfaces of which are presumably enriched in Mn. The number of Rh3 ensembles is therefore low and bridging CO ligands are not formed in the presence of CO.The possible relevance of these results for the catalytic conversion of CO–H2 mixtures on manganese-promoted Rh/NaY catalysts is discussed.

In-situ EXAFS investigation of non-acidic CVD-based Pt/KL catalyst under oxidation-reduction cycles

The structural evolution of non-acidic chemical-vapour deposition (CVD)-based Pt/KL catalyst has been investigated by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy. After in-situ reduction (H2, 400 °C), EXAFS results suggested the presence of small metal particles which may be localized inside the KL-zeolite channels. After the reforming of methylcyclopentane at 500 °C (and 1 atm), no coke deposition was evidenced by EXAFS, in agreement with the non-acidic character of this catalyst.Severe O2/H2 regeneration cycles did not alter the structure and location of Pt particles inside the zeolite.

Zeolite-supported metals by design: organometallic-based tin-promoted rhodium NaY catalysts

Abstract Rhodium–tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of α,β-unsaturated aldehydes. These model materials were obtained by chemical vapour deposition (CVD) and subsequent H 2 reduction of Sn(R) 4 (R=C 2 H 5 ; C 6 H 5 ) onto reduced Rh/NaY samples that were prepared by ion-exchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts containing appreciable amounts of proton, while non-acidic metal-in-zeolite samples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of decomposition is influenced by protons and by the tin precursor used. Carbonyl DRIFT spectra reveal clear evidences of a surface tin-enriched Rh–Sn phase only for proton-free CVD-based samples. In this respect, Sn(C 6 H 5 ) 4 leads to the formation of a higher tin coverage than that obtained from Sn(C 2 H 5 ) 4 . In the selective hydrogenation of citral (3,7-dimethyl-2,6-octadienal), the presence of protons was highly detrimental leading to the acetal formed by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be influenced by both monolayer and multilayer deposits of tin on Rh/NaY. The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnO x phases.