Osmar A. Ferretti

Surface organometallic chemistry on metals: II. Characterization of new bimetallic catalysts generated by reaction of Sn(n-C4H9)4 with silica-supported rhodium

Reaction of Sn(n-C4H9)4 with Rh supported on silica results in a new bimetallic Rhue5f8Sn catalyst which is extremely active and selective in the reduction of ethyl acetate to ethanol. Whereas Rh/SiO2 gives rise to a selectivity for ethanol of 57%, the Rhue5f8Sn catalyst obtained by the organo-metallic route results in a higher activity and a selectivity to ethanol as high as 98%. Above a Sn/Rh value of 0.3, the activity varies linearly with the tin content which suggests that the enhanced catalytic activity is due to a new intermetallic phase. The catalysts have been characterized at various steps of the preparation. The starting reduced catalyst Rh/SiO2 A with CO exhibits the typical infrared absorption bands of linear and bridged CO. Reaction of oxidized A with Sn(n-C4H9)4 in refluxing heptane occurs mostly between Rh2O3 and the organotin compound to give an unreduced RhIII7z.sbnd;SnRx bimetallic surface complex B, the existence of which has been suggested from mass balance, STEM, and IR spectroscopy. Reduction of B at 773 K under H2 leads to bimetallic particles with an average size of 2.2 nm and which do not contain any organic fragment (catalyst C). C chemisorbs only 0.1 H/Rht and 0.4 CO/Rht which is in sharp contrast with the values obtained with A (1.1 H/Rht and 1.1 CO/Rht). CO chemisorption on B gives only a single absorption band at 2000 cm−1 corresponding to linear coordination of CO. The presence of tin has apparently three effects: (i) it decreases significantly the amount of CO and H2 adsorbed; (ii) it apparently isolates rhodium atoms from their neighbors; (iii) it increases slightly the electron density on rhodium. Redox behavior of the Rhue5f8Sn/SiO2 toward O2 and silanol groups of silica has also been observed. With a fully reduced catalyst C, Rh(0) and Sn(0) are fully oxidized by O2 to Rh2O3 and SnO2. Thermal treatment of catalyst C under flowing He results in an oxidation of tin by surface silanol (or adsorbed water) to give a partially oxidized Sn species. H2 is evolved during this oxidation process. The origin of the high activity and high selectivity (without hydrogenolysis property) of these catalysts is ascribed to the presence of a new catalytic phase in which rhodium atoms are isolated from their neighbors without any “ensemble” able to cleave the Cue5f8C and Cue5fbO bonds of ethyl acetate.

Deactivation of steam-reforming model catalysts by coke formation: I. Kinetics of the Formation of Filamentous Carbon in the Hydrogenolysis of cyclopentane on Ni/Al2O3 Catalysts

Abstract Coke formation in the hydrogenolysis of cyclopentane on Ni/Al 2 O 3 catalysts in the temperature range 300–500°C was studied. A similar change in apparent activation energy for coking obtained with catalysts presenting different surface states coincides with the appearance of filamentous carbon on the catalyst surface. A definite carbon-to-surface nickel ratio seems to be necessary for the nucleation of the filaments. Coked catalysts reveal different activities for hydrogenolysis and hydrogenation reactions depending on the nature of the carbon deposit. The free-metal surface area decreases by two orders of magnitude during coke deposition accompanying cyclopentane hydrogenolysis. Nevertheless, as soon as the filamentous carbon appears on the catalyst, the nickel recovers a significant part of its initial activity, which shows that the accessible fraction of metal increases at the initial stage of the whisker formation.

Surface organometallic chemistry on metals: I. Hydrogen and oxygen interaction with silica-supported and alumina-supported rhodium

Abstract In order to understand the reactivity of Sn( n -C 4 H 9 ) 4 with supported rhodium, the chemisorption properties of silica-supported and alumina-supported rhodium toward O 2 and (or) H 2 has been investigated. It has been found that chemisorption of O 2 on rhodium is particle size dependent. At low particle size, bulk oxidation to Rh 2 O 3 occurs at 300 K. At high particle size, surface oxidation to surface Rh 2 O 3 occurs at 300 K and is followed by a slow bulk oxidation to Rh 2 O 3 . The rate of bulk oxidation can be increased considerably at 473 K. Chemisorption of H 2 on rhodium supported on silica or alumina occurs in two ways. Below 120 K, two forms of adsorbed hydrogen are present; above 260 K, only one form, an irreversibly adsorbed hydrogen, is observed. Both the results of chemisorption and those of thermodesorption suggest that below 120 K these two forms of hydrogen are present in equal amounts on the surface. For each form, the stoichiometry is close to one hydrogen atom per surface rhodium atom. Room temperature titration of the chemisorbed oxygen by hydrogen is possible according to the following equation, where y is the amount of adsorbed hydrogen (this value is pressure dependent): Rh 2 O 3 /support + (3 + y) H 2 → 3 H 2 O/support + 2 (Rh − Hy)/support.

Surface organometallic chemistry on metals: Part IV. Selective hydrogenation of ethyl acetate to ethanol on RhSn/SiO2 bimetallic catalysts: A mechanistic study

Bimetallic catalysts Rhue5f8Sn, Niue5f8Sn and Ruue5f8Sn, prepared via an organometallic route, are very selective for the hydrogenolysis of ethyl acetate to ethanol. This generation of catalysts, requires lower temperatures and lower pressures than the traditional copper chromites and constitutes an elegant method for the hydrogenolysis of (fatty) esters into (fatty) alcohols, which are important raw materials derived from natural resources. The objective of this work was to identify, via kinetic experiments, the primary products of the reaction in order to propose a mechanistic explanation for the selective hydrogenation of ethyl acetate to ethanol. Changing the Sn/Rh ratio drastically modifies both the activity and the selectivity of the catalysts: In the range 0 < Sn/Rh < 0.2, both the activity and the selectivity for hydrogenolysis to CO and CH4 decrease. On the other hand, for Sn/Rh ratios higher than ca. 0.2, both the activity and the selectivity for the ethanol formation increase almost linearly with the tin content while the rates of alkane and CO formation remain constant and very low. At Sn/Rh = 1.7, kinetic studies indicate that ethanol, acetaldehyde and ethane are the primary products, whereas CO and CH4 are secondary products. The rate low, which is of the form: r = k(PH2)0.5KEAPEA/(1 + KEAPEA + KEPE) n nis totally different from that observed on pure rhodium. These kinetic data, associated with previous physical observations leading to the concept of ‘site isolation’ of rhodium atoms in a matrix of tin atoms, lead to the proposal of a reaction mechanism involving a single rhodium atom.

Hydrogenation of crotonaldehyde on Pt/SiO2 catalysts modified with tin added via surface organometallic chemistry on metals techniques

The importance of the preparation procedure and the understanding of the mechanisms that control this stage on the final properties of the catalyst are evidenced for PtSn systems obtained via surface organometallic chemistry on metals (SOMC/M) techniques. The temperature of preparation reaction also plays a fundamental role as regards the nature of the active phase finally obtained, giving rise to catalytic phases having different levels of selectivity to unsaturated alcohols (UOL) in the crotonaldehyde hydrogenation. Organobimetallic catalysts lead to very high selectivities to crotyl alcohol (80% at 5% conversion).

Hydrogenation of carbonyl compounds using tin-modified platinum-based catalysts prepared via surface organometallic chemistry on metals (SOMC/M)

Abstract The catalytic behaviour of some compounds containing Cue605O and/or Cue605C bonds has been studied over silica-supported platinum-based catalysts, modified with tin. Tin was introduced by means of surface organometallic chemistry on metals (SOMC/M) techniques. The effect of the obtention conditions upon the catalytic performance was evidenced through the study of three systems having the same Sn/Pt atomic ratio (0.4), but prepared and activated at different temperature. In the hydrogenation of butyraldehyde and butanone, the adsorption of the η1-(O) type appears as highly favourable, both from a geometric and electronic point of view. In the benzaldehyde hydrogenation, the increase in the catalytic activity for PtSn-OM and PtSn-BM systems is quite more important than in the PtSn-OM∗ system, fundamentally by electronic effects associated with the presence of ionic tin and of the phenyl group. In the case of the cyclohexene, geometric and electronic, as well as steric effects lead to a strong reduction of the hydrogenation rate of Cue605C bond. These results can be extrapolated to explain the behaviour of the unsaturated α,β-aldehydes. The hydrogenation of the Cue605O group is promoted and the adsorption modes favourable to the Cue605C hydrogenation are inhibited by tin. The combination of both effects leads to the sequence of selectivity to UOL: Pt⪡PtSn- OM ∗ PtSn -BM

Stability promotion of Ni/α-Al2O3 catalysts by tin added via surface organometallic chemistry on metals: Application in methane reforming processes

Abstract This paper reports the effect of selective tin addition to nickel catalysts via a controlled technique (surface organometallic chemistry on metals, SOMC/M), and the performance of the resulting systems in methane reforming processes: partial oxidation (POM), CO 2 reforming (R) and mixed (CO 2 +O 2 ) reforming (MR), with particular emphasis on their resistance to coke formation. It is demonstrated that SOMC/M techniques allow to obtain well-defined bimetallic phases in a controlled way. It is found that there is a range of tin concentrations (0.01–0.05xa0wt.%) for which the stability of the bimetallic catalysts is markedly enhanced with respect to nickel catalyst, without affecting either the activity level or the selectivity to syngas. These facts are explained in terms of a more demanding nature in size of the active sites needed for carbon formation reaction, when compared to methane activation reaction to synthesis gas.

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

The behaviour of Ni supported catalysts, obtained using Ni(NO3)2 and Ni-acetylacetonate as precursor compounds, is analyzed. It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly. The systems show different carbon structures and sintering rates, depending on the precursor compound employed.

Hydrogenation of crotonaldehyde on Rh-Sn/SiO2 catalysts prepared by reaction of tetrabutyltin on prereduced Rh/SiO2 precursors

Abstract Rh and Rh-Sn/SiO2 catalysts have been prepared, characterized and used in the crotonaldehyde hydrogenation reaction. New bimetallic Rh-Sn catalysts were prepared by reaction of Sn(n-C4H9)4 over a prereduced Rh-SiO2 precursor. It was observed that the temperature of the preparation reaction also plays an important role in the nature of the Rh-Sn active sites. Thus, at lower temperatures, 363xa0K, Rh(SnBu4−x)y species remain adsorbed on the silica surface whereas at 773xa0K, the organometallic residue decomposes leading to Rh-Sn bimetallic catalysts. Tin addition causes a significant drop in hydrogen chemisorption capability but only a slight increase in metal particle size. Electron diffraction detected the presence of Rh0, RhSn2 and SnO2 phases for the bimetallic catalysts and Rh0 for the monometallic ones. XPS shows an important surface enrichment in tin suggesting that SnOx species would migrate and deposit on the metal crystals. The active sites generated upon these treatments allow the polarisation of the carbonyl group and consequently an enhancement in the selectivity to crotyl alcohol is obtained.

Characterization of Mo-MnO catalyst for propane oxidative dehydrogenation

Abstract Manganese oxide catalysts impregnated with molybdenum have been examined for the propane oxidative dehydrogenation. These catalysts exhibit catalytic activity and yield to propane at temperatures as low as 623xa0K. The catalysts were characterized by S BET , XRD, Laser Raman, TPR, EPR and XPS. Independently of molybdenum loading, none of the impregnated catalysts shows the patterns corresponding to molybdenum oxide by XRD. The reaction of molybdenum with manganese would probably occur starting from OH centers as reacting nuclei and it continued from there. Thus, the OH of manganese oxide surface would restrain the dispersion of the formed molybdate. However, the results of Raman spectroscopy analysis of catalysts before and after the catalytic test indicate that the reaction test could be responsible for modifications in the surface composition. Characterization before and after catalytic test corroborates the concept of the existence of a “dynamic feature” in which the reaction of the adsorbed phase plays a key role in the surface reconstruction. EPR results as well as XPS ones for catalysts after test have shown that independently of molybdenum loading, at a superficial level, all of them have similar characteristics. Evidently, the reaction test promote a surface reconstruction. XPS data together with those obtained by XRD which show that the greater the load, the more intense the MnMoO 4 signal indicate that MnMoO 4 accumulates in a definite place being the XRD signal more intense by localized mass growth. These evidences and the consequent description of the resulting surface architecture allow to understand the results of catalytic evaluation tests.