K. Matusek

The effect of catalyst treatment on the selective hydrogenation of acetylene over palladium/alumina

Abstract Hydrogenation of acetylene in an artificial acetylene-ethylene-deuterium mixture on three types of 0.04% Pd on alumina catalysts has been investigated. The effect of regeneration with oxygen, the hydrogenation temperature, ethylene fouling, and the effect of hydrogen vs deuterium were studied. It was established that supported palladium behaves similarly to the previously studied palladium black [Margitfalvi, J., Guczi, L., and Weiss, A. H., J. Catal. 72 , 185 (1981)] concerning the effect of carbonaceous deposits which affect the amount of surface hydrogen available for the formation of ethylidyne species. This form is responsible for the diminished selectivity. All factors removing this hydrogen result in an increased selectivity.

Spectroscopic and catalytic study on metal carbonyl clusters supported on Cab-O-Sil. I. Impregnation and decomposition of Fe3(CO)12

Impregnation of Fe3(CO)12 on Cab-O-Sil has been studied by ir spectroscopy, Mossbauer spectroscopy, and mass spectrometry. Isotope exchange between CO ligands of the impregnated sample and gas phase CO molecules was also investigated. On impregnation, two types of interaction can be distinguished: (i) interaction of the type FeCO…HOSi and CO…HOSi, shown by the shift to lower and higher ir frequencies for bridged and for terminal CO, respectively, and (ii) interaction between the metal framework and the support revealed in the oxidation of iron to form very small iron oxide particles. On impregnation a small amount of CO is evolved as a result of the interaction. CO exchange occurs faster with alumina-supported clusters than with silica-supported samples. On decomposition up to 370 K, the metal framework is retained and the cluster structure can be partly restored in a CO atmosphere. Above 420 K, Fe3(CO)12 is decomposed to form Fe2+ oxide on the surface. A possible mechanism for impregnation is discussed in terms of electron donation from the support oxygen to the iron d-bands as a result of which the metal-carbon bond strength is influenced. On decomposition the metallic iron interacts with the support OH groups causing oxidation and Fe2+ formation.

Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt–Sn/Al2O3 catalysts of different preparations

The platinum–tin interactions in Pt–Sn/Al2O3 catalysts were followed through several characterisation methods and modified by using two preparation procedures (1.5 wt% Pt, Sn:Pt=1:1): conventional coimpregnation with H2PtCl6 and SnCl4 (T sample) or by use of the bimetallic precursor [Pt(NH3)4]SnCl6, which was synthesised in the support porosity (N sample). The effects of these interactions on catalytic properties were displayed by the activity and selectivity in n-hexane rearrangement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to different temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. Insitu tin Mossbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as SnII; air treatment leads to a partial reoxidation of SnII to SnIV, sample N retaining more tin as alloy. X-Ray diffraction displays the simultaneous presence of PtSn, Pt3Sn and Pt with more alloys on sample N; the co-impregnated sample, which has a greater platinum phase, shows a better dispersion of tin (XPS data), in accordance with a high interaction with alumina. The catalytic activity was controlled by the platinum phase; for sample T, the influence of the addition of tin is restricted, whereas the catalyst prepared from the bimetallic precursor exhibits particular properties, attributable to the stabilisation of platinum in smaller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morphology of Pt ensembles present on various faces of Pt–Sn alloys. The lower amount of alloys in sample T can be related to a higher initial activity in C5 ring closure whereas the higher amount of these phases on catalyst N is in accord with a higher turnover frequency, and a good selectivity for the formation of olefins which are transformed into C6 saturated skeletal isomers in longer runs. The results are supplemented by thermodynamic data on the reduction of tin oxides and by the geometric properties of the low-index faces of PtSn and Pt3Sn alloys.

Sulfur adsorbed on Pt catalyst: Its chemical state and effect on catalytic properties as studied by electron spectroscopy and n-hexane test reactions

Abstract Pt black samples in a fresh state, deactivated by coking and treated with H2S were studied with electron spectroscopy. Their catalytic properties were tested in skeletal reactions of n-hexane. The Pt 4f XPS peaks show that Pt is predominantly in the metallic state after the treatments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amounts on sulfided samples. The S 2p band shows 4–8 at.-% S with respect to the total surface in two valence states: sulfate and sulfide, the latter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt—S interaction even after O2/H2 regeneration when the Fermi-edge Pt intensity rose to a height almost equal to that of a regenerated sulfur-free Pt. ISS shows that sulfur — as opposed to carbon and oxygen — is a surface component. Sputtering effect of He+ ions used in ISS can remove sulfate while a large part of sulfide is removed during O2/H2 regeneration and/or during n-hexane test runs. Both sulfidation and carbonization strongly decrease the overall catalytic activity. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containing sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O2/H2 treatment at 600 K but the same treatment — even repeated — restored the overall activity of a sulfided Pt only partially. We believe that sulfide acts mainly as a bonding modifier whereas sulfate can be a structural modifier influencing the selectivity similarly as coke or coke precursors.

Comparative study of various platinum catalysts in skeletal reactions of C6-hydrocarbons

Four platinum catalysts, Pt-black, PtC, PtSiO2, and PtAl2O3, have been compared in skeletal reactions of 3-methylpentane (3MP) and methylcyclopentane (MCP), in the presence of various partial pressures of hydrogen. Analogous hydrogen effects (defined as primary and secondary) have been observed for each catalyst. The selectivities could be explained almost exclusively by the influence of hydrogen. The selectivity of isomerization plus C5-cyclization vs fragmentation increased with increasing hydrogen pressure as did the ratio of saturated vs unsaturated C6 products. The 2-methylpentane vs n-hexane ratio (2MP/n-H) from both 3MP and MCP exhibited a strong hydrogen dependence (permitting separation of bond shift and C5-cyclic isomerization as a function of the hydrogen pressure), and significant crystallite size effects were also observed here.

Preparation, characterization, and catalytic activity of silica-supported highly dispersed PtFe catalysts

Abstract The catalytic and surface properties of highly dispersed PtFe catalyst supported on SiO2 have been investigated by Mossbauer spectroscopy, chemisorption, isotope exchange, and hydrogenolysis of ethane and n-butane. At low iron concentration Mossbauer spectra indicate PtFe formation and at the same time an increase in the adsorption of H2, O2, and CO. A parallelism between adsorption properties and catalytic activity can be interpreted by an increase in the number of surface metal atoms as a result of Pt-Fe formation. This is supported by the constancy of activation energy and the selectivity change in the reaction of n-butane hydrogenolysis. At higher iron loading iron is enriched on the surface to some extent and the reaction parameters mentioned are characteristic of iron itself, i.e., activity and activation energies decreased. Since the activity of PtFe SiO 2 is still higher by about three orders of magnitude than that of iron alone it is assumed that the activation of iron requires Pt neighbors by which either hydrogen can be supplied to iron sites or iron can be “diluted” thus retarding the deactivation process. These results are supported by the selectivity data.

Conversion of thiophene on molybdena–alumina catalysts containing Group 8–10 metals: effect of H2S uptake

Ageing and the effect of H2S on the activity and selectivity of Co-, Ru-, Pd-, Ir-and Pt-promoted and unpromoted MoOx–Al2O3 catalysts have been studied in relation to decomposition of thiophene. The overall activity increased on unpromoted and Pd-promoted molybdena–alumina, whereas on other catalysts it decreased substantially. The selectivity of production of butane and of fragmented hydrocarbons decreased on all catalysts. The changes in catalytic behaviour were caused by the uptake of H2S added to the system and produced in hydrodesulfurization (HDS) of thiophene. The results indicate no direct correlation between the propensity to interact with H2S and the HDS activity. It follows from data on changes in selectivity, that metal and molybdena act predominantly independently at the early stages of sulfidation.

Preparation and characterization of candidate catalysts for deep hydrodesulfurization of gasoils. Sulfidation and acidity characteristics of supported Ni/W and Ni/Mo catalysts

Abstract The uptake of 35 S -labelled H 2 S, the thiophene hydrodesulfurization (HDS) activity in pulse and flow system and the acidity measured by infrared (IR) band intensities of adsorbed pyridine were studied after various treatments over NiW/amorphous silica–alumina (ASA), NiW/Al 2 O 3 and NiMo/Al 2 O 3 catalysts. The Fourier Transformation Infrared (FTIR) measurements indicated that the NiW/ASA catalyst contained some Bronsted and much more Lewis acid sites, whereas less Bronsted acid sites were observed on the alumina supported samples. The concentration of Bronsted and Lewis acid sites decreased after sulfidation. The amount of total sulfur uptake of NiMo/Al 2 O 3 catalyst was about twofold of the one of NiW samples. The sulfidation level of the catalysts was much lower in the pulse system than in the flow one and also the thiophene conversion was much lower on NiW catalysts in the former setup than in the latter system. A band was detected in the radiochromatogram of NiW/ASA catalyst during desorption of H 2 S in the pulse system which was attributed to weakly adsorbed H 2 S on the Bronsted acidic sites. This weakly adsorbed H 2 S hindered the isomerization of n -butane in the flow system but not in the pulse system.

Surface structure and selectivity control in the CO + H2 reaction over FeRu Bimetallic catalysts

Abstract The structures of ruthenium and iron-ruthenium catalysts supported on alumina and on Cab-O-Sil were investigated by X-ray diffraction, photoelectron spectroscopy, temperature-programmed reduction, CO and O2 chemisorption and temperature-programmed desorption of hydrogen chemisorbed at various temperatures. Turnover frequencies and selectivities revealed in the CO + H2 reaction were correlated with the structures of the catalysts. It was established that carbon monoxide hydrogenation, being a structure-sensitive reaction, is primarily influenced by the particle size and by the addition of iron to ruthenium. As a secondary effect, the bonding mode of the surface hydrogen (activated chemisorption), which depends on the dispersion, support, calcination and addition of iron to ruthenium, greatly influences the catalyst selectivity. Weakly bonded hydrogen results in the formation mainly of methane, whereas substantial olefin formation is caused by hydrogen depletion. A mechanism for these effects and a possible interpretation of the promoter effect, which plays an important role in industrial catalysts, are given.

The effect of pretreatment on fresh and spent Pt/Al2O3 and Pt-Re/Al2O3 reforming catalysts

Abstract Effect of calcination, reoxidation and heating rate in an inert atmosphere on the subsequent reducibility of fresh and spent Pt/Al2O3 and Pt-Re/Al2O3 have been investigated. Catalytic activity was measured in the conversion of n-hexane. It has been established by using temperature programmed reduction (TPR) that after calcination the average oxidation state of the metals in Pt/Al2O3 and Pt-Re/ Al2O3 catalysts is 2.5 and 4.5 respectively. On reoxidation of the reduced catalysts at 720 K, the average oxidation state was 2 and 3.5, respectively, and TPR showed the presence of a separate platinum phase in the Pt-Re/Al2O3 catalysts. A separate Re phase was also observed in the spent Pt-Re/Al2O3 catalysts. In the pretreatment procedure the effect of sulphidation on the catalysts and its ability to retain sulphide was also investigated by a radioactive method using35S labelled H2S. Pt-Re/Al2O3 retains sulphide very strongly and its affect very much resembles to that of carbon formed at ageing. The behaviour of spent and fresh catalysts was also compared.