K. Foger

Reactions on ZSM-5-type zeolite catalysts

Catalytic reactions and sorption measurements have been carried out with ZSM-5 and silicalite catalysts which are distinguished by variation in skeletal heteroatom concentration. The catalysts were used in both the hydrogen-exchanged and sodium-exchanged forms. Sorption measurements were made with the hydrocarbons n-hexane, 3-methylpentane, and 2,2-dimethylbutane, and with the bases ammonia, n-butylamine, t-butylamine, and 4-methylquinoline. Catalytic reactions were carried out on both unpoisoned and base-poisoned catalysts using methanol, propylene, and 3-methylpentane reactants. In addition, the behavior of ethylene and ethanol reactants was also explored. The ease of base sorption and hydrocarbon sorption has been assessed in terms of effective molecular size in relation to the channel size of the catalyst, and this factor is also used as a basis for explaining the effectiveness of bases for poisoning the catalytic reactions. Temperature-programmed desorption (TPD) measurements with ammonia have been used to assess the energetic distribution of sorption sites for bases, and very strong binding sites with a TPD maximum for ammonia at about 780 °K have been identified as the probable sites used in the conversion processes. The main features of the catalytic conversion process are discussed. It is concluded that sorbed C3,C4 olefinic residues are general intermediates leading to aromatic formation. Under most circumstances, ethylene was relatively unreactive, and it is inferred that a sorbed C2 residue, which is related to ethylene by sorption and desorption, is an unlikely general intermediate. A mechanism is suggested for the formation of sorbed C3,C4 olefinic residues, and for initial carbon-carbon bond formation from methanol. Catalyst self-poisoning was observed with all catalysts except hydrogen-exchanged ZSM-5.

XPS investigation of strong metal-support interactions on Group IIIa–Va oxides

X-Ray photoelectron spectroscopy (XPS) has been used to measure the core level electron binding energies of Rh and Pt supported on Group IIIa–Va oxides after low (200 °C) and high (550 °C) temperature reductions. For 2% Rh/TiO2 we observe a small, reversible chemical shift (ΔEB = −0.2 ± 0.05eV) for the Rh 3d 52 peak indicating some electron transfer from Ti3+ to rhodium. This small value is in contrast to the large initial splitting of −0.6 eV which we found was due mostly to sintering of the rhodium during the initial reductions. At least one oxidation and reduction cycle was needed before a stable reversible set of binding energies was obtained. We estimate that the −0.2 eV value is a lower limit on the chemical shift, since there may be differences in the extra-atomic relaxation effects for the 200 and 550 °C catalysts due to a reversible change in the particle size or morphology. Further sputtering experiments on the pure oxides were used to disclose the nature of the surface species likely to cause the SMSI electron transfer. These were found to be Ti3+ (TiO2), Nb2–4+ (Nb2O5), Ta2–4+ (Ta2O5), and V2+ (V2O3). These reduced cations were readily oxidized in air at room temperature back to the stoichiometric oxide.

Reactions of neopentane and neohexane on platinum/Y-zeolite and platinum/silica catalysts

Isomerization and hydrogenolysis reactions of neopentane and neohexane have been studied in the presence of excess hydrogen at 455 to 625 K in a flow reactor over various Pt/silica and Pt/Y-zeolite catalysts having dPt in the range 1 to 20 nm. Additional catalyst characterization was provided by ESCA and by hydrogen TPD. Evidence is given to show that the reaction of neopentane was entirely confined to the platinum, even with Y-zeolite carrier. However, over Pt/Y-zeolite neohexane showed features which indicate dual-function catalytic behavior. As dPt decreased there was a general trend for the isomerization selectivity for neopentane (SI) to decrease and for the activation energy (Ea) to increase. From the way in which SI and Ea varied with dPt, it is concluded that there are two reaction pathways for neopentane, one on low index crystallite facets, and a second at platinum atoms of low coordination, the probable site for the latter being a single platinum atom. Hydrogen TPD revealed an increasing proportion of higher energy binding sites at very small values of dPt, and a correlation exists between SI and the concentration of adsorbed hydrogen under reaction conditions as estimated by TPD. ESCA examination of Pt/Y-zeolite (dPt ≈ 1 nm) showed the presence of relatively electron-deficient platinum, the extent of this being greater for Pt/(La)Y-than for Pt/(Na)Y-zeolite. For the neopentane reaction, SI and Ea were somewhat greater for Pt/(Na)Y- than for Pt/(La) Y-zeolite. Kinetic pressure dependence data showed that provided pH2 was high enough, hydrogen was an inhibiting gas. Relative to the strength of hydrogen adsorption, the surface reaction intermediate from neopentane was adsorbed more strongly on Pt/silica (dPt ≈ 4 nm) than on Pt/Y-zeolite (dPt ≈ 1 nm). The nature of the adsorbed intermediates and the reaction pathways are discussed.

An X-ray photoelectron spectroscopy and reaction study of PtSn catalysts

Abstract Platinum-tin catalysts prepared by different methods, and supported on γ-Al 2 O 3 have been studied with X-ray Photoelectron Spectroscopy (XPS), temperature-programmed reduction, and reaction measurements. Two major conclusions are drawn from the results. First, tin is present primarily in the oxidized form (Sn(II)) after high-temperature reduction (500 °C) of Pt-Sn γ- Al 2 O 3 catalysts, whereas alloy formation can occur on silica under the same conditions. No evidence for substantial amounts of Sn(0) is found on reduced γ-Al 2 O 3 -based catalysts. The spectroscopic reduction results are in agreement with TPR measurements in the present (and previous) work which show an average Sn reduction of 50%. Even PtSn complexes, which are readily reducible to Sn(0), are not reduced below Sn(II) on γ-Al 2 O 3 . Second, quantitative XPS measurements of surface tin concentration versus bulk loading reveal an inhomogeneous distribution of tin in the impregnated γ-Al 2 O 3 samples, with a large excess of tin on the external surface of Al 2 O 3 at loadings below 1 wt% Sn. The results show that impregnation of tin followed by platinum can result in separation of the two components due to the strong adsorption of tin ions in the outer surface of the γ-Al 2 O 3 particles. The best distribution of tin is found in the coprecipitated Sn-γ-Al 2 O 3 (Patent) preparations where the tin is uniformly distributed throughout the matrix. The reaction measurements of catalysts prepared via the different methods are compared and it was concluded that the activities and selectivities in methylcyclopentane and cyclohexane conversion are sensitive to the method of tin introduction. A catalyst prepared via the coprecipitation of Sn with γ-Al 2 O 3 followed by impregnation of platinum gave the highest sensitivities to activity and selectivity changes in the latter reactions. Whereas it is important to have platinum and Sn(II) in the vicinity of each other on the support, the mechanism of interaction is not well understood at this stage.

Hydrocarbon reactions on supported iridium catalysts

The reactions of ethane, n -butane, isobutane, neopentane, neohexane, 2,3-dimethylbutane, and methylcyclopentane have been studied over a range of dispersed iridium catalysts in which the iridium was supported on γ -alumina or Aerosil silica, and d Ir lay in the range ≤ 1 to 20 nm. The catalysts were characterized by electron microscopy, hydrogen adsorption, and temperature-programmed desorption of hydrogen. Hydrocarbon hydrogenolysis was the sole reaction pathway, except in the reaction of neopentane over catalysts with d Ir of 7 and 20 nm, where some isomerization to isopentane was observed, and in the case of isobutane, where there was some inferential evidence for isomerization to n -butane prior to hydrogenolysis. The hydrocarbons fell into one of two classes depending upon the activation energy and the type of hydrogenolysis reaction occurring. Reaction in a C 2 -unit mode (ethane the archetypal hydrocarbon) occurred with an activation energy in the region of 175 kJ mol −1 , and was the reaction mode for C I -C I , C I -C II , and C II -C II bond types. Reaction in an iso-unit mode (neopentane the archetypal hydrocarbon) occurred with an activation energy in the region of 235 kJ mol −1 , and was the reaction mode for C I -C IV , C I -C III , and C II -C III bond types. (C I indicates a primary carbon, etc.) The activation energy and the frequency factor (expressed as rate per surface iridium atom) for reaction in the C 2 -unit or the iso-unit mode was independent of d Ir . Provided the hydrogen pressure was kept sufficiently high, all reaction rates were proportional to P 1 HC P −3 H 2 . The reaction mechanism is discussed and is compared with the behavior of the corresponding reactions over platinum catalysts.

The effect of chlorine treatment on the dispersion of platinum metal particles supported on silica and γ-alumina

The reaction of Pt metal particles on SiO2 (Aerosil) and γ-Al2O3 supports with Cl2 (0.2–100% by vol.) in N2 or He in a flow system at temperatures (Tr) in the range 320–700 K has been studied with XRD, TEM, uv-diffuse reflectance spectroscopy, and temperature-programmed reduction (TPR) of the products. On SiO2, for 5–25% Cl2 only PtCl2 results, forming large (up to 500 nm diam.) singlecrystal sheets for 500 Tr ≥ 320 K. For 25–100% Cl2 various products form depending on Tr: (i) 320 ≤ Tr < 520 K a Pt(IV) chloride/support-surface complex [Pt(IV)Clx]; (ii) 520 ≤ Tr < 590 K PtCl4, (needles); (iii) 590 ≤ Tr < 670 K PtCl3 (needles); and (iv) 670 ≤ Tr ≤700 K PtCl2 (chunky). For Tr ≥ 600 K Pt is increasingly lost from the support for any concentration of Cl2. On γ-Al2O3, only support-surface bound [Pt(IV)Clx] is formed and no Pt is lost from the support for Tr≤ 700 K. On both supports, Pt metal is redispersed—its mean particle size is decreased on Cl2 treatment followed by reduction—only if [Pt(IV)Clx] is formed. Physically distinct chlorides, crystalline or amorphous, always reduce to low surface area aggregates.

Adsorption and temperature-programmed desorption of hydrogen with dispersed platinum and platinum-gold catalysts

Abstract Hydrogen adsorption has been studied by temperature-programmed desorption (TPD) and by static volumetric measurement (SVM) on a range of supported dispersed platinum and platinum-gold catalysts. For compositions in the range Pt 98, Au 2 to Pt 15, Au 85 mole%, a comparison of hydrogen adsorption data with predictions from surface enrichment theory combined with electron microscopic particle size measurement, led to the conclusion that after suitable thermal treatment, equilibrium surface enrichment by gold was achieved, although equilibrium was most difficult to achieve for the Pt 15, Au 85 composition. It was concluded from the nature of the hydrogen adsorption isotherms that, at higher pressures, some hydrogen could be adsorbed on surface gold atoms, probably as a result of spillover from the platinum component. The general shape of the TPD profiles on platinum-gold was independent of gold content; to a first approximation the gold acted as an inert diluent. The implications of this result for the mode of hydrogen chemisorptive bonding are discussed. With gold-free platinum catalysts there was a trend towards an extension of the high temperature tail of the TPD profile with decreasing d Pt in the range ⩽4 nm, due to the presence of hydrogen binding states of higher energy. Possible reasons for this are discussed. Illustrations are given of the technique of TPD-profile dissection using increased starting temperatures. A TPD peak with a maximum in the region 700–750 K has been assigned to desorption from the support, this binding state being populated via hydrogen spillover from the platinum.

Reactions of n-butane and neopentane on titania-supported iridium catalysts

Abstract The reactions of n -butane and neopentane have been studied over titania-supported iridium catalysts of varying dispersions (0.1 ≤ D ≤ 1.0). The catalysts were characterized by electron microscopy and adsorption and temperature-programmed desorption of hydrogen. After low temperature reduction (473 K) hydrogenolysis of n -butane occurred in the C 2 -unit mode and hydrogenolysis of neopentane in the iso-unit mode. Both reaction types are dependent on d Ir , and highly active sites, especially for the reaction in the iso-unit mode, are present in catalysts with d Ir ≥ 3.5 nm . Treatment in hydrogen above 600 K caused partial reduction of titania and Ti 3+ centers so produced interact strongly with the iridium particles (strong metal-support interaction, SMSI). Catalysts in the SMSI state are unable to adsorb hydrogen and for hydrocarbon reactions their catalytic activities fall by factors of 10 2 to 10 4 . The selectivity changes which are observed may be attributed to morphological differences between SMSI and non-SMSI catalysts. Treatment in 1% oxygen in helium at 473 K followed by reduction in hydrogen at 473 K successfully destroyed the SMSI state in the case of highly dispersed Ir/TiO 2 catalysts ( d Ir ≤ 1.0 nm ), but higher oxidation temperatures (673 K) are necessary to break the strong metal-support interaction in the case of less highly dispersed iridium catalysts. IrO 2 crystals are formed in oxygen at temperatures above 573 K and the catalysts undergo irreversible structural changes.

Oxidation of silica-supported Pt, Ir, and Pt/Ir catalysts

Abstract Pt and Ir impregnated alone or together on SiO2, Aerosil (Degussa), have been examined with X-ray diffraction, transmission electron microscopy, and temperature-programmed reduction as prepared and after heating in O2 or in 1% O2 in He at temperatures (Tox) in the range 100–700 °C. Mostly single-phase Pt/Ir alloy particles are formed only when the ratio of Pt to Ir is close to 50 50 . The surface composition of such particles is about Pt 73%, Ir 27%. On heating, no oxide is detected with Pt but the particle size increases. With Ir and Pt/Ir ( 50 50 ), a surface oxide, IrsO, is formed if Tox 300 °C. Pure Ir is completely oxidized at Tox = 550 °C but in Pt/Ir( 50 50 ) alloy only 65% of the Ir is oxidized at Tox = 700 °C when most oxide crystals have been separated from residual metal particles by gas phase transport of IrO2. The reduction of oxide is autocatalytic; once metal particles have been nucleated, atomic hydrogen is produced which nucleates further metal particles on the adjacent oxide. Agglomerates of heavily faulted Ir metal particles are formed on reduction. Thus oxidation and reduction appreciably diminish the metal surface area and destroy the single-phase alloy.

Skeletal reactions of hydrocarbons over supported iridium-gold catalysts

The reactions of 20:1 hydrogen/hydrocarbon (n-butane, neopentane, and 2,2-dimethylbutane) over silica-supported 0-86 mole % gold/iridium at < 10% conversion had activation energies of 170 kg/mole for n-butane and 2,2-dimethylbutane and of 242 kj/mole for neopentane, and the activation energies were independent of the gold content. The frequency factor of n-butane and 2,2-dimethylbutane decreased with increasing gold content of the catalyst and the product distributions indicated that the hydrogenolysis reaction was in the C/sub 2/-unit mode hydrogenolysis of neopentane had a frequency factor that was independent of the gold content and the reaction occurred only in the iso-unit mode. The iso-unit mode required a single iridium atom as the active site, and the C/sub 2/-unit mode more than one, probably two, adjacent iridium atoms. The surface mechanisms of the hydrogenolyses and the accompanying isomerization reactions are discussed.