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In the present paper the final goal is to use the hydrogenolysis of hydrocarbons as a diagnostic tool for characterizing the metallic surface, i.e. to give a technique for drawing conclusions about the character of a metallic surface itself from the observed kinetic parameters. The main features are as follows: Surface properties of a transition metal are essentially determined by the electron structure and geometry of the bulk, members of the group VIII metals possess, therefore, apriori activity in the hydrogenolysis of a given hydrocarbon. Vulcano shaped curves observed in the hydrogenolysis of ethane, n-pentane, may, therefore, be an indication that neither too strong, nor too weak interaction between the catalyst surface and the substrate is good for the optimum catalyst performance. In addition to the ability of a metal in the “weak” to “strong” type of interconvension conceptualized earlier, the kinetic parameters may be influenced by some additional factors e.g. temperature, hydrogen pressure and the aging of the catalyst. A systematic change in the catalyst structure of platinum and ruthenium results in a characteristic variation in the activity of ethane and n-butane hydrogenolysis as well as in selectivity. By carefully analysing the limiting factors mentioned the effect of different supports, metallic dispersion, impregnating substrate, alloying with iron and rhenium and adsorbed sulphide on the activity and selectivity was determined.

Hydrogenation of acetylene has been investigated on Au-iron oxide catalysts containing 4.5, 3.0, 5.6 wt% Au prepared by deposition–precipitation (DP) with urea, adsorption of Au sol (SA) and by co-precipitation (CP) of HAuCl 4 and Fe(NO 3 ) 3 , respectively. The latter method produced Au particles supported on 2-line ferrihydrite (Fe 5 HO 8 ·4H 2 O). Au/iron oxide catalyst precursors (Au/α-Fe 2 O 3 and Au/2-line ferrihydrite) were characterized by BET, TGA, TPR and XRD, and the Au particle size by TEM measurements. The TPR measurements confirm the ease of transformation of 2-line ferrihydrite to magnetite (Fe 3 O 4 ) and that the formation of magnetite from α-Fe 2 O 3 depends strongly on the size of the gold and the supporting oxide particles. Particle size effect of gold has been investigated in semi-hydrogenation of acetylene. Temperature of hydrogen treatment has been observed to play a crucial role in the activity and ethylene selectivity. Au/FeO x samples hydrogen treated at 353–473 K prior to introduction of the reactants (H 2 :C 2 H 2 = 140, 0.11% C 2 H 2 ) show very limited over-hydrogenation (ethane formation is less than ≈1%) and 2–4% selectivity of oligomer (C4+) formation at 353–473 K. Hydrogen treatment of the samples at T > 573 K but lower than the wustite (FeO) stability point (≈843 K) resulted in partial reduction of Fe 3 O 4 and formation of Fe 0 . The shift of Au(1 1 1) XRD line to higher 2 Θ provided evidence for the incorporation of Fe 0 into the Au particles. Modification of the Au/Fe 2 O 3 perimeter and appearance of Fe 0 co-catalyst and Fe–Au ensembles on the surface increased the hydrogenation activity but decreased the selectivity of ethylene formation.

Acetylene hydrogenation has been investigated on 1.8 wt.% Au(I)/SiO2 and 1.9wt.% Au(II)/SiO2 catalysts prepared by fixation of Au sol to SiO2 (Aerosil 200). The mean particle size measured by TEM is 3.7 and 6.1 nm, respectively. For the sake of comparison a 2.1 wt.% Au/TiO2 sample was prepared by deposition-precipitation (DP) technique (mean particle size of Au is 3.3 nm). Transformation of acetylene was measured at 5 K/min ramp rate with gas mixtures containing the reactants at H2/C2H2=2 and 70 ratios. The C2H2 content of the gas mixture was 0.11% (0.11 kPa C2H2). The activity sequence at 423 K was: Au/TiO2>Au(I)/SiO2≫Au(II)/SiO2. Both the partial pressure of hydrogen and the temperature significantly affect the activity (acetylene conversion) and ethylene selectivity. Above 500–550 K over-hydrogenation (ethane formation) and hydrogenolysis (methane formation) decrease the ethylene selectivity. Faster deactivation and larger amount of deposit was observed on Au/TiO2 than on Au(I)/SiO2. A reaction scheme is proposed suggesting formation of sigma bonded intermediates as sp carbon hybridises to sp2 and sp3.

In the presence of pre-synthesized Au nanoparticles both pulse radiolysis and 2-propanol assisted reduction of Pd (II) chloride to palladium have been investigated with the intention to check the possible formation of Au–Pd heteroaggregates. Pulse radiolysis of Pd2+ (50 Gy dose/pulse at 298 K in 270 mM 2-propanol to scavange OH radicals) produced Pd nanoparticles of 3.9–2.5 nm depending on the citrate concentration (2.8–12 × 10−4 M). UV–vis measurements and TEM investigations confirmed that part of the Pd particles were attached to the Au particles of 28 ± 6 nm forming Au–Pd heteroaggregates. Hydrothermal treatment of Pd hydrosol at 363–395 K in the presence of Au nanoparticles was observed to facilitate formation of Pd tendrils on Au and 2D Pd structures by aggregation process. Reduction of Pd2+ with 2-propanol (1.55–2.2 M) in Au hydrosol and sodium citrate as capping agent was also tested at 348–353 K. At pH 3.5–4.5 reduction of Pd2+ resulted in an effective decoration of the gold particles with small crystals of Pd. The observed structures definitely differ from the usual Au/Pd core/shell nanoparticles. The Pd–Au samples were characterized by UV–vis, transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). Hydrogenation of 1,3-cyclooctadiene in the liquid phase was applied for the additional characterization of the structurally different samples.

Pd hydrosol was synthesized by reducing PdCl2 solution with dissolved H2 in the presence of PDDA [poly(diallyldimethylammonium chloride)] as ionic stabilizer. O2 treatment resulted in the reappearance of ligand to metal charge transfer bands of PdCl 4 2− . Repeated oxidation and hydrogen treatments caused aggregation/agglomeration of the Pd particles yielding big spherical aggregates of 30–50 nm but still keeping the size of the 5–6 nm primary particles. Aggregation was attributed to the reduction of PdCl 4 2− anion having high local densities around Pd particles surrounded by PDDA polycation. Au–Pd bimetallic hydrosols were synthesized by reduction with 2-propanol and by pulse radiolysis technique in the presence of PDDA. Both reduction modes synthesize Au core–Pd shell structures. The mode of reduction was observed to affect the particle size and the thickness of the Au core and Pd shell. At the Au rich samples the differences in the optical spectra were attributed to different dispersions of the Au–Pd particles. Au–Pd bimetallic hydrosols were adsorbed on Aerosil 200 and the supported samples were tested in gas phase hydrogenation of acetylene.

Pd particles were prepared from PdCl 2 solution (>10 −3 M) in the presence of methanol, ethanol or 2-propanol using 60 Co radiation source to generate solvated electrons and organic reducing radicals. Temperature programmed oxidation (TPO), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize Pd particles. Radiolytic and thermal reduction of Pd 2+ appears to take place simultaneously resulting in the extensive poisoning of Pd sites as confirmed by TPO. The relatively high intensity of the C 1s spectra at 287–290 eV binding energy (BE) confirms the presence of keto and carboxylic groups on the surface. Faceted particles decorated with smaller ones have been observed by TEM measurements. Hydrogenation of 1-octyne in n -heptane has been used as a test reaction to characterize the surface sites. The increasing hydrogenation activity in repeated experiments can tentatively be explained by partial removal of surface impurities and by disintegration some of the adhered particles upon the corrosive action of 1-octyne.

Pd/SiO2 catalysts were prepared by adsorption of Pd sols on support Aerosil 200 and characterized by transmission electron microscopy (TEM), CO chemisorption and temperature programmed oxidation (TPO). The Pd hydrosols were generated from PdCl2 solution and ethanol as reducing agent in the presence of poly(diallyldimethylammonium chloride), PDDA. The pH of the solution was increased to ensure adsorption of the Pd sol with high concentration of PDDA. The catalytic activity was tested in benzene hydrogenation. The presence of organic impurities and the role of morphology affecting the activity have been discussed in terms of the preparation procedure applied.

Acetylene hydrogenation and formation of surface deposits have been investigated on two series of Pd and Pd-Au/SiO2 catalysts differing in metal particle size (D=0.47 and 0.08). Gold was deposited via ionization of pre-adsorbed hydrogen over pre-reduced Pd/SiO2 in order to ensure selective poisoning of the Pd surface. The non-steady-state regime of operation and the accumulation of hydrocarbonaceous overlayer were tested in pulse-flow experiments. Concentration of surface hydrocarbonaceous deposits accumulated during different treatments was also determined by temperature programmed oxidation (TPO). Hydrocarbon overlayer was observed to form immediately and its presence appeared to be a necessary requisite to get steady-state conversion and selectivity data. A large excess of hydrogen suppressed the formation of carbonaceous laydown and increased the over-hydrogenation of acetylene. Presence of Au decreased the carbon coverage and improved the ethene selectivity. Decoration of Pd by Au and the morphology of particles explain the ethene selectivity improvement.

A Pt-Sn/γ-Al 2 O 3 bifunctional catalyst was modified by tin, bismuth and tellurium using metal adsorption technique. The parent and the modified catalysts were tested in n -octane reforming under conditions similar to industrial applications. The added tin decreased the reformate yield a little, the aromatic content decreased and the isoparaffin content increased, the research octane number remained high. Tellurium and bismuth deposited greatly deactivated the metal function of the catalyst; the liquid yield of the product is far less than that over the parent platinum–tin catalyst. The isoparaffin and aromatic selectivity however changed favourably. More isoparaffin hydrocarbon was found in the liquid product over the modified catalysts. The reforming function of the metal was suppressed and, therefore, the reactions of alkenes occurring on the acid sites of the catalyst became dominant.

Results of the last decade with respect to the selective hydrogenation of hydrocarbons with multiple unsaturation (dienes and alkynes) over heterogeneous palladium catalysts are reviewed. Factors such as metal dispersion, carbon deposits, and the use of promoters and additives controlling catalytic activities and chemo-, regio- and stereo-selectivity are discussed. A detailed treatment of the status of the selective removal of hydrocarbon impurities with multiple unsaturation from industrial feedstocks is also given.

Pd nanoparticles were prepared by ‘controlled colloidal synthesis’ (CCS) using aerosil silica support. On SiO 2 , 1–2 nm thick ethanol-rich adsorption layer in ethanol–toluene and ethanol–water mixtures was formed into which Pd(II) ions diffused and were reduced there by ethanol. Using Pd(II)-acetate precursor in an ethanol–toluene mixture (sample G1) and PdCl 2 precursor in an ethanol–water mixture (sample G2), palladium particles of 7.0 and 8.8 nm size, respectively, were produced. Effects of treatments in air and hydrogen were studied on the Pd/SiO 2 samples. The Pd particles showed a twofold/threefold increase in the catalytic activity in benzene hydrogenation after treatment in air at 573 K. The activity increase cannot be explained by a change in the particle size as indicated by transmission electron microscopy (TEM) and CO chemisorption data. Augmentation of turnover number is interpreted by the removal of carbonaceous residues and by the restructuring of the surface caused by carbon removal followed by PdO/Pd transition in the subsequent hydrogen treatment.

Selectivity of product formation has been tested in hydrogenation of acetylene over 0.3 wt.% Pd/α-alumina and 0.5 wt.% Pd/TiO2catalysts. Non-steady-state regime of catalyst operation was tested in pulse-flow experiments. Significant carbon poisoning appears to be a necessaryrequisite for selective formation of ethylene. The effect of hydrogen and acetylene partial pressure has been tested on the selectivity of C4products. At 273–298 K the catalysts showed 26–35% selectivity for C4 hydrocarbons and

Using different reduction conditions Pd powders have been prepared and characterized by chemisorption and TPO (Temperature Programmed Oxidation) measurements. The amount of carbon deposits introduced during preparation seems to be peculiar to the preparation procedure. The O2 consumption curves attributed to PdO formation have shown two peak minima suggesting strong differences in the accessibility of Pd particles.

Formation of carbonaceous deposits from 1,3-butadiene has been investigated over a group of Pd catalysts. Hydrogen generated in decomposition of diene at 473–523 K participates in diene hydrogenation, resulting in the formation ofn-butenes. XRD measurements have confirmed formation of dissolved carbon (PdCx) phase. DRIFT measurements over 0.06 wt.%Pd/γ-Al2O3 have revealed bands at 1575 and 1464 cm−1, suggesting formation of carboxylate structures. Selectivity of the competitive hydrogenation in 1,3-butadiene and propene mixture R(BD)/R(Pr) has been measured on “fresh” and poisoned samples. Accumulation of deposits has decreased the R(BD)/R(Pr) ratio. The results have been interpreted by transport hindrance and a greater prevalence of non-selective low coordination sites on the poisoned surface. Measurements over Pd, Cu, Pt and Rh catalysts have shown that the highn-butane selectivity over Pt and Rh is also accompanied with low R(BD)/R(Pr) values, suggesting that thermodynamic and mechanistic factors are not entirely separable.

Gas phase hydrogenation of 1,3-butadiene, propene, and mixtures of 1,3-butadiene and propene has been investigated over 0.05 wt% Pd/Al 2 O 3 catalyst. The catalyst was deliberately poisoned by methane (513 K) or diene (473 K) and the effect of the hydrocarbonaceous materials was tested on the product selectivity and the rate of hydrogenation of diene–propene mixtures. It has been concluded that a large quantity of hydrocarbonaceous deposit decreases the surface fugacity of diene due to transport hinderance and in consequence enhances the over-hydrogenation of diene and permits the hydrogenation of propene in the presence of diene.

Pd—Ag catalysts poisoned by trapped hydrocarbons have been tested for selective hydrogenation of 1,3-butadiene (BD) at 284 K in a static recirculation reactor. Two levels of carbon poisoning were investigated: (i) the effect of self-poisoning (aging) in successive experiments at 284 K and (ii) the effect of deliberate poisoning. In the latter case first butadiene or acetylene was circulated over the catalyst above 373 K and then the activity and the selectivity of BD hydrogenation was tested over the poisoned surface at 284 K. The successive experiments performed at 284 K in H2/BD=2.2 mixtures resulted in the severe poisoning of the reaction sites. The steady state activity of the self-poisoned samples was about 1% of the initial activity. Vacuum or O2 treatment at T < 433K of the self-poisoned samples increased the hydrogenation activity. The result has been interpreted by surface restructuring of the hydrocarbonaceous deposits which apparently brings about re-creation of certain sites. Because of hydrogen's low surface fugacity and high stability of the multiple bonded probably oligomer species hydrogen treatment at T < 473K was not sufficient to restore the original activity. The selectivity of n-butane was initially less than 0.05% at 40–55% conversions and was not affected by self-poisoning. However, as the amount of polymers built up upon deliberate poisoning at 385–488 K following diene and acetylene treatments the n-butane selectivity gradually increased. The deliberate poisoning influenced only to a small extent the consumption of butadiene while the ratio: R1−B/RBD (rate of 1-butene consumption/rate of BD consumption) decreased from 3 to 0.1. Normal isotope effect of hydrogen addition was observed over both self-poisoned and deliberately poisoned catalysts: RH/RD was found to be less for n-butane formation than for BD consumption. Formation of n-butane on the catalyst has been interpreted by slow diene transport caused by the presence of oligomer layers on metal sites.

The effect of acidic sites on activity and product selectivity has been investigated in 1,3-butadiene hydrogenation over platinum catalysts prepared with high purity Al2O3 supports (Condea SB, Woelm N, Degussa Alumina C, ICN-N). Degassing the reduced samples at 723 K has increased the selectivity of n-butane formation on catalysts of high platinum dispersion (DCO=68–93%). Over Woelm and ICN aluminas isomerization of 1-butene formed on platinum sites to cis- and trans-2-butene can also be observed. Poisoning the catalysts with water, ammonia and ethylenediamine (EDA) suppresses isomerization of 1-butene, but only EDA hinders markedly formation of n-butane. The results have been interpreted by participation of Lewis acid sites and by contribution of electron deficient platinum sites in the vicinity of the metal-support interface.

The selective hydrogenation of 1, 3-butadiene was studied in the presence of a 10:1 excess of 1-butene over supported bimetallic catalysts containing palladium in a vapour phase atmospheric plug flow reactor in the temperature range 288–313 K. Addition of copper to alumina-supported Pd increases its selectivity for converting butadiene to n-butenes without saturating or isomerizing the n-butenes. For example, with atomic ratio Cu/Pd = 2, the bimetallic catalyst selectively converts > 99% of the diene to n-butenes while isomerizing or saturating less than 1% of the starting 1-butene. Under similar conditions, Pd alone maintains high n-butene selectivity to only about 50% conversion of the diene. Over both catalysts the kinetics of 1, 3-butadiene hydrogenation are first order in hydrogen and zero order in butadiene. Hydrogen sorption experiments showed that Cu addition decreases both the surface adsorption and the bulk absorption, the former suggesting surface decoration of Pd by Cu and the latter indicating significant bulk Cu-Pd interaction in the supported bimetallic samples.

Effects of hydrocarbonaceous deposits have been investigated in hydrogenation of 1,3-butadiene over Pd-Ag/Al2O3 catalysts. Studies on the effect of various treatments on self-poisoned samples revealed very different behavior of the surface deposits. Because of hydrogen's low surface fugacity, hydrogen treatment was not sufficient to remove surface species at T

The effect of the support in constraining the formation and defining the structure of bimetallic particles has been considered. Alumina and NaY zeolite supported platinum and cobalt bimetallic catalysts are illustrated to rationalize this effect. The surface composition and dispersion of the bimetallic particles are further correlated with the catalytic activity of bimetallic catalysts in CO hydrogenation.

The irreversible adsorption and coke formation from methane, n-hexane, 1-hexene and 1,5-hexadiene were investigated on Pt/Al2O3 and Pt-Sn/Al2O3 between and 450°C. Depending on temperature, chemisorption, coke precursor and coke formation have been observed. Coke precursors are formed on Pt sites and transferred to alumina. Two routes of coking were observed. Presence of tin decreases the adsorption bond strength of 1-hexene and at low concentrations increases the amount chemisorbed. This phenomenon is explained by the ensemble effect. A drain-off effect is proposed to explain the decrease of the coke coverage on the metallic sites upon addition of Sn.

Homologation of n-pentane and cyclopentane results in formation of benzene on Ru, Os, Re and Ir/SiO2 catalysts. Formation of benzene can be explained by the insertion of carbene into terminal vinilic carbon followed by C6-dehydrocyclization.

The effect of dispersity of Ni catalysts has been investigated in the hydrogenolysis of ethane. The kinetic order in hydrogen and the apparent activation energy are dependent on the dispersity. The experimental results reveal that the structure sensitivity of this reaction appears to a certain extent through the variation of the hydrogen order.

The self-poisoning effect of deuterium exchange reactions of C1−C6 straight chain aliphatic hydrocarbons as well as hydrogenolysis and dehydrocyclization of n-hexane in the presence of deuterium on a nickel powder catalyst has been investigated. The poisoning is enhanced with increasing carbon number. The results can be correlated to the ease of dissociative adsorption, i. e. to the formation of multiply bonded species. A tentative model is proposed to explain the “decrease” of deuterium exchange in n-hexane when the reaction is shifted towards hydrogenolysis and benzene formation. It is shown that at elevated temperatures the poisoning upon deuterium exchange is caused not only by blocking of the active sites with deposits but also by hindrance of desorption of the starting molecules in the original/exchanged form.

The reaction of 2-methylpropane has been investigated in presence of helium and hydrogen over a platinum black catalyst. The appearence of olefins in the gas phase indicates that the precursor of these unsaturated compounds adsorb ed on the surface may have some role in the self-poisoning observed during hydrogenolysis and/or isomerization.

Transformation of n-hexane has been investigated as a function of partial pressure of hydrogen at 563K over Pt catalysts including Euro-Pt-1. The carbon coverage and the fraction of free sites on the working catalysts together with product distributions have been determined. The conclusions are as follows: (1) The partial pressure of hydrogen regulates the coverage of the firmly held hydrocarbons which in turn for geometric/electronic reasons affect the product selectivity; (2) Independently of the size of the Pt particles, with the increase of the partial pressure of H 2 , the only reaction whose selectivity has been observed to increase continuously is fragmentation. Extrapolation of the selectivity values to large excess of H 2 seems to indicate that the structure sensitivity of the product selectivity ceases at large H 2 /nH ratios.

The effect of aging on the hydrogenation of a tail-end mixture (0.29 mol % C2H2, 0.44 mol % H2 balanced by C2H4) has been investigated on Pd-black and on various Pd/Al2O3 catalysts. Over supported catalysts the selectivity of ethylene formation decreases with time on stream in the first 40–60 hours of operation whereas the C2H2 conversion is not, or only slightly, influenced. With Pd-black the selectivity of ethylene formation changes in the opposite direction.14C-C2H2 experiments have revealed that the aging has only a minute effect on the intrinsic selectivity of C2H2 hydrogenation. The effect of the surface polymer and the nature of the ethylene hydrogenating sites are discussed. The results with supported catalysts have been interpreted by a polymer promoted hydrogen spillover.

The mechanism of dehydrocyclization has been investigated with a group of branched C6 and C7 isomeric hydrocarbons containing only four or five carbon atoms in the main chain on Ni and Co catalysts of small and medium dispersion in the temperature range 573–650 K. At low H2HC ratios transformation of 2-methylpentane, 3-methylpentane, and 3-ethylpentane yields toluene and ethylbenzene, respectively, besides benzene. Evidence is presented that the formation of benzene takes place via CHx-addition to a vinylic group, 1,6-ring closure, and CHx-abstraction in the adsorbed phase. In the proposed reaction sequence the rate of CHx insertion is regarded as the rate-limiting step of aromatization. Participation of the bond-shift or the C5-cyclization route of aromatization is not significant over Ni and Co.

The skeletal rearrangement reactions of butane, 2-methylpropane and 2,2-dimethylpropane were investigated over Al2O3- and SiO2-supported Pd, Pt, Ni and AgNi catalysts. As opposed to Pt, isomerization over the other catalysts proceeds mainly by carbene intermediates by a so-called “external C1-migration” pathway. This reaction is parallel with homologation. Hydrogen sensitivities and radiotracer studies confirm these suggestions.

The hydrogen sorption of carbon-supported platinum catalysts, studied by the potentiodynamic method, differs significantly from that of unsupported platinum. The difference depends on the preparation method and on the nature of the support. Possible explanations are suggested as (i) some kind of interaction between metal and support atoms during impregnation, (ii) high dispersity (amorphous structure) and (iii) hydrogen spillover onto the carbon support.

The exchange reaction between cyclohexane and deuterium gas and the reactions of dehydrogenation and hydrogenolysis in the presence of deuterium have been investigated over a wide range of temperature on platinum and nickel powder catalysts. The results are as follows: 1. i. In the temperature range 30–100 °C only deuterium exchange takes place. The activation energies are the same for both catalysts although the rate of exchange is about one order of magnitude higher on Pt than on Ni. 2. ii. Between 100 and 200 °C poisoning occurs due to extensive decomposition. This effect is characteristic of nickel and causes a decrease of two orders of magnitude in catalytic activity but leaves the activation energy unchanged. 3. iii. Above 200 °C besides exchange other reactions commence and result in the formation of n-hexane, benzene and toluene on Ni and benzene on platinum. In this temperature range the deuterium appears in equilibrium distribution not only in the products but in the parent molecule. It seems very likely that at high temperature the dissociative adsorption is not rate controlling and thus the exchange and the chemical transformation are controlled by the desorption and by the surface reaction, respectively.

Adsorption and hydrogenation of ethylene, 1-hexene, and benzene, and carbon monoxide adsorption on alumina-supported Pt and PtSn catalysts were studied. Gravimetric experiments and infrared (IR) absorption spectroscopy combined with chemisorption measurements were used for this purpose. Additionally fresh and coked catalysts were characterized by the IR spectra of CO and chemisorption data. Added tin causes a slight increase in benzene adsorption and a distinct decrease in ethylene and 1-hexene uptakes, whereas the hydrogenation activity is inhibited. Adsorption and catalytic data with benzene can be explained by a model of flat adsorption on Pt Al 2 O 3 and tilted adsorption on Pt-Sn Al 2 O 3 and are consistent with electronic modification of platinum by tin. Electronic interaction between platinum and tin is also indicated by the IR data. The most active sites for hydrocarbon decomposition on the platinum surface are the same as those on which carbon monoxide is multiply bonded. Deposited coke and tin block the same active sites on the platinum surface.

Pd-Based catalysts are used for the selective hydrogenation of alkynes and alkadienes to the corresponding alkenes on a large scale. Addition of Pb and Zn (Lindlar type catalysts) to Pd, and alloying of Pd with Ag, Cu, Pb improve the selectivity of alkyne hydrogenation. The presence of organic bases organic sulfides, or CO in the reaction mixture is known to inhibit hydrogenation of the alkene formed. Although structure sensitivity in alkene hydrogenation has been systematically investigated, only a few studies have been devoted to the effect of metal dispersion on the selectively of hydrogenation of an alkyne-alkene mixture. The work described in this article aimed to investigate the effect of Pd dispersion on the competitive hydrogenation of a mixture containing 0.3% C/sub 2/H/sub 2/, 0.5% H/sub 2/ balanced by C/sub 2/H/sub 4/ over Pd/alumina as catalyst. In particular, attempts have been made to clarify the extent of ethane formation produced directly from acetylene (intrinsic selectivity of the ethane formation) and that from ethylene in the presence of acetylene with the use of (/sup 14/C)C/sub 2/H/sub 2/. Isotopic labeling techniques have unambiguously demonstrated that ethane is generally formed from both acetylene and ethylene. 30 references.

Formation of carbonaceous deposits from n-hexane, methane and 1, 5-hexadiene has been investigated at 673 K over Pt/SiO2 catalysts of differing dispersion. The measurements with 1, 5-hexadiene, 2, 4-hexadiene and trans-hexatriene over SiO2 have confirmed the monofunctional behaviour of the catalysts. The initial C/Pts value is smaller, the subsequent rate of deposit formation and the fraction of “free” sites are higher on highly dispersed samples than on poorly dispersed ones. On catalysts of high Pt dispersion the C-C bond rupturing activity might impede the transformation of C6 entities into graphitic deposits.

Transformation of n-hexane has been investigated over 0.8 wt% Pt/Al2O3 and 15 wt% Pt/Al2O3 catalysts (metal dispersion is 66 and 6.3%, respectively) at 593 K as a function of the partial pressure of hydrogen. The surface composition of the working surface has been characterized by measuring the fraction of “free” sites, total hydrocarbon coverage, the coverage of the firmly held species. The experimental results confirm that the surface state of the catalysts is affected by the variation of the H2/n-hexane ratio. Under the experimental conditions investigated 10–20% of the surface platinum atoms might participate in forming the reaction sites responsible for the skeletal reactions.

The chemisorption and transformation of n-hexane have been investigated over Pt black in the presence of hydrogen between 450 and 600 K. Direct gravimetric measurements have confirmed that only 3–8% of the total hydrocarbon coverage can be removed by evacuation or by purging the system with an inert gas. Hydrogenation experiments have been used to separate reactive and irreversibly bound hydrocarbons; their coverages have been measured as a function of temperature and partial pressure of hydrogen. The variation of the product selectivity with the partial pressure of hydrogen has been interpreted considering the effect of the irreversibly bound hydrocarbons on ensemble size and on availability of hydrogen.

Hydrogen sensitivity of MCP ring opening on Ir/Al2O3 and that of n-hexane transformation on Pt black have been investigated. Comparison of the results on O2–H2-treated and ‘carbon’-precovered catalysts confirms that in the interpretation of the hydrogen effect on product distribution one has to regard the steric/electronic effect of trapped hydrocarbons on the surface metal atoms.

The adsorption and deuterium exchange of methane and ethane and the hydrogenolysis and isomerization of ethane, butanes and some C5 hydrocarbons have been investigated on Ru, Rh and Ir blacks. Product distribution patterns, together with data on chemisorption and deuterium exchange, suggest that surface transformations of hydrocarbons into strongly/multiply bonded species easily take place. The initial multiple rupture of hydrocarbons is a result of the slow product desorption, that seems to be the rate limiting step of the hydrogenolysis on these metals.

The chemisorption and transformation of n-hexane have been investigated over Pt black in the presence of hydrogen between 450 and 600 K. Direct gravimetric measurements have confirmed that only 3–8% of the total hydrocarbon coverage can be removed by evacuation or by purging the system with an inert gas. Hydrogenation experiments have been used to separate reactive and irreversibly bound hydrocarbons; their coverages have been measured as a function of temperature and partial pressure of hydrogen. The variation of the product selectivity with the partial pressure of hydrogen has been interpreted considering the effect of the irreversibly bound hydrocarbons on ensemble size and on availability of hydrogen.

Chain lengthening homologation of propane,n-butane,2-methylpropane,2-methylbutane and 2,2-dimethylpropane has been investigated over Pt, Pd and Ni catalysts. With both Pd and Ni the chain lengthening selectivity was observed to be commensurable to the isomerisation one. Addition of CHx onto the β-carbon atom in n-butane and propane as well as chain lengthening of 2-methylpropane and 2-methylbutane were observed. The mechanism of the skeletal rearrangement is discussed in terms of CHx addition.

Deuterium exchange reactions of methane, ethane, propane and neopentane, and the hydrogenolysis and isomerization of ethane, propane, n-butane and isobutane on platinum black have been studied in the temperature range 363–743 K. There is a gap of 150 K between the temperature of deuterium exchange and that of hydrogenolysis and isomerization. Only at high hydrogen pressure (about 50 kN m–2) and at low catalytic activity is the hydrogenolysis characterized by selective loss of a terminal methyl group, except for n-butane in which there is an equal chance of any of the C—C bond rupturing. The selectivity for isomerization has a maximum at high hydrogen pressure. These values together with kinetic data suggest that the ability of platinum in an exchange reaction to convert weakly bonded intermediates into strongly bonded ones is much less than that of nickel. The phenomenon of π-olefin and π-allyl bond conversion is applied in part to explain the results obtained. Brief comparison is given between the catalytic properties of platinum and those of nickel for the reactions investigated.