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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.

Au-TiO(2) interface on silica support was aimed to be produced in a controlled way by use of Au hydrosol. In method A, the Au colloids were modified by hydrolysis of the water-soluble Ti(IV) bis(ammoniumlactato)dihydroxide (TALH) precursor and then adsorbed on Aerosil SiO(2) surface. In method B, Au sol was first deposited onto the SiO(2) surface and then TALH was adsorbed on it. Regular and high-resolution transmission electron microscopy (TEM and HRTEM) and energy dispersive spectrometry (EDS) analysis allowed us to conclude that, in method A, gold particles were able to retain the precursor of TiO(2) at 1.5 wt % TiO(2) loading, but at 4 wt % TiO(2) content the promoter oxide appeared over the silica surface as well. With method B, titania was detected on silica at each TiO(2) concentration. In Au-TiO(2)/SiO(2) samples, the stability of Au particles against sintering was much higher than in Au/TiO(2). The formation of an active Au-TiO(2) perimeter was proven by the greatly increased CO oxidation activity compared to that of the reference Au/SiO(2).

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.

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.

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.

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.

Adsorption of cyclohexanol on Pt SiO 2 catalyst was investigated by means of ir spectroscopy. At room temperature hydrogen atoms from both OH and tertiary carbons split off easily, resulting in the formation of cyclohexanone held on SiO 2 by hydrogen bonds whereas on Pt it is held by a π-bond or through the lone electron pair of oxygen. Dissociation of α-hydrogens in cyclohexanone commences only at circa 353 K. At 393 K bands characteristic of an enolic system were observed, suggesting formation of a π-oxoallylic species during dehydrogenation of cyclohexanone.

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.

The mechanisms of aromatization of C 6 and C 7 alkanes have been investigated over Ni catalysts. On the basis of radioactive measurements with [3- 14 C]methylpentane it was concluded that benzene is formed both by a CH x addition—abstraction mechanism and by intramolecular rearrangements. A change of the product selectivity with hydrogen pressure has been interpreted in terms of a change in the ensemble size of Ni governed by strongly adsorbed hydrocarbon fragments.

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.

Data on chemisorption of ethane on transition metal blacks have been correlated with hydrogenolysis activity measurements. On Pt and Pd the rupture of the C−C bond seems to be a hindered process. Desorption of methane requires larger activation on Co and Ni than on Ru, Rh or or Ir blacks.

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.