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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.

Various Pd shell thicknesses (0.12–1.5 nm) were synthesized on preformed Au particles of 5 nm size by seeded growth technique (15–80 at% Pd) using sodium citrate and tannic acid. The sols were characterized by UV–vis spectroscopy, TEM and high-resolution TEM (HRTEM) measurements. HRTEM confirmed the pseudomorphic growth of the Pd shell on the Au core. The Au/Pd core/shell particles were fixed onto SiO2 (Aerosil 200) support by PDDA polycation. The catalytic activity in acetylene hydrogenation and selectivity of competition between acetylene and propene were tested after O2 and H2 pretreatment. The samples even in “as prepared” state hydrogenated acetylene. The thin Pd layer (15–30 at% Pd) on Au provided higher hydrogenation activity than the thick Pd shell. However, thermal treatment of the samples in H2 stream causing Au/Pd intermixing shifted the activity maximum to higher Pd concentration (68–80 at% Pd). Comparison of the TOF (1/s) and selectivity values allowed us to conclude that the homogenized particle with 68–80 at% Pd shows better hydrogenation activity and selectivity than the thin Pd shell (15–30 at% Pd) on the Au core.

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.

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

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.

Gas phase semi-hydrogenation of 1,3-butadiene has been investigated over Pd catalysts of egg-shell type containing 0.03–0.05 wt% Pd. The measurements concentrated on the effect of the hydrocarbonaceous deposits formed from methane or butadiene at 473–523 K. The carbonaceous material has influenced the over-hydrogenation selectivity ( n -butane formation) and the distribution of n -butenes. Under the reported conditions the deposits form three-dimensional islands and in the case of diene they are also present on the support sites. The poisoning experiments with Pd-powder confirmed the formation of PdC x phase from both methane and butadiene. Deliberate diene and prolonged methane poisoning at 513 K (18 h) increased the n -butane selectivity and decreased the 1-butene/2-butene and trans -/ cis -2-butene ratios with simultaneous decrease of the apparent activation energy of hydrogenation. The results are interpreted by transport hinderance which affects mainly the surface fugacity 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.

Selective semi-hydrogenation of 1,3-butadiene has been investigated in excess of hydrogen over Co, Cu and Pd—Ni catalysts. The results on 5 wt.-% Co/Al2O3 and Pd—Ni/Nb2O5 catalysts provide evidence that hydrocarbonaceous materials formed on these surfaces play an important role in governing the competition between 1,3-butadiene and n-butenes. Similar results have been observed on 5 wt.-% Cu/SiO2 modified by preadsorption of n-butylamine. On the poisoned surface hydrogenation and isomerization of n-butenes have been remarkably inhibited, but the sample remains active in semi-hydrogenation of 1,3-butadiene. It is proposed that the unusual high selectivity of alkene formation is ensured by firmly held adspecies (FHA) formed from diene or n-butylamine. FHA cover and modify metal sites and do not allow adsorption of n-butenes as they apparently cannot compete with FHA. Hydrogenation of diene has been interpreted by a compression-displacement model, in which the high strength of diene complexation plays a definite role. Butadiene ‘pushes away’ FHA for reason of its high complexation strength and finds (create) reaction sites for its semi-hydrogenation. Preadsorption of CH3OH on 5 wt.-% Cu/SiO2 affects the regionselectivity of n-butenes: on the modified surface the selectivity of cis-2-butene increases at the expense of 1-butene. These results have been interpreted by electronic modification of Cu sites by intermediates formed from decomposition of methoxy species.

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.

Hydrogenation of 1,3-butadiene was investigated on catalysts prepared from an amorphous ribbon of composition Pd2Ni50Nb48, activated by oxygen and hydrogen treatments at 683 K. The hydrogenation activity of the sample decreased gradually with time, indicating formation of carbonaceous materials (oligomers). As a consequence of oligomer formation the butene selectivity remained constant even at 100% butadiene conversion. Apparently n-butenes could not compete with oligomers. Restart experiments confirmed that diene, due to its high strength of complexation, is able to adsorb on metallic sites in the presence of oligomers. The results emphasize the role of oligomers in selective semihydrogenations.

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.

Catalytic behaviour of Pd 2 Ni 50 Nb 48 amorphous ribbon and Pd-Ni/SiO 2 catalysts has been investigated in hydrogenation of 1,3-butadiene. Pd-Ni/SiO 2 catalysts were prepared using vapor phase adsorption and decompostion, of Pd(acac) 2 over Ni/SiO 2 and Ni(CO) 4 over Pd/SiO 2 . Presence of Ni has been observed to increase the selectivity of competitive hydrogenation of 1,3-butadiene in propene or 1-butene. The high selectivity of Pd-Ni in competitive hydrogenation has been interpreted by geometric effects caused by dilution of Pd with Ni and and partial poisoning of Pd-Ni ensembles by firmly held adspecies.

Several catalysts with 1 w% Pd + x w% Ag (x=0, 0.1, 1, 2) have been supported on αAl 2 O 3 or Na modified Na-Al 2 O 3 , and tested for the selective hydrogenation of 1,3-butadiene (BD) in the presence of 1-butene (1-B). With all catalysts the activity decreases markedly with aging due to accumulation of carbonaceous residue on the surface. Alloying the Pd with Ag and neutralising the acid sites on the support with Na both have significant effects on the selectivity, i.e. converting BD directly to n-butenes without isomerizing or hydrogenating significant quantities of the 1-butene. Addition of Ag to the Pd decreases the hydrogenation activity but increases the selectivity for BD conversion to butenes, although solid state and chemisorption measurements indicated that mixing of the two metals is limited under these conditions. Experiments with 13 C-BD demonstrated that the major reaction pathway is to 1-B (〉50%). When normalized to the same pressure, much more of the product n-butane is formed from the BD than from 1-B. Alternate exposure to 13 C-labeled and unlabeled BD showed that the surface residue does not mix significantly with the reactants and is formed predominantly from the BD, not the n-butenes. The presence of Na in the support seems to inhibit Pd/Ag mixing and results in decreased selectivity; some of the Pd or Ag may be encapsulated in the Na 2 O.

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 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.

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.

Transformation of n-pentane, 2-methylbutane and 2,2-dimethylbutane has been investigated as a function of the H2/hydrocarbon ratio over a 10 wt % Ir/Al2O3 catalyst of 14 % dispersion. The selectivity of isomer formation has been observed to decrease with the increase of the H2/hydrocarbon ratio and follow the sequence n-pentane > 2-methylbutane 2,2-dimethylbutane. The effect of the experimental conditions on the product selectivity has been interpreted considering the actual surface state of the working catalyst.

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.