A. Sárkány

Hydrogenation of acetylene over low loaded Pd and Pd-Au/SiO2 catalysts

Abstract 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.

Structure sensitivity of acetylene-ethylene hydrogenation over Pd catalysts

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.

On the aging phenomenon in palladium catalysed acetylene hydrogenation

Abstract 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.

Adsorption and hydrogenation of ethylene, 1-hexene, and benzene and CO adsorption on PtAl2O3andPtSnAl2O3 catalysts

Abstract 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.

Hydrocarbon adsorption and coke formation on Pt/Al2O3 and Pt-Sn/Al2O3 catalysts

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.

Silica-supported Au nanoparticles decorated by TiO2 : Formation, morphology, and CO oxidation activity

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).

Green oil poisoning of a Pd/a1203 acetylene hydrogenation catalyst

The composition and deactivation effect of green oil on a Pd/A1203 catalyst (ICI 38-1) in the hydrogenation of a tail-end acetylene-ethylene mixture are discussed. Infrared and NMR measurements confirmed the presence of carboxylic acids in the oil. The green oil poisoned the acetylene hydrogenation sites but the product distribution pattern was not greatly affected.

Participation of support sites in hydrogenation of 1,3-butadiene over Pt/Al2O3 catalysts

Abstract 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.

Semi-hydrogenation of 1,3-butadiene on adspecies modified Pd—Ni, Co and Cu catalysts

Abstract 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.

Effect of hydrogen in the chemisorption of n-hexane over platinum black

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.