D. Shopov

Infrared spectroscopic study of the interaction between carbon monoxide and hydrogen on supported palladium

Preadsorbed carbon monoxide on alumina-supported palladium can be used as an indicator to examine the rearrangement of the metallic surface upon the action of hydrogen. The changes of the ir spectra from CO on PdAl2O3 when the sample is brought into contact with hydrogen show that the Pd(lll) plane is formed on disordered palladium crystallites even at room temperature, probably as a result of the formation of palladium hydride. Depending on the experimental conditions a number of ir bands are observed when a mixture of CO and H2 is passed over the catalyst. These bands are believed to belong to intermediates of carbon monoxide hydrogenation. It is suggested that the surface species is formed on palladium and after that is strongly adsorbed on alumina due to the spillover phenomenon. An attempt is made to explain the difference between the reaction pathways of methanation and methanol formation.

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.

Infrared spectra of adsorbed CO and catalytic conversion of cyclohexane on PtAl2O3 and PtPbAl2O3 catalysts

Abstract Pt Al 2 O 3 and PtPb Al 2 O 3 catalysts have been investigated by infrared (ir) spectroscopy of adsorbed carbon monoxide and selectivity for cyclohexane conversion. Addition of lead to Pt Al 2 O 3 catalysts decreases the frequency of the CO stretching vibration from adsorbed CO and affects the dependence of the respective band intensities upon the CO surface coverage. In catalysis the addition of lead to Pt Al 2 O 3 strongly suppresses fragmentation and deactivation in the cyclohexane conversion. The spectroscopic results are consistent with an s-electron transfer from lead to platinum. This modification of platinum has catalytic consequences. Especially the hydrogenolysis of cyclohexane is inhibited. The ir and catalytic data are consistent with the conclusion that alumina-supported PtPb catalysts exhibit both the ligand and ensemble effects.

Kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane

Abstract The kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane has been studied by the circulation flow method with a gradientless reactor. Dichloroethane inhibits the total oxidation reaction to a greater extent than the partial oxidation reaction. In this way it promotes the selectivity for ethylene oxide. The promoter was added to the feed gas stream in trace quantities ∮.2 to 28 ppm. The experiments were carried out under steady-state conditions, at atmospheric pressure and in the interval 240 – 292°C. Additional factors that could influence the selectivity, temperature, feed composition and degree of conversion of ethylene, were also investigated. It was established that the influence of the conversion is negligible. A linear model of the influence of the dichloroethane concentration on the selectivity could be applied to most series of the experiments. The selectivity-promoting effect of dichloroethane is stronger at higher temperatures. The selectivity is most sensitive to promoter concentration when the feed gas stream has the composition 50% O2, 25% C2H4 and 25% Ar. A kinetic model consisting of two rate equations - for the partial and for the complete oxidation is proposed, including a dependence on the partial pressure of dichloroethane. The model describes satisfactorily the influence of all factors on the selectivity. The role of dichloroethane is to inhibit the dissociative adsorption of oxygen on the silver surface.

A kinetic model of steady state ethylene epoxidation over a supported silver catalyst

Abstract A kinetic study of selective ethylene oxidation to ethylene oxide over a silver catalyst has been carried out by the circulation flow method. The silver was promoted by a Ca additive and was supported on alumina. The experiments were conducted under steady state conditions in the temperature interval 210 – 292°C and at atmospheric pressure. As a result of this kinetic study two rate equations, for the partial and for the complete oxidation reactions, are proposed. A reaction scheme is put forward, according to which adsorbed molecular oxygen produces ethylene oxide, whereas the atomic oxygen is responsible for the complete oxidation reaction. The empirical kinetic model corresponds to a Rideal-Eley type of mechanism. Selectivity decreased with temperature increase and with decrease of the oxygen content in the feedstock.

Synthesis and properties of hydrodesulfurization catalysts. IV. Thiophene hydrodesulfurization on molybdenum catalysts containing heteropoly compounds

Water soluble analogues of aluminium and cobalt heteropolymolybdates have been shown as possible precursors of active sites for thiophene hydrodesulfurization along with other molybdenum compounds. For the characterization of the catalysts Raman spectroscopy has been applied, and the catalytic activity of the samples is discussed.AbstractВодорастворимЫе аналоги гетерополимолибдата алюминия и кобальта оказались возможнЫми Предшественниками активнЫх центров вместе с другими молибденовЫми соединениями в гидродесульфурировании тиофена. БЫла использована спектроскопия Памана и измерение каталитической активности.

Some aspects of the bifunctional action of nickel-zeolite catalysts in the conversion of toluene

Abstract The bifunctional action of nickel-zeolite catalysts toward toluene conversion is studied. The additive behavior of both components of the catalysts is confirmed on the basis of the interpretation of their activity from three points of view: bifunctional catalysis, catalysis on the metal, and catalysis on the acidic component. New two-component catalysts of the type NiO CaNaY 3.2–5.0 (nickel content from 2.5 to 7.5 wt%) are developed. After reduction they are active for the dealkylation of toluene and under certain conditions for the disproportionation of toluene. The catalysts may be used for production of benzene and xylenes from toluene.

Spectroscopic study of chromia catalysts for dehydrogenation of secondary alcohols

Abstract Infrared, EPR and reflectance spectra in the uv and visible region were measured for six different chromium(III) oxide preparations. With some samples also the changes of spectra caused by the use of the catalysts for dehydrogenation of isopropanol to acetone at 350 °C were studied. The results are interpreted in terms of the change of coordination of surface Cr 3+ atoms and interaction between them due to preparation variables, surface hydration and influence of the reactants. A linear correlation was found between the Racah parameter calculated from the reflectance spectra and sensitivity of the reaction rate of dehydrogenation of secondary alcohols R · CHOH · CH 3 to the change in the structure of the group R.

Study of low-percentage alumina-supported nickel-molybdenum catalysts by ESR spectroscopy and magnetic measurements

Abstract Alumina-supported nickel-molybdenum catalysts (0.7–1.5 wt.-% nickel and 7–15 wt.-% molybdenum) were studied by ESR spectroscopy and magnetic methods. Thiophene hydrodesulphurization (HDS) was used to examine their activity. The effect of sample preparation on the state of nickel and molybdenum under reducing and sulphiding conditions was also investigated. The existence of a definite bond between nickel and molybdenum in the samples is responsible for a higher catalytic activity in HDS owing to the presence of nickel heteropolymolybdate.

Infrared spectroscopic study of Carbon monoxide oxidation over alumina-supported palladium

Abstract The interaction of carbon monoxide and oxygen over alumina-supported palladium was investigated by infrared spectroscopy under static and flowing conditions. An oxide phase, formed after high-temperature oxygen treatment, was only partially reduced at room temperature by subsequent CO titration. The transmission of IR radiation with the preoxidized samples was reduced due to the oxygen in the near-surface area. The subsurface oxygen could be desorbed under evacuation and heating at 773 K. The formation of an oxide phase under certain conditions caused a strong reduction of the rate of the CO oxidation reaction. At low and medial conversions this reaction proceeded via the Langmuir-Hinshelwood mechanism while the Eley-Rideal mechanism operated at maximum conversion when the catalyst surface was covered only by an oxygen layer. Probably, under definite reaction conditions, structural surface transformations and/or two-dimensional transitions occurred in the chemisorbed phase which gave rise to oscillations in the surface coverage and the reaction rate. During the CO oxidation the surface concentration of certain adsorbed CO species could be monitored only by IR spectroscopy. The data obtained by this technique indicate that under oscillatory conditions the reaction proceeded both via the LH and ER mechanisms. However, their relative participation varied. Oscillations were also observed with preoxidized samples but they occurred only on the reduced part of the surface. This finding is in disagreement with the mechanism proposed by some authors according to which under oscillatory conditions reduction and oxidation of the catalyst surface alternate together. Somewhat unexpectedly, a lack of synchronization between the oscillations in the concentration of two distinct surface species on the Pd(111) and Pd(100) faces was also observed by IR spectroscopy.