A. Palazov

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.

IR spectroscopy study of CO adsorption and of the interaction between CO and hydrogen on alumina-supported cobalt

IR spectroscopy was used to study CO adsorption on two samples of 5%Co/Al2O3. By variation of sample pretreatment, CO pressure, contact time and mode of hydrogen coadsorption, several surface species were identified: CO adsorbed on cobalt ions of different oxidation state, CO on metallic cobalt and polycarbonyls. Three species were distinguished which gave rise to bands in the 2050–2060 cm−1 region and could coexist on the catalyst, depending on the experimental conditions: CO linearly adsorbed on Co atoms with partial positive charge (Coδ+), hydrocarbonyl (CoHCO) and polycarbonyl [Co(CO)n]. The thermal stability and reactivity to hydrogen of the various species were studied and CO–Co0 was found to be the most stable.

Benzene adsorption and its interaction with carbon monoxide on alumina-supported platinum—An infrared spectroscopic study

Abstract Benzene and cyclohexane adsorption on Pt-Al 2 O 3 samples of optimum platinum contents was investigated, and benzene π-complex formation on the surface was directly observed when these hydrocarbons were in contact with the catalyst. The surface π-complex formed after adsorption on Pt-Al 2 O 3 sample had a considerable thermal stability, but reacted readily with hydrogen, and was removed by carbon monoxide subsequently adsorbed on the same sample. The direction of the change of the CO stretching frequency of the carbon monoxide adsorbed in its linear form served as an indicator of the type of the bonds formed when some other co-adsorbate was present on the same sample. For example, the difference between π- and σ-bonded coadsorbants could be registered in this way.

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.

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.

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.

Estimation of the number of co molecules adsorbed in various modes on different crystal planes of alumina supported polycrystalline palladium

The chemisorbed CO species formed on different crystal faces of polycrystalline palladium is not uniformly held on the metal surface. Parallel infrared and chemisorption measurements have demonstrated that the two-fold coordinated CO molecules on the Pd(111) plane are the most strongly adsorbed species. Under room-temperature evacuation of a sample with monolayer coverage about 20% of the double-bonded CO molecules on Pd(100) are desorbed. As to be expected, the weakest adsorbed CO species is the linear one on both the Pd(111) and Pd(100) faces. Using the variation in the CO bond strength some specially conducted experiments have made possible to find out a correlation between the integrated band intensities and the number of CO molecules adsorbed in various modes on the exposed crystal planes of the metal. Ratios between the integrated absorption coefficients of the IR bands from CO adsorbed in various modes were estimated. Empirical equations expressing a correlation between the number of CO molecules adsorbed on a definite face in a particular mode and the integrated intensities of the respective IR bands are proposed. Speculations concerning the crystal structure of the palladium particles are also presented.

An application of infrared spectroscopic data to investigation of the mechanism of some catalytic reactions of hydrocarbons on metals

Abstract The ir spectra of 1-hexene adsorbed on Ni SiO 2 , Pt SiO 2 and Pt Al 2 O 3 were obtained under different experimental conditions. An attempt to apply ir data for a better understanding of the mechanism of hydrogenolysis and related reactions has been made. The experimental results suggest that the different rate of CC bond rupture in the adsorbed on supported nickel and platinum hydrocarbons leads to a contrast between catalytic behavior of both metals in hydrogenolysis of paraffins. It has been found that the activation energy of hydrogenation-desorption of the surface carbon or/and hydrocarbon fragments is too low and cannot be a step which controls the rate of hydrocarbon hydrocracking on both nickel and platinum. The suggestion that CC bond rupture is the rate-controlling step, appears to be the most reasonable.

Infrared study of carbon monoxide adsorption and hydrogenation over PdFe catalysts

Abstract Carbon monoxide adsorption and hydrogenation over several alumina- and silica-supported Pd and PdFe catalysts have been studied by IR spectroscopy. A higher ratio of linear to bridged CO species was observed with the PdFe/Al2O3 samples. No methane formation was detected with the silica-supported catalysts. The PdFe/Al2O3 catalysts exhibited lower methanation activity, lower rates of formation and decomposition of the formate species and lower stability and reactivity of the adsorbed carbon monoxide. A mechanism of methane formation is proposed by which the formate species that occur on the support interact with hydrogen supplied from the metal by spillover.