G. Kadinov

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.

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.

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.

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.

Biogenic iron compounds: XRD, Mossbauer and FTIR study

AbstractMaterials based on biogenic iron oxides, which are a product of the metabolic activities of the neutrophilic iron-oxidizing bacteria (NIOB) from Sphaerotilus-Leptothrix group, were investigated. Natural microbial probes were collected from freshwater flow from Vitosha Mountain (Bulgaria) and cultivated under laboratory conditions in respect to select suitable cultures and conditions (nutrition media) for biomaterial accumulation of biogenic oxides. Samples were studied by physicochemical methods: X-ray diffraction, Mossbauer spectroscopy and IR spectroscopy. Their phase composition and physicochemical properties were obtained. Presence of both amorphous and crystal phase (ultra- and highly dispersed particles) was proved. Iron-containing compound in the natural biomass consists of α-FeOOH. The cultivated materials have more complex composition with iron-containing ingredients as α-FeOOH, Γ-FeOOH, Γ-Fe2O3 and Fe3O4. The sample of natural biomass was tested in reaction of CO oxidation and it showed potential to be used as catalyst support.

Study on the Composition of Biogenic Iron-Containing Materials Obtained Under Cultivation of the Leptothrix sp. on Different Media

The biogenic iron oxide/hydroxide materials possess useful combination of physicochemical properties and are considered for application in various areas. Their production does not require special investments because these compounds are formed during cultivation of neurophilic iron bacteria. Bacteria from genus Leptothrix develop intensively in the Sphaerotilus-Leptothrix group of bacteria isolation medium and feeding media of Fedorov and Lieske. These media are different in their composition which determined the present study as an attempt to clear up the reasons that define the differences in the composition of the laboratory-obtained biomasses and the natural biomass finds. FTIRS, Mössbauer spectroscopy, and XRD were used in the research. Comparative analysis showed that the biomass and control samples contain iron compounds (α-FeOOH, γ-FeOOH, β-FeOOH, γ-Fe2O3) in different ratios. The biomass samples were enriched in oxyhydroxides of higher dispersion. Organic residuals of bacterial origin, SO4, CO3, and PO4 groups were registered in the biogenic materials.

A study of the dispersity of iron oxide and iron oxide-noble metal (Me = Pd, Pt) supported systems

Samples of one-(Fe) and two-component (Fe-Pd and Fe-Pt) catalysts were prepared by incipient wetness impregnation of four different supports: TiO2 (anatase), γ-Al2O3, activated carbon, and diatomite. The chosen synthesis conditions resulted in the formation of nanosized supported phases—iron oxide (in the one-component samples), or iron oxide-noble metal (in the two-component ones). Different agglomeration degrees of these phases were obtained as a result of thermal treatment. Ultradisperse size of the supported phase was maintained in some samples, while a process of partial agglomeration occurred in others, giving rise to nearly bidisperse (ultra-and highdisperse) supported particles. The different texture of the used supports and their chemical composition are the reasons for the different stability of the nanosized supported phases. The samples were tested as heterogeneous catalysts in total benzene oxidation reaction.

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.

Transient Response Study of the Oxidation and Hydrogenation of Carbon Monoxide Adsorbed on Pd/Al2O3

Abstract IR spectroscopy was applied to study carbon monoxide oxidation and hydrogenation over a 5% Pd/Al2O3 catalyst. Upon oxidation at 338K the “island” mechanism operates, however, the structure of the CO adlayer in the “islands” is not homogeneous. In contrast, during hydrogenation at 433K a continuous rearrangement of the CO molecules occurs.