L. Borkó

Structure of Pt–Co/Al2O3 and Pt–Co/NaY Bimetallic Catalysts: Characterization by In Situ EXAFS, TPR, XPS and by Activity in Co (Carbon Monoxide) Hydrogenation

Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al2O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co2+ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al2O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.

CO hydrogenation and methane activation over Pd-Co/SiO2 catalysts prepared by sol/gel method

Abstract Silica supported cobalt, palladium and cobalt–palladium bimetallic catalysts were prepared by sol/gel technique and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES). The CO hydrogenation and low temperature methane activation under non-oxidative condition were used as test reactions. It was established that, in contrast to other Pd–Co systems, bimetallic particles were not formed, only a part of cobalt was reduced in the presence of palladium and the cobalt was segregated to the catalyst surface. In the CO hydrogenation over a catalyst with the ratio of Co/Pd=2, a synergism was observed, while over cobalt and palladium catalysts alone lower activities were measured. Over pure cobalt sample, which has a limited reducibility, only short chain hydrocarbons, mainly alkenes, are formed, whereas in the presence of palladium (sites for enhanced hydrogen activation), the amount of alkanes increased. The limited reducibility of the cobalt was explained by the small particle size due to the preparation technique. Palladium acts in the bimetallic system not only as a component, facilitating cobalt reduction, but as a source of hydrogen participating in the reaction.

Re-Co/NaY and Re-Co/Al2O3 bimetallic catalysts: In situ EXAFS study and catalytic activity

To reveal possible relations between the structure and catalytic activity, in situ EXAFS and catalytic studies complemented with XRD, XPS, and TPR measurements have been performed on the promotion of cobalt catalysts by rhenium prepared by the incipient-wetness technique on Al2O3 and NaY zeolite.In situ EXAFS data collected at the Co K-edge and at the Re LIII-edge provided direct evidence of the rhenium-cobalt bond formation. The degree of reducibility depends on the support. There are two structural features, that is, on Re-Co/NaY nearly all rhenium atoms are in contact with Co atoms, whereas the cobalt atoms are surrounded by cobalt atoms in the first coordination sphere. In the case of Re-Co/Al2O3 samples the rhenium in oxide form may prevent the development of the “cobalt surface phase” (CSP), which is hardly reducible.The rate, α value and olefin/paraffin ratio showing special features in the CO hydrogenation and CH4 conversion to higher hydrocarbons are in line with the structural architecture of the catalysts. Despite the difference in the degree of reducibility, the various activity of Re-Co/NaY and Re-Co/Al2O3 may be interpreted by the formation of mixed oxide on alumina preventing the deactivation and agglomeration of small metal particles. Furthermore, the rhenium promotes cobalt activity and the selective formation of higher hydrocarbon. In the mechanism the rhenium also prevents fast deactivation of cobalt.

Comparative study on hydrogen-assisted `one-step' methane conversion over Pd-Co/SiO2 and Pt-Co/NaY catalysts

Abstract In our laboratory, methane conversion to higher hydrocarbons in “one-step” process under non-oxidative condition at low temperature was first introduced and investigated over Pd–Co/SiO 2 prepared by sol/gel method [Guczi et al., Catal. Lett. 54 (1998) 33] and over Pt–Co/NaY [Guczi et al., Stud. Surf. Sci. Catal. 119 (1998) 295] bimetallic catalysts. It was found that methane conversion in one-step process is at least 2.5 times higher than that measured in “two-step” process on the same catalysts. In the present work, the two-step and one-step processes are compared. It has been established that in one-step process when methane dissociation occurs in the presence of hydrogen containing helium, not only the production of higher hydrocarbons increases but also the selectivity is shifted towards larger molecules. Palladium–cobalt system proved to be more efficient than the corresponding platinum–cobalt catalysts.

Hydrogenation of 1,3-butadiene over catalysts prepared from amorphous Pd2Ni50Nb48 ribbon: Effect of self-poisoning on competitive adsorption

Abstract Hydrogenation of 1,3-butadiene was investigated on catalysts prepared from an amorphous ribbon of composition Pd 2 Ni 50 Nb 48 , 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.

Low and high activity states in the oxidation of m-xylene on palladium catalysts

Abstract Low and high activity states in the carbon dioxide vs. inlet temperature transient curves in the catalytic combustion of m -xylenes over Pd 2 Ni 50 Nb 48 amorphous alloy ribbons ( I ), Pd/stainless steel flakes ( II ) and Pd/clinoptilolite ( III ) samples have been investigated. In addition, in both states hysteresis, in high activity state self-sustained oscillation in the CO 2 vs. time at full conversion and damped oscillation at low conversion were observed over samples ( II ) and ( III ). It was established that hysteresis is a general phenomenon in both states and is not affected by the catalyst structure, but it can be ascribed to Pd as active component. Transition between the low and high activity state may be due to change in the PdO structure as was indicated by XPS studies. Oscillation is indicative of the partial accumulation of carbonaceous deposits over the catalysts at full conversion, while damped oscillation might be a result of the change of local concentration due to overheating the catalyst sample during combustion.

CO Hydrogenation over Re-Co Bimetallic Catalysts Supported over SiO2, Al2O3 and NaY Zeolite

The promotion of rhenium on the reduction of Co2+ ions in cobalt based samples prepared by various methods, such as, sol/gel technique, incipient wetness impregnationof Al2O3 and NaY, has been investigated in the CO hydrogenation. Among the samples the reducibility of the sol/gel, Re-Co/Al2O3 and Re-Co/NaY(IM) were the highest compared to Re-Co/NaY (IE) [8]. In addition to facilitating reduction of Co2+ ions, the rhenium increased the rate of reaction and the formation ofC5+ hydrocarbons at 10 bar pressures. The samples pretreated in various manner, revealed an about five fold increase in activity and an increase in the chain length in the NaYsupported sample while the activity and selectivity changed in various way in the sol/gel prepared samples. Simultaneously, rhenium increased the amount of olefins of low carbon numbers.In the mechanism the rhenium provides an action preventing fast deactivation of cobalt.

Structure and catalytic activity of Co-based bimetallic systems in NaY zeolite: Low temperature methane activation

Dissociative chemisorption of methane over cobalt, platinum, ruthenium particles and over their bimetallic combinations inserted into NaY by ion exchange, as well as sequential hydrogenation of the CHx species formed have been investigated. Temperature programmed reduction (TPR), X-ray diffraction (TRD), X-ray photoelectron spectroscopy (XPS) and CO chemisorption indicated that reduction of Co2+ ions in Co/NaY can significantly be facilitated by the presence of platinum and ruthenium. In Co-Pt/NaY bimetallic Co-Pt particles were formed, whereas in Co-Ru/NaY no direct evidence is available for the formation of Co-Ru bimetallic particles. It is established that Co/NaY sample modified by platinum and ruthenium exhibits a superior behavior in methane activation and in its sequential hydrogenation to higher hydrocarbon. This is interpreted by two factors: (i) enhanced reduction of cobalt and (ii) stabilization of small bimetallic particles inside the zeolite supercage. Calcination in oxygen promotes migration of ruthenium particles to the external surface, but in the case of cobaltruthenium bimetallic particles this process is hampered by cobalt. Nevertheless, in both systems a synergistic effect exists and the amount of CHx species considerably increase at the temperature lower than those applied for e.g. Co-Ru/Al2O3. For the formation of higher hydrocarbons the CHx surface species with a value of 1

New features of sol/gel prepared ruthenium catalysts in CO hydrogenation and CH4 coupling

Abstract CO hydrogenation and the low temperature non-oxidative methane coupling to form higher hydrocarbons over Ru/SiO 2 and Ru/Al 2 O 3 prepared by incipient wetness method and Ru/Al 2 O 3 prepared by sol/gel method (denoted by catalysts I, II and III, respectively) have been compared. Catalysts II and III possess selectivity for the formation of C 5+ hydrocarbons higher than sample I and the energy of activation is 85.7 kJ/mol on the catalyst III, which is about 29–33 kJ/mol less than those measured for catalysts I and II. The striking feature of the Anderson-Schulz-Flory distribution (ASF) points to the existence of an umbilical cord mechanism and at lower temperature the reinsertion of α-olefins in concurrence with hydrogenolysis results in a sigmoid distribution. With increase in temperature insertion becomes more and more prevalent process and the sigmoid curve is transformed into two straight lines with different slopes. Similar behavior is valid for the “two-step reaction” in the methane coupling carried out over different ruthenium catalysts, of which sample III has the highest selectivity for C 2+ hydrocarbons.

The role of carbon nanospecies in deactivation of cobalt based catalysts in CH4 and CO transformation

Abstract Deactivation of Pt 10 Co 90 catalyst supported on Al 2 O 3 and NaY in non-oxidativemethane transformation and CO disproportionation that may have importance in H 2 -deficient FT process was investigated in a constant flow system at 1023 K, 10 bar. Link can be found between deactivation of catalysts, especially of bimetallic Pt-Co nanoparticles and the formation, decomposition of meta-stabile CoC x (XRD) and formation of carbon nanotubes (TEM). The appearance of encapsulated Co particles inside the nanotubes (EDS) proves the restructuring of the Pt-Co bimetallic particles resulting in irreversible deactivation.