O.S. Alekseev

Supported metallic catalysts obtained by anchoring metal complexes on carbon supports

Abstract Catalysts containing particles of metallic palladium have been synthesized by anchoring metal complex precursors on carbon supports (active carbon, carbon black, ultra-dispersed graphite and diamond). Anchored precursors were obtained by interaction of solutions of tetrachloride or allylcyclopentadienyl palladium complexes with the supports. Based on data on the peculiarities of HgPdCl 4 adsorption on carbon sorbents of different natures, three types of adsorption sites of Pd II ions have been identified and a quantitative evaluation of their strengths made. The size of the Pd crystallite formed upon reduction of the anchored complexes and its catalytic activity in cyclohexene hydrogenation depend upon the strength of the adsorption site. An increase in the strength of the adsorption site leads to a decrease both in the size and catalytic activity of Pd crystallites. The effect observed is associated with a modifying (ligand) action of atoms involved in the adsorption site. Fourier transform IR spectroscopic data indicate that the interaction of C 3 H 5 PdC 5 H 5 with an oxidized diamond surface occurs with participation of the carboxyl groups of the support. According to transmission electron microscopic data, Pd/C catalysts reduced with hydrogen at 300 °C contain Pd particles 10 – 20 A in size. On increasing the reduction temperature to 600 °C, the dispersion of Pd/diamond remains almost unchanged while the size of the metallic particles on Pd/graphite increases to 100 A. This difference is due to peculiarities in the interaction of Pd particles with graphite and diamond surfaces in a hydrogen atmosphere. In the structurally insensitive hydrogenation of benzene, the turnover number (TON) activity of the Pd/C catalyst is commensurable with that of Pd/SiO 2 , but in the structurally sensitive hydrogenolysis of ethane and hydrogenation of CO to methanol, the TON of Pd/C is one to two orders of magnitude lower, probably due to dissolution of carbon atoms in the Pd particles. Introduction of Zr ions into Pd/C leads to the appearance of a new absorption band at 1600 cm −1 in the IR spectrum of adsorbed CO, an increase in TON activity by 1 – 2 orders of magnitude in methanol synthesis and a drastic decrease in activity in ethane hydrogenolysis, the inhibiting effect of Zr ions being completely suppressed by high-temperature oxidation of the samples. These specific features are similar to those observed earlier for (Pd+Zr)/SiO 2 and Pd/ZrO 2 , and seem to be due to the formation of mixed Pd-Zr active sites in the systems under consideration.

Effects of the Interaction of Dispersed Metal Particles with the Support in Catalysts prepared using Organometallic Compounds. : V. Influence of Group IVB Ions on Adsorption and Catalytic Properties of Pd/SiO2

Abstract Results are reported of a systematic study of the effect of Group IVB transition metal (titanium, zirconium, hafnium) ions attached to the silica surface on the properties of dispersed palladium particles. Catalysts have been obtained from organometallic precursors. The following effects of the metal ions have been observed: (i) an increase in stability of palladium particles to sintering in hydrogen atmosphere at up to 873 K; (ii) a decrease in palladium surface coverage with chemisorbed hydrogen and carbon monoxide; (iii) an increase in TON activity and selectivity of palladium towards methanol synthesis from carbon monoxide and hydrogen; and (iv) a decrease in palladium activity in benzene hydrogenation and ethane hydrogenolysis. The data obtained have been compared with those for palladium supported on bulk TiO2 and ZrO2 and are discussed in terms of different hypotheses of the nature of a strong metal-support interaction.

Effects of the Interaction of Dispersed Metal Particles with the Support in Catalysts Prepared Using Organometallic Compounds : VI. Study of the State of Components in (Pd+Zr)/SiO2

Abstract Fourier transform infrared, X-ray photoelectron and extended X-ray absorption fine structure spectroscopy applied were to study the formation and state of components and forms of carbon monoxide adsorption with (Pd+Zr)/SiO2 catalysts obtained from organometallic precursors. Based on the results obtained, a model of active sites assuming direct interaction of highly dispersed palladium particles with surface zirconium ions is proposed. Taking into account the similarity of the adsorptive and catalytic properties of (Pd+Zr)/SiO2 and Pd/ZrO2 systems, which reveal the effects of a strong metal-support interaction (SMSI), the possibility of the application of this model to explain the mechanism of SMSI is discussed.

Formation and Properties of Dispersed Pd Particles Over Graphite and Diamond

Abstract According to the TEM and HT data, the size of Pd particles in Pd/C catalysts prepared via interaction of C 3 H 5 PdC 5 H 5 complex with the surface of carbon supports and reduced with hydrogen at 573 K ranges from 10 to 20 A . After increasing reduction temperature up to 873 K, dispersion of Pd/Altunit (ultra dispersed diamond) remains practically the same, while the size of metal particles in Pd/Sibunit (graphite-like carbon) increases up to 100 A . This difference is due to the peculiarities of the interaction between Pd particles and the surface of graphite and diamond in hydrogen atmosphere. In the reactions of acetylene and vinylacetylene hydrogenation the specific activity of Pd/C is comparable to those of Pd/SiO 2 , but in 1–2 orders lower in reactions of ethane hydrogenolysis and CO hydrogenation. The introduction of Zr ions to Pd/C results in the following effects: appearance of a new absorption band at 1600 cm −1 in the spectrum of adsorbed CO, increase in the specific activity in methanol synthesis by 1–2 orders but drastic decrease in the specific activity in ethane hydrogenolysis, while high temperature oxidation of the samples completely eliminates the inhibitor action of Zr ions. These peculiarities are similar to those previously observed for (Pd+ Zr)/SiO 2 as well as for Pd/ZrO 2 , and are likely to be due to mixed Pd-Zr n+ active sites formed in the systems.

Effects of the interaction of dispersed metal particles with the support in catalysts prepared using organometallic compounds. I. Influence of ions of group ivb elements on the properties of Pt/SiO2

Abstract The introduction of ions of group IVB elements (M IV = Ti, Zr, Hf) into Pt/SiO 2 catalysts leads to the same adsorption and catalytic effects (decrease in chemisorption of H 2 and CO, increase in activity in methanol synthesis, decrease in activity in benzene hydrogenation and ethane hydrogenolysis) as supporting the Pt on the bulk TiO 2 . The results indicate that the interaction of platinum clusters of less than 10Awith surface MIV ions is a possible reason for such effects.

Influence of Zr ions on the properties of Pd supported on graphite and diamond

Abstract FTIR, TEM and XPS methods have been applied to study the formation, dispersion, electronic and catalytic properties of the (Pd+Zr)/C system obtained by consecutive anchoring of (π-C 3 H 5 ) 4 Zr and C 3 H 5 PdC 5 H 5 on the surface of oxidized graphite and diamond. The interaction of (π-C 3 H 5 ) 4 Zr with the surface of oxidized diamond proceeds with participation of carboxyl groups of the support, and the supported Pd complex then seems to be coordinated with anchored Zr ions. According to the XPS data, Zr in the reduced (Pd+Zr)/C catalysts is found in oxidized states and Pd in metallic states. The size of Pd particles in (Pd+Zr)/C catalysts reduced in mild conditions ranges from 10 to 20 A. Pd sintering over graphite proceeds more rapidly compared to diamond. The introduction of Zr ions to Pd/C results in the following effects: appearance of a new absorption band at 1600 cm −1 in the spectrum of adsorbed CO, increase in the specific activity in methanol synthesis by 1–2 orders of magnitude but a drastic decrease in the specific activity in ethane hydrogenolysis, while high temperature oxidation of the samples completely eliminates the inhibitory action of Zr ions. These peculiarities are similar to those previously observed for (Pd+Zr)/SiO 2 as well as for Pd/ZrO 2 , and are likely to be due to mixed PdZr n + active sites formed in the systems.

EPR study of the nature of active sites and the type of their deactivation in CO hydrogenation on (Ni+Ti)/SiO2 catalysts

As shown by EPR, upon supporting Ni(π-C3H5)2 on Ti/SiO2, the Ni complex is coordinated to Ti3+ ions. Deactivation of (Ni+Ti)/SiO2 catalysts so obtained during CO hydrogenation is due to the oxidation of Ti3+ ions and metallic nickel particles by reaction products.AbstractМетодом ЭПР показано, что при нанесении Ni(π-C3H5)2 на Ti/SiO2 происходит координация комплекса Ni на ионах Ti3+. Установлено, что причиной дезактивации (Ni+Ti)/SiO2 катализаторов в гидрировании CO является окисление ионов Ti3+ и частиц металлического никеля продуктами реакции.