V. Ponec

On the selectivity of palladium catalysts in synthesis gas reactions

Abstract Reactions of CO H 2 mixtures have been studied on a series of Pd SiO 2 catalysts (2 wt% Pd) with and without a promoting additive. Mg and La compounds were used as promoters. It appeared that promoters (i) influence the activity of the catalysts and (ii) increase the selectivity for “oxygenates” and suppress the selectivity for CH 4 . The activity for methanol synthesis is linearly correlated with the amount of Pd extractable from the reduced and used catalysts as Pd-acetylacetonate. Since it is known that promoters also create a new type of ionic adsorption center for CO and NO adsorption, it is concluded that Pd “ions” are indeed the centers for activation of CO toward methanol whereas Pd metal supplies hydrogen atoms for hydrogenation.

Hydrogenation of carbonylic compounds: Part I: Competitive hydrogenation of propanal and acetone over noble metal catalysts

The hydrogenation of propanal and of acetone have been studied as model for reactions of aldehydes and ketones. It has been confirmed that on all metals studied here, propanal reacts more slowly than acetone, just opposite to the homogeneous catalysts. Competitive hydrogenation experiments brought results which strongly suggest that this finding should be related to the different adsorption modes, possible with either homogeneous or heterogeneous catalysts. A solid surface offers contiguous adsorption sites which induce a higher population of more firmly adsorbed (and less reactive) species. Observations made on ether formation could be explained by means of an older model, according to which two different species react on a large ensemble of surface atoms.

Selective isomerisation of n-butene by crystalline aluminophosphates

The results presented in this paper demonstrate that Me-AlPO4-11s are active, selective and stable catalysts for the selective isomerisation ofn-butene to isobutene. A suggestion is given for an active site. The results support a monomolecular reaction mechanism. Absorption experiments ofn-butene on Me-AlPO4-11 and Me-AlPO4-5 are presented, indicating that under the reaction conditions butene polymerisation occurs on Me-AlPO4-5 leading to a rapid deactivation.

Adsorption of carbon monoxide on platinum alloys: An infrared investigation

Abstract The effect of alloying on the absorption band frequency of CO adsorbed on Pt , v ( CO Pt ) , has been investigated with the alloys Pt-Pb, Pt-Sn, and Pt-Re. By use of the isotopic dilution method, it has been established that the downward shift of v ( CO Pt ) observed upon alloying Pt with Pb is caused partly by a dilution effect and partly by another, possibly electronic effect. Alloying Pt with Sn or Re, on the other hand, caused only a small shift of v ( CO Pt ) , which, moreover, could also be explained at least partly by a dilution effect. Oxidation of the alloy samples resulted in a phase separation of the components; the alloy situation could be restored again by a reduction procedure.

Selectivity of iridium catalysts in reactions of C6 hydrocarbons: The role of surface carbonaceous layers and metal particle size

Abstract Skeletal reactions of methylcyclopentane and hexane have been studied on Ir and the results are compared with earlier data on neohexane reactions. The conclusions are as follows: (1) Ir can catalyze the isomerization of hydrocarbons; (2) only under certain experimental conditions is the methylcyclopentane ring opening selective (“no” hexane); when covered by carbonaceous layers, Ir shows also a less selective ring opening. (3) the particle size dependence of the catalytic properties of Ir, as well as its sensitivity for self-poisoning, are much less pronounced than with Pt.

On the problem of the activity in methanol synthesis of supported, unpromoted copper catalysts

It is shown that a pure Cu catalyst is inactive in methanol synthesis. Activity can be invoked by small amounts of (alkali) promoters. Contaminations present in the current commercial supports are leached out during impregnation and promote Cu sufficiently to produce methanol. Contaminations-promoters slow down the reduction of Cu precursors.

Hydrogenation of acetylene over various group viii metals: effect of particle size and carbonaceous deposits

Abstract Supported Pt, Pd, Rh and Ir catalysts have been studied in the hydrogenation of acetylene and the effect of the particle size on this reaction was established. The metals studied can be subdivided into two groups, Pd and Pt on the one hand and Ir and Rh on the other. The latter group shows effects that can most likely be explained by an influence of the particle size on the intermediate of direct hydrogenation to ethane, whereas in the case of Pd and Pt the effect of deposition of carbonaceous layers prevails.

The effect of particle size in the adsorption of carbon monoxide on iridium: An infrared investigation

Abstract The effect of a varying particle size on the infrared absorption band frequency of CO adsorbed on Ir Al 2 O 3 has been investigated. The experimentally observed increase of the frequency with increasing particle size is ascribed to a weaker electron exchange of CO with the metal. This picture is confirmed by hydrogen and carbon monoxide adsorption measurements. By use of the isotopic dilution method, other possible effects, such as a varying dipole-dipole coupling, an electronic influence of the carrier, or electronic structure changes due to a varying metal particle size, could be discarded as possible explanations.

Selectivity of Rh catalysts in the syngas reactions. On the role of supports and promoters

Abstract Catalysts selective for C2-oxygenates can be prepared based on rhodium. However, a support and/or promoter is essential. It is shown that with V2O3 as a support, selective catalysts are only obtained when during preparation (wet steps, high temperature reduction) species are formed which finally appear on the metal surface. Blocking of the Rh surface has been observed by IR spectra of adsorbed CO and by determination of the CO/Rh ratio. UV/VIS and ESR spectra show that by interaction of V2O3 with various chlorides or HCl, VOCl2·5H2O is formed. This compound can be reduced into VOCl which is rather stable. Selective catalysts can also be prepared by adding VOCl2 to a non-selective Rh/Aerosil catalyst.

Selective isomerization of butene to isobutene

To identify the elements active for the skeletal isomerization ofn-butene, a search along the periods of the periodic system has been performed. It is shown that the group VIA (Cr, Mo and W) oxides and the Al oxide are potentially active catalysts. The reaction mechanism and the nature of the active sites are also discussed in relation to the information from the literature.