The role of carbon-hydrogen bond breaking in the epoxidation of ethylene

No abstract

Chemical interactions in multimetal/zeolite catalysts

For Pt/NaY catalysts our analysis of the mechanism of metal particle formation has enabled us to produce at will samples which contain either the majority of the Pt particles in supercages, without filling these cages completely, or the Pt particles bulge into neighboring cages. The catalytic selectivity is distinctly different for these preparations, in the former case molecules can enter a supercage which is partially filled by the Pt cluster, in the second case adsorption takes place through the cage window. Applying the same principles of catalyst preparation of bimetallic catalysts enables us to produce PtCu particles in supercages of NaY, which contain, initially a Pt core, surrounded by a Cu mantle. Earlier we have found that Ni ions migrate into hexagonal prisms during calcination of Ni/NaY; this process can be partially suppressed by first filling these prisms with Mn or Cr ions. In more recent work we found that addition of Pt strongly lowers the temperature of Ni reduction. Part of the Ni ions is reduced by hydrogen while still inside the smaller cages. This reduction process is, however, reversible; at elevated temperature and in an inert atmosphere protons re-oxidize the Ni atoms and dihydrogen gas is developed. In this way it seems possible to count the Ni atoms in small cages. The calcination stage in the preparation of zeolite supported metals has been studied in considerable detail for Pd/NaY. The Pd is introduced as a tetrammin complex; during calcination the ammine ligands are successively oxidized. Once three ammine ligands are destroyed, the Pd ions which carry only one ligand, surprisingly jump from the supercages to the sodalite cage

Chemical interactions in multimetal/zeolite catalysts

This report treats four subject areas: PtCu/NaY and Pd/Cu/NaY catalysts; reducibility of Ni in PdNi/NaY catalysts; CO hydrogenation over PdNi/NaY catalysts; and PdFe/NaY, Ga/H-ZSM5 and PtGa/H-ZSM5 catalysts

Chemical interactions in multimetal/zeolite catalysts

Mechanistic explanations have been found for the migration of atoms and ions through the zeolite channels leading to specific distribution of ions and the metal clusters. In this report, we summarize the state of understanding attained on a number of topics in the area of mono- and multimetal/zeolite systems, to which our recent research has made significant contributions. The following topics are discussed: (1) Formation of isolated metal atoms in sodalite cages; (2) differences of metal/zeolite systems prepared by ion reduction in channels or via isolated atoms; (3) rejuvenation of Pd/NaY and Pd/HY catalysts by oxidative redispersion of the metal; (4) formation of mono- or bimetal particles in zeolites by programmed reductive decomposition of volatile metal complexes; (5) cation-cation interaction as a cause of enhanced reducibility; (6) formation of palladium carbonyl clusters in supercages; (7) enhanced catalytic activity of metal particle-proton complexes for hydrocarbon conversion reactions; (8) stereoselectivity of catalytic reactions due to geometric constraints of particles in cages

Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a nontransition metal affects the bonds between the transition metal atoms in the surface and H or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to changes in delocalization of the metal electrons involved in the chemisorption bonds and changes in the no. of electrons available for bonds. Calcns. showed the effects of alloying on chemisorption are different for monocoordinated adsorbates (adatom on top of metal atom) and tricoordinated adsorbates (adatom equidistant to 3 transition metal atoms), resp. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. The ratio of monocoordinated to tricoordinated adsorbates, present in equil. at high coverage, is increased by this type of alloying. The increase is larger than expected on a geometrical basis only. Chemisorption on alloys of Group VIII metals with Group IB metals is discusse

Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a nontransition metal affects the bonds between the transition metal atoms in the surface and H or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to changes in delocalization of the metal electrons involved in the chemisorption bonds and changes in the no. of electrons available for bonds. Calcns. showed the effects of alloying on chemisorption are different for monocoordinated adsorbates (adatom on top of metal atom) and tricoordinated adsorbates (adatom equidistant to 3 transition metal atoms), resp. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. The ratio of monocoordinated to tricoordinated adsorbates, present in equil. at high coverage, is increased by this type of alloying. The increase is larger than expected on a geometrical basis only. Chemisorption on alloys of Group VIII metals with Group IB metals is discusse

Surface composition and selectivity of alloy catalysts

A review with 146 ref

Catalysis of molecular hydrogen-molecular deuterium equilibration by the oxygen-doped silver/potassium system

Metallic K does not enhance the rate of H2-D2 equilibration over Ag, but a very large promoting effect is found if the Ag is first covered with O2 and then contacted with K. The results of experiments performed in the absence of Ag suggest that this effect is due to an activation of potassium with oxygen