Russell F. Howe

Characterization of supported iridium catalysts prepared from Ir4(CO)12

Abstract The characterization of silica- and alumina-supported iridium catalysts prepared from Ir 4 (CO) 12 is described. The techniques employed are infrared spectroscopy, temperature-programmed desorption, and gravimetric chemisorption measurements. Supported Ir 4 (CO) 12 begins to decompose on heating in vacuo above 75 °C, and decomposition is complete at 350 °C. The major product of decomposition on silica is metallic iridium, whereas on alumina a fraction of the iridium becomes oxidized. Hydrogen reduction of both silica- and alumina-supported catalysts following decomposition of the parent complex in vacuo produces highly dispersed metallic iridium. From observed shifts in the infrared frequencies of adsorbed CO with coverage it is concluded that the iridium may be in the form of two-dimensional rafts, containing possibly 20 atoms or more.

Interaction of nitric oxide with supported chromium, molybdenum, and tungsten catalysts

The interaction of NO with alumina-supported catalysts prepared from Cr(CO)6, Mo(CO)6, and W(CO)6 has been investigated by means of infrared and EPR spectroscopy. Adsorption of NO on Cr(CO)6Al2O3 produces Cr2+ and Cr3+ nitrosyl species identical to those found on reduced CrO3Al2O3. A paramagnetic Mo+NO+ species has been detected on Mo(CO)6Al2O3, but the majority species on this catalyst is a Mo4+ dinitrosyl complex identical to that found on MoO3Al2O3. A corresponding W4+ dinitrosyl complex is formed on W(CO)6Al2O3.

A chemical and physical characterization of alumina-supported Rh6(CO)16

The decarbonylation and carbonylation reactions of the metal cluster carbonyl Rh6(CO)16 supported on Al2O3 were investigated using volumetric gas adsorption measurements, ir spectroscopy, isotopic measurements, transmission electron microscopy, and EPR spectroscopy. Complete decarbonylation by O2 and subsequent recarbonylation to Rh6(CO)16 could be achieved at room temperature provided sufficient physically adsorbed H2O was present. A two-step reaction scheme is presented which accounts for the experimental data; it is suggested that the Rh6 cluster remained intact during these reactions. When heated, in vacuo, above 250 °C, the catalyst lost its ability to undergo the reversible carbonylation/decarbonylation cycle, and the resulting material resembled a highly dispersed conventional Rh catalyst. TEM data showed that no larger crystallites (>10 A) of Rh are formed even when heated in vacuo to 450 °C.

EPR spectroscopy in surface chemistry: recent developments

Abstract A review is presented of some recent applications of EPR spectroscopy in surface chemistry. Emphasis is placed on new developments in the field: application to photocatalysis, spin trapping methods, mobility of adsorbed species, pulsed EPR experiments, and studies on well-defined surfaces.

Interactions of cobalt carbonyls with oxide surfaces: I. (CO)9Co3CCH3 on silicas, aluminas, and zeolites

Abstract The adsorption and subsequent reactions of the cobalt cluster compound μ 3 -ethylidine-tris-(tricarbonyl cobalt), I , on silica, alumina, and zeolite supports were investigated. The supports were pretreated, in vacuo , at temperatures between 295 and 700K. On silica, I appears to be physisorbed and reacts only slowly with O 2 to produce decarbonylated products. Adsorption on alumina produces physisorbed I and at least three cobalt carbonyl products of the reactions of I with the support. Aluminas pretreated at lower temperatures are the most reactive toward I , as evidenced by the greater tendency of I to react and form novel cobalt carbonyls on these aluminas. These reaction products are immediatley oxidized by O 2 to produce carbonate, bicarbonate, and oxidized cobalt. Compound I adsorbs at room temperature inside the supercages of a Na Y zeolite; it is subsequently converted, at elevated temperatures (350K), to the carbonyl species which were identified on alumina. Infrared and manometric data show that thermolysis and oxidation of I on zeolite ultimately lead to its total decarbonylation to produce carbonates, bicarbonates, and organic carbonyl fragments.

H2D2 equilibration over supported iridium catalysts prepared from Ir4(CO)12

X-Ray photoelectron spectroscopy (XPS) measurements have been performed on silica- and alumina-supported iridium catalysts prepared from Ir4(CO)12, and the activity of these catalysts for the H2:D2 equilibration reaction determined as a function of the activation treatment. The XPS results show that predominantly zerovalent indium is obtained upon decomposition of supported Ir4(CO)12 in vacuo. Maximum activity for H2:D2 equilibration is generated only after removal of all carbonyl ligands. The reaction is poisoned by readsorption of CO, and the recovery of activity upon desorption of CO parallels the original generation of activity.

The superoxide ion on supported tungsten: Evidence for surface mobility

Abstract The EPR spectrum of oxygen adsorbed on silica supported tungsten prepared from W(CO) 6 has been assigned to O − 2 on the basis of the g tensor and the hyperfine structure observed with 17 O enriched oxygen . A temperature dependent signal shape above 77 K indicates anisotropic motion of O − 2 on the surface.

Oxygen depletion of bismuth molybdates

Pure ..cap alpha..-phase bismuth molybdate (Bi/sub 2/Mo/sub 3/O/sub 12/), which is known to be weakly active for selective oxidation, and pure ..gamma..-phase bismuth molybdate (Bi/sub 2/MoO/sub 6/), which has good activity, were subjected to oxidation-reduction cycles with known amounts of hydrogen and oxygen, at 300/sup 0/-570/sup 0/C and with evacuation steps between treatments. The volume of oxygen consumed during reoxidation was equal to half the hydrogen consumed during the reduction on the ..cap alpha..-phase, which indicated that no hydrogen was retained during reduction. For the ..gamma..-phase, the oxygen consumption was greater than half of the hydrogen consumption and it increased with extent of reduction. The excess oxygen was apparently consumed by filling anion vacancies formed during outgassing subsequent to the reduction step. ESR spectroscopy and temperature-programed oxidation-reduction indicated that lattice oxide ions which bridge between bismuth and molybdenum layers of the koechlinite structure become more labile when the catalyst is in a partially reduced state, and that this effect is greater in the ..gamma..- than the ..cap alpha..-phase. Table and 15 references.

EPR STUDIES OF PHOTOCHEMISTRY AT MOLYBDENUM OXIDE SURFACES

We report an EPR investigation of paramagnetic species produced on MoO3-SiO2 catalysts by irradiation with uv light at low temperatures in H2 and O2. In H2 alone, the catalyst is photoreduced, giving Mo5+ species and adsorbed H atoms. In the presence of both H2 and O2, adsorbed O2 − and OH radicals are formed. These react on warming above 77 K to produce O− and HO2, which subsequently decay on warming to room temperature.