Gerard V. Smith

New Catalytic Materials from Amorphous Metal Alloys

Publisher Summary This chapter focuses on the new catalytic materials from amorphous metal alloys. Amorphous alloys can be produced with wide composition ranges not available in crystalline form, which permits the continuous control of their electronic properties. Their single-phase character and possible lack of surface segregation of the alloying elements ensure thatthe active species are in a uniform dispersion in a chemically homogeneous environment. There are indications that they have a high concentration of coordinatively highly unsaturated sites that makes them easier to study and characterize than ordinary catalysts. Their structure is nonporous, so diffusion limitations, often a problem in traditional heterogeneous catalysis, do not affect the surface reaction. All these features make amorphous alloys attractive materials in heterogeneous catalytic studies. Also, metallic glasses are a kind of missing link in the study of the nature of active sites in catalysis. The common thread running through theories of active sites is the concept of a small cluster of atoms being responsible for catalytic activity. Because metallic glasses have no long-range order, they offer the opportunity to examine reactions requiring little cluster order in the absence of those requiring high order.

Selective hydrogenation of alkynes over metallic glasses

Abstract The selectivities for half-hydrogenation of phenylacetylene, 1-octyne and 4-octyne were examined over PdSi and PdGe glassy (amorphous) and crystalline catalysts and, for comparison, over splat cooled pure Pd, reduced PdO 2 , Pd foil, Pt foil, Pd Al 2 O 3 , and Pd C . Only the Pd C and Pt foil gave less than 90% selectivities. Terminal alkynes comminute Pd structures and expose new active sites. These new sites are different on the rapidly cooled catalysts and the regularly crystallized catalysts. Although no significant changes are detected in alkyne hydrogenation selectivities after several hydrogenations, significant changes are revealed by (+)-apopinene (6,6-dimethyl-1 R ,5 R -bicyclo[3.1.1]hept-2-ene). On the terminal acetylene-treated foils and on the reduced Pd oxide the rates of addition and isomerization of (+)-apopinene increase, but the ratio of the two rates remain almost the same. In contrast, the splat cooled catalysts show a higher rate increase for isomerization than for addition.

Characterization of Pd-on-alumina and Pd−Si glasses by isomerization and hydrogenation of (+)-apopinene

The hydrogenation and isomerization of (+)-apopinene were performed using deuterium gas. In a typical experiment, a 500-..mu..l sample was injected onto a palladium catalyst which had been pretreated with deuterium for several h. Shaking the reaction vessel at 2200 rpm resulted in hydrogenation. After the desired % hydrogenation was accomplished, the reaction mixture was analyzed by gas chromatography (GC), resulting in good agreement between the amount of deuterium used and the amount of apopinene produced. The GC column was also used to separate the remaining apopinene from the reaction mixture for optical analysis using a polarimeter. The preparation of the palladium on alumina catalysts and the palladium-silicon splats are described. A series of these experiments was using different wt % of catalysts. Results indicate that (+)-apopinene has potential for the characterization of catalytic surfaces. Surface features have been revealed that are not readily available via surface techniques or have not been characterized by chemical means. The data seem to favor a glassy surface structure that is three-dimensionally random (hilly or rolling). 1 figure, 1 table.

Asymmetric hydrogenations over modified Raney nickel

Abstract Enantioselective Raney nickel catalyst systems are prepared by modification with asymmetric amino acids and hydroxy acids and used in the hydrogenation of prochiral ketones and prochiral CC double bonds. A substantial increase in asymmetric induction is accomplished by paying careful attention to modification and hydrogenation variables. Important factors are (i) the contact times of modifying and washing solutions during catalyst modification, and (ii) the contact times of modified catalyst and substrate before reaction. By employing combinations of deuterated Raney nickel, deuterium gas, and deuterated modifiers it is shown that hydrogen (deuterium) in the modifying molecule is involved in the geometry determining transition state.

Mössbauer studies of iron in Lurgi gasification ashes and power plant fly and bottom ash

Iron Mossbauer spectroscopy and X-ray diffraction methods were applied to the study of a selection of ashes produced in a Lurgi gasification plant and fly ash from a pulverized coal combustion. The ashes contained hematite, magnetite, and goethite. Sixty percent or more of the iron in these ashes was in the oxide form, with the remainder present in mullite and other silicate phases. Iron was divalent in the latter, and present as both Fe+2 and Fe+3 in mullite. Ratios of Fe+2 and Fe+3 varied from 0.3 to 0.7. By comparison, a water-quenched molten bottom ash was free of iron oxides and contained only amorphous silicate phases with virtually all iron in the divalent state.

The mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals: III. Effect of partial pressure of hydrogen on the selectivity of hydrogenolysis of oxacycloalkanes on Pt☆

The rates of hydrogenolysis and isomerization of methyloxirane, cis- and trans-2,3-dimethyloxirane and 2-methyloxetane were studied on a PtC catalyst, as functions of the hydrogen pressure and the temperature. A new phenomenon was observed in the case of 2-methyloxetane: the change in mechanism as the temperature was elevated led to a change in regioselectivity. There is no such effect for methyloxirane, for which the dissociative mechanism is not competitive at high hydrogen pressure. Likewise, the mechanisms of transformation of methyloxirane and the 2,3-dimethyloxiranes are not the same. Methyloxirane probably participated in edgewise adsorption on the surface of the catalyst, while cis-2,3-dimethyloxirane undergoes flat adsorption.

Hydrogenation and isomerization of (+)-apopinene over well-characterized Pt SiO2 and over Pt Al2O3

Abstract Six Pt SiO 2 and four Pt Al 2 O 3 catalysts were characterized by CO chemisorption and H 2 O 2 titration and by electron microscopy. Five of the Pt SiO 2 catalysts originated at Northwestern University and had been similarly characterized before. Over these catalysts the hydrogenation reaction of the complex molecular probe (+)-apopinene (6,6-dimethyl-1 R ,5 R -bicyclo[3.1.1]hept-2-ene) was used to evaluate the change in the mix of active sites as a function of percentage exposed Pt atoms (percentage dispersion, % D ). The ratio of isomerization to addition, k i k a , goes through a maximum at approximately 60% D , which corresponds to a maximum in edge sites (C 7 sites) on fcc octahedra. These sites are identified as Siegel type 2 M sites which catalyze essentially isomerization whereas adatoms and vertices (C 4 sites), Siegel type 3 M sites, catalyze addition and isomerization.

Characterization of palladium surfaces with (+)-apopinene: correlation of reaction paths with surface features

Abstract The molecular probe (+)-apopinene is chosen for the characterization of metal surface sites because of its unique structural features. These features allow clear distinctions of adsorption side and double bond position. By measuring the relative rates of double bond migration and addition, k i k a , it is shown that (+)-apopinene is sensitive to 17 different Pd catalyst preparations. Among these, a set of alumina-supported Pd catalysts ranging in dispersions from 0.055 to 1.00 (131 to 8.4 A by EM) yielded results consistent with the notions that 3M-type sites (highly coordinatively unsaturated) catalyze principally addition and 2M-type sites catalyze principally isomerization. (+)-apopinene distinguishes between reduced PdO2 samples which have undergone different extents of Surface reconstruction.

Mössbauer spectroscopic investigation of iron species in coal

Abstract A series of Herrin No. 6 coal and three coal-derived samples have been examined by Mossbauer spectroscopy. It is established that Mossbauer spectroscopy can be used to identify multiple iron species in a whole coal or an autoclaved char sample without the need to concentrate the minerals to enhance resolution. Our results indicate that there may be an association between the pyrite in raw coal and the coal matrix. This association appears to be broken down when the coal is heated to temperatures as low as 175 °C. It is also apparent that the iron sulphide present in the whole coal is converted to pyrite at these low temperatures. For our samples, the total quantity of iron species in different coal lithotypes is about the same, but they differ in their distributions. The fusain has the least amount of Fe 2+ species when compared to the vitrain or whole-coal sample used. At least two types of nonstoichiometric pyrrhotite are produced in the heat-treated samples. One of these pyrrhotites is unstable and contains dissolved sulphur which is apparently liberated as the temperature is increased.

Catalytic exchange and hydrogenolysis of thiophenes and related heterocycles

Abstract The exchange and hydrogenolysis (deuteriumolysis) of thiophene, 2-methylthio-phene, 3-methylthiophene, 2,5-dimethylthiophene, furan and γ-picoline were examined over a Mo γ- alumina catalyst. The exchange of thiophene and γ-picoline were examined over γ-alumina, Mo γ- alumina and CoMo γ- alumina catalysts. Three types of exchange functions seem to exist in the last two catalysts, random exchange and multiple exchange on γ-alumina and α-exchange on Mo. Poisoning with picoline and water suppresses γ-alumina type exchange but not α-exchange and desulfurization. Multiple exchange is correlated with hydrogenation/dehydrogenation activity and α-exchange is correlated with desulfurization. π-Complex formation seems important in multiple exchange but methyl group exchange requires, in addition, heteroatom activation. The role of Co may be to facilitate surface hydrogen mobility and thereby decrease coke formation.