Attila Wootsch

The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation

Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ x-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Additional studies of hydrogen content using in situ prompt gamma activation analysis, which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-saturated β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.

In Situ Determination of Hydrogen Inside a Catalytic Reactor Using Prompt γ [Gamma] Activation Analysis

Prompt gamma activation analysis (PGAA) has been further developed to analyze reacting components inside a chemical reactor. The new method, in situ PGAA, was used to determine the hydrogen-to-palladium molar ratio under various conditions of palladium-catalyzed alkyne hydrogenation. The H/Pd molar ratio was successfully measured in the range of 0.1-1.0 in an approximately 2-g catalytic reactor containing a few milligrams of palladium catalyst. The amount of hydrogen was only a few tens of micrograms, and the detection limit was approximately 5 microg, i.e., at ppm level compared to the whole reactor. The description of the device, methodological developments, a feasibility study, and results of a series of catalytic measurements are presented.

Surface and bulk structural response of Pt black upon its hydrogen treatment and catalytic reaction with n-hexane

Pt black has been studied by X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS) and XRD after reduction, after presintering and after subjection to various in situ treatments. All samples contained carbon and oxygen impurities. Sintering decreased the abundance of C. Samples introduced from the air contained a large amount of oxygen whose amount dropped markedly upon keeping the Pt in UHV for several hours. XPS and UPS revealed considerable amounts of nondissociatively chemisorbed CO after this treatment. An in situ hydrogen treatment at 603 K decreased the O content further but increased the concentration of surface carbon. The Pt 4f regions showed some oxidized Pt both before and after sintering. Pt reached an almost clean metallic state after UHV treatment. This state seemed to remain unchanged after further manipulations. Hardly any electronic interaction could thus be observed between Pt and its main impurity: C, which was present mostly as graphite and CxHy polymer after treatment with n-hexane plus hydrogen. n-Hexane alone produced mostly “disordered” surface carbon species. The intensity of higher-order X-ray reflections of Pt was suppressed upon sintering. This anisotropy was reversed after in situ H2 treatments, inducing recrystallization with preferential formation of higher Miller-index planes, (220) and (311). These reflections were again suppressed after exposure to n-hexane. Thus, adsorbate-induced solid-state rearrangement occurred as a result of the interplay of surface and subsurface impurities. The catalytic reactions of n-hexane over Pt black subjected to different pretreatments were different: abundant (220) and (311) reflections promoting isomerisation, C5-cyclisation and also hydrogenolysis. Carbon accumulation decreased the “intrinsic” activity of the Pt fraction detected by XPS. Selectivities, in turn, were governed by the crystallite structure as well as by the presence of composite platinum–carbon sites. “ Disordered” surface carbon species poisoned all skeletal reactions and favoured dehydrogenation to hexenes.

Reactions of n-alkanes on and removal of carbonaceous residues from Pt catalysts

Abstract Pt-black, 6% Pt/SiO2 and a 0.6% industrial Pt/A12O3 catalysts were compared in reactions of n-nonane. The main products were olefins and fragments. Of the three samples, Pt/A12O3 showed the lowest degradative activity. Products of direct C6- and C5-cyclization were present, together with minor amount of trimethylbenzene. Isomers contained mostly monobranched ones which could have been formed by the C5-cyclic or the bond shift route. The fragment composition pointed to metal catalysed hydrogenolysis. The C6 and C7 fragments also contained aromatic and cyclopentanic products. Long-term runs led to consumption of primary olefins into fragments (Pt-black and Pt/SiO2) and also into aromatics on Pt/A12O3. n-Nonane was, as a rule, less reactive than n-hexane which latter produced more nondegradative products (isomers, C5-cyclics and benzene). The composition of hydrocarbons removed by hydrogen treatment after run (with ca. 10% conversion)was determined first of all by the nature of the catalyst. It contained much methane in the case of Pt-black whereas much benzene was removed from supported catalysts, both after n-hexane and n-nonane reaction. In the latter case n-nonane was the only C9 hydrocarbon in the removed fraction. Higher hydrogen excess during catalysis led, as a rule, to higher conversions but hardly affected the amount and composition of the removed hydrocarbons with one spectacular exception: methylcyclopentane left EUROPT-1 after a run of with much H2. We propose that the composition of removed hydrocarbons may reflect the structure of hydrocarbonaceous adspecies during reaction with supported catalysts while it may transform into carbonaceous deposits on Pt-black. H2 can split them up to C1 units, appearing as methane in the gas phase. Surface electron spectroscopy is in agreement with this latter assumption.