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The construction of cell suitable for in situ monitoring of catalysts is described. The apparatus has been used to elucidate the nature of the acid sites responsible for coke formation during cyclohexene conversion on dealuminated HY zeolites. It is suggested that the most acidic Bronsted sites are primarily responsible for coke formation. Activation of cyclohexene probably occurs through hydride transfer between adsorbed unsaturated carbocations and gas phase cyclohexene.

Two types of factors control the selectivity of zeolites in catalytic applications : chemical and geometric. The chemical factors relate to the type of active functions present in the zeolite (strength and density of acid sites, metal, oxide or sulfide sites etc), while the geometric factors relate to the size and the connectivity of the channel system.

Spin-polarized self-consistent-field-local-spin-density-scattered-wave calculations for small Ni and Ni-Cu clusters, either alone or interacting with an adsorbed CO molecule have led to a detailed mechanism for the experimentally observed decrease in the saturation magnetization of Ni catalysts following chemisorption. It was also found that the magnetic response of Ni atom(s) directly involved in the chemisorption is highly sensitive to its (their) nearest-neighbor environment. Here this work is extended to small (Ni 9 , Ni 9 + CO) clusters either alone or interacting with models (Al 5 O 8 , Al 5 O 8 H) of a supporting medium in order to examine the possible influence of the support on (i) the Ni magnetic moments, (ii) the effect of CO chemisorption on these moments and (iii) support effects on the NiCO and CO binding. The calculations predict a decrease of the Ni moments for the supported (Ni 9 ) cluster compared with the unsupported aggregate. This support effect may be traced to a strong mixing between Ni 3d and O 2p Orbitals. In contrast to the situation for unsupported Ni clusters, chemisorption of CO on a supported Ni 9 cluster causes only minor changes of the magnetism. Furthermore, the CO-derived levels in the “ supported” case are very different from those calculated for Ni 9 CO.

Results of self-consistent field-local spin density-scattered-wave calcualtions are reported for small cluster (13–14 atoms) containing nickel and copper atoms in various proportions and for these clusters interacting with a CO molecule adsorbed on a top or a hollow site. The effect of alloying on the magnetism of the clusters in similiar to that found in the bulk case, namely (i) the Ni magnetic moments decrease with Cu content and (ii) the spin polarization on the copper atoms is very small. The effect of Cu is mainly limited to the fist nearest-neighbot Ni atoms. In agreement with experiment, adsorption of CO on pure Ni clusters causes a decerese in the magnetic moments of nearby Ni atoms, and we have recently proposed a detailed mechanism which explains this effect. The influence of Co on Ni magnetism for NiCu + CO clusters is investigated and is shown to be the same as in the pure Ni case as long as no Cu atoms are present in the first coordination shell of the Ni atom(s) involved in the bond. If Cu atoms are present in the first coordination shell, the stabilization of the Ni potential by CO chemisorption leads to increased NiCu covalent interactions. For the cases we have examined, this modified NiCu binding leads to a small increase rather than the usual decrease of the Ni moment for the atom directly involved in the chemisorption.

Spin-polarized self-consistent-field local-spin-density scattered-wave calculations have been performed for a variety of small nickel clusters and for these clusters interacting with CO adsorbed on top, bridge or hollow sites. In all cases, as is observed experimentally, adsorption causes a decrease in the magnetic moments of Ni atoms near the adsorption site. The mechanism for this change in the magnetism has been elucidated. Although the fine details vary somewhat from site to site, the primary cause is the same in all cases. Part of the Ni d density of states, in both spins, is destabilized through antibonding interactions with CO σ electrons and subsequently emptied. This triggers a spin-dependent rearrangement of the d electrons which leads to the decreased magnetism. These magnetic aspects have been integrated into an overall Ni-CO bonding scheme which is qualitatively consistent with available experimental data and with the results of more elaborate calculations when the latter are available.

Self-consistent Local Spin Density calculations for CO adsorbed on small Ni clusters have yielded microscopic insight into the effects of chemisorption on magnetism. The mechanism for the reduction of the Ni magnetic moments has been elucidated: specific Ni-CO antibonding interactions lead to destabilizing of a predominantly Ni(d) level. The spin-up electron in this level is consequently transferred to a previously empty spin-down Ni(d) level which is relatively unperturbed by the chemisorption.

Dealumination of HY zeolites by self steaming/acid leaching takes place nearly homogeneously throughout the solid, down to a scale of 20 nm, with creation of a secondary pore system consisting of large spherical pores connected by narrow restrictions. In contrast, isomorphous substitution occurs progressively from the exterior of the zeolite grains, but up to about 50% dealumination, preserves their microporous nature.

A wide class of solids, including modified and unmodified zeolites, aluminas, composite catalysts etc have been studied by N 2 adsorption at 77 K. Empirical correlations between parameters of different theoretical meanings such as the Dubinin micropore volume, the monolayer t volume, the BET surface area etc. have been established. Although these correlations can be rationalized on the basis of qualitative arguments, they remain to be explained quantitatively from the theories which are involved.

The catalytic performances of H-γ, H-Beta and H-Omega prepared by a steaming-acid leaching procedure, have been evaluated in cyclohexene and in Vacuum Gas Oil (VGO) transformation. The leaching is a key step, since depending on the operating conditions it can decrease or increase the conversion levels. In cyclohexene transformation, the acidity of the solids controls the activity and the following ranking is obtained: H-Beta>H-γ>H-Omega. In contrast, in VGO conversion accessibility of the acid centers has to be taken into account which means that H-γ leads to the best results. Moreover, optimization of the acidic properties of H-γ through a selective acid leaching improves significantly the conversion levels.

Infrared spectroscopy and TPD of ammonia have been used to compare the acidic properties of HY zeolites dealuminated by conventional treatments and isomorphous substitution. The steaming procedure leads to the formation of cationic Al species which poison the strongest structural acid sites; provided that an optimized acid leaching is retained, this poisoned acidity may be recovered. For similar Si/Al ratio, the isomorphous dealumination produces solids containing a large amount of Bronsted acid sites with very few Lewis sites.