Problems of analyzing by TEM and TPD (Temperature Programmed Desorption) the structure and micromorphology of small Pd particles vapor deposited in UHV onto clean oxide supports are discussed. Particle changes induced during extended exposures to high intensity electron irradiation of a number of electron transparent support materials such as Al2O3, SiO2, TiO2, MgO, and mica are examined. Different damage mechanisms are evaluated and experimental means of reducing the damage are explored. The possibility of extracting particle morphology information from a detailed analysis of CO thermal desorption spectra is also investigated. Evidence suggests that it may be possible to obtain micromorphology information down to very small particle sizes from TPD measurements if the effects of intrinsic particle morphology can be separated from the influence of diffusion of support species.

A colloidal route to particles of Pt100-xAux of. high dispersion is reported; their activities and selectivities in hydrocarbon conversion are consistent with alloy catalysts prepared by other routes.

Re-containing molecular bimetallic clusters were used as precursors for gamma alumina supported catalysts. Characterization by XPS and XAS after hydrogen reduction showed Re valence states in the order Re monometallic > Re-Os bimetallic > Re-Pt bimetallic > zero. These results suggest that industrial bimetallic catalysts are not best understood in terms of alloys, but rather as multifunctional, with each element contributing its own activity.

Hydrogenolysis of n-butane and CO hydrogenation were used to charac-terize the behavior of TiO2 supported Rh in the SMSI state. The Rh was supported on a model support consisting of nonporous spherical particles of TiO2. The simple geometry of the support permits the metal crystallites to be imaged ‘edge-on’ in a transmission electron microscope. This allows detailed examination of the metal surface and the metal-oxide interface as a function of pretreatment. After high temperature reduction in H2, there is no significant change in the morphology of the Rh crystallites. However, the presence of amorphous overlayers 0.2–0.4 nm thick is clearly evident on the Rh surface. These overlayers can only be partially removed after oxidation at 473 K. The presence of these overlayers is well correlated with the drop in hydrogen chemisorption and altered reactivity. While the butane hydrogenolysis activity of Rh is considerably suppressed in the SMSI state, the CO hydrogenation activity is increased by a factor of 3. The H2 uptake and the butane hydrogenolysis activity can be only partially restored after oxidation at 473 K. Treatment at 773 K in oxygen is necessary for restoring the pre-SMSI behavior.

The potential for carbon formation exists in any system in which hydrocarbons undergo thermal decomposition. It is well known that certain metals can increase the carbon yield by catalyzing the growth of both filamentous and graphitic types of deposit. The highest catalytic activity for carbon deposition is exhibited by the ferromagnetic metals and in particular, iron.

We have used a combination of controlled atmosphere and high resolution electron microscopy techniques to study the formation of the various types of carbon on metal surfaces. In this paper the emphasis is placed on the fundamental aspects surrounding the growth of filamentous carbon. The qualitative and quantitative data obtained from these studies have enable us to develop a mechanism for the growth of filamentous carbon and also provided insights into methods of inhibiting the growth of this form of carbon. Continuous observation of many filamentous carbon growth sequences has shown that both the addition of a second metal to the catalyst and also the strength of the metal-support interaction can have a profound effect on the mode by which carbon filaments grow.

Carbon filaments of macroscopic length may be produced by heating transition metal catalyst particles in a hydrocarbon-hydrogen mixture. Because the nanometer-sized catalyst particles contain relatively many surface atoms, they melt at a temperature considerably below that of bulk material; this depression of the melting point is difficult both to calcu-late and measure. This work addresses the fundamental question of whether the catalyst particles are in the liquid or solid state during filament growth.

We present measurements of the average number of filaments grown per unit area of substrate as a function of temperature which show that the the activity of the catalyst particles drops precipitously above 1030°C and nearly disappears above 1120°C. This compares with the melting point of the Fe-C eutectic of 1154°C. TEM observations show that the average dia-meter of an active catalyst particle is 15±7 nm. Calculations based on the model of Ross and Andres [1] for the melting temperature of small clusters of atoms predict a drop in activity above 1050°C similar to our observa-tions.

We describe theoretical results for part of the Gibbs free energy surface as a function of shape and temperature for small particles. We predict the existence of true ‘quasi-melting’ at temperatures much below the conventional melting points and also phase transitions between different stable forms as the temperature is varied. A phase diagram for small particles is constructed for the first time, showing the stability ranges for different particle shapes as a function of size and temperature.

This paper explores the possibility of using gas adsorption for particle-shape control in supported metal catalysis. Calculations are done to identify the kinds of shape changes which are possible due to treatment in an appropriate adgas. Experiments are done to try to verify the concepts. The results show that gas adsorption can be used for particle shape control in supported metal catalysis.

Specimens of highly dispersed Pt/Silicalite catalyst, decorated with 57Fe by impregnation in aqueous ferric nitrate solution, have been studied after decoration, and various pretreatments including exposure to carbon monoxide, by in situ Mössbauer spectroscopy. The form of Pt was followed by EXAFS analysis near the Pt LIII edge. The microstructural studies were combined with CO chemisorption and infrared absorption measurements. It is observed that iron in Pt clusters is very reactive even at ambient temperatures and that the interaction of Fe with CO is strongly accelerated by platinum.

A new approach to the surface electronic structure problem, based on a self-consistent scattering theory of point defects, permits 1st-principles calculations for an isolated cluster of adatoms and defects on an otherwise perfect infinite crystalline surface. A first numerical application of the method explains important observations concerning the interaction and diffusion of adatom dimers on Field Ion Microscope tips. Further studies will shed light on questions such as: What impurity species migrate to what kind of surface defects? What is the contribution of substrate atom positional relaxation to adsorption and diffusion barrier energies?

The considerable interest in metallic clusters spans a wide range. In this paper we discuss quantum effects and we suggest that there may be important interrelationships between such behavior and heterogeneous catalysis. Of course, the physical and chemical behavior of the metal particle are the key to understanding principles of catalytic activity. The quantum effects are found at progressively lower temperatures where the metallic properties change. In both cases a fundamental question is posed, “Do these particles exhibit metallic behavior?;” One of the common features of research in these two areas is the important role played by nuclear magnetic resonance(NMR) techniques since the signatures of a metal and an insulator are so dramatically different. The issue of metallic or nonmetallic behavior is therefore best addressed with this experimental approach and we will review the current situation by comparing a number of results on quantum size effects deduced from NMR experiments

The influence of Group 1A elements, and various alkyl ammonium cations, on a metastable faujasite synthesis at a silica/ alumina ratio of 9 are reported. Co-crystallization of faujasite, gmelinite and gismondine (P) occurs in reactions with Li+, K+ and Rb+, but the trapping of Cs+ in the FAU sodalite cage (CSZ-1/ CSZ-3) inhibits subsequent reactions to GIS, and blocks the formation of GME. Morphological effects of these cations include platelet formation and the promotion of small crystals. TMA addition promotes mazzite analogues; and DEDM stabilizes FAU, and at 140°C., induces the crystallization of ECR-1. The derivation of all these phases from a restricted number of secondary building units is discussed.

Raman spectroscopy of the solution, solid and gel phases present during crystallization of zeolite X was investigated. The vibrational data indicate that an amorphous aluminosilicate solid, composed primarily of four membered aluminosilicate rings is in contact with monomeric silicate ions during the prenucleation stages of the zeolite formation. No intermediate building blocks specific to zeolite X could be discerned from the vibrational spectra. The influence of a series of monovalent cations on the crystallization process was also examined, and a model of zeolite formation has been proposed.

By means of 29Si spectroscopy, it is established that the distribution of silicate anions in alkaline silicate solutions is a moderate function of base composition. At a fixed SiO2. concentration and silicate ratio, the proportion of Si present in oligomeric and cage-like structures increases in progressing from Li to Cs hydroxide. Interactions between alkali metal cations and silicate anions are investigated using NMR spectroscopy of the cations; in this way the concentration of ion pairs is measured as a function of cation size. As a result the silicate redistribution is ascribed to cation-silicate anion pairing and to a higher selectivity for ion pairing by large silicate anions as cation size increases.

Magic-angle spinning 13C NMR spectra of carbon monoxide adsorbed on rhodium/Y zeolites yield information about the proportioning of CO in the various possible adsorption states; linear, bridged and dicarbonyl. The relative amounts of these adsorbed types, particularly the ratio of bridged to linear CO is influenced by the nature of the majority cations present with the rhodium. Reduced Rh-Na(+) and Rh-Li(+) zeolites form all three CO surface species, while acidic Rh zeolites, formed by the introduction of the co-cations Ca(2+) and H(+), exhibit no bridged carbonyls. The suppression of the bridged moiety results from the withdrawal of electrons from rhodium by the acid centers making the metal electron deficient (more oxidized).

Rh(I) dicarbonyl species form on all samples studied, however these species are indistinguishable from the linear monocarbonyls based solely upon the isotropic chemical shift obtained from magic-angle spinning. The number of dicarbonyl species can be quantitatively determined by the Carr- Purcell-Meiboom-Gill sequence, the powder pattern or by selective exchange experiments. At room temperature the two CO molecules in the gemdicarbonyl appear to undergo a mutual hopping exchange. This motion is frozen out at 198K. The carbon-carbon internuclear separation in the gemdicarbonyl is 3.3 Å.

Catalysts pre-adsorbed with 13CO undergo exchange of the dicarbonyl species upon exposure at 198 K to 12CO, however they also react to form 13CO2. When exposed to CO at room temperature no CO2 formation is detected.

High resolution 27Al NMR spectroscopy has been used to study the dealumination of the A-type zeolite ZK4. The aluminum removed from the lattice during calcination undergoes reversible changes in coordination when treated in acidic and basic media. 2 5 Si NMR spectroscopy reveals that the changes are confined to the detrital aluminum species and do not affect the zeolite framework. Thermogravimetric analysis data combined with compositional data available from 29Si NMR spectra have been used to probe the state of charge of the detrital aluminum species and the nature of defects present in the lattice.

29Si chemical shifts, δ, are for aluminosilicate zeolites expressed as δSi-4Al (which can be estimated from the T-O-T angles observed crystallographically) plus terms involving b1 and b2, the quantitative influences of first and second neighbor aluminum atoms. The former, b1, is assumed to be the sum of a fixed electrostatic component, α, and a framework-specific angular term, β(0 < β < 0.9 ppm), that reflects the generally smaller Si-O-Al angles compared to Si-O-Si. The compositional dependence of δ is approximated by an equal influence, b2, per added second neighbor aluminum atom, the concentration of which is assumed to vary linearly with composition. Values of δS-nAl; n=0-3 for representative L (LTL-framework) and omega (MAZ) zeolites calculated on this basis agree well with those observed. Applying this description of the compositional dependence to as-synthesized and dealuminated CSZ-1 zeolites enables the measurement of framework aluminum contents in this four T-site case.

High resolution electron microscopy (HREM) is a powerful tool in materials research. This is especially true in the study of zeolites because most of the questions concerning the properties of zeolites require a detailed characterization of the local structure [1]. The potential of HREM is illustrated with studies on the growth of denser phases on Zeolite Y and Zeolite L.

In this paper we demonstrate the usefulness of a nearest neighbor force model for studying the structural dependence of the lattice dynamics of zeolites. Interpreting Raman scattering in the context of this model confirms earlier empirical vibrational assignments in zeolites and underlines the importance of short range forces in determining the frequencies of certain zeolite vibrational modes. The model describes fully the low frequency bond bending mode of A1 symmetry, although long range forces are shown to be required to model complete zeolite vibrational spectra.

Gallosilicate NAT-framework zeolites have been synthesized from Na2O:TMA2O:Ga2O3:SiO2:H2O gels and characterized by chemical analysis and powder X-ray diffraction. The structure of a hydrated material (Orthorhombic, Fdd2, a = 18.423(4) Å, b = 18.826(3) Å, c = 6.652(1) Å) was determined using powder neutron diffraction. The framework Si:T ratio, Si-Ga segregation and non-framework cation and water configurations are similar to those in the analogous aluminosilicates. The geometrical effects of framework gallium substitution are consistent with those observed for other zeolite framework topologies.