Transition metal crystallites

Figure 9.13. Alternative routes for alkali metal promotion. Three proposed locations for alkali metals in supported transition metal catalysts (a) is absorbed within the transition metal crystallite, possibly as a complex with nitrogen (b) is on the surface of the crystallite at a concentration fixed by the relative heats of adsorption on the metal and support (c) is adsorbed on the support, but in contact with the transition metal.

The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

One factor which should be noted for palladium, which also applies to the observation of the transition metals Is that not all crystallites have the same efficiency for diffracting electrons, l.e., as the atomic number decreases, the extinction distance for the crystallite Increases (13). Thus one would anticipate Chat as the mean atomic number decreases, the crystallites will have Co be progressively larger to enable visual observation on a support such as alumina. 

Supported transition metals. As mentioned previously, detection of transition metal oxides <4 nm on alumina by CTEM Is virtually Impossible because of the fundamental difficulty of providing sufficient contrast between support and crystallite (14, 15). 

Pd(lll). However, Pd(lll) shows little or no evidence for the stoichiometric 2Bi + L + 3L process. This could be due to the presence of longer range order on the single crystal than on the Pd particles, leading to processes more akin to two dimensional phase transitions on the Pd(lll) crystal surface, rather than a more local species conversion on the small metal crystallites. 

The finely divided cadmium may be stored for several months either as a slurry or in a dry state. (A dry 7 month-old sample was just as reactive as a fresh sample.) Strict anhydrous, airless conditions must be maintained, however. The cadmium crystallite sizes are variable but are generally in the 100-1000 A range. Particle sizes range up to several microns, with some particles even much larger. (Nontransition metals yield more nonuniform particle distributions than transition metals, which form stronger complexes with the solvent). 

A size-selective synthesis of nanostructured transition metal clusters (Pd, Ni) has been reported166, as has the preparation of colloidal palladium in organic solvents167, the latter of which is an active and stable catalyst for selective hydrogenation. The use of microwaves in the preparation of palladium catalysts on alumina and silica resulted in hydrogenation catalysts with improved crystallite size and activity168. 

In order to elucidate the importance of the role of in situ formed carbon in the formation of well-organized, highly crystalline mesoporous transition metal oxides, as-synthesized Ti02 was directly calcined under air to 700°C while keeping all other conditions the same as for the CASH method. As expected, the BET surface area of the resulting material was only 0.2 m2 g-1 and no porous structure could be detected by TEM imaging. This implies that the mesostructure completely collapsed. The crystallite size of this sample, heat treated to 700°C in air is 31.5 nm (calculated... 

Hydrogen decomposition desorption recombination (HDDR) process is the only top-down industrial process used for the preparation of coercive nanoparticles. This process applied to rare-earth transition-metal (RE-TM) alloys consists in heating the concerned alloy under hydrogen until it decomposes into a fine mixture of RE-hydride and TM. The hard magnetic phase is recombined with a much finer microstructure. This process was first developed to convert 100 microns sized cast Nd2Fei4B grains into 200-300 nm crystallites [18, 19]. Later, it has been applied to other RE-TM alloys [20, 21]. Recently, a new variation of this process has been developed towards developing texture in the final materials [22], It is briefly described below. 

A new concept has emerged that distinguishes between structure-sensitive and structure-insensitive reactions the specific rate of the latter is independent of particle size. This is a useful concept with many implications. A complete independence from crystal size, if applicable to the transition from solid to atom would, however, seem incongruous in the light of any electronic theory-physical or chemical. We must realize that the metal particles, even at the low end of the range investigated, are still relatively large a 20-A metal crystallite contains 300 to 400 atoms. 

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

Ross,33 and Beard and Ross34 had also been interested in electrocatalytic properties of Pt-3d transition metal binary-alloys, with a view that stable intermetallics could be formed. It was also their view that the catalytic enhancement shown by Pt-V, Pt-Cr, and latterly Pt-Co was due to the surface roughening of the platinum crystallites caused by leaching of the non-platinum elements from the surface. In the case of the Pt-Co alloy, they believed that a more stable alloy is formed that protects against further alloy degradation. 

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

---