Palladium crystallites

The catalytic activity of the pure /3-palladium hydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample prepare tion allowed one to avoid cracking the palladium crystallites, which would... 

Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a /3-phase lattice, an empty octahedral hole, or a free palladium atom—for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. 

This conclusion was additionally confirmed by Palczewska and Janko (67) in separate experiments, where under the same conditions nickel-copper alloy films rich in nickel (and nickel films as well) were transformed into their respective hydride phases, which were proved by X-ray diffraction. The additional argument in favor of the transformation of the metal film into hydride in the side-arm of the Smith-Linnett apparatus consists of the observed increase of the roughness factor ( 70%) of the film and the decrease of its crystallite size ( 30%) after coming back from low to high temperatures for desorbing hydrogen. The effect is quite similar to that observed by Scholten and Konvalinka (9) for their palladium catalyst samples undergoing the (a — j8) -phase transformation. 

X-Ray diffraction has an important limitation Clear diffraction peaks are only observed when the sample possesses sufficient long-range order. The advantage of this limitation is that the width (or rather the shape) of diffraction peaks carries information on the dimensions of the reflecting planes. Diffraction lines from perfect crystals are very narrow, see for example the (111) and (200) reflections of large palladium particles in Fig. 4.5. For crystallite sizes below 100 nm, however, line broadening occurs due to incomplete destructive interference in scattering directions where the X-rays are out of phase. The two XRD patterns of supported Pd catalysts in Fig. 4.5 show that the reflections of palladium are much broader than those of the reference. The Scherrer formula relates crystal size to line width ... 

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. 

Supported palladium. The STEM and TPD analyses as well as composition of the materials used Is shown In Table 11. Observation of palladium crystallites by STEM for sample H proved to be very difficult, whereas sample J proved to be relatively easy. In both cases... 

Two approaches were taken to provide Insight to the uniformity of these samples 1) perform extensive visual Inspection of the materials to ensure that no large palladium crystallites were present, which was the case for both samples, and 2) perform surface area measurements using hydrogen TPD to establish the relationship between the observed data by STEM and the estimated surface area from theoretical considerations. 

Sample H was evaluated In this manner, and showed higher H/Pd ratios than anticipated, 0.6 versus 0.3 for the crystallites observed. This result would suggest that the palladium crystallites are thin, thus explaining the Instability In the electron beam and lack of diffraction contrast. 

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. 

Table II. Palladium Crystallite Size and Dispersion Data...

Supported platinum-palladium. This aspect of the study focused on the characterization of platinum and palladium on alumina. The analytical capability of STEM Is fully demonstrated In a problem of this type, because of the rapid manner In which crystallite composition can be analyzed. This study Is especially Interesting because of Che use of platinum and palladium combinations In automotive catalysis. 

This Study has shown that reasonably uniform platinum crystallites can be made on y-alumlna, and that platinum and palladium can be segregated and maintained In that form for the most part even after exposure to high temperature oxidation-reduction conditions. Highly dispersed clusters of palladium, nickel, cobalt, and Iron can be observed. Cluster size determination could not be accurately made because of the lack of contrast between the cluster and the support. The marginal detectability by EDS for these clusters enabled elemental Identification to be made, however, mass uniformity determinations could not be made. 

In contrast to platinum, palladium nickel, and gold, with silver only crystallites with an approximately circular plan shape are seen (S3) and there is no worthwhile evidence of angularity the likely general shape (S3, 34) is that of a flatish curved dome. 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

Hence, the decision to use a heated substrate with simultaneous evaporation of the component metals as an aid to homogenization requires consideration of whether or not it might have an adverse effect, i.e., causing preferential nucleation of one component, which interdiffusion may not be able to remedy. It was believed (60) that in preparing Pd-Rh alloys by simultaneous deposition on a substrate at 400°C, rhodium nucleated preferentially and that crystallites grew by the addition of palladium (and rhodium) atoms. The diffusion of palladium atoms into this kernel formed a phase with 88 =t 5% Rh (phase II). The outer shell of the crystallite, phase I, was in effect a solid solution deficient in rhodium compared with the overall film composition, and the Rh content of phase I therefore increased as the Rh flux was increased. 

Alternatively, it may be possible to demonstrate for the pure metals that the catalytic activity is independent of film weight in a certain weight range. For example, rates of ethylene oxidation were constant over pure palladium films, deposited and annealed at 400°C and weighing between 4 and 40 mg (73). Then, if electron micrographs show that the crystallite size is relatively independent of composition, a satisfactory comparison of catalytic activity can be made at the various alloy compositions. Finally, surface area measurements are less urgently needed when activity varies by orders of magnitude, or where the main interest lies outside the determination of absolute reaction rates. 

The exchange of a number of compounds in this category with deuterium has been examined by Burwell and his colleagues. n-Heptane has been exchanged over nickel-keiselguhr (43), reduced nickel oxide (29), a series of nickel catalysts of varying crystallite size (37), and over palladium supported on 7-alumina (43). Less extensive studies were also made with 2,3-dimethylbutane (29, 43) and n-hexane (42). 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 800°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface. 

Johnson and co-workers (62) have come to the conclusion that interaction of lead with Pt crystallites results in the formation of an inactive phase in which the Pt atoms are ionized and soluble in HC1. These data were derived from engine tests, in which the catalysts were exposed to fuels with 0.03-0.1 g Pb/gal. The amount of crystalline Pt in these catalysts was smaller than in catalysts run on lead-free fuels. The authors indicate that noncrystalline forms of Pt are present on A1203 supports under certain conditions, and that lead stabilizes such forms. The question whether the noncrystalline, ionic Pt is a surface or a bulk phase remains unanswered. Bulk mixed Pt-Pb oxides have been described (98, 99), but, again, the dispersed forms of noble metals supported on A1203, which lead (and other elements) may stabilize, are known to be associated with the surface only. Palladium can be expected to form such noncrystalline dispersed phases to a still greater extent since it is more easily oxidized than Pt. 

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. 

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