Surface state dispersions metals

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

When the catalyst contains more than one component, selective gas chemisorption methods are normally used for analysing the surface area associated with a particular component. In this procedure, a gas (H2 or CO for Group VIII metals) is adsorbed on only the component of interest. The method is also particularly useful for studying the dispersion state and surface areas in highly dispersed metallic systems. 

The key to most of the functional properties reported is a fine microstructure of the metal particles (i.e. in the nanometer scale) which is uniformly dispersed within a ceramic matrix. In some cases the particle size needed is in the range of a few nanometers in order to enhance the surface properties, while in other cases optimization is needed between the demand for single domain particles while minimizing unwanted surface states. 

Table 1. Properties of L-gap surface states of noble metal (111) surfaces obtained from a parabolic fit to the ARPES dispersion [44].

In all of this work there was little suggestion that the surface states of the palladium might behave differently from bulk states. Selwood (17) indicated that, from some sorption-magnetic susceptibility data for hydrogen sorbed on palladium which was finely dispersed on alumina gel, the ultimate sorption capacity was approximately at the ratio 2H/Pd. Trzebiatowsky and coworkers (25) deposited palladium on alumina gel in amounts ranging from 0.46 to 9.1% of gel weight. They found the palladium to be present in a normal crystal lattice structure, but its susceptibility was less than for the bulk metal. This suggested to the present authors that the first layer of palladium atoms laid down on the alumina gel underwent an interaction with the alumina, which has some of the properties of a semiconductor. Such behavior was definitely shown in this laboratory (22) in the studies on the sorption of NO by alumina gel. Much of this... 

Within a single secondary washcoat particle, the distribution of the precious metals can be assumed to be relatively homogeneous. The precious metals are typically present in a highly dispersed state. Dispersions measured by CO chemisorption methods are typically in the range 10-50% or even higher, for fresh catalysts. This means that the precious metals are present as single atoms or as small clusters of about ten atoms. For a catalyst with about 1.8 g precious metal per liter of catalyst volume, this corresponds to a precious metal surface area in the range of about 3-30 m 1 catalyst volume. 

UV is able to generate, on solid surfaces, new states of transition metal ions that can serve as active sites for thermal catalytic reactions, A typical example here is generation of long-lived low valence states of vanadium and copper on the surface of Si02 [31, 32], These states catalyze the thermal oxidation of alkenes even at room temperatures. Other examples of generation with UV light of active catalytic sites on the surface of dispersed oxides are reviewed in Ref. 33. 

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