Metal-loading for catalysis

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

The maximum frequency of the C-O vibrational band and the overall band shape reflect the average electronic environment of the platinum atoms. Changes in band intensity, frequency and shape depend on the cluster size, exposed crystallographic planes, location (inside versus outside zeolite pores) and oxidation state of the metal. The number of publications for CO adsorption on zeolite supported noble metals are too numerous to list here, but a few examples that illustrate the type of information that can be obtained are included below. 

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Metal incorporation into the zeolite using metal loaded seed materials. The combination of catalyst metal with zeolite catalyst is one of the most intriguing subjects for bifunctional catalysis. The achievement of prominent effect of the seed crystals on the crystallization of ZSM-34 type catalyst induced an idea that the seed material on which a catalyst metal had been supported previously would also be effective for rapid crystallization. 

Metal clusters supported on refractory oxides are used extensively in catalysis for the production of chemicals and petroleum-derived transportation fuels. Catalysts in this class typically have metal loadings of less... 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

The electronic metal-support interactions can also account for the lesser extent of SMSI with respect to H. chemisorption and catalysis when the metal loading is increased at constant crystallite size, since for an equivalent quantity of free electrons created by reduction in the support, the higher number of metal atoms present leads to weaker electronic perturbations per atom concerned. 

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