Nature of the Support Material

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

In the following, we summarize the main features of practical relevance. The position and amplitude of the activity hysteresis depends mainly on partial pressure of O2 and on the nature of the support material. The threshold temperatures of both decomposition and re-formation of PdO increase markedly with O2 partial pressure. Data on the effect of the support material are reported in Table 12.2. 

Some of the important parameters which affect surface cleaning are (a) the size and nature of the support material and the coating, (b) the surface conditions of the support, (c) the time of treatment, and (d) the method by which the coating was applied. This last point is of crucial importance since the consistency of the method of coating determines the reproducibility and the accuracy of the method. It should be carefully standardized to allow comparative studies. The size of the item to be cleaned is also important. If it is too small it will be difficult to measure the amount of removed soil, and if it is too large it will dramatically disturb the ultrasonic field. A method used by ultrasonic cleaner manufacturers to assess cleaning power is to remove Tipp-Ex fluid from the screw threads. 

The actual chemical nature of the support material may be and often is of direct importance to its usefulness as a support material.15 Silicas can react with small nucleophiles such as F-, OH- and CN. Thus, silica-supported fluorides are inactive, both as nucleophilic fluorinating agents and as bases. Similarly, silicas are not effective support materials for cyanides due to the formation of strong Si-CN bonds. For different reasons, an acidic clay would not be a suitable support for cyanides, due to the possible formation of toxic HCN. Charcoal is the most effective support material for stabilising Cu(I), probably due to its aromatic character.16 For many chemisorbed supported reagent catalysts, silicas are preferred since they give relatively strong surface bonds. However, Si-O-C bonds are hydrolytically vunerable and direct Si-C bonds are preferred.17... 

In these studies, the observed enhancement of the activity was attributed to the ability of the gas-phase process to achieve high dispersion of the active phase, independent of Ae nature of the support material [13,15], and to result in a stronger interaction [14]. 

Partitioning effects the equilibrium substrate, or effector concentrations within the support may be different from those in the bulk solution. Such effects, related to the chemical nature of the support material, may arise from electrostatic or hydrophobic interactions between the matrix and low-molecular weight species present in the medium, leading to a modified microenvironment, l.e., to different concentrations of substrate, product or effector, hydrogen and hydroxyl ions, etc., in the domain of the immobilized enzyme particle. 

The nature of the supporting material can determine the final catalytic performance. Interestingly, gold catalysts supported on various carbons displayed differences even in the same reaction, which implies that the catalytic performance was affected not only by the nature of the carbon but also by the preparation method [47]. One of the most relevant parameters for the choice ofthe support seems to be the type and distribution of surface groups, as these can favor or inhibit the grafting of the nanoparticles. Hence, acidic or basic treatments of supporting materials... 

In general, the nature of the support materials is a major selectivity-determining factor, while the composition of the binding buffer is an auxiliary factor that can tune the selectivity to a certain extent. 

Quats are commonly used at a level of 6% in combination with compatible nonionic surfactants. These household products are called disinfectant cleaners. The efficacy of these products has to be evaluated for each formula, regardless of quat content. Optimum germ inactivation is obtained with a ratio of 1 part nonionic for 2.5 parts of cationic. Increasing the nonionic level inhibits the disinfecting efficacy by trapping the quat inside the nonionic micelles. The rinsability of quats is also better at this ratio, regardless of the nature of the support material [7]. 

The liquid stationary phase in a GLC packed column is adsorbed on the surface of a solid substrate (also called the support). This material must be inert and finely divided (powdered). The typical diameter of a substrate particle is 125 to 250 ft, creating a 60- to 100-mesh material. These particles are of two general types diatomaceous earth and Teflon . Diatomaceous earth, the decayed silica skeletons of algae, is most commonly referred to by the manufacturer s (Johns Manville s) trade name, Chromosorb . Various types of Chromosorb, which have had different pretreatment procedures applied, are available, such as Chromosorb P, Chromosorb W, and Chromosorb 101-104. The nature of the stationary phase as well as the nature of the substrate material are both usually specified in a chromatography literature procedure, and columns are tagged to indicate each of these as well. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

---