Determination of Metal Dispersion

The chemisorption of hydrogen onto supported metals is inevitably more complex than onto unsupported macroscopic forms for three reasons (i) small supported metal particles differ even from their unsupported counterparts because of the very presence of the support to which they are attached and with which they 

The importance of the manner of interaction of hydrogen with supported metals is two-fold (1) it provides means for estimating the number of surface metal atoms, and hence with certain assumptions the exposed metal area and the mean particle size and (2) it informs us of possible states in which hydrogen may exist on the catalyst during catalytic reactions in which it is a partner. The first consideration is treated in this section, and the second in the following section. 

Some however prefer to work at low temperature (e.g. 77 K) to ensure that a complete monolayer remains on the metal and that spillover is absent. It is then necessary to estimate the amount of physical adsorption that has to be subtracted from the total, either by measuring the adsorption isotherm for the support alone, or, more reliably the isotherm on the catalyst, following the first adsorption and a short evacuation. This method is referred to as the back titration method (Eigure 3.13), and even at ambient temperature the second isotherm often reveals some weak adsorption , the nature of which may vary from one system to another, and which needs to be taken from the total in order to isolate the amount of the strong form . This often leads to an isotherm that is flatter in the high-pressure 

One way of circumventing this problem is to fit the results to the appropriate linearised form of the Langmuir equation for dissociative adsorption  

The volumetric method has very often been used with platinum catalysts for which quite satisfactory results are generally obtained it is usual to assume that the monolayer volume or amount, obtained as just described or by extrapolation corresponds to an H Ms (hydrogen atom to metal surface atom) ratio of 1 1. Some justification for this assumption is to be found, at least for particles of moderate size, in the adsorption stoichiometry shown by films and single crystals, but for very small particles and at high pressures the H/Mj ratio can exceed unity quite substantially this is especially so with rhodium and iridium (see below). Care is however needed with palladium because of the risk of forming the hydride however, monolayer coverage is obtained at pressures below which dissolution starts. The base metals iron, cobalt and nickel have been 

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In this section the following points will be addressed 1) the application of HREM in the determination of metal dispersion 2) the structural features of metal-support interaction effects and their evolution under reducing environments in the temperature range 473 K-1173 K and, finally, 3) the reversibility of the interaction effects with oxidation treatments. 

Determination of Metal Dispersion in NM/Ce(MX)2.x Catalysts by HREM. Evolution of Dispersion under Reducing Conditions. 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

Examples of the latter are demonstrated along with the clarification of certain other questions for bimetallic systems (3.4). It is not our intention to describe a catalytic process, but rather the materials alterations that seem to occur during common set up procedures, such as oxidation and reduction. We will also report on a unique ESCA based method that seems to determine the important feature of metals dispersion in the alumina matrix, a feature that may be disturbed as a result of the "abuse" experienced by a catalyst (1). 

There are situations in which crystallites are readily visible, especially on supports which do not offer excessive electron scatter. In these cases, metal content can be quantitatively determined for areas which have highly dispersed metal and agglomerated metal. This information in conjunction with the crystallite size distribution provides the microscopist with the information required to make an estimate of metal dispersion (13). These estimates are valuable especially in situations where conventional gas adsorption measurements cannot be made on the metal, i.e., when the crystallites are contaminated, have multiple oxidation states, or are poisoned. 

The flask can then be evacuated to remove last traces of petroleum ether, then re-weighed to determine the exact quantity of metal. With the flask connected to the inert gas system by a 3-way tap, reaction solvents and other reagents can be added directly to the metallic reagent. If you need to separate the oil from a quantity of metal dispersion... 

PlOC-4 The kinetics of self-poisoning of Pd/Al203 catalysis in the hydrogenolysis of cyclopentane is discussed in J. Catal, 54, 397 (1978). Is the effective diffu-sivity used realistic Is the decay homographic The authors claim that the deactivation of the catalyst is independent of metal dispersion. If one were to determine the specific reaction rate as a function of percent dispersion, would this information support or reject the authors hypotheses ... 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Figure 4.1. Single-line FIA manifold for the determination of metal ions by flame atomic absorption spectrometry (AA). The sample (5) is injected into a carrier stream of diluted acid (5 X 0 M sulfuric acid), propelled forward by pump P, and transported to the nebulizer of the AA system, the distance between the injection valve and the AA instrument being reduced as much as possible (length 20 cm) in order to secure limited dispersion of the injected sample.

Some aspects related to catalysts characteristic and behaviour will be treated such as determination of metal surface area and dispersion, spillover effect and synterisation. A detailed description of the available techniques will follow, taking in consideration some aspects of the gas-solid interactions mechanisms (associative/dissociative adsorption, acid-base interactions, etc.). Every technique will be treated starting from a general description of the related sample pretreatment, due to the fundamental importance of this step prior to catalysts characterisation. The analytical theories will be described in relation to static and dynamic chemisorption, thermal programmed desorption and reduction/oxidation reactions. Part of the paper will be dedicated to the presentation of the experimental aspects of chemisorption, desorption and surface reaction techniques, and the relevant calculation models to evaluate metal surface area and dispersion, energy distribution of active sites, activation energy and heat of adsorption. 

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