Chemisorption methods

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Improved chemisorption methods ( self-assembled monolayers ) and physi-sorption methods (Langmuir-Blodgett films). 

Fig. 3 shows a topographic image of a Pt/y-A s catalyst. Contrast from particles is clearly separated from the substrate topography. On the other hand pores on the substrate are well defined. If the aperture includes some portion of the dark field spot then the resolution for small particles is improved. Fig. 4 shows an image of a 100% dispersed catalyst (as measur ed by chemisorption methods) in which particles of about 5 A can be seen. 

There is a one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

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. 

Ng = number of surface sites, determined by chemisorption methods. 

In principle, this chemisorption method should enable the investigator to count surface sites that are catalytically active. In practice, this does not appear to be the case for most amines. Even in the case of a highly... 

Chemisorption, transmission electron microscopy, and XRD line broadening do not necessarily result in the same calculated dispersion for a given catalyst. Chemisorption may be biased toward a lower average crystallite size and line broadening toward a higher size. In fact, line broadening and chemisorption methods are not directly comparable unless Fourier analysis is applied to the X-ray data. Chemisorption and transmission electron microscope results are directly comparable. 

To determine whether a change in dispersion or in the type of catalytic sites is responsible for these different effects of phosphate, Jian and Prins (59, 75) investigated the kinetics of these hydrogenation and elimination reactions. Unfortunately, no simple chemisorption method has proved capable of determining the dispersion of supported metal sulfides (6). Therefore, an indirect method, involving the determination of rate and adsorption equilibrium constants (the first proportional to the number of sites and the second dependent only on the type of site) had to be used. 

Oxygen chemisorption methods were used to titrate surface vanadium sites in these studies. Raman, X-ray diffraction and isotopic labeling were done to support the dispersion results from chemisorption. A further conclusion was that as the % V increased for ethane oxidation reactions that the catalytic activity and selectivity was similar to that of unsupported vanadia. 

Two methods have been proposed for the preparation of organosilane monolayers. One is chemisorption from organosilane solution [4-6], and the other one is Langmuir-Blodgett (LB) method or water-cast method [7-23], Fig. 1 shows the film formation mechanism of the organosilane by LB method (a) and chemisorption method (b). In the case of LB method (Fig. 1(a)), the toluene solution of organotrichlorosilane was spread on the water surface (pH 5.8) at a controlled subphase temperature. To attain quasi-equilibrium state of the monolayer, the monolayer was kept on the water subphase under a given... 

Fig. 1. Film formation mechanism of the organosilane by (a) LB method and (b) chemisorption method.

BET area (Table V) and the copper area from oxygen chemisorption. Table VII summarizes the copper and zinc oxide areas so determined for the whole compositional range. The oxygen chemisorption method suffers from the uncertainty that some oxygen may be adsorbed on the copper solute and on defects in the zinc oxide surface that are formed only in the presence of copper. There is indirect evidence from a comparative study of carbon monoxide and oxygen chemisorption, however, that this is not the case and that oxygen titrates only the copper metal surface. 

John Sinfelt feels "that the most generally valuable tool for a long time has been the measurement of adsorption isotherms, including the BET method for determination of total surface area and various selective chemisorption methods for determining the amount of surface associated with particular component. The BET method has been widely used by catalytic chemists for almost half a century for the characterization of catalytic materials. It is among the foremost developments in surface science during the twentieth century". 

Selective chemisorption methods have been used with success for the determination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quantitative piece of information about the catalyst surface. 

It is evident from the above discussion that catalyst characterization is an activity important for scientific understanding, design, and troubleshooting of catalyzed processes. There is no universal recipe as to which characterization methods are more expedient than others. In the opinion of the writer, we will see continued good use of diffraction methods and electron microscopy, surface analysis, IR spectroscopy, and chemisorption methods, increased use of combined EM and ESCA analyses for determining the dopant dispersion, increased use of MAS-NMR and Raman spectroscopies for understanding of solid state chemistry of catalysts, and perhaps an increased use of methods that probe into the electronic structure of catalysts, including theory. 

Essentially, the picture emerging from CO adsorption studies is that IR spectroscopy of species adsorbed on the supported clusters and chemisorption methods give only indirect information about the surface of clusters, and the surface may be modified by the adsorbates used. 

In the first group belong the techniques which are also used in heterogeneous catalysis for determining the surface area of catalysts. Two such techniques are widely used The Brunauer-Emett-Teller (BET) method, based on the physical adsorption of N2 or Ar at very low temperatures [8, 44] and the H2 or CO chemisorption method [8, 44], The first method leads to the total catalyst surface area, whereas the second leads to the specific (active metal) surface area. In the case of supported electrocatalysts (e.g., Pt/C electrocatalysts used as anodes in PEM fuel cells) the two techniques are complementary, as the former can lead to the total electrocatalyst surface... 

Titanium sulfate supported on zirconia catalysts were prepared by drying of powdered Zr(OH)4 with titanium sulfate aqueous solution followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by the measurement of surface area. The addition of titanium sulfate to zirconia shifted the phase transition of ZrOa from amorphous to tetragonal to higher temperature because of the interaction between titanium sulfate and zirconia. The catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. 

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. 

The cobalt catalyst prior to catalysis experiments have been shown to be less than 10 A in size by several methods including chemisorption methods, ferromagnetic resonance and spin echo nuclear magnetic resonance techniques (11, 20, 27). The transmission... 

Particle size determined from TEM, FMR, SENMR and chemisorption methods, in Angstroms. 

Nickel dispersion, metal area and crystallite size were determined by hydrogen chemisorption method [5]. 

Column 2 of Table 2 shows the dispersion values which we measured previously using conventional volumetric adsorption techniques. For the Pt/Rh catalyst, where it was necessary to use H2 as the probe molecule due to a strong and irreversible interaction of CO with one of the washcoat components (not present in the Pd/Rh catalyst), the volumetric chemisorption data agree fairly well with the CO-H2 methanation technique. However, for the Pd/Rh catalyst, the volumetric chemisorption method employing CO gave a nonsensical dispersion of 174% for the fresh catalyst and an implausible dispersion of 96% for the vehicle-aged catalyst. 

Taking into account the size of all the particles observed by CTEM, a calculation was made to estimate the arithmetic mean diameter of these particles, the equivalent diameter dgq comparable to the mean diameter obtained by chemisorption methods and calculated by comparing surface and volume of the particles, the specific surface area, and the dispersion, which represents the munber of accessible metal atoms [14-16],... 

After characterization of the metal by electron microscopy and chemisorption methods such as hydrogen, oxygen, and carbon monoxide adsorption, the reaction with tetrabutyl tin was performed and followed by GC and volumetry. The reaction... 

P-08 - Different chemisorption methods applied to zeolite supported Pt-catalysts... 

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