Chemisorption line broadening

Iron particle size from X-ray line broadening and percentage-reduction/CO-chemisorption ... 

Reduction Metal content after reduction (%) d , from x-ray line broadening (A) dvS, fromH2 Metal particle chemisorption size distribution at room temp. from electron (A) micrographs Number of B5 sites per mg of Ni Number of B6 sites per cm2 of Ni surface... 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

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. 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

For supported metal catalysts, no simple calculation is possible. A direct measurement of the metal crystallite size or a titration of surface metal atoms is required (see Example 1.3.1). TWo common methods to estimate the size of supported crystallites are transmission electron microscopy and X-ray diffraction line broadening analysis. Transmission electron microscopy is excellent for imaging the crystallites, as illustrated in Figure 5.1.5. However, depending on the contrast difference with the support, very small crystallites may not be detected. X-ray diffraction is usually ineffective for estimating the size of very small particles, smaller than about 2 nm. Perhaps the most common method for measuring the number density of exposed metal atoms is selective chemisorption of a probe molecule like H2, CO, or O2. 

A few short papers have dealt with the zero-field NMR of small ferromagnetic nickel and cobalt particles. The physical basis of such measurements is more complicated than that of NMR in an applied field, and this is not discussed here. Suffice it to say that for Ni a considerable size-dependent line broadening is found (189), which could possibly be used to investigate the effect of chemisorption on the surface magnetism of such particles. The Co zero-field NMR can distinguish between superparamagnetic, FCC, and hexagonal close packed (HCP) cobalt (190). 

State of Dispersion of Metal. Chemisorption of carbon monoxide at 23 °C and x-ray diffraction line broadening have been used to measure... 

Adsorption measurements with different supports or catalysts were carried out by using a mixed solution of cinchonidine and substrate 1 (4 mmol/1 for each) in solvent B. After stirring at 298 K for 1 h, the amount of each compound adsorbed was determined from the decrease in the concentration of the solution, The concentrations were monitored by HPLC. The mean crystallite sizes of Pd in the reduced catalysts were obtained from XRD line broadening. The total surface areas (Sbet) and the areas of Pd metal surface (Spa) were determined using the nitrogen adsorption at 77 K and by the CO chemisorption at 323 K, respectively. 

The metal in a supported metal catalyst may be characterized by electron microscopy (providing a distribution of crystallite sizes), X-ray diffraction line broadening (providing an average size for crystallites larger than about 4 nm), and specific chemisorption (titration) with compounds such as H2 or CO [providing an estimate of the exposed metal area (v5. the total surface area of metal plus support)]. 

From the extent of reduction and the surface area of the metal, as calculated from the extent of chemisorption, the mean size of the metal can be calculated. The broadening of the maxima in the X-ray diffraction pattern also measures a mean particle size. Usually the mean particle size calculated from the X-ray line-broadening is larger than that calculated from the extent of reduction and the surface area of the metal particles. The difference is because X-ray line-broadening provides a weight-mean particle size, Ln d l Ln- di, whereas the extent of chemisorption and the extent of reduction result in a volume-surface mean diameter, Lnid /Lrijdj. ... 

A chemisorption teclmique developed by Koinai et al., based on CO methanation, was successfrilly used to analyze noble metal dispersions of both fresh and vehicle-aged Pt/Rli and Pd/Rli commercial automotive three-way catalysts. The teclmique is relatively rapid (< 2 hours), extremely sensitive, and largely free from complications due to adsorption of CO on non-noble metal components of the washcoat (support, promoters, stabilizers, etc.). Particle sizes of the vehicle-aged catalysts, calculated by applying the spherical particle assumption to the dispersions measured by the CO methanation method, agreed well with particle sizes calculated from x-ray diffraction line-broadening data. These results indicate that the CO methanation teclmique can be applied routinely to obtain fast and accurate measurements of noble metal surface areas in automotive catalysts retrieved from tlie field, even tliose with metal dispersions ca. 2% or less. 

Assuming that spillover can be either suppressed or accounted for, hydrogen is usually the recommended adsorptive for determining Pt dispersion as the H Pt stoichiometry is well established from combined chemisorption, TEM and X-ray line broadening experiments as being i i However, there are indications that... 

Loader 38) studied the Raman spectra of styrene adsorbed on different silicas—chromatographic grade silica gel, Cab-O-Sil, and Aerosil 380. The author utilized the fact that chemisorption will bring about marked changes in the spectrum whereas physical adsorption will cause only a broadening of the Raman lines accompanied in some cases by a frequency... 

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