Hydrogen is used as a probe molecule for characterisation of Brsnsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTlR (OH--H2 complex formation), simultaneously. The physisorption of hydrogen is a function of the pore size of a zeolite whereas the OH. * .H2 interaction reflects the strength of the acid sites. Hydrogen complexes observed by FTlR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H , bands. For the zeolites studied the behaviour of these parameters is consistent and shows the following order of acidity: SAPO-37 < H-Y < H-EMT < H-ZSM-5 < H-MOR. Hydrogen is also compared with carbon monoxide, another probe widely used for acidity determination.
Silicoaluminophosphate molecular sieves of SAPO-5 type are synthesized with variable Si/(AI + P + Si) mole ratios using triethylamine as the organic additive. The crystalline products are characterized by several physico-chemical methods. Incorporation of silicon into the framework of SAPO-5 is demonstrated by the variation in the unit-cell volume with silicon mole fraction, chemical analysis and magic-angle-spinning nuclear magnetic resonance. When Si/(AI + P + Si) < 0.077, silicon substitutes predominantly for phosphorus. For compositions where Si/(AI + P + Si) > 0.077, it appears that a combination of mechanisms, substitution of one silicon for one phosphorus and substitution of two silicons for aluminium-phosphorus pairs, takes place. Surface analysis using depth profiling shows an inhomogeneous distribution of silicon at higher silicon content. Preliminary catalytic results in 0 -and rn-xylene isomerization are discussed. The aluminophosphate-based molecular sieves are an important relatively new class of adsorbents and catalytic material^.'^^ Microporous materials containing framework silicon are denoted by the acronym SAP0.2,3 The generation of SAPOs is considered to follow a mechanism by which silicon substitutes into the corresponding AlPO, framew o r k ~. ~ This substitution3 can involve (1) replacement of one aluminium by one silicon (SM1); (2) replacement of one phosphorus by one silicon (SM2) or (3) replacement of aluminium-phosphorus pairs by two silicons [SM3 Ho (Ho = homogene~us)].~ The substitution of silicon for phosphorus leads to the formation of Brnnsted-acid sites of medium acid strength. Increasing the amount of silicon in the framework of these SAPO materials can lead to high activity in certain acid-catalysed reactions. It is therefore very important to synthesize SAPOs with high silicon content. Additionally, it is highly desirable to control the synthesis so as to aim at SAPO catalysts with silicon content optimized to produce the maximum number of Brnnsted-acid sites of medium strength acidity.Recent papers have appeared on the synthesis and/or characterization of SAPO-55-'2 and a few exist on the incorporation of silicon into the AIPO, framework. Mertens et aL5 studied the incorporation of silicon into a series of SAPO-5 materials with silicon mole fraction, x = 0.01, 0.15, 0.28 and 0.36. These authors suggest silicon substitution by SM2 when x = 0.01 and substitution by combined SM2 and SM3 He (He = heterogeneous) processes when x 3 0.15. It is of interest to know the value of x corresponding to the change in mechanism which occurs when x is increased from 0.01 to 0.15. Moreover, it is important to know whether the build-up of siliceous regions results in composition gradients in the crystals. These questions are addressed in the present work by considering SAPO-5 synthesized to have a silicon mole fraction in the range 0.05-0.17. These materials are characterized using several physical methods and a preliminary catalytic evaluation is made. Simple cluster M O calculations are used t...
A model for the quantitative plulysis of XPS data from coated spherical particle8 is dwdbed. INTRODUCI'IONInhomogeneous materials in powder form in which the surface composition plays a vital role are very common in industry, e.g. as catalysts, pigments and fillers. Particularly in the latter two cases, surface coatings are present and qualitative analysis of these by techniques such as XPS has become relatively routine. However, to obtain quantitative analyses one requires to know the surface composition, the extent of coverage and the degree of homogeneity, and this is more difficult to achieve.In the electron spectroscopy study of overlayer/substrate systems the relevant equations for peak intensities are well characterized for the case of ideal flat surfaces; in particular the application of the angular rotation effect, which results in enhancement of the surface overlayer intensity at low electron take-off angles, has been extensively investigated.*'2 Recently Chang and Boulin3 reported SiOz and A203 overlayer thickness determinations from Auger spectra analysis, using flat surface equations. Also, Dreiling4 has developed equations for use in the X P S study of ideal multilayer systems, to estimate relative surface concentrations, and has applied these to the analysis of metal oxides. Many materials of interest are particulate in form andhence exhibit an exceedingly 'rough' surface. To our knowledge there exists no derivation of equations describing the intensity of photoemission from coated spherical particles. In this paper we present a model for the analysis of overlaid spherical particles, discuss the limitations of the flat surface equations in this context and compare the model predictions with results from an XPS study of silica-coated titania pigments. THEORY AND MODEL Plane d a c e intensity equationsThe general expression for the number of electrons received from a given subshell of element z is given byss6 where J is the incident photon flux, 4 the angle this makes to the sample surface, 0, is the atom density of element z, a, is the photoelectric cross-section for the relevant subshell, the first exponential term represents the electron attenuation factor, 8 is the electron take-off angle (to the sample surface), A, is the electron mean free path, K is the instrumental response factor, and the second exponential term represents the attenuation of the z electrons by a contaminant layer of thickness d. This equation can be applied to derive expressions for the intensity of photoemission from a multilayer system such as Fig. 1.From Eqn (1) the intensity of photoemission from an infinite thickness of z (uncontaminated) is given by
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