Adsorbate xenon

LEED, namely one with a, c(2x2) and one with a, p(2x2) superstructure. They are compatible with CusPt and CusPta layers. The first atomic layer was in both cases found by means of photoemission of adsorbed xenon to be pure copper. Details of the experimental work can be found in ref. 9 and 10. A schematic view of both structures can be seen in figure 1. Both consist of alternating layers of pure copper and of mixed composition. In the CuaPt case, the second and all other evenly numbered layers have equal numbers of copper and platinum atoms, whereas in the CusPta case the evenly numbered layers consist of thrice as many platinum as copper atoms. 

J. Fraissard, T. Ito, 1988 (Xe-129 NMR-study of adsorbed xenon - a new method for studying zeolites and metal-zeolites), Zeolites 8, 350. 

Photoemission of adsorbed xenon, abbreviated as PAX, is a site-selective titration technique, in which the UPS spectrum of physisorbed Xe reveals the nature of the Xe adsorption site [52, 53]. Since we are dealing with a weakly physisorbing atom, these experiments have to be done at cryogenic temperatures on the order of 50-60 K. We first explain the theory behind the PAX method and then illustrate the technique with an example. 

Its capability to titrate sites on heterogeneous surfaces makes photoemission of adsorbed xenon in principle a particularly attractive technique for investigating the surfaces of catalysts. Unfortunately, the technique has its limitations, because the Xe 5p /2 peak has a finite linewidth of about 0.4 eV. If a surface possesses more than three to four different adsorption sites, the spectra may become too... 

The NMR spectra of adsorbed xenon were obtained on a Bruker MSL-300 spectrometer operating at 83.0 MHZ and 295K. Typically, 2000-40000 signal acquisitions were accumulated for each spectrum with a recycle delay of 0.3s between 90 pulses. The Xe NMR chemical shifts were referenced to that of external xenon gas extrapolated to zero pressure using Jameson s equation [11]. All resonance signals of xenon adsorbed in zeolites were shifted downfield from the reference but were taken to be positive in this report. 

Ripmeester (346) used MAS to study xenon adsorbed on zeolites Na-X and H-mordenite. In the case of faujasite containing excess sorbate, separate lines from liquid, solid, gaseous, and sorbed xenon could be distinguished (see Fig. 67). The presence of a line from adsorbed xenon at 160 K shows that sorbed xenon does not freeze at the bulk xenon melting point. The line from liquid xenon measured at 170 K shifts to high field (Fig. 67b), suggesting that sorbed xenon is more dense than bulk liquid. 

Xe NMR experiments were performed according to previously described methods (19) on a Bruker CXP 100 apparatus at 24.3 MHz. The chemical shift (6) of adsorbed xenon is given relative to the value for gaseous 129Xe extrapolated to zero pressure. 

Because it would be difficult to compare isothermal adsorption results for multiphase samples, the studies were restricted to NaY zeolite and the compound for which R = 0.05. For both solids, the logarithmic representations for the relation log N = f (log P), where N is the number of adsorbed xenon atoms per gram of dehydrated zeolite and P is the xenon equilibrium pressure, are straight lines. The line for the sample with R = 0.05 is below that of NaY zeolite (Figure 3). The log N value extrapolated to zero abscissa is smaller, and the slope is only a little larger. 

Figure 4. 129Xe chemical shift versus the number of adsorbed xenon atoms per gram of anhydrous zeolite N. 

This is the case which Frenkel had in mind when he derived Eq. (37). We shall illustrate this ease with a few examples. Cassel and Neugebauer (195) estimated the adsorption of xenon on mercury. An evaluation of their data (196) leads to the conclusion that the adsorbed xenon molecules are not in their lowest state of vibrations perpendicular to the surface,... 

Fig. 3.23 Photoemission of adsorbed xenon (PAX) spectra at 60 K of 0.27 monolayer of Ag deposited on Ru(001) followed by annealing, showing that Xe first populates Ru, and then Ag. The top spectrum corresponds to a complete monolayer ofXe on the Ag/Ru(001) sample and can be used for quantitation. (From [73]).

Fermi level of the metal [19]. This is the value that one measures with scanning tunneling microscopy (see Chapter 7) and with photoemission of adsorbed xenon (see Chapter 3). Thus, on a heterogeneous surface we have local work functions for each type of site, and the macroscopic work function is an average over these values. 

The adsorption behavior of benzene on dehydrated NaX and NaY zeolites has been investigated directly by H and 13C NMR measurements of the adsorbed benzene and indirectly by the combination of 129Xe NMR and isotherm measurements of the co-adsorbed xenon. Powdered zeolite samples of various Si/Al ratios and with varied adsorbate concentrations were investigated. Detailed macroscopic and microscopic adsorption phenomena of benzene in NaX and NaY zeolites, including the loading capacity, mobility, and sites of adsorption are presented in terms of measurements of NMR linewidths and chemical shifts. 

Xenon Adsorption Experiments. Gaseous xenon was co-adsorbed onto the samples on a vacuum manifold the xenon equilibrium pressure was measured by an absolute-pressure transducer (MRS Baratron) capable of measuring pressure with accuracy 0.1 torr. The adsorption isotherms of the co-adsorbed xenon in the samples were measured volumetrically at 22 °C. 

XH and 13C NMR Measurements. In order to confirm these 129Xe NMR results obtained indirectly from the co-adsorbed xenon, we performed separate NMR experiments on the adsorbed benzene. Specifically, the and 13C NMR of benzene were measured with the same samples used in the 129Xe... 

Through the analysis of adsorption isotherms and 129Xe NMR results of the co-adsorbed xenon, we have shown that the dispersal of benzene molecules depends on not only the cation distribution but also the amount of benzene adsorbate within the supercage of zeolite adsorbents. We have also demonstrated for the first time that this well known indirect technique has the capability not only to probe the macroscopic distribution of adsorbate molecule in zeolite cavities but also to provide dynamic information about the adsorbate at the microscopic level. Conventional H and 13C NMR which directly detect the adsorbate species, although providing complimentary results, are relatively less sensitive. 

NMR spectrometry of Xenon-129 adsorbed in coked samples of the totally protonated H-ZSM-5 zeolite and the modified Na, H-ZSM-5 showed variations attributable to differences in coke distribution. 129Xe NMR spectrometry is extremely useful for probing microporous materials. Ito et al.(2) demonstrated, for example, that NMR spectrometry of adsorbed xenon in coke-fouled H-Y zeolite could probe the deposits after coking and the nature of the internal surfaces after decoking. The NMR results in this study are consistent with a distribution of coke restricted by size selectivity of the acidifying medium. 

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