Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 04-NMR of Physisorbed 129Xe Used as a Probe to Investigate Molecular Sieves

NMR of Physisorbed 120XE Used as a Probe to Investigate Molecular Sieves

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λi and µi are the probabilities of xenon collisions with Si and Na – Y, respectively.

In general, such a situation can arise whenever there are, in the zeolite crystallites, zones that are clearly differentiated either by the nature of the particles or by the distribution of the particles that can, in this case, be of the same type.

Let us consider now a catalyst Mx– βG – Na – Y containing n metal particles per gram of sample (each containing on average x atoms of metal M) with a total amount β of adsorbed gas G. There is a distribution of atoms G (or molecules) over the totality of the metal, the number, i, of G atoms (or molecules) per particle being possibly different from one particle to another. Now, the chemical nature of the surface of the metal particle will be indicated by changes in the number, i, of G atoms (or molecules) per particle. For each number, i, there is therefore a corresponding term δi = δMx+iG characteristic of the collision between Xe and the particles Mx + iG.

As we have seen above, the form of the spectrum of xenon adsorbed on such a sample will depend on the Xe–(Mx + iG) interaction, on the numbers i, j... of atoms or molecules of G chemisorbed on the various particles and on the distribution of Mx + iG, Mx + jG ... particles within a zeolite crystallite.

The main applications of this technique concern the study of the distribution of phases chemisorbed on supported metal particles and the determination of the particle size.

2.5.2

Chemisorption of Hydrogen

The spectrum of xenon adsorbed at 300 K on a Ptx– Na – Y sample consists of a single line (denoted a) whose chemical shift δ(a) is always much greater than δNa–Y whatever the xenon pressure [58] (Fig. 27). This signal is due to the coalescence of the line of high chemical shift, δPt, due to xenon adsorbed on the platinum particles, and that of shift δNa–Y corresponding to Xe atoms colliding with the walls of the supercages or with other Xe atoms. δPt is very high, about 1000 ppm, and practically independent of the particle size [182]. For the sake of simplicity, we shall say that line a is characteristic of Xe – Pt collisions.

After chemisorption of a very small amount of hydrogen (the number, nH2 , of hydrogen molecules being much smaller than the number of metal particles determined by electron microscopy, nEM) a second signal (denoted b) is detected; its shift, δ(b), is between that of δ(a) and δNa–Y, and it corresponds to Xe atoms adsorbed on particles that have chemisorbed hydrogen.

The existence of these two signals proves that the distribution of H atoms on the particles divides the zeolite crystallite into two zones. At ambient temperature, hydrogen is chemisorbed by the first particles encountered when it penetrates the zeolite crystallites, thus defining two zones: a central one (a)

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Fig. 27 Spectra of xenon adsorbed on Pt-βH-NaY at different numbers, nH2 , of chemisorbed hydrogen molecules; 0 ≤ nH2 ≤ npart.; ◦ ◦ ◦ bare Pt particles; (• • •) Pt particles with chemisorbed H2 (see text). (Reprinted from [2–4], with permission from Elsevier Science)

Fig. 28 δ = f (N) variation for Pt – Y zeolites. a line a, b line b, c line c. See text. (Reprinted from [180], with permission from P.A. Jacobs)

corresponding to bare particles and a peripheral one (b) containing particles that have chemisorbed hydrogen (Fig. 28).

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Furthermore, when nH2 increases while remaining very small, line b increases at the expense of a, but without any change in their NMR characteristics (such as chemical shift and line width), showing that each particle in b bears the same amount of hydrogen, or at least that the average distribution of the chemisorbed hydrogen is constant in zone b, regardless of the size of this zone. The intensity, Ia, of the signal falls to zero when that of b, Ib, is a maximum. Considering the literature data concerning the variation of the heat of chemisorption with the coverage and the evolution of δ with the chemisorbed H2 concentration, the first authors [58] concluded that the amount chemisorbed at the beginning was 2H per particle. In this case, Ib is maximum when the total number of chemisorbed H2 molecules, nH2 , is equal to the real number of particles, np, which one can determine exactly from nH2 . One finds that np is always much greater than nEM determined by electron microscopy, confirming thus that, except in very rare cases, this latter technique is unable to detect very small particles. When nH2 becomes greater than np, a signal c appears (δ(c) < δ(b)), generally poorly resolved from b and corresponding to particles that have more than 2H on their surface.

Boudart et al. have extended this study to various Pt/Na – Y samples. They observed that the number of Pt atoms per metal particle determined by 129 Xe NMR was always smaller than that obtained by other techniques: X-ray diffraction, EXAFS, WAXS and TEM [183, 184], if it is assumed, as in [58], that the clusters are sufficiently small for Xe colliding with them to “see” the first two hydrogen atoms adsorbed on the clusters no matter where it strikes. They propose a second explanation, namely, that the Pt clusters are large so that they almost fill the supercages of Y-zeolite. Then Xe will interact only with Pt atoms exposed in each of the four windows of the supercages. If, in addition, the first two H atoms adsorbed on Pt facing a window stay at that window, the endpoint indicates the point at which there is one H2 adsorbed per window of the supercage. Accordingly, there are four times as many Pt atoms per cluster as obtained by the first assumption. The second assumption gives results which are compatible with the results given by other physical techniques.

In fact, 1H NMR has shown, at least in the case of Pt on Al2O3 [185] or SiO2 [186] that H2 is chemisorbed at 300 K by the first particles encountered, with a degree of coverage at equilibrium of about 0.5. It is only beyond this coverage that H2 diffuses easily in the sample. Ib therefore reaches a maximum when the coverage of each particle is close to 0.5. Another solution consists of chemisorbing H2 at high temperature or, which would be equivalent, homogenizing the phase initially chemisorbed at 300 K over the whole sample, by raising the temperature (if possible to 673 K in order to overcome spillover, at least partially). As an example, Fig. 29, line 1 shows the spectrum of a Pt,Na – Y sample which has adsorbed a small amount of H2 at 300 K [181]. When the sealed sample (therefore at constant total H2 concentration) is heated , there is a parallel decrease in Ia and δa of line a, and an increase in Ib and δb of line b. This shows that zone b extends at the ex-

(2) 368 K, (3) 393 K, (4) 448 K (nH2

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pense of a, as a result of desorption from b-type particles and diffusion of H2 throughout the entire sample. When the temperature and the heating time are sufficient, the H2 is homogeneously distributed throughout the sample and a single line is obtained (Fig. 29, line 4). If the total H2 concentration is very low, the probability that there are more than 2H per particle is negligible. The sample consists then of particles which are either bare or carry 2H. In this case δb is linearly dependent on the H2 concentration and goes through a critical point where all the particles carry 2H, from which the number of particles can be determined [181, 184, 187].

129 Xe NMR is therefore an interesting technique for determining the average number of atoms per particle. Moreover, this technique gives detailed information about the cluster distribution within the zeolite crystallites. A narrow Lorentzian signal of xenon adsorbed on samples without H2 indicates a homogeneous distribution of clusters in the Y crystallites. In the opposite case, a broad peak and sometimes even a second peak is detected. Finally, if the situation arises 129 Xe NMR can be used to determine the concentrations

of Pt located inside and outside the crystallites [183].

Chmelka et al. have shown that 129 Xe NMR can be used to monitor the location of metal clusters and cluster precursors as a function of calcination conditions for Na – Y zeolite-supported platinum catalysts [75, 188]. Their results indicate that for the reduction conditions imposed, the formation of highly dispersed platinum clusters within the Y-zeolite matrix is best achieved by employing a calcination temperature close to 673 K. Incomplete

Fig. 29 Spectrum of xenon adsorbed on Ptx – βH-NaY after heat treatment at (1) 300 K, < npart), (◦ ◦ ◦) bare Pt particles, (• • •) Pt particles with chemisorbed H2 (see text). (Reprinted from [2–4], with permission from Elsevier

Science)

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decomposition of the ion-exchanged Pt(NH3)2+4 complex during calcination at 473 K results in migration of nearly all platinum to the exterior surface of the zeolite crystallite during reduction. Calcination temperatures significantly above 673 K induce decomposition of the shielded precursor species and subsequent migration of the metal into the sodalite cavities. A substantial amount of the platinum confined within the sodalite cavities migrates back into the supercages during reduction at 673 K [75, 188]. A similar study was carried out by Yang et al. [189].

Studying the surface of platinum supported on alumina, Boudart et al. have shown that metals supported on nonmicroporous materials can be probed by Xe NMR as previously demonstrated for zeolites. In particular, the amounts of adsorbed hydrogen and oxygen, as well as the reaction of adsorbed hydrogen with dioxygen, can be followed by this technique [190].

2.5.3

Chemisorption of other Gases (G)

The distribution of the first molecules of a gas G chemisorbed on Pt particles depends above all on the nature of this gas. For example, at 300 K, oxygen behaves very similarly to hydrogen.

In the same way, at 300 K, carbon monoxide half saturates the first Pt particles encountered when entering the zeolite crystallite (apparent stoichiometry 1 CO/2 Pt) [191]. In contrast, at 673 K one can obtain at low coverage a homogeneous distribution corresponding to one CO molecule per particle.

Thus, by means of this technique, it is possible in all cases to determine quantitatively the distribution of gases chemisorbed on metal particles and the distribution within the Y crystallites of Pt particles distinguished by the amount chemisorbed.

Now, in fundamental research it is important to know the particle coverage. For example, by means of the results obtained by 129Xe NMR spectroscopy for the local distribution of CO chemisorbed on the very small platinum particles (six atoms on average) supported on Na – Y zeolite, it has been possible to determine precisely the effects of back-donation from the metal to CO and of dipole-dipole coupling between chemisorbed CO on the variation of the stretching frequency, νCO , with surface coverage [192].

There is a further point of interest concerning the chemisorption of CO at 300 K. Under the experimental conditions employed, the xenon technique can detect only changes occurring inside the supercages. It is, therefore, insensitive to the chemisorption of CO on the Pt particles located on the external surface of the Y crystallites. If very thin layers of the solid are used, the xenon will detect only the first CO molecules chemisorbed on the internal metal particles when the outer particles are saturated. Assuming a stoichiometry of one CO per 2 Pt atoms it is then possible to determine the number of Pt atoms located on the surface of the external Pt particles [180].

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Finally, this technique can also be used to determine the distribution of several gases chemisorbed on zeolite-supported metal particles [181].

2.5.4

Metals other than Platinum

129 Xe NMR has been applied to the study of catalysts based on metals other than Pt. For example, by means of this technique, Coddington et al. [193] have shown that adsorption of Mo(CO)6 in zeolite Na – Y followed by decomposition at 473 K produces uniformly dispersed Mo2 clusters in the zeolite supercages; heating to 673 K causes sintering to an average cluster size of three or four Mo atoms.

Finally, 129 Xe NMR seems to be particularly useful for studying bimetallic catalysts, amongst other things, to determine the size of the particles and their distribution in the Y crystallites, as well as to estimate their electron deficiency (metal-support interaction), as has been shown by Ichikawa et al. on Rh6–x Irx/Na – Y bimetals with x = 0.6 [194].

2.6

Xenon Diffusion

2.6.1 Introduction

The study of the diffusion of xenon in a microporous system is particularly interesting for the characterization of the solid itself and for the study of the diffusion of other adsorbates. It can be performed by comparing chemical shifts with various standards or by means of the now-classical pulse-field gradient NMR (PFG NMR) technique.

We think it is worthwhile recalling first of all the characteristics of pure xenon in order to better understand the effect of the adsorbent. Reference [195] is the first systematic investigation of self-diffusion in

a monoatomic gas over a large density and temperature range. Within experimental error the results are the same with 129 Xe and 131 Xe. For example, at

298 K and 25 amagat, D = 2.054 × 10–7 m2 s–1. D is inversely proportional to the density and increases with the temperature.

2.6.2

Self-Diffusion of Xenon by the PFG NMR Technique

Pulse-field gradient NMR is a versatile tool for studying molecular transfer in zeolitic adsorbent-adsorbate systems. In particular, it allows the direct measurement of the translational molecular mobility inside the crystallites, represented by the self-diffusion coefficient D. This technique is based

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on the application of radio frequency pulse sequences for generating spin echos [196, 197], where during two time intervals two inhomogeneous magnetic fields with the field gradient, of intensity g, are superimposed on the constant magnetic field.

In general, under the influence of these field gradient pulses, the magnitude of the NMR signal of adsorbed species (the spin-echo intensity) is reduced by the factor

ψ(g, δ , ∆) = [1 – γ (∆)] exp – γ 2δ 2g2			r		2	(	∆)	intra
							6
	r	2		∆)
+ γ (∆) exp – γ 2δ 2g2		(					inter		,	(26)
					6

with ∆ denoting the time separation between the two identical field gradient pulses, chosen to be much larger than the pulse width δ . γ is the gyromag-

netic ratio of the considered nuclei. r2(∆) intra and r2(∆) inter represent the mean square displacements of the two following subgroups: those diffusants

that remain inside the individual crystallites over the whole observation time, and those that may leave their crystallites and that are able, therefore, to cover large diffusion paths through the intercrystalline space. γ (∆) denotes the relative amount of the latter subgroup.

For Na – X and ZSM-5 zeolites the ψ(δ , ∆) = f (δ 2) relationship is represented by the first term of Eq. 26 and all the information is contained in the mean square displacement r2(∆) intra (Fig. 30a and b) [198]. For the A zeolite the observed echo attenuation is represented by the superposition of the two terms of Eq. 26 which contains two types of information: r2(∆) intra obtained from the shape of the second more slowly decaying part of the Ln ψ vs. δ 2 plot and the quantity γ (∆) (Fig. 30c).

According to the Einstein relation

r2(∆) = 6D∆,

(27)

the self-diffusion coefficient may be determined from the slope of the mean square displacement versus the observation time, provided that the root ofr2(∆) is still smaller than the root of the mean square radius, R2 , of the adsorbent particle.

Table 6 gives the coefficient D determined for sufficiently short observation

times. Also are included the intracrystalline mean lifetime τintra calculated from γ (∆) and τintradiff defined by [199]:

τintradiff = R2 /15D,

(28)

Direct information about the existence of surface barriers is provided by

comparing τintra and τintradiff which represent the minimum possible lifetime calculated by Eq. 26 from the intracrystalline diffusivities. For methane these

two quantities are in reasonable agreement [198], proving that molecular ex-

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Fig. 30 Plots of the 129Xe NMR signal intensity (spin echo amplitude) versus the square of the width of the gradient pulses for different observation times ∆, for zeolites NaX (crystallite diameter = 50 µm), ZSM-5 (crystallite dimensions 100 × 30 × 30 µm3), and Na6.6Ca2.7A (crystallite diameter = 13 µm). (Reprinted from [198], with permission from Elsevier Science)

change is controlled mainly by intracrystalline diffusion. On the contrary, in all NaCa – A specimens as well as in the considered specimen with the smaller crystallites, xenon desorption is significantly retarded by factors other than intracrystalline diffusion, i.e. by additional transport resistances on the external surface or in the surface layer of the crystallites. This result may be related to the kinetic diameter of xenon atoms, which is greater than that of CH4, making them suitable for probing the surface permeability of adsorbents with limiting free diameter of this order of magnitude (0.4–0.5 nm for NaCa-A, 0.55 nm for ZSM-5).

For Na – X zeolite no surface resistance is observed, since the diameter of xenon is much lower than that of the windows of supercages ( 0.75 nm). This result has been confirmed by 129Xe PFG NMR tracer desorption measurements [200].

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In Na – X and ZSM-5 zeolites the self-diffusion coefficients of xenon decrease with increasing concentration while for NaCa – A zeolites they are essentially constant [65]. In the case of Na4.4Ca3.8 A, the observed diffusivity (1.5 × 10–9 m2 s–1) is several orders of magnitude larger than literature data based on uptake experiments (1.2 × 10–11 and 1.10 × 10–14 in [201] and [202], respectively) or computer simulation (3.3 × 10–12 in [203]). Computer simulations of xenon diffusion in ZSM-5, however, are found to be in satisfactory agreement [65].

Figure 31 shows the Arrhenius plots of the self-diffusion coefficients of Xe in Na – X, silicalite and Na4.4Ca3.8 A. The diffusivities follow the simple exponential dependence [204]

D = D0 exp(– E/RT)

(29)

with D0 and E denoting the preexponential factor and the activation energy of D, respectively. E is a measure of the energetic barriers that the molecules have to overcome on their diffusivity path in the intracrystalline space.

Table 6 indicates the influence of extra-framework cations on the magnitude of E. D0 is the self-diffusivity that would be attained at infinitely high temperatures, i.e. under conditions where energetic influences are not any longer of importance and the diffusivity is entirely determined by the geometrical restriction of the adsorbed particles within the zeolite. Then D0 decreases with the free diameter of the pores (X zeolites > silicalite A zeolite) (Table 6).

Fig. 31 Arrhenius plots of the intracrystalline self-diffusion coefficient D of xenon in:

• NaX; silicalite; + Na4.4Ca3.8A.

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Table 6 (adapted from [198] and [204], with permission): Self-diffusion coefficients, D, and intracrystallite mean lifetimes τ (see text) of adsorbed xenon. Activation energy, E, and pre-exponential factor of D

	Adsorbent				Adsorbed Xenon
	Limiting	Mean			τ diff
Zeolite	free	crystallite		D		τ		D	0	E
					intra	intra
	diameter	diameter		(10–9 m2 s–1) (ms)		(ms)		(10–8 m2 s–1) (kJ mol–1)
	(nm)	(µm)
NaCaA	0.4–0.5
Ca2+ %				1 ± 0.3	3 ± 0.1		± 20
45%		13				80		0.9 ± 0.3		8 ± 3
63%		20		1.5 ± 0.4	4 ± 1	45	± 10
80%		5		1.5 ± 0.4	0.4 ± 0.2	25	± 8	8 ± 3		6 ± 1.5
NaX	0.75	50		5 ± 1.5	8 ± 2	15	± 10
		20		5 ± 1.5	1.5 ± 0.5	5 ± 3
Silicalite	0.55	25		0.9 ± 0.3	11 ± 3	> 40
(a)		100×		0.9 ± 0.3	160 ± 40			0.9 ± 0.2		5 ± 1
Silicalite	"					> 70
(b)		30	× 30

mean crystallite dimensions (µm3)

In the case of silicalite, as a consequence of diffusion anisotropy [197], the correct dependence of the spin-echo attenuation on the gradient intensity deviates from the pattern provided by the first term of Eq. 26. However, because of the low signal/noise ratio the diffusivities, Dx, Dy and Dz, in the three principal directions were not determined, whereas this had been possible for hydrocarbons adsorbed in oriented crystallites [205]. The principal elements of the diffusion tensor of xenon adsorbed in silicalite have been determined by molecular dynamics simulation [206] (1.3, 4 and 0.28 × 10–9 m2 s–1 for Dx, Dy and Dz, respectively).

Finally, in the case of xenon adsorbed in 5A zeolite there is a good agreement between the limiting (zero concentration) transport diffusivities measured by the ZLC technique and by PFG NMR [207]. This agreement is sufficiently rare to be worth pointing out.

2.6.3

Dynamics of Adsorbed Xenon and 129Xe Chemical Shift

2.6.3.1

Encumbering of Zeolite Pores: Location of Transport Resistance

The use of the 129 Xe chemical shift generally gives qualitative information about xenon diffusion and especially its dependence on various parameters.