Article pubs.acs.org/JPCC
Exploring the Complex Porosity of Transition Aluminas by 129Xe NMR Spectroscopy Erika Weiland,†,‡ Marie-Anne Springuel-Huet,*,† Andrei Nossov,† Flavien Guenneau,† Anne-Agathe Quoineaud,‡ and Antoine Gédéon*,† †
Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7574, Laboratoire de Chimie de la Matière Condensée de Paris, 11 place Marcelin Berthelot, F-75005, Paris, France ‡ IFP Energies nouvelles, Etablissement de Lyon − Rond-point de l’échangeur de Solaize - BP3, 69360 Solaize, France S Supporting Information *
ABSTRACT: Crystalline mesoporous γ-, δ-, and θ-alumina samples with complex porosity and various surface areas (70 to 330 m2/g) were characterized by 129Xe NMR spectroscopy and nitrogen adsorption. Experimental conditions have been optimized to measure 129Xe NMR chemical shifts independent of the nature of the surface and solely dependent on the pore size. Xenon adsorption constants have been determined from xenon adsorption isotherms and from the fit of the chemical shift versus volume to surface ratio (V/S) curves with a simple exchange model. The discrepancy observed between those values has been attributed to the rugosity of the alumina pore surface. A direct correlation between the observed chemical shift and the alumina pore diameter was obtained for the whole series of alumina samples.
1. INTRODUCTION Alumina is widely used as support for catalysts in refining, petrochemicals and fine chemical processes.1−3 This support is extensively used because of the low cost of production and its ability to be easily and differently shaped (powder, extrudate, sphere). The support selection is made taking into account the constraints of the process, such as pressure drop, diffusional limitations and surface properties.4 Gamma alumina (γ-Al2O3) is, for instance, predominantly used in reforming5 and hydrotreatment,6 delta (δ-Al2O3) or theta (θ-Al2O3) aluminas in automotive exhaust catalysis.7,8 The support has a major impact on catalyst performance in terms of activity, selectivity and life-span. First, at macroscopic scale, the shape of the catalyst pellet is optimized according to the industrial application and the type of reactor. For example, the optimal filling of the reactor, the maximum pressure range between the top and the bottom of the reactor, the wettability of the pellet are to be considered. Furthermore, the mechanical properties must be sufficient to minimize the impact of catalytic cycles on the duration. At mesoscopic scale, the porosity of the support affects the genesis of the active phase and the catalytic performance. The pore size distribution and connectivity condition the intragranular diffusion of the reactants and the products of the catalytic reaction. Change in the texture of catalyst support is one of the main causes of diminished catalytic activity or selectivity. At atomic scale, the surface properties are essential in terms of active phase genesis. Acid− base surface sites have a direct impact on the dispersion of the © XXXX American Chemical Society
metallic active phase and its interaction with the support surface. The characterization of the textural properties of alumina supports is of prime importance to elaborate the final catalysts. Different classes of alumina supports in terms of pore size distribution can be distinguished: purely mesoporous or more complex structure made by a mix of mesopores and macropores. Furthermore, the textural properties of a support are characterized by their total porous volume (in cm3/g), their porous distributions and their surface areas (in m2/g). So far, the conventional techniques to characterize porosity are nitrogen adsorption−desorption at 77 K which allows one to obtained hystereses representative of the pore shape and distribution and mercury porosimetry which can investigate mesoporosity (pores from 2 to 50 nm) and macroporosity (pores above 50 nm). These techniques give average measures of pore size but the interpretation is under discussion; in particular the pore size distribution and the tortuosity are accessible through geometric models of the pores. Recently, owing to technologic progress, new characterization methods of porous materials have emerged such as imaging techniques which allow 3D cartography of the porous network. Among these new techniques, one can find X-ray microtomography, focused ion beam coupled to scanning Received: April 2, 2015 Revised: June 4, 2015
A
DOI: 10.1021/acs.jpcc.5b03211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. BET surface areas, pore volumes, C coefficients, xenon adsorption constants, and
a
2
a
129
Xe NMR chemical shifts
Xe adsorption constant Kads 1012 atom/(Pa·m2)b
chemical shift δ ppm
185 147 151 123 139
4.6 4.0 6.2 2.6 3.7
2.29
152
4.5
0.69 0.64 0.55
2.75 2.69 2.46
111 117 107
4.2 2.6
0.62 0.66 0.52 0.60 0.65 0.65 0.66 0.79 0.74 0.67
3.04 3.36 2.85 3.33 4.17 6.47 7.29 8.85 9.52 9.14
131 153 118 143 144 168 124 132 145 212
6.6 5.0 5.2 6.8 5.5 5.6 5.3 6.0 5.7 9.2
93.4 90.0 96.8 78.3 95.2 85.4 92.1 83.1 78.0 84.8 86.7 76.5 76.0 73.6 76.4 75.2 69.6 59.3 53.6 51.3 45.7 47.4
3
pore volume V cm /g
a
sample
BET surface S m /g
V/S ratio nm
Al_332_T Al_326_T Al_309_T Al_282_T Al_276_C
332 326 309 282 276
0.72 0.51 0.59 0.73 0.61
2.17 1.58 1.91 2.61 2.20
Al_263_C
263
0.60
Al_250_T Al_237_T Al_224_C
250 237 224
Al_204_C Al_198_C Al_183_C Al_179_C Al_156_C Al_100_T Al_90_T Al_89_T Al_78_T Al_73_T
204 198 183 179 156 100 90 89 78 73
a
C coef.
a
From N2 adsorption isotherm at 77 K. bFrom Xe adsorption isotherm at 295 K.
washed. Shaping involves the passage from a boehmite powder to support pellets. The extrudates can be cylindrical or trilobal. Their diameter ranges from 1.2 to 2 mm and the lengths from 2 to 6 mm. A thermal treatment at high temperature (from 798 to 1248 K) was performed to obtain the final support. The aim of these thermal treatments is to optimize the particle size, which increases with temperature, the average pore diameter, the total pore volume, and the surface area. The porosity was characterized by nitrogen adsorption− desorption isotherms at 77 K on a BELSORP-max (BEL Japan, Inc.) porosimeter. All the isotherms present hysteresis loops of various shapes. Isotherms of some samples are shown in Supporting Information (Figure S1). Brunauer−Emmett− Teller (BET) surface areas were determined from adsorption isotherms at relative pressures of 0.05 < P/P0 < 0.35. The mesopore volume is calculated from the quantity of nitrogen adsorbed at P/P0 = 0.99. The t-plot analyses do not show any presence of micropores. Xe adsorption isotherms were measured at 295 K with a homemade volumetric apparatus in the pressure range 1.3 to 133.3 kPa. In this range, all the isotherms are linear and the adsorption constants, Kads, were determined with Henry’s law: Nads = KadsSP. Results of nitrogen adsorption measurements (BET surface areas (S), total pore volumes (V), ratios V/S, C constants of BET equation), xenon adsorption constants, and 129Xe NMR chemical shifts are given in Table 1. The samples are denoted Al_X_Y, where X is the BET surface area and Y is the shape of the pellets (T for trilobe and C for cylinder). 2.2. 129Xe NMR Measurements. Before NMR experiments, all the samples were evacuated under vacuum (∼10−2 Pa) at room temperature for 30 min and then at 573 K overnight (heating rate 100 K/h). Xenon was adsorbed at room temperature using a homemade apparatus. 129 Xe NMR spectra were recorded at 295 K with a Bruker Avance 300 spectrometer operating at 83.02 MHz for Xe
electron microscopy (FIB-SEM), electron tomography, NMR imaging and more recently X-ray microscopy.9 In addition to the difficulty of implementation, these techniques present some limitations: limited size of the objects observed, limited number of the objects, pore range of some nanometers to some micrometers. 129 Xe NMR of adsorbed xenon has been successfully used to study the porosity of materials.10,11 The technique has been initially applied to crystalline microporous oxides, like zeolites. In particular, a correlation between the isotropic chemical shift of xenon (δ) and the mean free path, λ, of xenon atoms in the pore structure, defined as the average distance traveled by a Xe atom between two successive collisions with the pore surface, has been established for these solids.12 λ is evidently correlated to the pore size but also takes into account the pore shape and the pore connectivity. Since then, the technique has been extended to amorphous mesoporous materials, such as silicas and alumina silicas mesostructured (M41S, SBA-15, etc.) or not (silica gels, silica glasses, etc.) and another relationship between the chemical shift and the pore size (D) has been established for these mesoporous silica systems.13 On the other hand, the applications of 129Xe NMR to materials with low surface area, in small amount or with long relaxation time of xenon have been largely improved by the use of hyperpolarized xenon.14 The aim of this work is to investigate whether an analogous δ-D correlation can be derived for crystalline mesoporous aluminas in which the mesoporosity originates from the aggregation of crystallized plate-like crystals of ca. 15 nm size. In these solids, the pore shape has no defined geometry and the resulting porosity is complex.
2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Transition Aluminas. For all alumina pellets, boehmite γ-AlOOH was used and obtained by precipitation in aqueous solution of aluminum salts. The boehmite precipitate was filtered and B
DOI: 10.1021/acs.jpcc.5b03211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. 129Xe chemical shift versus xenon pressure (A) and NMR spectra at pressure ca. 10 kPa, arbitrary intensities (B), for Al_237_T treated at various temperatures.
pressures between 1.3 and 133.3 kPa. The very short T1 values make hyperpolarized 129Xe NMR not suitable, and all the chemical shift measurements at room temperature were performed with thermally polarized 129Xe. From 1000 to 100000, scans were accumulated with 10 μs 90° pulses and 0.25 s repetition times. Longitudinal relaxation rates were measured using π−τ−π/2 inversion−recovery sequences. The chemical shifts are referred to xenon gas at zero pressure.
sites (SAS) for Xe. At low Xe loading, the Xe atoms mainly interact with those SAS, and the strong interaction results in a high chemical shift (δSAS) at very low pressure. As the xenon pressure increases, a more and more important part of the Xe atoms interact with the alumina surface and present a smaller chemical shift (δS). The observed chemical shift being a weighted average of δSAS and δS, it decreases when pressure increases. When the alumina is activated at higher temperature (823 K), the effect of those SAS is even more important, and there is a shift of the whole curve toward the high values of chemical shift (Figure 1). When the temperature of activation is low (573 K), the effect of the SAS is small, and the chemical shift is rather constant for P above 80−100 kPa. The increase in chemical shift at low pressures is accompanied by a broadening of the signal. Undoubtedly, the dehydration/dehydroxylation of the surface makes some adsorption sites available for a strong interaction with the xenon atoms, which perturbs the chemical shift. These sites have been first suspected to be paramagnetic elements because the longitudinal relaxation time, T1, of adsorbed Xe measured in these aluminas is short (a few ms to a few tens of ms for most aluminas depending on the temperature of pretreatment and on the Xe loading; see Supporting Information, Table S1). In fact, electron spin resonance (ESR) spectra have revealed the presence of Fe3+ and Cu2+ species, and another, not identified, species (see Supporting Information, Figure S4) probably due to the synthesis via precipitation of aluminum salt in aqueous solution. Nevertheless, experiments performed on aluminas free of paramagnetic elements have shown the same typical shape of the chemical shift variations at low Xe loadings (see Supporting Information, Figure S2). Therefore, the increase in chemical shift at low decreasing Xe pressures should be attributed to aluminum species more or less unsaturated (Lewis sites). A geometric effect may also be considered: the contact between two or more alumina platelets forms adsorption sites with small sizes, which play the role of strong adsorption sites (Figure 4). To minimize the influence of these strong adsorption sites, the pretreatment temperature of 573 K was chosen (see Experimental section). The chemical shift has been measured for all the alumina samples as a function of xenon pressure. The very short T1 values make hyperpolarized 129Xe NMR not suitable, and all the chemical shift measurements at room temperature were performed with thermally polarized 129Xe. All the variations have similar shape and, for the sake of clarity, only some of them are reported in Figure 2. It can be seen that
3. RESULTS AND DISCUSSION To correlate the chemical shift of adsorbed xenon to the size of the pores, one should verify whether the chemical shift depends only on the interaction of Xe atoms with the pore surface. In particular, for microporous solids like zeolites, the chemical shift increases with the concentration of adsorbed xenon due to xenon−xenon interaction inside the pores. In this case, the chemical shift at zero pressure, δS, should be determined from the chemical shift variation versus xenon loading. When the NMR experiments are performed with hyperpolarized xenon under continuous-flow conditions, the helium−xenon-nitrogen gas mixture contains a small fraction of xenon, usually 1%. In these conditions, xenon−xenon interactions are negligible, and the chemical shift measured can be considered as that of a single adsorbed atom. Nevertheless, at low Xe loading, the chemical shift may be polluted, even at a great extent, by interaction of the xenon atoms with some specific strong adsorption sites such as highly charged cations, paramagnetic impurities, etc., and it is always useful to study the variation of the chemical shift with pressure to detect the presence of such adsorption sites. For mesoporous materials, the Xe−Xe interactions inside the pores are similar to that of xenon atoms in gas phase. These interactions cause an increase of 0.55 ppm/amagat (one amagat being the concentration of atoms at 0 °C and 1 atm, that is 4.49 × 10−5 mol/cc for xenon).15 Their effect on the chemical shift is therefore negligible, and the chemical shift is quasiindependent of the Xe pressure in the pressure range usually investigated. In order to verify the independence of the shift with Xe pressure, we studied the variation of chemical shift with pressure. In the first experiments, the alumina samples were evacuated at 673 K. The chemical shift dependence with Xe pressure is reported for Al_237_T in Figure 1. It shows a typical shape characteristic of the presence of strong adsorption C
DOI: 10.1021/acs.jpcc.5b03211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C δobs =
δa 1+
V K adsSRT
(2)
where V and S are the pore volume and the pore surface area, respectively. Kads is the Henry adsorption constant (Na = KadsPS). The V/S ratio is proportional to the pore diameter, D, through a geometric factor η (V/S = D/η) if the pore geometry is known (η = 2 for slit pores, η = 4 for cylindrical pores, η = 6 for closed spheres for example). For aluminas, the pore geometry is not well-defined, and we plotted the chemical shift, measured at pressure ca. 80 kPa, as a function of the ratio V/S (Figure 3). All the results show a hyperbolic variation, confirming the validity of the simple exchange model.
Figure 2. 129Xe chemical shift versus xenon pressure at 295 K for some aluminas treated at 573 K.
the presence of strong adsorption sites is always detected at low xenon loadings, but their effect on the chemical shift is negligible for P above 80 kPa and the pore surface can be considered homogeneous from the point of view of xenon adsorption. The value of the chemical shift measured at P ≈ 80 kPa was taken as the value characteristic of the pore size. One can see that the chemical shift depends on the textural properties of the samples. For other porous solids, correlations between the chemical shift, δ, and the pore size, D, have been established. The first one was obtained for zeolites and related materials.16 To take into account the pore size and the connectivity of zeolite pore structures, the relation was established between the chemical shift and the mean free path of a xenon atom in the pore structure.17 For amorphous silica gels, a qualitatively similar, but yet distinct, δ−D relationship have been observed.18−21 The origin of this difference is not fully understood. The high chemical shifts observed with amorphous materials, compared to crystalline zeolites, have been tentatively attributed to the rugosity, at atomic scale, of the pore surface responsible of a higher Xesurface interaction.22 This interpretation was in good agreement with the high value of the chemical shifts obtained at low temperature for mesoporous materials (ca. 120−130 ppm) compared to that usually observed for zeolites (ca. 100 ppm for FAU or MFI structures).23 To rationalize the hyperbolic dependence of chemical shift variation versus the pore size, a simple model has been initially used for zeolites and then for mesoporous materials as well.12,13 It considers that the measured chemical shift, δobs, arises from the chemical exchange between adsorbed and bulk phase. Therefore, δobs =
Naδa + Nvδv Na + Nv
Figure 3. 129Xe NMR chemical shift versus V/S ratio for aluminas (our measurements), silica gels (data from ref 18) and MCM-41 (data from refs 20, 24, and 25). The lines are the nonlinear least-squares fits of the data with eq 2.
For comparison, the results obtained by Terskikh et al. on silica gels18 and those obtained on mesostructured silicas (MCM-41) from different sources20,24,25 are also reported in Figure 3. It can be observed that the curve relative to γ-alumina is different from that obtained with silica gels and MCM-41, the chemical shift of the different aluminas being larger whatever the V/S value. The values of δa and Kads obtained from the nonlinear leastsquare fit with eq 2, are given in Table 2. Table 2. Values of the Chemical Shift, δa, Characterizing the Interaction Xe-Surface and of the Xenon Adsorption Constant, Kads, for Aluminas, Silica Gels, and MCM-41
a
materials
δa (ppm)
Kads (atom·Pa−1 m−2)
Aluminas Silica gels MCM-41
117 ± 9 118 ± 5a 120 ± 12
1.60 (±0.40) × 1012 0.76 (±0.17) × 1012a 0.36 (±0.05) × 1012a
Obtained from the results of refs 18, 20, 24, and 25.
It is therefore observed that the δa value, characterizing the Xe-surface interaction, is similar for all the oxides. This can be explained by the fact that the shielding of xenon nuclei (σR) is mainly driven by the nature of the atoms that Xe atoms are approaching, which are essentially oxygen atoms and hydroxyl groups for all the oxides. It must be restated here that the experimental conditions (temperature of pretreatment and xenon pressure used for NMR measurements) have been chosen in order to minimize the effect of strong adsorption
(1)
where Na, δa and Nv, δv are the xenon populations and chemical shifts of xenon at the surface and in the pore volume, respectively. The chemical shift δv is that of xenon gas, taken as reference, thus one has δv = 0. The populations Na and Nv can be derived from Henry’s law and the ideal gas equation, respectively. Therefore, eq 1 becomes D
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these crystallographic phases, the particles are bigger and result from the sintering of the smaller γ-phase particles. The topotactic transition γ−δ−θ implies a sintering along lateral faces, thus increasing the proportion of basal faces with respect to lateral ones. It was suggested that the basal faces present a greater amount of strong adsorption sites,27 which may be responsible of high values of Kads. In order to relate the chemical shift to a more common physical quantity, as pore size, the chemical shift has been plotted versus the mean diameter determined from the pore size distribution (according to cylindrical BJH model) obtained by N2 adsorption (Figure 5). In case of bimodal porosity, two
sites (partially coordinated aluminum cations) that may appear on the surface, vide supra. At Xe−O distances characteristic of the interaction potential well, the Xe shielding does not vary much with the distance.26 Therefore, the divergence of the curves in Figure 2 arises essentially from the different adsorption constants between the aluminas and the different silicas (SiO2 aerogels and MCM-41). In this case, local surface topology may play an important role in the adsorption constant value. To elucidate the relevance of the mean Kads values obtained from the fits of the δ−V/S variations, we measured Xe adsorption isotherms on all the alumina samples and determined the experimental Kads values which are given in Table 1. The values range between 2.6 × 1012 and 9.2 × 1012 atom/ (Pa·m2), which is always larger than that obtained from the fit (1.6 × 1012). What is the origin of this discrepancy? One reason may arise from the experimental conditions of the NMR measurements. To avoid the influence of strong adsorption sites, the spectra have been recorded at Xe pressures around 80−93 kPa. At these pressures, the “small pores” formed at the contact between alumina platelets are filled with Xe atoms. Therefore, the Xe atoms exchanging between adsorbed and gas phase in the pores interact with a surface smaller than that measured by N2 adsorption used in the V/S coordinate of Figure 3 as schematically represented in Figure 4. In these
Figure 5. 129Xe NMR chemical shift versus the pore diameter, D(BJH) for aluminas (our work), silica gels (data from ref 18), and MCM-41 (data from refs 20, 24, and 25).
mean pore sizes were determined and associated with the two signals observed in the NMR spectra. For an alumina series (ca. 20 samples) presenting complex mono/bimodal porosity, a direct correlation between the observed chemical shift and the alumina pore diameter was obtained. This representation implies an arbitrary value of the geometric factor η, which is difficult to evaluate. Actually, the determination of both parameters, η and D, demands the development of theoretical porosity models. However, the unambiguous correlation between δobs and D shows that, at least, the porosity of all alumina supports can be described by a common geometry factor. One can wonder about the differences between the curves of aluminas, silicas, and MCM-41. One hypothesis may be the “rugosity” of the pore surface that increases from MCM-41 to alumina. In contrast to zeolites for which the chemical shift is taken at “zero coverage” (value obtained by extrapolation of the chemical shift variation with xenon loading to zero), the chemical shift is often measured at high pressure (around 1 atm or more) since its value does not depend much on the pressure. As already mentioned to explain the differences between the values of Kads obtained from the fit of the δ−V/S curves or from adsorption isotherms, in these conditions, the adsorption sites situated in the atomic scale “pockets” on the surface are occupied by Xe atoms. The surface area accessible to Xe atoms exchanging between adsorbed and gas phase inside the pores is therefore smaller than that used in the ratio V/S. The greater the difference between these surface areas, i.e., the greater the rugosity, the more shifted the δ−V/S curves. It can be anticipated that the surface rugosity in mesostructured silicas (MCM-41) is a smooth undulation. It becomes more excavated
Figure 4. Schematic representation of the apparent surface (dotted blue line) accessible to Xe atoms diffusing in the mesopores.
conditions, the chemical shift value measured at high pressure should correspond to a smaller surface area that is a greater value of V/S and the curve should be shifted to higher V/S values. This shift leads to an increase in the Kads value determined from the fit of the curve. Another reason may come from the use of the ideal gas equation to determine the number of atoms in gas phase inside the pores (NV). Actually, Xe gas is not an ideal gas, and moreover it is confined inside the pores. Therefore, the real density should be greater than the density of an ideal gas. The quantity PV/RT underestimates NV, which leads to a shift of the δ−V/S curve to the left (smaller V/S values). The Kads value obtained from the fit of the curve is then also underestimated. This is true also for silicas and MCM-41. Terskikh et al. mentioned the value of 5.6 × 1012 atoms Pa−1 m−2 measured from the adsorption isotherm, higher than the value obtained from the fit of the curve (7.6 × 1011 Pa−1 m−2).13 We measured Kads from xenon adsorption isotherms on MCM-41 and obtained values around 3.2 × 1012 atoms Pa−1 m−2, whereas the fit gives 3.6 × 1011 Pa−1 m−2. One can notice that the higher values of Kads measured from xenon adsorption isotherms correspond to aluminas synthesized at high temperatures that is to δ or θ alumina forms. In E
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in silica gels in which the porosity comes from the agglomeration of more or less spheroidal amorphous particles. The surface rugosity of alumina supports is directly linked to the size of platelet aggregates and the agglomeration mechanism. The sintering of platelets can be involved in the increase in the surface homogeneity and the decrease of the rugosity.28 The interaggregate space may constitute a higher volume fraction and, therefore, the accessible surface area may be significantly reduced.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]; tel: 33 1 44 27 55 37. *E-mail:
[email protected]; tel: 33 1 44 27 71 43/61 21. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Dr. Yann Le Du for performing the ESR experiments and Pr. Didier Gourier (Ecole Nationale Supérieure de Chimie de Paris) for the interpretation of the ESR spectra.
4. CONCLUSION In order to gain a deeper insight on the complex pore structure of alumina materials, experimental conditions have been optimized to measure 129Xe NMR chemical shifts independent of the nature of the surface and dependent on the pore size, solely. To minimize the adsorption heterogeneity of the pore surface and its effect on the chemical shift, a rather low temperature (573 K) of pretreatment of the samples and a xenon pressure of 80 kPa were chosen. Twenty transition aluminas with various surface areas (70 to 330 m2/g) were studied. In unexpected ways, the chemical shifts measured for all the samples show a nice hyperbolic variation with the surface-to-volume ratio of pores, according to a simple model of fast exchange between adsorbed and bulk xenon phases. A hyperbolic variation is also obtained when plotting the chemical shift versus the pore size determined from N2 adsorption involving a common geometric parameter characterizing the pore geometry of all the aluminas. The fit of the δ−V/S curves with a simple exchange model allowed us determining a mean adsorption constant. The discrepancy observed between that value and those obtained from xenon adsorption isotherms has been attributed to the rugosity of the pore surface. The atomic scale voids constituting the surface rugosity are filled with xenon atoms at the pressure used for NMR measurements. Then, the surface area accessible to Xe atoms exchanging between adsorbed and gas phases inside the pores is reduced and leads to a biased value of Kads obtained by fitting the δ−V/S curves. The variation, δ = f(V/S), has been compared to that obtained with silica gels and MCM-41, previously published. Three different curves are observed for the three types of solids. The fit of these curves with the same exchange model gives similar values of the chemical shift, δa, characterizing the Xe− surface interaction, but different values of the adsorption constant, Kads. The rugosity may also be at the origin of the difference observed between the δ−V/S curves of the three solids, each type of solids presenting a specific one, which increases from mesoporous structured silicas (MCM-41), silica gels to aluminas.
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REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
N2 adsorption−desorption isotherms at 77 K of some alumina samples. Chemical shift variation versus xenon pressure of Al_237_T and Al_198_C treated at 573, 673, and 823 K. Spectra of xenon adsorbed at various pressures on Al_237_T treated at 573, 673, and 823 K. ESR spectra of Al_237_T, Al_282_T, and Al_198_C. Values of T1 relaxation time of xenon adsorbed in some samples. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03211. F
DOI: 10.1021/acs.jpcc.5b03211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.5b03211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
