6938
Langmuir 2001, 17, 6938-6945
Copper Site Energy Distribution of de-NOx Catalysts Based on Titanosilicate (ETS-10) Paolo Carniti* and Antonella Gervasini* Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via C. Golgi n. 19, I-20133 Milano, Italy
Aline Auroux Institut de Recherches sur la Catalyse, CNRS, 2 av. A. Einstein, F-69626 Villeurbanne, France Received January 25, 2001. In Final Form: July 24, 2001 This study deals with copper-based catalysts prepared on a crystalline titanosilicate (ETS-10) matrix. Two samples prepared by ionic exchange, obtained by depositing different amounts of copper (6 and 11 wt %, corresponding to partial and total exchange, respectively), were selected for this study. Nitrogen monoxide was chosen as adsorbate to probe the surface properties of copper sites in terms of NO-Cu interaction energy. Microcalorimetric analysis of NO adsorption gave a direct measure of the NO-Cu energy of interaction and a view of the energy distribution of the copper sites from the differential and integral adsorption heat curves as a function of NO coverage. Initial heats of adsorption were around 110-130 kJ‚mol-1 and fell down to 30-40 kJ‚mol-1 at higher NO coverage. Volumetric isotherms of NO adsorption were measured at different temperatures (19-70 °C). A thermodynamic model was applied to the isotherms, making it possible to describe the NO-Cu site interaction energy by means of a function giving the number of Cu sites vs adsorption enthalpy, ∆aH. The high-loading copper sample had more energetic interaction with NO (-171 < ∆aH/kJ‚mol-1 < -44), with a broader distribution, than the lowloading sample (-111 < ∆aH/kJ‚mol-1 < -41). Parameters of adsorption, such as adsorption constant, molar entropy of adsorption, and half-coverage temperature at unit pressure (To1/2), were obtained for each type of Cu site. The properties of the Cu sites were related with the activity measured in the reaction of selective catalytic reduction (SCR) of NO with ethylene.
Introduction In the past decade there has been a lot of interest in the process of selective catalytic reduction of NOx (SCR). Several important papers of Iwamoto et al.1,2 proposed copper-exchanged ZSM-5 zeolite as a catalyst of particular relevance for NO decomposition and NO-SCR with C1-C3 hydrocarbons acting as reducing species. Besides ZSM-5, large and medium pore zeolite structures exchanged with Cu and other cations have been studied for reducing NO with hydrocarbons.3-6 The adsorption and catalytic properties of zeolites and other molecular sieves are closely related to their ability to adsorb a variety of molecular species into their structure; this behavior is influenced by the framework composition and the nature of the charge-balancing cations.7 Recently, the synthesized ETS-10 microcrystalline material (titanosilicate)8,9 has been used as support matrix for the preparation of copper-based catalysts in comparison with Cu-ZSM-5 systems.10,11 Several aspects such as the * Corresponding authors. E-mails:
[email protected];
[email protected]. (1) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, Y.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95, 3727. (2) Iwamoto, M.; Yahiro, H. Catal. Today 1994, 22, 5. (3) Armor, J. N. Appl. Catal., B 1992, 1, 221. (4) Gopulakrishnan, R.; Stafford, P. R.; Davidson, J. E.; Hecker, W. C.; Bhartolomew, C. H. Appl. Catal., B 1993, 2, 165. (5) Tabata, T.; Kokitzu, M.; Okada, O. Catal. Today 1994, 22, 147. (6) Corma, A.; Forne´s, V.; Palomares, E. Appl. Catal., B 1997, 11, 233. (7) Rakic, V. M.; Hercigonja, R. V.; Dondur, V. T. Microporous Mesoporous Mater. 1999, 27, 49. (8) Kuznicki, S. M. U.S. Patent No. 4853202, 1989, assigned to Engelhard Corp. (9) Kuznicki, S. M. U.S. Patent No. 5011591, 1991, assigned to Engelhard Corp.
influence of the host matrix in stabilizing the copper species, the copper loading, and the electronic situation of the different kinds of copper centers were studied by performing adsorption measurements using CO, NO, and C2H4 as adsorbates. Despite the great number of papers in the literature which have been devoted to the characterization of the surface species formed by direct interaction of probe molecules (CO, NO, O2, and hydrocarbons) with the multivalent metallic species, mainly by infrared spectroscopy,12-17 the energy of surface centers and their distribution on the surface have been less studied.18,21 The existence of a heterogeneous distribution of active centers on a catalyst surface is an old concept in (10) Auroux, A.; Picciau, C.; Gervasini, A. Stud. Surf. Sci. Catal. 1999, 125, 555. (11) Gervasini, A.; Picciau, C.; Auroux, A. Microporous Mesoporous Mater. 2000, 35-36, 457. (12) Hoost, T. E.; Laframboise, K. A.; Otto, K. Appl. Catal., B 1995, 7, 79. (13) Shimokawabe, M.; Tadokoro, K.; Sasaki, S.; Takezawa, N. Appl. Catal., A 1998, 166, 215. (14) Hadjiivanov, K.; Dimitrov, L. Microporous Mesoporous Mater. 1999, 27, 49. (15) Henriques, C.; Ribeiro, M. F.; Abreu, C.; Murphy, D. M.; Poignant, F.; Saussey, J.; Lavalley, J. C. Appl. Catal., B 1998, 16, 79. (16) Amorim de Carvalho, M. C. N.; Passos, F. B.; Schmal, M. Appl. Catal., A 2000, 193, 265. (17) Park, S.-K.; Kurshev, V.; Luan, Z.; Lee, C. W.; Kevan, L. Microporous Mesoporous Mater. 2000, 38, 255. (18) Borgard, G. D.; Molvik, S.; Balaraman, P.; Root, T. W.; Dumesic, J. A. Langmuir 1995, 11, 206. (19) Carniti, P.; Gervasini, A.; Ragaini, V. J. Chem. Soc., Faraday Trans. 1997, 93, 1641. (20) Kanougi, T.; Tsuruya, H.; Oumi, Y.; Chatterjee, A.; Fahmi, A.; Kubo, M.; Miyamoto, A. Appl. Surf. Sci. 1998, 130-132, 561. (21) Zhanpeisov, N. U.; Matsuoka, M.; Mishima, H.; Yamashita, H.; Anpo, M. J. Mol. Struct. 1998, 454, 201.
10.1021/la0101301 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/02/2001
Cu Site Energy Distribution of de-NOx Catalysts
Langmuir, Vol. 17, No. 22, 2001 6939
Table 1. Main Physicochemical Characteristics of the Samples Cu loading sample
wt %
mmol‚g-1
exchange (%)
Cu/Si (molar ratio)
Cu/Ti (molar ratio)
surfaceb (m2‚g-1)
Vµ (cm3 g-1)
Davc (Å)
5.8 11.4
0.91 1.78
46 90
0.08 0.16
0.39 0.74
480 394 307
0.155 0.123 0.078
nd 100
ETS10a Cu-ETS-L Cu-ETS-H c
a Composition in wt %: K O, 5.1; Na O, 9.7; TiO , 18.8; SiO , 66.4. b Determined by “t-plot” approach (Harkins-Jura reference equation). 2 2 2 2 Average particle size of CuO aggregates as determined from XRD (Scherrer law).
heterogeneous catalysis but one which is more often assumed than actually verified.22 The study of surface heterogeneity is of considerable interest both for the development of the theory of adsorption and for practical scopes in applied catalysis science. The most direct method for studying a heterogeneous surface, that is, the distribution of surface sites, is the measurement of the differential heat of adsorption, directly determined from volumetric-calorimetric experiments, as a function of surface coverage. The shape of the calorimetric curves is dependent on the adsorbent and on the adsorbed species. As a general trend, heats of adsorption are always observed to decrease as a function of adsorbate surface coverage, independently of the adsorbate-adsorbent couple. This behavior can be ascribed either to lateral interactions between adsorbed species or to surface heterogeneity.23 In most cases, the differential heats measured do not coincide with the real heats, that is, the enthalpies of adsorption. This is because the adsorption occurs simultaneously on centers of different enthalpies, and the corresponding measured heat represents an average quantity on the basis of the amount of adsorbate retained on centers of each type. There is also some evidence that under certain experimental conditions and depending on the adsorbate-adsorbent couple, the different types of surface sites are filled successively starting from the most enthalpic ones. This assumption can hold only if large differences among the adsorption constants are involved.24 Moreover, as the achievement of thermodynamic equilibrium between the adsorbate species and the adsorbent centers is a prerequisite for the soundness of calorimetric measurements, the temperature at which the adsorption is conducted is very critical. The choice of adsorption temperature is constrained and involves a compromise between using high temperatures to reduce equilibration times of the experiments or using low temperatures to achieve high surface coverage on the pressure scale of calorimetric measurements. Lower temperatures are also preferred to ensure that the adsorbate species do not decompose on the surface. Regarding kinetic considerations, at low adsorption temperature, the most enthalpic sites may not be activated or, even if activated, may be occupied at an adsorption rate too slow to achieve equilibrium during the experiments.25,26 Performing the adsorption calorimetric and/or volumetric experiments at different temperatures gives rise to different calorimetric/volumetric curves. When the temperature range and the time employed in the measurements ensure that the adsorption equilibrium has been obtained on all kinds of sites, and provided that no modification of the surface occurs in the temperature range considered, the differences among the observed curves (22) Point, R.; Petit, J. L.; Gravelle, P. C. J. Catal. 1977, 48, 408. (23) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Surfaces; Academic Press: London; 1992; Chapters 8, 9. (24) Carniti, P.; Gervasini, A.; Auroux, A. J. Catal. 1994, 150, 274. (25) Cardona-Martinez, N.; Dumesic, J. A. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: San Diego, CA, 1992; Vol. 38, p 149. (26) Auroux, A. Top. Catal. 1997, 4, 71.
have to be ascribed to the influence of temperature on the thermodynamic parameters of adsorption and, particularly, on the adsorption constants.24 This study is based on the adsorption of the NO probe molecule over copper-based catalysts on a microcrystalline titanosilicate (ETS-10) matrix, utilizing adsorption volumetric data collected at various temperatures and successively elaborated through a thermodynamic model recently proposed in the literature.27 Results in terms of copper site energy distribution (number of Cu sites vs enthalpy of adsorption, ∆aH) were obtained by this method. It is important from both an academic and an applied point of view to obtain data on the adsorption capacity and on the energetics of modified materials, which can act as catalysts in different reactions, with respect to adsorbates that can take part in the catalytic process. Moreover, relations between the nature and the energy of the copper surface species and the NO reagent, involved in the selective catalytic reduction (SCR) reaction, are tentatively proposed. Experimental and Computational Details Samples and Analyses. The Na+/K+ microcrystalline form of titanosilicate (ETS-10, from Engelhard) with formula28 (Na,K)2Si5TiO13 was chosen as matrix for preparing copper-based materials acting as catalysts in the selective catalytic reduction of NO. A large series of copper catalysts with Cu loadings in the range 3-24 wt % was prepared by a base-exchange procedure from aqueous solutions of copper acetate. Details on the preparation have been reported in ref 11. Two samples with Cu loadings of 5.8 and 11.4 wt %, corresponding to 46 and 90% exchange capacity of the ETS-10 matrix, respectively, have been chosen for this study. The samples with low and high Cu content are labeled below as Cu-ETS-L and Cu-ETS-H, respectively (Table 1).The crystallinity checking of the two copper samples and the detection of copper oxide aggregates were performed by X-ray diffraction (XRD) analysis on a Philips PW1877 diffractometer using Cu KR radiation filtered by Ni (λ ) 1.5418 Å). The textural properties were determined from N2 (99.9995% purity, from Tecnogas) adsorption isotherms measured at -196 °C by a Sorptomatic 1900 apparatus (CE Instruments) working with a static volumetric technique. Prior to the experiments, the samples (ca. 300 mg) were outgassed at 350 °C for 16 h under vacuum (0.1-1 Pa). Coupled microcalorimetric-volumetric measurements of NO (99.995% purity, from Air Liquide) adsorption were performed in a differential heat flow calorimeter (C80, from Setaram) equipped with a standard adsorption-desorption device.26 Fresh samples (ca. 100 mg) were outgassed at 400 °C for 16 h under vacuum (ca. 10-3 Pa). Then, they were exposed to small quantities of NO at a constant temperature of 30 °C. The equilibrium pressure was measured after each step of adsorption by a capacitance manometer (Datametrics). The adsorption was performed up to an equilibrium pressure of about 150 Pa. NO (99.5% purity, from Sapio) adsorption isotherms up to 1500 Pa were measured on fresh samples (ca. 300-400 mg) outgassed at 350 °C for 16 h under vacuum (ca. 10-3 Pa). An automatic static volumetric apparatus (Sorptomatic 1900 chemi(27) Carniti, P.; Gervasini, A. React. Kinet. Catal. Lett. 1994, 52, 285. (28) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347.
6940
Langmuir, Vol. 17, No. 22, 2001
Carniti et al.
sorption version, from CE Instruments) was used. Adsorption was performed at various temperatures in the range 20-70 °C. The equilibrium pressure was determined by capacitance manometers (0-1.3 and 0-13 kPa, from MKS) as the point at which during 7 min the pressure was stable within 2 Pa. Following this procedure, each adsorption experiment lasted about 20 h. The catalytic activity in the reaction of NO reduction with ethylene performed in oxygen-rich atmosphere (NO-C2H4-O2) was carried out in a fixed-bed microreactor. Temperatures in the range 150-500 °C were used. Experimental details have been presented in ref 29. The feed gases consisted of 0.4% of both NO and C2H4 and 4% O2, the balance being He. Typical runs were performed at a space velocity of 7500 h-1 (GHSV). The outlet stream from the reactor was analyzed with a gas chromatograph mounting a 60/80 Carboxen-1000 (Supelchem) column and equipped with a thermal conductivity detector (TCD). Mathematical Model and Computations. The data obtained from the adsorption isotherms of NO measured at different temperatures were used in a model which made it possible to obtain the values of the thermodynamic parameters of adsorption of the surface sites together with their dependence on temperature.27 The NO adsorption data were treated by assuming true reversible adsorption equilibrium. The experimental isotherms were interpreted as summations of single Langmuir isotherms, each of them relevant to the sites of a definite type:
n)
∑ n ) ∑n i
i
max,ibip/(1
+ bip)
(1)
i
where n is the amount adsorbed (expressed as moles) at pressure p, ni and nmax,i are the actual and the maximum amount adsorbed on sites of the ith type, respectively, and bi is the Langmuir adsorption constant of the sites of the ith type which depends on temperature. Such a dependence is given by the following equation:
bi ) bo,i exp(-∆aHi/RT)
(2)
if the temperature range is narrow enough for the adsorption enthalpy (-∆aHi) to be considered constant. The preexponential factor, bo,i, can be related to the parameter half-coverage temperature at unit pressure, To1/2,i, whose meaning and derivation has been defined in previous works: 24,27
bo,i ) exp(∆aHi/RTo1/2,i) A relation between To1/2,i and ∆aHi was also found:
1/To1/2,i ) A/∆aHi + B∆aHi
(3) 24
(4)
where A and B are two constant coefficients. From the experimental isotherms at various temperatures, the parameters of the model for each type of site, nmax,i, ∆aHi, and To1/2,i were evaluated by a computation program including the optimization subroutine OPTNOV,30 which minimized the following objective function:
Φ)
∑∑(n
jk,calc
j
- njk,expl)2/njk,expl2
(5)
k
where njk,expl refers to the experimental values at a given temperature (index j) and pressure (index k) and njk,calc refers to the corresponding values calculated by eqs 1-4. Assuming an adsorption stoichiometry NO/Cu ) 1, the parameter nmax,i of eq 1 corresponds to the total number of sites of the ith type, which is a fraction of the overall monolayer coverage of the samples. Because NO can be adsorbed as mononitrosyl (NO/Cu ) 1) and dinitrosyl (NO/Cu ) 2) species,31-33 (29) Gervasini, A.; Carniti, P.; Ragaini, V. Appl. Catal., B 1999, 22, 201. (30) Buzzi Ferraris, G. Ing. Chim. Ital. 1968, 4, 171, 180. (31) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. (32) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97, 1204.
the assumption of the NO/Cu ) 1 stoichiometry could lead to an overestimation of the number of copper adsorption sites of the sample. However, mononitrosyl species are predominant at low NO pressure (pNO < 2 kPa).34 Only data with pNO e 1 kPa were taken into account in the computations. On the basis of the crosssectional area of the NO molecule (12.6 Å2) and of the surface area of the samples (Table 1), the maximum amount of NO adsorbed at monolayer was calculated as 5.02 and 3.91 mmolNO‚gcat-1 for Cu-ETS-L and Cu-ETS-H, respectively. The computation program based on the above-described model that was used in the previous works19,24,27 employed discrete values of adsorption enthalpy. For the present work, the program has been modified introducing the possibility that the energy site distribution can be described as a sum of Gaussian curves. In this case, the means (∆aHi,max) and variances (σi2) of the different Gaussian curves were optimized instead of the discrete values of ∆aHi.
Results and Discussion Characteristics of the Samples. The ETS-10 material is a recently synthesized titanosilicate with a threedimensional 12-ring pore system, containing octahedrally coordinated Ti(IV).8 The octahedrally coordinated environment about the Ti(IV) sites renders this titanosilicate inactive as oxidation catalyst, although it still possesses acidity and molecular sieving properties.9,35 The structure of ETS-10 consists of a combination of tetrahedral silica and octahedral titanium units with intersecting tubular pores with a maximum aperture of 8 Å. It has a very high ion-exchange capacity; the exchange sites are associated with doubly charged titanium atoms.28 The ETS-10 material has been loaded with given amounts of copper acetate deposited by a ion exchange procedure to obtain a large series of copper samples; their characterization in terms of structure and texture has been already reported in refs 10 and 11. It has been observed that the ETS-10 structure does not support high loadings of copper since the structure collapsing started from 15 wt % of Cu. The two copper samples chosen for this study, with Cu contents of 5.8 (Cu-ETS-L) and 11.4 (Cu-ETS-H) wt % did not exceed 15 wt % of Cu. The XRD pattern of Cu-ETS-L was indistinguishable from that of the parent ETS-10; no peak related to copper oxide aggregates could be detected, confirming the presence of particles with size below 2-3 nm. In the case of Cu-ETS-H, a decrease in crystallinity could be detected by comparison with the diffractogram of the parent matrix. A thorough inspection of the 2θ region of 36-39° revealed broad and very low intensity peaks attributable to the presence of CuO of size estimated around 100 Å, calculated on the basis of the broadening of the CuO X-ray peaks (Table 1). The study of the texture of the copper samples confirmed the preservation of the microporous characteristics of the samples despite the high copper loading. The micropore volume of Cu-ETS-L was 80% of that of the parent matrix. A more remarkable loss of microporous volume was observed for Cu-ETS-H, about 50% (Table 1). This behavior could be explained by micropore plugging through formation of CuO aggregates and/or by incipient structure collapse, as revealed by XRD. Adsorption of NO followed by Microcalorimetry. Microcalorimetric experiments of NO adsorption were carried out to determine the amount of NO adsorbed on the surface sites in relation with the energy of adsorption. The ETS-10 support did not adsorb NO significantly. The (33) Lei, G. D.; Adelman, B. J.; Sarkany, J.; Sacgtler, W. M. H. Appl. Catal., B 1995, 5, 245. (34) Cheung, T.; Bhargava, S. K.; Hobday, M.; Foger, K. J. Catal. 1996, 158, 301. (35) Saxton, R. J. Top. Catal. 1999, 9, 43.
Cu Site Energy Distribution of de-NOx Catalysts
Figure 1. Differential heat (qdiff) of NO adsorption versus coverage (molNO‚molCu-1) for the Cu-ETS-L (b) and Cu-ETS-H (9) samples.
presence of Cu greatly increased the NO uptake. The NO adsorption measurements of the Cu-loaded ETS-10 samples, on which the extent of NO uptake varied with copper loading, have already been reported.11 The differential heats (qdiff) of NO adsorption for the two copper samples are plotted in Figure 1 as a function of the amount of NO adsorbed; the data have been reported as mol of NO adsorbed/mol of Cu in the catalyst (molNO molCu-1), for a better comparison. The two samples adsorbed approximately the same volume of NO, expressed as molNO molCu-1. Considering that Cu-ETS-H contained twice the amount of copper in Cu-ETS-L (0.91 and 1.78 mmolCu‚gcat-1, respectively), it adsorbed an amount of NO almost double of that of ETS-L, comparing the adsorption on the two samples under the same equilibrium pressure. The qdiff curves of Figure 1 present a continuous decreasing trend without a well-defined plateau, indicating an heterogeneity of copper sites for both samples, Cu-ETS-H presenting a less pronounced decrease. For Cu-ETS-L, qdiff fell down over a very short uptake range from its initial value around 130 kJ‚molNO-1 down to 30-20 kJ‚molNO-1. Curves of Figure 1 show that the adsorption heat is higher for the high copper content sample (Cu-ETS-H) than for that with a low copper amount (Cu-ETS-L). For example, under a NO coverage of 0.015 molNO‚molCu-1, about 60 kJ‚molNO-1 was evolved from the surface of Cu-ETS-H and only about 40 kJ‚molNO-1 from that of Cu-ETS-L. This behavior suggests the presence of many low-energy copper sites and very few highly energetic ones over Cu-ETS-L, while Cu-ETS-H possesses a population of copper sites with broader energy distribution. Adsorption Isotherms of NO at Different Temperatures. The NO adsorption isotherms were measured at several temperatures chosen to have a low physisorption and an adsorption rate sufficiently high to achieve equilibrium during the experiments. The range 20-100 °C has been already confirmed as suitable when the NO probe is concerned. It is known that the adsorption of NO on copper surfaces can give rise to both mono- and dinitrosyl species, depending on the NO pressure as well as on the oxidation and aggregation state of copper centers (i.e., dimeric and oxygen-bridged copper complexes, oxo cations, and copper oxide particles), as evidenced by many infrared studies.33,37 On zeolitic and zeolite-like materials loaded with an amount of copper not greater than the maximum capacity of exchange of the matrix, Cu might (36) Gervasini, A.; Auroux, A. J. Phys. Chem. 1993, 97, 2628. (37) Beutel, T.; Sa´rka´ny, J.; Lei, G.-D.; Yan, J. Y.; Sachtler, W. M. H. J. Phys. Chem. 1996, 100, 845.
Langmuir, Vol. 17, No. 22, 2001 6941
Figure 2. Equilibrium isotherms of NO adsorption on Cu-ETS-L at different temperatures: symbols, experimental values; lines, calculated isotherms using the optimized parameters of Table 2.
Figure 3. Equilibrium isotherms of NO adsorption on Cu-ETS-H at different temperatures: symbols, experimental values; lines, calculated isotherms using the optimized parameters of Table 2.
be mainly present as isolated Cu2+ ions, as observed by spectroscopic analyses.38,39 Several studies on copper zeolite catalysts characterized by adsorption of NO6,40 indicated the presence of NO adsorbed species (1810 cm-1 band) up to ca. 1 kPa. When the amount of NO adsorbed is increasing, the intensity of the 1810 cm-1 band increases while new bands appears associated with NO2+, NO2Cu+, and (NO)2-Cu+ with maxima at 1896 and 1630 cm-1 appear.6,31,32 These results show that the adsorption of NO when increasing the NO pressure causes oxidation of Cu centers and dimerization of NO monomeric species. Mononitrosyl species bound to Cu2+ centers are predominant at NO pressure not greater than 2 kPa.34 At low NO coverage, the dependence of the amount of NO adsorbed on temperature (exothermic effect) was evident for the Cu-ETS-L and Cu-ETS-H samples (Figures 2 and 3). For higher NO coverage, the isotherms did not follow a clear trend according with the adsorption temperature. In this pressure range, physisorption played a remarkable role. The low-pressure branch curves reveal (38) Torre-Abreu, C.; Ribeiro, M. F.; Henriques, C.; Ribeiro, F. R. Appl. Catal., B 1997, 11, 383. (39) Ma´rquez-Alvarez, C.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A.; Haller, L.; Ferna´ndez-Garcı´a, M. J. Am. Chem. Soc. 1997, 119, 2905. (40) Bell, A. T. Catal. Today 1997, 38, 151.
6942
Langmuir, Vol. 17, No. 22, 2001
Figure 4. Isosteres of adsorption of NO on Cu-ETS-L at different surface coverage from 0.016 to 0.033 molNO‚molCu-1.
Figure 5. Isosteres of adsorption of NO on Cu-ETS-H at different surface coverage from 0.014 to 0.022 molNO‚molCu-1.
a Langmuirian form, as confirmed from the calculated isotherms by the parameters obtained considering the experimental curves as summations of single Langmuir isotherms, as discussed in the following paragraph. The amount of NO adsorbed, expressed in terms of molNO‚ molCu-1, was higher for the low copper loading sample. The experimental isotherms were transformed into the corresponding isosteres for surface coverages from 0.016 to 0.033 molNO‚molCu-1 on Cu-ETS-L and from 0.014 to 0.022 molNO‚molCu-1 on Cu-ETS-H (Figures 4 and 5). The isosteric heats of adsorption, Qst, were evaluated from the slopes of the various lines obtained by plotting ln P vs 1/T, in accordance with the Clausius-Clapeyron equation, assuming that Qst is independent of temperature.41,42 Higher values of Qst were calculated for Cu-ETS-H (4237 kJ‚mol-1) than for Cu-ETS-L (30-28 kJ‚mol-1). No clear decreasing trend of Qst as a function of NO coverage was observed in any case. The Qst values are not able to enlighten the differences in heterogeneity of Cu sites of the two samples. The differential calorimetric and isosteric heats of NO are very different since the calculated Qst values correspond to medium-high uptakes and therefore lower NO adsorption heats are expected compared with the measured qdiff. Copper-Site Energy Distribution. The volumetric isotherms of NO adsorption were measured at different (41) Koubek, J.; Pasˇek, J.; Volf, J. J. Colloid Interface Sci. 1975, 51, 491. (42) Sircar, S. Langmuir 1991, 7, 3065.
Carniti et al.
temperatures, and the mathematical model described above was applied in order to derive the Cu-site energy distribution of the surfaces. For each catalyst, the volumetric data obtained at the different temperatures were taken into account at the same time. Only the initial parts of the isotherms corresponding to the chemisorption of NO were utilized in the computations. In this initial region, the Cu-NO interaction would be almost uniquely in the form of the mononitrosyl complex,34 and physisorption, mainly due to the support, is low. In preliminary computations, the values of the adsorption enthalpies were tested in the range from -14 down to -120 kJ mol-1. Values higher than -14 kJ mol-1 were not taken into account as this value corresponds to the condensation enthalpy of NO. The range of enthalpy was divided into several (up to nine) bands, attributing to each site the mean value of the enthalpy of its band. These calculations showed that a significant amount of sites were associated with a ∆aHi of -14 kJ mol-1 relevant to physisorption, while the chemisorption sites, associated with Cu species, were centered around no more than two ∆aHi values, the most important one being between -40 and -50 kJ mol-1 and the other one at lower values (
