Microcalorimetric Study of the Acidic Character of Modified Metal

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Langmuir 1996, 12, 5356-5364

Microcalorimetric Study of the Acidic Character of Modified Metal Oxide Surfaces. Influence of the Loading Amount on Alumina, Magnesia, and Silica A. Gervasini Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy

J. Fenyvesi and A. Auroux* Institut de Recherches sur la Catalyse, CNRS, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France Received May 2, 1996. In Final Form: July 23, 1996X The modification of the acid/base properties of a series of oxide supports (alumina, magnesia, and silica) modified by increasing loadings of additive ions (Li+, Ni2+, and SO42-) from 1% to 50% of the support surface coverage is reported using adsorption microcalorimetry of NH3 and SO2 as acidic and basic probe molecules. The curves of differential heat of adsorption versus the coverage have shown that the acidic properties of alumina were weakly modified when 1% of the support surface was covered by the guest ion. The acid/base properties of silica and magnesia were much more easily enhanced by the additives. A high guest oxide loading (50% of surface coverage) led to very strong modifications of the surface acid-base behavior depending on the support and the additive. Volumetric measurements allowed us to quantify the number of acid/base sites, to express them in terms of ion specific effect, and also to search for correlations, in the case of high loadings, with Tanabe’s and Seiyama’s hypotheses on the acidity of mixed oxides.

1. Introduction Supported metal oxide catalysts have developed much attention because of their wide application as oxidation catalysts and/or as precursors to supported metal and sulfide catalysts. Studies on the nature of the interaction between the dispersed metal oxide species and the support have shown that their catalytic behavior and their acidbase properties are strongly affected by the inductive effect of the metal ions in the solids.1,2 It has also been established in the literature that the support influences strongly the nature and extent of metal oxide-support interaction and that the physicochemical properties of dispersed metal oxides are usually very different from those of the corresponding bulk phases.3,4 Besides the effect of the support, the loading amount of metal oxide has also a very strong impact on the nature of the interaction between metal oxide and support5 and on the acid-base properties of these systems.6,7 High-valent, not fully coordinated metal ions or anionic vacancies are supposed to act as Lewis acidic centers, and OH groups act as Bro¨nsted acidic sites, while oxygen ions O2- account for the basic character of these catalysts. Several works, in particular on mixed oxide catalysts for selective oxidation of hydrocarbons, have been concerned with searching correlations between the acidobasic properties of these systems and activity/selectivity in oxidation reactions.8,9 However, none, to our knowledge, * To whom correspondence should be addressed: tel, 33472445398; fax, 33-472445399; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Tanabe, K. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1987; Vol. 8, pp 232-271. (2) Chen, Y.; Zhang, L. Catal. Lett. 1992, 12, 51. (3) Cardona-Martinez, N.; Dumesic, J. A. Adv. Catal. 1993, 38, 149244. (4) Berteau, P.; Delmon, B. Catal. Today 1989, 5, 121. (5) Turek, A. M.; Wachs, I. E.; DeCanio, E. J. Phys. Chem. 1992, 96, 5000. (6) Youssef, N. A.; Youssef, A. M. Bull. Soc. Chim. Fr. 1991, 128, 864. (7) Gervasini, A.; Auroux, A. J. Catal. 1991, 131, 190.

S0743-7463(96)00436-2 CCC: $12.00

is referring to a systematic study of the acid-base properties when varying both the nature of the support and the amount of loading oxide. In previous papers,10,11 we have reported the results of microcalorimetric and catalytic studies of the acidic character of pure oxide supports (alumina, magnesia, and silica) and the same oxides doped by small amounts of metal ions (Ca2+, Li+, Nd3+, Ni2+, SO42-, Zr4+) in a concentration of 0.1-0.3 µmol/m2, which corresponds to a surface coverage of about 0.5-1% of mole of metal ion per mole of support. The results have been rationalized according to an acidity/basicity scale defined by the specific ionic effect.10 The aim of the present paper has been to extend our investigation to supported oxide samples by varying the amount of the additive on the oxide support (from 3% up to 50% in ion surface coverage of the support). In this approach we have considered both the nature of the guest metal oxide and the surface structure of the host support, which include the following: (a) Three different supports, such as an amphoteric γ-alumina, a weakly acidic silica, and a basic magnesia. These supports provide different capacities for the formation of highly dispersed species for many metal oxides and induce metal oxide-support interactions of various strength. (b) Three different guest metal ions such as lithium, nickel, and sulfate ions which exhibit various acid-base properties especially at low loadings. (c) Three estimated values of metal oxide loadings supposed as about 1%, 10%, and 50% of the monolayer capacity of the supports. In fact, the corresponding (8) Ai, M. J. Catal. 1978, 52, 16. (9) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. In New Solid Acids and Bases, Their Catalytic Properties; Delmon, B., Yates, J. T., Eds.; Elsevier: Amsterdam, 1989; Vol. 51. (10) Gervasini, A.; Bellussi, G.; Fenyvesi, J.; Auroux, A. J. Phys. Chem. 1995, 99, 5117. (11) Gervasini, A.; Bellussi, G.; Fenyvesi, J.; Auroux, A. In New Frontiers in Catalysis; Proceedings of the 10th International Congress on Catalysis; Guczi, L., et al., Eds.; Elsevier: Amsterdam, 1993; pp 2047-2050.

© 1996 American Chemical Society

Modified Metal Oxide Surfaces

experimental values were not always close to the calculated values, mainly on silica which involves different fashions of the oxide phase interactions. However the sample loadings will be referred to as 1, 10, or 50 for simplification. If we consider the above compounds in terms of their electron donor or acceptor properties, the determination of their acido-basic properties is a determining step of their characterization. In spite of numerous works, no general correlation between acido-basic properties and amounts of added oxide has been so far formulated. In particular, how does the acidity change when, for a particular ion, the loading varies from a very low amount, creating a ion-doping effect, to an added oxide content up to a monolayer coverage? One of the most accurate methods for characterizing the acido-basic properties of a catalyst is adsorption microcalorimetry of acid/base probe molecules which gives access to both the number and the strength of the acid/ base sites.3,12,13 New hypotheses regarding the acidity generation of binary oxides have been proposed by Tanabe1,14 and Seiyama,15 which predict what kind of binary oxides will show acidic properties (Bro¨nsted or Lewis acid) but do not predict the acid strength. According to their postulates we will check whether their hypotheses are applicable to our samples and how the theoretical values fit with our experimental determination of the acidity. 2. Experimental Section Preparation of the Doped and Supported Oxides. The three host supports, γ-Al2O3 (from Akzo-Chemie), MgO (from Carlo Erba), and SiO2 (from Grace Catalysts & Carriers) were the same oxides as those used in the previous works.10,11 Besides the doped oxides with Li+, Ni2+, and SO42- additives on γ-Al2O3 (referenced as Li(Ni,S)-Al-1), on MgO (referenced as Li(Ni,S)Mg-1), and on SiO2 (referenced as Li(Ni,S)-Si-1) prepared as reported in ref 10, three new series of samples with different amount of guest oxides have been prepared. They have been referenced as Li(Ni,S)-Al(Mg,Si)-10 or as Li(Ni,S)-Al(Mg,Si)50 depending on the amount of guest oxide on the support expressed as percent of mole of metal ion per mole of support. The composition of the loaded oxides ranged in the interval 3-10% for Li(Ni,S)-Al(Mg,Si)-10 and 15-50% for Li(Ni,S)-Al(Mg,Si)50. Table 1 collects the samples, their qualitative and quantitative composition, expressed in ionic concentration and percentage of surface coverage of the guest ions,10,16 as calculated from the surface area values and the chemical analyses of the samples. The doped and the supported oxides were prepared by incipient wetness impregnation of weighed amounts of the host oxides in powder form (impregnation point of Al2O3, MgO, and SiO2 of 1, 4, and 3 mL/g, respectively) with aqueous solutions containing known concentrations of the salts of the three additives. As salt precursors, lithium acetate (Strem), nickel formate (J. Matthey), sulfuric acid (Prolabo, for the doped oxides), or ammonium sulfate (Prolabo, for the supported oxides) were chosen. Water was gently removed from the final solutions at 120 °C. Eventually the dry solutions were calcined at 500 °C during 3 h in air for the doped oxides, Li(Ni,S)-Al(Mg,Si)-1, and during 6 h in pure oxygen for the supported oxides, Li(Ni,S)-Al(Mg,Si)-10 and Li(Ni,S)-Al(Mg,Si)-50. The optimal temperature of calcination was chosen on the basis of the temperatures of decomposition of the precursor compounds resulting from thermal analysis experiments (thermogravimetric analysis and differential thermal analysis). The decomposition temperature of the precursors impregnated on the supports corresponded with those of the relevant compounds. (12) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (13) Andersen, P. J.; Kung, H. H. In Catalysis; The Royal Society of Chemistry: London, 1995; Vol. 11, pp 441-466. (14) Tanabe, K.; Sumiyoshi, T.; Shibata, K.; Kiyoura, T.; Kitagawa, J. Bull. Chem. Soc. Jpn. 1974, 47, 1064. (15) Seiyama, T. In Metal Oxides and Their Catalytic Actions; Kodansha: Tokyo, 1978. (16) Jira`tova`, K.; Bera`nek, L. Appl. Catal. 1982, 2, 125.

Langmuir, Vol. 12, No. 22, 1996 5357 Table 1. Composition of the Loaded Oxides ionic surface surface concna coverageb (µmol/ (% mol of ion/ area m 2) mol of support) (m2/g)

sample

composition M (wt %)

Al Li-Al-1 Li-Al-10 Li-Al-50 Ni-Al-1 Ni-Al-10 Ni-Al-50 S-Al-1 S-Al-10 S-Al-50

Al2O3 (100) Li(0.015)-Al2O3 Li(0.22)-Al2O3 Li(1.75)-Al2O3 Ni(0.135)-Al2O3 Ni(2.43)-Al2O3 Ni(10.0)-Al2O3 S(0.115)-Al2O3 S(0.60)-Al2O3 S(2.94)-Al2O3

0.10 1.49 13.25 0.11 2.05 9.46 0.18 0.85 3.67

0.50 7.12 56.7 0.52 9.31 38.3 0.80 4.2 20.6

208 210 212 190 203 202 180 201 219 250

Mg Li-Mg-1 Li-Mg-10 Li-Mg-50 Ni-Mg-1 Ni-Mg-10 Ni-Mg-50 S-Mg-1 S-Mg-10 S-Mg-50

MgO (100) Li(0.007)-MgO Li(0.08)-MgO Li(0.85)-MgO Ni(0.10)-MgO Ni(0.80)-MgO Ni(3.45)-MgO S(0.10)-MgO S(0.15)-MgO S(1.36)-MgO

0.08 7.19 87.36 0.15 1.05 4.94 0.26 0.33 2.49

0.23 2.78 29.5 0.41 3.3 14.2 0.76 1.12 10.23

110 125 16 14 112 129 119 119 139 170

Si Li-Si-1 Li-Si-10 Li-Si-50 Ni-Si-1 Ni-Si-10 Ni-Si-50 S-Si-1 S-Si-10

SiO2 (100) Li(0.020)-SiO2 Li(0.34)-SiO2 Li(2.79)-SiO2 Ni(0.26)-SiO2 Ni(3.67)-SiO2 Ni(17.3)-SiO2 S(0.072)-SiO2 S(0.15)-SiO2

0.10 1.75 16.12 0.14 2.22 8.80 0.08 0.17

0.46 7.84 64.4 0.70 10.0 47.2 0.36 0.63

310 280 279 249 307 281 335 276 281

a Concentration of the metal ion deposited on pure Al O , MgO, 2 3 and SiO2 oxides. b Surface ion coverage with respect to pure Al2O3, MgO, and SiO2 expressed in metal ion moles to oxide support moles.

The formation of Li2O and NiO oxides on alumina and silica occurred at 400 and 300 °C, respectively, and were associated with exothermic reactions. The decomposition of (NH4)2SO4 was an endothermic reaction; on silica, before the complete decomposition to sulfate (confirmed by XPS analysis), the dimer compound (NH4)3H(SO4)2 was formed at 300 °C. On magnesia support, a remarkable loss of mass was observed at 350 °C due to Mg(OH)2 decomposition. On this support, the decomposition of the cationic precursors was observed at 350 °C16,17 and that of sulfate at 420 °C,18 in agreement with literature. Therefore, calcination performed at 500 °C ensured the complete decomposition of the three precursor compounds on the supports as well as that of surface Mg(OH)2, preventing the evolution of Li2O and the decomposition of sulfate. Physicochemical Measurements. X-ray powder diffraction patterns (XRD) of the samples were collected on a computercontrolled Phillips diffractometer equipped with nickel-filtered Cu KR radiation (λ ) 1.541 78 Å). BET surface areas were determined from conventional N2 adsorption isotherms. The samples were heated at 400 °C for 2 h under vacuum (increasing rate of temperature ) 2 °C/min) before BET analysis. Ammonia and sulfur dioxide were chosen as probe molecules to perform calorimetric and volumetric gas-solid titrations of the acid and basic sites of the samples, respectively. Sulfur dioxide and ammonia (Air Liquide, purity >99.9%) were purified by successive freeze-thaw pumping cycles before use. Ammonia was previously dried on sodium wires. The samples (≈100 mg) were put in quartz calorimetric cells and pretreated at 500 °C with a 2 °C/min increase rate under vacuum overnight. (17) Perrichon, V.; Durupty, M. C. Appl. Catal. 1988, 42, 217. (18) Amin, A.; Hanafi, S.; Selim, S. A. Thermochim. Acta 1982, 53, 125. (19) Waqif, M.; Bachelier, J.; Saur, O.; Lavalley, J. C. J. Molec. Catal. 1992, 72, 127.

5358 Langmuir, Vol. 12, No. 22, 1996 The NH3 and SO2 heats of adsorption were measured in differential heat flow microcalorimeters of Tian-Calvet type, C80 and HT from Setaram. The adsorption temperature was maintained at 80 °C in order to limit physisorption. A glass volumetric line linked to the calorimetric cells permitted the introduction of successive small doses of the gases onto the samples until a final equilibrium pressure of 133 Pa was obtained. The equilibrium pressure relevant to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). Both the calorimetric and the volumetric data were stored and analyzed by microcomputer processing. In order to calculate the irreversibly chemisorbed amount (Virr), the sample was pumped at 80 °C at the end of the first adsorption, and a second adsorption was then performed at the same temperature. Virr was determined by the difference between the primary and secondary isotherms. While microcalorimetry is a very efficient method for the determination of the number and strength of sites, it is not suited to distinguish specifically between Bro¨nsted and Lewis acidic or basic sites. It must be associated to infrared spectroscopy measurements to discriminate accurately both types of sites. However, it is known that on oxide materials the protonic acidity is weaker than the Lewis acidity, so that it is sometimes possible to interprete the differential heat curves in terms of nature of the sites.

3. Results Composition of Doped and Supported Oxides. The host oxides which were selected are commonly used as well as supports and catalysts. Considering that magnesia, alumina, and silica take part in most of the acidbase reactions, to find a way to regulate or modify their surface properties appears as a fundamental objective. This can be effected either by adding very small amounts of metallic ions which will act as doping ions or by covering a larger surface of the support to give rise to supported oxides. The selected additives (Li+, Ni2+, SO42-) were chosen on the basis of their very different behaviors, as a basic alkali ion, a transition metal ion, and an acidic sulfate ion. The ions were added on each support oxide in a concentration range between 0.1 and 16 µmol of metal ion per surface area which corresponds to a surface coverage of the support between 0.2 and 64% metal ion mole per support oxide mole. Details on the composition of the samples and various surface coverages can be seen in Table 1. The interaction between the support and the additives, in terms of anchor points, determined the experimental surface coverage obtained. This can explain the failure in obtaining a high degree of surface coverage in particular cases (i.e., S-Si-10, no anchorage point on silica surface, etc.). The alumina series oxides did not display important modifications in the surface area values with reference to pure alumina even when a high degree of surface coverage was realized (Table 1). Starting from 208 m2/g of pure alumina, among the aluminas modified with cationic additives, the lower surface values were those of Ni-Al50 and Li-Al-50, with values of 180 and 190 m2/g (decrease of 11%). When alumina was modified with sulfate, an increase of the surface was observed, in particular when high sulfate loading was realized (250 m2/g for S-Al-50, increase of 18%). This can be due to the open structure of the sulfate groups deposited on the alumina surface. The analogous light influence of the additives on silica surface area values was observed (Table 1). The values ranged from a minimum of 249 m2/g for Li-Si-50 up to 335 m2/g for Ni-Si-50, 18% and 8% of decrease and increase of areas, respectively, with respect to pure silica (310 m2/g). When the magnesia surface was doped with small amounts of additives (1% of surface coverage), no

Gervasini et al.

important modification was created. Moreover, there was observed a strong decrease of the surface area values increasing the lithium loading (i.e., Li-Al-10 and LiAl-50). Due to the small ionic radius of Li+,20 it could hinder the microporosity of MgO leading to the observed loss of about 80% of the MgO surface. On magnesia too, the loading with a high amount of sulfate (S-Mg-10 and S-Mg-50) caused enhanced values of surface areas. All the modified oxides which have been studied remained amorphous (SiO2) or crystalline (γ-Al2O3 and MgO periclase) phases of the relevant host oxides independent of the additive loading. In fact, XRD of the samples did not reveal well detectable lines due to segregated oxide islands even for 50% of surface coverage. This could indicate that the guest oxides were well dispersed on the three host supports. However it is likely that small local spinel or nickel silicate spots could be formed which escaped XRD detection. One can also infer that the difficulty in detecting separate oxide phases was due to the difference between the mass of the guest (in particular for Li2O and SO3) and the host oxides (Al2O3, MgO, and SiO2). Only for Ni-Al(Mg,Si)-50 samples, on which the deposition of a high amount of NiO (heavy atom) was performed, were very light XRD lines typical of NiO phase revealed. From BET measurements it can also be suggested that Li+ forms a compound or solid solution with the MgO support. However, XRD did not detect crystals of this compound because they are likely to be very small and located only at the surface of MgO particles. Anyway, it can be concluded that the guest oxides were fairly well dispersed on the host support oxides. As said above, the acid-base properties have been determined by adsorption microcalorimetry using NH3 and SO2 to probe the surface. Adsorption Microcalorimetry. Table 2 summarizes the thermodynamic results observed from the calorimetric experiments. The differential heat of adsorption at initial point of the measurements (Qinit) is shown in this table. As explained in the Experimental Section, the irreversibly adsorbed volume (Virr) is determined from the difference of volume between the primary and secondary isotherms. This volume corresponds to the amount held by strong chemisorption at adsorption temperature over these samples. The integral heat of adsorption corresponding to this volume is Qint. Another representation particularly indicative of the evolution of the partial acidities lies in the calculated specific effect of ions on acidity which is defined4,16 as follows acidity of ion-modified support - acidity of reference support unit concentration of ion into support

and expressed in µmol (probe)‚m-2/µmol (ion)‚m-2. This definition leads to a specific effect on acidity of zero for the reference support. In applying the above notion, we can propose a classification of the ion-modified supports related to an acidity scale when using NH3 or to a basicity scale when using SO2 as probe molecule. Figures 1-3 represent the differential heats of adsorption (Qdiff) of NH3 and SO2 versus the respective coverage by the probe on the series of alumina samples. The curves concerning SO2 adsorption on Li(Ni,S)-Al samples are roughly composed of three regions. At the very beginning a sharp decrease in Qdiff is generally observed, which should be assigned to the adsorption on a few very strong Lewis base sites. In the next region a relatively slight decrease in Qdiff or even a plateau is (20) Shannon, R.-D. Acta Crystallogr. 1976, 32, 751.

Modified Metal Oxide Surfaces

Langmuir, Vol. 12, No. 22, 1996 5359 Table 2. Volumetric and Calorimetric Data NH3

SO2

sample

Qinit (kJ/mol)

Virr (µmol/m2)

Qinta (J/m2)

Al Li-Al-1 Li-Al-10 Li-Al-50 Ni-Al-1 Ni-Al-10 Ni-Al-50 S-Al-1 S-Al-10 S-Al-50

217 180 216 146 211 190 196 236 196 142

1.34 1.58 1.66 1.29 1.58 1.62 1.98 1.46 1.58 1.86

0.21 0.20 0.25 0.15 0.23 0.22 0.28 0.22 0.22 0.22

Mg Li-Mg-1 Li-Mg-10 Li-Mg-50 Ni-Mg-1 Ni-Mg-10 Ni-Mg-50 S-Mg-1 S-Mg-10 S-Mg-50

36 120 18

0.25 0.35 1.03

0.008 0.028 0.018

1.26 0.108

55 57 60 57 80 88

0.39 0.30 0.29 0.26 0.25 0.34

0.021 0.017 0.014 0.012 0.018 0.026

0.97 0.047 0.008 0.04 0 0.036

Si Li-Si-1 Li-Si-10 Li-Si-50 Ni-Si-1 Ni-Si-10 Ni-Si-50 S-Si-1 S-Si-10

83 52 120 101 99 131 131 47 66

0.070 0.089 0.388 0.253 0.104 0.198 0.353 0.119 0.090

0.004 0.004 0.038 0.021 0.007 0.019 0.037 0.004 0.005

0.18 0.181 0.0114 0.24 0.0575 0.032 0.60 0.120

a

Qinta (J/m2)

ISEb

Qinit (kJ/mol)

Virr (µmol/m2)

2.27 0.214 -0.004 2.08 0.137 0.068 0.67 0.281 0.142

194 174 235 210 188 200 160 164 170 131

1.68 1.80 1.91 2.86 1.73 1.93 0.04 1.84 0.83 0.14

0.25 0.27 0.28 0.43 0.26 0.29 0.01 0.26 0.11 0.02

1.14 0.154 0.089 0.46 0.122 -0.173 0.91 -0.996 -0.420

215 251 229 305 210 190 203 200 191 218

2.69 3.64 3.16 5.34 5.31 3.97 3.92 4.12 3.73 2.70

0.44 0.62 0.44 1.24 0.84 0.60 0.42 0.62 0.58 0.32

11.64 0.065 0.030 17.24 1.212 0.249 5.46 3.095 0.004

58 85 135 185 99 167 163 102

0.006 0.012 0.151 0.306 0.006 0.062 0.224 0.007

4 × 10-4 9 × 10-4 0.019 0.041 5 × 10-4 0.009 0.031 6 × 10-4

0.05 0.0827 0.0186 -0.01 0.025 1.313 0.01

ISEb

Integral heat corresponding to Virr of adsorbed NH3 and SO2. b Ion specific effect4,16 expressed in [µmolNH3/SO2‚m-2/µmolion‚m-2].

Figure 1. Differential heat of ammonia and sulfur dioxide adsorption on Li-Al samples versus the adsorbed amount.

observed, corresponding to the heats released during adsorption on the predominant sites. Then the decrease of Qdiff with the adsorbed volume becomes fast. In some cases (Ni-Al-50 and S-Al-50) the curves become so steep that the second region or the plateau disappears and only a sharp decrease line is observed. The curves concerning NH3 adsorption on Li(Ni,S)-Al samples show a relatively continuous decrease with less trend to describe a plateau around 150 kJ/mol than alumina alone. Figure 1 shows the differential heats of adsorption of NH3 and SO2 on samples with lithium. When comparing the heats at very low coverage of NH3, Li-Al-1 and LiAl-10 display similar heats up to 5% of NH3 coverage whereas Li-Al-50 displays a much lower initial heat (146 kJ/mol). The acidic character of the support is essentially modified in the strong sites domain. The curve of differential heat of Li-Al-10 is close to that of alumina up to 1 µmol (NH3)/m2 coverage and then higher than the

support in the medium acid sites domain. Table 2 shows that the irreversibly adsorbed volume of NH3 is similar between Li-Al-50 and Al but higher for Li-Al-1 and LiAl-10 compared to the support. For SO2 adsorption, Figure 1 gives evidence of the very strong basicity of Li-Al-50 compared to the other samples. The addition of lithium at a very low percentage interacts with the very strong basic sites of alumina and reduces the initial heat (174 kJ/mol). The two other percentages increase the initial heat (235 and 210 kJ/mol for Li-Al-10 and Li-Al-50 respectively). A 10% amount of lithium ion (Li-Al-10) increases the number of basic sites in the domain of mean sites (around 100 kJ/mol). The evolved heats (Qint) corresponding to the irreversibly adsorbed volume are superior to those of alumina in the three cases and they increase proportionally to the amount of additive (Table 2). To conclude, the addition of lithium in very low amount (1%) decreases the acid strength but does not affect the basicity of the support. The coverage of about 10% of the surface increases both the mean and weak acidity and basicity. A higher amount of lithium (Li-Al-50) destroys the strong acidity and increases considerably the basicity of the support. The number of strong basic sites increases in all cases. Figure 2 presents the differential heats of adsorption of NH3 and SO2 on Ni-Al samples. The sample Ni-Al-1 presents a number of strong acid sites similar to those of alumina up to 1 µmol (NH3)/m2 of NH3 coverage and then in higher amount than on alumina at higher coverage. Ni-Al-10 and Ni-Al-50 display the same types of strong acid sites as Ni-Al-1 but with a strength lowered of 20 kJ/mol. The strength of the mean sites of Ni-Al-50 is considerably increased compared to the other samples. The initial heat is however weakly affected by nickel additives (Table 2).

5360 Langmuir, Vol. 12, No. 22, 1996

Figure 2. Differential heat of ammonia and sulfur dioxide adsorption on Ni-Al samples versus the adsorbed amount.

Figure 3. Differential heat of ammonia and sulfur dioxide adsorption on S-Al samples versus the adsorbed amount.

Concerning the basicity of these samples, the strength is not modified by addition of 1% or 10% of nickel on the alumina surface up to 1 µmol (SO2)/m2 of coverage. Above this volume, the strength and number of sites increase slightly. However, the addition of a large amount of nickel (Ni-Al-50) creates a huge decrease of the number of basic sites (0.2 µmol (SO2)/m2). The initial heats are similar (188 kJ/mol for Ni-Al-1 and 200 kJ/mol for Ni-Al-10) to that of the support, but for Ni-Al-50, Qinit ) 160 kJ/mol. To conclude, 1% of nickel (Ni-Al-1) modifies neither the acidity nor the basicity of the support up to 1 µmol/m2 of coverage. Above this value, a small increase of the mean and weak acid strength occurs without modification of the number of acid or basic sites. A 10% amount of nickel (Ni-Al-10) decreases the strength of strong acid sites but increases the strength of mean and weak acid sites as for Li-Al-1. However Ni-Al-10 shows an increase of the number and strength of basic sites. For Ni-Al-50, the strong acidity is reduced but the number and strength of mean and weak sites are considerably increased. This amount of nickel destroys nearly all the basicity. When the alumina surface coverage is increased, a decrease of strong acidity and an increase of weak and mean acid strength occur. Small quantities of nickel (1% and 10%) (Ni-Al-1 and Ni-Al-10) increase slightly the basicity whereas 50% nickel poisons all the basic sites of the support. When sulfate ions are added on alumina, the differential heats of ammonia adsorption versus the coverage (Figure 3) show that the strength of acid sites was modified compared to the host alumina. A very low amount of sulfate (S-Al-1) increases the strength of the very strong acid sites (between 236 and 170 kJ/mol). On the contrary the differential heat curve for S-Al-10 is below the alumina curve until 1 µmol/m2 in ammonia coverage and

Gervasini et al.

Figure 4. Differential heat of ammonia and sulfur dioxide adsorption on Li-Mg samples versus the adsorbed amount.

then slightly above it. The addition of 3 wt % of sulfate (S-Al-50) leads to an unexpected curve. The first doses of ammonia are totally and irreversibly adsorbed without or with a very low evolved heat, then after a few doses, the heat increases and the curve becomes similar to the others above 1 µmol/m2. It is worth noticing that the adsorption isotherm does not show any abnormal behavior. This strange phenomenon can be interpreted by an endothermic reaction associated to an exothermic adsorption. No pressure increasing was observed due to a possible dissociation of ammonia and leading to an endothermic evolved heat. However, it is likely that a dissociative adsorption occurs even if we were unable to prove it. The very strong sites due to sulfate ions are supposed to create either a strong or dissociative chemisorption. The formation of an ammonium sulfate occurring on the surface cannot also be totally excluded even if presumably too exothermic (∆H°f ((NH4)2SO4) ) -901 kJ/mol). Lavalley et al.19 have shown that the acidity and stability of alumina are strongly modified by sulfation, which enhances the strength of the weakest Lewis acid sites but poisons the strongest. As evidenced by SO2 adsorption, the initial heats and basicity of the strong sites of the samples decrease proportionally to the sulfate amount (Figure 3). A 10% amount of sulfate (S-Al-10) already affects considerably the number and strength of the basic sites: the strong sites plateau is around 130 kJ/mol only and the number of basic sites is nearly divided by two (Table 2). Figures 4-6 display the differential heats of adsorption of NH3 and SO2 versus the coverage on the series of magnesia samples. The curves concerning SO2 adsorption on Li(Ni,S)-Mg samples are roughly composed of three regions. At the very beginning a sharp decrease in Qdiff is observed, followed by a kind of plateau around 150 kJ/ mol and then again a sharp decrease at high coverage. The variety of sites mostly affected in number by the additives is mainly those of mean strength close to the previously cited plateau. Due to the lack of acidity of magnesia, only sulfate ions modify noticeably the surface of the support. Figure 4 represents the differential heats of adsorption of ammonia and SO2 on the series with lithium additive. A very small amount of lithium ion increases significantly the acidity of the system (Qinit ) 120 kJ/mol) until a coverage of 0.2 µmol (NH3)/m2 is reached. Above this volume, Li-Mg-1 displays only a physisorption behavior similar to that of the support. A higher amount of lithium destroys completely the few remaining acid sites, and no adsorption occurs for Li-Mg-50. Concerning SO2 adsorption, Figure 4 shows two types of strong basic sites for sample Li-Mg-1: a small amount

Modified Metal Oxide Surfaces

Figure 5. Differential heat of ammonia and sulfur dioxide adsorption on Ni-Mg samples versus the adsorbed amount.

of very strong sites with a strength between 250 and 170 kJ/mol and a much higher amount of strong sites forming a plateau around 170 kJ/mol. Li-Mg-10 shows an initial heat of 229 kJ/mol close to that of the support (215 kJ/ mol) but a plateau of lower strength around 130 kJ/mol only. A higher amount of lithium increases considerably the basic sites in both number and strength as shown by Li-Mg-50 which presents a population of very strong basic sites around 300 kJ/mol and a plateau around 250 kJ/ mol. Doping magnesia with lithium (Li-Mg-1) increases both the acidity and basicity. The increasing of acidity can be interpreted by a substitution of Mg2+ ion by Li+ ion. Lithium ion has a stronger Lewis acidic character than Mg2+. The addition of lithium in higher amounts destroys all the acidity created by traces of lithium. However the very strong basic sites are not affected by lithium addition. It has been shown (Table 1) that the surface area of the samples Li-Mg-10 and Li-Mg-50 were decreased to only 16 m2/g after calcination which corresponds to a surface totally coated by Li2O for sample Li-Mg-50. Figure 5 shows the differential heats of adsorption of both probes on samples with nickel additives. The initial heats of ammonia adsorption (Table 2) are increased up to 60 kJ/mol when nickel is added and remain very close to each other whatever the amount of additive. A very low amount of nickel (Ni-Mg-1) increases the strength and number of acid sites. The differential heats of SO2 adsorption as a function of coverage show that the strongest population of sites disappears when nickel is added, then the curves are identical up to 2 µmol of SO2/ m2. Above this volume, Ni-Mg-1 and Ni-Mg-10 show a huge increase in the number of medium strength sites (plateau at 150 kJ/mol) whereas a further addition of nickel leads to a smaller increase. Ni-Mg-50 leads to a slight increase of the strength (plateau around 180 kJ/mol) but a large decrease in number. To conclude, nickel additive in very low amount increases both acidity and basicity. A larger amount of nickel reduces both the acidity and basicity compared to Ni-Mg-1. Ni-Mg-50 shows a large poisoning of the strong basic sites. Figure 6 displays the differential heats of NH3 and SO2 adsorption versus the adsorbed amount on samples with sulfate ions. The acidity increases in number and strength with the sulfate uptake, moderately for S-Mg-1 and, more importantly, giving very close values for the two other percentages. S-Mg-1 and S-Mg-10 display very similar differential heats of SO2 adsorption with a much larger number of sites than the magnesia support. On the contrary a higher amount of sulfate (S-Mg-50) decreases both the strength (plateau around 155 kJ/mol) and number

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Figure 6. Differential heat of ammonia and sulfur dioxide adsorption on S-Mg samples versus the adsorbed amount.

Figure 7. Differential heat of ammonia and sulfur dioxide adsorption on Li-Si samples versus the adsorbed amount.

of strong basic sites. Sulfate ions are attached on the basic sites of the magnesia support, which means on the oxygen ions. A 3 wt % amount of sulfate poisoned most of the basic sites. Concerning the series on silica, the very weak acid or basic character of this support enhances the influence of the additives (except for sulfate ions) as can be seen in Figures 7-9, which represent the differential heats versus the probe coverage. Sample Li-Si-1 displays a lower initial heat of NH3 adsorption (52 kJ/mol) than the support and remains close to the support, whereas those of the two other samples are much higher (Table 2). An important population of relatively strong acid sites has been created (above 90 kJ/mol) in samples Li-Si-10 and Li-Si-50 (Figure 7). For the basicity, the initial heats of SO2 adsorption and the number and strength of the sites increase widely and proportionally to the coverage of silica by lithium ions (58, 85, 135, and 185 kJ/mol initial heats for Si, Li-Si-1, Li-Si-10, and Li-Si-50, respectively). A low amount of nickel additive also widely increases the acidity (Figure 8) both in strength and number, and a larger amount (Ni-Si-50) multiplies by five the number of strong acid sites. The number and strength of basic sites varies also proportionally to the uptake in nickel ion and comparatively the basicity is more enhanced by the additive than the acidity. Sulfate ions (Figure 9) do not create much acidity or basicity compared to the support. A 10% amount of sulfate increases slightly the acidity and destroys totally the weak basicity, whereas 1% has nearly no effect. The interaction between sulfate ions and silica (which has very few active sites) is very weak and can explain our lack of success in stabilizing a higher amount on the silica surface.

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Figure 8. Differential heat of ammonia and sulfur dioxide adsorption on Ni-Si samples versus the adsorbed amount.

Figure 9. Differential heat of ammonia and sulfur dioxide adsorption on S-Si samples versus the adsorbed amount.

4. Discussion The chemical properties of the three supports have been changed substantially by the presence of added ions. The strong and total acidity of the pure alumina are equal to 1.34 and 2.6 µmol‚m-2, respectively. A certain increase is observed upon addition of nickel cations in the domain of medium and weak acidity. In contrast, alkali ion (Li+) above a certain loading decreases the total acidity in all the strength domains. Basicity of modified aluminas is much more affected than acidity. On increasing the concentration of lithium, the strength (much more than the number) of the very strong basic centers increases enormously. A high concentration of nickel provokes a huge decrease of the number and strength of basic sites. The presence of sulfate ions decreases both the number and strength of the basic sites. If as reported by Chen et al.2 the dispersion capacity of a M2+O type oxide can be estimated to be around 9-9.8 M2+ ions/nm2 (which corresponds to about 15 µmol of M2+ ions/m2), the loading amount of our alumina-supported oxides is always lower than the dispersion capacity of the cations, and this amount has a strong impact on the nature of the interaction between metal oxide and support and affects mainly the strength and number of the very strong acid sites. For the higher loadings (as for Ni-Al-50, for example), the nickel ions might occupy most of the tetrahedral and octahedral sites (around 2/3) of γ-alumina. The Lewis basic sites of the amphoteric alumina, which are the exposed lattice oxygen, become unaccessible, and the basicity is totally decreased. Kania and Jurczyk21 reported that the (21) Kania, W.; Jurczyk, K. Appl. Catal. 1987, 34, 1.

Gervasini et al.

amount of chemisorbed NH3 expressed in moles of NH3/ m2 increased with the amount of NiO. They reported a reduction in the number of strong acid sites, compared to pure alumina, accompanied by an increase in the concentration of centers of moderate and weaker strength, which is in agreement with the curves plotted in Figure 2. The authors showed also that an increase from 1 to 5 wt % of NiO on alumina provoked a decrease of the number and strength of strong basic sites.22 With the same NiO on alumina system, Youssef et al.6 have taken the irreversible adsorption of various bases as a measure of the acidity. Each base measured a high total acidity but low strong acidity. The acidity and stability of alumina are strongly modified by sulfation which enhances the strength of the weakest Lewis acid sites but poisons the strongest.19 The quasi-neutral surface of silica, which has a saturated structure and a very low dispersion capacity, is strongly modified in both number and strength by the addition of nickel or lithium. So we can assume the existence of a strong metal oxide-support interaction for silica-supported systems, especially when the oxide loading is low. On magnesia, a great number of base centers of moderate or weak strength found on the surface were modified with sulfate or nickel oxide. The hydroxylation on the surface of MgO is certainly greatly affected by the second oxide loading. For Li-Mg samples, the variation in basicity can be elucidated by looking at the structural features of the Li2O system. It is known that Li+ introduced into the MgO lattice forms a solid solution, closing up the structural defects.23 This is accompanied by a sharp decrease in the specific surface area. The centers possessing a high affinity for Lewis acids are O2ions with low coordination, therefore both their number and strength must decrease sharply as the number of defects decreases,23 which is in agreement with our results for 5-10 M2O mol % upon MgO domain. In order to clarify the nature of acid-base sites of our series of samples, the infrared spectra of the hydroxyl region and that of surface chemisorbed CO2 and pyridine species have been investigated and will be published in a further article. Returning to the calculation of the specific effect of ions as described above (Table 2) and to our previous paper concerning the doping ion effect,10 it appeared to us worth checking if this parameter could also fit with acidity and basicity measurements for higher coverages than 1%. As examples, Figures 10 and 11 represent the basicity and acidity, as determined by adsorption microcalorimetry of SO2 and NH3 and related to the additive only by subtracting the contribution of the support (Table 2, Virr columns), as a function of the calculated specific effect of ions for samples with 10% and 50% loadings, respectively. Only samples Li-Mg-10 and S-Mg-50 do not fit very well with the dashed lines, certainly because the loading of these two samples (2.8 and 10.2%, respectively) is too far from the theoretical value, and in fact the S-Mg-50 acidity or basicity fits better with the other 10% samples. The fact that only the contribution of the addition is taken into account enhances the difference with the theoretical stoichiometry. Of course, for each series, this specific effect is decreased when the loading increased (see Table 2). Finally we have tried to compare our experimental results with the predictions of acidity generation on binary metal oxides of Tanabe and Seiyama. Tanabe et al.1,14 (22) Jurczyk, K.; Kania, W. Appl. Catal. 1989, 56, 253. (23) Yu.Sinev, M.; Filkova, D. G.; Yu.Bychkov, V.; Ukharskii, A.; Krylov, O. V. Kinet. Katal. 1991, 32, 157.

Modified Metal Oxide Surfaces

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Figure 12. Tanabe (a, b, c) and Seiyama (d, e, f) hypotheses for Al2O3 supported oxides. Figure 10. Specific effect of ions on basicity on 10% samples.

Figure 13. Tanabe (a, b, c) and Seiyama (d, e, f) hypotheses for MgO supported oxides.

Figure 11. Specific effect of ions on acidity on 50% samples.

have proposed the hypothesis that the acid sites on chemically mixed oxides are formed by an excess of a negative or positive charge in a model structure of binary metal oxide and that whether the charge is an excess or not, and whether it is positive or negative, the acid sites are determined by the coordination numbers C and valences V of the positive and negative elements in the model structures pictured according to two postulates: (i) The coordination numbers of a positive element of a metal oxide, C1, and that of a second metal oxide, C2, are maintained even when mixed. (ii) The coordination number of oxygen of a major component oxide is retained for all the oxygens in a binary oxide. Their hypothesis explains the mechanism of the acidity generation of binary oxides and predicts whether the acid sites will be of the Bro¨nsted or the Lewis type. If the coordination number of oxygen is the same in each component oxide, no excess or defect of charge occurs and the acid character of the catalyst is not modified. Seiyama15 has presented a different model for the acidity generation of binary metal oxides. He assumes that acidity appears at the boundary where two oxides contact. He calculated the effective charge of oxygen as the sum of the boundary charges in each of the two oxides. According to the Tanabe or Seiyama hypotheses, model structures have been pictured for our samples in Figures 12, 13, and 14, in which a, b, and c are devoted to the Tanabe model and schemes d, e, and f are reserved to the Seiyama model. Figures 12-14 summarize the calculations of the charge difference ∆ around the boundary oxygen. Using Tanabe and Seiyama’s hypotheses the charge difference, ∆ is calculated by

Figure 14. Tanabe (a, b, c) and Seiyama (d, e, f) hypotheses for SiO2 supported oxides.

∆)

(

)

VA VO NCA NCA NCO

and ∆)

(

)

VA VS + -2 NCA NCS

respectively, where V/NC is the ionic valence to coordination number ratio, and A, S, and O subscripts concern the positive element of the added oxide, the positive element of the major component oxide (support), and the negative element (oxygen) of the major component oxide, respectively. Lewis acidity is assumed to appear upon the presence of an excess of positive charge and Bro¨nsted acidity for an excess of negative charge. Despite the fact that our samples are not chemicallymixed binary oxides nor mechanically-mixed oxides but supported oxides, we searched to find which hypothesis would fit better with our own determinations of the acidity for the samples containing the highest amount of guest oxide. Indeed, the possible presence of Ni-Al-O solid solutions at the surface of the catalyst Ni-Al-50, reported by earlier studies,24 suggests that Tanabe’s model might be applicable for this sample. However, the Seiyama model, which assumes that the acidity appears at the

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the charge difference (∆) from Seiyama’s model. The more negative the charge difference is, the higher the generation of basicity in the system and the weaker the acidity.

Figure 15. Basicity determined by microcalorimetry as a function of the Seiyama charge difference.

boundary where two oxides contact, seems to be more adapted to the case of our supported oxides. Figure 15 represents the number of basic sites determined by adsorption microcalorimetry (Virr(SO2)) as a function of (24) Cimino, A.; Lo Jacono, M.; Schiavello, M. J. Phys. Chem. 1975, 79, 243.

5. Conclusion Modification of alumina, silica, or magnesia with various amounts of other oxides has a considerable influence on their acid/base properties. The change in these properties depends on the nature and amount of the modifying oxide introduced, as shown by adsorption microcalorimetry of ammonia and sulfur dioxide adsorption. Qualitatively, it has been demonstrated that there are nonlinear changes in the number and character of acid-base sites with the addition of lithium, nickel, or sulfate ions to alumina, silica, or magnesia. That is, for example, the addition of very small amounts of additives to alumina has very slight impact on the heat of adsorption and the density of acid/ base sites until there is a sudden and dramatic change when half of the surface is covered. However, the effects of modification are more pronounced on silica due to its neutral behavior. The observed acid/base site strengths and numbers have been correlated with the ion specific effect and the charge difference according to Seiyama’s hypothesis. LA960436L