MgO Systems by XRD, TPR, and 1H MAS NMR

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Langmuir 1999, 15, 1192-1197

Study of MgO and Pt/MgO Systems by XRD, TPR, and 1H MAS NMR Marıá Angeles Aramendıá, Jose´ Antonio Benı´tez, Victoriano Borau, Ce´sar Jimeńez,* Jose´ Marıá Marinas, Jose´ Rafael Ruiz, and Francisco Urbano Departamento de Quı´mica Orga´ nica, Universidad de Co´ rdoba, Avda San Alberto Magno s/n, E-14004 Co´ rdoba, Spain Received July 15, 1998. In Final Form: October 19, 1998 We studied changes in magnesium oxide upon rehydration under operating conditions similar to those used in the subsequent impregnation of a metal salt on its surface. The ensuing transformations were monitored by using the XRD, TPR, and 1H MAS NMR techniques. On the basis of the results, magnesium oxide (periclase phase) is rehydrated to magnesium hydroxide (brucite phase) and dehydrated back to periclase when the calcination temperature is raised to 600 °C. At this point, the transformation occurs simultaneously with changes in the textural and basic properties of the solid. Pt/MgO catalysts prepared by impregnation of magnesium oxide exhibit variable metallic and surface properties depending on whether Pt(NH3)4(NO3)2 or H2PtCl6 is used as the impregnating salt.

Introduction Magnesium oxide is a solid of high technical significance and widespread use as a refractory material. Its catalytic interest lies in its essentially basic surface character, which makes it an effective catalyst and catalyst support. Magnesium oxide is usually obtained by dehydration of magnesium dihydroxide. The reaction has been extensively studied by the X-ray diffraction technique,1-3 microscopy,4,5 and IR spectroscopy.6 Because of its low specific surface area, magnesium oxide is seldom used as a support in metal catalysts. In recent years, our group has conducted extensive research into the preparation of solids with high specific areas7-9 with a view to their subsequent use as catalysts or catalyst supports. The transformation of MgO (periclase) to Mg(OH)2 (brucite) and its reverse has been extensively studied and forms the basis for the topotactic synthesis of high surface area MgO powders.10 Magnesium oxide is also interesting because it has the ability to stabilize metals in unusual oxidation states and to avoid sintering and evaporation of the metal atoms.11,12 There are many examples where metal reactivity is affected by the use of MgO as support.13-15 Papers published over the present decade have revealed MgO-supported platinum catalysts to be (1) Ball, M. C.; Taylor, H. F. Miner. Magn. 1961, 32, 754. (2) Guilliatt, I. R.; Brett, N. H. Philos. Mag. 1971, 23, 647. (3) Terauchi, H.; Ohga, T.; Naono, H. Solid State Commun. 1980, 35, 895. (4) Gordon, R. S.; Kingery, W. D. J. Am. Ceram. Soc. 1966, 49, 654. (5) Utamapanya, S.; Klabunde, K. J.; Schlup, J. Chem. Mater. 1991, 3, 175. (6) Freund, F. B. Dtsch. Keram. Ges. 1970, 47, 739. (7) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Romero, F. J.; Navıó J. A.; Barrios, J. J. Catal. 1995, 157, 97. (8) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Porras, A.; Urbano, F. J. J. Catal. 1996, 161, 829. (9) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Porras, A.; Urbano, F. J. J. Mater. Chem. 1996, 6, 1943. (10) Volpe, L.; Boudart, M. Catal. Rev. Sci. Eng. 1985, 27, 514. (11) Schwank, J.; Galvano, S.; Parravano, G. J. J. Catal. 1980, 63, 415. (12) Driessen, J. M.; Poels, E. K.; Hinderman, J. P.; Ponec, V. J. Catal. 1983, 82, 26. (13) Pande, N. K.; Bell, T. J. Catal. 1986, 98, 7. (14) Dayte, A. K.; Schwank, J. J. Catal. 1985, 93, 256. (15) Doi, Y.; Miyake, H.; Soza, K. J. Chem. Soc., Chem. Commun. 1987, 5, 595.

highly selective toward the aromatization of alkanes.16-18 This, together with the absence of acid sitesswhich effect hydrogenolysis in the re-forming of naphthassfrom the support, makes MgO especially interesting for the obtainment of gasolines with appropriate octane numbers from petroleum. This peculiarity of Pt/MgO catalysts appears to stem from interactions between Pt and the basic sites of the support that alter the electron density of the metal. In recent years, high-resolution solid-state NMR spectroscopy has grown dramatically in use as a complement to electron diffraction techniques for the characterization of crystalline solids. Solid-state NMR spectroscopy has also become a useful tool for examining amorphous solids, which escape the X-ray diffraction technique, as it allows one to characterize the local environment of the atom or atoms that seemingly constitute active sites. Characterizing the chemical nature of surface active sites and their properties are two major goals in catalysis research that have so far been pursued using a variety of physical techniques including IR, UV, visible, and Raman spectroscopies, ESR, ESCA, EXAFS, and XRD, among others. Specifically, 1H MAS NMR spectroscopy was recently used for the structural elucidation of alumina,19-21 titanium oxide,22 vanadium oxide-aluminum orthophosphate systems,23 sepiolites,24 Mg/Ga double-layered hydroxides,25 magnesium oxide-magnesium orthophosphate systems,26 and zeolites.27-29 (16) Davis, R. J.; Derouane, E. G. Nature 1991, 349, 313. (17) Davis, R. J.; Derouane, E. G. J. Catal. 1991, 132, 269. (18) Clarke, J. K. A.; Bradley, M. J.; Garvie, J. A. L.; Craven, A. J.; Baird, T. J. Catal. 1993, 143, 122. (19) Doremieux-Morin, C.; Martin, C.; Bregeault, J. M.; Fraisard, J. Appl. Catal. 1991, 77, 149. (20) DeCanio, E. C.; Edwards, J. C.; Bruno, J. W. J. Catal. 1994, 148, 76. (21) Mastikhin, V. M.; Nosov, A. V.; Terskikh, V. V.; Zamaraev, K. I.; Wachs, I. E. J. Phys. Chem. 1994, 98, 13621. (22) Crocker, M.; Herold, R. H. M.; Wilson, A. E.; Mackay, M.; Emeis, C. A.; Hoogendoorn, A. J. Chem. Soc., Faraday Trans. 1996, 92, 2791. (23) Jhansi-Lakshmi, L.; Srinivas, S. T.; Kanta-Rao, P.; Nosov, A. V.; Lapina, O. B. Mastikhin, V. M. Solid State NMR 1995, 4, 59. (24) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Ruiz, J. R. Solid State NMR 1997, 8, 251. (25) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Romero, F. J.; Ruiz, J. R. J. Solid State Chem. 1997, 131, 78. (26) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Romero, F. J.; Ruiz, J. R. J. Solid State Chem. 1998, 135, 96.

10.1021/la9808972 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/30/1999

MgO and Pt/MgO Systems

While commercial, low surface area MgO (15-30 m2‚g-1) can be readly transformed to high surface area MgO (100300 m2‚g-1)10 by hydrating MgO and then calcining at temperatures > 400 °C, catalysts prepared from MgO typically possess low surface areas comparable to that to the starting material. The reason for this anomaly is that chloride derived from the precursor salts typically used for catalyst preparation has a detrimental effect on the surface area of the resulting MgO.30 Since the porosity and surface area are important parameters for supported metal catalyst. The primary aims of this work were to obtain a magnesium oxide with good textural properties and to compare a Pt-supported catalyst based on this oxide and obtained from a chloride salt with another prepared from a Pt precursor containing no chloride. The solids were characterized not only in textural terms but also from the results of XRD, 1H MAS NMR, CO2 chemisorption, and H2 chemisorption tests. Experimental Section Preparation of Catalysts. The magnesium oxide support was obtained by calcining commercially available magnesium hydroxide (Merck ref 105.870) at 600 °C in the air for 2 h. The solid thus obtained, designated MgOS, had a very low specific surface area, so it was rehydrated in refluxing water for 6 h. The resulting new solid, MgOW, was air-dried and calcined at 200, 400, 600, and 800 °C in the air for 2 h to obtain solids MgO200, MgO400, MgO600, and MgO800, respectively. Finally, Pt was deposited onto solid MgO600 by impregnation in a Pt(NH3)4(NO3)2 or H2PtCl6 solution containing the amount of salt required to obtain a catalyst with 1 wt % of metal. The Pt solution used in each case (10 mL/g support) was added to the support, and the mixture was stirred at room temperature for 6 h, after which the solvent was evaporated and the catalyst dried and reduced in a hydrogen stream of 500 °C at a flow rate of 50 mL/min for 1 h. The catalysts thus obtained were designated Pt/MgO(N) and Pt/MgO(Cl), in accordance with the impregnating salt used [Pt(NH3)2(NO3)2 and H2PtCl6, respectively]. Textural Properties. The textural properties of the solids (viz. specific surface area, pore volume, and mean pore radius) were determined from nitrogen adsorption-desorption isotherms recorded at liquid nitrogen temperature on a Micromeritics ASAP2010 porometer. Surface areas were calculated by the BrunauerEmmett-Teller (BET) method,31 and pore distributions were established by the Barrett-Joynet-Halenda (BJH) method (adsorption branch, cylindrical pores open on one side only and adsorbed layer thickness as calculated by the Halsey method).32 All samples were degassed at 150 °C at a pressure below 3 µm of Hg. Recording of TPR Profiles. Thermal programmed reduction (TPR) profiles were recorded on a Micromeritics TPD/TPR 2900 analyzer equipped with a thermal conductivity detector. The experimental protocol was as follows: an amount of solid between 150 and 300 mg at 4 °C was subjected to a temperature ramp of 10 °C/min up to 610 °C while a stream containing 10.6% of hydrogen in argon was passed at a flow rate of 50 mL/min. A cold trap (liquid nitrogen and isopropyl alcohol) was used to prevent reaction byproducts (mostly water) from reaching the detector. Hydrogen Chemisorption Measurements. Hydrogen chemisorption measurements were made by the dynamic pulse method,33 which had been used by our group in previous work.34,35 The experimental procedure was as follows: Once the TPR profile (27) Hunger, M.; Freude, D.; Pfeifer, H. J. Chem. Soc., Faraday Trans. 1991, 87, 657. (28) Hunger, M.; Ernst, S.; Weitkamp, J. Zeolites 1995, 15, 188. (29) Hunger, M.; Horvath, T.; Engelhardt, G.; Karge, H. G. Stud. Surf. Sci. Catal. 1985, 94, 756. (30) Leofanti, G.; Solari, M.; Tauszik, G. R.; Garbassi, F.; Galvano, S.; Schwank, J. Appl. Catal. 1982, 3, 131. (31) Brunauer, S.; Emmett, P. H.; Teller, E. J. J. Am. Chem. Soc. 1951, 60, 73. (32) Barrett, E. P.; Joyner, L. S.; Halenda, P. P. J. Am. Chem. Soc. 1964, 73, 373. (33) Freel, J. J. Catal. 1972, 25, 139.

Langmuir, Vol. 15, No. 4, 1999 1193 was recorded, the solid was maintained at the final temperature in an argon stream for 20 min in order to remove residual adsorbed hydrogen; then, the catalyst was cooled to room temperature, at which measurements were made. The catalyst weight (0.5-2.0 mg of Pt) and pulse volume used (100-250 µL of 10.6% H2 in Ar) were chosen in such a way that they would allow the insertion of several pulses before the catalyst became saturated. At that point, the solid surface was flushed by passing an argon stream at 600 °C for 20 min. This cycle was repeated as many times as required to obtain reproducible values. Carbon Dioxide Chemisorption Measurements. Recently, our group developed a procedure for determining basic sites in catalytically active solids that uses thermal programmed desorption (TPD) in combination with mass spectrometry (MS).36 The amount of chemisorbed CO2 is determined on a Micromeritics 2900 TPD/TPR analyzer. Prior to analysis, each sample is heated at 600 °C in an argon stream for 1 h. The samples used to examine the metal catalysts in this work were the same as those employed to obtain the TPR profiles and hydrogen chemisorption. Measurements were made at room temperature by passing argon containing 5% CO2 and pure argon alternately over the sample. The amount of chemisorbed CO2 was thus determined from the difference between the first adsorption peak (physisorbed plus chemisorbed CO2) and the desorption peak. The amount of physisorbed CO2 was evaluated by subjecting the sample to successive adsorptions and desorptions of CO2-containing argon. The amount of carbon dioxide adsorbed or desorbed under these conditions was considered to be physisorbed at the operating temperature. X-ray Diffraction. X-ray diffraction patterns for the solids were recorded on a Siemens D-500 diffractometer using Cu KR radiation. Scans were performed over the 2θ range from 5 to 80°. Solid-State NMR Spectroscopy. 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) experiments were carried out at 400.13 MHz (9.4 T) on a Bruker ACP-400 spectrometer using zirconia rotors. All measurements were made at room temperature. Spectra were recorded by using an excitation pulse of π/2 (5 µs) and a recycle time of 3 s. Longer recycle times had a negligible effect on signal intensity since the spin-lattice relaxation times for these solids are all shorter than 600 ms.37 An overall 1000 free induction decays were accumulated. Chemical shifts were measured relative to an external tetramethylsilane (TMS) standard. Samples were dehydrated by evacuation in a BET apparatus at 100 °C at a pressure below 3 µm of Hg overnight prior to NMR experiments. They were transferred in a nitrogen atmosphere to a moisture-free nitrogen glovebox and then loaded into the zirconia rotors. The rotors were spun at 4 kHz during measurements. 1H MAS NMR spectra were interpreted on the assumption that no atmospheric water penetrated the rotors and reached such highly hygroscopic samples. To confirm this assumption, spectra for a sample calcined at 600 °C were recorded immediately upon transfer to the rotor and several days later. The two spectra turned out to be identical and rather different from those for samples exposed to moisture, so the likelihood of water reaching the samples was negligible. The 1H background resonance from the probe itself, identified by recording the 1H MAS NMR spectrum for an empty rotor, consisted of a broad, very weak resonancesapparently a static resonance. All 1H MAS NMR spectra used and reported in this paper were corrected for such background resonance by subtracting the “empty rotor” spectrum.

Results and Discussion Table 1 shows the specific surface areas and mean pore diameters for the solids studied. As can be seen, calcining Mg(OH)2 at 600 °C to obtain MgOS caused no significant (34) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Moreno, A. Colloids Surf. 1996, 106, 161. (35) Aramendıá, M. A.; Benı´tez, J. A.; Borau, V.; Jimeńez, C.; Marinas, J. M.; Moreno, A. React. Kinet. Catal. Lett. 1997, 62, 23. (36) Aramendıá, M. A.; Borau, V.; Jimeńez, C.; Lafont, F.; Marinas, J. M.; Porras, A.; Urbano, F. J. Rapid Commun. Mass. Spectrom. 1995, 9, 193. (37) Sears, R. E. J.; Kaliaperumal, R.; Manogaran, S. J. Chem. Phys. 1988, 88, 2284.

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Table 1. Chemical Textural Properties of Magnesium Oxides and the Catalysts Obtained from Them solid

SBET (m2/g)

dp (Å)a

Mg(OH)2 MgOS MgOW MgO200 MgO400 MgO600 MgO800 Pt/MgO (N) Pt/MgO (Cl)

14 15 11 10 41 116 27 98 33

204 241 232 198 234 77 303 112 179

nb (µmol of CO2/g)b

Db (µmol of CO2/m2)c

29 ( 4

1.9 ( 0.1

3(1 54 ( 3 257 ( 1 64 ( 1 389 ( 1 11 ( 1

0.3 ( 0.1 1.3 ( 0.1 2.2 ( 0.1 2.4 ( 0.1 4.0 ( 0.1 0.3 ( 0.1

a Mean pore radius. b Total number of basic sites. c Basic site density.

Figure 2. XRD patterns for solids MgOW (A), MgO200 (B), MgO400 (C), MgO600 (D), and MgO800 (E).

Figure 1. XRD patterns for commercially available Mg(OH)2 (A) and solid MgOS (B).

changes in specific surface area. Nor did rehydration of MgOS to MgOW. On the other hand, calcining the last solid at various temperatures considerably increased its specific surface area, which peaked at 600 °C and then decreased above that temperature. Simultaneously with the increased surface area, calcination at 600 °C considerably reduced the mean pore size of the solid, even though its surface structure continued to consist largely of bulky pores. These results are consistent with previously reported findings (e.g. that hydrating MgO and subsequently calcining it increases its specific surface area).38 Regarding basic properties, raising the calcination temperature for the rehydrated sample to 600 °C increased its basicity; above that temperature, however, the solid basicity started to decrease. A comparison of surface basic site density, defined as the number of micromoles of CO2 chemisorbed per square meter of surface area, reveals that results were similar among temperatures; however, calcination at 800 °C, which led to a simultaneous decrease in specific surface area and basicity, resulted in no significance decrease in surface basic site density. Therefore, the rehydration treatment yields a new magnesium oxide with excellent chemical textural properties that make it highly suitable for use as a metal catalyst support. Figure 1 shows the X-ray diffraction patterns for the starting, commercially available Mg(OH)2 and the product of its calcination at 600 °C (MgOS). As can be seen, the commercially available hydroxide (Figure 1A) exhibits a brucite-like structure (2θ ) 38.1, 18.6, 50.9, 58.7, 62.1, 68.3, 72.1, 32.9°). Calcination of this hydroxide at 600 °C produces MgOS, the XRD patterns for which (Figure 1B) (38) Holt, T. E.; Logan, A. D.; Chakraborti, S.; Dayte, A. K. Appl. Catal. 1987, 34, 199.

suggests that the solid consists exclusively of highly crystalline periclase magnesium oxide (2θ ) 43.0, 62.4, 78.7, 37.0, 74.8°). In previous work, we showed that calcining Mg(OH)2 at this or higher temperatures gives a periclase magnesium oxide of high crystallinity.26 Figure 2 shows the XRD patterns for solid MgOW and the products of its calcination at various temperatures. The first salient finding is that, following rehydration, the magnesium oxide becomes magnesium hydroxide with a brucite-like structure (Figure 2A). This phenomenon, known as a “memory effect”, has been previously demonstrated in hydrotalcite-like compounds.39 When the solid is calcined at 200 °C, it preserves its brucite Mg(OH)2 structure (Figure 2B). However, when the calcination temperature is raised to 400 °C (Figure 2C), the brucite layers are thoroughly dehydroxylated and a periclase magnesium oxide that undergoes no appreciable changes in its crystal structure on calcination at 600 or 800 °C (Figure 2D,E, respectively) is obtained. This is consistent with previous results of Mackenzie et al.,40 who showed Mg(OH)2 to be transformed into MgO with no intervening phase over the temperature range 350-400 °C. Figure 3 shows the 1H MAS NMR spectra for the starting Mg(OH)2 and the product of its calcination at 600 °C (MgOS). As can be seen, brucite Mg(OH)2 (Figure 3A) exhibits two signals centered at 1.6 and 4.5 ppm, in addition to several spinning sidebands. On the basis of its downfield shift, the signal at 4.5 ppmsthe strongerscan be assigned to OH groups in Mg(OH)2, where brucite layers are still hydroxylated. According to Brunet and Schaeller,41 the signal at 1.6 ppm can be assigned to hydroxyl groups bonded to the second layer of magnesium atoms. Mastikhin et al.42 show the plot of δ values of a variety of oxide and zeolite catalysts vs their proton affinities. This show the general trend of the chemical shift increase while going from the less to more acidic OH. Therefore, on the basis of the previous chemical shifts for Mg(OH)2, the OH groups in this solid have a scarcely basic character. This is consistent with the results of Table 1, which suggest that the solid adsorbs no CO2, i.e., that it lacks basic sites. Therefore, both NMR signals can be assigned to acid OH groups. As shown by the XRD patterns, when Mg(OH)2 (39) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 173. (40) Mackenzie, K. J. D.; Meinhold, R. H. Thermochim. Acta 1993, 230, 39. (41) Brunet, F.; Schaeller, T. Am. Miner. 1996, 81, 385. (42) Mastikhin, V. M., Colloids Surf. A: Physicochem. Eng. Aspects 1993, 78, 143.


Langmuir, Vol. 15, No. 4, 1999 1195

Figure 3. 1H MAS NMR spectra for commercially available Mg(OH)2 (A) and solid MgOS (B). Asterisks denote spinning sidebands.

is calcined at 600 °C, it becomes periclase magnesium oxide, the 1H MAS NMR spectrum for which (Figure 3B) exhibits two main signals centered at 4.6 and 0.2 ppm; the latter signal, which is also the stronger, corresponds to “basic” OH sites judging by its shift.43 The signal at 4.6 ppm suggests the presence of hydrogen bonds in some OH groups, which, like those in brucite, are of acid character. Figure 4 shows the NMR spectra for rehydrated magnesium oxide (MgOW) calcined at different temperatures. As previously observed from the XRD patterns, the water treatment causes a structural rearrangement that leads back to brucite Mg(OH)2 (Figure 4A). When the solid is heated at 200 °C, it preserves its brucite structure (see signals at 4.5 and 1.6 ppm in Figure 6B); i.e., no dehydroxylation between layers takes place up to that temperature. At 400 °C, the solid becomes periclase magnesium oxide, the spectrum for which is similar to that of Figure 3Bsonly that signals are slightly shifted upfield. It exhibits an additional, weaker signal at 4.3 ppm. When the calcination temperature is raised to 600 °C, the 1H MAS NMR spectrum (Figure 4D) contains a broad signal in the δ region from 10 to -4 ppm, with peaks at about -1.8, 1.0, and 4.5 ppm; this is in contrast with the spectrum for solid MgOS (Figure 3B). Therefore, judging by the shielded signal at δ ) -1.8 ppm, OH groups in this solid are more basic than in the previous ones, which confirms the results of the basicity measurements based on CO2 chemisorption. Also, the signal at 4.5 ppm is stronger than in the previous solids, which is consistent with a larger population of acid OH groups. If heating continues up to 800 °C (Figure 4E), the number of OH groups is considerably decreased, as clearly shown by the signal-to-noise ratios for the spectra of Figure 4D,Esthe (43) Bond, G. M.; Konig, P. J. Catal. 1982, 77, 309.

Figure 4. 1H MAS NMR spectra for solids MgOW (A), MgO200 (B), MgO400 (C), MgO600 (D), and MgO800 (E). Asterisks denote spinning sidebands.

downfield signal is the strongest in the latter, so the Bro¨nsted basicity of the solid is lower relative to the previous ones. This decreased basicity was also inferred from the CO2 chemisorption results (see Table 1). After the magnesium oxide support was characterized, it was impregnated with the metal salt precursors studied. For this purpose, solid MgO600 was chosen as it was that

1196 Langmuir, Vol. 15, No. 4, 1999

Figure 5. XRD patterns for the precatalysts obtained from Pt(NH3)4(NO3)2 (A) and H2PtCl6 (B).

Aramendıá et al.

Figure 7. TPR profiles for catalysts Pt/MgO(N) (A) and Pt/ MgO(Cl) (B).

Figure 6. XRD patterns for catalysts Pt/MgO(N) (A) and Pt/ MgO(Cl) (B).

exhibiting the most suitable chemical textural properties among the magnesium oxide systems studied. As stated in the Experimental Section, we used two different platinum salts as metal precursors. Figure 5 shows the XRD patterns for the precatalysts following impregnation with both metal salts in aqueous solutions. As expected, both resulting solids exhibited a brucite-like structure, consistent with previous findings in Pt catalysts supported on hydrotalcite-like compounds.44 The transformation is complete in the precatalyst obtained from chloride; on the other hand, the solid obtained from the nitrate salt contains residual periclase. Figure 6 shows the XRD patterns for the two catalysts after reduction in a hydrogen stream at 500 °C. The pattern for the catalyst obtained from Pt(NH3)4(NO3)2, designated Pt/MgO(N), is consistent with the presence of both brucite and periclase phases (Figure 6A). On the other hand, the pattern for catalyst Pt/MgO(Cl) (Figure 6B) exhibits the diffractions for periclase only; this catalyst is more crystalline than the previous one. These results show that Pt(NH3)4(NO3)2 affects the crystallization and decomposition of brucite, into which the support is transformed during the impregnation treatment (Figure 5); as a result, the conversion into periclase is still incomplete at 500 °C. The chemical textural properties of the catalysts differ from those of support MgO600 (Table 1). Thus, catalyst Pt/MgO(N) possesses a slightly lower specific surface area (44) Alvarez, W. E.; Resasco, D. E. J. Catal. 1996, 164, 467.

Figure 8. 1H MAS NMR spectra for catalysts Pt/MgO(N) (A) and Pt/MgO(Cl) (B). Asterisks denote spinning sidebands.

than its parent support. However, it contains more basic sites and at a higher density. These results show that treatment with the nitrate salt produces new basic sites. On the other hand, treatment with hexachloroplatinic acid causes a dramatic decrease in both specific surface area and the number of basic sites; the resulting catalyst possesses the bulkier pores. Figure 7 shows the TPR profiles for the two Pt/MgO systems. That for catalyst Pt/MgO(N) (Figure 7A) contains two groups of reduction bands. One exhibits a main band centered at 221 °C and accompanied by two shoulders at 157 and 164 °C; the other consists of a single band centered at 421 °C. On the other hand, solid Pt/MgO(Cl) exhibits three broad bands centered at 184, 329, and 443 °C (Figure 7B). In the former case, both bands can be assumed to be due to the reduction of the same species (Pt2+), interacting in a different way with the support. The profiles reveal that the reduction of the metal phase is decisively influenced by the interaction of the precursor complex with the support. Similar results were previously obtained by our group using other supports.34 Some of the bands in the Pt/MgO(Cl) spectrum might be due to the reduction of Pt4+ to Pt2+.


The H2 volumes measured by the pulse method were used to calculate the H/Pt atom ratio, following a previously reported procedure.34,35 The results ranged from 0.41 to 0.46 for Pt/MgO(N) and from 0.16 to 0.19 for Pt/MgO(Cl). On the basis of them, the latter catalyst exhibits less marked dispersion of the metal on the support, which can be ascribed to Pt being encapsulated by sintered MgO particles. Solid Pt/MgO(Cl) undergoes partial encapsulation in the MgO support because the support is partially redissolved during impregnation owing to the highly acid pH provided by H2PtCl6. On evaporation to dryness, the metal and support are redeposited together so the catalyst exhibits low dispersion. On the other hand, the impregnating solution used to obtain solid Pt/MgO(N), Pt(NH3)4(NO3)2, provides an alkaline pH, so the support is not redissolved at all but rather thorougly impregnated. This results in increased dipersion relative to Pt/MgO(Cl) solid and hence in a catalyst featuring a high hydrogen uptake.

Langmuir, Vol. 15, No. 4, 1999 1197

The 1H MAS NMR spectra for these two catalysts (Figure 8) exhibit two main signals centered at about -1.0 and 4.5 ppm that suggests decreased basicity of OH surface groups relative to support MgO600, which exhibits a single signal at -1.8 ppm. Also, the signal at -1.0 ppm is broad in both spectra, so the OH surface groups vary widely in strength. As in magnesium oxide, the signal at 4.5 ppm can be assigned to OH groups involved in hydrogen bonds and hence of an acid character. Acknowledgment. The authors wish to express their gratitude to Spain’s DGICyT for financial support awarded for the realization of this work as a part of Project PB920816 and to the staff of the Nuclear Magnetic Resonance Service of the University of Co´rdoba for their invaluable assistance in recording the NMR spectra. LA9808972

MgO Systems by XRD, TPR, and 1H MAS NMR

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