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Langmuir 1997, 13, 2600-2602
Electron-Donating Property of Sodium Metal-Doped Magnesium Oxide Hiromi Matsuhashi* and Kenneth J. Klabunde† Department of Science, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040, Japan, and Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received July 1, 1996. In Final Form: March 10, 1997X The electron-donating property of Na metal-doped MgO (Na/MgO) was studied by the ESR technique. The spin concentration of (CO)nx- ((CO)2- and/or (CO)63-) radicals formed from adsorbed CO on a Na/MgO surface was much higher than that of the surface paramagnetic center formed by an electron released from sodium metal and trapped in an anion vacancy. The ESR signal intensity of the trapped electron paramagnetic center was not changed by the CO adsorption treatment, and so the trapped electron was not transferred to the adsorbate. However, the surface one-electron-donating property was increased dramatically by the trapped electron, apparently due to an inductive effect.
Introduction In recent years the study of solid acids and bases has taken on an added exciting dimension with the introduction of superacids and superbases. One of the most general ways of generating superbasicity is by doping metal oxides with zero-valent alkali metals.1-3 Upon electron donation to the lattice, generally the free electron is believed to take up residence at a defect site, such as an oxygen vacancy (Fs+ center):
However, it is unclear if the Fs+ site becomes the site of superbasicity or if this electron influences other basic sites as well. Indeed, it is known that the Fs+ center has a very strong electron-donating property. Kijenski and co-workers have studied the electron-donating properties of Na/MgO toward adsorbed tetracyanoethylene and nitrobenzene. They have also reported that catalytic activities for dehydrogenation of isopropylbenzene and hydrogenation of alkenes were most probably connected with the presence of very strong single-electron donor sites.3 In addition, Giamello and co-workers have reported that the number of O2- species formed on Li/MgO and Mg/MgO during O2 adsorption was larger than that of Fs+ sites expected.4 These results prompted us to examine CO as an adsorbate, since it is known that a variety of telomeric reduced species are formed on MgO.5-9 Herein we show that the Fs+ site is not the electron-donating site but that its presence has a great influence on other electron donor sites. * To whom correspondence should be addressed at Hokkaido University of Education. † Kansas State University. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Suzukamo, G.; Fukao, G.; Minobe, M. Chem. Lett. 1980, 585. (2) Baba, T.; Handa, H.; Ono, Y. J. Chem. Soc., Faraday Trans. 1994, 90, 187. (3) Kijenski, J.; Malinowski, S. J. Inst. Catal. Hokkaido Univ. 1980, 28, 97. (4) Giamello, E.; Ferrero, A.; Coluccia, S.; Zecchina, A. J. Phys. Chem. 1991, 95, 9385. (5) Smart, R. St. C.; Slager, T. L.; Little, H. L.; Greenler, R. G. J. Phys. Chem. 1973, 77, 1019. (6) Zecchina, A.; Coluccia, S.; Spoto, G.; Scarano, D.; Marchese, L. J. Chem. Soc., Faraday Trans. 1990, 86, 703.
S0743-7463(96)00651-8 CCC: $14.00
Figure 1. Change of the ESR signal of (CO)nx- radicals on Na/MgO. Amount of Na: 0.36 mmol g-cat-1; 1 Torr ) 133.3 Pa.
Experimental Section Magnesium oxide used in this study was prepared as follows.10 Pure MgO (Merck, analytical grade) was placed in a beaker and boiled for 1 h with distilled water to obtain Mg(OH)2, followed by drying at 373 K and powdering to 32-60 mesh. Then Mg(OH)2 was placed in a preparation apparatus made of glass. (7) Ito, T.; Sim, R.-B.; Tashiro, T.; Toi, K.; Kobayashi, H. In AcidBase Catalysis II; Hattori, H., Misono, M., Ono, Y., Eds.; Kodansha: Tokyo, 1994; p 177. (8) Morris, R.; Klabunde, K. J. J. Am. Chem. Soc. 1983, 105, 2633. (9) Giamello, E.; Murphy, D.; Marchese, L.; Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3715. (10) Matsuhashi, H.; Arata, K. J. Phys. Chem. 1995, 99, 11178.
© 1997 American Chemical Society
Letters
Langmuir, Vol. 13, No. 10, 1997 2601 and kept under high vacuum. The amount of Na doped on the MgO surface was determined by atomic absorption spectrometry. The sample was weighed and dissolved in dilute HCl solution for atomic absorption analysis. Carbon monoxide telomerization reduction to (CO)nx- was chosen as a probe reaction to estimate the electron-donating property of Na/MgO.8,11 After the catalyst preparation, CO was introduced to the ESR tube and adsorption was carried out at room temperature for 30 min. Before the ESR measurement, gas phase CO was evacuated at room temperature. The ESR data were obtained on a JEOL JES-FE1XG ESR spectrometer with 0.1 mW microwave power. The spin concentration was determined by comparing the integrated signal area of the sample with that of 2,2-diphenyl-1-picrylhydrazyl.
Results and Discussion
Figure 2. Change of the ESR signal of FA+ centers on Na/ MgO. The sample was the same as that of Figure 1.
Figure 3. Change of the spin concentration of FA+ centers and (CO)nx- radicals on Na/MgO against CO pressure. The activation temperature of MgO was 876 K. ([, b) (CO)nx- radicals; (9, 2) FA+ centers. Amount of Na: (b, 9) 0.36 mmol g-cat-1; ([, 2) 0.65 mmol g-cat-1. Thermal decomposition of Mg(OH)2 and activation were carried out in a vacuum to obtain a high-surface-area oxide. Three temperatures were chosen for the activation: 773, 873, and 973 K. Heat treatment was conducted for 3 h. After the activation, MgO was cooled to room temperature and mixed with NaN3 powder kept in another part of the preparation apparatus. The mixed powder was heated again at 673 K in vacuum until NaN3 decomposition was complete. The color of the sample turned from white to light blue-violet when a large amount of NaN3 was used. Then, the sample was transferred to an ESR tube with a breakable seal for the ESR measurement. The sample was sealed
An ESR spectrum of paramagnetic centers produced by Na/MgO is shown in Figure 1. In our study, two kinds of paramagnetic centers were observed on Na/MgO,10 which we label as FA+ and FB+. These centers were formed from the electron released from Na metal and trapped in different kinds of defect sites, probably oxygen vacancies. The oxygen vacancy of the FA+ was formed on an unstable surface,12 probably on a not well-dehydrated surface. In the case of the FB+, this may involve a newly formed vacancy generated during the Na ion adsorption process.4 The formation of the FA+ sites is very likely connected with the construction of superbase sites.10 The trapping electron in anion vacancy provides an increase of the apparent coordination number of Mg2+. As the result of this effect, the absolute positive charge of Mg2+ is increased. Further, the absolute negative charge of O2that is located at the next position of Mg2+ is also increased by this effect, resulting in O2- showing superbasicity. After the CO adsorption, the ESR signal CO polyanion radicals appeared rapidly, as is shown in Figure 1. Carbon monoxide adsorbed on MgO is converted into (CO)nx- (x is an even number).6 A small part of those changes to (CO)nx- radicals is as a result of electron transfer from surface electron-donating sites.8 From the shape and g value of ESR spectra, it was assigned to (CO)2- and/or (CO)63- radicals.8,9 It is impossible to distinguish clearly between them, because the obtained spectra were similar to those for both of them. The radical of (CO)63- is formed by one-electron transfer from the surface to (CO)62-, which is formed without electron transfer.13 The radical of (CO)2is also formed by one-electron transfer.9 Therefore it is possible to count the amount of transferred electrons by measurement of the ESR signal intensity. The signal of FA+ centers was observed on the peak of (CO)nx- radicals. Usually, a sample was allowed to stand for 1 week to obtain enough sensitivity for ESR signal detection.11 The surface one-electron-donating property of MgO was enhanced by doping of Na metal. The spin concentration of (CO)nx- radicals was much higher than that of FA+ and FB+ centers. The spin concentration of (CO)nx- radicals increased with increasing CO pressure, but that of the signal of FA+ was not changed, as is shown in Figures 2 and 3. Figure 2 shows the enlarged ESR spectra of the FA+ signal region. The spin concentration of the FA+ centers was calculated from the signal area obtained by assuming a smooth base line. An example is shown on the spectrum of 20 torr of CO adsorbed. The changes of the spin concentration of (CO)nx- radicals and FA+ centers against the CO pressure are shown in Figure 3. The spin concentration of FA+ centers was almost constant. This indicates that the FA+ centers must be formed below the (11) Klabunde, K. J.; Matsuhashi, H. J. Am. Chem. Soc. 1987, 109, 1111. (12) Lunsford, J.; Jayne, J. P. J. Phys. Chem. 1966, 70, 3464. (13) Garrone, E.; Zecchina, A.; Stone, S. J. Catal. 1980, 62, 396.
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Figure 4. Changes of the spin concentration of FA+ centers and (CO)nx- radicals on Na/MgO against Na content. Activation temperature of MgO: (9) 773 K; (b) 873 K; (2) 973 K.
surface of MgO and that the electron in the anion vacancy has an indirect interaction with CO adsorbed on the MgO surface. From these results, it is clear that the surface one-electron-donating property was increased dramatically by doping of Na metal. Figure 4 shows changes of the spin concentrations of the FA+ centers and (CO)nx- radicals formed on Na/MgO against the amount of Na. The maximum of the spin concentration of the FA+ centers was obtained around 0.20.6 mg g-cat-1 of Na. A large amount of Na provided a low spin concentration of the FA+ centers probably due to trapping of a second electron; the FA+ centers are converted into diamagnetic FA centers by the second electron.4 In contrast to that, the spin concentration of the (CO)nxradicals kept on increasing with increasing Na content, and that was found to be constant or decrease around high Na content. Small Na metal particles are known to be formed on the MgO surface as the amount of Na increases.14 The formation of Na metal particles is presumed in our experiments when a large amount of Na was doped on the surface. However, the result of Figure 4 shows that Na metal particles are not a reducing agent for the formation of (CO)nx- radicals. Giamello et al. asserted the existence of “ESR-silent” (14) Murphy, D.; Giamello, E.; Zecchina, A. J. Phys. Chem. 1993, 97, 1739.
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reducing centers on the alkali metal/MgO.4 These centers should be considered as a source of electrons. They also showed that a quick electron transfer occurred from surface F centers.9 The F centers they noticed compared to the FB+ centers in our sample form the comparison of the g value of FB+ with that reported. Indeed, the FB+ center disappeared when CO was adsorbed, as is shown in Figure 1. The FB+ centers must be another source of electrons. Small metal particles are not the source of electrons, as is discussed above. The number of all reducing centers, which are FB centers (ESR-visible and ESR-silent), could be estimated from the data of Figures 3 and 4. The amount of (CO)nx- radicals was 0.56 mmol g-cat-1 when 0.65 mmol g-cat-1 of Na was doped on MgO activated at 873 K. The degree of increase of FA+ centers was low in the range 0-0.28 mmol g-cat-1 of Na. It seems that the doped Na was mainly used for the reduction of surface OH groups in this region.10 For the formation of FA+ and FA centers, ca. 0.08 mmol g-cat-1 of Na was used (0.06 mmol g-cat-1 for the first electrons and 0.02 mmol g-cat-1 for the second electrons). Therefore, the amount of electrons of the reducing centers was less than 0.29 mmol g-cat-1. However, the amount of (CO)nx- radicals was 0.56 mmol g-cat-1, which was larger than that of reducing centers. It can be concluded that the active sites for the formation of (CO)nx- radicals were newly formed on the MgO surface by doping of Na metal: the number of those being more than 0.27 mmol g-cat-1. A relation between the amount of (CO)nx- radicals and that of the FA+ centers was observed, as is shown in Figure 4. At all activation temperatures, the amount of Na that gave the maximum activity for the CO telomerization was almost twice as much as the quantity that gave the maximum signal intensity of FA+ centers. The generation of active sites for CO telomerization is supposed to be connected with the formation of FA+ and FA centers. The effect of FA centers is larger than that of FA+ centers. The number of newly formed active sites was more than 0.27 mmol g-cat-1, as is described above. The concentration of formed active sites was much higher than that of FA+ centers (0.08 mmol g-cat-1). This means that one FA+ or FA center activates several sites around it. In conclusion, the presence of the FA+ and FA centers can exert a great influence on other electron donor sites. The data in Figures 1 and 2 suggest that the FA+ and FA centers exist mainly below the surface and then electronically influence a large number of surface sites that then become capable of (CO)nx- radical formation. These sites are probably low-coordination oxygen anions on edges or corners that are made more basic by the presence of the underlying FA+ and FA centers. Acknowledgment. K. J. Klabunde acknowledges the Army Research Office for support of this and related metal oxide surface chemistry. LA9606512
