10350
2009, 113, 10350–10353 Published on Web 05/21/2009
Generation of O- Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water? Sergei E. Malykhin, Alexander M. Volodin, Alexander F. Bedilo,* and George M. Zhidomirov BoreskoV Institute of Catalysis SB RAS, NoVosibirsk 630090, Russia ReceiVed: May 6, 2009
O- radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ ) 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450 °C, this process is initiated by selective excitation of 3-coordinated complexes [Mg2+-O2-]3C containing chemisorbed water. A new mechanism for generation of the O- radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an •OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH- · · · Mg2+-O-]3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O-3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required. Long-distance separation of charges or radical fragments is a necessary but poorly understood stage of various thermal and photostimulated processes resulting in the formation of ion radicals on the surface of oxide materials. Such processes include the formation of ion radicals after adsorption of aromatic compounds1-3 and reduction of nitroaromatics3,4 on the acceptor and donor sites of oxide systems as well as photostimulated formation of oxygen radical anions under illumination into the surface absorption band of oxide dielectrics.3,5-7 A common feature of all such processes is the generation of isolated paramagnetic species with S ) 1/2 stabilized on the surface, which can be detected by EPR. It has been reliably established that O- radical anions can be formed as hole sites during illumination of MgO.3,5-7 Two types of radical species can be simultaneously observed in the presence of oxygen: O2- radical anions and [O- · · · O2] complexes formed on electron and hole sites, respectively. The suggested mechanism of their formation includes light absorption by lowcoordinated (LC) surface structures [Mg2+-O2-]LC followed by long-distance separation of charges and stabilization of the radical anions.3,5-7 It is important to note that the generation and decay of the excited state of the surface complex [Mg+-O-]LC* are local processes that are not accompanied by charge separation. The possibility of charge separation with participation of the conductivity or valence bands is very low for such dielectric material as MgO. A viable alternative for the charge separation mechanism is a “chemical” mechanism ascribing the generation of the ion radicals to the formation of mobile radical species. Radicals •H and •OH formed from the fragments of chemisorbed water H+ and OH-, respectively, are the most likely candidates for such species. A similar mechanism related to migration of •H radicals * To whom correspondence should be addressed.
10.1021/jp9042123 CCC: $40.75
accounts for the formation of electron-rich sites during UV illumination of MgO in the presence of hydrogen.8,9 Meanwhile, chemisorbed water is the most likely cause for the presence of hydroxyl groups on the MgO surface.10-12 However, their participation in the formation of ion radicals is not discussed in the literature. In the current paper we studied the formation of O- radical anions under illumination with monochromatic UV light on partially hydroxylated MgO surface dehydrated at moderate temperature 450 °C. All of the experiments were carried out using the EPR “in situ” technique discussed in detail in previous publications.3,13 The MgO sample used in the study was prepared by MgCO3 decomposition under evacuation at 600 °C and had the surface area of 80 m2/g. The EPR spectra registered after the MgO illumination with monochromatic light of different wavelengths under vacuum and in the O2 atmosphere are presented in Figure 1. If oxygen is present, the illumination results in the formation of [O- · · · O2] complexes.5-7 They were destroyed by evacuation at 298 K. The major species observed by EPR after this procedure are O- radical anions with g⊥ ) 2.042, g|) 2.002 (Figure 1a). Based on the previously reported data,5-7 this signal can be attributed to three-coordinated radical anions O-3C. The maximum concentration of O-3C radical anions in our experiments was about 5 × 1017 g-1. Taking into account the results on the MgO dehydration at various temperatures,10-12 there is no doubt that all threecoordinated oxygen atoms at the corners of cubic MgO nanoparticles activated at 450 °C are covered with hydroxyl groups. Meanwhile, it is known that such hydroxylation shifts the absorption band of 3-coordinated surface complexes [Mg2+-O2-]3C observed at λ ) 280 nm to longer wavelengths.12,14 So, light with λ ) 280 nm and, especially, λ ) 303 nm will be absorbed by the 3-coordinated complexes containing adsorbed 2009 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10351
Figure 1. (a) EPR spectra obtained after illumination at 298 K using monochromatic light under vacuum (dashed lines) and in 0.1 Torr O2 (solid lines) followed by evacuation for 30 min and cooling to 100 K. (b) EPR spectrum observed after illumination under vacuum at 100 K.
hydroxyl groups. This primary process corresponds to the formation of oxygen radical anions O-3C presented in Figure 1a. The illumination of the initial MgO sample under vacuum at 100 K by light with λ ) 280 nm results in the appearance of a narrow (∆Hp-p ∼1 G) singlet EPR signal with g ) 1.999 (Figure 1b). The signal is similar to the one observed for electron-rich sites generated during the photoreduction in hydrogen of MgO sample activated at not very high temperature.15 Also note simultaneous appearance of weak components with g ∼ 2.03-2.05 typical for g⊥ of the radical anions O-3C. All paramagnetic species formed in the processes discussed above have well-resolved and relatively narrow EPR lines. The average distance r between paramagnetic sites in powder samples with randomly oriented paramagnetic sites in cubic lattice can be estimated from the width of the EPR lines ∆H using formula ∆H ∼ 4µe/r3.16,17 In most cases the width of the individual lines in the EPR spectra of photoinduced sites does not exceed 0.5-2.0 G. Using these values the distance between the paramagnetic particles can be estimated as r > 2-4 nm. For nanocrystalline MgO samples this distance is comparable with the dimensions of the primary nanocrystals.18,19 Other possible mechanisms of line broadening16 that probably predominate imply that the actual distance is even larger. The highest concentrations of oxygen radical anions observed on the MgO surface in all known cases do not exceed 1017-1018 cm-3. Meanwhile, the concentration of cubic MgO nanocrystals with dimensions 3-4 nm is about 1019 cm-3. This means that on the average less than one ion radical is stabilized in a single nanocrystal. So, the ion radicals formed during illumination in the absorption band of [Mg2+-O2-]LC complexes separated by r > 2-4 nm are likely located in different nanocrystals. Moreover, the separation of the radical fragments to different nanoparticles may be the major factor preventing their recombination. The mechanism suggested in this paper is based on the model of electron-rich sites formed during photoreduction of the MgO
surface due to adsorption of •H radicals.9 Meanwhile, it is obvious that the presence of hydrogen is not required for generation of •H radicals. They can also be formed during thermal or photo dissociation of chemisorbed water. The second fragment that would be formed in such process is an •OH radical. Therefore, the latter can be rather naturally related to stabilization of hole sites. Thus, the formation of electron [Mg+-O2· · · H+]LC and hole [OH- · · · Mg2+-O-]LC radical sites observed on the MgO surface after illumination can be related to the longdistance separation of •H and •OH radicals resulting from homolytic dissociation of chemisorbed water. The suggested scheme of the process is presented in Figure 2. The initial state of a system consisting of two MgO nanocrystals is shown in the left side of the scheme. The upper nanoparticle is partially hydroxylated with a proton stabilized at the corner oxygen atom and a hydroxyl anion stabilized on the neighboring magnesium atom. The lower nanoparticle acts as a hydrogen atom acceptor. The final state of the system with separated electron and hole centers localized on different particles is schematically shown in the right side of the scheme. Thus, this process corresponds to the homolysis of chemisorbed water followed by long-distance separation of the •H and •OH radicals. The •H radical leaves to form an electron site at a remote place, whereas the surface structure [OH- · · · Mg2+-O-]3C remaining after its detachment consists of the •OH radical stabilized near the corner oxygen atom. The imminent local charge transfer generates a hole O-3C stabilized by a nearby hydroxyl group (Figure 2). It was important to make sure that such a structure is stable and its properties are qualitatively similar to those of the oxygen radical anion O-3C. This structure was simulated within Gaussian 03 program20 using B3LYP hybrid DFT functional and 6-31G(d) basis set. A cluster consisting of 9 magnesium and 9 oxygen atoms was used as a model of an MgO nanoparticle. Figure 3 presents the optimized structures of the structures discussed in the paper: the original Mg9O9 cluster (C), cluster with chemi-
10352
J. Phys. Chem. C, Vol. 113, No. 24, 2009
Letters
Figure 2. Scheme of hydrogen atom migration resulting in stabilization of electron and hole sites. Blue spheres, O atoms; red spheres, Mg atoms.
Figure 3. Optimized structures of Mg9O9 cluster (C), same cluster with chemisorbed water (A), radical resulting from detachment of a hydrogen atom (B), and the same cluster with additional hydrogen atom stabilized on the edge (D).
sorbed water (A), the radical resulting from the detachment of the hydrogen atom from the corner oxygen atom (B), and electron-rich site resulting from adsorption of a hydrogen atom on the edge oxygen atom of the cluster (D). The optimized geometries, Mulliken charges and spin densities of all species are reported in Tables S1-S4. In the radical (B) the three-coordinated oxygen atom deviates from its initial position, and most of the spin density (0.95) is localized on it. Thus, such structure appears to reproduce the main characteristics of the radical anions O-3C. Detailed discussion of its spectroscopic parameters and reactivity will
be reported later. The charge of the hole is compensated by the nearby hydroxyl group. The overall structure is neutral. So, no long-distance electron transfer is required. Now let us briefly discuss the energetics of the process. The easiest way is to compare the calculated energies of the species shown in Figure 3. The overall process corresponds to the formation of two radicals presented in the right part from the two neutral species shown in the left part. The calculated energies are 2555.3284, 2478.8540, 2554.6503, and 2479.4152 au for species A, C, B, and D, respectively. So, the overall energy required for reaction A + C ) B + D is 0.1169 au or
Letters 3.2 eV. This simple estimation suggests that the energy of absorbed light equal to 4.1 eV for λ ) 303 nm should be sufficient to carry out this reaction. Further studies are required to determine the possible reaction pathway. In conclusion, let us emphasize that under real experimental conditions the surface of oxides always contains some chemisorbed water. So, the model for the generation of ion radicals on a partially hydroxylated oxide surface due to homolytic dissociation of chemisorbed water may adequately describe the mechanisms of many thermal and photostimulated processes resulting in stabilization of ion radicals on oxide surfaces. Acknowledgment. Financial support by RFBR (Grant 0603-72251-CNRSL) is acknowledged with gratitude. We are also grateful to Professor G. I. Panov for valuable discussions. Supporting Information Available: Tables S1-S4 with optimized geometries, Mulliken charges, and spin densities. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Garcia, H.; Roth, H. D. Chem. ReV. 2002, 102, 3947–4008. (2) Bedilo, A. F.; Volodin, A. M. Kinet. Catal. 2009, 50, 314–324. (3) Volodin, A. M.; Bolshov, V. A.; Konovalova, T. A. Mol. Eng. 1994, 4, 201–226. (4) Morris, R. M.; Klabunde, K. J. Inorg. Chem. 1983, 22, 682–687. (5) Volodin, A. M. React. Kinet. Catal. Lett. 1991, 44, 171–177. (6) Volodin, A. M. Catal. Today 2000, 58, 103–114. (7) Sterrer, M.; Berger, T.; Diwald, O.; Kno¨zinger, E.; Allouche, A. Top. Catal. 2007, 46, 111–119. (8) Tench, A. J.; Nelson, R. L. J. Colloid Interface Sci. 1968, 26, 364– 373. (9) Chiesa, M.; Paganini, M. C.; Spoto, G.; Giamello, E.; Di Valentin, C.; Del Vitto, A.; Pacchioni, G. J. Phys. Chem. B 2005, 109, 7314–7322. (10) Coluccia, S.; Marchese, L.; Lavagnino, S.; Anpo, M. Spectrochim. Acta 1987, 43A, 1573–1576.
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10353 (11) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. J. Phys. Chem. 1996, 100, 12142– 12153. (12) Bailly, M.-L.; Costentin, G.; Lauron-Pernot, H.; Kraft, J. M.; Che, M. J. Phys. Chem. B 2005, 109, 2404–2413. (13) Bolshov, V. A.; Volodin, A. M.; Zhidomirov, G. M.; Shubin, A. A.; Bedilo, A. F. J. Phys. Chem. 1994, 98, 7551–7554. (14) Muller, M.; Stankic, S.; Diwald, O.; Knozinger, E.; Sushko, P. V.; Trevisanutto, P. E.; Shluger, A. L. J. Am. Chem. Soc. 2007, 129, 12491– 12496. (15) Moreira, I. P. R.; Wojdel, J. C.; Illas, F.; Chiesa, M.; Giamello, E. Chem. Phys. Lett. 2008, 462, 78–83. (16) Wertz, J. E.; Bolton, J. R. Electron Paramagnetic Resonance. Elementary Theory and Practical Applications; McGraw-Hill: New York, 1972; Chapter 9. (17) Raitsimring, A. M.; Salikhov, K. M. Bull. Magn. Res. 1985, 7, 184– 217. (18) Richards, R.; Li, W.; Decker, S.; Davidson, C.; Koper, O.; Zaikovski, V.; Volodin, A.; Rieker, T.; Klabunde, K. J. J. Am. Chem. Soc. 2000, 122, 4921–4925. (19) Mishakov, I. V.; Bedilo, A. F.; Richards, R. M.; Chesnokov, V. V.; Volodin, A. M.; Zaikovskii, V. I.; Buyanov, R. A.; Klabunde, K. J. J. Catal. 2002, 206, 40–48. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003.
JP9042123
