J. Phys. Chem. B 2001, 105, 10457-10460
10457
Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide Mario Chiesa,† Maria Cristina Paganini,*,‡ Elio Giamello,‡ and Damien M. Murphy† National ENDOR Centre, Department of Chemistry, Cardiff UniVersity, P.O. Box 912, Cardiff CF10 3TB, and Dipartimento di Chimica IFM, UniVersita` di Torino e Unita` INFM di Torino, Via P. Giuria 7, 10125 Torino, Italy ReceiVed: May 14, 2001; In Final Form: August 14, 2001
Evaporation of Cs atoms onto dehydrated polycrystalline MgO leads to the formation of surface color centers in correspondence of surface point defects. EPR spectroscopy has revealed that the adsorbed Cs atoms are partially ionized, and a fraction of the electron spin density is delocalized onto a surface oxygen vacancy or trap. The observed defect can thus be written as Csδ+(trap) δ- . These results give evidence of the preferential interaction of the metal atoms with specific surface defect sites in the early stages of the metal-support interaction. The reaction of these centers with molecular oxygen leads to bleaching of the surface with formation of the O2- superoxide radical anion. A fraction of the adsorbed superoxide ions are adsorbed on “regular” Mg2+ sites while the remaining ones are adsorbed on top of Cs+ ions.
The nature of the metal-support interaction in heterogeneous catalysis has both intrigued and stimulated researchers for many years.1 To unravel the complexities of these interactions, elaborate experimental and theoretical methodologies have developed, in particular by studying the deposition of single metal atoms or small metal clusters on metal oxide thin films.2-5 These investigations have revealed that, far from being an inert host, the support can influence the electronic properties of the deposited atoms through specific interactions with surface sites, and in some cases enhanced catalytic reactivity has been observed.3,4 These studies, carried out on well-defined oxide surfaces, are extremely important because they provide clear evidence for the size-dependent catalytic activity of small clusters and the relationship with the intrinsic electronic and geometric structure of the cluster.3-6 The changes in catalytic activity have been interpreted by assuming a size-dependent interaction of the small clusters with the substrate.3,7 In other words, the morphological defects such as low coordination ions and point defects clearly play a crucial role in stabilizing metal atoms at the metaloxide interface and ultimately influencing the catalysis of the supported metal particles. Investigations into the metal-oxide interface, and the changes to the electronic structure of the supported clusters, has been explored for a number of years also over polycrystalline metal oxide surfaces. Important concepts related to this area, for instance, have been advanced by studying the interaction of low ionization energy metals (essentially alkali metals) with highly ionic polycrystalline oxides such as zeolites.8-10 Also in the case of nonporous ionic oxide like MgO the study of the early stages of the interaction between low ionization energy metals and the surface provides interesting evidence about the defects present at the surface and their reactivity. Me-MgO systems (Me ) Li, Na, K, Rb, Mg) have been investigated by our group * Author to whom correspondence should be addressed at Universita` di Torino, Dpt. Chimica IFM, Via Giuria 9, 10125 Torino, Italy. Fax: ++390116707855. E-mail:
[email protected]. † Cardiff University. ‡ Universita ` di Torino e Unita` INFM di Torino.
over the years using EPR spectroscopy, and a number of different paramagnetic centers were identified on the oxide surface depending on the amount and type of metal vapor used.11-16 At low levels of added metal vapors, it was found that the atoms are completely ionized by the ionic surface, and the released electrons are trapped or confined to surface anion vacancies producing surface F centers (labeled FS+ and FS for the paramagnetic and diamagnetic centers, respectively, on MgO). In these “dilute” conditions the charge separated pair (metal cation and ionized electron) are widely separated from each other, so that no interaction is observed between them. At higher loading of alkali metals, other unusual paramagnetic centers are formed depending on the particular doping metal used, including monomeric and trimeric metal centers. In both cases, the observed centers were identified as an aggregation of one or three partially ionized metal atoms, with appreciable delocalization of the unpaired electron onto a suitable surface vacancy situated both at planar faces and at defect sites on the oxide surface, such as steps or edges. These partially ionized atoms attracted our attention not only because of their unusual state but also because of the information which may be gained by studying these systems on the initial interactions occurring at the metal-support interface. The present contribution therefore reports the first observation of a partially ionized cesium atom on the surface of highly dehydrated MgO, describes the spin resonance properties of the surface center formed upon interaction of the metal vapors with the support and, finally, its reactivity with molecular oxygen. The polycrystalline magnesium oxide used in this study was prepared by a chemical vapor deposition method17 and kindly supplied by Prof E. Kno¨zinger (Technishe Universitat, Wien). Cesium atoms were produced by thermal decomposition of the corresponding CsN3 (ex Aldrich). The MgO powder, with a surface area of ≈300 m2/g, was thermally activated at 1173 K for 1 h under a dynamic vacuum (10-5 Torr). During this thermal activation, the metal azide was held in a separate part of the experimental cell. The two powders were subsequently mixed together (under vacuum) and heated to 673 K under
10.1021/jp0118470 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001
10458 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Letters
Figure 1. X-band EPR spectrum of Cs-doped MgO.
TABLE 1: Spin-Hamiltonian Parameters of Monoalkali Centers at the Surface of MgO and of the Cs+ O2- Superoxide Adduct center
I
%Ab
aiso/mT
Cs/MgO
7/2 5/2
100 75.5
45.1
3/2
27.8
3/2
93.1
Rb/MgO K/MgO Na/MgO
A°/mT 82.0 36.1
17.2
3/2
100
B/mT
B°/mT
Cs2%
Cp2%
g//
g⊥
ref.
-
23.2
55.0
-
2.0000 1.9970
1.9998 1.9948
This work
49
6.4 1.9965 1.9992
1.9951 1.9995
1.9992 2.0017
1.9993 2.0010
0.35
4.26
122.0 4.09 6.466
8.25
0.06
31.6
0.166
1.06 3.22
47 20.5
5 5
g Tensor O2- on Cs/MgO
16 16 14
A Tensor (mT)
gxx
gyy
gzz
Axx
Ayy
Azz
2.0068
2.0045
2.1197
1.24
1.15
0.78
dynamic vacuum to achieve complete decomposition of CsN3. The white MgO powder immediately developed a blue coloration, after contact with the Cs vapors. The resulting EPR spectrum (recorded at 77K on an X-band Bruker EMX spectrometer) is shown in Figure 1. The blue color is intrinsically related to surface paramagnetic (color) centers, because exposure of the sample to molecular oxygen results in the complete and immediate destruction of the blue color and the simultaneous formation of surface adsorbed O2- ions having a typical EPR spectrum (vide infra). The spectrum in Figure 1 is characterized by a multiplet of eight lines spread over a magnetic field range of about 300 mT. This pattern of lines is that expected for one unpaired electron interacting with a single 133Cs nucleus (133Cs; I ) 7/2; Ab. 100%). The separation between the lines increases dramatically with increasing magnetic field strength, and the eight spectral lines show remarkably different intensities. The line-widths of the individual components also increase toward the spectral wings with the concomitant decrease in line intensity. Both trends of line width and intensity have been observed previously in the case of Na/MgO 14 and Rb/MgO 16 and can be accounted for by the second-order effects in the hyperfine coupling as described by the Breit-Rabi equation.18
The spin Hamiltonian parameters of the paramagnetic center formed on Cs/MgO are listed in Table 1 (including data previously obtained for the corresponding Na/MgO, K/MgO and Rb/MgO samples). These parameters were obtained via simulation of the experimental spectrum, using different computer simulation programs, in an attempt to accurately account for the pronounced second-order effects, and included matrix diagonalization of the spin-Hamiltonian. All programs however gave an unsatisfactory fit for the exact position of the first two hyperfine lines (-7/2, -5/2), which resonate at a low field, in a region where a non negligible mixing of spin states occurs. Nevertheless the parameters listed in Table 1 for Cs/MgO (derived by simulating the intensity and position of the remaining six lines) represent a satisfactory analysis of the experimental spectrum. This has been confirmed by comparing the observed spectrum with that of metal Cs atoms confined in adamantane by the matrix isolation technique 19 which presents similar trends of both line shape and intensities and line spacing between each line. This latter value in particular is always about twice that observed in the spectrum in Figure 1. The unpaired electron density on the Cs atom can be calculated by analysis of the hyperfine coupling constants. The A value is slightly anisotropic, so that both aiso and B can be
Letters SCHEME 1
derived from the axial A tensor (A⊥)aiso-B and A//)aiso+2B), with the total spin density on Cs, expressed as CS2+CP2 (where aiso/A°)Cs2 and B/B°)Cp2). From computer simulation of the spectrum in Figure 1 the value of aiso ) 45.1 mT was determined. Besides, the B contribution is so small (about 0.1 mT) that can be neglected in the spin-density computation. Since A° for 133Cs is 82.0 mT than the electron spin density on the valence 6s orbital of 133Cs is aiso/A° ) 0.550. The center responsible for the signal in Figure 1 can thus be ascribed to a surface color-type center experiencing a significant interaction with the parent alkali metal (Cs), or simmetrically, as a Cs atom that upon contact with an ionic surface results partially ionized. The rather amazing fact is, however, the degree of this partial ionization that amounts to about half of the electron charge so that the cesium species can be seen as a sort of “emication”. These unusual findings are consistent with previous results obtained for other alkali metals doped on MgO (Na, K, Rb),14,16 in which partial ionization of the adsorbed metal has occurred.
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10459 The nature of the paramagnetic centers formed on these doped samples was discussed in previous publications,14,16 and classified as Mδ+(trap)δ- centers. This formal nomenclature emphasizes the partial ionization of the metal atom (δ+) and delocalization of the remaining electron spin density onto a suitable surface trap, such as an anion vacancy. It is well-known that the surface morphology of polycrystalline MgO contains a number of localized point defects, such as anion vacancies, in addition to larger defects such as edges, steps, and kinks. The role of the localized point defects, particularly the single anion vacancies (or monovacancies), in trapping and stabilizing excess electrons with the resulting formation of surface color centers, has been clearly established.21-23 Recently, however, a growing number of theoretical considerations have suggested that cationanion di-vacancy pairs, can also exist on the surface showing that they are also capable of electron trapping.24-26 Therefore for Cs-doped MgO, both mono- and/or di-vacancy sites, which are present at various location of the surface, allow partial delocalization of the electron density away from the parent alkali atom and onto the MgO surface. As the size of the parent atom increases, the residual spin density on the atom also increases (Table 1) due to the finite size of the surface vacancy which hosts the partially ionized unpaired electron. Two possible models of the Csδ+(trap)δ- defects are reported in Scheme 1 involving a single and double vacancy at a step of MgO (100). It has to be outlined, however, that the proposed models, though quite realistic, are not the result of a direct observation. The validity should be tested in the future by theoretical calculations in order to verify their stability and the agreement between experimental and calculated values of spin density. Upon contact with molecular oxygen the blue sample of cesium-doped MgO turns white and the new EPR spectrum reported in Figure 2 is observed at 77 K. This new spectrum can be interpreted as the superimposition of different surface
Figure 2. X-band EPR spectrum of O2- superoxide ions at the surface of the bleached Cs/MgO sample.
10460 J. Phys. Chem. B, Vol. 105, No. 43, 2001 adsorbed superoxide O2- species, and its formation indicates the total electron transfer from all surface traps. The EPR spectrum of an adsorbed O2- anion is orthorhombic with three principal values gxx, gyy, and gzz, the z direction being that of the O2- internuclear axis.27 The gzz value depends on the energy splitting (∆) between the two π* orbitals of this 13 electrons Π radical which in turn depends on the electrostatic field felt by the superoxide anion:
gzz ) ge +
2λ ∆
where λ is the spin-orbit coupling constant for oxygen and ∆ is the energy splitting between the π* orbitals.27 Three different gzz components are clearly visible in the spectrum shown in Figure 2. Two of them (gzz ) 2.091 and gzz) 2.077) are easily assigned to “classic” superoxide ions on Mg2+ differing in their local coordination.28 The third one, on the basis of its gzz value (2.119, in the range typical of monovalent cations) and of the complex 8-lines Cs hyperfine structure evidenced by the stick diagram, can be unambiguously assigned to a O2- radical anion stabilized onto a Cs+ ion. The spectrum reported in Figure 2 (a rare example of adsorbed O2- showing superhyperfine interaction with the adsorption site) thus provides the first evidence of a surface O2--Cs+ adduct. The spin Hamiltonian parameters of the Cs+O2- spectrum are reported in Table 1. The g values are very close to those observed for matrix-isolated Cs superoxide.29 Regardless of the precise nature of the electron trap in the Csδ+(trap)δ- center, the current results are of interest from two perspectives. First, the results demonstrate that partial ionization of the alkali metal atom occurs on MgO with delocalization of a fraction of electron density into a suitable point defect at the surface. Second, for low metal loading on the MgO surface, the interaction of the atoms appears to be selective toward the vacancy defects present at the surface. A similar process of atom adsorption, prior to metal nucleation into larger clusters, occurs for other types of metals (such as transition metals) interacting with oxides such as alumina.5 At low coverage, the role of surface defects in activating the supported metal 5 and both the chemisorptive and catalytic activity of the metal 3-5 is markedly influenced by the electronic perturbations induced in the atom or cluster by interaction with the defect. For these two combined reasons, the current results converge in indicating the importance of the EPR data in providing a better definition of the early steps involved in the metal-oxide interaction.
Letters Acknowledgment. This work is part of ISADORA a PRA project founded by Istituto Nazionale di Fisica della Materia (INFM). M.C. thanks the EC for a Marie Curie research Fellowship (ERBFMBICT98 3508). Funding by EPSRC for the National ENDOR Center is gratefully acknowledged. References and Notes (1) Freund, H. J. Faraday Discuss. 1999, 114, 1. (2) Sanchez, S.; Abbet, U.; Heiz, W.-D.; Schneider, H.; Hakkinen, R. N.; Barnett, U.; Landman, J. Phys. Chem. A 1999, 103, 9573. (3) Abbet, S.; Sanchez, A.; Heiz, U.; Schneider, W.-D.; Ferrari, A. M.; Pacchioni, G.; Rosch, N. J. Am. Chem. Soc. 2000, 122, 3453. (4) Ferrari, A. M.; Giordano, L.; Rosch, N.; Heiz, U.; Abbet, S.; Sanchez, A.; Pacchioni, G. J. Phys. Chem. B 2000, 104, 10612. (5) Frank, M.; Kuhnnemuth, R.; Baumer, M.; Freund, H. J. Surf. Sci. 1998, 427-428, 288. (6) Leisner, T.; Rosche, C.; Wolfe, S.; Granzer, F.; Woste, L. Surf. ReV. 1996, 3, 1105. (7) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W.-D. J. Am. Chem. Soc. 1999, 121, 3214. (8) Kasai, P. H.; Bishop, R. J., Jr. J. Phys Chem. 1973, 77, 2308. (9) Edwards, P. P.; Harrison, M. R.; Klinowski, J.; Ramadas, S.; Thomas, J. M.; Johnson, D. C. J. Chem. Soc. Chem. Commun. 1984, 982. (10) Edwards, P. P.; Woodall, L. J.; Anderson, P. A.; Armstrong, A. R.; Slaski, M. Chem. Soc. ReV. 1993, 305. (11) Zecchina, A.; Scarano, D.; Marchese, L.; Coluccia, S.; Giamello, E. Surf. Sci. 1988, 194, 513. (12) Giamello, E.; Ferrero, A.; Coluccia, S.; Zecchina, A. J. Phys. Chem. 1991, 95, 9385. (13) Giamello, E.; Murphy, D. M.; Ravera, L.; Coluccia, S.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1994, 90, 3167. (14) Murphy, D. M.; Giamello, E. J. Phys. Chem. 1994, 98, 7929. (15) Murphy, D. M.; Giamello, E.; Zecchina, A. J. Phys. Chem. 1993, 97, 1739. (16) Murphy, D. M.; Giamello, E. J. Phys. Chem. 1995, 99, 15180. (17) Kno¨zinger, E.; Jacob, K.-H.; Singh, S.; Hofmann, P. Surf. Sci. 1993, 290, 388. (18) Breit, G.; Rabi, I. I. Phys. ReV. 1931, 38, 2082. (19) Jones, R.; Howard, A.; Joly, H. A.; Edwards, P. P.; Singer, R. J. Magn. Res. Chem. 1995, 33, S98. (20) Koh, A. K.; Miller, D. J. At. Data Nucl. Data Tables 1985, 33, 235. (21) Paganini, M. C.; Chiesa, M.; Giamello, E.; Martra, G. M.; Coluccia, S.; Murphy, D. M. Surf. Sci. 1999, 421, 240 (22) Murphy, D. M.; Yacob, A.; Purnell, I. J.; Farley, R. D.; Rowlands, C. C.; Paganini, M. C.; Giamello, E. J. Phys. Chem. B 1999, 103, 1944. (23) Giamello, E.; Paganini, M. C.; Murphy, D. M.; Ferrari, A. M.; Pacchioni, G. J. Phys. Chem. B 1997, 101, 971. (24) Ojama¨e, L.; Pisani, C. J. Chem. Phys. 1998, 109, 10984. (25) D’Ercole, A.; Pisani, C. J. Chem. Phys. 1999, 111, 9743. (26) D’Ercole, A.; Ferrari, A.; Pisani, C. Submitted. (27) Che, M.; Tench, A. J. AdV. Catal. 1983, 32, 1. (28) Giamello, E.; Ugliengo, P.; Garrone, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1373. (29) Lindsay, D. M.; Herschbach, D. R.; Kwiram, A. L. Chem. Phys. Lett. 1974, 25, 175.
