Surface Color Centers on Calcium Oxide: An Electron Paramagnetic

The reactivity of all centers with oxygen to form adsorbed superoxide radical anions has .... overflow="scroll" xmlns:xocs="http://www.elsevier.com/xm...
0 downloads 0 Views 271KB Size
Download PDF
5306

Langmuir 1997, 13, 5306-5315

Surface Color Centers on Calcium Oxide: An Electron Paramagnetic Resonance Investigation Mario Chiesa, Maria Cristina Paganini, and Elio Giamello* Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via P. Giuria 9, 10125 Torino, Italy

Damien M. Murphy Department of Chemistry, University of Wales Cardiff, Cardiff CF1 3TB, U.K. Received April 4, 1997X Various types of surface color centers at the surface of high surface area calcium oxide have been investigated by electron paramagnetic resonance. The defect centers, basically consisting in surfacetrapped electrons, were generated either by UV irradiation of the solid in hydrogen (or deuterium) atmosphere or by addition to the oxide of low ionization energy metal (Mg, K) vapors. In the former case Fs(H)+ defects have been obtained and characterized by magnetic interaction between the trapped electron and the proton of a nearby surface hydroxyl. In the second case Fs+ centers and, in the case of the addition of potassium, a different center labeled Fs(K)+ and characterized by interaction of the unpaired electron and a single potassium nucleus have been observed. The reactivity of all centers with oxygen to form adsorbed superoxide radical anions has been also investigated. The structural features of the adsorbed superoxide ion have been determined in the case of the reaction between Fs(H)+ centers and O2, again on the basis of the observed magnetic interaction between the adsorbed moiety and a nearby hydoxyl group. The reported data are strictly analogous to earlier data on MgO so that the whole set of experimental results can be generalized and interpreted in terms of the global surface defective properties of the alkaline-earth metal oxides.

Introduction By definition an F center (also known as a color center) is a defect typical of ionic crystals. They essentially consist of electrons trapped in anion vacancies or Schottky defects in the solid and can usually be generated by exposure of the oxide to radiation of suitable wavelength (known as radiative coloring) or by metal addition at high temperature (additive coloring). Investigation of these centers is quite extensive and 50 years ago Wood and Joy expressed this interest nicely; “This defect is one of the simplest which can occur in ionic crystals, and in the physics and chemistry of lattice defects it occupies a position of importance roughly comparable to that of the hydrogen atom in ordinary chemistry”.1 For many years the study of F centers remained firmly within the domain of physics until the late 1960s when the surface counterparts (symbol FS) of the bulk centers (F) were discovered and explored for the first time.2-9 These surface centers were predominantly generated on alkalineearth metal oxides by exposure of the oxide to γ irradiation or UV irradiation under H2. Containing an abundance of surface stabilized electrons, the radiatively colored materials soon attracted a great deal of interest from the wider chemical community since the formation of surface radical anions by electron transfer from the F center to various adsorbates could easily be studied.9-11 Interest * Author for correspondence. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Wood, R. F.; Joy, H. W. Phys. Rev. 1946, 136, 451A. (2) Tench, A. J.; Nelson, R. L. J. Colloid Interface Sci. 1968, 26, 364. (3) Nelson, R. L.; Tench, A. J.; Harmsworth, B. J. Trans. Faraday Soc. 1967, 63, 1427. (4) Tench, A. J.; Nelson, R. L. Trans. Faraday Soc. 1967, 63, 2254. (5) Henderson, B.; Wertz, J. E. Adv. Phys. 1968, 17, 749. (6) Tench, A. J. Surf. Sci. 1971, 25, 625. (7) Nelson, R. L.; Tench, A. J. Trans. Faraday Soc. 1967, 63, 3039. (8) Nelson, R. L.; Harmsworth, B. J.; Tench, A. J. Trans. Faraday Soc. 1968, 64, 2521. (9) Nelson, R. L.; Hale, J. W. Discuss. Faraday Soc. 1971, 52, 77. (10) Lunsford, J. H.; Jayne, J. P. J. Phys. Chem. 1965, 69, 2182.

S0743-7463(97)00352-1 CCC: $14.00

in both the chemical properties and magnetic features of the surface centers became intense, almost competitive, and was actively pursued for many years.9-11 Some fundamental questions on the exact nature of the surfacetrapped electron centers have however remained unresolved. Surface color centers can also be prepared by additive coloring using alkali metal vapors at moderate temperatures. By this method the activated oxide is exposed to the vapors of low ionization energy metals; the metal atoms become ionized upon contact with the ionic surface and both the metal cations that are formed and the released electrons are subsequently stabilized by the ionic matrix. Unlike the case of radiatively colored materials, the surface F centers generated by additive coloring were only sparingly investigated in the past.12 Therefore systems prepared by doping an alkaline-earth oxide with alkali metal vapors has been actively pursued in our laboratories over the past few years in order to provide a better description on the spectroscopic and chemical properties of these previously poorly understood surface color centres.13-21 (11) Lunsford, J. H.; Jayne, J. P. J. Phys. Chem. 1966, 70, 3464. (12) Kijenski, J.; Malinowski, S. J. Chem. Soc. Faraday Trans. 1 1978, 74, 230. (13) Zecchina, A.; Scarano, D.; Marchese, L.; Coluccia, S.; Giamello, E. Surf. Sci. 1988, 194, 513 (14) Giamello, E.; Ferrero, A.; Coluccia, S.; Zecchina, A. J. Phys. Chem. 1991, 95, 9385. (15) Murphy, D.; Giamello, E.; Zecchina, A. J. Phys. Chem. 1993, 97, 1739. (16) Murphy, D.; Giamello, E. J. Phys. Chem. 1994, 98, 7929. (17) Giamello, E.; Murphy, D.; Ravera, L.; Coluccia, S.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1994, 90, 3167. (18) Giamello, E.; Murphy, D. In Radicals on Surfaces; Lund, A., Rhodes, C., Eds., Kluwer Academic Publications: Dodrecht, 1995, Vol. 13, p 147. (19) Murphy, D.; Giamello, E. J. Phys. Chem. 1995, 99, 15172. (20) Giamello, E.; Murphy, D. M.; Paganini, M. C. Colloids Surf., A 1996, 115, 157.

© 1997 American Chemical Society

Surface Color Centers on Calcium Oxide

There are several reasons for this ongoing research into surface color centers generated by both radiative and additive coloring. (i) The nature and detailed structure of these surface centers are poorly characterized. A greater understanding of the defects holding the excess electron should consequently provide new insights into problems of surface morphology and surface defectivity of ionic oxides (e.g., nature, location, and concentration of the surface anion vacancies capable of electron trapping).21 (ii) Reactivity of the surface-trapped electrons with molecules of an external gaseous phase has drawn interest. This initiates a series of chemical reactions involving electron transfer from the color center to adsorbed molecules providing new insights into the activation process of small inorganic molecules and the formation and stabilization of some otherwise very short-lived reactive radicals.18,22-24 (iii) Generating not only the “classic” surface FS color centers but also new families of surface-trapped electron species may be possible. Addition of alkali metals to the surface of MgO produces these new types of trapped electron centers which are characterized by a significant interaction between the added metal cations and suitable surface defect sites.15,16,19,20 A stimulating analogy exists between these results on MgO (concerning the reactions of alkali metal vapors with MgO)19 and the results obtained by either inclusion of alkali metals into the tridimensional framework of zeolites25,26 or dissolution of the same metals in liquid ammonia.27 (iv) Coupling experimental investigations on MgO with theoretical calculations is also possible. MgO in fact is the object of several recent theoretical investigations as a model oxide. This is due to its simple structure and morphology which have allowed detailed calculations not only on the properties of its regular surface28-30 but also on the energy and electron distribution for the various possible models of the surface color centers.21,31 The coupling of theoretical calculations with experimental data on MgO has been a fruitful approach, providing a deeper definition on the structural features of surface F centers.21,22,31 For the above mentioned reasons we have concentrated our attention in the past 5 years to the interaction of alkali and alkaline-earth metal vapors with MgO, focusing in particular on the paramagnetic and chemical properties of these centers.13-21 The present paper extends this research to provide a detailed description on the various types of surface color centers on calcium oxide, generated both by UV irradiation under H2 (or D2) and by addition of alkali (K, Na) and alkaline-earth metals (Mg). The reactivity of these trapped electrons with molecular oxygen and the structural features of the resulting adsorbed superoxide anion (as revealed through the EPR spectra) will be treated in detail. As described below, the nature (21) Giamello, E.; Paganini, M. C.; Murphy, D.; Ferrari, A. M.; Pacchioni, G. J. Phys. Chem. B 1997, 101, 971. (22) Pacchioni, G.; Ferrari, A. M.; Giamello, E. Chem. Phys. Lett. 1996, 255, 58. (23) Giamello, E.; Murphy, D.; Garrone, E.; Zecchina, A. Spectrochim. Acta. 1993, 49A, 1323. (24) Giamello, E.; Murphy, D.; Marchese, L.; Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3715. (25) Edwards, P. P.; Anderson, P. A. Acc. Chem. Res. 1996, 29, 23. (26) Anderson, P. A.; Edwards, P. P. J. Am. Chem. Soc. 1992, 114, 10608. (27) Edwards, P. P. Adv. Inorg. Chem. Radiochem. 1982, 25, 135. (28) Colburn, E. A. Surf. Sci. Rep. 1992, 15, 281. (29) Catlow, C. R. A.; Faux, I. D.; Norgett, M. J. J. Chem. C (Solid State Phys.) 1976, 9, 419. (30) Mackrodt, W. C.; Stewart, R. F. J. Chem. C (Solid State Phys.) 1979, 12, 431. (31) Ferrari, A. M.; Pacchioni, G. J. Phys. Chem. 1995, 99, 17010.

Langmuir, Vol. 13, No. 20, 1997 5307

and reactivity of the surface-trapped electron species on CaO are strictly comparable to those found on MgO. The information collected for MgO and CaO (both exhibiting the same NaCl structure) can now be generalized and interpreted in terms of the global surface defective properties of the alkaline-earth oxides. In the current study electron paramagnetic resonance (EPR) spectroscopy is used to characterize the surface color centers on CaO. This technique is one of the best methods available for examining paramagnetic states and has played practically a historic role in characterizing color centers in various systems. Although the majority of the color centers on metal-doped MgO are diamagnetic (neutral centers containing two trapped electrons, labeled FS), a sufficient fraction of them are paramagnetic (charged centers containing a single trapped electron, labeled FS+) and provide insights into the overall metal vapor-oxide interaction. Experimental Section High surface area CaO was obtained by the slow decomposition (≈16 h under a dynamic 10-4-10-5 Torr vacuum at 843 K) of high-purity CaCO3 powder. This was carried out in a special vacuum cell holding the EPR tube.18 The oxide was then activated under vacuum at 1173 K (i.e., Tdehy ) 1173 K) for 1 h to obtain a completely dehydrated material. The surface area of the activated oxide is about 70 m2 g-1. Decomposition of the carbonate was the preferred route for preparation of the high surface area CaO since decomposition of the corresponding hydroxide gave less reproducible results. Two types of experimental procedures were adopted to generate the surface color centers on activated CaO: (i) radiative coloring and (ii) additive coloring with metal vapors. (i) Radiative coloring. FS+(H) or FS+(D) color centers (see the following section for description of these centers) were generated on the surface of the dehydrated oxide as follows. Hydrogen or deuterium (≈100 Torr) was added to the sample at 298 K and the powder cooled to 77 K by placing the cell in liquid nitrogen. The sample was then exposed to a UV low-pressure mercury vapor lamp for about 1 h, with intermittent mixing of the contents in order to expose fresh CaO surface to the lamp. The excess H2 or D2 was then slowly evacuated at 298 K from the now slightly colored sample (pale blue) prior to EPR spectra recording. (ii) Additive coloring. A small ribbon or piece of the metal was added to the activated oxide under vacuum and heated to an appropriate temperature for metal vapor production (≈850870 K for Mg, 533-553 K for Na, and 413-433 K for potassium). Depending on the exposure time of the oxide to the metal vapors, the sample becomes colored to varying degrees, pale blue for short exposure times to dark blue for longer exposure times. All the metal vapor treated systems are labeled M/CaO, where M refers to the doping metal (e.g., K, Na, Mg). K/CaO indicates therefore a potassium-treated calcium oxide sample. A complete description of the identical experimental procedures adopted on metal vapor doped MgO are given elsewhere.14-20 X-band CW-EPR spectra were recorded at 298 and 77 K on a Varian E-109 spectrometer and/or a Bruker ESP 300E series spectrometer, both operating at a 100 kHz field modulation. The Varian EPR spectrometer was interfaced to a CS-EPR Stelar data acquisition station, allowing spectra recording and careful double integration of the signals. Varian pitch (g ) 2.0028) was used for g value calibration in a dual cavity of the Varian spectrometer. The EPR computer simulations were obtained using a program for a personal computer derived by the SIM14S program (QCPE 265). Optimization routines based on the Simplex minimization procedure were adopted to refine the simulations.

Results and Discussion Trapped Electron Centers on MgO. An Overview. Before describing in detail the nature and reactivity of surface color centers on CaO, it will be instructive to briefly describe the features of the analogous MgO system. It was shown in previous investigations that trapped electron

5308 Langmuir, Vol. 13, No. 20, 1997

Chiesa et al.

Chart 1

Chart 2

centers on MgO can be generated by exposure of the oxide to UV or γ radiation in the presence of H2 or alternatively by exposure of the oxide to vapors of low ionization energy metals.2-4,13-20 The former method leads to the formation of a particular type of surface center which is characterized by a magnetic interaction between the trapped unpaired electron in the surface vacancy and the proton belonging to a nearby hydroxyl group; these species are labeled FS+(H) centers or FS+(D) centers if D2 is used instead of H2.2,17,21 In the absence of a neighboring proton (i.e., γ irradiation in the absence of H2), the surface paramagnetic trapped electron center is simply labeled FS+.17 An illustration of the one-electron paramagnetic FS+ and FS+(H) centers is presented in Chart 1. On alkali metal doped MgO, both “classic” FS+ centers and a new family of surface excess electron centers are created. These new surface centers are characterized by an appreciable hyperfine interaction between the trapped electron and the parent alkali cation.15,16,19,20 They can be visualized as surface excess electron traps formed between the adsorbed alkali cation and a coordinatively unsaturated surface defect. The spatial arrangement of the adsorbed cation and the lattice cations is such that an unpaired electron can be delocalized into the space between this collective aggregate of cations.19 A schematic illustration of these surface color centers is shown in Chart 2. Materials containing surface-trapped electrons are extremely reactive and capable of reducing adsorbed molecules to the corresponding radical anion. The reactivity of the surface centers with oxygen and the consequent electron transfer to this molecule forming superoxide radical anions on MgO have been investigated in depth by EPR.18,20,22,23 The results provide detailed information not only on the surrounding surface morphology of the adsorbed anion but also on the total amount of trapped electrons present; both diamagnetic FS centers (EPR silent) and paramagnetic FS+ centers (EPR active) easily reduce O2 to O2- thereby quantifying the number of surfacetrapped electrons present. The nature of the color centers formed by UV irradiation in H2 or D2 is presented in part 1 of the present paper, the color centers formed on metal-doped CaO are presented in part 2, and the structural features of the superoxide radicals formed on the resulting CaO materials are presented in part 3. The similarities and contrasts

Figure 1. EPR spectra of FS+(H) color centers formed on (a) CaO and (b) MgO, by UV irradiation at 77 K of the activated oxides under excess H2.

between these results and those found on MgO (summarized above) will be highlighted and explained. Few studies are available in the literature concerning surface color centers on CaO, and on those that were reported, the features of the EPR spectra were never thoroughly discussed.2-5 CaO which, like MgO, exhibits the typical NaCl structure of alkaline-earth oxides, can be prepared in polycrystalline form with a surface area definitely lower than MgO (by about half). The absence of significant amounts of nonzero spin nuclei in CaO (MgO contains 10.13% of 25Mg nuclei; I ) 5/2) on the one hand reduces the amount of structural information than is usually gleaned from hyperfine/superhyperfine data but, on the other hand, simplifies the spectral features of the centers. (1) EPR Spectra of FS+(H) Centers on CaO. UV irradiation of activated CaO in the presence of H2 produces a pale blue colored powder, and simultaneously the EPR spectrum shown in Figure 1a can be observed. This spectrum is clearly analogous to that obtained using the same procedure on thoroughly dehydrated MgO (Figure 1b for comparison) and is characterized by the presence of a pair of hyperfine lines due to the H nucleus. The main difference between the two spectra is the magnitude of the doublet hyperfine structure. As evidenced below in greater detail, the spectrum in Figure 1a is a composite of different surface species and is quite reproducible. Only the relative intensities of the signals corresponding to the different species present vary slightly from one experiment to the next. UV irradiation of the CaO in the presence of D2 also produces a pale blue colored sample and the corresponding spectrum shown in Figure 2b. For comparison and clarity another spectrum obtained on the UV/H2-treated CaO is shown in Figure 2a whose features are slightly different from those of the spectrum in Figure 1a. The respective

Surface Color Centers on Calcium Oxide

Langmuir, Vol. 13, No. 20, 1997 5309

parallel to that already reported for MgO.21

H• + O2-...FS2+-CaO f OH-...FS+-CaO

Figure 2. Experimental (Exp.) and computer simulated (Sim.) EPR spectra for (a) the FS+(H) and (b) FS+(D) color centers on CaO.

computer simulations of each spectrum are also shown. The two spectra presented in Figure 2 confirm the origin of the observed doublet (about 0.4 mT, Figures 1a and 2a) on UV/H2-treated CaO as arising from a hyperfine interaction with a proton (I ) 1/2 w doublet) since it is not observed when D (I ) 1 w triplet) is employed. Since the ratio between the magnetic moments (µ) of deuterium and hydrogen is µD/µH ) 0.15, the hyperfine splitting of the deuterium triplet is thus expected to be about 0.06 mT and so not resolved in Figure 2b, being buried under the intrinsic line width of the signal. The computer simulations allow one to accurately extract the spin-Hamiltonian parameters of the surface centers from the powder EPR spectra (Table 1). The simulation indicates the presence of three different and overlapping EPR signals. The dominating species (I) exhibits axial g and A (hyperfine) tensors while species (II) has an axial g tensor but no A hyperfine tensor. The axial nature of the tensors and the high reactivity of both centers with adsorbed acceptor molecules clearly indicates that these centers are localized at the surface.14,18,20,23,24 A third minor species (III) was also evidenced from the simulation. This species has isotropic symmetry (typical of bulk trapped electrons) and based on the similarity in the g value and saturation behavior can be assigned to a subsurface electron center in analogy with that observed on MgO.17 The main paramagnetic species in Figure 2a (species I in Table 1) can be easily assigned to a surface Fs+(H) center on CaO, for a number of reasons. The symbol indicates as usual a single surface-trapped electron in magnetic interaction with the nucleus of a proton (from a nearby surface hydroxyl group). These hydroxyl groups are formed during the generation of the color centers and are not observed on MgO or CaO samples radiatively or additively colored in the absence of hydrogen. The following reaction scheme likely occurs on CaO, strictly

(1)

This involves electron transfer from the H• atom (generated in the gas phase by the UV irradiation of H2) to an empty surface anion vacancy (FS2+) with resulting stabilization of the ionized hydrogen atom on a surface oxide O2- ion as an hydroxyl group. It is this group which magnetically interacts with the trapped unpaired electron in the vacancy. Species II has the same spin Hamiltonian parameters as species I but for the absence of a superhyperfine structure with a proton. It can be assigned to the smaller concentration of surface FS+ centers which are not in magnetic interaction with a proton. As reported by Tench2,4 the g values of the color center on CaO are smaller than the free electron ge value (ge ) 2.0023) and also slightly but significantly smaller than those measured for the same center on MgO.17 This can be explained by the increased spin-orbit coupling constant (λ) for calcium, which is higher than magnesium and indicates a partial mixing of the Ca2+ orbitals in the wave function of the trapped electron. The actual interaction of an unpaired electron with an I * 0 cation can be directly evidenced and measured from the magnitude of the hyperfine interaction, as in the case of the trapped electrons on MgO which interact with 25Mg nuclei (I ) 5/2) and produce a weak hyperfine structure of six lines.17,21 No hyperfine interaction could be observed on CaO because of the negligible abundance of the I ) 7/2 calcium isotope (43Ca, 0.13%). It is the shift in the g values which therefore evidences the interaction of the electron with Ca2+. The superhyperfine interaction of the surface electrons with the hydroxyl proton is characterized by the rather large A⊥ component (A⊥ ) 0.395 mT, Table 1). This is about twice that measured for the same FS+(H) centers on MgO (A⊥ ) 0.207 mT).17,21 The measured H superhyperfine tensor can be decomposed into an isotropic and anisotropic part as follows (neglecting the opposite signs of the parallel and perpendicular component)

Aexp ) aiso +

[ ] B B -2B

(2)

From the measured A values, aproximated from those given in Table 1, this corresponds to

Aexp )

[

]

(0.395 (0.395 ) (0.27 + (0.010

[

(0.13 (0.13 -0.26

]

(3)

The 0.27 mT value of aiso (the isotropic coupling constant) is very small and thus its analysis is not straightforward. In terms of spin delocalization toward the 1s hydrogen orbital it corresponds to about 0.53% spin density (i.e., C1S2 ) aiso/A0 ) 0.27/50.6). The aiso value could then be explained by a small spin polarization contribution from the hydroxyl OH bond induced by the trapped unpaired electron. Analysis of the anisotropic coupling (B) is on the other hand far more informative. The energy corresponding to the dipolar interaction between the electron and the proton can be expressed in terms of a simple pointdipole approximation and in magnetic field units21 as

5310 Langmuir, Vol. 13, No. 20, 1997

Chiesa et al.

Table 1. Spin-Hamiltonian Parameters of the Various Paramagnetic Centers Reported in the Text

B(θ) )

gnβn(3 cos2 θ - 1) r3

(4)

where gn is the nuclear g factor (for the proton), βn the nuclear magneton, r the distance between the two dipoles, and θ the angle between the external magnetic field and the vector between the two dipoles. This corresponds, for the data derived in (3) to a distance r of 2.77 Å. This estimated value is lower than the distance between the center of the anion vacancy and the nearest hydroxyl proton which would be 3.53 Å away (assuming the Ca2+O2- distance given by the lattice parameter is 2.40 Å and the O-H distance 0.96 Å). There are two fundamentally important reasons for this apparent discrepancy. Firstly, relaxation of the surface defects can occur and this will undoubtedly result in a non-negligible perturbation to the above interionic distances.32 Secondly the electron density in the vacancy is significantly polarized by the positive charge of the proton; this was unambiguously shown by quantum mechanical calculations on the same system on MgO.21 In other words the center of the spin distribution does not exactly correspond to the geometric center of the anion vacancy but is in fact asymmetrically distributed and found to be somewhat closer to the hydroxyl group.21 On the basis of these observations it can be concluded that, whatever the detailed geometric features of the FS+(H) centers on CaO, these color centers definitely consist of an electron, trapped in a surface anion vacancy and experiencing a weak interaction with the proton of neighboring hydroxyl group. The polarizing effect of the hydroxyl group on the electron is sufficient to distort the electron spin distribution in the vacancy. Although this distortion of the spin distribution cannot be directly observed, it can be strongly inferred by comparison with the same center on MgO, where an anomalously high (32) Tasker, P. W.; Duffy, E. M. Surf. Sci. 1984, 137, 91.

hyperfine interaction between the unpaired electron and the Mg2+ ions closest to the surface hydroxyl was observed and confirmed by ab initio quantum chemical calculations.21 In that case the H superhyperfine interaction was smaller (A⊥ ) 0.207 mT) compared to that found on CaO (A⊥ ) 0.395 mT), indicating that a stronger interaction, and therefore a stronger polarization of the electron density in the trap, occurs on the latter system. The structural features and magnetic properties of the FS+(H) color centers on CaO are thus analogous to those described for MgO and suggest that these centers can be considered as excess electrons trapped in a class of surface defects common to the alkaline-earth oxides. (2) Color Centers Generated on CaO by Metal Addition. We now turn our attention to the surfacetrapped electron centers that are generated on metal vapor doped CaO. When very low levels of magnesium or potassium metal vapors are exposed to CaO, the solid immediately turns pale blue in color and a spectrum typical of surface FS+ color centers is produced. The EPR spectra of these centers are shown in parts a and b of Figure 3 for K- and Mg-doped CaO, respectively. The spectra are characterized by a narrow axial signal at g ≈ 2.000 and a line width of about 0.35 mT. Similar to that previously reported for MgO, no hyperfine structure indicative of an appreciable interaction between the released electron and the parent alkali or alkaline-earth cation was observed on K/CaO or Mg/CaO after exposure to low doses of metal vapors.17 Addition of Na to CaO also visibly colored the oxide, but in this case no EPR spectrum was ever detected, even for high doses of added metal. This suggests that the color centers formed on Na/CaO are diamagnetic. The tendency of the Na-doped system to form diamagnetic color centers was also observed on Na/MgO.17 Evidence for the existence of diamagnetic surface color centers is supported, not only by the absence of an EPR signal but also by the reactions of the Na/CaO samples with molecular oxygen

Surface Color Centers on Calcium Oxide

Langmuir, Vol. 13, No. 20, 1997 5311

Figure 4. EPR spectra of K/CaO after exposure of the oxide to higher doses of metal vapors.

Figure 3. EPR spectra of CaO after exposure to low doses of K and Mg metal vapors: (a) K/CaO and (b) Mg/MgO.

(part 3). All metal-treated CaO samples, including the Na/CaO samples, react with molecular oxygen to produce the superoxide (O2-) anions. This reaction occurs by a single electron transfer from the surface-trapped electron to a single adsorbed O2 molecule forming O2-. The color centers are thus destroyed and the sample is said to be “bleached”.23 The resulting O2- signal intensity is then indicative of the total number of surface electrons. Although no EPR signal was observed for Na/CaO, the integrated O2- signal intensity was very strong, clearly indicating the presence of surface electrons. This observation means that sodium has a clear propensity to produce exclusively diamagnetic surface color centers on CaO (in contrast to the other metals which lead to both paramagnetic and diamagnetic centers). The diamagnetic centers can most likely be described as electron pairs sharing the same surface oxygen vacancies (FS). The g values for the paramagnetic centers on K/CaO and Mg/CaO (Figure 3) are listed in Table 1. These values are typical of surface FS+ centers with the expected negative ∆g shifts (their average g value is lower than ge). In comparison with the analogous FS+ centers on K/MgO and Mg/MgO, the g values for the K- and Mg-doped CaO centers are slightly smaller. As discussed above for the ∆g shift of the FS+(H) centers, this can be explained by the higher spin-orbit coupling constant (λ) of Ca2+ in comparison with that of Mg2+, which will result in a larger ∆g shift from ge. All these results are therefore consistent with the view that the excess electrons produced by ionization of the metal atoms are trapped and stabilized in surface anion vacancies (i.e., FS2+ vacancies) on CaO. As the amount of K vapor doped onto the CaO surface is progressively increased, the above EPR spectrum of the FS+ center (Figure 3a) is replaced by a new and complex spectrum shown in Figure 4. Higher doses of Mg vapors

did not produce such a complex spectrum; in this case the FS+ signal intensity simply grew as the Mg-doping level was increased. The new EPR spectrum is best interpreted in terms of two overlapping quartets centered on the same g value and separated by ≈3.0 and ≈2.5 mT, respectively (Figure 4). A similar set of quartets was also observed on K/MgO and explained by the existence of two similar surface centers characterized by an appreciable hyperfine interaction with a surface potassium cation (39K, I ) 3/2) and assigned to a “mononuclear” Kδ+(trap)δ- species. The notation adopted tries to account for the observed fact that the electron spin density was partially localized on the 4s K orbital and partially delocalized toward a particular electron trap on the MgO surface. Some of the possible structural models to account for these new centers were discussed in detail in ref 19. Essentially this new surface electron trap on K/MgO was constructed from a particular group of lattice Mg2+ cations and the parent K+ cation with the electron been delocalized into the space between the collective aggregate of positively charged cations.19 On the basis of the similarity between the EPR spectra for the K+ quartets on MgO and CaO, the spectrum shown in Figure 4 for K/CaO may therefore also be assigned to the presence of two slightly different surface mononuclear Kδ+(trap)δ- species. For convenience a more simpler notation of the Kδ+(trap)δ- species may now be proposed. In the case of the FS+(H) color centers, as the unpaired electron maintains some residual interaction with the parent hydrogen atom (i.e., H• f H+ + e-), the Fs+(H) terminology highlights the interaction between the trapped electron (FS+) and the proton (H+) of the hydroxyl group. On the basis of this terminology the Kδ+(trap)δ- center may now be more conveniently described as FS+(K), illustrating the partial interaction of the surface trapped electron with the parent K atom (i.e., K f K+ + e-) (Chart 2). The tendency to form these new surface centers at high loadings of alkali metal vapors may be explained by the limited number of surface FS2+ vacancies available for trapping excess electrons. In other words when all available FS2+ anion vacancies are saturated and filled with the excess electrons (forming paramagnetic FS+ or diamagnetic FS centers), the extended morphological defects become increasingly more important for the stabilization of both the released electrons and the alkali cations. Reconstructed anion vacancies then appear to form, into which an extensive electron delocalization can occur, the electron then maintaining an appreciable spin localization on the parent cation. An analogy may be drawn between the chemistry of alkali metals deposited

5312 Langmuir, Vol. 13, No. 20, 1997

onto alkaline-earth oxides and the ionization of alkali metals in liquid ammonia. The latter system forms intensely colored solutions containing solvated electrons and metal cations depending on the amount of dissolved metal. The ionized electron and the parent cation exhibit different degrees of mutual interaction in the liquid ammonia depending on the metal concentration,27 starting from dilute solutions in which there is no electron-cation contact (“loose electron-ion pairs” analogous to the FS+ center case) to more concentrated solutions in which there is a relevant residual interaction (“contact ion pairs” analogous to the FS+(K) species observed on K/CaO). Spin Hamiltonian parameters for the two observed quartets on CaO (Figure 4), as derived by computer simulation, are presented in Table 1. The hyperfine structure is characterized by a slightly axial 39K hyperfine tensor with very similar parallel and perpendicular A values. From the principal values of the A tensors, both the aiso (Fermi contact term) and B (dipolar interaction) terms can be evaluated so that the total spin density on the metal cation can be calculated (i.e., spin density ) C4s2 + C4p2 ) aiso/A0 + B/B0, where A0 ) 8.25 mT and B0 ) 1.06 mT for the 4s and 4p orbital of 39K).19 This indicates a spin density on potassium of approximately 0.40 and 0.34 for the two FS+(K)a and FS+(K)b quartets, respectively. Both species, because of the slight anisotropy of the A tensor exhibit a small but non-negligible p character (about 3%) behaving similarly to the corresponding centers on K/MgO.19 Comparison of the two FS+(K) centers on K/CaO with the corresponding centers on K/MgO indicates that the electron spin density on the K+ cation is lower for the center on CaO (≈40% and 34%) compared to that on MgO (≈55% and 52%). The interpretation of such a datum is not straightforward in the absence of a more detailed model of the center (i.e., number and geometrical arrangment of the calcium ions) and of a precise knowledge of the relaxation at the surface trap. In the case of a series of well-defined homologous centers (the bulk F centers in alkali chlorides), it was observed, passing from LiCl to NaCl and KCl, that the hyperfine coupling constant measuring the electron spin density on the cations sourronding the vacancy increases with increasing the cation size.33 This was explained in terms of the progressive volume reduction of the bulk vacancy caused by the progressively larger cations. In the case discussed here the situation is more complex, and we limit ourselves to the following general considerations. The electron spin density in the case of Fs+(K) centers is mainly localized on three distinct entities which are (a) the volume of the trap, (b) the Me2+ cations (Ca2+, Mg2+) sourrounding the trap itself, and (c) the added potassium ion. The 39K spin density decrease observed passing from Fs+(K) on MgO to the analogous centers on CaO essentially means, at the present state of our knowledge, that the surface trap on CaO and the relative Ca2+ surrounding cations can, together, host a larger spin density in comparison with the corresponding entities on MgO. Some minor features in the spectrum of K/CaO (Figure 4) can also be observed and assigned to the presence of other surface-trapped electron centers. These include a minor abundance of residual FS+ centers (which are responsible for the extra lines at the center of the spectrum) and traces of a trimeric K3δ+(trap)δ- species (responsible for the additional weak lines at the spectrum center), both of which are observed on K/MgO at slightly elevated doses of added K metal vapor. (33) Ayscough, P. B. Electron Spin Resonance in Chemistry; Methuen & Co. Ltd.: London, 1967; p 386.

Chiesa et al.

Figure 5. Low-temperature (77 K) EPR spectra of the superoxide O2- anion adsorbed on CaO samples previously containing (a) surface FS+(H) centers and (b) FS+(D) centers.

A final note concerns the overall similarities between the K-doped MgO and CaO systems. On both oxides two FS+(K) centers are reproducibly formed, both having axial hyperfine tensors with a predominantly 4s character in the electron wavefunction and only a minor 4p contribution. Despite the complexity in controlling the amount of metal doped onto the surface, the sometimes tedious nature of extracting accurate spin Hamiltonian parameters from the powder EPR spectra, and the difficulties of inherent surface heterogeneity of the alkaline-earth oxide, nevertheless the reproducibility and similarities in the results on both oxides illustrate the methodological surface chemistry between alkali metal atoms and the common defect sites on the surface of polycrystalline MgO and CaO powders. (3) Reactivity of the Surface-Trapped Electron Centers with Oxygen. All of the above samples containing surface-trapped electron centers (FS+, FS+(H), or FS+(K)) react with molecular oxygen to give the superoxide radical anion (e.g., FS+ + O2 f FS2+ + O2-). The characteristic EPR spectrum of the trapped electron center is immediately replaced by the well-known EPR spectrum of the superoxide radical.22,23,34-36 The resulting O2- spectra on CaO samples radiatively colored (UV-H2 or UV-D2/CaO) or additively colored (Mg/CaO) are shown in Figures 5 and 6 respectively. The O2- anion is only visible when adsorbed on a diamagnetic surface cationic site whose electrostatic field removes the degeneracy between the two π* oxygen (34) Giamello, E.; Ugliengo, P.; Garrone, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1373. (35) Che, M.; Tench, A. J. Adv. Catal. 1983, 32, 1. (36) Giamello, E.; Garrone, E.; Ugliengo, P.; Che, M.; Tench, A. J. J. Chem. Soc. Faraday Trans. 1 1989, 85, 3987.

Surface Color Centers on Calcium Oxide

Langmuir, Vol. 13, No. 20, 1997 5313

Figure 7. Magnified and expanded spectra of the gxx and gyy region of Figure 5.

Figure 6. EPR spectrum at 77 K of O2- adsorbed on Mg/CaO.

orbitals. This results in an orthorhombic spectrum with three principle g values of gxx, gyy and gzz, the z direction being that of the O2- internuclear axis and the y direction that perpendicular to the adsorption site.34,35 The extent of the energy splitting (∆) between the two π* orbitals depends on the electrostatic field felt by the superoxide anion, which in turn determines the value of the z component of the g tensor; it is gzz which is most sensitive to the electrostatic effects.37 Neglecting second-order terms, the resulting value for gzz is

gzz ) ge +

2λ ∆

(5)

where λ is the spin-orbit coupling constant for oxygen and ∆ the energy splitting between the π* orbitals.35 The gzz value can therefore assume different values depending not only on the nominal charge of the cationic adsorption site but also on the local environment of the cation. In other words, values of gzz will distinguish between cations with the same nominal charge (e.g., M2+) which because of different coordinative environments (e.g., 5C, 4C, 3C) exert different electrostatic fields on O2- producing a heterogeneity of gzz components on the same oxide surface.23,24,38 In the case of O2- formed by electron transfer from surface color centers on MgO to molecular oxygen, the gzz value was found to vary depending on the method used to generate these color centers.22,23 Predominant gzz values of 2.091 and 2.077 were recorded for O2- radicals formed on additively colored samples (M/MgO) and radiatively colored samples (UV-H2/MgO), respectively. On the basis of the current findings a similar situation occurs on CaO. The main gzz component for the O2- radicals formed on metal vapor doped CaO is higher (gzz ) 2.098; Figure 6 and Table 1) compared to the samples irradiated under H2 or D2 (gzz ) 2.095; Figure 5 and Table 1). The overall gzz values on MgO are therefore lower compared to those found on CaO. This is due to the smaller ionic radius of (37) Kanzig, W.; Cohen, M. H. Phys. Rev. Lett. 1959, 3, 509. (38) Che, M.; Giamello, E. In Studies in Surface Science and Catalysis; Fierro, J. L. G., Ed.; Elsevier Science Publishers: Amsterdam, 1987; Vol. 57B, p 265.

the Mg2+ cations which create a higher attractive field producing a larger ∆ splitting and thus a smaller gzz value (e.g., ∆ ≈ 0.37 eV on Mg2+ with gzz ≈ 2.077; ∆ ≈ 0.29 eV on Ca2+ with gzz ≈ 2.095).35 The EPR spectra of O2- formed by reaction of O2 with color centers on CaO samples UV irradiated under H2 and D2 (parts a and b of Figure 5, respectively) are characterized by interaction of the unpaired electron of O2- with a H (or D) nucleus. The superhyperfine splitting on the x and y components are magnified and presented in Figure 7 (the magnified gzz region of the spectrum is shown in Figure 5). This interaction, which was already observed for O2- on MgO,36 is intrinsically interesting as it is quite uncommon to observe superhyperfine interactions between surface bound radicals and the nuclei of other species not directly associated with the radical or its adsorption site. In this case the proton, which is responsible for the superhyperfine interaction, is in close proximity to the O2- anion but is independent from both the O2- radical and the Ca2+ adsorption site (it comes from a separate surface group). It is most likely that this interacting proton is the same hydroxyl proton responsible for the superhyperfine interaction with the trapped electron of the FS+(H) center discussed above. This suggests that the site for O2- adsorption is close to the actual surface electron trap donating the electron and the mobility of the anion very limited after electron transfer. A structural investigation of the adsorbed O2- species may be performed based on an analysis of the superhyperfine splitting. The three principle A values of the H superhyperfine structure, directly measured from the spectrum in Figure 7, are listed in Table 1. Although the absolute sign of the various A components cannot be directly measured from the powder spectra, the sign of the largest component (Axx ) -0.450 mT) is clearly opposite to those of the other two components (Ayy ) (0.270 mT, Azz ) (0.180 mT). This has two important consequences from a structural viewpoint. (i) The structure of the superhyperfine tensor is traceless indicating that the Fermi contact term is negligible and the whole interaction can essentially be described as dipolar in nature. (ii) The structure of the tensor indicates a “T-shaped” geometry for the O2-...H system (Chart 3). This is so because no other geometrical arrangement is compatible with the situation where the superhyperfine tensors are described as Axx = |Ayy + Azz|. The tensor structure can be employed once again based on the point dipole approximation (as done above for the FS+(H) center). Chart 3 illustrated above is in fact in

5314 Langmuir, Vol. 13, No. 20, 1997

Chiesa et al.

Chart 3

Figure 8. Different schematic views of the superoxide O2- ion stabilized on a Ca2+ ion at the surface of CaO and interacting with a nearby hydroxyl. The O-O internuclear distance is 1.35 Å and the OH distance in the hydroxyl is 0.96 Å.

analogy with that previously reported for the O2-...H/MgO system, in which it was assumed that the 1 electron spin density was equally divided into two point-dipoles localized on the two oxygen nuclei.36 This latter assumption is particularly suitable for our model as the spin density of O2- is in a π* orbital made up by two lobes. The energy of the dipolar coupling in magnetic field units can again be expressed using eq 4. In particular, in the present model for O2-...H on CaO (Chart 3), the equations relating the three elements of the B tensor can be explicitely written as follows

Bxx )

gnβn (3 cos2 β - 1) r3 Byy ) -

Bzz )

gnβn r3

gnβn (3 cos2 R - 1) r3

(6)

(7)

(8)

with the following relations existing between the various geometrical parameters of the problem; R ) cos-1(d/r) and β ) π/2 - R. The O-O distance for the superoxide anion is defined as 2d. If 2d is fixed as 1.35 Å (as derived by crystallographic data), a least-squares fitting of the three experimental B parameters can be performed by varying the electron-nucleus distance r.36 The best results were obtained for a value of r ) 2.20 Å (R ) 72°) which corresponds to a B tensor of Bxx ) 0.449 mT, Byy ) -0.262 mT, and Bzz ) -0.187 mT. Taking into account the crude approximation adopted using the simplified point dipole method, these calculated B values are in good agreement with the experimentally derived values. The overall scheme which describes the model of the surface O2- anion based on the above calculation is presented in Figure 8. The O-H distance of 2.20 Å and the corresponding R and β angles implies a distance between the H and the O2- molecular axis of about 2.10 Å. This distance is considerably shorter than the lattice parameter of CaO which is 2.40 Å. Although accurate lattice distances for a surface vacancy are difficult to calculate, due to relaxation effects,32 the fact that the present estimated O-H distance is slightly shorter than the lattice parameter is not surprising considering the

strong ionic character of the O-H bond at the surface of alkaline-earth oxides.21 The partial positive charge present on the proton is probably high enough to attract the O2- anion (see Figure 8). The fact that the O-H distance found on MgO was higher (and the H superhyperfine values lower) is likely due to the fact that O2- is less tightly bound to the Ca2+ adsorption site (gzz ) 2.095) compared to the Mg2+ site (gzz ) 2.077). The attractive capability of the positively charged H is then high enough to account for the shift of O2- anion from the vertical position on top of the Ca2+ ion. No superhyperfine structure was observed for the O2anion on the Mg/CaO sample, as expected since H2 was absent in the generation of these additively colored FS+ centers. The slightly higher gzz value of O2- on Mg/CaO (gzz ) 2.098) compared to the UV-H2/CaO sample (gzz ) 2.095) is also in line with that observed on MgO.23 This was assigned to the presence of different adsorbed sites for the radical, reflecting the varying surface morphology of the differently pretreated MgO, and in this case CaO, samples. Conclusion The above results concerning the nature and reactivity of surface-trapped electrons centers on CaO are strictly comparable to earlier results obtained on MgO. UV irradiation of CaO in the presence of adsorbed H2 (or D2) results in the formation of surface FS+(H) color centers. The most distinguishing feature of these trapped electrons is their minor superhyperfine interaction with a nearby hydroxyl proton. Analysis of the EPR spectra indicates that the electron spin distribution in the surface anion vacancy is distorted due to the polarizing effect of the surface hydroxyl, in analogy with that observed on MgO. Surface-trapped electron species are also generated on CaO additively doped with metal vapors. The types of centers formed in this case are again similar to those observed on MgO, including FS+ color centers and a new family of surface-trapped electron centers labeled FS+(M), where M is the doping metal. Reactivity of the surface electrons with molecular oxygen has been examined. Careful analysis of the superhyperfine splitting from the readily formed O2- anion and the interacting proton of the nearby hydroxyl group reveals a model of the adsorbed oxygen radical as being remarkably similar to that observed on MgO. As before the strong polarizing effect

Surface Color Centers on Calcium Oxide

of the hydroxyl is sufficient to attract the negative charge of the O2- anion thereby distorting it from a vertical position on top of the Ca2+ cation. The EPR spectra of both the color centers themselves and the oxygen radicals generated from these centers have provided some detailed information for the structural investigation of the surface defect sites on MgO and CaO. In particular, the structural parameters of the FS+(H)

Langmuir, Vol. 13, No. 20, 1997 5315

centers and the O2-...H complex (revealed by the H superhyperfine interaction) will be verified and interpreted in structural terms by means of quantum chemical calculations. The results directly evidence the similarities in the common types of surface defects which are formed on thermally activated alkaline-earth oxides. LA970352S