J. Phys. Chem. C 2011, 115, 751–756
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Identification and Distribution of Surface Ions in Low Coordination of CaO Powders with Photoluminescence Spectroscopy Hugo Petitjean,†,‡,§ Jean-Marc Krafft,†,‡,§ Michel Che,†,‡,§,| Helene Lauron-Pernot,†,‡,§ and Guylene Costentin*,‡,§ Laboratoire de Re´actiVite´ de Surface, UPMC, UniVersity Paris 06, UMR 7197, F-75005 Paris, France., Laboratoire Re´actiVite´ de Surface, CNRS, UMR 7197, F-75005 Paris France, and Institut UniVersitaire de France ReceiVed: October 25, 2010; ReVised Manuscript ReceiVed: December 2, 2010
Oxide ions in low coordination (O2-LC) play a major role in adsorption and catalysis properties of CaO powders. They also give specific photoluminescence features that could be used to characterize the distribution of these ions at the molecular level, but up to now, the photoluminescence of defective CaO powders was assigned incompletely. In this study, photoluminescence spectra of defective CaO powders prepared in various ways are recorded at 77 K with unprecedented resolution. A new feature is thus evidenced with an excitation at 320 nm and an emission at 490 nm. Combining infrared and photoluminescence spectroscopies, we show that this feature does not originate from remaining hydroxyl groups, but it completes the set of photoluminescence fingerprints of O2-LC ions on CaO. A revisited assignment of this set is proposed, consistent with the dependence of transition energy on coordination numbers. An application example shows how the photoluminescence spectroscopy can be used as an original tool to compare surface morphologies of CaO samples at the molecular level and under in situ conditions. 1. Introduction Oil chemistry has mainly used acidic supports for catalysis. Now, biomass chemistry develops many reactions involving oxygen-rich molecules (e.g., transesterification or etherification), which often require basic heterogeneous catalysts. Even though alkaline-earth oxides (AEOs) have been widely used in fine chemistry for their basic catalytic properties,1 the exact nature of the active site (oxide ions in low coordination (O2-LC), surface hydroxyl, or partially dehydroxylated site) is still a matter of investigation. For the establishment of structure-activity relationships on AEOs, characterization methods need to be developed in situ and at the molecular level. On AEOs, photoluminescence spectroscopy is a highly sensitive technique that can provide information on oxide ions in low coordination (O2-LC)2-6 and, accordingly, on the surface morphology at the molecular level. All the AEOs have the same electronic structure of valency and crystallize according to NaCl structure. AEOs surfaces mainly exhibit (100) planes, with mostly ions in 5-fold coordination. Highly divided AEOs also exhibit defects such as steps, corners and kinks, with ions in 4-fold or 3-fold coordination. O2-LC ions exhibit photoluminescence features that directly depend on the coordination number. The distribution of O2-LC ions leads to composite spectra. The shape and intensity of their components can be used to characterize the distribution of O2-LC ions, but only if each photoluminescence component is assigned accurately. On MgO, excitation and emission phenomena were thoroughly investigated to determine the couple of wavelengths (λEXC, λEM) associated with the excitation and the emission of * Corresponding author. Tel: +33 1 44 27 60 05. Fax: +33 1 44 27 60 33. E-mail:
[email protected]. † E-mail: H.P.,
[email protected]; J.M.K.,
[email protected]; M.C.,
[email protected]; H.L.P.,
[email protected]. ‡ University Paris 06. § CNRS. | Institut Universitaire de France.
each type of low-coordinated ion. Molecular modeling5,7 and recent advances in experimental setups8-11 allowed authors to refine the assignments of excitation and emission spectra of MgO. Now photoluminescence is used to characterize the coordination number and distribution of O2-LC ions in catalysis studies on MgO.12 On CaO, the technique is not so accomplished because the photoluminescence spectra of powders are assigned incompletely. Yet, CaO surfaces exhibit interesting basic properties in adsorption and catalysis,1,13-20 often superior to MgO.13,14 Tench and Pott21 and Coluccia et al.22 reported the first photoluminescence spectra of CaO powders. They showed that the photoluminescence features were similar to those of MgO. Even though the bands of the spectra were not resolved enough to be assigned to specific surface sites, the bands were systematically shifted to higher wavelength for CaO. Garrone et al. used the electrostatic mode of Levine and Mark23 to predict this upshift value for each transition wavelength from the ratio of the Madelung constants.24 MacLean and Duley evidenced two couples of wavelengths (λEXC, λEM) on CaO but did not assign them to specific sites.25 Recently, Stankic et al. proposed an elegant approach to improve and simplify spectra by using CaO nanoparticles obtained by chemical vapor deposition (CVD), which exhibit fewer surface defects than common CaO powders.26 Nevertheless, some photoluminescence features observed for highly divided CaO powders are missing with these model nanoparticles. Interestingly, the same group evidenced that CVD nanoparticles of MgO doped with Ca2+ ions27 or functionalized with CaO deposits28 give additional bands compared to those observed with CaO or MgO CVD nanoparticles. The authors suggested that these new bands can be related either to new types of CaO defects or to original patterns involving both Mg2+ and Ca2+ ions. Now, to assign all the photoluminescence features of CaO powders unambigously, we have to cope with samples
10.1021/jp110193k 2011 American Chemical Society Published on Web 12/13/2010
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J. Phys. Chem. C, Vol. 115, No. 3, 2011
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Figure 1. Excitation and emission spectra (on the left and right, respectively) recorded at 77 K for Carb-vac. For excitation and emission spectra, λEM and λEXC are set at (a) λEM ) 350 nm and λEXC ) 250 nm, (b) λEM ) 285 nm and λEXC ) 395 nm, and (c) λEM ) 490 nm and λEXC ) 320 nm, respectively.
that are as rich in surface defects as powders commonly used in adsorption and catalysis applications. The aim of this study is to assign all the surface photoluminescence features of CaO powders and to show that this assignment can be easily used to characterize morphologies of CaO surfaces at the molecular level. We investigated four samples of CaO powders prepared from different routes. To assign their photoluminescence features, the spectra were recorded at low temperature (77 K), which minimized energy transfer between luminescent species11,29 and substantially improved the spectral resolution. To determine the chemical nature of the photoactive surface centers newly evidenced, the role of the thermal pretreatment on dehydroxylation was studied by photoluminescence and infrared. To characterize and compare the morphologies of the four samples at the molecular level, we present a method for the processing of photoluminescence spectra, which could be easily used in further studies of CaO surfaces. 2. Experimental Section 2.1. Synthesis of CaO Powders. Four CaO samples were synthesized by varying the precursor and decomposition conditions: - Carb-vac is calcium carbonate (CaCO3, Aldrich, 98%) decomposed under primary vacuum at 1173 K (ramp 1 K min-1) for 3 h; - Hydrox is calcium hydroxide (Ca(OH)2, Aldrich, 99.995%) decomposed under primary vacuum at 1173 K (ramp 1 K min-1) for 3 h; - Oxal is calcium oxalate decomposed under primary vacuum at 1173 K (ramp 1 K min-1) and calcined under oxygen flow at 923 K as described earlier;30 - Carb-fl is calcium carbonate (CaCO3, Aldrich, 98%) decomposed under air flow (20 mL min-1) at 1073 K (ramp 10 K min-1) for 3 h. TGA experiments showed that the precursors used were converted into CaO phase during the four syntheses. From X-ray diffractograms after synthesis (not shown), Carb-vac and Oxal were weakly crystallized whereas Hydrox and Carb-fl exhibited the lime structure (JCPDS 37-1497). 2.2. Photoluminescence. Photoluminescence experiments were performed in a special cell described earlier.8 The CaO sample was put in this cell and was in situ outgassed (ramp 1 K min-1) up to 1023 or 1273 K and kept at these temperatures for 3 h under dynamic vacuum (10-6 Torr). The spectra were recorded under dynamic vacuum (10-6 Torr) at 77 K. This temperature was maintained by immersing the lowest part of the cell (containing the sample) in liquid nitrogen placed in a dewar made of suprasil quartz. This low temperature was used
to increase the resolution of the spectra and to decrease the energy transfer between the luminescent sites. Composite bands were decomposed in several Gaussian curves with the Levenberg-Marquardt algorithm implemented in ORIGIN 6.1. 2.3. Infrared. A self-supported wafer of 20-30 mg was placed in a quartz cell, designed to reach 1173 K. The sample was heated (ramp 1 K min-1) under vacuum up to 1023 or 1173 K. Then, the wafer was brought from the furnace part of the cell to the optical one (fitted out with ZnSe windows), at room temperature. All spectra were registered at room temperature using a Bruker FTIR Vector 22 spectrometer, fitted out with a DTGS detector (resolution 2 cm-1, 128 scans/spectrum). 2.4. TEM and Nitrogen Physisorption. TEM characterizations were performed on a JEOL JEM 100 CXII UHV using a 100 keV electron beam. Physisorption of N2 was followed with a Micromeritics apparatus (ASAP 2010) after outgassing the sample at 473 K for 15 h and at 573 K for 2 h. Specific surface areas were calculated from the BET method. 3. Results and Discussion To assign the photoluminescence features of CaO, we proceeded in three steps: first, we identified the excitation and emission wavelengths (respectively λEXC, λEM) that are coupled, i.e., corresponding to the same photoluminescence process (absorption/excitation and direct emission); second, we investigated with FTIR and photoluminescence spectroscopies if the identified wavelengths correspond to either hydroxylated or dehydroxylated sites; and third, we assigned each (λEXC, λEM) couple to a defined surface site. To show how the new assignment can be used to characterize morphologies of CaO surfaces at the molecular level, we propose a method to process the photoluminescence spectra of four powdered CaO samples and to rank these samples according to the distribution of oxide ions in low coordination. 3.1. Identification of (λEXC, λEM) Couples. The excitation spectrum is composite and exhibits excitation contributions centered on different wavelengths. The relative intensities of these contributions depend on the emission wavelength selected for the recording. The emission spectrum exhibits the same composite character. Two wavelengths λEXC and λEM are considered as “coupled” when the contribution centered at λEM on the emission spectrum is maximized with the excitation wavelength set at λEXC and if, reciprocally, the contribution centered at λEXC on the excitation spectrum is maximized with the emission wavelength set at λEM. Figure 1 shows the excitation and emission spectra recorded at 77 K on Carb-vac, previously dehydroxylated at 1273 K under dynamic vacuum: these conditions greatly improve the resolu-
Surface Ions in Low Coordination of CaO Powders
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TABLE 1: Photoluminescence Features of CaO Obtained by Thermal Decomposition of Different Precursors or by CVD: Identification of (λEXC, λEM) Couples and Proposed Assignment authors Coluccia et al.
origin
22
λEXC (nm)
CaCO3
MacLean and Duley25
Ca(OH)2 and CaCO3
Stankic et al.26
CVD
this work
CaCO3 (vacuum or air), Ca(OH)2, and CaC2O4
TABLE 2: Experimental Excitation and Emission Wavelengths for CaO (This Work) and MgO,9 Calculated Valuesa for CaO, and Assignment CaO (meas)
MgO (meas)
CaO (calc)
λEXC
λEM
λEXC
λEM
λEXC
λEM
assignment
250 285 320
350 395 490
