Fourier Transform Infrared Study of Low-Temperature CO Adsorption

Fourier Transform Infrared Study of Low-Temperature CO Adsorption on ... Wang , Yonglan Luo , Abdullah M. Asiri , Abdulrahman O. Al-Youbi , and Xuping...
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Langmuir 2004, 20, 730-736

Fourier Transform Infrared Study of Low-Temperature CO Adsorption on CuMgAl-Hydrotalcite S. Kannan,†,* Tz. Venkov,‡ K. Hadjiivanov,‡ and H. Kno¨zinger§ Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, GB Marg, Bhavnagar 364 002, India, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Department Chemie, Physikalische Chemie, LMU Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨ nchen, Germany Received June 19, 2003. In Final Form: October 22, 2003 Single-phase CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized by coprecipitation. Thermoanalytical studies of this sample showed transformations in three stages in the temperature range up to ca. 900 K yielding mainly CuO phase. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements showed the presence of carbonates even after calcination of the sample at 973 K. The genesis of Cu+ sites during thermal treatment in vacuo at different temperatures for this sample was followed by IR spectroscopy of CO adsorbed at low temperature. Essentially no Cu+ sites are present on a sample calcined at 723 K, consistent with X-ray photoelectron spectroscopic (XPS) data. However, sample subjected to activation (1 h of O2 treatment at 723 K followed by 1 h of evacuation at the same temperature) upon CO adsorption at 85 K unambiguously showed the presence of Cu+ sites. 12CO-13CO coadsorption studies confirmed the presence of dicarbonyls, which are converted to linear Cu+-CO species during evacuation at 85 K. Concentration of the accessible Cu+ sites increased with the increase in activation temperature up to 873 K and decreased with a further temperature rise. The copper sites on the sample are heterogeneously distributed and their distribution depends on the activation temperature. Two routes of reduction of Cu2+ to Cu+ are proposed: (i) autoreduction during evacuation and (ii) reduction by CO.

1. Introduction Hydrotalcite-like (HT-like) compounds constitute a class of two-dimensional materials, receiving increasing attention because of their diverse applications. Structurally, they consist of alternating positively charged mixed-metal hydroxide sheets and negatively charged interlayers containing anions for charge compensation. They are represented by the general formula [M(II)1-xM(III)x (OH)2][Ax/nn-]‚mH2O, where M(II) is a bivalent metal ion, M(III) is a trivalent metal ion, A is the interlayer anion, and x can have values between 0.2 and 0.4. Although various bivalent metal ions can be incorporated in a HTlike network, recently there has been significant interest in Cu-containing hydrotalcites owing to their various catalytic applications.1,2 In general, it is difficult to synthesize pure Cu-containing binary hydrotalcites because of the Jahn-Teller distortion. However, copper can be incorporated in HT-like networks in the presence of another cation, thereby yielding a ternary hydrotalcite. Although many systems with different cobivalent metal ions with copper are known, Mg as cobivalent metal ion is interesting because it is a nonredox species and can form well-crystallized hydrotalcite by itself with Al. Corma and co-workers3 have utilized CuMgAl mixed oxides derived from CuMgAl ternary hydrotalcites for decomposition and selective catalytic reduction (SCR) of nitrogen oxides. It was inferred from X-ray photoelectron spec†

Central Salt and Marine Chemicals Research Institute. Bulgarian Academy of Sciences. § LMU Mu ¨ nchen. ‡

(1) Trombetta, M.; Ramis, G.; Busca, G.; Montanari, B.; Vaccari, A. Langmuir 1997, 13, 4628. (2) Dubey, A.; Rives, V.; Kannan, S. Phys. Chem. Chem. Phys. 2001, 3, 4826. (3) Shannon, R. J.; Rey, F.; Sankar, G.; Thomas, J. M.; Maschmeyer, T.; Waller, A. M.; Palomares, A. E.; Corma, A.; Dent, A. J.; Greaves, G. N. J. Chem. Soc., Faraday Trans. 1996, 92, 4331.

troscopic (XPS) measurements that Cu+ centers are the active sites for NO reduction with hydrocarbons while Cu0 centers are the active sites for NO decomposition. The influence of added copper (523 K) as seen from the appearance of reflections mainly due to CuO. When the temperature of calcination exceeded 773 K, a spinel phase also crystallizes (along with CuO), although the intensity of the reflections at 2θ around 36° and 42° were very weak. The crystallinity of this phase improved with increasing calcination temperature. It is difficult to ascertain the nature of this spinel. It should most likely be present as

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Figure 2. TG-DTA traces of CuMgAl-HT (the inset presents QMS analysis of H2O and CO2).

CuAl2O4 (crystallization of MgAl2O4 only occurs at temperatures higher than 1073 K), because of their poor intensities and minimal variations in the peak positions for various spinels.27 A sharp increase in crystallinity of CuO was noted above 1023 K (Figure 1, pattern o), due to incipient crystallization of CuO around this temperature, probably through decomposition of a Cu-rich intermediate phase.28 3.1.2. Thermogravimetry-Differential Thermal Analysis. The results of the TG-DTA analysis of the sample are given in Figure 2. Three stages of thermal transformations can be recognized for this sample. The first weight loss occurring around 450 K is generally attributed to removal of interlayer water molecules. It is to be noted that CuO was formed well below this temperature, as mentioned above. The second stage of weight loss, which occurred in a wide temperature window in the range 500-700 K, involves both dehydroxylation of metal hydroxide sheets and partial decarbonation resulting in complete loss of the layered network. Analysis of the gases evolved (given in the inset of Figure 2) substantiated this observation, showing evolution of both H2O and CO2 in this temperature regime. Finally, a well-defined sharp endotherm was noted around 873 K, which is accompanied by a weight loss and the evolution of mainly CO2, probably arising from a copper oxycarbonate network. It should be remembered that improved crystallization of CuO occurs at temperatures greater than 873 K. 3.1.3. XPS. X-ray photoelectron spectroscopy (XPS) also supported the presence of carbonates at such high temperatures (see Figure 1S, Supporting Information) by a C1s emission at 288.9 eV.29 In addition, O1s spectra (see Figure 2S, Supporting Information) also supported the variation in the phase composition upon calcination. The sample calcined at 473 K showed a sharp peak around 531.3 eV, usually ascribed to hydroxyl groups,30 with a shoulder at 529.9 eV, which is attributed to oxide oxygen, probably from CuO (cf. PXRD results, which showed only peaks due to CuO at this temperature). With increasing calcination temperature, the oxidic peak at 529.9 eV becomes more symmetric and increases in intensity, corroborating the formation of well-crystallized CuO, as evidenced from PXRD. Calcination at 1073 K leads to a shift of the O1s emission to 530.2 eV, which is inferred to characterize a spinel phase.31 Copper is always present (28) Kannan, S.; Rives, V.; Knozinger, H. J. Solid State Chem. (Published online on 12/03/2003). (29) Khassin, A. A.; Yurieva, T. M.; Kaichev, V. K.; Bukhtiyarov, V. I.; Budneva, A. A.; Paukshtis, A.; Parmon, V. N. J. Mol. Catal., A 2001, 175, 189. (30) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208.

Low-Temperature CO Adsorption on CuMgAl-Hydrotalcite

Figure 3. In situ DRIFT spectra of CuMgAl-HT activated in air at 303 (a), 323 (b), 373 (c), 423 (d), 473 (e), 523 (f), 573 (g), 623 (h), 673 (i), 723 (j), 773 (k), 873 (l), and 973 K (m); ex situ calcined for 773, 873 and 973 K.

as Cu2+ irrespective of the calcination temperature, as evidenced by the observed spin-orbit splitting.32 3.1.4. DRIFT Spectra in KBr Pellets. To follow the sample evolution with the calcination temperature, we have studied samples heated at different temperatures by IR spectra in KBr pellets by the DRIFT mode of Fourier transform infrared spectroscopy. The spectrum of the sample calcined at 303 K (Figure 3, spectrum a) showed broad bands in the region 3600-3100 cm-1 showing extensive hydrogen-bonded hydroxy network and adsorbed water, a band around 1650 cm-1, attributed to deformation mode of water molecules, and a band at 1390 cm-1, ascribed to ν3 asymmetric stretching mode of carbonate present in the interlayer of hydrotalcite. Calcination of the sample at 323 K (Figure 3, spectrum b) showed significant variations in the carbonate region, wherein the carbonate band splits into two bands appearing around 1525 and 1385 cm-1. This kind of split in carbonate vibration was earlier reported by Cabrera et al.33 for CuZnAl hydrotalcite (however, above 373 K) and by us for CuM(II)Al hydrotalcite.28 Such a split is ascribed to a change in the carbonate coordination upon loss of interlayer water molecules. Further, the intensity of the deformation band decreased in its intensity and completely disappeared above 423 K. Increase in the calcination temperature decreased the intensity of hydroxy vibration, although no significant drop in the intensity for carbonate vibration was noted till 673 K (Figure 3, spectra d-i). Further increase in the temperature resulted in the decrease in the intensity of carbonate vibration; however, weak bands were noted even at 973 K (Figure 3, spectrum, m). 3.2. Low-Temperature CO Adsorption. To follow the creation of Lewis acidity and Cu+ cations on the sample surface, we have activated the sample at different temperatures and recorded the carbonyl infrared spectra of adsorbed CO. Three sets of CO adsorption experiments were made with the sample. The first set deals with samples calcined at increasing temperatures starting from 673 K. For these experiments, the sample was activated by 1 h of heating in an oxygen atmosphere (5 kPa) followed by 1 h of evacuation at the activation temperature. Subsequently, the sample was cooled to 85 K and CO adsorption was performed. (31) Cocke, D. L.; Johnson, E. D.; Merrill, R. P. Catal. Rev.sSci. Eng. 1984, 26, 163. (32) Ertl, G.; Hierl, R.; Kno¨zinger, H.; Thiele, N.; Urbach, H. P. Appl. Surf. Sci. 1980, 5, 49. (33) Cabrera, I. M.; Granados, M. L.; Fierro, J. L. G. Phys. Chem. Chem. Phys. 2002, 4, 3122.

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Figure 4. FTIR spectra of CO adsorbed at 85 K on a CuMgAlHT sample activated at 673 K. Equilibrium CO pressure of 50 Pa (a) and evolution of the spectra in the conditions of dynamic vacuum (b-f).

The second set of experiments aimed at establishing the effect of evacuation temperature on the creation of Cu+ cations. For this purpose the sample was initially activated in oxygen at 723 K and then the evacuations were performed at different temperatures. The third set aimed at proving the existence and reducibility of Cu2+ ions on the sample. For better interpretation of the spectra, some experiments involving 12CO-13CO coadsorption were also performed. 3.2.1. Sample Activated at 673 K. The IR spectrum of CO adsorbed at 85 K on this sample displays three intense bands, their maxima being located at 2168, 2146, and 2101 cm-1 (Figure 4, spectrum a). Decrease of the equilibrium pressure and evacuation result in a fast decline of the bands at 2168 and 2145 cm-1, the latter disappearing a little faster (Figure 4, spectra b-f). In addition, the band at 2101 cm-1 also decreases in intensity and is shifted to 2106 cm-1. Further evacuation slightly affects this band; it only slightly decreases in intensity (Figure 4, spectra d-f). Subsequent readsorption of CO almost restores the initial spectrum (not shown). Evacuation at 85 K and at higher temperatures leads to changes very similar to those already described. In this case, however, the band that is resistant to evacuation is registered at higher wavenumbers, i.e., at 2112 cm-1. According to data from the literature,6,9,12-16 the bands in the 2112-2100 cm-1 spectral region are unambiguously assigned to copper carbonyls. Indeed, this band has not been found with a MgAl-HT sample.34 In this region both carbonyls of metallic copper and Cu+ cations can be observed. In our case, however, it is not likely to have reduced copper after the activation treatment. In addition, the carbonyls of metallic copper are unstable and easily decomposed during evacuation. Hence we infer that the observed bands characterize carbonyls of Cu+ cations. A band at 2150 cm-1 has been found after lowtemperature CO adsorption on MgAl-HT sample and assigned to CO bonded to surface OH groups.34 This assignment is consistent with the low stability and the low C-O stretching frequency of the carbonyls. Evidently, the band at 2147 cm-1 observed with our sample is at least partly due to H-bonded CO. However, a careful analysis of the spectra shows that this band can also be associated with the blue shift of the low-frequency band at 2101 cm-1. This effect can be more clearly seen with the (34) Kannan, S.; Kishore, D.; Hadjiivanov, K.; Kno¨zinger, H. Langmuir 2003, 19, 5742.

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Figure 5. FTIR spectra of CO (equlibrium pressure of 300 Pa) adsorbed on a CuMgAl-HT sample activated at 723 K: sample temperature of 273 (a), 223 (b), 203 (c), 173 (d), 153 (e), 113 (f), and 85 K (h).

samples activated at higher temperatures. For this reason we assign a component of the band at 2146 cm-1 to the symmetric mode of Cu+(CO)2 species, the respective antisymmetric vibrations being at 2101 cm-1. During evacuation these species are converted into linear carbonyls absorbing at 2106 cm-1. Additional arguments in favor of this interpretation will be provided below. The shift of the band at 2106 cm-1 to 2112 cm-1 when the sample was evacuated at higher temperatures is attributed to reduction of some Cu2+ sites and creation of a new fraction of Cu+ cations. The higher frequency band at 2168 cm-1 is due to CO adsorbed on Lewis acid sites. This band undergoes a blue shift of about 10 cm-1 with the decrease in the coverage. This effect can be explained by lateral interaction between the CO molecules adsorbed on a separate phase. On the basis of the IR spectra presented in Figure 4, it is not possible to determine unambiguously the nature of these Lewis acid sites. A similar band was registered with a copper-free MgAl-HT and assigned to CO adsorbed on Mg2+ sites.34 However, a careful analysis of the spectra suggests that the band has a complex contour. This suggests that it is not excluded that some Cu2+-CO species contribute to this band. 3.2.2. Sample Activated at 723 K. The only carbonyl band registered after room-temperature CO adsorption on the 723 K activated sample is at 2123 cm-1 (Figure 5, spectrum a). Then the temperature was gradually decreased. This resulted in an enhancement of the 2123 cm-1 band in intensity initially without shift of its position (Figure 5, spectra b and c). At lower temperatures a band at 2162 cm-1 starts to develop (Figure 5, spectrum c) and the 2123 cm-1 band shifts to 2117 cm-1 (Figure 5, spectra d-h). The decrease of the temperature also induces the appearance and rise in intensity of a band at 2182 cm-1 (Figure 5, spectra d-h). Evacuation at 85 K reverses the described trends. Computer deconvolution indicates that the bands in the 2165-2100 cm-1 region have complex contours. The computer-deconvoluted carbonyl spectrum at high CO coverage is presented in Figure 6. Here, the existence of bands at 2158, 2149, 2119, and 2105 cm-1 is clearly seen. At low coverages, the carbonyl band resistant to evacuation also consists of two components, namely, at 2127 and 2113 cm-1 (not shown). These results suggest the existence of two families of Cu+ sites on the sample. The first family forms monocarbonyls characterized by a band at 2127 cm-1 that are converted into dicarbonyls with bands at 2158 and 2119 cm-1. The monocarbonyls formed with the second family of Cu+ sites absorb at 2113 cm-1 and the

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Figure 6. Computer deconvolution of the FTIR spectrum registered after adsorption of CO (equlibrium pressure of 300 Pa) at 85 K on a CuMgAl-HT sample activated at 673 K.

Figure 7. FTIR spectra of a 12CO-13CO isotopic mixture (molar ration ca. 1:2) adsorbed on a CuMgAl-HT sample activated at 723 K. Equilibrium pressure of 100 Pa CO (a) and evolution of the spectra in the conditions of dynamic vacuum (b-e). Spectrum f is registered after 12CO adsorption only (100 Pa of CO followed by evacuation).

corresponding dicarbonyls are observed at 2149 and 2105 cm-1. In fact, the real heterogeneity is even greater. Thus, at the initial stages of dicarbonyl formation a band at 2162 cm-1 is clearly observed. Parallel cyclic temperature programmed reduction-reoxidation experiments also confirmed the possible presence of such heterogeneous nature of copper sites in this sample (Figures 3S and 4S, Supporting Information). The TPR profile of fresh hydrotalcite (first cycle) showed a single peak (around 600 K) with a weak low-temperature reduction peak, while subsequent cycles (oxidic samples) showed a single peak around 523 K. Variations in the temperatures with cycles are possibly due to textural variations. However, reoxidation of these oxidic phases showed two well-defined peaks occurring near 423 and 523 K, suggesting the presence of two different families of reduced copper sites. To obtain more information about the nature of the band at 2165-2100 cm-1, we have studied 12CO-13CO coadsorption. Let us first consider the expected possibilities. If dicarbonyls are formed on our sample, coadsorption of 12 CO and 13CO should result in formation of Cu+(12CO)13 ( CO) mixed species. Let us suppose the existence of dicarbonyls. Analysis of the spectra presented in Figure 7, spectrum a, indicates that their IR modes (at relatively low coverages) are at 2162 and 2117 cm-1. By use of an approximate force field model,35 it is easy to calculate the force and interaction constant in the dicarbonyls. On this (35) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, 1975.

Low-Temperature CO Adsorption on CuMgAl-Hydrotalcite

basis, the frequencies of the Cu+(12CO)(13CO) species should be at 2148 and 2082 cm-1. It will be difficult to follow the changes of the band expected at 2082 cm-1 since it should superimpose with the bands due to linear Cu+13 CO species and the intense antisymmetric band of the Cu+(13CO)2 dicarbonyls. Both bands are expected at similar frequencies. However, the band expected at 2148 cm-1 could be well separated from other bands, especially at coverages at which the dicarbonyls just start to be formed. In addition, to suppress the formation of Cu+(12CO)2 species, it is necessary that the concentration of 12CO in the isotopic mixture should be smaller than the concentration of 13CO. Some spectra registered after coadsorption of a 12CO13 CO isotopic mixture (molar ratio ca. 1:2) are presented in Figure 7. It is clearly seen that no band around 2162 cm-1 (registered after adsorption of 12CO) is formed. A band at 2149 cm-1 appears instead. These results strongly support the idea about the formation of dicarbonyls. The band at 2106 cm-1 is due to the superimposition of the bands due to the antisymmetric modes of some Cu+(12CO)2 species (expected in low concentration), the symmetric stretching band of Cu+(13CO)2 dicarbonyls, and some OH-13CO modes. The band at 2072 cm-1 is mainly due to the antisymmetric modes of Cu+(13CO)2 species. It should be noted that at low coverage only the bands due to monocarbonyls are detected. However, the 12CO band is at somewhat lower frequencies than registered in the experiments involving 12CO adsorption. This is most probably due to the lack of dipole-dipole interaction between the adsorbed 12CO molecules since they are diluted in 13CO. Indeed, in this case only the static shift should be observed and the band should be detected at lower frequencies. These results show that the static shift in this case is -6 cm-1. No shift in the band position was noted after 12CO adsorption, indicating that both static and dynamic shifts (having opposite signs) should have the same absolute values. In other words, one can conclude the dynamic shift to be +6 cm-1. All these considerations explain the deviation observed between the bands at 2117 and 2076 cm-1: In an ideal case they had to differ by a factor of 0.977 77, i.e., the higher-frequency counterpart of the 2076 cm-1 band should be detected at 2123 cm-1. By analysis of the spectrum in the 13C18O region, one can derive additional arguments in favor of the above assignment. At low coverage, a Cu+-13C18O band is observed at 2024 cm-1. The concentration of 12C18O is too low, so only mixed ligand complexes are expected at higher coverages. Hence, no band around 2020 cm-1 should appear at higher coverage, which was indeed observed. 3.2.3. Samples Activated at 773-973 K. The IR spectra of CO adsorbed at low temperature on the sample activated at higher temperatures (Figures 8-10) show the following peculiarities: The concentration of the Cu+ sites (estimated by the intensity of the IR carbonyl bands) increases for samples activated at temperatures up to 873 K, which is explained by the copper autoreduction (Figures 8 and 9). Unfortunately, the concentration of the Cu2+ sites cannot be followed accurately since at these high activation temperatures the concentration of the accessible Al3+ Lewis sites (forming carbonyls with bands coinciding with the Cu2+-CO bands) increases. After activation at 973 K the intensity of the carbonyl bands of Cu+ slightly decreases (Figure 10), which may be explained by sample sintering. The copper sites become heterogeneous and the carbonyl stretching frequencies are blue-shifted. This could be explained by the discrete phase segregation of aluminamagnesia-MgAl mixed oxides as supports for copper.

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Figure 8. FTIR spectra of CO adsorbed at 85 K on a CuMgAlHT sample activated at 773 K. Equilibrium pressure of 500 Pa (a) and 20 Pa (b) of CO and evolution of the spectra in the conditions of dynamic vacuum (c-j).

Figure 9. FTIR spectra of CO adsorbed at 85 K on a CuMgAlHT sample activated at 873 K. Equilibrium pressure of 500 Pa (a) and 20 Pa (b) of CO and evolution of the spectra in the conditions of dynamic vacuum (c-k).

Figure 10. FTIR spectra of CO adsorbed at 85 K on a CuMgAlHT sample activated at 973 K. Equilibrium pressure of 500 Pa (a) and 20 Pa (b) of CO and evolution of the spectra in the conditions of dynamic vacuum (c-h).

However, even for the samples activated at high temperatures, the expected changes in the spectra due to dicarbonyl-monocarbonyl transformation are observed. 3.2.4. Sample Oxidized at 723 K and Evacuated at Different Temperatures. Adsorption of CO (85 K, 900 Pa equilibrium pressure) on the sample evacuated at room temperature results in the appearance of a relatively strong band at 2149 cm-1 having a high-frequency shoulder at 2165 cm-1 and a weaker band at 2109 cm-1 (Figure 11, spectrum a). The band at 2149 cm-1 has been already assigned to H-bonded CO and that at 2165 cm-1

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Figure 11. FTIR spectra of CO (equilibrium pressure of 900 Pa) adsorbed at 85 K on a CuMgAl-HT sample calcined at 723 K in oxygen and evacuated for 1 h at 293 (a), 473 (b), and 673 K (c).

to CO adsorbed on Lewis acid sites (Mg2+ and/or Cu2+). The band at 2109 cm-1 with a component of the 2149 cm-1 band is assigned to dicarbonyls of Cu+ sites. The IR spectra of CO adsorbed on the sample evacuated at 473 K are presented in Figure 11, spectrum b. In this case again the principal carbonyl band is that at 2149 cm-1 due mainly to OH-CO species. The band due to Cu+(CO)2 species here is definitely more intense than with the sample evacuated at ambient temperature. This is explained by the enhanced autoreduction of copper at elevated temperatures. These changes are even more pronounced with the sample activated at 673 K (Figure 11, spectrum c). In this case the band at 2111 cm-1 is very intense. The band at 2152 cm-1 is somewhat reduced in intensity, which is associated with an additional sample dehydroxylation during the evacuation at 673 K. In agreement with this, the fraction of Lewis acid sites (band at 2172 cm-1) has increased. These results clearly demonstrate that Cu+ cations on our sample are generated exclusively by sample autoreduction at elevated temperatures. It is known that Cu2+ sites are easily reduced to Cu+ in a CO atmosphere. This could be a second route for generation of Cu+ sites. To check this possibility we have adsorbed CO at low temperature on an activated sample. The sample was then reduced with CO at 523 K and cooled again to 85 K. The intensity of the band around 2176 cm-1 has decreased after reduction while both bands, at ca. 2150 and 2108 cm-1, have gained intensity (Figure 12). These results clearly demonstrate that (at least part of) the band at 2176 cm-1 is associated with carbonyls of Cu2+ cations while the bands at 2150 and 2108 cm-1 are related to Cu+ sites. 4. Conclusions CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized

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Figure 12. FTIR spectra of CO adsorbed (230 Pa) at 85 K on CuMgAl-HT (a) calcined at 723 K or (b) calcined at 723 K and subjected to reduction by CO at 523 K for 5 min. The difference spectrum (b - a) is presented in the inset.

by coprecipitation. PXRD showed crystallization of discrete CuO phase at low temperatures well below the first stage of thermal transformation, as deduced from TG analysis. DRIFT and XPS studies showed the retention of carbonate even for the samples calcined above 973 K. CO adsorption measurements revealed Cu2+ for calcined sample, while activated samples showed the presence of Cu+ at low temperatures. Concentration of Cu+ enhanced with an increase in activation temperatures up to 873 K. Isotopic adsorption measurements revealed formation of unstable dicarbonyls of Cu+ upon low-temperature CO adsorption, which were converted into linear Cu+-CO species during evacuation at 85 K. CO adsorption also evidenced a heterogeneous distribution of the Cu+ sites on a metal oxide matrix, whose heterogeneity was influenced by the activation temperature. Two routes of reduction of Cu2+ to Cu+ were identified, namely, autoreduction during evacuation and interaction with CO (which by itself acts as a reductant). Scope for tailoring the concentration of Cu+ may provide some clues in optimization of reaction conditions for specific catalytic reactions. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and the Fonds der Chemischen Industrie. S.K. and K.H. are indebted to the Alexander-von-Humboldt Foundation, Bonn, Germany, for research fellowships. Supporting Information Available: C1s and O1s photoelectron spectra of CuMgAl-HT and temperature-programmed oxidation and reduction profiles of CuMgAl-HT. This information is available free of charge via the Internet at http://pubs.acs.org. LA035086Z