Activated Carbon Catalysts for the

Department of Physics, West Virginia University, Morgantown, West Virginia ... Department of Chemistry, University of Utah, Salt Lake City, Utah 84112...
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Energy & Fuels 2002, 16, 182-188

Characterization of CuCl2/PdCl2/Activated Carbon Catalysts for the Synthesis of Diethyl Carbonate A. Punnoose,† M. S. Seehra,*,† B. C. Dunn,‡ and E. M. Eyring‡ Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315, and Department of Chemistry, University of Utah, Salt Lake City, Utah 84112-0850 Received July 20, 2001. Revised Manuscript Received September 26, 2001

X-ray diffraction (XRD), low-temperature electron spin resonance (ESR) spectroscopy, and magnetometry are employed to determine the electronic and structural properties of over a dozen samples of fresh and KOH-treated CuCl2/PdCl2/activated carbon (AC) catalysts used in the synthesis of diethyl carbonate (DEC) from ethanol and CO. The percentage yields of DEC from the preceding paper by Dunn et al. are compared with the results from XRD and ESR to determine the nature of the active species. Temperature variation (4 to 300 K) of the ESR spectra of CuCl2/ AC for different loadings of Cu reveals two Cu2+ species: Cu2+-carbon and nanoclusters of CuCl2 precipitated during impregnation. An excellent correlation is observed between the ESR intensity of Cu2+-carbon species and weight percent of DEC produced, although the maximum yield for CuCl2/AC is only about 4% for Cu loading of 9% (wt). For the CuCl2/PdCl2/AC catalysts, the yield increases to 10%, signifying the important role of Pd2+. For the CuCl2/PdCl2/AC catalysts treated with KOH for different ratios of OH/Cu ) x between 0 and 5, wt % of DEC produced increases to 18% for x ) 1, but drops to negligible amounts for x ) 3 and 5. In XRD studies, the intensity of the paratacamite peaks correlates well with the % yield of DEC for all values of x, showing paratacamite as the active species in the KOH-treated catalysts. For x ) 3 and 5, paratacamite converts to calumetite, corresponding to the drastic drop in the yield for DEC conversion.

Introduction In the preceding paper by Dunn et al.,1 experimental results on the synthesis of diethyl carbonate (DEC) from ethanol and carbon monoxide using the heterogeneous CuCl2/PdCl2 catalysts supported on activated carbon (AC) are reported. These results show that CuCl2/AC catalysts provide about 4% yield of DEC for about 6 to 9% loading of Cu. Addition of PdCl2 increases the DEC yield to about 10%, whereas pretreatment of CuCl2/ PdCl2/AC with KOH further increases the DEC yield to about 18% for x ) OH/Cu ) 1. For x ) 3 and 5, the yield drops dramatically to negligible amounts. In this work, we have carried out detailed investigations of the CuCl2/PdCl2/AC catalysts used in the above investigations, using X-ray diffraction (XRD) studies at room temperature and variable temperature (4 K to 300 K) electron spin resonance (ESR) spectroscopy. The goal of these investigations was to determine the active species by comparing our results with the percentage yield of DEC obtained in ref 1. The PdCl2/CuCl2 catalysts supported on alumina, silica, and carbon supports have been used by others in a number of reactions, viz., oxidation of CO,2-6 and synthesis of aldehydes, ketones, * Corresponding author. E-mail: [email protected]. † West Virginia University. ‡ University of Utah. (1) Dunn, B. C.; Guenneau, C.; Hilton, S. A.; Pahnke, J.; Eyring, E. M.; Dworzanski, J.; Meuzelaar, H. L. C.; Hu, J. Z.; Pugmire, R. J. Energy Fuels, 2002, 16, 177. (2) Choi, K. I.; Vannice, M. A. J. Catal. 1991, 127, 465-488. (3) Park, E. D.; Choi, S. H.; Lee, J. S. J. Phys. Chem. B 2000, 104, 5586-5594.

dimethyl carbonate, and ethyl dichloride.7-12 However, the results reported in ref 1 are the first such experiments for the synthesis of DEC. Consequently, it is of interest to determine whether the reaction mechanisms for the synthesis of DEC are different from those cited above.2-17 To determine the role of the various constituents (viz., PdCl2, CuCl2, AC, and KOH treatment) of the catalysts on the percentage yield of DEC, we have carried out experiments on four sets of samples: (i) CuCl2/AC with Cu loading between 0 and 9 wt %, but without PdCl2 and KOH treatment; (ii) PdCl2/AC without CuCl2 and (4) Park, E. D.; Lee, J. S. J. Catal. 2000, 193, 5-15. (5) Park, E. D.; Lee, J. S. J. Catal. 1998, 180, 123-131. (6) Desai, M. N.; Butt, J. B.; Dranoff, J. S. J. Catal. 1983, 79, 95103. (7) Leofanti, G.; Padovan, M.; Garilli, M.; Carmello, D.; Zechina, A.; Spoto, G.; Bordiga, S.; Palomino, G. T.; Lamberti, C. J. Catal. 2000, 189, 91-104. (8) Carmello, D.; Finocchio, E.; Marsella, A.; Cremaschi, B.; Leofanti, G.; Padovan, M.; Busca, G. J. Catal. 2000, 191, 354-363. (9) Finocchio, E.; Rossi, N.; Busca, G.; Padovan, M.; Leofanti, G.; Cremaschi, B.; Marsella, A.; Carmello, D. J. Catal. 1998, 179, 606618. (10) Tang, H. G.; Sherrington, D. C. J. Catal. 1993, 142, 540-551. (11) Rao, V.; Datta, R. J. Catal. 1988, 114, 377-387. (12) Pacheco, M. A.; Marshall, C. L. Energy Fuels 1997, 11, 2-29. (13) Blanco, J.; Fayos, J.; Garcia De La Banda, J. F.; Soria, J. J. Catal. 1973, 31, 257-263. (14) Gracia, C. L.; Resasco, D. E. Appl. Catal. 1989, 46, 251-260. (15) Fortini, E. M.; Gracia, C. L.; Resasco, D. E. J. Catal. 1986, 99, 12-18. (16) Leofanti, G.; Padovan, M.; Garilli, M.; Carmello, D.; Marra, G. L.; Zechina, A.; Spoto, G.; Bordiga, S.; Lamberti, C. J. Catal. 2000, 189, 105-116. (17) Yamamoto, Y.; Matsuzaki, T.; Ohdan, K.; Okamoto, Y. J. Catal. 1996, 161, 577-586.

10.1021/ef010180d CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2001

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Table 1. Details of the Samples Investigated in This Work sample code

description

wt % of Cu

wt % of Pd

OH/Cu ratio

AC 0.048CA 0.25CA 0.6CA 1.36CA 3.0CA 6.0CA 9.0CA 4.8PA CPA2 CPA1 0.05OH/Cu 0.1OH/Cu 0.5OH/Cu 1.0OH/Cu 3.0OH/Cu 5.0OH/Cu

activated carbon support CuCl2‚2H2O + AC CuCl2‚2H2O + AC CuCl2‚2H2O + AC CuCl2‚2H2O + AC CuCl2‚2H2O + AC CuCl2‚2H2O + AC CuCl2‚2H2O + AC PdCl2 + AC CuCl2‚2H2O + PdCl2 + AC CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC KOH + CuCl2‚2H2O + PdCl2 + AC

0 0.048 0.25 0.6 1.36 3.0 6.0 9.0 0 14.0 3.0 3 3 3 3 3 3

0 0 0 0 0 0 0 0 4.8 1.8 0.27 0.27 0.27 0.27 0.27 0.27 0.27

0 0 0 0 0 0 0 0 0 0 0 0.05 0.1 0.5 1.0 3.0 5.0

KOH treatment; (iii) untreated CuCl2/PdCl2/AC; and (iv) PdCl2/CuCl2/AC with KOH treatment to yield OH/Cu ratio between 0 and 5. For set (i), ESR studies show the presence of nanosized CuCl2 clusters and Cu2+ species attached to the AC support, the ESR intensity of the latter being proportional to the limited (=4%) yield of DEC. For set (ii) samples of PdCl2/AC, XRD shows the presence of Pd nanoparticles and these samples have negligible DEC yield. For the bimetallic CuCl2/PdCl2/AC samples of set (iii), ESR again shows both CuCl2 nanoclusters and Cu2+-carbon species and increase in the DEC yield to about 10%. Finally, for the KOH-treated bimetallic samples of set (iv), we see the presence of paratacamite, Cu2Cl(OH)3, in XRD whose line intensity has excellent correlation with the % yield of DEC up to the maximum of 18% for OH/Cu ) 1. For OH/Cu > 1, paratacamite converts to other phases such as calumetite Cu(OH,Cl)2‚2H2O and DEC yield drops to negligible amounts. These results show that although Cu2+-carbon has some activity, the presence of PdCl2 and paratacamite provides the best yield. Details of these results and their discussion are presented below. Experimental Details The procedures for the preparation of the catalysts are described in the preceding paper.1 XRD spectra were recorded at room temperature on a Rigaku D/Max diffractometer with Cu KR source (λ ) 1.5418 Å) using the standard procedures for powder diffractometry. Analysis of the data was accomplished using a JADE software and the JCPDS library. The ESR measurements were carried out using a standard reflection-type x-band (9.56 GHz) spectrometer equipped with a Varian microwave cavity and a variable temperature (4 to 300 K) cryostat system obtained from Oxford instruments. The line positions were determined accurately using an NMR probe. The line widths ∆H reported here are the peak-to-peak separations in the absorption derivative. The intensities of ESR lines quoted in this paper were obtained by double integration of the derivative signal in carefully weighed samples. Measurements of the magnetization were carried out on a commercial superconducting quantum interference device (SQUID) magnetometer. For measuring the magnetization under zero-field cooled (ZFC) case, the samples were cooled to 5 K in zero field, a measuring field of 400 Oe was then applied followed by data acquisition at increasing temperatures by stabilizing the temperature at each point. For the field-cooled (FC) case, the sample was cooled in an applied field of 400 Oe from 300 to 5 K, followed by data acquisition in the above

Figure 1. XRD spectra of pure activated carbon(AC) support, 3.0 wt % Cu:0.27 wt % Pd:AC (CPA1), 14 wt % Cu:1.8 wt % Pd:AC (CPA2), 3.0 wt % Cu:AC (3.0CA), and 4.8 wt % Pd:AC (4.8PA) samples. Line spectra from the PDF files for CuCl2‚ 2H2O, silica, and Pd are also shown for comparison. manner. The samples on which experimental studies are reported in this paper are listed in Table 1.

Results and Discussion A. X-ray Diffraction. The XRD spectra of activated carbon (AC) support, 3.0 wt % Cu:0.27 wt % Pd:AC (CPA1), 14 wt % Cu:1.8 wt % Pd:AC(CPA2), 3.0 wt %Cu:AC (3.0CA), and 4.8 wt %Pd:AC (4.8PA) samples are shown in Figure 1. The XRD pattern of activated carbon support shows two very broad peaks centered around 2θ ) 25° and 45° which are commonly observed in nanosized carbons.18 The sharp lines superimposed on the broad carbon peaks are identified as silica, perhaps an unavoidable impurity in AC. Similar peaks could also be found in the XRD data of various CPA (18) Manivannan, A.; Chirila, M.; Giles, N. C.; Seehra, M. S. Carbon 1999, 37, 1741-1747.

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Figure 2. XRD spectra of KOH-treated CPA(3.0 wt % Cu:0.27 wt % Pd:AC) catalysts with different OH/Cu ratios from 0.05 to 5. Peaks corresponding to different phases are marked on the spectra.

catalysts reported in the literature although they were seldom identified as silica and sometimes its two major peaks were misinterpreted as carbon.3-5 Loading the activated carbon with CuCl2‚2H2O (3.0CA) as well as with both CuCl2‚2H2O and PdCl2 (CPA1) up to moderate levels did not yield any peaks which could be unambiguously assigned to any known compounds. This suggests that the loaded compounds exist as amorphous materials or as nanodispersed crystallites with sizes of the order of few nanometers. But for the heavily loaded CPA2, few additional sharp lines are observed that could be readily assigned to CuCl2‚2H2O. In the 4.8PA sample with only PdCl2 loading, clear peaks due to metallic Pd are observed. From the width of the Pd peaks, a particle size of 10 nm is obtained. Note that simply by impregnating on to the activated carbon support, PdCl2 got reduced to metallic Pd nanoparticles. Conversion of PdCl2 to Pd upon impregnation on AC has been reported in the literature.17 XRD patterns of CuCl2/AC catalysts (CA) with Cu loading of up to 9 wt % did not show any additional clear peaks due to the loaded copper species. But with increasing Cu loading, a clear rise in the baseline could be observed, resulting in the gradual diminishing of the peak due to carbon and silica indicating the increased presence of nanodispersed/amorphous copper species. CuCl2 loading on AC differs considerably from a similar loading of PdCl2 since PdCl2 gets easily converted to metallic Pd0 just by the impregnation whereas CuCl2 did not show reduction to metallic Cu0. The X-ray diffraction spectra of CPA catalyst (3.0 wt % Cu:0.27 wt % Pd/AC) treated with KOH with the OH/ Cu ratio ranging between 0.05 and 5.0 are shown in Figure 2. Here the general features are similar to that of the untreated CPA sample with some additional peaks depending on the OH/Cu content. On KOH treatment, new broad peaks due to paratacamite (marked “p” in Figure 2) appear which gradually strengthen on increasing the OH/Cu ratio to e 1. Above this level, the paratacamite peaks completely disappeared and new sharp peaks due to calumetite (marked “c”) are ob-

Figure 3. ESR spectra of pure activated carbon(AC) support, 3.0 wt % Cu:0.27 wt % Pd/AC (CPA1) and activated carbon samples impregnated with different percentages of CuCl2(CA samples) and PdCl2(PA sample) showing the effect of metal loading on the carbon signal. All conditions of the experiment were identical and spectra were recorded from equal quantities(∼7.5 mg) of samples.

served. For OH/Cu g 1, strong lines due to KCl were observed, while additional KOH peaks also were present for the sample with OH/Cu ) 5. Thus XRD clearly indicates that the added KOH reacts with the copper species in the CPA catalyst and forms paratacamite and its concentration increases with increasing OH/Cu ratio up to x ) 1, and converting to calumetite for x > 1. Note that the paratacamite peaks are quite broad yielding crystallite size of 10 to 20 nm. These nanosized crystallites will have large surface areas (compared to bulk paratacamite) providing enhanced catalytic activity. B. ESR Spectroscopy of Untreated Samples. Carbon Signal. ESR spectra of these samples show signals from carbon and copper species. At 4 K, a very sharp ESR signal at g ) 2.003 and with line width ∆H ) 8 Oe is observed in pure activated carbon support (Figure 3). On increasing the temperature, the signal gradually diminished with temperature T following approximately the 1/T variation of Curie law. The resonance position and line width indicate that the sharp signal is the usual free radical signal from carbon.18,19 Earlier studies on carbon samples have shown that this ESR signal originates from the uncompensated dangling bonds on the carbon surface.18 On loading AC with copper chloride, the carbon signal showed significant changes. This sharp signal at g ) 2.003 could easily be missed in CuCl2-loaded carbon samples due to its very small line width of ∼8 Oe and since its position coincides with a strong sharp signal at g ∼ 2 due to the Cu2+ species. So CA samples (CuCl2/ AC) with Cu loading of 0.25, 0.60, 1.36, 3.0, 6.0, and 9.0 were very carefully examined by scanning a very (19) Manivannan, A.; Punnoose, A.; Seehra, M. S. Mater. Res. Soc. Symp. Proc. 2000, 593, 365-369.

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Figure 5. Plot showing the temperature variations of the ESR line width and resonance field of the broad signal observed from CPA1.

Figure 4. ESR spectra of 3.0 wt % Cu:0.27 wt % Pd:AC (CPA1) catalyst at a few selected temperatures. Receiver gains are suitably chosen for graphical reasons.

small magnetic field range at 4 K (shown in Figure 3). The intensity of this signal determined from accurately weighed samples was found to reduce gradually and systematically with Cu loading and became nonobservable in samples with Cu loading g 6.0 wt %. This shows that the copper chloride species may be interacting with the uncompensated dangling bonds on the carbon surface by forming some kind of surface species and the unpaired spins on the carbon surface get compensated in this process. For Cu loading g 6.0 wt %, absence of the carbon signal indicates the compensation of all the dangling bonds available on the carbon surface. The bond between the Cu2+ species and the carbon surface could be achieved via intervening O or Cl ions and possible presence of such bonds between the Cu2+ and the support via O or Cl have been suggested by others also.7,14,15 In an earlier report19 it has been shown that on exposing AC to oxygen, the ESR signal intensity reduces gradually due to the interaction of paramagnetic oxygen with the dangling bonds, thus reducing the number of unpaired spins on the carbon surface. Park and Lee4 have shown that the nature of the carbon support surface is important in deciding the catalytic activity of the sample. ESR spectra of PdCl2 loaded on AC(4.8PA) showed a very different behavior in which the carbon signal was not affected by the PdCl2 impregnation (Figure 3). This is interesting because when AC is loaded even with a much smaller concentration of CuCl2, the carbon signal showed a large reduction in intensity. From XRD, we have observed earlier that PdCl2 is reduced to Pd0 nanoparticles on interaction with activated carbon. Although the mechanism for this conversion is not clear, it is obvious from the above ESR results that PdCl2 and CuCl2 interact quite differently with activated carbon. Cu2+ Signals. ESR signals due to Cu2+ were observed in all samples loaded with copper chloride, in addition to the carbon signal described above. Representative ESR spectra of CPA1 catalyst at selected temperatures are shown in Figure 4. By carefully monitoring the

temperature variation of the spectrum, two distinctly different ESR signals could be identified. At room temperature, only a broad signal centered near g ) 2.024 with line width ∆H ) 690 Oe could be observed. On lowering the temperature, this signal gradually broadened and shifted to lower fields and in addition to this signal, a new sharp signal started appearing at g ∼ 2, superimposed on the broad signal. Due to the extensive broadening below 100 K, the broad signal became very weak at the lowest temperatures. Parallel with this process, the sharp signal gradually intensified and started showing a 4-line hyperfine structure on lowering the temperature to 4 K. At 4 K, the spectrum shows only the axially symmetric sharp signal with a peak-to-peak width ∆H ) 200 Oe and with g⊥ ) 2.050 and g| ) 2.292. Only the parallel component of the signal showed resolved hyperfine components with A| ) 160 Oe. It is well-known that Cu2+ with electronic spin S ) 1/2 and nuclear spin I ) 3/2 will give rise to an ESR signal with 4 line hyperfine structure. The low line width and the presence of hyperfine splittings are clear evidence that the Cu2+ species giving rise to the sharp signal is a localized isolated species, most probably attached to the carbon support surface. In the recent ESR studies on alumina-supported CuCl2 catalysts, Leofanti et al. 7 have also observed a similar signal with g⊥ ) 2.06 and g| ) 2.32 and A| ) 150 Oe. They have attributed this signal to Cu2+ located in the octahedral vacancies on the alumina surface. The large line width and the absence of any hyperfine structure in the broad signal are clear indications of the extensive spin-spin interaction suggesting that this signal is from concentrated Cu2+ clusters. This signal shifted to lower fields and broadened extensively on decreasing the temperature from 300 K to 4 K as illustrated in Figure 5. This temperature variation is similar to the expected and observed behavior of magnetic nanoparticles.20,21 The absence of any Bragg peaks due to crystalline CuCl2‚2H2O in our XRD investigations is consistent with this interpretation of the presence of CuCl2 nanoparticle clusters in these samples. High (20) Seehra, M. S.; Punnoose, A.; Roy, P.; Manivannan, A. IEEE Trans. Magn. 2001, 37, 2207-2209. (21) Nagata, K.; Ishihara, A. J. Magn. Magn. Mater. 1992, 104107, 1571-1573.

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Figure 6. Temperature variation of the magnetic susceptibility of CPA1 for field-cooled and zero-field cooled samples measured in an applied field of 400 Oe.

dispersion and small size of nanoclusters makes the Bragg peaks too broad for detection. Several workers7,13 have observed similar broad ESR signals in aluminasupported copper chloride catalysts which were attributed to precipitated copper chloride. Presence of copper chloride particles precipitated during the impregnation have been detected in several experiments.7,16 Additional support for the presence of nanoclusters comes from the temperature variations of the magnetization M of the sample CPA1 (Figure 6). The data for the magnetic susceptibility χ ) M/H under the fieldcooled (FC) and zero-field-cooled (ZFC) conditions begins to bifurcate at 350 K with a broad maximum near 300 K suggesting that the sample consists of a broad distribution of particle sizes with average blocking temperature of about 300 K. This bifurcation of the FC and ZFC susceptibility data is typical of magnetic nanoparticles.22 The effects of progressively higher loadings of Cu on the ESR spectra of the Cu2+ species are shown in Figure 7. Here we show the 4 K ESR spectra of the CuCl2/ activated carbon (CA) samples, loaded with 0.25, 0.60, 1.36, 3.0, 6.0, and 9.0 wt % of Cu. For the weakly loaded samples, viz., 0.25CA, 0.6CA, 1.36CA, and 3.0CA, the four-line hyperfine structure is clearly visible. However, for high Cu loadings, the hyperfine structure disappears and the line width broadens because of the well-known effect of Cu2+-Cu2+ exchange interaction which become important as the Cu2+ concentration increases. The presence of the broad line due to nanoclusters is only barely evident at 4 K, consistent with the results of Figure 4. This effect is useful in accurate determinations of the intensities of the sharp structured signal because of the noninterference from the broad signal at 4 K. The change in intensity of the ESR signals at 4 K as a function of Cu loadings in carefully weighed samples of equal mass is shown in Figure 8. Initially the intensity increases rapidly with Cu loading, saturating at about 6%. As noted earlier, the intensity of the carbon ESR line decreases with increase in Cu loading, disappearing at about 6% Cu. This inverse correlation between the intensities of the carbon ESR and the Cu2+ ESR signals (22) Seehra, M. S.; Babu, V. S.; Manivannan, A.; Lynn, J. W. Phys. Rev. B 2000, 61, 3513-3518.

Figure 7. ESR spectra of the CuCl2/activated carbon catalysts(CA) with Cu loading of 0.25, 0.60, 1.36, 3.0, 6.0, and 9.0 wt % recorded at 4 K illustrating the changes in the structured sharp signal with Cu loading. Receiver gains are suitably chosen for graphical reasons.

Figure 8. Plot showing the variation of the normalized intensity of the sharp structured Cu2+ ESR signal and the wt % of DEC produced as a function of Cu loading on activated carbon.

suggests that Cu2+ species are attached to carbon dangling bonds responsible for the carbon signal. C. ESR Spectroscopy of KOH-Treated Samples. The KOH treatment of the CPA (CuCl2/PdCl2/activated carbon) catalysts is observed to make significant changes in the ESR spectra of both the Cu2+-carbon species and the Cu2+ nanoclusters. Also, the changes depend on the x ) OH/Cu ratio. In Figure 9, we show the 4 K ESR spectra for different values of the ratio x. Not evident from Figure 9 is the drastic reductions in the intensity of the structured-spectra because of the changes in the gain settings of the spectrometer. However, it is quite evident that as x increases toward 1, the resolution of the hyperfine structure improves because of the reduction in the spin-spin interactions as a result of the

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Figure 10. ESR spectra of KOH-treated CPA catalysts with different OH/Cu ratios recorded at 150 K. Receiver gains are suitably chosen for graphical reasons. Figure 9. ESR spectra of KOH-treated CPA catalysts with different OH/Cu ratios recorded at 4 K. Nearly equal quantities (11 mg) of samples were used for the measurements and the receiver gains for each measurement are indicated on the spectra (higher gains means lower intensity).

lowered density of Cu2+-carbon species. So an increase in x results in three effects: lowered intensity, improved resolution of the A| components, and appearance of the hyperfine splitting of the g⊥ components for x > 1. Appearance of the A⊥ components for x ) 3 and 5 shows that the concentration of the Cu2+-carbon species is extremely low, resulting in near elimination of the spin-spin interactions. For x ) 3, a new signal at g ) 2.191 is also observed whose intensity increases for x ) 5 (Figure 9). Since the XRD data show the formation of calumetite, Cu(OH,Cl)2‚2H2O, for x ) 5, this new line at g ) 2.191 is very likely due to the formation of calumetite, whose high concentration as detected by XRD is responsible for the absence of a hyperfine structure. We note that even for x ) 5, ESR from the Cu2+-carbon species, although quite weak, is still present. To investigate changes in the broad signal due to Cu nanoclusters, we show in Figure 10 the ESR spectra measured at 150 K, at which temperature the broad signal is easily distinguishable from the narrow signal from the Cu2+-carbon species. It is evident from Figure 10 that the broad signal also diminishes in intensity as the x ) OH/Cu ratio increases. From these observations, it is inferred that the concentrations of both types of ESR-active Cu2+ species decrease with increase in x, most likely due to their conversion to some ESR-inactive species. Since our XRD studies in this range of x values show the formation of paratacamite, Cu2Cl(OH)3, we investigated ESR spectroscopy of pure phase, laboratory-prepared paratacamite sample, which was found to be ESR-silent in the 4 K to 300 K range. It is noted that some other Cu2+ compounds are also found to be

ESR-silent for a variety of reasons.23,24 Summarizing the above observations, our ESR and XRD results can be explained if the ESR-active Cu2+ species are converted to paratacamite as x increases from 0 to 1. For x ) 3 and 5, paratacamite is converted to calumetite. D. Comparison with Catalytic Activity for DEC Conversion. Here we compare the results obtained from the above investigations with the % yield of diethyl carbonate (DEC) obtained by Dunn et al.1 using the CuCl2/PdCl2/AC catalysts, with and without the KOH treatment. Studies by Dunn et al. were carried out at 150 °C, and at these temperatures the catalysts are thermally stable.1 Consequently, this comparison has validity since the surface/structural properties of the catalysts at 150 °C are not expected to be different from our case. As noted earlier, CuCl2/AC without the PdCl2 loading does have a maximum activity of about 4% for DEC production. In Figure 8, we compare the relative changes in the normalized ESR intensity of the structured-line due to Cu2+-carbon species and the weight percent of DEC produced as a function of weight percent of Cu loading in the CuCl2/AC catalysts. An excellent correlation is obtained, suggesting that the ESR-active Cu2+-carbon species may indeed be responsible for the catalytic activity, albeit small, for the CuCl2/AC catalysts. For the Pd-loaded, KOH-treated catalysts, the % yield for DEC production depends on the OH/Cu ratio, with a maximum of about 18% for OH/Cu ) 1. At the same time, ESR-silent paratacamite is detected in our XRD studies whereas the ESR intensity of the lines from the Cu2+ species decreases dramatically. This suggests that a correlation between the concentration of paratacamite and the % yield of DEC may exist. In Figure 11, we show the comparison between the relative changes in the (23) See review article, Punnoose, A.; Singh, R. J. Int. J. Mod. Phys. B 1995, 9, 1123-1157. (24) Okazaki, M.; Toriyama, K.; Tomura, S.; Kodama, T.; Watanabe, E. Inorg. Chem. 2000, 39, 2855-2860.

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Figure 11. The normalized XRD intensity of the paratacamite peak at 2θ = 16° and the DEC conversion activity of the KOHtreated CPA catalysts as a function of the OH/Cu ratio.

intensity of the main paratacamite Bragg peak near 2θ = 16° and % yield for DEC production with change in the OH/Cu ratio. An excellent correlation is obtained in the whole range, suggesting the key role played by paratacamite in the production of DEC. For OH/Cu > 1, XRD studies show the conversion of paratacamite to calumetite, accompanied by the dramatic drop in the DEC % yield to insignificant levels.

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As noted earlier, bimetallic CuCl2/PdCl2 catalysts have been used in other reactions such as the lowtemperature oxidation of CO to CO2.2-6 To explain this reaction, the Walker chemistry has been used in which PdCl2 is converted to Pd0 by the oxidation of CO, and the role of Cu2+ from CuCl2 is to regenerate Pd0 to Pd2+ to continue the reaction. A similar mechanism may be valid in the DEC conversion, in which paratacamite is more efficient than CuCl2 for regenerating Pd0 to Pd2+. Interestingly, Park and Lee 3-6 have suggested paratacamite as superior to other copper phases for CO oxidation. It is noted that Pd species seems to be effective only with a Cu2+ source since PdCl2 loaded on AC did not show any appreciable activity for DEC.1 Since our XRD results show that PdCl2/AC converts to Pd0, it follows that the catalytic activity is due to Pd2+ and not Pd0. For the CuCl2/AC catalysts without PdCl2 loading, the observed small activity is assigned to the Cu2+-carbon species. In this regard, the activated carbon support may be playing a crucial role since CuCl2 impregnated in an identical manner on alumina and silica supports did not show any significant activity.1 Acknowledgment. This research was supported in part by the U.S. Department of Energy (Contract No. DE-FC26-99FT40540). Listing of any commercial equipment in this paper does not imply endorsements by the authors, their employers, or the U.S. Department of Energy. EF010180D