Nonthermal RF Plasma Modifications on Pd - ACS Publications

Texas A&M UniVersity-Commerce, Chemistry Department, P.O. Box 3011, Commerce, Texas 75429-3011. Nonthermal RF plasma modification has been ...
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Ind. Eng. Chem. Res. 2006, 45, 5879-5884

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Nonthermal RF Plasma Modifications on Pd/γ-Al2O3 for Selective Hydrogenation of Acetylene in the Presence of Ethylene Chunkai Shi and Ben W.-L. Jang* Texas A&M UniVersity-Commerce, Chemistry Department, P.O. Box 3011, Commerce, Texas 75429-3011

Nonthermal RF plasma modification has been applied to alumina-supported Pd catalysts for selective hydrogenation of acetylene in the presence of ethylene. High ethylene selectivity and good acetylene conversion activity were obtained at low temperatures on the H2 plasma-treated catalyst without the in situ reduction procedure. The results suggest some Pd nitrate was reduced to Pd metals during H2 plasma treatments at room temperature. XRD, FT-IR, and TPR results confirm the reduction effect of plasma treatments. The results also suggest that the H2 plasma treatment produces more active sites on the surfaces of aluminasupported Pd catalyst for selective hydrogenation of acetylene to ethylene and limits the production of ethane and green oil than the catalysts prepared using the conventional procedures. It demonstrates that the RF nonthermal plasma treatment is an effective way to manipulate surface properties and the interaction between metals and supports for the design of novel catalysts for selective hydrogenation of acetylene. 1. Introduction Selective hydrogenation of acetylene in the presence of ethylene is an important industrial process to purify ethylene for polymerization. A small increase of ethylene selectivity or yield represents a large savings for the industrial process. Alumina-supported Pd is a well-known catalyst for this industrially important application. Significant results have been reported based on alumina-supported Pd catalysts with additional components, such as Ag, SiO2, TiO2, etc., and with pretreatments of oxygen or oxygen-containing compounds.1-6 However, further development of novel catalysts with an increased selectivity, limited formation of the green oil production, and reduced cost of operation is still desirable for commercial applications. Nonthermal plasma-based techniques for catalyst preparation have recently attracted significant attention for catalyst design and development.7-9 Highly active and selective catalysts have been synthesized via various nonthermal plasma techniques. For example, alumina-supported nickel catalysts modified by nonthermal RF plasma treatments were investigated for benzene hydrogenation. Air and H2 plasma treatments, either before or after impregnation, can significantly change the structure of a Ni precursor supported on alumina and the interaction between them.9 Both plasma modifications before and after impregnation of metal precursors are effective in improving the activity and stability of Ni catalysts for benzene hydrogenation. The plasma effect on the activity and stability of Ni catalysts for benzene hydrogenation is proposed due to the modified metal-support interaction induced by plasma modifications.9 Another example is the unique interface produced by nonthermal plasma treatments.10,11 It is reported that Ni(NO3)2/Ta2O5 and Ni(NO3)2/ ZrO2 catalysts treated with argon plasmas followed by hightemperature calcination and reduction show completely different metal-support interfaces from the catalysts exposed only to high-temperature procedures. Much cleaner metal-support interface with negligible interdiffusion was observed on the catalysts with plasma treatments. Although the nonthermal plasma treatments were operated on catalysts at room temper* To whom correspondence should be addressed. E-mail: ben_jang@ tamu-commerce.edu.

ature, the effect lasted through high-temperature calcinations and reductions.10 Others use various nonthermal plasmas for the preparation of gold-doped bimetallic zeolite catalysts,12 Fe2O3/ZSM-5 catalyst,13,14 and Ni-Fe/Al2O3,15 for FischerTropsch synthesis and partial oxidation of natural gas to syngas. Examples of plasma techniques specifically for Pd catalysts and acetylene hydrogenation include reports by Zea et al. and Chen et al.16,17 Highly dispersed Pd metal supported on carbon catalysts was produced with the high-temperature plasma torch system.16 Better selectivity for acetylene hydrogenation was found over the plasma-produced Pd catalysts. However, a low activity was reported. Chen et al., on the other hand, used a glow discharge nonthermal plasma to modify R-Al2O3-supported Pd catalysts.17 Although the activity and selectivity of plasmamodified catalysts are better than the conventional catalysts for acetylene hydrogenation, the study was not investigated in the presence of ethylene. This study focuses on using the RF nonthermal plasma system with a 360° rotating chamber to produce unique alumina-supported Pd catalysts for selective hydrogenation of acetylene in the presence of ethylene. 2. Experimental Section 2.1. Catalyst Preparation. One-eight inch bimodal γ alumina pellets (from Alfa Aesar), medium pore of 70 µm and 5000 Å, with a 1.14 cm3/g pore volume were crushed and screened to 20-40 mesh sizes. The 20-40 mesh alumina particles were then dried at 200 °C for 12 h followed by cooling in a desiccator to room temperature. The resulted alumina particles were impregnated with calculated palladium nitrate solution (Alfa), based on the incipient wetness technique for 1% Pd metal loading. The resulted material followed by drying at 120 °C for 12 h in static air is designated as uncalcined Pd/Al2O3 catalyst. The uncalcined 1%Pd/Al2O3 catalysts that were treated with H2 plasmas are designated as H2-plasma Pd/Al2O3. The uncalcined 1%Pd/Al2O3 that was further calcined at 500 °C in air for 3 h is designated as calcined Pd/Al2O3. 2.2. Plasma Treatment. Plasma treatments for catalytic materials were carried out in the custom-designed 360° rotating RF plasma system. The details of the system and the schematic are described in a previous publication.9 A system pressure of 400 mTorr was used for plasma treatments in this study.

10.1021/ie0602512 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/26/2006

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Typically, 1 g of catalysts was loaded into the plasma chamber for plasma treatments. The duration of plasma treatment was set at 30 min with a continuous waveform of 160 W. 2.3. Catalyst Characterization. 2.3.1. Temperature-Programmed Reduction (TPR). TPR tests in H2 were carried out with the automatic catalyst characterization system, AMI-200 (Altamira Instruments). About 100 mg of samples was treated at 150 °C for 60 min and then cooled to 30 °C in argon followed by exposure in 33% H2/Ar for baseline stabilization of thermal conductivity detector (TCD). Finally, the sample bed temperature was linearly increased from 30 to 600 °C at a ramping rate of 10 °C/min. The consumption of H2 during TPR was measured by a TCD. 2.3.2. CO Chemisorption. All catalysts were reduced in situ in hydrogen at 300 °C for 2 h before CO chemisorption at 30 °C using the AMI-200 system. The CO/Pd ratio was assumed to be 1:1 for dispersion calculation. 2.3.3. Fourier Transform Infrared Spectroscopy (FT-IR) Analysis. 20-40 mesh fresh or used catalysts were grounded and pressed into pellets for FT-IR transmission analyses using the Nicolet Nexus 470 system. Typically, a 1:40 ratio of catalysts to KBr was mixed for pellet preparation. 2.3.4. X-ray Diffraction (XRD) Analyses. X-ray powder diffraction patterns were recorded on a Siemens D500 diffractometer using Ni-filtered Cu KR radiation with λ ) 1.5418 Å in θ-2θ scan mode. 2.4. Reaction Studies. All catalysts were tested for the selective hydrogenation of acetylene from 25 to ∼100 °C. A 1/ in. stainless steel reactor housed in a GC oven with 4 temperature-programmed heating and cooling capabilities at low-temperature ranges was used. One group of catalysts underwent purging with UHP nitrogen at room temperature before the reaction testing was done. A temperature-programmed procedure with a 0.2 °C/min ramping rate was used. The other group of catalysts was reduced in UHP hydrogenation at 300 °C for 2 h followed by purging with nitrogen before testing. Reaction study was carried out with a space velocity of 84000 sccm/h-g catalyst over ∼30 mg of catalyst; 40 sccm of the premixed gas consisting of 98.8% C2H4 and 1.2% C2H2 was mixed with 2 sccm of UHP H2 to introduce into the microreactor. All flows were delivered by mass flow controllers. The analysis of the gas components from the microreactor was performed by an on-line Shimadzu GC 17A equipped with a 30 m × 0.32 mm (i.d.) × 1.50 µm GS-CARBONPLOT capillary column operating at 80 °C and a FID detector. Possible products for acetylene hydrogenation are ethylene and ethane. The conversion of acetylene to ethylene is desirable, but the conversions of acetylene to ethane and ethylene to ethane are not economical since they both reduce the net ethylene content. Acetylene/ethylene conversion to green oil is considered negligible in this study since the reaction time is short. The selectivity to ethylene is defined as

% C2H4 selectivity ) (1 produced C2H6/converted C2H2) × 100 Positive % C2H4 selectivity refers to a net gain of ethylene during the hydrogenation process and negative selectivity results in ethylene loss. 3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. Pd Dispersion Measurement. Table 1 summarizes the Pd dispersion results of uncalcined, calcined, and H2-plasma Pd/Al2O3 catalysts. The

Table 1. Pd Dispersion of Catalysts catalyst

Pd dispersion (%)

H2-plasma Pd/Al2O3 calcined Pd/Al2O3 uncalcined Pd/Al2O3

29 19 22

Figure 1. XRD patterns of uncalcined, calcined, and H2-plasma Pd/Al2O3.

H2-plasma Pd/Al2O3 catalyst shows a somewhat higher Pd dispersion value than other catalysts, suggesting that the H2 nonthermal RF plasma treatment can improve the Pd dispersion on γ-Al2O3, similar to the results reported by Zea et al., though the effect is not prominent.16 The effect is likely due to the reduction of some Pd(NO3)2 to Pd metals at room temperature during the H2 plasma treatment as discussed in the following sections. 3.1.2. XRD Analyses. To explore the possibility of decomposition and reduction of Pd(NO3)2 supported on Al2O3 during the H2 plasma treatment, the XRD analyses of uncalcined, calcined, and H2-plasma Pd/Al2O3 catalysts were carried out. The results are shown in Figure 1. All samples show typical diffraction peaks of γ-Al2O3 between 2θ ) 20-80° (JCPDS 10-425). The catalyst calcined at 500 °C also shows a characteristic peak at 2θ ) 33.9° attributed to the {101} crystalline plane of PdO phase (JCPDS 6-0510). Interestingly, a diffraction peak around 2θ ) 40° attributed to the {111} planes of metal Pd is detected from the XRD patterns of H2plasma Pd/Al2O3.2 This confirmed that the surface Pd(NO3)2 on γ-Al2O3 support can be effectively reduced to metal Pd particle under the H2 plasma treatment condition while the γ-Al2O3 crystallization structure is not affected. 3.1.3. FT-IR Analyses. The decomposition and reduction of Pd(NO3)2 supported on γ-Al2O3 are further studied by FT-IR. The spectra of uncalcined, calcined, and H2-plasma Pd/Al2O3 samples are shown in Figure 2. As shown in Figure 2, the uncalcined and H2-plasma Pd/Al2O3 samples show a peak around 1384 cm-1 assigned to asymmetric stretching vibration mode of NO3-, in addition to the peaks at 3480 and 1633 cm-1 attributed to IR spectra of H2O from samples.18 The 1384 cm-1 peak of the uncalcined Pd/Al2O3 is significantly higher than the same peak of H2-plasma Pd/Al2O3. Together with the XRD results, it suggests that part of the surface Pd(NO3)2 on γ-Al2O3 support can be reduced to Pd metal under the H2 plasma treatment condition. On the other hand, the 1384 cm-1 peak is not detected for the calcined sample, which suggests the palladium nitrate is completely converted to PdO after the calcination at 500 °C for 3 h.

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Figure 2. FT-IR spectra of uncalcined, calcined, and H 2-plasma Pd/Al2O3.

Figure 3. TPR of uncalcined, calcined, and H2-plasma Pd/Al2O3.

3.1.4. TPR Analyses. Figure 3 compares the results of TPR over three catalysts. A low-temperature negative peak at around 122 °C is the only observed peak in the TPR profile of calcined Pd/Al2O3. It is reported that PdO is easily reduced to metallic Pd with hydrogen at room temperature or below and the reduced Pd metals absorb hydrogen to form hydride under the same condition.19-21 Thus, it is expected that PdO on the calcined Pd/Al2O3 sample will be reduced with hydrogen and then absorbs hydrogen during the TCD detector stabilization period of the TPR analysis process before starting H2-TPR. As shown in Figure 3a, only a negative low-temperature peak was detected. The low-temperature peak is attributed to the hydrogen release from the reduced metal Pd particles. Similar desorption results were reported in the literature.4,22,23 On the other hand, it is anticipated that the TPR profile of H2-plasma Pd/Al2O3 should show a similar low-temperature H2 desorption peak as the calcined Pd/Al2O3 because H2 plasma can reduce part of the supported Pd(NO3)2 to Pd metals, evident by the XRD and FT-IR results. Indeed, as shown in Figure 3b, H2-plasma Pd/Al2O3 also shows the negative peak at 122 °C similar to the calcined Pd/Al2O3. In addition, two H2 consumption peaks are also shown on the TPR profile. Those two peaks are due to the reduction of unreduced Pd(NO3)2 after the H2 plasma treatment during TPR.

Figure 4. Activities of uncalcined, calcined, and H2-plasma Pd/Al2O3 without the in situ reduction.

In the case of uncalcined Pd/Al2O3, the TPR result, as shown in Figure 3c, shows a profile similar to that of H2-plasma Pd/ Al2O3 with two high-temperature H2 consumption peaks at ∼195 and 400 °C and also with the negative peak at 122 °C. The negative peak of TPR over the uncalcined Pd/Al2O3 is somewhat surprising because there is no literature indicating that Pd(NO3)2 could be reduced around room temperature in hydrogen and only Pd(NO3)2 is expected to be on the surfaces of uncalcined Pd/Al2O3. However, the TPR result strongly suggests that the reduction of Pd(NO3)2 to Pd metals on alumina supports is likely in our case as it has been speculated that Pd ion in its nitrate form may be reduced to metal Pd at low temperature.24 To further explore the nature of two consumption peaks of TPR profiles of uncalcined and H2-plasma Pd/Al2O3, additional FT-IR analyses on these two samples after reduction in hydrogen at 300 °C for 2 h were performed. The presence of the peak of NO3- at 1380 cm-1 on FT-IR spectra of these two samples suggests that the supported Pd(NO3)2 on γ-Al2O3 is not completely reduced with hydrogen, even at 300 °C for 2 h. It further suggests that two possible Pd(NO3)2 species may exist on γ-Al2O3 surfaces, one reduced at a lower temperature than 300 °C and one at a higher temperature than 300 °C. Based on the additional FT-IR analyses and the TPR results in Figure 3, it is proposed that the two high-temperature peaks, at ∼195 and 400 °C in Figures 3b and 3c, are attributed to hydrogen consumption for the reduction of different Pd(NO3)2 species on γ-Al2O3. However, further investigation is needed to identify the difference of the palladium species. 3.2. Catalytic Reaction Study. 3.2.1. Catalytic Performance without in Situ H2 Reduction. Acetylene conversion and the selectivity to ethylene over uncalcined, calcined, and H2-plasma Pd/Al2O3 catalysts were measured in the presence of ethylene. Before the reaction testing, all catalysts were only purged with an ultrahigh purity nitrogen flow without any reduction. The activity and selectivity results are shown in Figures 4 and 5, respectively. The acetylene conversion activity of H2 plasmatreated Pd/Al2O3 is significantly higher than those of uncalcined or calcined Pd/Al2O3. As shown in Figure 4, the activity over H2-plasma Pd/Al2O3 gradually increases from 38% to 100% when the reaction temperature increases from 25 to 45 °C. Uncalcined and calcined catalysts, on the other hand, do not show any significant activities below 31 °C, but their activities increase quickly with increasing reaction temperature. As for the ethylene selectivity, the H2-plasma Pd/Al2O3 catalyst shows

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Figure 5. Selectivities of uncalcined, calcined, and H2-plasma Pd/Al2O3 without the in situ reduction.

high selectivity, up to 90%, at temperatures below 31 °C and with good acetylene conversion activities. The selectivities of uncalcined and calcined Pd/Al2O3 remain low, ranging from 20% to 40%, in this temperature range, though a subsequent selectivity increase takes place with the increasing reaction temperature from 30 to 40 °C and then a sudden drop in selectivity occurs. The H2-plasma Pd/Al2O3 catalyst shows both much higher acetylene conversion and superior ethylene selectivity at low temperatures compared to other catalysts. This is due to H2 nonthermal RF plasma treatment reducing part of Pd(NO3)2 on γ-Al2O3 to active Pd metals, which is verified by our XRD, FT-IR, and TPR results. Uncalcined and calcined catalysts showing much lower activity and selectivity in the lower temperature region are due to the lack of reduced Pd in the beginning of the testing process. As the testing continues, PdO and Pd(NO3)2 are reduced to Pd metals and the activity and selectivity increase. However, as the reaction temperature increases, the selectivity to ethylene decreases, possibly due to the thermal effect of hydrogenation reactions. 3.2.2. Catalytic Performance of Catalysts with in Situ H2 Reduction. Figures 6 and 7 summarize the results of acetylene conversion and selectivity to ethylene over uncalcined, calcined, and H2-plasma Pd/Al2O3 catalysts with the in situ hydrogen reduction at 300 °C for 2 h. Compared to the uncalcined and calcined Pd/Al2O3 catalysts, the H2-plasma Pd/Al2O3 catalyst shows higher acetylene conversion, which increases from ∼19% to 100% between 25 and 60 °C, and a similar ethylene selectivity, with more than 90% between 25 and 45 °C. The selective hydrogenation of acetylene occurs when acetylene adsorbs on different active Pd metal surfaces. Three active Pd metal sites have been proposed to explain acetylene selectivity to ethylene, ethane formation by direct acetylene hydrogenation, and oligomer formation from acetylene.3,25,26 Apparently, improving Pd active sites for selective hydrogenation of acetylene to ethylene on catalyst surfaces is most desirable. On the other hand, the metal-support interaction could also be an important factor. Plasma treatment effects on the modifications of interaction between surface metals and supports have been reported.9,10,16,17 The XRD, FT-IR, and TPR results suggest that the H2 plasma treatment not only partly reduces Pd(NO3)2 to Pd metals but also possibly changes the interaction between Pd(NO3)2 and supports which affects the resulting TPR profile.

Figure 6. Activities of uncalcined, calcined, and H2-plasma Pd/Al2O3 with the in situ reduction.

Figure 7. Selectivities of uncalcined, calcined, and H2-plasma Pd/Al2O3 with the in situ reduction.

It is speculated that the H2 plasma treatment may not only improve the population of active sites for selective hydrogenation of acetylene to ethylene but also limit two other active sites for ethane formation and oligomer (green oil) formation from acetylene. This is consistent with the activity and selectivity results of catalysts in Figures 6 and 7. In addition, all catalysts’ selectivity dramatically decreases after 45 °C. It is found that the FT-IR spectra (not shown) of used catalysts show absorbance at 2960, 2930, 2880, and 2860 cm-1 for the asymmetric C-H stretch vibrations of CH3 and CH2 after hydrogenation reaction.27 This suggests that some limited oligomer was formed on catalysts during acetylene hydrogenation under our reaction conditions. The oligomer formed can block active sites of selective acetylene conversion to ethylene and consumes feedstock acetylene which can also lead to the selectivity decrease. It is suggested that ethylene hydrogenation occurs on the support via the hydrogen transfer mechanism. The hydrogen transfer from Pd to supports is furnished by the deposited carbonaceous as the hydrogen transfer bridge.3 This may lead to a further decrease of acetylene selectivity to ethylene. On the other hand, compared to the activity and selectivity of catalysts without in situ hydrogenation (Figures 4 and 5), the H2-plasma catalyst, with in situ hydrogen reduction (Figures

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6 and 7), shows lower activity and higher selectivity while the uncalcined and calcined catalysts show higher activities and selectivity. The activities and selectivity increase for uncalcined and calcined catalysts can be simply attributed to the increase of surface metallic Pd after the in situ reduction. In the case of H2-plasma Pd/Al2O3, the decrease in activity may be attributed to the decrease in the two active sites for ethane formation and oligomer formation, and the selectivity increase can also be attributed to the same effect or to an increase in the active sites for acetylene selective hydrogenation to ethylene after the in situ high-temperature hydrogen reduction at 300 °C. This is consistent with the literature because reduction in high temperatures have been extensively reported for Pd catalysts to show higher ethylene selectivity in acetylene hydrogenation reaction, but normally resulting in lower activities.5,6 The fact that the H2 plasma treatment on Pd/Al2O3 catalyst, without the in situ reduction, can increase not only the selectivity but also the activity for acetylene selective hydrogenation at low temperatures compared to two other catalysts both with or without the in situ reduction is phenomenal. The Pd nitrate reduction to Pd metals can explain only part of the unusual activity and selectivity of H2-plasma Pd/Al2O3 because the percentage of Pd nitrate reduced during the H2 plasma treatment is small based on the thermogravimetric analysis result. It is speculated that a special metal-support interaction between Pd metals/Pd nitrate and alumina is induced by the plasma treatment. And the plasma effects on catalysts last through the in situ reduction at 300 °C. Similar phenomena were observed on other plasma-treated catalysts.9-11 It is reported that the oxygen vacancies and/or surface defects of metal oxide surfaces will affect the nucleation, the growth, and the stability of metal clusters.28,29 Similar defects and vacancies are possibly and likely generated by bombardments of electrons, radicals, and ions during the plasma treatments in this study. Therefore, the metalsupport interaction will be altered depending on the detailed order and conditions of pretreatments, including plasma treatments, before reactions. However, additional high-resolution surface analyses are needed to clarify the speculation. 4. Conclusion It is reported that the RF nonthermal plasma is effective to modify the alumina-supported Pd catalysts for selective hydrogenation of acetylene in the presence of ethylene. High ethylene selectivity and good acetylene conversion activity were obtained on the H2 plasma-treated catalyst at low temperatures without the in situ reduction procedure. It is concluded that the H2 plasma treatment can reduce some Pd nitrate on alumina surfaces to Pd metals at room temperature. XRD, FT-IR, and TPR results confirm the reduction effect of H2 plasma treatments. The results also suggest that the H2 plasma treatment is likely to produce more active sites for selective hydrogenation of acetylene to ethylene and to limit the production of ethane and green oil on supported Pd catalysts than the conventional procedures. The interaction between Pd metals/Pd nitrate and alumina support is proposed to be modified by the RF plasma treatments. It demonstrates that the RF nonthermal plasma treatment is an effective way to manipulate surface properties for the design of novel catalysts for selective hydrogenation of acetylene. Acknowledgment The authors wish to acknowledge the financial support partly by the Welch Foundation and the Research Enhancement Grant

of Texas A&M Univeristy-Commerce. XRD analyses were carried out by Dr. Zhongchun Wang at the University of New Mexico. Literature Cited (1) Liu, R. J.; Crozier, P. A.; Smith, C. M.; Hucul, D. A.; Blackson, J.; Salaita, G. Metal sintering mechanisms and regeneration of palladium/ alumina hydrogenation catalysts. Appl. Catal., A 2005, 282, 111. (2) Praserthdam, P.; Ngamsom, B.; Bogdanchikova, N.; Phatanasri, S.; Pramotthana, M. Effect of the pretreatment with oxygen and/or oxygencontaining compounds on the catalytic performance of Pd-Ag/Al2O3 for acetylene hydrogenation. Appl. Catal., A 2002, 230, 41. (3) Lamb, R. N.; Ngamsom, B.; Trimm, D. L.; Gong, B.; Silveston, P. L.; Praserthdam, P. Surface characterisation of Pd-Ag/Al2O3 catalysts for acetylene hydrogenation using an improved XPS procedure. Appl. Catal., A 2004, 268, 43. (4) Ngamsom, B.; Bogdanchikova, N.; Borja, M. A.; Praserthdam, P. Characterisations of Pd-Ag/Al2O3 catalysts for selective acetylene hydrogenation: effect of pretreatment with NO and N2O. Catal. Commun. 2004, 5, 243. (5) Kang, J. H.; Shin, E. W.; Kim, W. J.; Park, J. D.; Moon, S. H. Selective Hydrogenation of Acetylene on TiO2-Added Pd Catalysts. J. Catal. 2001, 208, 310. (6) Jin, Y.; Datye, A. K.; Rightor, E.; Gulotty, R.; Waterman, W.; Smith, M. The Influence of Catalyst Restructuring on the Selective Hydrogenation of Acetylene to Ethylene. J. Catal. 2001, 203, 292. (7) Liu, C.-J.; Vissokov, G. P.; Jang, B. W.-L. Catalyst Preparation Using Plasma Technologies. Catal. Today 2002, 72, 173. (8) Jang, B. W.-L., Hammer, T., Liu, C., Eds. Plasma Technology and Catalysis. Catal. Today 2004, 89 (1-2). (9) Ratanatawanate, C.; Macias, M.; Jang, B. W.-L. The Promotion Effect of Nonthermal RF Plasma on Ni/Al2O3 for Benzene Hydrogenation. Ind. Eng. Chem. Res. 2005, 44, 9868. (10) Liu, C.; Zou, J.; Yu, K.; Cheng, D.; Han, Y.; Zhan, J.; Ratanatawanate, C.; Jang, B. W.-L. Plasma Application for More Environmentallyfriendly Catalyst Preparation. Pure Appl. Chem., in press. (11) Zou, J.-J.; Liu, C.-J.; Zhang, Y.-P. Control of the Metal-Support Interface of NiO-Loaded Photocatalysts via Cold Plasma Treatment. Langmuir 2006, 22, 2334. (12) Diamy, A.-M.; Randriamanantenasoa, Z.; Legrand, J.-C.; PolissetThfoin, M.; Fraissard, J. Use of a Dihydrogen Plasma Afterglow for the Reduction of Zeolite-Supported Gold-based Metallic Catalysts. Chem. Phys. Lett. 1997, 269, 327. (13) Dadashova, E. A.; Yagodovskaya, T. V.; Beilin, L. A.; Shpiro, E. S.; Lunin, V. V. Modification of Fe2O3/ZSM-5 Catalyst of Fischer-Tropsch Synthesis by Glow Discharge in Oxygen and in Argon. Kinet. Catal. 1991, 32, 1350. (14) Dadashova, E. A.; Yagodovskaya, T. V.; Shpiro, E. S.; Beilin, L. A.; Lunin, V. V.; Kiselev, V. V. The Synthesis of Fe2O3/ZSM-5 Catalyst for Carbon Monoxide Hydrogenation I Glow Discharge of Oxygen and Argon. Kinet. Catal. 1993, 34, 670. (15) Wang, J.; Liu, C.; Zhang, Y.; Yu, K.; Zhu, X.; He, F. Partial Oxidation of Methane to Syngas over Glow Discharge Plasma Treated NiFe/Al2O3 Catalyst. Catal. Today 2004, 89 (1-2), 183. (16) Zea, H.; Chen, C. K.; Lester, K.; Phillips, A.; Datye, A.; Fonseca, I.; Phil, J. Plasma torch generation of carbon supported metal catalysts. Catal. Today 2004, 89, 237. (17) Chen, M. H.; Chu, W.; Dai, X. Y.; Zhang, X. W. New palladium catalysts prepared by glow discharge plasma for the selective hydrogenation of acetylene. Catal. Today 2004, 89, 201. (18) Ehrhardt, C.; Gjikaj, M.; Brockner, W. Thermal decomposition of cobalt nitrato compounds: Preparation of anhydrous cobalt(II)nitrate and its characterisation by Infrared and Raman spectra. Thermochim. Acta 2005 432, 36. (19) Sica, A. M.; Gigola, C. E. Interaction of CO, NO and NO/CO over Pd/γ-Al2O3 and Pd-WOx/γ-Al2O3 catalysts. Appl. Catal., A 2003, 239, 121. (20) Sandoval, V. H.; Gigola, C. E. Characterization of Pd and Pd--Pb/R-Al2O3 catalysts. A TPR-TPD study. Appl. Catal., A 1996, 148, 81. (21) Zhang, Q.; Li, J.; Liu, X.; Zhu, Q. Synergetic effect of Pd and Ag dispersed on Al2O3 in the selective hydrogenation of acetylene. Appl. Catal., A 2000, 197, 221. (22) Diaz, E.; Ordonez, S.; Vega, A.; Coca, J. Adsorption properties of a Pd/γ-Al2O3 catalyst using inverse gas chromatography. Microporous Mesoporous Mater. 2004, 70, 109. (23) Chou, C. W.; Chu, S. J.; Chiang, H. J.; Huang, C. Y.; Lee, C. J.; Sheen, S. R.; Perng, T. P.; Yeh, C. T. Temperature-programmed reduction study on calcination of nano-Palladium. J. Phys. Chem. B 2001, 105, 9113.

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ReceiVed for reView March 1, 2006 ReVised manuscript receiVed June 23, 2006 Accepted June 28, 2006 IE0602512