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Ind. Eng. Chem. Res. 2007, 46, 4390-4395
Promotion Effects of Air and H2 Nonthermal Plasmas on TiO2 Supported Pd and Pd-Ag Catalysts for Selective Hydrogenation of Acetylene Chunkai Shi, Ryan Hoisington, and Ben W.-L. Jang* Chemistry Department, Texas A&M UniVersitysCommerce, Commerce, Texas 75429-3011
Selective hydrogenation of acetylene in rich ethylene was investigated over uncalcined, air plasma, and H2 plasma Pd/TiO2 and Pd-Ag/TiO2 catalysts. Air and H2 plasma treatments have different modifying effects on Pd/TiO2 catalyst for selective hydrogenation of acetylene to ethylene. With additional Ag loadings, 1% and 3%, on Pd/TiO2, plasma treatments shift the temperature of maximum C2H4 yield to lower temperatures as compared to Ag shifting the maximum yield temperature to higher temperatures. Air plasma treatment alone on Pd/TiO2 produces the highest maximum C2H4 yield at the lowest temperature among all catalysts tested. H2 plasma, on the other hand, is more effective in enhancing the performance of the Pd-1%Ag/TiO2 catalyst. Both air and H2 plasmas show the capability to shift the maximum yield temperatures of Pd/TiO2 and Pd-Ag/TiO2 catalysts to lower temperatures. It provides a new avenue to fine-tune the catalytic reactivity of supported Pd catalysts with or without the Ag promoter to selectively convert acetylene to ethylene in excess of ethylene to meet various industrial demands. Introduction Selective hydrogenation of acetylene in the presence of ethylene is an important industrial process for the polyethylene industry since acetylene poisons the ethylene polymerization catalyst. Supported Pd catalysts are widely used to selectively convert acetylene to ethylene in the presence of ethylene.1-7 Therefore, the interest of further development of Pd-based catalysts with improved selectivity and activity for acetylene hydrogenation under ethylene rich conditions remains strong. It has been reported that Pd-based catalysts with TiO2 as either supports or additives showed good activity and high selectivity for the acetylene hydrogenation reaction.5-7 Moon and coworkers found enhanced ethylene selectivity on TiO2-modified Pd catalysts for the acetylene hydrogenation reaction in the presence of ethylene and suggested that the selectivity enhancement was ascribed to charge transfer from Ti species to Pd, which leads to weak ethylene adsorption on Pd.5,6 Panpranot et al. also reported that Ti3+ in Pd/TiO2 catalysts can effectively improve ethylene selectivity.7 On the other hand, plasma technologies for catalyst synthesis and modification have been demonstrated to be an effective way for enhancing low-temperature performance of catalysts in hydrogenation reactions.8-11 It was reported that air and H2 plasma treatments effectively improved the activity and stability of alumina-supported Ni catalyst for benzene hydrogenation.8 Zea et al. reported that plasma torch derived Pd catalyst showed better selectivity but lower activity for acetylene hydrogenation.10 Recently, it has been demonstrated in our laboratory that H2 plasma treatment can significantly improve both acetylene conversion activity and ethylene selectivity of Pd/Al2O3 at low reaction temperatures.11 In this study, uncalcined and RF nonthermal H2 and air plasma treated Pd/TiO2 and Pd-Ag/TiO2 catalysts are prepared and studied for selective hydrogenation of acetylene to ethylene in the presence of a large excess of ethylene. 2. Experimental Section 2.1. Catalyst Preparation. Titania pellets (1/8 in., anatase, from Alfa Aesar) are crushed and sieved to 20-40 mesh sizes. * To whom correspondence should be addressed. E-mail:
[email protected].
These particles are then dried at 200 °C for 12 h followed by cooling down to room temperature in a desiccator. The resulting particles are impregnated with calculated, mixed solution of palladium nitrate and silver nitrate (Alfa), based on the incipient wetness technique. The obtained materials after drying at 120 °C for 12 h are designated as uncalcined Pd/TiO2, Pd1%Ag/TiO2, and Pd-3%Ag/TiO2. Pd and Ag metal loadings on TiO2 catalysts are 1 wt % and 1% and 3%, respectively. Additional plasma treatments on uncalcined catalysts are carried out in a custom-made 360° rotating RF plasma system, as described in the previous publications.8,11 400 mTorr pressure, 160 W output, and a continuous wave are used for H2 and air plasma treatments in this study. The time for plasma treatment is set at 30 min. Typically, 1 g of uncalcined catalysts is loaded in the chamber for plasma treatments. The uncalcined Pd/TiO2 catalysts with additional H2 and air plasma treatments are designated as H2 plasma and air plasma Pd/TiO2, respectively. Similarly, uncalcined Pd-Ag/TiO2 catalysts (1% or 3%) with additional H2 and air plasma treatments are designated as H2 plasma and air plasma Pd-Ag/TiO2, respectively. 2.2. Catalytic Reaction. A catalytic reaction test for the selective hydrogenation of acetylene is carried out in a 1/4 in. stainless steel reactor housed in a GC oven. Before the start of each test run, the catalyst, ∼30 mg, is purged with 50 cm3/min of UHP nitrogen for 30 min at 25 °C. After the above pretreatment, the reaction over the catalyst is performed using a feed composition of 1.14% C2H2, 4.76% H2, and the balance C2H4 (mixture of 1.2% acetylene in ethylene from Scott Specialty Gases and UHP H2 from Matheson) with a space velocity of 84 000 h-1. All gas flow rates are controlled by mass flow controllers. The reaction temperature is set from 25 to 95 °C and linearly increased at a heating rate of 0.2 °C/min. Prior to the reaction, the feed is analyzed 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 flame ionization detector (FID) five times to obtain the average composition. The product compositions are analyzed every 10 min. In this study, the acetylene selectivity to ethylene, the acetylene conversion, and the ethylene yield are defined as follows:
10.1021/ie0701468 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
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% C2H4 selectivity ) (1 - produced C2H6/converted C2H2) × 100 % C2H2 conversion ) (decreased C2H2/C2H2 in feed) × 100 % C2H4 yield ) % C2H2 conversion × % C2H4 selectivity Positive % C2H4 selectivity and % C2H4 yield refer to net gains of ethylene during the hydrogenation process, and negative results represent ethylene loss. 2.3. BET Surface Area/Pore-Size Distribution. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and average pore size of the samples were measured by nitrogen adsorption at 77 K with a Micromeritics Tristar 3000 instrument. All samples were degassed under a vacuum of about 50 mTorr at 150 °C for 4 h before measurements. 2.4. FTIR. A 1:50 ratio of Pd catalysts and KBr was mixed and grinded. 50 mg of resulting powders were pressed at 8 ton/ in.2 to make pellets. Fourier transform infrared (FTIR) spectra were taken using the Nicolet Nexus 470 FTIR spectrometer with 4 cm-1 resolution and 32 scans. 3. Results and Discussion 3.1. BET Surface Area/Pore-Size Distribution. Measurements of BET surface area, pore volume, and average pore size were taken on TiO2 support, uncalcined Pd/TiO2, uncalcined and air plasma treated Pd-1%Ag/TiO2, and uncalcined and H2 plasma treated Pd-3%Ag/TiO2. The results are summarized in Table 1. There is no significant difference of the properties measured for TiO2 support, uncalcined Pd/TiO2, and uncalcined Pd-1%Ag/TiO2. The ranges of BET surface areas, pore volume, and average pore size are in the range of 136.1-139.1 m2/g, 0.341-0.359 cm3/g, and 95.2-99.4 Å, respectively. All properties decrease slightly from TiO2 to Pd/TiO2 and then to PdAg/TiO2 because of the filling of an increasing amount of metal precursors to the pores of TiO2. An additional 3% Ag did decrease the surface area, pore volume, and average pore diameter more considerably to 127.7 m2/g, 0.318 cm3/g, and 95.0 Å, respectively. Air and H2 plasma treatments are not expected to change the surface areas and pore-size distributions of catalysts significantly based on the previous results on Pd/Al2O3 catalysts. However, air plasma Pd-1%Ag/TiO2 shows slightly lower surface area and pore volume than the uncalcined Pd-1%Ag/TiO2. On the other hand, H2 plasma Pd-3%Ag/TiO2 shows considerably higher pore volume and average pore size than the uncalcined Pd-3%Ag/TiO2. The results of the opposite trend of changes caused by plasma treatments with different loadings of Pd and Ag nitrates indicate that plasma treatments at room temperature can indeed significantly change the structure and morphology of Pd precursors in the pores. It is further evidenced by the FTIR results in the following section. 3.2. FTIR. FTIR spectra of three Pd/TiO2 catalysts, including uncalcined, H2 plasma, and air plasma Pd/TiO2, are listed in Figure 1. The nitrate peak of the uncalcined Pd/TiO2 is clearly at ∼1380 cm-1 as shown in Figure 1. Air plasma treatment decreases the absorbance of Pd/TiO2 at 1380 cm-1, indicating ∼20% of nitrate has been decomposed or reduced. However, based on the color of the air plasma Pd/TiO2, which is light gray, it is suggested that part of TiO2 supported Pd nitrate (the color is brownish) has been reduced to Pd metals. A similar reduction capability of nonthermal plasmas using nonreducing
Table 1. BET Surface Areas and Pore-Size Distribution of TiO2 and TiO2 Supported Catalysts catalyst/properties
BET surface area (m2/g)
pore volume (cm3/g)
average pore size (Å)
TiO2 uncalcined Pd/TiO2 uncalcined Pd-1%Ag/TiO2 air plasma Pd-1%Ag/TiO2 uncalcined Pd-3%Ag/TiO2 H2 plasma Pd-3%Ag/TiO2
139.1 138.6 136.1 130.9 127.7 131.5
0.359 0.352 0.341 0.329 0.318 0.337
99.4 97.0 95.2 96.6 95.0 98.3
gas for plasma generation has also been reported by Liu and co-workers.12 H2 plasma treatment, on the other hand, almost completely reduces Pd nitrates to Pd metals, as evidenced by its dark gray color and the complete disappearance of the nitrate peak at 1380 cm-1. The results suggest that both air and H2 nonthermal RF plasmas can reduce TiO2 supported Pd nitrates to Pd metals, as supported by the preliminary scanning transmission electron microscopy (STEM) results. However, H2 plasma is more efficient than the air plasma for the reduction of Pd nitrate on TiO2 supports. 3.3. Reaction Study. 3.3.1. Uncalcined, Air Plasma, and H2 Plasma Pd/TiO2. The C2H2 conversion activity results of the uncalcined, air plasma Pd/TiO2, and H2 plasma Pd/TiO2 for the acetylene hydrogenation reaction are compared in Figure 2. As shown in Figure 2, the conversion activities of all three catalysts increase with temperature. The uncalcined Pd/TiO2 shows negligible activity at 25 °C but increases with temperature quickly and then slows down at temperatures above 50 °C. The conversion reaches 93% at 95 °C. With H2 and air plasma treatments, the acetylene conversion activity of Pd/TiO2 was effectively enhanced at temperatures below ∼55 °C. The effect of H2 plasma treatment is most significant at much lower temperatures. The initial activity of H2 plasma Pd/TiO2 at 25 °C is 31% versus 1% of the uncalcined Pd/TiO2, while air plasma Pd/TiO2 shows 11% conversion at the same temperature. However, the enhancement of the air plasma treatment increases faster than the effect of H2 plasma treatment as temperature increases. At temperatures between 31 and ∼55 °C, the activity order is air plasma Pd/TiO2 > H2 plasma Pd/TiO2 > uncalcined Pd/TiO2 versus H2 plasma Pd/TiO2 > air plasma Pd/TiO2 > uncalcined Pd/TiO2 at temperatures below 31 °C. Above ∼55 °C, the activity of the uncalcined one is slightly better than the plasma treated catalysts. No enhancement effect of plasma treatment was observed. The behavior of Pd/TiO2 in response to plasma treatments is similar to the Pd/Al2O3 catalyst, as reported earlier.11 Figure 3 summarizes the selectivities to ethylene of acetylene hydrogenation over air plasma Pd/TiO2, H2 plasma Pd/TiO2, and uncalcined Pd/TiO2. Most literature of supported Pd catalysts for selective hydrogenation of acetylene reported high ethylene selectivity at low temperatures and decreasing selectivity with temperature. Indeed, it is the case for both the uncalcined and the air plasma treated Pd/TiO2. To our surprise, the selectivity of H2 plasma Pd/TiO2 is extremely low,
