Ind. Eng. Chem. Res. 1997, 36, 2533-2536
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Catalytic Reduction of NO over Cu/C Eli Ruckenstein* and Yun Hang Hu Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14226
The catalytic reduction of NO to N2 and CO2 by C over Cu/C was investigated and a relationship between the conversion of NO and Cu loading obtained. An optimum conversion of 100% was obtained at 410 °C over a Cu/C sample that contained 20 wt % Cu. For higher Cu contents the conversion decreased. The selectivities to N2 and CO2 at 360 and 410 °C were 100% over the samples that contained 5-30 wt % Cu. As the reaction time increased, the conversion of NO first decreased and later increased. An explanation is provided for this behavior. H2 titration was employed to determine the change of the exposed surface area of Cu during reaction. The change of the surface area and of the pore size distribution of Cu/C during the reaction was also investigated. 1. Introduction The presence of nitrogen oxide (NOx) in the combustion exhaust streams is of concern because it constitutes a major pollutant (Iwamoto et al., 1981; Iwamoto and Hamada, 1991; Li and Armor, 1992a,b; Viswanathan, 1992; Armor, 1994; Petunchi and Hall, 1994; Wood, 1994; Yamashita and Vannice, 1996). Although NO is thermodynamically unstable, it exhibits a high resilience to decomposition. The selective catalytic reduction of NOx with ammonia
4NO + 4NH3 + O2 f 4N2 + 6H2O 2NO2 + 4NH3 + O2 f 3N2 + 6H2O is the only commercially employed catalytic process for NOx emission control from stationary sources (Bosch and Janssen, 1988). Although high NOx reduction is achieved, it has a number of drawbacks, such as ammonia “slip” and equipment corrosion (Li and Armor, 1992a). In addition, the process is expensive. Therefore, an efficient NOx reduction process that does not employ ammonia as reductant is needed. Hydrocarbons and carbon constitute attractive, alternative reductants to ammonia (Sotoodehnia-Korrani and Nobe, 1970; Hamada et al., 1990; Kikuchi et al. 1991; Misono and Kondo, 1991; Li et al., 1993; Cha, 1993). Compared to hydrocarbons, carbon is relatively inexpensive and can be easily accommodated to any exhaust source. The use of transition metals as catalysts for the NO-carbon reaction was suggested by Shelef and Otto (1969), who observed a catalytic effect of the impurities present in carbon on NO reduction, and by Watts (1958), who noted that the presence of copper increases the rate of carbon oxidation and modifies the product distribution. Inui et al. (1982), combining the first series of transition metals (Ni, Fe, Co) with lanthanum oxide and precious metals (Pt, Ru, Rh, Pd), and Yamashita et al. (1991), using copper and nickel, have observed a noticeable increase in NO conversion. Recently, Illan-Gomez et al. (1995) studied, using the temperature-programmed reduction (TPR), the effect of transition metals as catalysts on the NO reduction by carbon. Although the catalytic reduction of NO by C had already been studied, the effect of the catalyst loading as well as the change in activity, surface area, and pore size distribution with reaction time had not yet been * To whom correspondence should be addressed. S0888-5885(96)00904-9 CCC: $14.00
investigated. The objective of this paper is to examine the effect of Cu loading on the reduction of NO over Cu/C and the change of NO conversion, surface area, and pore size distribution of Cu/C with reaction time. The main conclusion is the existence of an optimum Cu loading of 20 wt %, for which the reduction activity and the selectivity for CO2 and N2 are 100%. Obviously, carbon should be used to reduce NO only for exhausts free of oxygen, such as those of nitric acid plants. 2. Experimental Section 2.1. Catalyst Preparation. The catalysts were prepared by impregnating the activated carbon (Aldrich Chemicals, 20-40 mesh) with an aqueous solution of Cu nitrate (Alfa Chemical), followed by drying at room temperature in air and finally by decomposition at 300 °C in He for 3 h. 2.2. Reaction. The catalytic reaction was carried out under atmospheric pressure, in a flow system, using a vertical quartz tube (6 mm inside diameter) reactor. The Cu/C particles (weight: 0.5 g) were held on quartz wool. The feed gas, with a gas hourly space velocity (GHSV) ) 2400 cm3 g-1 h-1, contained 4 mol % NO (in He). The analysis of the reactant/product mixtures was performed with an in-situ gas chromatograph equipped with Porapak Q and 5A molecular sieve columns. 2.3. BET Surface Area and Pore Size Distribution Measurements. The surface area and the pore size distribution were determined via nitrogen adsorption, using a Micromeritics ASAP 2000 instrument. The surface area was calculated using the BET method, while the pore size distribution curve was obtained from the adsorption branch of the N2 isotherm by the BJH method. The sample was degassed at 300 °C in high vacuum before measurements. 2.4. H2 Chemisorption on Cu/C. The H2 chemisorption was carried out at room temperature, using a Micromeritics ASAP 201C instrument. The Cu surface area of the Cu/C sample was determined by assuming a 1:2 (H2:Cu) stoichiometry. The sample was degassed in He for 60 min and in a high vacuum of 10-5 Torr for 30 min, at 300 °C before performing the H2 chemisorption experiment. 3. Results 3.1. Reduction Reaction of NO. The reduction reaction of NO was carried out over C and Cu/C (Figures 1-3). Figure 1 shows that there is little activity over © 1997 American Chemical Society
2534 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
Figure 1. Relationship between NO conversion and temperature for NO reduction over pure C (reaction conditions: P ) 1 atm, GHSV ) 2400 cm3 g-1 h-1, NO (in He) ) 4%).
Figure 4. Mesopore size distribution of Cu/C before reaction. Figure 2. Relationship between NO conversion and reaction time for NO reduction over Cu/C (reaction conditions: T ) 360 °C, P ) 1 atm, GHSV ) 2400 cm3 g-1 h-1, NO (in He) ) 4%).
Figure 3. Relationship between NO conversion and reaction time for NO reduction over Cu/C (reaction conditions: T ) 410 °C, P ) 1 atm, GHSV ) 2400 cm3 g-1 h-1, NO (in He) ) 4%).
pure C until 515 °C; there is, however, high activity over Cu/C at and even below 410 °C (Figures 2 and 3). This indicates that Cu plays an important catalytic role. The activity, determined for about 8 h, decreases first and then increases with reaction time; of course for very long times the activity should decrease with reaction time. At 360 °C, for 5 wt % Cu/C, the initial NO conversion was 45%; it decreased to 30% after 2 h, increased to 35% after 6 h, and then remained almost constant. For 10 wt % Cu/C, the initial NO conversion was about 58%; it decreased to 46% after 1.5 h, increased to 62% after 7 h and then remained almost constant. For 20 wt % Cu/C, the initial NO conversion was about 83%; it decreased to 67% after 1.3 h, increased to 77% after 6.5 h, and then remained almost constant. For 30 wt %
Table 1. Surface Areas of Various Cu/C Samples sample
surface area, m2/g
pore volume, cm3/g
C 5 wt % Cu/C 10 wt % Cu/C 20 wt % Cu/C 30 wt % Cu/C
603 595 589 498 437
138 136 135 114 100
Cu/C, the initial NO conversion was about 70%; it decreased to 45% after 2 h, increased to 54% after 4.5 h, and then remained almost constant. The NO conversion with reaction time at 410 °C had a trend similar to that at 360 °C. However, at 410 °C, the conversion of NO after about 3 h was about 75% for 5 wt % Cu/C, about 96% for 10 wt % Cu/C, about 99% for 20 wt % Cu/C, and about 95% for 30 wt % Cu/C. Consequently, the Cu/C containing 20 wt % Cu provides the optimum activity. The selectivities to N2 and CO2, defined as [N2/ (N2 + NOx)] × 100% and [CO2/(CO2 + CO)] × 100%, were 100% at both 360 and 410 °C for all Cu loadings. 3.2. Surface and Pore Size Distribution of Cu/ C. The N2 adsorption data indicate that the surface areas of the Cu/C samples decrease with increasing Cu content (Table 1). For the 20 wt % Cu/C sample, Table 2 shows that as the reaction time increases, the surface area remains first almost constant but then decreases. The pore size distribution of the Cu/C samples in the range of mesopores (20-500 Å, determined by the BJH method, is given in Figures 4 and 5. These figures show that the pore size distribution hardly changes with the Cu content. Moreover, the results from the t-plot indicate that the surface area provided by the micropores (
