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Aqueous Catalytic Disproportionation and Oxidation of Nitric Oxide

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Aqueous Catalytic Disproportionation and Oxidation of Nitric Oxide Environ. Sci. Technol. 1998, 32, 876-881 Noncatalytic oxidation/reduction and absorption tech- Aqueous Catalytic niques (chemical scrubbing) have the advantage of being able to eliminate both NOx and SOx simultaneously (5, 6). Disproportionation and Oxidation of The primary impediment to their industrial application has Nitric Oxide been the low solubility of NO in aqueous solution. Because nitric oxide is 90-95% of the NOx present in typical flue gas streams (7), it has been necessary to preoxidize NO to NO2 JOSEPH H. MACNEIL, before the scrubber. Direct oxidation with O occurs slowly POLLY A. BERSETH, 2 at the low NO concentrations present in exhaust streams. GLENN WESTWOOD, AND The additional complexity and expense of using alternative WILLIAM C. TROGLER* - oxidants, such as OCl and H2O2, have prevented widespread Department of Chemistry and Biochemistry, University of use of this method. Other approaches to treating NO in California at San Diego, La Jolla, California 92093-0358 aqueous scrubbers have included the addition of heavy metal chelators to sequester nitric oxide for subsequent removal (8) and even the addition of yellow phosphorus emulsions Nitric oxide, a byproduct of combustion exhaust, is a key and O2 to oxidize nitric oxide to a combination of nitrite and nitrate salts (9). species that leads to urban photochemical smog. Nitric oxide exhibits low aqueous solubility, and it has proved In this paper we report a novel catalytic approach for difficult to remove NO from gas streams by aqueous scrubbing removing NO in aqueous solution with supported palladium or platinum metal. The catalysts display activity at ambient methods. A catalyst for the aqueous disproportionation temperatures and are not poisoned by SO2. of NO to nitrous oxide and aqueous nitrite has been developed with wood-derived activated carbons. Addition of palladium Experimental Section or platinum metal to the carbon support significantly accelerates catalysis. The carbon-supported catalyst was Reactions were performed with standard Schlenk techniques not poisoned by sulfur oxides. The reaction is thought under anaerobic conditions, except where it is noted otherwise. UV/vis spectra were recorded with an HP 8452A to involve the reduction of surface-bound nitric oxide dimers. diode array spectrometer in 1.0 cm quartz cells. IR data The reducing equivalents stem from the oxidation of NO -1 - were acquired at 2 cm resolution with a Nicolet 510 FTIR to NO2 . Addition of oxygen as an alternative electron spectrometer fitted with a MCT liquid nitrogen-cooled acceptor during the reaction immediately arrested nitrous detector. Pyrex cells with CaF2 windows and 5 or 10 cm path oxide formation, and all the nitric oxide was removed from lengths were used for gas sampling. Gas chromatograph the gas stream as aqueous nitrite. This demonstrates the (GC) analyses were performed with the use of a Hewlett- × × feasibility of removing SOx and NOx simultaneously by low- Packard 5890 GC that was fitted witha6ft 1/8 in. o.d. 13 temperature aqueous scrubbing. molecular sieve column (45/60 mesh) and a thermal con- ductivity detector. Data were output to a HP-3392A integrator. The NO (Liquid Air, 99% min purity) used in the majority of the experiments was spectroscopically free (<5 ppm) of Introduction higher nitrogen oxides but contained trace quantities of N2O. Because of nitrogen oxide's key role in photochemical smog Dilute nitric oxide (Matheson, 0.493% in NO, 1 ppm NO2) and in acid rain formation, NOx controls are a priority in was used for the recirculation experiments. Liquid nitrogen urban regions (1). In the United States, the 1990 Clean Air boil-off was the source of gaseous N2. Aqueous solutions Act Amendments mandated stringent NOx emissions reduc- were prepared with degassed HPLC-grade water. The tions. Stationary combustion sources, primarily electric following wood-derived activated carbons (used as received - power plants, generate large amounts of NO byproducts from commercial suppliers) were tested: Darco granular (20 x < that account for more than half the total U.S. NO emissions. 40 mesh), Darco KB (100 mesh), Darco-KB-B ( 100 mesh), x < Emerging legislation for zero-emission vehicles (electric cars) Darco G-60, Norit SA3 ( 100 mesh), and Engelhard CP-97. will further shift the NOx burden onto stationary combustion Palladium (0.5%, 1%, 5%, 10%) on activated carbon, platinum sources. (0.5%, 3%) on activated carbon, Pearlman's catalyst [20% Pd(OH) on carbon], and Lindlar catalyst (5% Pd on CaCO ) The implementation of postcombustion technologies is 2 3 were obtained from Aldrich. PdCl , palladium (5%, 10%) on especially problematic for coal-fired utilities. Selective 2 carbon, palladium (0.5%) on alumina, and palladium(II) (5%) catalytic reduction (SCR) of NO by NH at 300-400 °Cisthe 3 on barium sulfate were obtained from Strem Chemicals, Inc. best available NOx control technology for stack gases (2); however, difficulties with fly ash poisoning, ammonia slip- Standard Reaction Conditions. A Schlenk flask contain- page, and high overall operating expenses have prompted a ing 0.10 g of catalyst in 10.0 mL of 3 M NaOH was pressurized search for alternative methods. Selective noncatalytic re- to 840 Torr with NO (350 mL of headspace). Rapid stirring duction (SNCR) of NOx to N2, employing ammonia or urea, was used to facilitate NO uptake into the aqueous phase. is also in use. This requires operating temperatures in the The flask was repressurized with argon as the NO was range 800-1200 °C. Emission reduction levels of 50-70% depleted. The course of the reaction was monitored by are typically obtained with this process. Attention has also periodic GC analysis of 50-µL samples extracted from the focused on decomposing NO to N2 and O2 by Cu-doped headspace. Calibration curves were used to determine the ZSM-5 zeolites and related systems (3, 4); however, these are relative quantities of NO, N2O, and argon in each sample. not yet viable for stationary combustion sources. The temperature was 22 °C for all the rates reported herein, unless otherwise noted. * Corresponding author telephone: 619-534-6175; fax: 619-534- Low NO Concentrations. Recirculation reactions were 5383; e-mail: [email protected]. performed in a closed loop system consisting of the reactor 876 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 7, 1998 S0013-936X(97)00743-8 CCC: $15.00 1998 American Chemical Society Published on Web 02/10/1998 vessel, a gas-phase IR cell, and a peristaltic recirculation pump linked by Tygon tubing. The system was evacuated and purged three times with nitrogen, before filling with 1000- 6000 ppm NO. Nitrogen was used to bring the final pressure up to 780 Torr. In some experiments O2 (1-5%) was also added. The rate of reaction at 22 °C was monitored by FTIR spectroscopy; both the depletion of NO and the increase in N2O were calculated from calibration curves. Complete recirculation of the headspace volume occurred every 45- 50 s. Varying the recirculation rate did not change the reaction rate, which implies that NO saturation of the aqueous phase was maintained. Preparation of an Activated Carbon-Supported Pal- ladium Catalyst. Palladium dichloride (0.4936 g) was dissolved in 50.0 mL of 0.20 M HCl and stirred with 4.08 g of activated carbon (Engelhard) for 65 h. After extraction of FIGURE 1. Catalytic enhancement of the rate of eq 1 by activated a small aliquot for UV/vis analysis, the solution was filtered, carbon and by 1% palladium-loaded activated carbon. Rates are and the precipitate was washed with HPLC-grade water until given as moles of N2O product formed vs reaction time (0.1 g of the supernatant was free of chloride (tested with AgNO3). catalyst, 10 mL of 3 M NaOH, 840 Torr of NO, 22 °C). The palladium concentrations of the solution immediately before filtration and after washing were determined from 2- ) ( -1 TABLE 1. Rates (22 °C) of NO Disproportionation Catalyzed UV/vis analyses at 474 nm (max (PdCl4) 160 10 cm M-1). A total of 0.274 g of palladium had been adsorbed by by Various Activated Carbons in 3 M NaOH with 840 Torr of the carbon, corresponding to 6.7 wt %. The sample was NO - - dried overnight under vacuum. A 1.0-g sample of the PdCl2- carbon sample (mol of NO) min 1 (g of C) 1 treated carbon was added to 25 mL of 3 M NaOH and shaken × -5 under 3 atm of H for 18 h. The Pd0/C product was filtered Darco Granular 10 10 2 Darco KB 7.2 × 10-5 and dried under vacuum. Darco KBa 10 × 10-5 Calculation of Turnover Data. As indicated in the Darco KB-B 8.4 × 10-5 discussion to follow, 4 mol of nitric oxide was required to Darco KB-Ba 12 × 10-5 - produce 1 mol of N2O (and 2 mol of nitrite). The nitrite Darco G-60 18 × 10 5 -5 stoichiometry was determined by purging NO and N2O from Norit SA3 17 × 10 × -5 the solution with N2 and filtering the reaction solution through Engelhard CP-97 10 10 a syringe tip filter to remove catalyst. An aliquot was diluted a Commercial sample was prewetted. Data have been recalculated - 25-fold with 1 M NaOH, and the concentration of NO2 was to compensate for the mass of water. determined from its UV absorption maximum at 354 nm using a calibration curve for NaNO2 constructed for the range Table 1 contains data obtained for catalysis experiments - 0.002 0.1 M. Turnover data were calculated as [(moles of performed with wood-derived activated carbons; all samples × N2O 4)/moles of palladium]. This rate, based on the were weakly active catalysts in the absence of palladium.
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