Catalytic Nitrate and Nitrite Reduction with Pd-Cu/PVP Colloids in Water: Composition, Structure, and Reactivity Correlations Kathryn A. Guy, † Huiping Xu, ‡ Judith C. Yang, ‡ Charles J. Werth, § and John R. Shapley* ,† Department of Chemistry and WaterCAMPWS, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Department of Materials Science and Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, and Department of CiVil and EnVironmental Engineering and WaterCAMPWS, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: NoVember 14, 2008; ReVised Manuscript ReceiVed: April 4, 2009 A set of bimetallic Pd-Cu/PVP (PVP ) poly(N-vinylpyrrolidone)) colloids, with copper proportions ranging from 0 to 50 atom %, has been examined as catalysts in a batch reactor with flowing hydrogen for the reduction of aqueous nitrate and/or nitrite. The encapsulated Pd-Cu nanoparticles were characterized by powder XRD, TEM, EDX, and IR of adsorbed CO. A significant decrease in average particle diameter and changes in the Pd-Cu crystallinity occurred above ca. 30% copper content, and this transition corresponded with a significant increase in observed nitrate reduction rates. The strong dependence on composition suggests that specific Cu n ensembles on the surface of the Pd-Cu nanoparticles are needed for effective nitrate-to-nitrite conversion. In contrast, nitrite reduction rates were only minimally enhanced by the presence of copper. Increasing pH had little effect on the nitrate reduction rates, but it strongly inhibited the rate of nitrite reduction. The requisite protonation of a palladium-nitrite surface intermediate is proposed. Introduction Elevated levels of nitrate are often found in drinking water sup- plies, especially in agricultural areas using nitrate rich fertilizers. 1-4 To reduce the possibility of adverse health effects, such as methemoglobinemia in infants, the EPA and various governmental agencies have set limits on the amounts of nitrate and nitrite in drinking water. 1,2,5-8 Interestingly, however, recent research has shown that increased nitrate and nitrite levels in the blood serum of adults may actually be beneficial by reducing the amount of muscle damage accrued during a heart attack. 9,10 The removal of nitrate from drinking water is challenging due to its high solubility and high stability. 3,5,6,11 Current technologies for removing nitrate include ion exchange, 4,12,13 reverse osmosis, 14 and biological denitrification. 15 However, these techniques often generate cost issues due to specific requirements for supervision and maintenance or disposal of concentrated brines. 3,14-17 The selective reduction of nitrate by using metal catalysts is a promising alternative for effective removal of this contaminant from drinking water. 18 In the late 1980s, Vorlop and co-workers discovered that nitrate could be reduced primarily to dinitrogen with supported bimetallic hydrogenation catalysts; nitrite was an observed intermediate and ammonia was a coproduct. 19,20 Palladium was found to be the most active and selective metal for the reduction of nitrite, but the reduction of nitrate required a second metal as a cocatalyst. 19,20 The stepwise mechanism shown in Scheme 1 has been proposed for the reduction of nitrate through the formation of nitrite and other intermediates. 19 Although only NO 2 - and N 2 O have actually been detected as intermediates, 21-23 the formation and further reaction of NO adsorbed on the catalyst surface has commonly been suggested as the key step in determining the selectivity for dinitrogen vs. ammonia. 19,24,25 The most frequently examined system for catalytic nitrate reduction is the combination of palladium and copper deposited on alumina or other supports. 18-32 Such studies have shown that both the preparation method and the operating conditions affect the activity and selectivity of bimetallic Pd-Cu nitrate reduction catalysts. 21,23,26,29,32 The distribution of palladium and copper on the support surface ranges from isolated phases to alloys depending on the preparation method. 26 It has been suggested that mixed Pd-Cu sites show higher catalytic activity compared to the separated metal sites, 23,24,33 but the relationship between the catalyst structure on the atomic level and the overall activity/ selectivity characteristics is not well established. The formation of nanoparticles offers a way to prepare bime- tallic clusters with narrow size and composition distributions. 34-40 Bimetallic Pd-Cu colloidal materials were prepared by Esumi and co-workers in 1990 by thermal decomposition of palladium and copper acetates in high-boiling organic solvents. 41 Later researchers modified this method to use 2-ethoxyethanol as the solvent and reducing agent. 34 Additionally, the reduction of the metal acetates was carried out in the presence of a protecting polymer such as poly(N-vinylpyrrolidone) (PVP) to control particle size and to increase stability. These polymer-protected bimetallic Pd-Cu colloids, which are dispersible in water, raised the possibility of studying how the nanoparticle composition and metal distribution affect catalytic nitrate reduction. We have adopted these procedures to prepare a set of Pd-Cu/PVP colloids with Pd:Cu atomic ratios ranging from 50:50 to 100:0, * To whom correspondence should be addressed. Phone: 217-333-0297. Fax: 217-244-3186. E-mail: shapley@illinois.edu. † Department of Chemistry and WaterCAMPWS, University of Illinois at Urbana-Champaign. ‡ Department of Materials Science and Engineering, University of Pittsburgh. § Department of Civil and Environmental Engineering and Water- CAMPWS, University of Illinois at Urbana-Champaign. SCHEME 1: Stepwise Reduction of Nitrate to Dinitrogen J. Phys. Chem. C 2009, 113, 8177–8185 8177 10.1021/jp810049y CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009