Enhanced Activity and Selectivity of Carbon Nanofiber Supported Pd Catalysts for Nitrite Reduction Danmeng Shuai, †,§ Jong Kwon Choe, †,§ John R. Shapley, ‡,§ and Charles J. Werth* ,†,§ † Department of Civil and Environmental Engineering, ‡ Department of Chemistry, and § Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States * S Supporting Information ABSTRACT: Pd-based catalyst treatment represents an emerg- ing technology that shows promise to remove nitrate and nitrite from drinking water. In this work we use vapor- grown carbon nanofiber (CNF) supports in order to explore the effects of Pd nanoparticle size and interior versus exterior loading on nitrite reduction activity and selectivity (i.e., dinitrogen over ammonia production). Results show that nitrite reduction activity increases by 3.1-fold and selectivity decreases by 8.0- fold, with decreasing Pd nanoparticle size from 1.4 to 9.6 nm. Both activity and selectivity are not significantly influenced by Pd interior versus exterior CNF loading. Consequently, turn- over frequencies (TOFs) among all CNF catalysts are similar, suggesting nitrite reduction is not sensitive to Pd location on CNFs nor Pd structure. CNF-based catalysts compare favorably to conventional Pd catalysts (i.e., Pd on activated carbon or alumina) with respect to nitrite reduction activity and selectivity, and they maintain activity over multiple reduction cycles. Hence, our results suggest new insights that an optimum Pd nanoparticle size on CNFs balances faster kinetics with lower ammonia production, that catalysts can be tailored at the nanoscale to improve catalytic performance for nitrite, and that CNFs hold promise as highly effective catalyst supports in drinking water treatment. ■ INTRODUCTION Nitrate is one of the most common groundwater and surface water contaminants in the United States, 1 due to intensive fer- tilizer use in agriculture 2,3 and feedlot runoff. 4 Nitrate contami- nation of drinking water resources is a serious health concern because its transformation product, nitrite, can cause methemo- globinemia, 5 or blue baby syndrome, as well as create carcin- ogenic N-nitroso compounds in the human body. 6 The US EPA has established maximum contaminant levels (MCL) for nitrate and nitrite at 10 mg L -1 as N and 1.0 mg L -1 as N, respectively. 7 The most widely used approach to remove nitrate and/or nitrite from drinking water is ion exchange. This approach results in the production of a brine waste that re- quires further disposal. 2 Biological treatment is also possible, but concerns for pathogens and challenges associated with maintaining performance under varying treatment conditions have limited its application. 2 Pd-based catalytic reduction has emerged as a promising and potentially more sustainable tech- nology to treat drinking water and ion exchange brines. 3,8-10 Catalytic nitrate reduction proceeds initially through nitrite and then potentially through nitric oxide and nitrous oxide. The primary product is dinitrogen, but ammonia is also a possible and regulated end product. 3,8,9,11,12 Promoter metals (e.g., Cu, Sn, and In) are necessary to initiate nitrate reduction, 8,9,11,13-18 but reduction of nitrite and subsequent intermediates only re- quires Pd. Concerns regarding the cost of Pd-based catalysis were recently addressed in a pilot study of trichloroethylene (TCE) reduction in groundwater at Edwards Air Force Base in California. 19 While these results indicate that Pd-based catalytic treatment can be competitive in terms of both cost and perfor- mance, nitrate reduction rates are at least an order of magni- tude less than those for TCE, and maximizing selectivity for dinitrogen versus ammonia is a concern. Hence, new catalysts with higher activity for nitrate reduction, and with greater selec- tivity for dinitrogen over ammonia for nitrate and nitrite reduc- tion, are needed to promote further development of practical treatment systems. Various catalyst properties have been found to affect nitrate and nitrite reaction rates, as well as selectivity for dinitrogen, including metal loading, metal nanoparticle size, metal location, and catalyst support electronic properties. 8,9,14,20-23 Most pre- vious work has been performed by using conventional catalyst supports, e.g., activated carbon, 21 alumina, 11,13-16 and silica, 15,16 although more novel supports have also been used, e.g., TiO 2 , 24 ZrO 2 , 22 SnO 2 , 22,25 organic resins, 26 conducting polymers, 23 and carbon nanofiber foam. 27,28 Conventional sup- ports are often thick relative to the size of Pd nanoparticles and Received: September 12, 2011 Revised: January 29, 2012 Accepted: January 31, 2012 Published: January 31, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 2847 dx.doi.org/10.1021/es203200d | Environ. Sci. Technol. 2012, 46, 2847-2855