Applied Catalysis B: Environmental 125 (2012) 1–9 Contents lists available at SciVerse ScienceDirect Applied Catalysis B: Environmental jo ur n al homepage: www.elsevier.com/locate/apcatb Monometallic Pd/Fe 3 O 4 catalyst for denitrification of water Wuzhu Sun a,b , Qi Li a,∗ , Shian Gao a , Jian Ku Shang a,c a Materials Center for Water Purification, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b University of Science and Technology of China, PR China c Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA a r t i c l e i n f o Article history: Received 30 November 2011 Received in revised form 18 April 2012 Accepted 15 May 2012 Available online 22 May 2012 Keywords: Pd/Fe3O4 Catalytic denitrification Superparamagnetic catalyst Fe(II)/Fe(III) redox couple a b s t r a c t A magnetite supported monometallic Pd catalyst was synthesized by a co-precipitation process followed with the reduction in pure hydrogen at 453 K. The catalyst was composed of ultrafine Pd nanoparticles (∼2 nm) highly dispersed on the surface of superparamagnetic Fe 3 O 4 nanoparticles. Aside from its roles as the catalyst support and the magnetic separation medium, Fe 3 O 4 was found to be a good promoter for the nitrate reduction, where nitrate was firstly reduced to nitrite by the Fe(II)/Fe(III) redox couple, and subsequently reduced to nitrogen and ammonium. Further mechanistic studies demonstrated that besides the Pd sites, active sites for the nitrite reduction also exist on the surface of Fe 3 O 4 . Part of the nitrite reduction occurred on the surface of Fe 3 O 4 , which may also be attributed to the Fe(II)/Fe(III) redox couple. In the present study, ammonium was the main product because of the different denitrification mechanisms compared with bimetallic catalysts. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Due to the intensive agricultural activities, especially over- fertilization, nitrate concentrations in both surface and ground water have increased in many locations throughout the world, and the problem is becoming increasingly important with the fast increase of agricultural and industrial activities in recent years [1–3]. Because of its widespread presence and toxicity, nitrate becomes one of the most common groundwater contaminants in many countries. Nitrate could cause various problems to the health of human beings, such as blue baby syndrome, high blood pressure, diabetes, and liver damage. It is also the precursor of nitrosamine, which could cause various cancers [3–8]. Therefore, the World Health Organization (WHO) recommended that the maximum con- taminant level (MCL) for nitrate in drinking water should be below 50 ppm [9], and many countries have formulated their own MCLs according to their domestic conditions. Many treatment methods have been proposed to remove nitrate from contaminated water, including both biological and physi- cal chemical denitrification approaches [10–15]. Among various physical–chemical denitrification processes, such as ion exchange, reverse osmosis, electrodialysis and catalytic reduction by hydro- gen or other reducing agents, the catalytic reduction by reducing agents had been considered as one of the most promising ∗ Corresponding author at: 72 Wenhua Road, Shenyang, Liaoning Province 110016, PR China. Tel.: +86 24 83978028; fax: +86 24 23971215. E-mail address: qili@imr.ac.cn (Q. Li). methods to treat nitrate in water [16–19]. Since Vorlop and Tacke [20] demonstrated for the first time that the catalytic reduction of nitrate by reducing agent was achievable, extensive research efforts have been made in this approach [16–33]. Most catalysts for the nitrate reduction are composed of a noble metal and a promoter, which may be a transition metal or a metal oxide. The function of the promoter is to reduce nitrate to nitrite by a redox process to start the catalytic process, while the noble metal plays an important role in maintaining transition metal in metallic state and reducing nitrite by activated hydrogen [3,16–18]. Currently, most research efforts are focused on bimetal denitrifica- tion catalysts, in which the noble metals usually are Pd, Pt, Ru, Rh, or Ir, and the promoters are Cu, Sn, Ag, or In [16–29]. By compar- ison, a limited number of studies have been conducted in noble metal/metal oxide denitrification catalysts, in which a metal oxide acts as the promoter. Till now, only CeO 2 , TiO 2 , and SnO 2 were found to be good promoters of the noble metal in the denitrification pro- cess [3,30–33]. Based on the current understanding of the catalytic denitrification mechanism, it appears that any catalyst associating a noble metal, capable of chemisorbing hydrogen, with a compo- nent, having a redox behavior may catalyze nitrate reduction [33]. Thus, novel catalyst material systems for denitrification could be developed by following this approach, and by incorporating other functions to enhance water treatment performance in engineering devices. Because of its easy separation from an aqueous environment under the external magnetic field, the utilization of magnetic magnetite (Fe 3 O 4 ) has been explored for various water treat- ment applications, for example, dehalogenation, heavy metal ion 0926-3373/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2012.05.014