Rong Wang, Mufei Yue, Rihong Cong, Wenliang Gao**, Tao Yang* 

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, PR China

abstract

Photocatalytic reduction of nitrate (NO3 ) is a green and potentially inexpensive technique for reducing NO3 pollution in ground water. TiO2-based photocatalysts have been studied extensively for this purpose. In the present study, the semiconducting catalyst CuFe0.7Cr0.3S2 was applied to NO3 reduction. Loading this catalyst with metal co-catalysts (Ru, Au, Cu, Ag, Pt, and Pd) greatly increased the rate of NO3 reduction and the N2 selectivity. In addition, there was a synergistic enhancement of the photocatalytic performance when the catalyst was loaded two co-catalysts. For example, the catalyst loaded with Pd and Au at mass fractions of 0.75% and 3%, respectively, could photocatalyze the complete reduction of NO3 in a 100 ppm N aqueous solution with 100% N2 selectivity in less than 5 h with UV irradiation. However, with an inner irradiation from a full-arc Xe lamp, the NO3 conversion rate reduced to 0.065 mg N/h, probably because of the low density of the photoexcited electrons. The results show the potential of metal sulfifides for photocatalytic reduction of NO3 , and the possibility of use of visible light.

1. Introduction

For health reasons, the World Health Organization recommends that the concentrations of nitrate (NO3 ), nitrite (NO2 ) and ammonium (NH4þ) in drinking water are below 10, 0.03, and 0.4 ppm, respectively. Ground water is a major source of drinking water, and methods for removal of excess NO3 from ground water are required. Various methods have been applied to remove NO3 from aqueous solutions, including ion exchange, electrodialysis, and catalytic processes [1]. Among these methods, catalytic processes are most promising because the NO3 can be converted into harmless nitrogen gas. Physicochemical reduction of NO3 by monometallic or bimetallic co-catalysteloaded catalysts, such as PdeCu/TiO2, has been studied extensively and the reduction rate has been optimized to ~10 mg of nitrogen per hour (mg N/h) or approximately 62 mg of NO3 per hour [2,3]. The selectivity for N2 production can also be optimized by choosing an appropriate combination of metal co-catalysts such as Cu, Ni, Fe, Sn, In and Ag [4,5].

Photocatalytic reduction of NO3 is of interest because it could utilize solar energy. In the fifirst study of photocatalytic reduction of NO3 , Kudo et al. used Pt-, Pd-, and Rh-loaded TiO2 catalysts with no sacrifificial reagent [6]. Ranjit et al. studied the photocatalytic activity of metal-loaded TiO2 under UV irradiation in an aqueous solution containing CH3OH, CH3CH2OH and EDTA [7]. In both cases, the conversion rates were low, which is not surprising because many electrons are needed to convert NO3 into N2. Therefore, a high density of electrons is needed at the catalytic site when NO3 ions are adsorbed on the surface.

In 2005, Zhang et al. reported that Ag doped nanoparticles of TiO2 could photocatalyze reduction of NO3 at a very high rate (approximately 50 mg N/h), which is almost one order of magnitude higher than the rate achieved in traditional physicochemical catalytic processes [8,9]. To the best of our knowledge, most studies have focused on the photocatalytic activities of modifified TiO2 catalysts [10e30], and only several other oxides have been studied for photocatalysis of the reduction of NO3 , such as KxGaxSn8-xO16 (x ¼ 1.8) [31,32], KTaO3 [33], BaLa4Ti4O15 [34,35], NaTaO3 [33,36], and SrTiO3 [11,37]. Very recently, two chalcopyrite type sulfifides (CuInS2 and CuFe1 xCrxS2, 0  x  0.4) were found to be effective catalysts under UV irradiation without co-catalysts [38,39]. Preliminary results showed that the unmodifified metal sulfifides could adsorb NO3 and catalyze its reduction. However, the N2 selectivity was not reported. In the present study, we performed a systematic investigation of the catalytic activities and N2 selectivity of CuFe0.7Cr0.3S2 (CFCS) catalysts loaded with various metal co-catalysts.

CFCS prepared by the solvothermal method has been used to effectively catalyze the reduction of NO3 under UV irradiation, and it has a much higher activity than that of unmodifified TiO2 [6]. Therefore, there must be adsorption sites for NO3 on the surface of CFCS, and the density of photoexcited electrons on these catalytic sites must be high enough for the reduction of NO3 . Generally, NO3 reduction occurs in two successive reactions, from NO3 to NO2 and then from NO2 to N2. The fifirst reaction requires two electrons, and the second reaction requires three electrons. Every two nitrogen radicals that are produced will combine to form N2, which then leaves the catalyst surface in the gas phase. In this study, we aimed to further improve the photocatalytic efficiency and the N2 selectivity of CFCS by loading it with co-catalysts (e.g. Ru, Au, Cu, Ag, Pt, and Pd). Eventually, CFCS loaded with 0.75 wt% Pd and 3 wt% Au offered a high photocatalytic effificiency, and the NO3 in a 100 mL aqueous solution (100 ppm N) was completely converted into N2 in 4.5 h.

2. Materials and methods

2.1. Preparations of the catalysts

Black CFCS powder samples were prepared by the thioureaoxalic acid solvothermal method as described in the literature [38]. In this method, CuCl2 2H2O (0.01 mol), FeCl3 6H2O (0.007 mol) and CrCl3 6H2O (0.003mol) were mixed with oxalic acid (1.50 g) and an excess of thiourea (0.20 mol). Then, the mixture was transferred into a 50 mL Teflon autoclave, and the autoclave was sealed. After heating at 220  C for 5 d, a black solid was obtained. This solid was collected and washed with water to remove any contaminant.

Various co-catalysts were investigated to see if they improved the photocatalytic activity of CFCS. For example, to load CFCS with a co-catalyst, 0.2000 g of CFCS, 1.4 mL of H2PtCl66H2O (1.48 mg/mL), and 20 mL of distilled water were placed in a 100-mL beaker. This solution was mixed by ultrasonication for 20 min. Then, dilute aqueous KBH4 was added to the solution very slowly to obtain Pt particles. The obtained powder sample was washed with water and dried at 60  C in an oven before further characterizations. Additional co-catalysteloaded CFCS catalysts were prepared using the same method with RuCl3, AgNO3, HAuCl4 4H2O, PdCl2, and CuCl2 6H2O aqueous solutions.

2.2. Characterizations

Powder X-ray diffraction (XRD) data were collected on a PANalytical X'pert diffractometer equipped with a PIXcel 1D detector (PANalytical, Almelo, the Netherlands) with Cu Ka radiation (1.5406 Å). The operation voltage and current were 40 kV and 40 mA, respectively. Scanning electron microscopy images were recorded using a JEOL JSM-7800F electron microscope (JEOL, Tokyo, Japan) at a working distance of 4 mm.

2.3. Photocatalytic activity evaluation

The photocatalytic activities of the prepared catalysts were mostly tested in a sealed circulation system equipped with a vacuum line (LabSolar-IIIAG system, Perfect Light Ltd. Co., Beijing, China), a 150-mL Pyrex glass reactor (enwrapped with 5  C recycling water to maintain cool), and a gas sampling port that was directly connected to a gas chromatograph (GC7900, Shanghai Techcomp, Shanghai, China). The gas chromatograph was equipped with a thermal conductivity detector and a column packed with 5A molecular sieves. Helium was used as the carrier gas to detect the so-produced N2 online. 500 W Hg or 300 W Xe-lamp were used to provide UV- or visible light irradiation (CEL-M500 or CEL-HXF300, Beijing AuLight Ltd. Co.), which was applied from the top of the reaction vessel (See Fig. S1 in the Electronic Supplementary Information, ESI). Sodium oxalate (0.01 mol/L) was used as the sacrifificial agent. During the reaction, a small amount of the solution was withdrawn periodically, the catalyst was immediately separated by centrifugation, and the supernatant was analyzed to determine the residual concentration of NO3 and NO2 with an UVeVis spectrophotometer. Ammonia was not detected in our case. To further evaluate the visible light activity by enhancing the incident light intensity, we also applied a setup with an inner irradiation lamp. The photocatalytic reaction was carried out in a double-walled quartz cell cooled by water with a 500 W full-arc Xe lamp (CEL-LAX500) as the light source. A schematic view of the setup can be found in ESI.

3. Results and discussion

The phase purities of the prepared catalysts were all verifified by powder XRD (Fig. 1). There is no standard XRD pattern available for CuFe0.7Cr0.3S2, instead the XRD pattern for CuFeS2 is provided in Fig. 1 for comparison purposes because it has a similar pattern to CuFe0.7Cr0.3S2 [38]. The CFCS catalysts both without and with cocatalysts showed almost identical XRD patterns. In addition, the catalysts recovered after the photocatalytic experiments showed similar patterns to the fresh CFCS catalyst, which indicates the catalysts are very stable in the photocatalytic reactions and are not degraded. Only a small quantity of Fe2O3 was observed as an impurity, and this did not affect the photocatalytic reaction in our study. SEM images (Fig. 2 and Fig. S3 in ESI) showed that the CFCS catalyst had nanoscale plates (average diameter 50 nm, thickness 15 nm) on its surface.

To investigate the activities of the various co-catalysts, aqueous solutions of NO3 (25 ppm N) were irradiated for 1 h under UV light and the percentage of NO3 converted to NO2 /N2 was calculated (Fig. 3). The results showed that both Ru and Au co-catalysts decreased the conversion of NO3 , while Cu did not affect the reduction. The co-catalysts Ag, Pt and Pd increased the conversion of NO3 to between 26.5 and 34.5%. For comparison purposes, an aqueous solution containing no catalyst was irradiated under UV-light. In this solution, after 2 h of irradiation, the NO3 concentration remained constant within the experimental error. The low conversion rate in this blank experiment was believed to arise from the low photon density of the irradiation source used in this study. Comparison of the catalytic experiments to the blank experiment indicated that all the catalysts could reduce NO3 with high efficiency

The adsorption and desorption of substrates at catalytic sites is a dynamic equilibrium process. We speculated that loading the catalyst with Ag, Pt or Pd at a mass fraction of 0.75% would increase the number of binding sites and delay the desorption of NO3 by lowering the potential barrier. This could increase the conversion rate of NO3 and allow the deep reduction to N2. Loading with cocatalysts could also facilitate photo-excited charge separation based on the photocatalytic mechanism, and therefore allow a higher density of electrons at the catalytic sites. These synergetic effects could greatly increase the photocatalytic reduction of NO3 in aqueous solution.

To better understand the effects of the different metallic co-catalysts on the reduction, NO2 reduction experiments were performed (Fig. 4). In most cases, loading with a co-catalyst increased the conversion of NO2 to N2. The barrier to NO2 adsorption was lowest with Au as the co-catalyst, which gave a conversion rate of 16.8%. The mechanism of this enhancement would be similar to that discussed above, with the co-catalysts aiding both charge separation and adsorption of substrates. By comparing to Fig. 3, we would expect that the Au cocatalyst has a strong binding ability to nitrite ions and may be benefificial for the N2 production.

To investigate co-catalyst loading with two metals, we tried combining PteAu, AgeAu, and PdeAu. These combinations were selected because the individual metals all gave good results as cocatalysts. Among these co-catalyst combinations, the highest conversion rate and N2 selectivity were achieved with a mass fraction loading of 0.75% Pd and 2% Au (Fig. 5). Increasing the mass fraction of Au, while holding that of Pd constant, resulted in higher N2 selectivity. The N2 selectivity of 52% (in 1 h irradiation) was achieved with a mass fraction loading of 0.75% Pd and 3% Au. It is understandable that the increasing the content of Au could increase the production of N2 because the increasing metal content provides more catalytic sites and particularly, Au is helpful to adsorb NO2 for deep conversion to N2. With further increasing the mass fraction loading of 1.5% Pd and 4% Au, a complete conversion of NO3 and N2 selectivity of 65% were achieved with 1 h of irradiation. From the industrial view of point, the usage of the noble metal should be as low as possible for the economic purpose. In our case, we did not further increase the content of the cocatalysts, which probably would give an even better performance.