Formic acid as the in-situ hydrogen source for catalytic reduction of nitrate in water by PdAg alloy nanoparticles supported on amine-functionalized SiO2
Graphical abstract
Introduction
Nitrate pollution in water is becoming a severe problem all over the world due to its intensive release from both agricultural and industrial activities [1]. Excessive nitrate presence in water could cause water eutrophication under proper temperature conditions, resulting in the indiscriminate killing of aquatic organisms and the destruction of the local ecological equilibrium [2]. It could also cause series of damages to human health, including blue baby syndrome, high blood pressure, diabetes, liver damage, and various cancers [3]. Accordingly, regulatory agencies around the world had set up the maximum contaminant level (MCL) for nitrate in drinking water according to their domestic conditions to ensure the health of people. For example, the European Drinking Water Directive set its MCL for nitrate in drinking water at 50 mg/L [4], while a more stringent standard was recommended by the World Health Organization at 25 mg/L [5]. To meet its drinking water standard, various processes had been developed to remove nitrate from water, including ion exchange [6], reverse osmosis [7], electrodialysis [8], photocatalytic reduction [9], catalytic reduction [10], and biological methods [11].
Among these techniques, catalytic reduction of nitrate by reducing agents had been considered as one of the most promising methods because it could convert nitrate to harmless nitrogen gas with a high efficiency [12]. Most catalysts for the nitrate reduction are composed of a noble metal and a promoter, in which the promoter reduces nitrate to nitrite by a redox process to start the catalytic process and nitrite is reduced by activated hydrogen on the noble metal [13]. Since the first successful demonstration of catalytic reduction of nitrate by Vorlop and Tacke [14], extensive research efforts have been made on this approach and hydrogen gas was used as the reducing agent in most of these works [15], [16], [17], [18], [19], [20], [21]. However, the use of H2 in the catalytic reduction of nitrate may cause problems, especially when considering its potential for industrial applications. Due to its low solubility in water, large amounts of H2 were wasted in this process and could be released into the environment, which only increased the cost of the operation but also posed a latent danger of explosion. In addition, the pH value of the treated water inevitably increased due to the formation of hydroxide ion during the catalytic reduction process with H2 as the reducing agent [22], which could deteriorate the activity and selectivity of catalysts. Thus, CO2 was needed to be bubbled into the solution with H2 to stabilize its pH value at weakly acidic conditions, which increased the complexity of the apparatus and the operation.
Formic acid (HCOOH) is the simplest carboxylic acid, which is an important intermediate in chemical synthesis and could be produced as a by-product in the manufacture of other chemicals [23]. It could be decomposed into H2 and CO2 by noble metal catalysts [24], [25], [26], [27], which makes it an in situ H2 source for the enhancement of H2 utilization efficiency in various reactions [28], [29]. The simultaneously produced CO2 could serve as the buffer to stabilize the solution pH in the catalytic reduction of nitrate [22], [30]. Thus, a few attempts had been made to use HCOOH as the reducing agent precursor for the catalytic reduction of nitrate [9], [22], [30], [31], [32], in which Pd was used as the noble metal component and the promoter component examined were Cu, Sn, and In [31], [13]. Till now, no report was available on the use of Ag as the promoter in the catalytic reduction of nitrate with HCOOH as the reducing agent precursor, while it had been reported that Pd/Ag could serve as a bimetal denitrification catalyst with H2 as the reducing agent [33]. Furthermore, it had been reported that the existence of Ag could enhance the decomposition activity of Pd on HCOOH and the catalytic hydrogenation of Pd compared with other metals due to the larger difference of work function values between Pd and Ag [34], [35]. Thus, it is of interest to examine the catalytic reduction of nitrate by Pd/Ag bimetal catalyst with HCOOH as the reducing agent precursor.
In this work, a PdAg/SiO2-NH2 catalyst was developed, and its catalytic nitrate reduction performance was examined with HCOOH as the reducing agent precursor. The surface of SiO2 catalyst support was first modified by amine groups, which had been found to beneficial for HCOOH decomposition [36], [37], [38], [39], [40]. PdAg alloy nanoparticles were loaded onto the SiO2-NH2 catalyst support by first loading Pd with a typical impregnation and subsequent reduction process [41], followed by a controlled surface reaction [42] to load Ag and create PdAg alloy. The PdAg/SiO2-NH2 catalyst was found to be able to effectively reduce nitrate with HCOOH as the reducing agent precursor for the first time. The catalytic nitrate reduction performance by the PdAg/SiO2-NH2 catalyst was found to be better in a closed reaction system than in an open reaction system. The surface NH2 modification of SiO2 catalyst support largely enhanced its nitrate reduction efficiency, and PdAg alloy was superior to PdCu alloy as the catalyst for the nitrate reduction with HCOOH as the reducing agent precursor The optimized Pd/Ag atomic ratio was found to be different between the catalytic HCOOH decomposition and the catalytic nitrate reduction.
Section snippets
Materials and chemicals
All chemicals were of analytical grade and used without further purification. Silica gel was purchased from Qingdao Haiyang Chemical Corporation (Qingdao, P. R. China). (3-aminopropyl) triethoxysilane (H2N(CH2)3Si(OC2H5)3, APTS), palladium chloride (PdCl2), silver nitrate (AgNO3), copper chloride (CuCl2), potassium borohydride (KBH4), potassium nitrate (KNO3), ethanol (C2H5OH), formic acid (HCOOH, 98%), sodium formate (HCOONa) were purchased from Sinopharm Chemical Reagent Corporation
Structure, morphology and composition of PdAg/SiO2-NH2 catalysts
Fig. 1a compares the FTIR spectra of SiO2 support before and after NH2 surface modification. Their spectra were generally alike, while two peaks occurred at 2984 cm−1 and 2937 cm−1 on the FTIR spectrum of SiO2 support after NH2 surface modification which could be attributed to the CH stretching of APTS. Thus, it suggested that APTS was successfully loaded onto SiO2 support to modify its surface with NH2 [43]. Fig. 1b shows the XRD patterns of PdAg/SiO2-NH2 catalysts with different Pd:Ag molar
Conclusions
In summary, a PdAg/SiO2-NH2 catalyst was developed, which demonstrated an effective catalytic nitrate reduction performance with HCOOH as the reducing agent precursor for the first time. The surface NH2 modification of SiO2 catalyst support largely enhanced its catalytic nitrate reduction performance because it could facilitate the rupture of OH bond in HCOOH. PdAg alloy nanoparticles were superior to PdCu alloy nanoparticles as the catalyst for the nitrate reduction with HCOOH as the reducing
Acknowledgements
This study was supported by the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No. Y4N56R1161 and Y4N56F2161), and the National Natural Science Foundation of China (Grant No. 51502305).
References (54)
- et al.
Desalination
(1987) - et al.
J. Catal.
(2005) - et al.
J. Catal.
(2002) - et al.
Enzyme Microbiol. Technol.
(1992) - et al.
Appl. Catal. B: Environ.
(2011) - et al.
J. Mol. Catal. A: Chem.
(2001) - et al.
Catal. Today
(1999) - et al.
Appl. Cat. B: Environ.
(2001) - et al.
J. Catal.
(2001) J. Catal.
(2004)
Appl. Catal. B: Environ.
Desalination
Appl. Catal. B: Environ.
Water Res.
Catal. Today
Appl. Cat. B: Environ.
Catal. Today
J. Environ. Sci.
Appl. Catal. B: Environ.
J. Catal.
Appl. Cat. B: Environ.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
J. Hazard. Mater.
Anal. Chim. Acta
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Cited by (73)
Lignin-coordinated highly dispersed Pd[sbnd]Zn alloy nanocluster supported on N-doped nanolayer carbon and its application in hexavalent chromium detoxification
2023, International Journal of Biological MacromoleculesDevelopment of Pd-based catalysts for hydrogenation of nitrite and nitrate in water: A review
2023, Journal of Hazardous MaterialsFluorescent paper-based sensor integrated with headspace thin-film microextraction for the detection of acyclic N-nitrosamines following in situ photocatalytic decomposition
2023, Analytica Chimica ActaCitation Excerpt :Furthermore, it has been reported that the combination with noble metals, such as Pd and Pt, increases the catalytic activity of TiO2 [44]. In this work, nanostructured Pd (Pd nanoparticles, Pd NPs) was selected as metal catalyst, since it can provide increased surface area yielding higher reactivity [45]. Firstly, to evaluate their catalytic behavior, 0.5 mg mL−1 TiO2 and 0.7 μg mL−1 Pd NPs were tested both separately and combined.
Photocatalytic removal of nitrate from water using activated carbon-loaded with bimetallic Pd-Ag nanoparticles under natural solar radiation
2022, Journal of Photochemistry and Photobiology A: Chemistry
- 1
These authors contributed equally to this work.