ARTICLE pubs.acs.org/JPCC
Photocatalytic Oxidation of Organic Pollutants Catalyzed by an Iron Complex at Biocompatible pH Values: Using O2 as Main Oxidant in a Fenton-like Reaction Xi Chen, Wanhong Ma,* Jing Li, Zhaohui Wang, Chuncheng Chen, Hongwei Ji, and Jincai Zhao* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
bS Supporting Information ABSTRACT: A red iron(II) 4,40 -dicarboxy-2,20 -bipyridine complex ([FeII(dcbpy)3]) was investigated as an extraordinary Fenton catalyst capable of activating much more molecular O2 to mineralize organic pollutants in water at biocompatible pH values under visible irradiation. Eight representative organic pollutants were effectively degraded in the presence of this catalyst with high turnover number (368-2000). The flexible bifunctional coordination mode (N donor for ferrous ion and O donor for ferric form) devoted by the ligand dcbpy should be responsible for the preservation of iron(II/III) catalysis in such a neutral pH condition, whereas any substitution of the 4,40 carboxylic groups in dcbpy by other groups such as ether, alcohol, nitroyl, or methyl groups resulted in nearly total loss of catalytic stability. More important, the present [FeII(dcbpy)3] catalyst can dramatically change the traditional role of H2O2 as main oxidant in the general Fenton reaction and make molecular O2 become the main oxidant in the mineralization of organic pollutants instead. Through the simultaneously quantitative measure of the actual consumption of molecular O2 and H2O2 as well as the corresponding mineralization yields of substrates (2,4-DCP and Org II), respectively, we found that the usage of O2 not only is almost twice the H2O2 depletion for the mineralization of substrates but also rigorously accords with the mineralization yield of substrates in terms of stoichiometric relation. No matter whether H2O2 is in excess or not, O2 participates in the mineralization of substrates and acts as the main oxidant. For the same amount of H2O2 consumption, the O2 consumption was only 2.5% and 8.13% relative to the H2O2 usage in the controlled general Fenton reaction and UV-Fenton reaction (pH = 3.0), respectively. This clearly indicates that the present catalyst is able to use much more O2 to eliminate organic pollutants in water under visible irradiation. Visible irradiation is crucial for the oxidative degradation and mineralization process. The extraordinary ability of bifunctional coordination sites offered by this ligand provides a promising design paradigm for Fenton-like catalyst to eliminate organic pollutants by using more solar energy and O2 in air.
1. INTRODUCTION Effective and low-cost oxidative elimination of deleterious organic pollutants in water has been one of the major challenges in environmental remediation.1-3 Recently, the catalytic degradations of organic pollutants in water at neutral pH by new transition metal complexes have attracted much attention,1-5 which essentially originated from the Fenton reaction under acidic conditions (the alleged Fenton-like reactions: iron complex and H2O2). In these systems, H2O2 is commonly preferred as an environmentally friendly oxidant since its byproduct is H2O.6 Among these synthetic iron complex catalysts, so far the most effective and accessible iron-complex catalysts are limited to all-nitrogen donor complexes. For example, an excellent catalyst Fe-TAML reported by Collins’ group can rapidly decompose pentachlorophenol (PCP) and 2,4,6-trichlorophenol (TCP) pollutants even within several minutes and lead to the mineralization of pollutants with the nearly stoichiometric ratio of H2O2 to substrates.4,5 Namely, this 100% efficiency of H2O2 as the oxidant in the mineralization of pollutants means that O2 in air, a lower-cost and environmentally benign oxidant, participates r 2011 American Chemical Society
scarcely into the degradation of pollutants. So, it is still urgent to develop new robust catalysts and/or systems that enable the use of O2 in air as the main oxidant instead of H2O2 to finish some Fenton-like reactions. In fact, the participation of O2 into the oxidative transformation and mineralization of substrates is commonly observed in both the classical Fenton reaction7-14 and the iron complex catalyzed Fenton-like reaction. In the classical Fenton reaction, the consumption of O2 may result from the direct incorporation into organic intermediates by the reaction with organoradicals and a little O2 is incorporated into the mineralization of substrates. This pathway of O2 participation is very analogous to that of the Fenton-like case (eq 3) wherein the oxidative degradation of substrates by O2 is much less than that of H2O2. For the UV-Fenton reaction, a considerable amount of oxidative degradation of substrates is attributed to the O2 participation, even reaching as high as 36% when the Received: October 27, 2010 Revised: January 21, 2011 Published: February 18, 2011 4089
dx.doi.org/10.1021/jp110277k | J. Phys. Chem. C 2011, 115, 4089–4095
The Journal of Physical Chemistry C
ARTICLE
concentration of initial iron ions is equivalent to that of substrates so as to meet the needs of a stoichiometric UV-Fenton reaction.15 In this reaction, the degradation of substrates by O2 is due to an additionally paralleling reaction that UV light reduces Fe(OH)2þ to produce •OH radicals, and then the •OH radicals can produce extra R• radicals (eq 2a), which will incorporate more O2 into its end products as indicated in eq 3. For the most general Fenton-like reaction, O2 is incorporated into products generally through the well-known Russell reactions with transient organic carbon-centered radicals (eq 3) which are produced from H-atom abstraction of substrates by either •OH radical or high-valence oxo-iron species (eq 2).16-19 FeII Ln þ H2 O2 f FeIII Ln þ • OH þ OH-
ð1aÞ
FeII=III Ln þ H2 O2 f Ln FeIV=V dO þ H2 O
ð1bÞ
½FeIII OH2þ þ hν f FeII þ • OH
ð1cÞ
RH þ • OH f R • þ H2 O
ð2aÞ
RH þ • OH f RHþ• þ OH-
ð2bÞ
RH þ Ln FeIV dO f R • þ Ln FeIII OH
ð2cÞ
RH þ Ln FeIV=V dO f ROH þ FeII=III Ln
ð2dÞ
R • O2 f ROO• f ketones þ alcohols þ O2
ð3Þ
FeIII Ln þ H2 O2 f FeII Ln þ • OOH þ Hþ
ð4Þ
The incorporation of O2 into the products through the reaction of eq 3 may reach a relatively high level, even yielding 70% ketone and alcohol products from O2,20 but for the overall oxidative degradation and mineralization of substrates, the yield of R• radicals as well as ROO• radicals commonly is inappreciable (less than 5% equivalent to H2O2 consumption) among all of the subsequent reactions delivered from H2O2. It is mainly because of the generally short-life R• radicals and the very limited O2 concentration; also, the rigid N donor coordination in iron complex is unfavorable to let molecular O2 close to its catalytic metal center. However, some iron complexes with special ligands that have both pyridine N and carboxyl O donors, such as the Giftype system (H2O2/FeIII picolinic acid/HAc, pH ∼1), can exhibit an important property: they can efficiently use O2 as co-oxidant in the catalytic oxidation of substrates by H2O2.21-23 Although the accurate discrimination of a Gif-type mechanism is still oscillating between the predominating reaction by highvalence FeIVdO and •OH radicals, the elusive coordination of a carboxylic group to the central iron ion indeed differs from that of macrocyclic-N donor rigid coordination and affords the use of O2 as oxidant. Nevertheless, strong reducing agents such as Zn and Pd/H2 have to be used in the Gif system for reducing the catalyst from the FeIII to the FeII state because FeIII species are very stable and difficult to be reduced by the environmentally friendly oxidant H2O2 (eq 4).24,25 In this work, we report a new photocatalytic system containing an iron 4,40 -dicarboxy-2,20 -bipyridine complex [FeII(dcbpy)3], which can effectively use O2 as main oxidant. No matter whether H2O2 is in excess or not, the usage of O2 not only is almost twice
more than that of the H2O2 depletion for the mineralization of substrates but also rigorously accords with the mineralization yield of substrates in terms of stoichiometric relation for the mineralization of various organic pollutants at neutral pH under visible irradiation. The free carboxyl group of dcbpy shows a weak affinity to the low valence state of the metal center, which was analogous to those of N, N, and O ligands in an iron-enzyme system.26 Such a structural flexibility of iron complex is somewhat like the case of mononuclear non-heme iron enzymes where 2-His-1-carboxylate facial triad coordination ensures a series of O2-dependent reactions at physiological pH.27-29 Thus we expect that using dcbpy as the initial ferrous chelator can realize an exquisite and reversible shift of the coordination sites during the iron center catalytic cycle, in which the N donors of bipyridine moiety for ferrous ion makes the red catalyst facile to be excited by visible light irradiation,30 and the O donors of carboxylic group for ferric ion increase the stability of catalyst at neutral pH. This structurally designed strategy of catalyst will provide an extraordinary approach to use more O2 in air and less H2O2 for the elimination of organic pollutants in the Fenton-like reaction.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Iron(II) perchlorate hydrate and iron(III) perchlorate hydrate were purchased from Aldrich. 2,20 -Bipyridine, 4,40 -dicarboxy-2,20 -bipyridine, and 4,40 -dimethyl-2,20 -bipyridine, which were used as the ligand for the iron complex catalysts, were purchased from Simopharm Chemical Reagent Co. (China), Acros Organics, and Fluka, respectively. The other ligands, for example, 4,40 -dinitro-2,20 -bipyridine, 4,40 -bis(methoxycarbonyl)-2,20 -bipyridine, and 4,40 -bis(hydroxymethyl)2,20 -bipyridine were all purchased from Nanjing Chemzam Pharmtech Co.(China). Dyes (Orange II, Alizarin yellow GG, Malachite green, Rhodamine B and Acridine Orange), 2,4dichlorophenol, o-phthalic acid, salicylic acid, and 5-sulfosalicylic acid were all of laboratory reagent grade and used without further purification. Horseradish peroxidase (POD), which was used for the measurement of H2O2, was purchased from the Simopharm Chemical Reagent Co. (China), the N,N-diethyl-p-phenylenediamine sulfate (DPD) reagent was from Aldrich Chemical Co. The pH of the solutions was adjusted by dilute aqueous solutions of either NaOH or HClO4. Barnstead UltraPure water (18.3 MΩ) was used for all experiments. 2.2. Methods. Preparation of the Photocatalysts. [FeII(dcbpy)3] complex which has strong and broad absorption in the visible region (for [FeII(dcbpy)3], ε542nm ∼ 8 103 L mol-1 cm-1) (see Figure S1 in the Supporting Information) was prepared by mixing Fe(ClO4)2 in water with stoichiometric 4,40 -dicarboxy2,20 -bipyridine ligand as described previously in the literature.31 The mixture was refluxed for about 20 min until a deep red solution formed. The solution was then filtered, and the resulting metal complex was precipitated by addition of sodium perchlorate and recrystallized from warm water. The other iron complexes (in Table 1) and [FeIII(H2O)3(dcbpy)3] complex were all prepared according to a similar procedure. Photodegradation Processes. A 500 W halogen lamp with a light filter to cutoff light of wavelength
