Oxidative Degradation of Phenol using <em>in situ</em> Generated Hydrogen Peroxide Combined with Fenton’s Process
Oxidative Degradation of Phenol using in situ Generated Hydrogen Peroxide Combined with Fenton’s Process
Supported palladium-iron catalysts for the removal of model contaminants from wastewater
Oxidative destruction of organic compounds in water streams could significantly reduce environmental effects associated with discharging waste. We report the development of a process to oxidise phenol in aqueous solutions, a model for waste stream contaminants, using Fenton’s reactions combined with in situ synthesised hydrogen peroxide (H2O2). Bifunctional palladium-iron supported catalysts, where Pd is responsible for H2O2 synthesis while Fe ensures the production of reactive oxygen species required for the degradation of phenol to less toxic species is reported. A comparison is made between in situ generated and commercial H2O2 and the effect of phenol degradation products on catalyst stability is explored.
Phenolic compounds are present in waste streams produced by a variety of industries; including petrochemical, pharmaceutical, plastics and resin manufacturing (1, 2). Due to their high toxicity to both humans and aquatic life, even at concentrations as low as 2 mg l–1 (3), it is essential to remove phenolic compounds from these waste streams before discharge to the mains water system or the wider environment. One approach to destroy the contaminants is through oxidative degradation to less toxic compounds or total mineralisation to carbon dioxide (CO2). Fenton’s reaction, the catalytic formation of hydroxyl and hydroperoxy radicals by the disproportionation of hydrogen peroxide, can be considered one of the most efficient catalytic systems for degradation of organic pollutants in wastewater streams at low to moderate concentrations (4–9). Comparison of different oxidation processes for phenol degradation by Esplugas et al. reports the efficacy of Fenton’s reagent over approaches including photocatalytic approaches and ozonation (10). There are a number of advantages associated with the use of Fenton’s reactions to treat wastewater streams, such as its simplicity (being operated at room temperature and at atmospheric pressure) and its lack of toxicity, with H2O2 degrading to environmentally benign species such as H2O and O2.
A range of catalytically active iron species have been utilised as Fenton’s reagents, these can consist of metal salts (Fe(II) (11, 12) or Fe(III) (13)), metal oxides (Fe2O3, Fe3O4) (14) and zero-valent iron (Fe(0)) (15, 16). A range of heterogeneous iron based catalysts have also been reported, including Fe-doped zeolites (Fe-zeolite-Y (17), Fe-ZSM5 (18) and Fe-beta (18)) Fe2O3 intercalated between layered clays such as laponite (19, 20) and bentonite (21) as well as Fe containing SBA-15 (22) and other Fe containing catalysts (22–25).
It is currently accepted that the oxidation of phenol follows the mechanism proposed by Devlin and Harris (26), where phenol is first decomposed into aromatic compounds, such as catechol and hydroquinone. These initial products in turn undergo oxidation to produce carboxylic acids, such as oxalic, acetic and formic acids, the intermediates formed in the production of these acids often have greater toxicity than phenol itself (27). However, catalyst deactivation, especially due to metal leaching of Fe into aqueous solution, presents new challenges including the need for removal of ferrous or ferric contaminants from the waste stream, in addition to the continual replenishment of the heterogeneous catalyst. Besides iron, a range of other transition metals with multiple redox states such as cobalt (28, 29), copper (30–32), manganese (33) and ruthenium (34) can all decompose H2O2 into •OH radical species through Fenton-like pathways. However, due to economic and environmental concerns iron is still often the preferred choice for the process.
Nevertheless, a number of drawbacks are associated with using commercial hydrogen peroxide for this process that remain to be solved, namely the costs associated with the anthraquinone process, the means by which H2O2 is produced industrially. Although this process can be considered highly efficient the requirement for the constant replacement of the anthraquinone and the energy costs associated with H2O2 concentration, transportation, storage and dilution at source prior to use has led to the investigation of alternative means of H2O2 production in situ for the application of wastewater treatment (35–38). A number of studies have investigated the efficacy of H2O2 generated in situ over Pd-Fe based catalysts utilising a range of hydrogen sources such as formic acid, hydroxylamine and hydrazine (35–38, 40). Herein we investigate the activity of Pd-Fe bimetallic catalysts supported on titania (TiO2) and silica (SiO2) for the degradation of phenol via the in situ synthesis of H2O2 from molecular H2 and O2.
In a typical preparation of Pd-Fe/TiO2, the requisite amount of PdCl2 aqueous solution (6 mg ml–1 Pd, 0.58 M HCl) and FeCl3 aqueous solution (6 mg ml–1 Fe) were added to a 50 ml round bottom flask. Water was then added to achieve a total solution volume of 16 ml. The solution was then heated to 60°C with 1000 rpm stirring and the required amount of support TiO2 (P25 Degussa) was added. After complete addition of support the temperature was raised to 95°C with stirring (1000 rpm) for 16 h to allow complete evaporation of the water. The dried catalyst was then ground and reduced under flowing 5% H2/Ar (500°C, 4 h, 10°C min–1). An analogous preparation method was employed for Pd-gold, Pd-manganese and Pd-copper where HAuCl4.3H2O (Strem Chemicals, 99.8%), Mn(NO3)2.xH2O (Sigma-Aldrich, 99.99%) and Cu(NO3)2.xH2O (Sigma-Aldrich, 99.99%) were utilised as metal precursors.
Oxidation of Phenol via in situ Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen
Phenol oxidation reactions were performed in a Parr stainless steel autoclave with a nominal volume of 50 ml and a maximum working pressure of 14 MPa. In a typical test the autoclave was charged with catalyst (0.01 g, 1 g l–1) and phenol (1000 ppm in H2O, total mass 8.5 g). The charged autoclave was then purged three times with 5% H2/CO2 (0.7 MPa) before filling with 5% H2/CO2 to a pressure of 2.9 MPa, followed by the addition of 25% O2/CO2 (1.1 MPa). The reactor was then heated to 30°C followed by stirring (1200 rpm) of the reaction mixture for typically 2 h. After 2 h the reaction solution was collected and filtered, followed by analysis by high performance liquid chromatography (HPLC) fitted with an Agilent Poroshell 120 SB-C18 column.
Oxidation of Phenol Using Preformed Hydrogen Peroxide
Phenol oxidation reactions were performed in a Parr stainless steel autoclave with a nominal volume of 50 ml and a maximum working pressure of 14 MPa. In a typical test the autoclave was charged with catalyst (0.01 g, 1 g l–1) and phenol (1000 ppm in H2O, total mass 8.5 g) and either stabilised hydrogen peroxide (50 wt% in H2O stabilised, Fluka™) or unstabilised hydrogen peroxide (30 wt% in H2O, ACROS). The charged autoclave was then purged three times with 25% O2/CO2 (0.7 MPa) before filling with 25% O2/CO2 (4 MPa). The reactor was then heated to 30°C followed by stirring (1200 rpm) for typically 2 h. After 2 h the reaction solution was collected and filtered, followed by analysis by HPLC fitted with Agilent Poroshell 120 SB-C18 column.
X-ray photoelectron spectroscopy (XPS) analyses were made on a Kratos Axis Ultra DLD spectrometer. Samples were mounted using double-sided adhesive tape and binding energies were referenced to the C 1s binding energy of adventitious carbon contamination that was taken to be 284.7 eV. Monochromatic Al Kα radiation was used for all measurements; an analyser pass energy of 160 eV was used for survey scans while 40 eV was employed for detailed regional scans. The intensities of the Pd 3d and Fe 2p features were used to derive metal loadings. Microwave plasma atomic emission spectroscopy (MP-AES) analysis of post reaction solutions were carried out using an Agilent 4100 MP-AES. Samples were investigated for the presence of Pd and Fe using multiple wavelength calibrations for each individual element.
Results and Discussion
Our initial investigations into removing phenol by utilising in situ H2O2 production combined with Fenton’s chemistry examined the activity of bimetallic Pd based catalysts for the degradation of phenol, via the in situ generation of H2O2. In our choice of catalyst combinations we utilised Pd as the H2O2 generating species and varied the second metal to metals that have known activity as Fenton type catalysts. We compare these catalysts to a well-established catalyst used for the direct synthesis of H2O2, 2.5% Pd-2.5% Au/TiO2 (41, 42).
Figure 1 shows the extent of phenol conversion over time utilising bimetallic catalysts that contain the H2O2 degradation component in the presence of 1000 ppm phenol. Over the reaction period, phenol conversion was limited (less than 5%) for all Pd-X/TiO2 catalysts other than Pd-Fe/TiO2, which demonstrated a significant conversion of phenol (78%) after a reaction time of 2 h despite both Mn (33, 43) and Cu (44, 45) being studied as Fenton-like catalysts in their own right. Post reaction investigations of the reaction medium by MP-AES revealed a significant amount of Fe leaching from the catalyst support, with 38% Fe leached after 120 min. Building on these initial findings we investigated the role of both Pd and Fe in the degradation of phenol (Figure 2). It was determined that both monometallic Pd and Fe catalysts offer limited activity towards the degradation of phenol, 5% and 8% conversion respectively, indeed the extent of phenol conversion is comparable to that observed when no catalyst is present (Table S.1 in the Supplementary Information). This limited conversion can be related to the inactivity of Fe to synthesise H2O2. In comparison, Pd supported catalysts are well known for their activity towards the degradation of H2O2 to H2O (46, 47), as such the production of reactive oxygen species (•OOH and •OH) responsible for the degradation of phenol is limited.
A physical mixture of Pd/TiO2 and Fe/TiO2 catalysts resulted in a substantial improvement in phenol conversion to 35% compared to the monometallic Pd and Fe catalysts alone supporting the synergistic effect of having both components in the reaction. However, this was still far lower than the phenol conversion achieved when employing the bimetallic catalyst (78%) despite containing the same amount of each metal component. This demonstrates the need for either an alloyed Pd-Fe species or the requirement for both metals to be in close proximity, by being supported on the same support, to limit non-selective H2O2 decomposition during diffusion between sites responsible for H2O2 generation and degradation to reactive oxygen species, likely •OOH and •OH. Georgi et al. (48) have reported accelerated rates of Fenton’s reaction when heterogeneous Pd is utilised alongside homogeneous Fe, in the presence of H2, due to improved rates of Fe3+ reduction to Fe2+ and we suggest the increased rate of phenol conversion observed for the bimetallic Pd-Fe catalyst may be, at least in part, attributed to this enhancement in Fe reduction rate.
The effect of the Pd:Fe ratio on catalytic activity towards phenol conversion was investigated, with total Pd loading fixed at 2.5 wt% and varying the Fe content of the catalyst between 0.5–2.5 wt% (Figure 3). A correlation can be drawn between total Fe loading and activity towards phenol conversion, with the activity of the 2.5% Pd-2.5% Fe/TiO2 catalyst approximately five times greater than that of the 2.5% Pd-0.5% Fe/TiO2 catalysts.
Investigation of the post reaction solution by MP-AES revealed leaching of Fe in all Pd-Fe/TiO2 catalysts, ranging from 6 ppm in the high Fe loaded samples to <1 ppm in low Fe loading samples (Figure S.1 in the Supplementary Information). The total Fe leaching was closely related to phenol conversion in all reactions with both low Fe loaded catalysts (2.5% Pd-1% Fe/TiO2 and 2.5% Pd-0.5% Fe/TiO2) showing minimal Fe leaching despite reasonable rates of phenol conversion (15% and 23% respectively). Investigation into the relationship between phenol conversion and Fe leaching reveals a close relationship between the two. Monitoring the extent of Fe leaching over the course of a reaction using the 2.5% Pd-2.5% Fe/TiO2 catalyst reveals Fe content dramatically increases as phenol conversion increases beyond 30% (Figure 4). This indicated that while conversion levels below 20% could be reached with minimal Fe leaching the generation of further oxidation products such as carboxylic acids or alcohols resulting from the degradation of phenol may have been responsible for accelerated leaching of Fe from the catalyst. It is well known that the Fenton process is highly dependent on solution pH, with activity reduced at elevated pH due to the formation of inactive iron oxohydrides (49) in addition to increased decomposition of H2O2 (50). We have previously reported the beneficial effect of the CO2 diluent in promoting the stability of H2O2, through the formation of carbonic acid in solution. We report a decrease in reaction solution to pH 4 when a CO2 rich atmosphere is utilised, which is comparable to the pH reported as optimal for the Fenton reaction (49, 51).
XPS investigation of the surface concentration of Pd and Fe in 2.5% Pd-2.5% Fe/TiO2 before and after treatment with the diol catechol and oxalic acid as examples of degradation products of phenol (1000 ppm), both of which we have detected as reaction intermediates by HPLC, can be seen in Table I. It was observed that for the catechol treated catalyst, the surface concentration of Fe was similar to that of the fresh. However, for the oxalic acid-treated catalyst, there was a decrease from 2.66 at% to 1.40 at%. In the case of Pd, there was a large loss in surface concentration for both the catechol and oxalic acid treated catalysts. When the catalyst was treated with catechol the surface concentration of Pd decreased from 0.82 at% to 0.47 at%, while the analogous treatment with oxalic acid Pd content is reported to decrease to 0.17 at%.
|Element||Fresh catalyst, at%||Catechol treated, at%||Oxalic acid treated, at%|
To better understand the role of the support and to identify if improved rates of phenol conversion could be achieved, a SiO2 support with a significantly higher surface area than the TiO2 was used to prepare an analogous 2.5% Pd-2.5% Fe/SiO2 catalyst in an attempt to suppress Fe leaching. As can be seen in Table II the extent of phenol oxidation was much greater for the 2.5% Pd-2.5% Fe/SiO2 catalyst (92%) compared to 2.5% Pd-2.5% Fe/TiO2 (78%). Analysis of the post reaction medium by MP-AES revealed that total Fe leaching increased for the 2.5% Pd-2.5% Fe/SiO2 catalyst in comparison to the 2.5% Pd-2.5% Fe/TiO2 catalyst. We ascribe this to an increase in the extent of phenol conversion and a greater concentration of further oxidation products such as oxalic acid which we have shown is capable of leaching Fe species from the catalyst by chelation to form Fe-oxalate.
|Catalyst||Phenol conversion, %||Fe leaching, ppm|
In an attempt to increase the dispersion of metals active towards the direct synthesis of H2O2 as well as those sites responsible for the production of reactive oxygen species, total metal loading was decreased to 0.5% Pd-0.5% Fe/SiO2, achieving 99% phenol conversion, under identical conditions. By normalising catalyst mass to ensure the total amount of metal utilised was identical to the 2.5% Pd-2.5% Fe/SiO2 catalyst, total Fe leaching was observed to be similar to that of the 5% Pd-Fe/SiO2 catalyst, at 12 ppm and 11 ppm respectively. This supports the hypothesis that phenol conversion and Fe leaching are closely linked in all cases. The excellent rates of phenol conversion observed led us to investigate the efficacy of the 0.5% Pd-0.5% Fe/SiO2 catalyst at shorter reaction times, as seen in Figure 5. We report very high rates of phenol conversion over 60 minutes, 96%, significantly higher than that of the standard 2.5% Pd-2.5% Fe/TiO2 discussed above, with a phenol conversion of 65% over this time scale, despite five times greater metal content.
Evaluation of the catalytic activity of Pd-Fe/SiO2 toward phenol degradation using pre-formed, commercial, H2O2 compared to that observed when utilising H2O2 produced in situ can be seen in Table III. It is observed that phenol conversion is limited in the presence of preformed H2O2 and O2 (6%), and H2 (81%), which Georgi et al. have previously accredited to an enhancement in the Fe redox cycle (48). However, the utilisation of H2O2 synthesised from molecular H2 and O2 resulted in an enhanced phenol conversion, beyond that seen with commercially available H2O2 (96%). We ascribe the dramatic enhancement in catalytic activity to the absence of compounds such as phosphoric acid and acetanilide, known to be utilised as H2O2 stabilising agents (52), and the generation of a greater flux of reactive oxygen species responsible for phenol degradation.
|Reaction conditions||Phenol conversion, %|
|5% H2/CO2 + 25% O2/CO2a||92|
|0.5 wt% H2O2 (preformed) + 25% O2/CO2b||6|
|0.5 wt% H2O2 (preformed) + 5% H2/CO2b||81|
In order to establish the key species required for the oxidation of phenol a number of hot filtration experiments were performed (Table S.2 in the Supplementary Information). From these experiments a number of conclusions can be drawn, namely that in situ generation of H2O2 is required to achieve any activity in the phenol oxidation reaction. The extent of phenol conversion was far greater when H2O2 was generated from H2 and O2 compared to the use of preformed H2O2, possibly due to the presence of stabilising agents found in commercial H2O2 (52) or self-termination of radical species when all the H2O2 is added at the beginning of the reaction. In addition it is observed that homogeneous Fe species present in the solution can catalyse the degradation of phenol but only in the presence of a heterogeneous Pd catalyst, although the catalyst activity was far less than when the 0.5% Pd-0.5% Fe/SiO2 catalyst was employed for the reaction, suggesting that while there is a homogeneous reaction occurring with the leached Fe species there is also a heterogeneous component to the reaction rate.
In conclusion, we have demonstrated the efficacy of Pd-Fe supported catalysts towards the oxidation of phenol using H2O2 generated in situ from molecular H2 and O2, in a relatively short reaction time, with superior results achieved in comparison to using preformed H2O2. We have also identified some of the phenol oxidation intermediates that are responsible for the leaching of active metals from the surface of the catalyst support, namely Fe. We consider that through additional catalyst optimisation and reactor design it may be possible to ensure minimal leaching of metal while maintaining the excellent rates of phenol conversion reported.
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Ricci Underhill gained his BSc and MSc from Cardiff University, UK, with his PhD completed in 2018. His research interests include the development of heterogeneous catalysts for the treatment of waste-stream contaminants and the direct synthesis of hydrogen peroxide.
Richard Lewis received his degree from Cardiff University in 2012 and compaleted his PhD at the Cardiff Catalysis Institute (CCI) in 2016. He is currently a post-doctoral research associate within the CCI with research interests including the design and synthesis of precious metal based catalysts for liquid phase oxidation and waste-stream treatment.
Simon Freakley gained his degree from Durham University, UK, in 2009 and completed his PhD in 2012 in heterogeneous catalysis at the Cardiff Catalysis Institute. He is currently a Sêr Cymru research fellow appointed by the Welsh government. His research interests include precious metal catalysts, water purification and utilising renewable energy in catalysis.
Mark Douthwaite is a post-doctoral research associate at the Cardiff Catalysis Institute. He received his BSc, MSc and PhD at Cardiff University. His fundamental academic interests are focused towards the design, synthesis and characterisation of heterogeneous catalysts for liquid and gas phase applications in green chemistry.
Peter Miedziak is a lecturer in Chemistry at the University of South Wales, UK. He previously worked at Cardiff Catalysis Institute as part of Professor Graham Hutchings’ research group. His area of expertise is using supported nanoparticular metals as catalysts, particularly gold, with an interest in the effect of changes in the preparation parameters and analysis by microscopy.
After a PhD at the Institute of Research on Catalysis and Environment of Lyon, France, Ouardia Akdim joined the Laboratory of Multimaterial and Interface at the University Claude Bernard of Lyon as a postdoc in 2008. In 2010 she worked as a research associate at the European Institute of Membrane of Montpellier, France. Since 2014 she has worked as a senior research associate at the Cardiff Catalysis Institute with Professor Hutchings. Her areas of expertise are, among others: heterogeneous catalysis, hydrogen for fuel cells, materials synthesis, hydrogen peroxide, waste water treatment and disinfection.
Jennifer Edwards is a senior research fellow in the Cardiff Catalysis Institute, based in the School of Chemistry at Cardiff University. She has published over 90 papers and patents, and has a particular interest in the design and application of precious metal catalysts. Her efforts in the field have been recognised by the International Precious Metal Institute (Carol Tyler award 2011) and the German catalysis network Unicat (Clara Immerwahr award 2013). She was a finalist in the Science category for the Women of the Future awards in 2015.
Graham Hutchings is Regius Professor of Chemistry and Director of the Cardiff Catalysis Institute. He studied chemistry at University College London, UK. His early career was with ICI and AECI Ltd where he became interested in gold catalysis. In 1984 he moved to academia and has held chairs at the Universities of Witwatersrand, Liverpool and Cardiff. He was elected a Fellow of the Royal Society in 2009, and he was awarded the Davy Medal of the Royal Society in 2013.