Polymeric materials for catalysis applications

Resumen

Chapter I. Introduction The increasing use of nitrogenous fertilizers is responsible for nitrogen species permeation through the soil layers and contamination of groundwater. Methemoglobinemia and cancer are diseases produced by the human consumption of water exceeding the maximum permitted level of nitrate of 50 mg·L-1. Nitrates may be reduced to nitrites within the human body. Nitrite combines with hemoglobin, which contains the ferrous (Fe2+) ion, to form methemoglobin, which contains the ferric (Fe3+) form of iron. The increased affinity for oxygen of methemoglobin leads to an overall reduced ability of the red blood cells to release oxygen to tissues. When methemoglobin concentration is too high in red blood cells, tissue hypoxia may occur. This disease, known as the blue baby syndrome, is fatal to the new born. Physico-chemical, biological and catalytic processes are available for removing nitrates from water. Physico-chemical methods as ion-exchange, reverse osmosis and electro-dialysis remove nitrates from drinking water, but concentrate them elsewhere, with the subsequent disposal problem of the generated nitrate waste brine. Biological denitrification transforms nitrates into molecular nitrogen but is difficult to operate and may become another source of contamination of water with bacteria. Catalytic reduction of nitrate species to form nitrogen has been considered as an alternative technology for nitrate abatement. Nitrate reduction is generally carried out with hydrogen (H2) in the presence of metal catalysts. Monometallic systems based on Pt or Pd can be used, and also bimetallic systems combining a hydrogenating metal (Pd, Pt) and a promoter metal (Cu, In, Sn) dispersed on different supports with a relatively high surface area as activated carbons, carbon nanotubes, zeolites and metal oxides, as well as cation exchange resins. It has been demonstrated that the reaction progresses through intermediate nitrite (NO2−), and that nitrogen (N2) and ammonium (NH4+) are the principal products of the catalytic reduction of nitrate (NO3−) with dyhidrogen (H2). In many cases, the efficiency of the studied catalysts is not satisfactory, as high concentrations of toxic nitrites or ammonia by-products instead of the desired N2 are produced. The activity and selectivity of the catalysts is highly dependent on the preparation method, on the way the noble metal is promoted, on the metal/promoter ratio and on the operation conditions. Moreover, the support also affects the catalytic performance. The aim of this research work is to determine the ability of some conducting polymers (polypyrrole, polyaniline and polythiophene), for removing nitrates from water by adsorption and reduction, preferably without the need of a metal catalyst and producing molecular nitrogen as the only product. Chapter II. Experimental In this chapter, the synthesis of the materials and the characterization techniques used in this research work are described, as well as the experimental procedure used to study their ability for removal of nitrates in water. Chapter III. Plasma-assisted preparation of polypyrrole-supported catalysts and their application to nitrate removal in water This chapter is extracted from the publication: R. Buitrago-Sierra, M.J. García- Fernández, M.M. Pastor-Blas, A. Sepúlveda-Escribano, “Environmentally friendly reduction of a platinum catalysts precursor supported on polypyrrole”, Green Chemistry. 2013, 15, 1981-1990. Supported metal catalysts are traditionally prepared by impregnating a support material with the metal precursor solution, followed by reduction in hydrogen at high temperatures. In this chapter, a polymeric support has been considered. Polypyrrole (PPy) has been chemically synthesized using FeCl3 as a doping agent, and it has been impregnated with a H2PtCl6 solution to prepare a catalyst precursor. The restricted thermal stability of polypyrrole does not allow using the traditional reduction in hydrogen at elevated temperature, and chemical reduction under mild conditions using sodium borohydride implies environmental concerns. Therefore, cold RF plasma has been considered as an environmentally friendly alternative. Argon (Ar) plasma leads to a more effective reduction of platinum ions in the chloroplatinic complex anchored onto the polypyrrole chain after impregnation than reduction with sodium borohydride, as has been evidenced by XPS. The increase of RF power enhanced the effectiveness of the Ar plasma treatment. A homogeneous distribution of platinum nanoparticles has been observed by TEM after the reduction treatment with plasma. The PPy/Pt catalyst reduced by Ar plasma at 200 W effectively catalyzed the aqueous reduction of nitrates with H2 to yield N2, with a very low selectivity to undesired nitrites and ammonium by-products. Chapter IV. Optimization of the platinum loading and the argon plasma treatment The size and distribution of the metal nanoparticles on the support depend on the nature and concentration of the reducing agent, the reduction procedure and the metal loading. In the previous chapter, a metal loading of 1 wt. % was selected and reduction using sodium borohydride (NaBH4) as a mild chemical reducing agent has been compared with Ar cold plasma reduction. It has been concluded that electrons in the plasma are responsible for the more effective reduction of platinum ions to the metallic state than reduction with borohydride. Different RF powers (100 W, 150 W and 200 W) and also a repetitive plasma treatment were studied. The experimental results show that the increase of the Ar plasma power results in a more effective reduction of platinum ions into is zero- valent metallic state, and that the manual mixing between repetitive treatments assure an even exposure to the plasma. In this chapter, the RF power has been set to 200 W, and the influence of the length of the plasma treatment and the platinum loading has been analysed. As a result, the optimal experimental conditions have been established: 2 wt. % platinum loading and an argon plasma treatment at 200 W carried out for 3 h (5 min x 36 times) with manual mixing between treatments to assure an even exposure to the Ar plasma. Chapter V. Comparative study of the nitrate removal in water using monometallic and bimetallic catalysts supported on polypyrrole This chapter is extracted from the publication: M.J. García-Fernández, R. Buitrago-Sierra, M.M. Pastor-Blas, O.S.G.P. Soares, M.F.R. Pereira, A. Sepúlveda- Escribano, “Green synthesis of polypyrrole-supported metal catalysts: application to nitrate removal in water”, RSC Advances. 2015, 5, 32706-32713. Pt and Pt/Sn nanoparticles supported on polypyrrole (PPy) have been prepared using Ar plasma to reduce the metal precursors dispersed on the polymer. The PPy support was synthesized by chemical polymerization of pyrrole with FeCl3·6H2O, this leading to the conducting form of the polymer (assessed by conductimetric measurements). The Ar plasma treatment produced a partial reduction of platinum ions, anchored as platinum chloro-complexes to the PPy chain, into metallic platinum. A homogeneous distribution of Pt and Sn nanoparticles was observed by TEM. The catalytic activity of the PPy-supported catalysts was evaluated in the reduction of aqueous nitrate with H2 at room temperature. Nitrate concentration in water below the maximum acceptable level of 50 mg·L-1 was achieved with all catalysts. However, considering not only efficiency in nitrate reduction, but also minimized concentrations of undesired nitrite and ammonium, the monometallic Pt catalyst seems to be the most promising one. Chapter VI. Metal-free procedure for the removal of nitrates from water: effect of the oxidant used in the synthesis of polypyrrole Polypyrrole (PPy) has been synthesized by chemical polymerization of pyrrole (C4H5N) using ferric chloride (FeCl3·6H2O) or potassium peroxydisulfate (K2S2O8) as oxidants and dopants. The influence of the counterion acting as dopant, chloride (Cl−) or sulfate (SO42−), in the process of nitrate removal by adsorption/reduction has been determined, and it has been observed that the ion-exchange and the redox properties of PPy are strongly affected by the oxidant used in the polymer synthesis. The initial oxidation degree of the polymer is determined by the oxidant, and it defines the ability of the polymer to carry out the reduction of nitrate by electron transfer from the polymeric chain. The reduction process, the selectivity to desired nitrogen and to un-desired nitrite and ammonium are also affected by the oxidant used. Chapter VII. Proposed mechanisms for the hydrogenation of nitrate catalyzed by platinum nanoparticles supported on polypyrrole and polyaniline This chapter is extracted from the publication: M.J. García-Fernández, M.M. Pastor-Blas, F. Epron, A. Sepúlveda-Escribano, “Proposed mechanisms for the removal of nitrate from water by platinum catalysts supported on polyaniline and polypyrrole”, Applied Catalysis B: Environmental. 2018, 225, 162-171. Platinum nanoparticles have been synthesized on polyaniline (PANI) and polypyrrole (PPy) as supports using H2PtCl6 as metal precursor and a reducing treatment with cold Ar plasma. The catalytic activity of the polymer-supported catalysts in the reduction of aqueous nitrate with H2 at room temperature was evaluated. These systems are able to considerably decrease the concentration of nitrate in water in only 5 minutes. The mechanism of the nitrate abatement process is determined by the nature of the conducting polymer. The nitrogen functionalities in polyaniline are external to the ring system, and favor nitrate retention at the platinum complex either by the formation of an adduct or by nitrate participating as a ligand. In contrast, polypyrrole possesses aromatic nitrogen atoms with a considerably more important steric hindrance. In this case, ion exchange between the counterions in the doped polymer (SO42−) and nitrate ion from water is produced, followed by reduction of nitrate by hydrogen chemisorbed on the platinum nanoparticles. Chapter VIII. Surfactant-assisted synthesis of conducting polymers and their application to the removal of nitrates from water This chapter is extracted from the publication: M.J. García-Fernández, S. Sancho-Querol, M.M. Pastor-Blas, A. Sepúlveda-Escribano, “Surfactant-assisted synthesis of conducting polymers. Application to the removal of nitrates from water”, Journal of Colloid and Interface Science. 2017, 494, 98-106. Three different conducting polymers, polythiophene (PT), polypyrrole (PPy) and polyaniline (PANI) have been synthesized via oxidative chemical polymerization in aqueous media, in such a way that the synthesis protocol did not involve any toxic solvents. They have been tested in the abatement of nitrates from an aqueous solution without the need of any metal catalyst. The N-containing polymers (PANI and PPy) were able to remove nitrates to a level that accomplishes the European legislation requirements; however, the nature of each polymer greatly influenced the process mechanism. Whereas ion exchange between Cl− and SO42− counterions in the polymer and NO3− from water is the main responsible for the effective nitrate removal in PANI, as assessed by FTIR and XPS analyses, the nitrate removal mechanism on PPy is based in an electron transfer from the polymer to nitrate through N sites located in the pyrrolic ring. On the other hand, PT was not able to exchange nitrate unless it was synthesized with FeCl3 as oxidant/dopant and an anionic surfactant (sodium dodecyl sulfate -SDS-) is used. In that case, the electrostatic attraction between sulfate (OSO3−) groups from the surfactant and Fe3+ ions from FeCl3 produced the anchoring of Cl− to the oxidized PT growing chain, this favoring ion exchange with nitrate in the aqueous solution, followed by a redox process. Chapter IX. Synthesis of conducting polymer-TiO2 hybrid materials for their application in the removal of nitrate from water This chapter is extracted from the publication: J.J. Villora-Picó, V. Belda-Alcázar, M.J. García-Fernández, E. Serrano, A. Sepúlveda-Escribano, M.M. Pastor-Blas, “Conducting polymer-TiO2 hybrid materials: application in the removal of nitrates from water”, Langmuir. 2019, 35, 6089-6105. Materials able to produce the reduction of nitrate from water without the need of a metal catalyst and avoiding the use of gaseous hydrogen have been developed by combining the synergistic properties of titania and two conducting polymers. Polymerization of aniline and pyrrole on titanium dioxide in the presence of two different oxidants/dopants (iron trichloride or potassium peroxydisulfate) has been evaluated. The resulting hybrid materials have good thermal stability imparted by the titania counterpart, and a considerable conductivity provided by the conducting polymers. The capability of the hybrid materials of reducing aqueous nitrate has been assessed and compared to the catalytic hydrogenation of nitrate using a platinum catalyst supported on these hybrid materials. The mechanism of nitrate abatement implies adsorption of nitrate on the polymer by ion-exchange with the dopant anion, followed by the reduction of nitrate. The electron transfer from titania to the conducting polymer in the hybrid material favors the reductive ability of the polymer, in such a way that nitrate is selectively reduced with a very low production of undesirable side products. The obtained results show that the activity and selectivity of the catalytic reduction of nitrate with dihydrogen in the presence of a platinum catalyst supported on the hybrid materials is considerably lower than those of the metal-free nanocomposites.