Electrocatalytic hydrogen peroxide formation on mesoporous non-metal nitrogen-doped carbon catalyst☆
Graphical abstract
Nanoporous nitrogen doped carbon (NDC) can be an electrocatalyst for the selective reduction of oxygen to hydrogen peroxide. A strong dependence on the pH is observed with a very effective peroxide generation at neutral pH.
Introduction
Direct synthesis of chemicals from its pure elements or bimolecules under ambient conditions often represents the most atomically-economic reaction route. The understanding of the elemental processes as well as the identification of all relevant experimental and intrinsic parameters and steps for many (electro) catalytic reactions are, however, highly complex and still poorly understood to date. The electrochemical direct conversion of pure hydrogen and oxygen to hydrogen peroxide is such a prominent example. Hydrogen peroxide is among one of the 100 most important chemicals in the world [4], because it is used in a lot of different fields of chemical and chemistry-related industries e.g. for pulp and paper bleaching, wastewater treatment or as “green” detergents and in different chemical syntheses [5]. Today, hydrogen peroxide is industrially produced via the anthraquinone process in large-scale plants [6], [7]. Alternative and “on the place of use” located production methods are also discussed in the literature e.g. membrane, fuel cell or plasma reactors [8], [9], [10].
In general, the reaction of pure hydrogen and oxygen leads to the formation of two main products, namely water and/or hydrogen peroxide. Since the direct conversion of hydrogen and oxygen is strongly exothermic, electrochemical membrane reactors, e.g. fuel cells, are typically employed to separate the two half-cell reactions; that are the hydrogen oxidation reaction [11], [12] on the anode and the oxygen reduction reaction (ORR) on the cathode side [8], [13], [14]. From the perspective of the oxygen electroreduction, three reaction pathways are possible: (a) the direct two-electron process to generate H2O2, (b) the overall four-electron process consisting of H2O2 formation followed by consecutive reductive decomposition of H2O2 to H2O and finally (c) the direct four-electron process for the formation of H2O [15], [16], [17].
In fuel cell research, different cathode electrocatalyst concepts are discussed in terms of activity and stability for the efficient electrochemical conversion of oxygen. These concepts largely involve the utilization of metal nanoparticles dispersed on high surface area carbon support materials. The metal nanoparticles, such as pure platinum or platinum alloys with e.g. Cu, Co, Ni largely are the catalytically active reaction centers for the oxygen electroreduction. Improved ORR activities have been reported for dealloyed core-shell nanoparticle catalysts [18], [19], [20], [21], [22], [23], Pt skin catalysts [24], [25], Pt monolayer catalysts [26], [27] or non-noble metal catalysts, e.g. Fe/Co/N/C or nitrogen-doped carbons [28], [29], [30], [31], [32], [33]. Recently, pure nitrogen-doped carbon catalysts have attracted great attention due to their high performance for the electrochemical O2 reduction toward H2O without usage of costly precious metals [34], [35]. On the other side, only a handful of electrocatalyst concepts for the electrocatalytic hydrogen peroxide production are reported to date. Most of the H2O2 electrocatalysts involve complexes (e.g. N-ligands like porphyrin or chlorin) containing metals like Fe, Pd or Co [36], [37] and supported metal (e.g. Pd, Au-Pd, Pt-Hg, Pd-Hg) nanoparticles [2], [36], [38], [39], [40], [41], [42], [43], [44]. In comparison, typical Pt nanoparticle fuel cell catalysts, polycrystalline Pt and single crystals of Pt show a H2O2 yield of less than 5% in acidic environment [22], [45], [46], [47].
The long-term performance of electrocatalysts for the peroxide formation strongly suffers from the reaction conditions, such as aggressive H2O2-containing electrolyte, high potentials as well as from heat and pressure. For instance, the decomposition of peroxide leads to the release of OH· and OOH· radicals, resulting in an accelerated catalyst aging. A further challenge for the successful design of electrocatalysts is the selectivity toward H2O2 at high yields. In particular, the series reaction of H2O2 toward H2O via two-electron transfer should be largely eliminated.
Recent results of theoretical studies suggest, that hydrogen peroxide only shows physisorption to N-doped graphene and that the presence of protons facilitates O–O bond cleavage, i.e. peroxide reduction [48]. Inspired by investigations on the electron transfer process of mesoporous metal-free nitrogen-doped carbon catalyst using ionic liquid N-butyl-3-methylpyridinium dicyanamide (referred to as meso-BMP) in acidic and electrochemical environments, we have performed a comprehensive study on highly selective and active meso-BMP catalyst to highlight the influence of pH values and nature of electrolyte (acid–HClO4, alkaline–KOH and neutral–KClO4) for the electrochemical formation of H2O2 using the rotating ring disk electrode (RRDE) technique. To support our electrochemical results in terms of activity, selectivity and durability for H2O2 formation, we used additionally an independent photometric method to monitor the H2O2 formation rate over the reaction time at a certain applied potential. We show that the H2O2 formation rate is strongly influenced by the supporting electrolyte, pH values and applied voltage range. The observed difference in the reactivity-selectivity characteristics investigated on the meso-BMP catalyst is largely related to the pH dependent mechanisms and kinetics of the electrochemical H2O2 formation. More importantly, the metal-free catalyst showed considerable stability during the long-term RRDE experiments, highlighting a high resistance of the active catalyst in the presence of preformed peroxide.
Section snippets
Experimental
All chemicals were used as received without further purification.
Metal-free mesoporous nitrogen-doped carbon catalyst
The metal-free mesoporous nitrogen-doped carbon (meso-BMP) catalyst was synthesized by pyrolysis of ionic liquids and contains a relatively high nitrogen content of around 17 wt% obtained from the element analysis. The nitrogen adsorption BET (Brunauer-Emmett-Teller) surface area of the meso-BMP is around 320 m2/g. The total pore volume derived from Non Localized Density Functional Theory (NLDFT) calculation is 0.749 cm3/g, with a mesopore volume of 0.09 cm3/g. It is noted that no metal was used
Conclusions
A metal-free mesoporous nitrogen-doped carbon catalyst showed a high electrocatalytic activity, durability and selectivity toward peroxide by electrochemical converting of O2 in a non-corrosive neutral as well as in acidic reaction medium. For the first time, the effects of the pH on the electrochemical formation of peroxide were demonstrated by using two different independent experimental methods (RRDE and UV–VIS). We showed that the H2O2 selectivity and formation rates strongly depend on the
Acknowledgments
The authors thank Annette Wittebrock for laboratory support (TU Berlin, Germany), Sören Selve for TEM measurement (TU Berlin, Zentraleinrichtung Elektronenmikroskopie, Germany) and Carmen Serra for XPS measurement (Universidad de Vigo, Spain).
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This work was supported by the Technische Universität Berlin, the Max Planck Society and the Cluster of Excellence “Unifying Concepts in Catalysis (UniCat)”.