Preparation, performances and reaction mechanism for the synthesis of H2O2 from H2 and O2 based on palladium membranes
Introduction
Hydrogen peroxide is a commodity which, today, is almost exclusively produced by the anthraquinone process [1]. This process is economically feasible only on large-scale plants. Additionally, the final price of H2O2 produced is deeply influenced by the complexity of the process and by the costly separation and concentration steps needed. Direct synthesis, which could be a more economic and “green” alternative, has been known since the beginning of the XXth century [2] and many patents have been filed even during the last 30 years [3], [4], [5], [6], [7], [8], [9], [10], [11], however only a few scientific papers have been published in the open literature [12].
To date, no industrial application has ever been accomplished, even if some successful attempts have been realized at DuPont during 1980's [3]. This is because two severe drawbacks have to be overcome: (i) the formation of explosive H2/O2 mixtures that should be avoided and (ii) the selectivity of the reaction (whose main product is water) that should be improved. Recently, the use of especially designed catalytic membranes could have been suggested to overcome both problems [10], [11], [12].
Section snippets
Experimental
Two types of tubular membranes have been prepared, loaded with Pd and tested in the direct synthesis of H2O2.
α-Alumina asymmetric membranes (AAM)
A dense Pd film was deposited on such supports by EPD, as stated in the Section 2.
After thermal treatment in inert atmosphere at different temperatures, two kind of surfaces could be obtained: (i) a rough and disordered surface after a thermal treatment in inert gas at 800 °C, as shown in Fig. 2(a) and (ii) a surface formed of well developed crystallites after a thermal treatment at 500 °C (Fig. 2(b)). A notable difference in reactivity was observed for these two different surfaces (Fig. 3):
Conclusions
Tubular catalytic membranes are able to catalyze H2O2 synthesis under mild conditions (2–3 bar H2, 5–25 °C) with a fair productivity (∼2 mmol H2O2 gPd−1 min−1). Regardless of the type of membrane, catalytic activity depends on: (i) the oxidation state of surface Pd atoms (a pre-oxidation step is needed); (ii) the temperature (temperatures as low as 2–5 °C favor H2O2 synthesis over decomposition). In addition, the decomposition of H2O2 on tested catalysts mainly proceeds via its reduction by H2.
Acknowledgements
Financial support from EU (contract No. G5RD-CT2002-00678) is gratefully acknowledged. The authors are indebted to HITK e.V. and Mast Carbon Ltd. for kindly providing the α-alumina tubular membranes and the carbon coating. Finally the authors thank Dr. Ing. Roland Dittmeyer and Mr. Karel Svajda from DECHEMA e.V. for useful discussions.
References (15)
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2015, Catalysis TodayCitation Excerpt :It is important to avoid the contact between concentrated H2 and O2 by working outside the explosive region or by physically separating the gases. One reactor system that is capable of working with high gas concentrations are permeable membrane reactors [70–74]. Typical selectivities reported using an optimised Pd based membrane system can be between 80 and 90% with reaction rates of 16.8 mol h−1 m−2 in a methanol solvent [72].