Highly concentrated H2O2 preparation, stabilization and decomposition over manganese oxide-based catalysts for space propulsion

Abstract

The European Chemical Agency is planning to ban the use of the currently most used space propellant, hydrazine, due to its high toxicity. As a response, the EU-funded GRASP research consortium nominated high concentration hydrogen peroxide (HP) as a suitable non-toxic replacement. However, to date still several issues impede the implementation of HP as a space propellant, which have been addressed in this research. First of all, high concentration HP is either poorly available or only at high cost. SolvGE has invented a safe and user-friendly method to carry out purification of easily available low concentration to propellantgrade HP. Even though this purification process already has an acceptable efficiency, the waste stream still contains HP. Since the efficiency of the concentration process itself is limited by Raoult’s law, it is only possible to increase the overall process efficiency by recuperation of HP from the effluent flow. The company desired that the proposed solution would not affect the current process, and would add minimal complexity to the system. In addition, the solution should not affect neither the passive nature of the system nor the system’s autonomy. It was found that subsequent cooling and condensation of the waste stream would be the optimum approach to achieve this goal. In order to gain fundamental understanding of this process, a theoretical model was set up that would predict the required condenser properties. Then, experiments were conducted that supplied evidence to validate both the key assumptions in the model and the model itself. Though, it was found that the model would considerably underestimate the required condenser size, which can be attributed to both the relatively low concentration of HP in the waste stream and the relatively low turbulence in the flow, as well as the large uncertainty in the actual flow rate. Ultimately, the research efforts did not culminate into a detailed design solution, yet they did provide SolvGE with further understanding of the problem and a starting point for further development. Secondly, at this time no catalyst is available that can ignite HP - ethanol mixtures and withstand the environment (up to 2000°C and over 20 bar) in a rocket engine for prolonged periods of time. Current research efforts have focused on the synthesis of a HP decomposition catalyst: the hence liberated heat then serves to ignite ethanol. Generally, when manganese oxide (MnOx) catalysts would be operated under these conditions the active phase will rapidly deactivate. Yet, Serra Maia’s group has shown that properly supported MnOx catalysts can be operated in decomposition for twenty-five minutes without noticeable degradation. Though, these catalysts most likely cannot withstand the operation conditions of a HP - ethanol engine. This research utilized drop-testing to investigate reactivity and deactivation behavior of MnOx catalysts on different supports. Drop-testing is a common method to investigate hypergolicity, as opposed to engine testing. It can also be employed to study decomposition behavior, for instance of HP. Even though the resemblance to decomposition chamber conditions is poor, this method allows for a fast screening of catalysts. In addition, it mimics repeated engine starts, and can thus simulate long-term deactivation behavior. In droptesting a drop of HP is released from a set height onto the catalyst, and the subsequent reaction is monitored. Here the reactions were monitored using high speed imaging and a fast thermal data recorder. Ceramic catalysts pellets (cercaps) were prepared by mixing 1.5 wt.% MnO2 with different metal oxides and subsequently firing at high temperature. Of the tested support materials (i.a. silica, titania, kaolin and magnesium spinel), only yttria-stabilized zirconia (YSZ) and γ-alumina (yAl) showed measurable acti