Green Synthesis and Catalytic Activity Assessment of Bespoke Nano-Catalyst for Eco-Friendly Green Propellant Systems based on Hydrogen Peroxide

Much effort has been devoted to replace pollutant, toxic, cancerogenic hydrazine-based propellants. Hydrogen peroxide could offer promising characteristics as well as high density speci�c impulse. Effective catalysts with high durability are required to initiate H 2 O 2 decompositions, or to inherit hypergolic nature with different fuels. This study reports on the novel green synthesis of silver nanoparticles (18 nm) via hydrogen evolved by water electrolysis. MnO 2 (20 nm) was developed in a sustainable manner via green hydrothermal processing. High crystalline, mono-dispersed particles were developed. Catalytic activity was assessed via precise measurements of liquid temperature pro�le (LTP) to reach the boiling point of hydrogen peroxide. Whereas silver demonstrated LTP peak at 20 seconds; MnO 2 experienced LTP of 40 seconds. Catalyst survivability was recorded via precise measurement of life time mass loss rate upon catalyst addition to hydrogen peroxide. While silver nanocatalyst demonstrated high performance at the reaction start; silver was found to be poisoned with crystalline phase change to silver oxide within 20 seconds. On the other hand, manganese oxide experienced high durable catalytic action. Consequently, MnO 2 could be candidate for H 2 O 2 monopropellant thrusters as catalyst bed. On the other hand, silver nanoparticles could be candidate for a single use in bipropellant to inherit hypergolicity.

Alternative green propellant with high speci c impulse is highly appreciated for space propulsion. Ionic liquids and hydrogen peroxide are gained a great attention as green alternative to hydrazine [5].
Compared with hydrazine, ionic liquids exhibit higher volumetric and gravimetric speci c impulse along with safe handling and easy storage [4,6]. However, such propellants face many challenges [7].
Production technology of ionic liquid is extremely expensive, high ignition delay, high combustion temperature could cause thermal failure to the catalyst [8]. On the other hand, hydrogen peroxide could be the best alternative to hydrazine. Hydrogen peroxide can be thermally or catalytically decomposed into steam and oxygen with the evolution of 191.2 KJ/mol [9]. Highly concentrated hydrogen peroxide can serve as monopropellant. Additionally, it can act as oxidizer in bipropellant systems [10]. Hydrogen peroxide is very reactive; number of stabilizers must be added to enhance its storability [8]. Hydrogen peroxide experiences higher density than hydrazine. Additionally, it saves many expenses due to safe handling and transportation. Furthermore, the catalyst bed doesn't require any further preheating [11]. Hydrogen peroxide 90% (232 s) shows similar density speci c impulse as hydrazine (230 s) [12,13]. In green bipropellant systems, hydrogen peroxide can be used as oxidizers with hydrocarbon fuels [14].
Hydrogen peroxide lack of hypergolicity with hydrocarbon fuels.
Catalyst particles play a fundamental rule in hydrogen peroxide decomposition. Catalyst bed can initiate hydrogen peroxide decomposition in mono-propellant systems, and can inherit hypergolicity nature in bi-propellant systems. Heterogeneous catalysts including Nobel metals like silver (Ag), platinum (Pt), cobalt(Co) and transition metal oxides such as (CuO,MnO 2 , Mn 2 O 3 ) are the most common catalyst for space propulsion applications [3]. Catalyst should secure reasonable price cost, high activity, and long-life time. Silver and manganese oxide are the universal catalysts for hydrogen peroxide. Silver is one of the most widely heterogenous catalysts for H 2 O 2 decomposition due to its high activity and facile synthesis.
Silver melting point (962°С) is very close to decomposition temperature of 90% H 2 O 2 thus silver demonstrates weight loss through operation cycles [13]. On the other hand, manganese oxide (MnO 2 ) experiences a good activity towards H 2 O 2 decomposition but at high temperature it may be change to another oxidation state, that have lower activity than MnO 2 [15]. Currently, nanoparticles have a great attention in catalysis application due to high interfacial surface area [16]. Metallic silver nanoparticles can be easy fabricated via conventional chemical reduction methods; silver reduction methods depend on highly toxic chemical agents [17]. On the other hand, classical batch synthesis techniques for metal oxides are time-consuming; they cannot secure high quality mono-crystalline particles with controlled This study reports on the green synthesis of silver nanoparticles via water-electrolysis. Water molecules were split into oxygen and hydrogen via Hoffman voltameter. The evolved green hydrogen was adopted for reduction of silver with the evolution of colloidal silver nanoparticles. On the other hand, manganese oxide was developed via green hydrothermal technology in a sustainable manner using super-critical water. The developed colloidal nano-catalysts were characterized via TEM, XRD, and SEM. Comparative catalytic activity assessment for hydrogen peroxide was conducted. While silver was found to be more effective; its lifetime was limited to 20 seconds due to catalyst poisoning. By contrast, manganese oxide was found to be more durable catalyst.
Hydrogen peroxide (H 2 O 2 , 30 v %) (CAS: 7722-84-1) was employed for manganese oxide synthesis. All chemicals were purchased from Merck. All chemicals were used without any further puri cation.

Green of silver nanoparticles
Silver nanoparticles were developed via green batch synthetic method, where the conventional toxic reducing agents were replaced by elemental hydrogen gas. At rst 1mM of silver nitrate solution (500 ml H 2 O) was developed. Subsequently, capping agent was added to the solution. Hydrogen gas was generated from water Using Hofmann voltameter at rate 4ml/min. The generated hydrogen gas was bubbled in silver nitrate solution (4 ml/min) with continuous stirring and heating at 70 0 C [19]. After about 30 minutes, the color of solution changed to pale yellow then it becomes darker ( Figure 1).
Samples of colloidal silver particles were taken for UV-vis every 30 minutes to record concentration changes.

Green synthesis of manganese oxide nanoparticles
Manganese oxide nanoparticles are synthesized by hydrothermal technique. manganese acetate (0.5M) act as metal salt is introduced inside the reactor at 10 ml/min. on the other hand, supercritical water at 400C and 240 bar act as supercritical uid is ow in the reactor at 20 ml/min. inside the reactor, nanoparticles is produced at the boundary of two uids. Further details about hydrothermal processing can be found in the following reference [20].

Nano-catalyst characterization
UV-Vis spectroscopy was conducted for colloidal silver characterization using Shimadzu-1650 UV with range 300:800 nm with interval 0.5 nm using deionized water was taken as reference. Moreover, Scan Electron Microscope (SEM) (Zeiss EVO-10, Carl Zeiss Corporation) and Transmission Electron Microscope (TEM) (JEM-HR-2100, Joel Corporation) were used to study the morphology, size, and shape of manufactured Ag and MnO 2 nanoparticles. Energy Dispersive X-ray was used to analyze the nanoparticles composition before and after utilization of catalyst in H 2 O 2 decomposition. The crystalline phase of manganese oxide and silver was examined using XRD D8 advance by Bruker Corporation with a scanning rate of 5 0 C per minute.

Catalytic activity assessment
Analytical hydrogen peroxide (30% V/V) was concentrated to 85% using Rotary Evaporator (Buchi R-200, Switzerland) [21]. Catalysts' reactivity was evaluated via liquid temperature measurement upon catalyst addition. 0.3 g of nano-catalyst were added to 30 g of 85% H 2 O 2 in rounded bottom ask supported with thermocouple K-type. Temperature measurement was performed using WIKA TC40 1X K-type thermocouples with 2 mm diameter. Thermocouples data was collected and carried to a laptop using a National Instruments acquisition board number NI 9171 to record temperature via lab-view and National instruments ( Fig. 2-a).
For catalytic activity durability, a constant atmospheric pressure batch reactor with assessment criteria based on a Life time mass loss (LTML) technique [12]. Precisely weighed amount of 85% H 2 O 2 (30 g) was put in conical ask. The conical ask with H 2 O 2 was kept on analytical balance. Prescribed amount of nano-catalyst (0.3 g) was added to H 2 O 2 . The weight loss with time, due to the evolution of exothermic decomposition products, was recorded till complete decomposition of H 2 O 2 ( Fig. 2-b).
Additionally, to assess the catalysts' survivability and repeatability, 0.2 g of nano-catalyst was added to 10 g of 85% H 2 O 2 using Lifetime mass loss (LTML) (Fig. 2-b). The later experiment was repeated 5 runs for the same catalyst sample to evaluate catalyst lifetime. The catalyst was cooled down in a clean container before inserting it in the next run. The experiment for certain sample catalyst was ended if any of the following criteria matched: catalyst failed to decompose about 10% of H 2 O 2 or when it lasted for more than 10 minutes [12,22].

Result And Discussion
3.1 Characterization of silver nanoparticles UV-Vis spectroscopy is a simple quick way to identify metal nanoparticles due to their unique optical property. The interaction between the conduction electrons on the metal nanoparticles' surface with incident light is known as surface Plasmon Resonance phenomena. This interaction relies on the shape and size of nanoparticles as well as the dielectric constant of dispersing media. The peak intensity and position on UV-Vis spectrum gives rapid indication about size, shape, and concentration of metal nanoparticles. UV-Vis spectrum of developed colloidal silver nanoparticles experienced Plasmon absorbance band peak around 400 nm (Fig. 3).
It is obvious that UV-Vis absorbance intensity at 400 nm increases with hydrogen gas bubbling time.
There's no red or blue shift due to surface plasmon resonance band. It can be concluded that bubbling H 2 gas in silver nitrate solution was accompanied with increase of silver concentration. Time of hydrogen gas bubbling did not affect the particle size and shape. TEM micrographs of synthesized colloidal silver elucidate the evolution of silver nanoparticles of 18 nm (Fig. 4). respectively. Silver XRD pattern was found to be in good agreement with standard face centered cube structured silver XRD diffraction card (JCPDS le No. 04-0783) (Fig. 5) [23].
SEM micrographs of developed as-prepared silver demonstrated spherical in shape. Moreover, clusters of silver nanoparticles are formed over the drying process (Fig. 6).
Elemental mapping via Energy dispersive X-ray (EDX) analysis revealed a sharp peak around 3 Kev that typical of elemental silver [24]. Uniform dispersion of silver with no interfering foreign substances was assessed via EDAX analysis (Fig. 7) [25].

Characterization of MnO 2 nanoparticles
TEM micrographs of colloidal MnO 2 particles revealed high quality mono dispersed MnO 2 particles of 20 nm (Fig. 8). are clearly existed which may be formed during centrifuge and drying process (Fig. 10).
Elemental mapping via EDAX analysis demonstrated uniform distribution of (Mn, O) elements without any foreign component. Thus, MnO 2 nanoparticles prepared by hydrothermal technique are highly pure and highly crystalline (Fig. 11).

Catalyst activity assessment
Generally, reaction rate constant (K) is considered as one of the most important indicators about catalyst performance. Reaction rate constant is highly dependent on the reaction temperature; concentrated hydrogen peroxide decomposition occurred over a wide range of temperature. It is too di cult to keep temperature constant due to liberating a large amount of energy. Hydrogen peroxide liquid temperature was employed as catalyst activity indicator, based required time to reach the maximum liquid temperature [12].

Liquid temperature pro le
An innovative activity parameter is introduced by Rusek based on the time needed to reach the maximum temperature of the liquid H 2 O 2 [28]. As the reaction proceeds, the liberating energy causes temperature of liquid to increase rapidly. Additionally, the mass of H 2 O 2 decreases due to formation of decomposition gases products. More H 2 O 2 mass loss indicates higher catalyst activity. It's impossible for temperature to exceed the boiling point of liquid H 2 O 2 (85%) at 140 0 C. Catalytic activity assessment was conducted via liquid temperature measurements. Silver nano catalyst experienced maximum liquid temperature within 20 seconds (t max =20 sec); Manganese oxide experienced maximum liquid temperature within 40 seconds (t max =40 sec) (Fig. 12).
It can be concluded that silver could expose high catalytic activity at the reaction start. Silver could be poisoned or oxidized within 20 seconds during hydrogen peroxide decomposition. Therefore, liquid temperature decreased after 20 seconds. At the reaction start, silver activity was much higher than that of MnO 2 but this activity rapidly deteriorated.

Mass loss
Catalytic activity was further assessed via mass loss of hydrogen peroxide upon catalyst addition. Silver nano catalyst demonstrated high mass loss rate for 20 seconds; laterally MnO 2 experienced higher mass loss rate (Fig. 13).
Silver nano catalyst could have higher activity for the rst 20 seconds; subsequently silver could be oxidized, or the catalyst active sites may be poisoned with stabilizers existing in H 2 O 2 . These ndings were found to be in good agreement with liquid temperature measurements.

Catalyst Durability
In rocket propulsion, survivability, and repeatability of catalyst over operational range must be taken in our consideration. To assess catalyst durability lifetime mass loss (LTML) technique was employed several times on the same amount of catalyst. In this test, 0.2 g catalyst was employed several times. Each time fresh hydrogen peroxide (85%. wt,10 g) was added to the same catalyst from previous run. The time required to 90% mass loss was measured for each run. Generally, the activity of the catalyst deteriorates with the number of runs. The required time to accomplish 90% mass loss was recorded for each run (Fig. 14).
Manganese oxide experienced less time to achieve 90% mass loss compared with silver nano-catalyst. In general, manganese oxide experienced higher durability than silver nano-catalyst. However, silver experienced signi cant improvement of catalyst reactivity after run 2. This enhancement was related to that silver catalyst needs to be activated associated with roughen the surface of catalyst as reaction proceed [8,29]. MnO 2 experienced extraordinary behavior after run 1. Enhanced catalytic activity and faster reaction to 90% mass loss was accomplished after run 1. This behavior may be related to propelling catalyst nanoparticles by decomposition gases which reduce the contact surface area. Studying each run separately is not a good indicator of catalyst performance. Consequently, the total amount of H 2 O 2 decomposed all over the experiment is used as catalyst assessment indicator. LTML showed that MnO2 decomposes more H 2 O 2 in less time than silver. Time required to reach the maximum liquid temperature all over the runs was employed to indicate the catalytic activity and survivability [22]. Silver nanoparticles activity signi cantly deteriorates but MnO 2 maintain its performance. Also, silver nanoparticles showed higher reactivity in the rst run (Fig. 15).

Catalyst Poisoning
Stabilizers were adopted during H 2 O 2 manufacture to enhance its storage and prevent selfdecomposition [30]. The catalyst active sites are highly affected by these stabilizers. It's clearly that silver catalyst is highly affected by these stabilizers much larger than MnO 2 due to large deterioration in its activity. Consequently, silver Catalyst is highly susceptible to poisoning. Elemental mapping of exploited silver catalyst via EDAX detector, demonstrated the presence of oxygen, phosphorus, chlorine, and sulfur on the catalyst surface (Fig. 16).
Elemtnal quanti cation analysis spectra for silver nanoparticles revealed a sharp peak related to the presence of phosphorus, sulfur, chlorine along with oxygen (Fig. 17). These foreign elements could withstand silver poisoning via stabilizers in hydrogen peroxide. XRD analysis of poisoned Ag nanoparticles revealed the formation of silver oxides. Poisoned silver demonstrated seven clear characteristic peaks at 81.9°,77.8°,64.8°,55.1°,44.8°,38.6°,33.1°. These peaks have the similarity with the peaks in standard silver oxide XRD diffraction card ((JCPDS no. 00-001-1041) (Fig. 18)

Conclusion
Hydrogen peroxide is an ideal eco-friendly replacement for hydrazine. Hydrogen peroxide lack of hypergolicity with all types of fuels. Nano catalyst particles (Silver and manganese oxide) are mandatory for green propellants. Highly crystalline silver nanoparticles (18 nm) were developed via water electrolysis. Sustainable continuous hydrothermal process was employed for green synthesis of MnO 2 (20nm) nanoparticles. Reaction rate constant, Lifetime mass loss (LTML) technique, and time to reach maximum temperature provided accurate indication to assess the catalysts from activity and durability. It was reported that MnO 2 nano-catalyst is more durable and survivable along with less susceptibility to poisoning. MnO 2 nano-catalyst is the best candidate to use in H 2 O 2 based monopropellant thruster catalyst beds. On the other hand, silver nanoparticles showed higher activity at the onset of the reaction, but it rapidly deteriorates. consequently, as-prepared silver nanoparticles are the best choice for a single used catalyst that can be imbedded inside gelled hydrocarbons to from hypergolic mixture with H 2 O 2 .      Elemental mapping and Energy dispersive X-ray (EDAX) analysis of silver nanoparticles Elemental mapping micrographs of synthesized MnO 2 nanoparticles Figure 12 Comparative liquid temperature pro le for silver and manganese oxide.  Elemental mapping micrographs of synthesized Ag nanoparticles after decomposition Figure 18 XRD analysis for Ag catalyst after decomposition reaction.