Properties of Mn0.4Zn0.6Fe2O4 and Mn0.6Zn0.4Fe2O4 as Nanocatalyst for Ammonia Production

Ammonia synthesis requires high pressure and high temperature process. Unfortunately, the capital intensive cost resulting low yield of ammonia by using recent catalyst which is iron oxide. Therefore, manganese zinc ferrite as a soft ferrite material will be introduced as a new nanocatalyst to enhance the ammonia yield. As a new nanocatalyst for ammonia production, study of comparasion two different concentration of MnZn Ferrite is very important. This paper will compare the yield of ammonia by using two different nanocatalyst which are Mn0.4Zn0.6Fe2O4 and Mn0.6Zn0.4Fe2O4. Both were synthesized by sol-gel method and has been characterize by using FESEM (morphology), XRD (phase identification), EDX (elemental analysis) and TPR (oxide reduction). The ammonia was produce with and without magnetic field applied. The result shows that the ammonia yield is higher for Mn0.4Zn0.6Fe2O4 nanocatalyst than Mn0.6Zn0.4Fe2O4 by using magnetic field applied. 67.2% of yield has been achieved by using new nanocatalyst Mn0.6Zn0.4Fe2O4 and magnetic field applied at ambient environment.


Introduction
Ammonia is a chemical substance which has been used as an additive in various applications such as explosives, detergent and about 76% from the total production of ammonia has been applied in fertilizer industry. Unfortunately, the current production is only capable to generate 10-20% of ammonia yields with capital and energy intensive. In order to overcome these drawbacks, nanotechnology is seen as an excellent solution. By introducing nanocatalyst which is manganese zinc ferrite (Mn 0.4 Zn 0 .6Fe 2 O 4 and Mn 0.6 Zn 0.4 Fe 2 O 4 ) with the new concept of applied magnetic field, the catalytic activity can be induced and the yield can be enhanced.
MnZn-ferrites are considered as one of the great important soft ferrite ceramic materials. This type of ceramics find extensive applications in electronic and telecommunication industry and catalyst. Spinel ferrites combine interesting soft magnetic properties with rather high electrical resistivities [1]. Because of the chemical composition and crystal structure of Mn-Zn ferrite, this material has a high initial permeability, saturation magnetization, and a relatively low eddy current losses compared with alloy cores.
Various processing techniques including conventional and nonconventional have been developed for the synthesis of ferrites. However, each one of these techniques has its specific limitations. Non-conventional techniques such as co-precipitation, thermal decomposition, sol-gel, and hydrothermal, self-propagating high temperature synthesis (SHS) and other wet chemical techniques were widely used [2][3][4][5]. The main disadvantage concerns economic and environmental. Therefore, the manufacturing of near fully dense manganese zinc ferrites through minimum number of steps with high magnetic properties will be the focus of this research.
The properties of manganese zinc ferrites depend on their microstructure. The grain size and porosity of the sample will affect the strength performance of the material. One of the main problems during conventional sintering of manganese zinc ferrites is the elimination of porosity. It is well known that the domains structure of magnetic materials is affected by crystal structure, saturation magnetization, magnetocrystalline anisotropy and magnetostriction as well as the size and shape of grains, porosity and crystal defects, which are determined by the processing route [6]. This works deals with the synthesis of manganese zinc ferrite (Mn 0.4 Zn 0.6 Fe 2 O 4 and Mn 0.6 Zn 0.4 Fe 2 O 4 ) and the ammonia synthesis by using magnetic induction (1Tesla). Both nanocatalyst will compare which one is the best nanocatalyst for ammonia synthesis due to their mole fraction difference.

Nanocatalyst Preparation
The precursor for Mn 0. 4 3 .9H 2 O with of 65% HNO 3 . As-prepared samples were stirred for 1 week and gradually heating until the gel formed at 70 o C. The sample was then dried in an oven at 110 o C for 24 hours and annealed at 700 o C, 800 o C, 900 o C, 1000 o C, and 1100 o C in inert environment for 4 hours.

Nanocatalyst Characterization
The as-synthesized nanocatalysts were characterized by using X-Ray Diffraction (XRD) with CuKD and O=1.5418Å, Field Emission Scanning Electron Microscopy (FESEM) SUPRA 35VP, Energy Dispersive X-ray Spectroscopy (EDX), and Temperature Program Reduction (TPR). XRD was used for the identification of phases, crystalline and crystallite size determination. The FESEM gives the microstructure evaluation on the sintered powder. While TPR gives the information about reduction temperature and hydrogen consumption of the samples.

Phase Identification
Phase identification and lattice parameter were observed using XRD result. The single phase Mn 0.4 Zn 0.6 Fe 2 O 4 and Mn 0.6 Zn 0.4 Fe 2 O 4 obtained at 1100 o C sintering temperature (Fig 1 and  Fig 2). Intensity of major peak revealed the FWHM, d-spacing and 2 theta degree which also explained the crystallite structure in term of lattice parameter as shown at Table 1     The samples of 1-700, 1-800, and 1-900 are not single phase since there are another peak appearing as hematite peak. This is because the annealing temperature is not high enough to arrange the atoms to occupied at their own lattice. Whereas, at 1000 o C the as-synthesized sample shows as single phase with [311] plane. By using the Scherer equation, the crystallite size can be obtain as mention at table I. The crystallite size is getting bigger since the annealing temperature increased. All sample for Mn 0.4 Zn 0.6 Fe 2 O 4 shows the hexagonal structure according to the value of a=b=c. The smallest particle size is for sample 1-700 which is 70.8nm. Figure 2 shows the XRD pattern for Mn 0. 6 Figure 3 shows the morphology of samples 1 with different annealing temperatures. At 700 o C, the average particle size is 43nm and shows the agglomeration particle. At 800 o C the average particle is 52nm. At 900 o C the average particle size is 63nm. At 1000 o C and 1100 o C the samples were not in the range of nanoparticle size since the particle size is more than 100nm. Figure 4 shows the morphology of samples 2 at five different annealing temperature. The particle size for 2-700 is 34nm while 2-800 has average particle size 43nm. The other sample, 2-900 shows the average particle size 150nm, 2-1000 has bigger crystallite size which is 250nm and 2-1100 has average crystallite size 350nm. All nanoparticles show the agglomeration and increasing annealing temperature deal with the increasing of the crystallite size.

Oxide reduction
Temperature Programmed Reduction (TPR) was identify reduction of the oxide catalyst to the metallic state. The highest flow rate has been used in this process at 20 ccm/min.  Figure 5 shows the reduction profile for sample 1 and sample 2. It shows that sample 1 needs higher hydrogen consume to reduce the oxide to metallic state. Due to the XRD and FESEM result, sample 2 has smaller particle size (37.5nm) compared to sample 1 (70.5nm). Therefore, the percentage hydrogen consumption to reduce is higher for sample 2. Table 3 provides the information for percentage hydrogen and phase behavior at reduction process for samples 1 and 2.   [7]. At this figure, sample 1 shows the middle peak at 559 o C that can be speculated as the reduction temperature for Fe 3 O 4 to FeO. Sample 2 shows at temperature 555 o C the reduction process for Fe 3 O 4 to FeO has been occurred. There has been mentioned that amount of zinc will decrease the hydrogen consumption for reduction process [8]. This can be seen from Figure 5 that zinc contain for sample 1 is higher than sample 2. Thus, the hydrogen consumption for reduction process at sample 1 is lower than sample 2. At 800 o C all the oxide has been reduce completely to the metallic state.

Ammonia formation
The formation of ammonia was detected using Kjeldhal Method. The experiment was taken at ambient pressure under five different temperature which were 28 o C , 68 o C, 108 o C , 148 o C , and 188 o C to measure the highest yield. Figure 6 shows the value of ammonia yield using nanocatalyst Mn 0.4 Zn 0.6 Fe 2 O 4 . The highest yield 67.2% was obtain at room temperature (28 o C). The yield was decreasing as the temperature increasing. This is due to the effect of currie temperature of nanocatalyst below 100 o C [9]. Both nanocatalyst was obtain the highest yield at room temperature and ambient pressure by applied magnetic field. It has been proven by other researcher that by using applied magnetic field, the yield will increase tremendously compared to the reaction with the absence of magnetic field [10]. This work has a similar condition with other researcher who has been obtain the higher yield of ammonia in the presence of magnetic field by using nanocatalyst Mn 0.8 Zn 0.2 Fe 2 O 4 [11]. It was speculated that Mn 0.4 Zn 0.6 Fe 2 O 4 posses better spin waves which enhanced the catalytic reaction. The interaction between homogeneous magnetic field in the vicinity of the active centers and magnetic moments of the gaseous particles. In the case H 2 and N 2 , had elevated the production of ammonia in the ambient pressure and room temperature. It is because the electrons in a good alignment which increase the chemical reaction of the process [12]. The best nanocatalyst is Mn 0.4 Zn 0. 6