Development of novel supported iridium nanocatalysts for special catalytic beds
In the present paper, an experimental study of the catalytic decomposition of hydrous hydrazine was investigated on the different structural forms of the catalyst. The synthesized iridium catalysts have been usually used directly and have not been evaluated in the laboratory reactor. This study includes the preparation of iridium-based catalysts supported on spherical (alumina), honeycomb monoliths (cordierite) and foams (alumina) for the evaluation of catalytic activity in the laboratory reactor. The characterizations of these catalysts were evaluated by the TGA, FESEM and BET analysis. The result of the catalytic characterization of monolithic support was shown a homogeneous distribution of active metal without any problem of sintering (average size 25 nm) on the support surface. While the surface of the spherical and foam supports were shown non-uniform distribution of nanoparticles on the support skeleton (average size 55 nm). The monolithic catalyst exhibits higher decomposition rate and H2 selectivity than other supports due to uniform in shape and particle size distribution.
KeywordsIridium nanoparticles Support shape Catalyst activity Laboratory reactor
The catalyst efficiency relates to the active phase and carrier effects and also the operating properties . Depending on the support type of catalyst, the reaction rate of the catalyst differs, although it does not directly participate in the catalytic reaction [2, 3, 4, 5, 6]. In general, an ideal support should have high thermomechanical strength, and high specific surface area in harsh operation . Although the catalyst support plays a key role in enhancing the overall rate of catalytic decomposition, very few comparative studies have been carried out on the efficiency of different catalyst supports in hydrazine decomposition. The reason is that most researchers have done their studies using one form of support rather than using different carriers for a desired reaction [8, 9, 10, 11]. Perhaps, because the commercial catalyst (iridium on gamma-alumina) for hydrazine decomposition has a spherical form [12, 13, 14], less has been paid to various forms of catalyst support in the present papers, but in the species such as hydrogen peroxides, there are some reports about the influence of the shape of catalyst support on the decomposition rate [15, 16, 17].
Some researchers were showed the grain size has a strong influence on the properties, the surface increases when the size of the grains decreases which leads to a higher activity [18, 19]. In other work, An and co-workers showed in hydrogen peroxide bed, the performance was better when using alumina pellets than when using monolith support because of higher bed pressure and temperature . Hence, an appropriate form and structure of the supported iridium catalysts based on the catalytic decomposition of hydrazine or hydrazine borane can be a good alternative to existing catalysts [21, 22]. The direct effect of surface area and structure on the catalytic activity of iridium is quite evident. However, no papers as far as we known have reported the comparative properties of the iridium nanoparticles by introducing iridium ions on the surface of different supports in the area of catalytic decomposition of hydrazine monohydrate.
Iridium catalyst alone is intended for hydrazine decomposition and the addition of nickel to it is not appropriate. Nickel or cobalt catalyst along with iridium is suitable for the hydrogen storage materials. Few researches have been focused on the iridium catalyst for hydrazine decomposition in special forms. One of the major challenges in high pressure is the destruction of supports under high operating temperatures and pressures [23, 24]; therefore, our studies are warranted to complete research about the use of these catalysts for hydrazine decomposition. Since an acceptable catalyst support is required for the purposeful and proper operation, synthetic catalyst optimization is needed.
A suitable decomposition iridium catalyst is reported, which carries on-spherical granules, honeycomb monoliths and foams. The characteristics of the synthesized catalytic were compared to each other in the selective hydrogenation and reaction rate of hydrazine decomposition. Notably, granular, foam, and monolith showed different activity during decomposition of hydrazine monohydrate, mainly because of the various distributions and availability of iridium nanoparticles. Different sizes of nanoparticles formed on different supports affect the rate of hydrazine degradation.
The supports were dried at 100–120 °C in the muffle oven for 12 h to remove moisture. Then, an aqueous solution of dihydrogen hexachloroiridate(IV) hydrate (H2IrCl6·xH2O, Sigma-Aldrich) precursor was ready at room temperature during the overnight.
First, to increase the internal surface area, the as-received monolith (cordierite with 400 cells per square inch) and foam (alumina with 20 pore per inch) were washcoated into the α-alumina, γ-alumina, and polyvinyl alcohol sol with equal weight, and suspended in water for 1 h with careful control of the temperature and viscosity. Up to three washcoat stages can be done to get a given porous washcoat support, and the washcoat layers were dried at 100 °C during 12 h and calcined at 700 °C for 4 h.
The washcoat supports and γ-alumina spherical granules were loaded with iridium nanoparticles by wet impregnation using the H2IrCl6·xH2O solution as the precursor for five times. The catalysts were dried at 80 °C for 24 h and finally calcined at 400 °C for 3 h. The iridium loadings were controlled at 20 wt% of support and denoted as Ir@S, Ir@M, and Ir@F for corresponding catalysts with spherical granular, monolith, and foam supports, respectively. The iridium catalysts were used for decomposition of hydrazine monohydrate (N2H4·H2O, Merck) in laboratory reactor.
Results and discussion
BET properties of catalysts with different supports
Pore volume (cm3/g)
Pore diameter (nm)
The NH3 dissociation with high activation temperature should be adjusted, since the working point moves away from the optimum at high dissociation, thus affecting bed performance. Therefore, decomposition becomes more important when the NH3 mass fraction begins to decrease. With the further decomposition of hydrazine and ammonia, the amount of product gases increases, and this high pressure of H2 prevents further ammonia decomposition .
The Ir@M catalyst shows its activity by heat transfer and mass transfer in 15 min, referring to a turnover frequency (TOF) of 310 h−1. For Ir@F, a low activity is shown with a TOF value of 185 h−1 in 30 min. Ir@S catalyst achieves a λ value of 1 after 23.5 min, with a lower TOF of 212 h−1. The significant drop in reaction rate and hydrogen selectivity of the Ir@F catalyst might be originated from the structural change of support. In this context, morphology with different shapes was developed on the textural properties of cordierite monolithic and foam substrates.
Hydrazine monohydrate decomposition properties of different catalysts
Making a high-strength catalyst can be a quick and easy way to access it for high operating temperatures and pressures. Hence, three types of catalyst with different support shapes (spherical granule, monolith, and pellets) were prepared, characterized and tested in the laboratory reactor for decomposition of hydrazine monohydrate. In all cases, total hydrogen selectivity is reached below 30%, in spite that the NH3 conversion depends on both support and the active phase nature. TOF of the monolithic catalyst (310 h−1) was excellent, due to the following two reasons. First, fast and direct contact of the washcoat layer on all monolithic surfaces could be increased reaction rate; however, it was difficult to penetrate deeply inside the pores of the spherical granule by hydrazine monohydrate. Second, heat and mass transfer were effective in the monolithic catalyst, due to the channel structure. The time profile figure show that the monolithic is generally more desirable catalyst than that of spherical granule which is commonly used.
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