Effect of Silica Content on Support-Iridium Active Phase Interactions on the Nanocatalyst Activity

INTRODUCTION
The synthesis process and applications of nanocatalysts are an integral part of nanotechnology and catalysis science that reported in pervious researches [1–4]. The catalyst carriers play an important role in determining the rate of the catalytic reaction although it does not directly participate in the reaction [5–7]. In general, a suitable support with high specific surface area should have various superior properties such as high physical/chemical stability and high thermal/mechanical strength [8]. There is some controversy in the literature about the role and the influence of the support on the activity of active phase nanoparticles [9]. Among the different types of supports used in heterogeneous catalysis, alumina and aluminosilicate are the good carries due to their superior properties [10,11]. Ultra high temperature ceramics and their composites can be used as a supports with excellent heat resistance in the thruster, but the cost will be very high [12–15]. The most important and cheapest support is gamma-alumina (γ-Al₂O₃) with uniform pore size distribution. This kind of support is object-specific in high porosity and surface area to volume ratio, and also the chemical, thermal and mechanical stability [16,17]. The base line support was the granular activated alumina used in standard Shell-405 catalyst manufacture. But granular alumina containing approximately 6% SiO₂ was produced by Esso Research and Engineering Company [18]. MCM-41 as acidic support with low to moderate Brønsted acidity is suitable material for catalyst preparation. The surface of silica support, unlike alumina, contains hydroxyl groups, which exhibits no Brønsted acidity, but its acidity can be tuned by incorporating Al into the silica framework, thus generating weak to moderately acidic sites. It is possible to adjust the acid content of aluminosilicate by changing the Si/Al ratio [19].
Unfortunately, there are no effort data associated with the use and different comparison of silica–alumina as a catalyst carrier for the hydrazine decomposition. Hydrazine (N₂H₄) is a fuel with high energy content and hydrogen-rich product [20]. N₂H₄ is a very active compound, and can be decomposed to N₂ and H₂ on catalysts at room temperature [21]. Various catalyst supports have been used for hydrazine decomposition [14–16]. Among the metal centers active as catalysts for the N₂H₄ decomposition reaction, iridium (Ir) has been preferred for developing new catalytic systems because of previous screening tests performed by the authors on the metal supported that indicated iridium as the most promising catalyst for hydrazine decomposition [25–27]. 20–40 wt% iridium active phase is used as catalyst for N₂H₄ decomposition. Iridium nanoparticles exhibit low H₂ selectivity [28]. Therefore, the development of efficient and selective catalysts with special supports for gas generation with high temperature production from hydrazine is critical importance in monopropellant thrusters.
In the present paper, some catalysts with different silica–alumina composition were employed to study the characteristics of N₂H₄ as well as its decomposition gases and intermediate products on Ir based catalysts with same active phase contents (20 wt%) and different supports (alumina with different silica content) to elucidate the changes in surface structure and the hydrazine decomposition reaction. We found a drastic change in iridium dispersion by changing the support type. Because varieties of iridium catalysts with different Ir dispersions could be prepared by controlling the silica content alone, dispersion effects on the catalytic activity during decomposition were readily identified using catalysts prepared through the same procedure, eliminating the effects of preparation variables.

MATERIALS AND METHODS
Catalyst preparation by porous impregnation
Hydrazine monohydrate (N₂H₄·H₂O, Merck), dihydrogen hexachloroiridate(IV) hydrate (H₂IrCl₆.xH₂O, Sigma-Aldrich), Hydrochloric acid fuming (HCl, Merck, 37%), gamma alumina and alumina/silica (SBET=187-200 m2g-1, Homemade products), and aluminosilicate (MCM-41, SiO₂ (97 wt%): Al₂O₃ (3 wt%), SBET=920 m2g-1, Homemade product) were all used as received without further purification. Characterizations of the supports are summarized in Table 1.
The support materials were dried at 120^oC for 12 h in the muffle oven flowing to remove moisture. One way of preparing supported iridium catalysts is the wetness aqueous impregnation method. First, an aqueous solution of H₂IrCl₆.xH₂O precursor was prepared at room temperature and remained overnight. In this procedure, about 1 g of catalyst support (gamma-alumina and aluminosilicate) was impregnated with the solution of hexachloroiridic acid at the appropriate concentration to obtain the desired percentage of metal, and allowed to stand with occasional stirring at 60oC. In addition, HCl was added in the hexachloroiridate solution to decrease the pH. An oxide surface contains terminal hydroxyl groups that protonate or deprotonate depending of the acidity of the impregnating solution. Cations are absorbed at high pH on alumina, and anions are absorbed at low pH. The impregnation was followed by drying in air at 80oC for 12 h, and then calcined at 400oC for 3 h at a rate of 2^oC/min. For high loaded iridium catalysts, the whole procedure was repeated three times to obtain the successive loading levels. Prior to the catalytic reaction, the catalysts were reduced with pure H2 at 400oC for 2 h. The starting weight ratio iridium/support was maintained constant for all preparations and corresponds to iridium load (related to the mass of the carrier) of 20% (w/w). The final samples were denoted as Ir-XAlYSi, which are listed in Table 1.

Characterization of the catalyst samples
The specific surface area of the carriers and catalysts was evaluated by means of Brunauer–Emmett–Teller (BET) analysis. The N₂ adsorption–desorption isotherms were recorded on a BELSORP Mini, Microtrac Bel Corp. The catalysts were heated to remove physically bonded impurities from the surface at 150°C in inert gas for 5 h. TPR data were obtained in a flow system. The catalyst was placed in a TPR reactor and pretreated in Ar at 300^oC for 1 h. When the reactor was cooled to room temperature again, an H₂–Ar mixed gas was introduced. The temperature was raised at a constant rate, and the amount of H₂ uptake during the reduction was measured by a TCD. X-ray Diffraction (XRD) studies were carried out in a PANalytical X’Pert diffractometer equipped with a Co Ka radiation. A field emission electron microscope coupled to an energy-dispersive spectrometer (FESEM-EDS) model MIRA3 Tescan, FEG was used to evaluate the morphology and the surface chemical composition of catalyst samples, both before and after the test campaigns. High resolution transmission electron microscopy (HRTEM) analysis was performed with an FEI Tecnai F20 microscope. Thermogravimetric analysis (TGA) was carried out using Linseys, STA 1600 apparatus. Thermal treatments were determined in a temperature range of 25-1000°C at the rate of 10°C min−1. Catalysts resistance to mechanical compression was verified by placing spherical particles on a steel surface. The surface of granular was exposed to increasing pressures by a piston. Six runs were performed for each catalyst, and averaged.

Catalytic experiments
The self-designed system was applied for the decomposition of hydrazine monohydrate, with a sealed stainless steel reactor, a gas outlet, a safety valve, and an injector syringe. First, 0.1 g of granules was placed in the reactor. The decomposition was initiated by introducing 10 mL N₂H₄·H₂O into the reactor. The N₂ and H₂ gaseous product was passed through a trap containing 1 M HCl to the absorption of ammonia, and volume of gases measured via an electronic balance by a gravimetric water-displacement method, and recorded by data acquisition software. The gas volume was used to calculate the number of gas moles from the molar ratio in Eq. (1):

The selectivity of H2 generation (X) was calculated following Eq. (2).

The turnover frequency value (TOF) or the reaction rate (h-1) was calculated in the 80% conversion of N₂H₄·H₂O as follows:

where, P is the atmospheric pressure, V is the volume of generating gases (H₂ and N₂), R is the universal gas constant, T is the reaction temperature, n_metal is the mole number of iridium.

RESULTS AND DISCUSSION
Four different supporting materials were taken into account: alumina and alumina-silica with different molar ratios (97:3, 3:97, 50:50). The choice of the catalyst supports was driven by the aim of validating the effectiveness of the proposed iridium deposition method on supports with the same values of specific surface area and, at the same loading of the different carriers. Table 1 shows the physical properties of the supports used in the present study. Spheres with a diameter of 2 mm were used as catalyst supports.
Also, Table 1 presents H2-TPR data of the iridium catalysts. For Ir-100Al catalyst, the hydrogen consumption peak was centered at 234oC, which represented the reduction of IrO2 to metallic iridium. For Ir-97Al3Si catalyst, the main hydrogen consumption peak was one single peak around 354oC, much higher than that for Ir/γ-Al₂O₃ catalyst, indicating that the addition of SiO₂ significantly promoted reduction of Ir. The reason for this promotion was larger clusters of iridium. Therefore, this effect leads to the simultaneous reduction of Ir at higher temperatures. After calcination of Ir-3Al97Si at 400^oC, hydrogen consumption maxima appeared at 227oC. TPR-H₂ profile of Ir-50Al50Si shows a small peak assigned to the reduction of iridium strongly bonded with the support surface at approximately 215oC and the peak in the range 331^oC, which testify the presence of different IrOx species. The metal–support interactions are indicated by the presence of larger clusters of iridium supported on Ir-97Al3Si and Ir-50Al50Si.

Surface area analysis
The BET surface area and pore volume before and after catalyst coating in four different supports are summarized in Table 2. The nitrogen adsorption–desorption isotherms of mesoporous alumina and aluminosilicate supports with the same loading amounts all yield type IV curves with H1-shaped hysteresis loops (Fig. 1a), suggesting their uniform cylindrical pores. It exhibits a Brunauer-Emmett-Teller surface area of 187 m²/g for 100Al, 198 m²/g for 97Al3Si, and 200 m²/g for 50Al50Si. But, the support made from 97 wt% silica will have a total surface area of approximately 920 m²/g. On a weight basis, the addition of Ir reduces the total surface area, measured by N₂ adsorption. Such behaviour can be explained by considering the blockage of support pores with iridium nanoparticles during impregnation [29]. Barrett-Joyner-Halenda (BJH) calculations derived from the desorption branches reveal a narrow pore size distribution centered at 6-12 nm, (Fig. 1b).
The large surface areas, and narrow pore size distributions combined with well-dispersed iridium nanoparticles enhance the hydrazine decomposition of these ordered mesoporous gamma-alumina. Also, mechanical resistances of the aluminosilicate supports are less when compared to gamma-alumina support (Table 2). An increasing amount of porosity decreases the mechanical resistance of the catalyst, while the sufficient strength of the support is subjected to extremely rough conditions [25, 26]. The lower mechanical resistance of the aluminosilicate supports (compared to alumina support) is attributed to the type of interaction and porosity.

Thermal gravimetric analysis
The TGA curves of the catalysts in the air atmosphere reveals that the total weight losses given are: 8.48 wt % (Ir-100Al), 0.5 wt% (Ir-97Al3Si), 4.2 wt % (Ir-3Al97Si), and 8.31 wt % (Ir-50Al50Si), see Fig. 2. However, the TGA curves for total samples showed the same trend of decrease mass loss as the temperature approached 1000oC. In each catalyst, the weight loss occurs from room temperature to about 300oC. In this range, the evaporation of the solvents and the burning of residual organic molecules usually occur. The evaporation of water in open pores in the samples and the combustion and decomposition of the organic compounds are continued. Since the catalyst was calcined at 400oC, the peaks above this temperature are partly due to the dehydroxylation of the support OH groups.[32]. Then a small increase in mass of about 400–600oC occurs, that can be assigned to slowly oxidizing iridium metal, followed by a rapid mass loss [33]. However, the weight changes of all catalysts are generally low at this temperature range.

X-Ray diffraction analysis
The identification of the crystallizing phases has been assessed by X-ray diffraction analysis. The analysis of the XRD profiles allows the acquisition of information about the mean size of the crystallites. Fig. 3 shows the patterns of the Ir catalyst as function of support compositions. The XRD patterns showed three major peaks at ~40.5°, 47° and 69°, which correspond to three crystal faces of iridium: (1 1 1), (2 0 0), and (2 2 0), respectively, according to the XRD standard card (JCPDS Card No. 06−0598). These peak positions were assigned to Ir metal phase in Ir-100Al, Ir-97Al3Si, and Ir-3Al97Si. The sharper diffraction peak at 2θ=40.5° corresponds to the Ir (1 1 1) plane [34]. XRD profile of Ir-50Al50Si catalyst showed the peaks of IrO2 with a JCPDS (No. 15-870) standard pattern. No diffraction peak was observed for iridium metal in the XRD profile of Ir-50Al50Si catalyst. In the XRD pattern of Ir-50Al50Si, the prominent peaks of IrO2 appeared at 2θ ~27°, 34°, and 53°, and this pattern was also obtained with the significant peaks at 2θ ~45° and 65° which were indexed as γ-Al₂O3. Reflections of Ir-50Al50Si catalyst are indexed as (1 1 0), (1 0 1), (2 0 0) and (2 1 1). The sharper diffraction peak centered at 2θ=34° in the XRD spectrum can be attributed to the structure (1 0 1) of IrO2 [35]. The spectra clearly indicate the oxidation of iridium to iridium oxide in Ir-50Al50Si catalyst after reduction in hydrogen, because of physical mixture of the components. It was noted that adding the silica in alumina resulted in a decrease in the average crystallite size, except for Ir-50Al50Si. The mean iridium particle sizes were determined from the line broadening of the diffraction lines, using the Scherrer equation, which ranged from 19 nm (Ir-100Al), 16 nm (Ir-97Al3Si), 16 nm (Ir-3Al97Si) to 28 nm (Ir-50Al50Si). Notably, the aluminosilicate support perturbs the crystal growth of Ir nanocrystallites, which would affect the catalytic activity. It seemed that smaller particle size was not favorable for obtaining high activity in decomposition of hydrazine hydrate. Hence, the metal particle size of supported iridium catalysts is an important factor affecting catalytic behavior.

High-resolution transmission electron microscopy
The HRTEM images in Fig. 4 clearly showed the dispersion of iridium nanoparticles with an average size of around 15-20 nm, which is in accordance with the XRD result. It is found a good agreement between the results from XRD analysis and TEM analysis and hence, the results from TEM analysis are shown the smaller particle size of Ir-97Al3Si and Ir-3Al97Si catalysts (~15-16 nm), and Ir-100Al catalyst has bigger particle size (~20 nm) than both these catalysts. The particle shape seemed to be spherical, but also smaller size of iridium crystallite exists on Ir/Aluminosilicate than on Ir/γ-Al₂O₃. There exists a strong interaction between iridium and γ-Al2O3 in Ir-100Al, which leads to a better anchoring of iridium on γ-Al₂O₃ than on SiO₂. Thus,  γ-Al₂O₃ seems to be a good support for dispersing iridium with high loading [36]. Most notably the dispersion of the active phase decreased in aluminosilicate compared to alumina. These changes could be connected with the observed data in catalyst activity.

Scanning electron microscopy analysis
The surfaces of the prepared samples were investigated by FESEM. Fig. 5a presents the spherical morphology of iridium on  γ-Al₂O₃ granules. For Ir-97Al3Si, Ir-3Al97Si, and Ir-50Al50Si catalysts, metal particles were observed of different shapes and sizes and were not uniformly distributed on the alumina surface (Fig. 5b, c and d). In contrast to the aluminosilicate catalyst (Fig. 5c and d), Ir-100Al show the homogenous dispersion of iridium nanoparticles over the surface of  γ-Al₂O₃ support due to the strong interaction iridium and alumina, which was later manifested by hydrazine catalysis over this optimal catalyst. Here, high metal dispersion of nanoparticles in alumina surface corresponded to the better catalytic activity of iridium particles [37].

Catalytic activity evaluation
The catalytic properties of Ir/ γ-Al₂O₃ catalyst for the decomposition reactions of N₂H₄·H₂O were examined and compared with the corresponding Ir/aluminosilicate catalysts. It has been reported that the catalytic performance was quite sensitive to the carriers [31–33]. N2H4 molecules can be adsorbed on Ir surface accompanied with decomposition through intramolecular or intermolecular reaction. Hydrazine bonds via both N atoms with the N-N axis paralleling to the iridium surface. N-N bond cleavage will facilitate ammonia formation.
Hydrazine molecules through exothermic reactions decompose into NH₃, H₂, and N₂ on the iridium catalyst. NH₃ and N₂ are the main gas products at low temperatures, and the H₂ selectivity is low [41]. Ammonia is not only a product but also an intermediate. Iridium nanoparticle is an active center for intermediate ammonia decomposition. The reaction can be described in two steps [42]:

Decomposition of ammonia is endothermic, and the temperature decreased after this decomposition [43]. Thereby, it is a great challenge of developing efficient catalysts for the selective decomposition of hydrazine to H₂ production. The NH₃ dissociation should be adjusted, since the working point moves away from the optimum at high dissociation, thus affecting thruster performance. Once the reaction is catalyzed, hydrazine starts decomposing, and inevitably, is accompanied by increasing ammonia, which is along with ammonia decomposition. The decomposition of NH3 is slow due to the high activation temperature of this reaction. Hence, when mass fraction of ammonia starts decreasing, decomposition is more important. Hydrogen and nitrogen gases will increase with hydrazine and ammonia decomposition, and this high pressure of hydrogen causes to inhibit the rate of ammonia decomposition [44]. Iridium active phase supported on four different supports –alumina and aluminosilicate- had the ability to convert hydrazine completely into products at 25^oC. With increasing silica content, the activity decreased. As the active phase and its composition are the same for all studied catalysts, the variations are due to the action of the support such as effect on dispersion of the active phase. As shown in Fig. 6a, Ir-50Al50Si catalyst was very inactive, and other Ir/aluminosilicate catalyst (Ir-97Al3Si and Ir-3Al97Si) exhibited low catalytic activity, and moderate H2 selectivity in catalytic decomposition of N₂H₄·H₂O. However, Ir/ γ-Al₂O₃, the resulting showed drastically enhanced catalytic performance, it can be suggested that the  γ-Al₂O₃ catalyst similar to Shell 405 more efficient in the decomposition of hydrazine. The  γ-Al₂O₃-supported iridium catalyst affording mostly decomposition products, it can be inferred to be have higher dispersion due to stronger iridium-alumina interaction. These findings emphasise the interaction between iridium atoms, and also iridium and Al2O3 on the high-loading Ir/γ-Al2O3 catalysts [36]. Hydrazine molecules can be adsorbed on iridium nanoparticles in different conformations, then decomposition occurs through intramolecular or intermolecular reaction, depending upon the nature and structure of metal catalysts [45]. Specifically, N₂H₄ bonds to the surface via both N atoms with the N-N axis paralleling to the surface (cis conformation), rendering the N-N bond more perturbed and more favorable to be cleaved. N-N bond cleavage will facilitate NH₃ formation (low H₂ selectivity) via a lower-energy intermolecular reaction path [46]. Interestingly, NH3 conversions on aluminosilicate catalysts are much lower than those on alumina counterparts.
Fig. 6b plots the hydrogen selectivity and the turnover frequency (TOF) of catalysts versus the support types. It can be seen these that the TOF of the iridium catalysts dropped with the increase in silica content of support. This plays an important role on release of gas product. The H₂ selectivity over Ir/ γ-Al₂O₃ catalysts was decreased from 42% to over 25% and 17% after addition of 3 and 97 wt% SiO₂ into the support (Ir-97Al3Si and Ir-3Al97Si), respectively. Ir-50Al50Si catalyst did not exhibit any activity in hydrous hydrazine decomposition. Thus, the activity of Ir toward hydrazine decomposition was greatly changed by support, which was beneficial for H2 selectivity. The rate of catalytic reaction per unit mole of iridium for the Ir-100Al catalyst is ~306 h-1, which stands out as the top level of this N2H4·H2O decomposition catalysts. The H2 selectivity was 42% over Ir-100Al, which is surprisingly high because Ir nanoparticle itself exhibited high activity for hydrazine decomposition to produce NH₃ instead of H₂. This implies that  γ-Al₂O₃ exhibits certain ammonia decomposition activity. These results imply that iridium sites are also an active center for intermediate NH₃ decomposition in hydrazine decomposition reaction. Interestingly, the NH3 conversion on Ir/ γ-Al₂O₃ catalyst is much higher than those of Ir/Aluminosilicate counterparts, and iridium nanoparticles are dispersed more homogeneous on the surface of  γ-Al₂O₃. Thus, it is obvious that higher NH₃ conversion on Ir/ γ-Al₂O₃ catalyst, this could be attributed to highly dispersed iridium active sites. The results shown in Fig. 6b clearly show that the operation of catalyst is strongly dependent on the nature of the support. The interaction between reactant and catalyst play an important role in chemical reactions, especially the bond cleavage reaction [46]. According to the bond order conservation theory, weaker intramolecular bonding causes stronger adsorption, thus leading the catalyst higher activity towards a dissociative reaction. The data obtained indicate the suitability of  γ-Al₂O₃, as compared to aluminosilicate as supports. In this context, one issue that has attracted attention in the literature is whether or not the carrier type influences the catalytic activity or it is just a way of achieving different iridium dispersion, to address specifically this point.
The XRD patterns for the iridium catalysts after decomposition of hydrazine monohydrate are shown in Fig. 7. Three catalysts (Ir-100Al, Ir-97Al3Si, and Ir-3Al97Si) exhibit the main reflection lines of iridium nanoparticles (2θ = 40.5°, 47° and 69°). This suggests that after hydrazine decomposition, iridium nanoparticles are active in the three catalysts without becoming oxidized. As observed for the catalysts before hydrazine monohydrate decomposition, the main lines characteristic of IrO₂ (2θ = 27°, 34°, and 53°) were detected for the Ir-50Al50Si catalyst. IrO₂ particles in the Ir-50Al50Si catalyst remain in the same oxidized phase, although they did not participate in the decomposition reaction. These phase stabilities are essential for the long lifetime of the iridium catalysts. Also, stable particle on surface is important during the decomposition reaction that the FESEM analysis used to detect it. The FESEM images of the catalyst surfaces before and after hydrazine decomposition are quite different (Fig. 8). It is noted that before and after decomposition of hydrazine in Fig. 7a, the surface character of 100Al remained almost the same. This result strongly indicates that the type of support iridium catalyst does not blocked and the interaction between iridium and support is suitable; the results agree with BET surface area in Table 3. Significant growth of the particles on catalyst surface with Ir-50Al50Si was similar before and after being used in reactor, indicating that the sintering of iridium particles may be the deactivation reason of this catalyst, as shown in Fig. 7c. For the Ir-97Al3Si and Ir-3Al97Si catalysts (Table 3), the surface area was increased after decomposition of hydrous hydrazine, indicating that the addition of SiO₂ significantly promoted sintering of Ir and poor interaction. There is the possibility that the release of confinement of nanoparticles inside the pore channels of the catalyst resulting an increase in the BET surface area. This growth are also shown in the FESEM images (Fig. 8b and d). The Ir-97Al3Si have surface areas of 63 m²/g after decomposition of hydrous hydrazine, compared with 74 m²/g before decomposition of hydrous hydrazine. Similarly, the Ir-3Al97Si has a surface area of 391 m²/g after test, compared with 413 m²/g before test. However, we may suggest that the different interaction occurring between iridium and support is probably related to the variation of the surface structures resulting from different silica content in alumina.

CONCLUSIONS
Four different supports (alumina and aluminosilicate) were used for preparation of iridium catalysts to be used in the hydrazine monohydrate decomposition. A reasonable explanation to justify the large differences in activity between the catalysts prepared following the same method, but different silica content in the support relies on accepting that the free surface of iridium nanoparticles plays a role in catalysis. Most notably the dispersion of the active phase decreased in the following order Ir-50Al50Si >Ir-3Al97Si > Ir-97Al3Si > Ir-100Al. Moreover, the different active phase cluster size, acidic sites and support nature were reflected in differences inactive phase reducibility and in the character of iridium species. The aluminosilicate [SiO2 (97 wt%): Al2O3 (3 wt%)]-supported Ir catalyst exhibited smaller extent of decomposition reaction and a larger extent of ammonia. As a result, ammonia was the main product, and less gas products were yielded in comparison with the other two catalysts. On the other hand, the Al2O3-supported Ir catalyst exhibited increased selectivity to hydrogen, plausible due to the better active phase distribution. The results provide clear evidence that the most active catalysts for the hydrazine decomposition are those that contain the lowest silica percentage on the alumina support.

CONFLICT OF INTERESTS
The authors declare that there is no conflict of interests regarding the publication of this paper.

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