γ-Al2O3 Supported Bimetallic Catalysts: Synthesis and Applications for Hydrazine Decomposition

Abstract

The main objective of the present research work was to explore the potentially unique properties and possible synergistic effect in supported bimetallic catalysts which were synthesized in two stages by adopting simple route. γ-Al2O3 granules were selected as a support material and prepared by sol gel process followed by oil drop method for granulation purpose. Co was chosen as low cost, basic metal and four noble metals (M): Ir, Ru. Pt, and Pd were selected as promoters. Four series of γ-Al2O3 supported bimetallic catalysts, (CoMx/γ-Al2O3) were synthesized via wet impregnation by loading ~ 20 wt% of Co metal and 0-5 wt% of promoter metals onto γ-Al2O3 in each series. On the basis of mole fraction (x) of promoter metals, the catalyst series were named as CoIrx/γ-Al2O3, CoRux/γ-Al2O3, CoPtx/γ-Al2O3, and CoPdx/γ-Al2O3. The prepared catalysts were characterized by various techniques i.e., Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) along with EDX analysis, transmission electron microscopy (TEM), surface area analysis, and temperature programmed reduction/oxidation (TPR/TPO) processes. In order to explore the diverse role, the synthesized catalysts were applied for hydrogen generation from two model reactions i.e., catalytic decomposition of hydrazine in a self-designed reactor and electro-oxidation of hydrazine by cyclic voltammetry (CV) technique. The catalytic decomposition of hydrazine (0.5 M) was performed at 25 oC and volume of gaseous products (H2+N2) was measured by a gravimetric water displacement method. The catalytic efficiency was evaluated in terms of turn over frequency (TOF) and selectivity (X) towards hydrogen generation. The optimal composition from each series was selected on the basis of activity and selectivity. Hydrazine decomposition reaction was further tested at various temperatures and activation energy (Ea) values were calculated from the kinetic profiles using Arrhenius equation. In particular, the kinetic studies depicted an increase in rate of hydrogen generation with an increase in temperature up to 65 oC. The effect of support was also studied by loading the selected optimal composition on various supports including MgO, ZnO, SiO2, and zeolite; (γ-Al2O3 proved to be the best iv support candidate). For electrochemical studies, glassy carbon electrode was modified with each catalyst powder by drop-casting and used for investigating electro-oxidation of hydrazine analyte in 0.1 M phosphate buffer solution (pH 7.2). Cyclic voltammetry was performed to elucidate the redox behavior of synthesized electroactive materials. A linear current-voltage response was observed on the modified electrodes in each catalyst series. The stability and reproducibility of all the catalytic materials was substantiated by the catalytic activity as measured in many successive cycles. All bimetallic catalysts showed potentiality for hydrogen generation and electrochemical applications. The combination of high TOF and selectivity for hydrogen generation as well as prominent current response in CV studies rendered CoIr0.081/γ-Al2O3, CoRu0.11/γ-Al2O3, CoPt0.034/γ-Al2O3, and CoPd0.093/γ-Al2O3 catalysts as optimal compositions in respective series. Out of these optimal compositions, CoPt0.034/γ-Al2O3 bimetallic catalyst exhibited maximum performance for hydrogen generation from hydrazine decomposition as well as for electro-oxidation of hydrazine. CoPt0.034/γ-Al2O3 catalyst presented 100% H2 selectivity with TOF of 190 h-1, and activation energy of 29 kJ mol-1. All bimetallic catalysts also showed robust electrocatalysis and presented good peak current response for electro-oxidation of hydrazine. These results reflected better alloying effect between Co and Pt metals in addition to presence of more active sites and better metal dispersion. The electroanalytical activity was in the order of: CoPt0.034/γ-Al2O3 > CoPd0.093/γ-Al2O3 > CoIr0.081/γ-Al2O3 > CoRu0.11/γ-Al2O3, with peak current values of 183.2 μA, 59.4 μA, 50.3 μA, and 46.1 μA, respectively. The maximum performance of the optimal catalysts can be attributed to tuning of catalyst’s properties by synergistic effect of two metals, better metal dispersion, metal-support interactions besides the excellent features of γ-Al2O3 granular support as confirmed by XRD, H2 chemisorption, BET, SEM, TEM, TPR and TPO analyses. In addition, noble metals in low contents promoted the activity and selectivity tremendously, and were quite suitable for making the process cost effective for selective decomposition of hydrazine, a toxic material into clean future energy fuel (H2 + N2).