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Water Oxidation Catalyst via Heterogenization of Iridium Oxides on Silica: A Polyamine-Mediated Route to Achieve Activity and Stability Nagaraju Shilpa, Joydeb Manna, Parasmani Rajput, and Rohit Kumar Rana ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00966 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016
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ACS Catalysis
Water Oxidation Catalyst via Heterogenization of Iridium Oxides on Silica: A Polyamine-Mediated Route to Achieve Activity and Stability Nagaraju Shilpa,†,‡ Joydeb Manna,†,¶ Parasmani Rajput,§ and Rohit Kumar Rana*,†,‡ †
Nanomaterials Laboratory, I & PC Division, CSIR – Indian Institute of Chemical Technology,
Hyderabad 500007, India ‡
Academy of Scientific and Innovative Research, CSIR – Indian Institute of Chemical
Technology, Hyderabad 500007, India §
Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai
400085, India Corresponding Author *Email:
[email protected] ABSTRACT. Heterogenization of nanostructured iridium-based catalysts to simultaneously achieve activity and stability in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant is reported herein. We demonstrate that a polyamine-mediated assembly process to disperse iridium species on mesoporous silica spheres facilitates fabrication of nano-
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sized iridium oxides under optimal thermal treatment. From a comprehensive morphological and electronic structure studies including electron microscopy, UV-vis spectroscopy, XANES and EXAFS, we show that the influence of polyamine is crucial in stabilizing catalytically active iridium oxides in the mesoporous silica matrix. Whilst the functionalization of silica surface with polyamine facilitates interaction with the negatively charged iridium precursor, the presence of polyamine further enables control on the dispersion and crystallization of the generated iridium oxides during the thermal treatment at 573 K. As a consequence, the catalyst exhibits enhanced activity with higher TON along with desirable stability for it to be recycled keeping the activity intact. The activity and stability of the synthesized catalyst as compared with that of IrCl3 and IrO2 reveal that balancing between the dispersion and crystallization of iridium oxides is crucial in heterogenization of the catalyst.
KEYWORDS. nanostructures, oxides of iridium, heterogeneous catalysis, polyamine, surface functionalized silica, water splitting INTRODUCTION In the pursuit to have clean, sustainable and renewable source of energy for tackling anthropogenic climate changes, water splitting using solar energy is seen to be one of the most exciting and environmentally acceptable options.1-3 Resembling the natural photosynthesis, water splitting comprises oxidation of water to O2 generating protons and electrons as required for the fuel formation. Water oxidation is a highly energy demanding reaction as it necessitates rearrangement of bonds between two water molecules accompanying a multi-electron transfer process. Hence an effective catalyst needs to stabilize various intermediates involved in the reaction in order to overcome the kinetic barrier for a faster rate of oxygen production.4
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Amongst different WOCs (water oxidation catalysts), iridium based catalysts have been considered as one of the most active materials.5-9 Although a recent trend has been to use more abundant elements for water-oxidation catalysis, the activity and especially the stability of these non-iridium catalysts in acidic conditions are the major obstacles.4,10 Therefore, it is still advantageous to develop WOCs based on iridium, which in its higher oxidation states is capable of exhibiting high rate of O2 evolution with low kinetic barrier.11 Many iridium containing homogeneous catalysts for the oxidation of water have been studied extensively, and as reported and reviewed the stability of catalyst is a concern.4 Particularly, strongly acidic or oxidizing conditions favor the transformation of the molecular catalysts, which eventually leads to precipitation of metal-based particles.12-15Although attempts are made to stabilize molecular compounds on solids,16-19 often the active species retain only some of the structural features of the precursor.19 There have been many efforts to develop alternative methods for heterogenization of the catalyst.20-24 However, the challenge has been to achieve both stability and activity together.24 Herein, we demonstrate that, surface functionalization of a silica surface with polyamines is a promising way to disperse and stabilize iridium oxides in order to develop heterogeneous catalysts for effective catalytic water oxidation (Figure 1A). Linear-chain polyamines have been previously shown to aid in designing nanoparticle-assembled functional materials.25-27 In this work, we establish the ability of these polyamines in assembling and stabilizing the catalytic active sites on mesoporous silica spheres. RESULTS AND DISCUSSION Assembly Strategy. Assembling the iridium precursor on to the synthesized mesoporous silica spheres was found to be influenced by the charge on silica surface. The surface of mesoporous silica (MS) has a negative charge, while iridium chloride in solution exists as anionic species
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(Figure S1). 28,29 As a result, interaction between them was wayward and hence a large amount of the iridium was leftover in solution. Therefore, further modification of silica surface with a desired functional group was necessitated in order to tailor their interactions. In a resemblance to the biosilica structure, which ensembles polyamines as the proteinaceous matrix in a porous silica structure, we used poly(allylamine) (PAH) to modify the silica surface. The PAH by virtue of their electrostatic interaction, can allow assembly of catalytic species forming stable heterogenized materials.30,31 The mesoporous silica having readily hydrolysable groups on its surface can also facilitate an effective binding with the positively charged amine moieties of polyamine. As seen from the zeta potential measurements (Figure 1B), the silica particles which had negative surface potential, after functionalization with polyamine (PAH-MS) resulted in a charge reversal. Consequently, upon addition of the iridium precursor, the functionalized silica could take up 95% of the iridium from the solution. A simultaneous change in the surface charge to negative potential further indicated the presence of negatively charged iridium species on the
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surface. The iridium loading in the functionalized mesoporous silica was 3.66 wt.%.
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Samples
Figure 1. (A) Schematic representation of WOC synthesis; (B) Variation in the zeta potential of the material at different stages of the WOC synthesis. However, our initial catalytic testing showed that the as-prepared iridium containing MS prepared at room temperature (Ir@MS-RT), had a substantial leaching of iridium with
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supernatant becoming slightly brown in color. ICP-OES analysis revealed leaching of about 11.58 % of the total iridium, evidencing that the iridium species were not held strongly on to the support. Formation of oxides of iridium can improve their stability as reported for electrocatalytic applications.32,33 Therefore, we carried out a systematic study to stabilize the catalyst by thermally treating Ir@MS-RT at a medium (573 K, Ir@MS-MT) and high temperature (773 K, Ir@MS-HT). FT-IR analysis confirmed the change in chemical functionalities on the silica surface, while the dynamic light scattering measurement showed hydrodynamic sizes of the particles in the range of 150-350 nm (Figures S2 & S3). There was a reduction in surface charge for the thermally treated samples, plausibly signifying formation of iridium oxides (Figure 1B). Structural Characterization of WOC. FE-SEM (Field Emission Scanning Electron Microscopic) images of as-prepared WOC depict spherical morphology of mesoporous silica (Figures 2A–C). With thermal treatment the morphology remained intact, though there was slight increase in agglomeration of the spheres at higher temperature. Transmission Electron Microscopic (TEM) images of the catalyst treated at various temperatures is shown in Figures 2D-F. Apart from the spherical particles in the range of 100-200 nm diameters, the porous structures of the individual spheres are clearly evident in the TEM images. The surface with openings for the pores is seen as well in the FE-SEM image taken at a higher magnification (Figure 2G). FE-SEM imaging with a Low Angle Back Scattered electron (LABE) detector along with Energy Dispersive X-ray spectroscopy (EDX) indicated the dispersion and siteisolation of the iridium species (Figures 2H, S4 and S5). It also corroborates well with the relatively darker spots of
