Exsolution of Catalytically Active Iridium Nanoparticles from Strontium Titanate

Abstract

The search for new functional materials that combine high stability and efficiency with reasonable cost and ease of synthesis is critical for their use in renewable energy applications. Specifically in catalysis, nanoparticles, with their high surfaceto-volume ratio, can overcome the cost implications associated with otherwise having to use large amounts of noble metals. However, commercialized materials, i.e. catalytic nanoparticles deposited on oxide supports, often suffer from loss of activity due to coarsening and carbon deposition during operation. Exsolution has proven to be an interesting strategy to overcome such issues.Here the controlled emergence, or exsolution, of faceted iridium nanoparticles from a doped SrTiO3 perovskite is reported and their growth preliminary probed by in situ electron microscopy. Upon reduction of SrIr0.005Ti0.995O3 the generated nanoparticles show embedding into the oxide support, therefore preventing agglomeration and subsequent catalyst degradation. The advantages of this approach are the extremely low noble metal amount employed (~0.5% weight) and the catalytic activity reported during CO oxidation tests, where the performance of the exsolved SrIr0.005Ti0.995O3 is compared to the activity of a commercial catalyst with 1% loading (1% Ir/Al2O3). The high activity obtained with such low-doping shows the possibility for scaling up this new catalyst, reducing the high cost associated with iridium-based materials.Image analysis and calculations; XPS supplementary data; STEM-EDX maps of as-synthesised Ir0.5-STO (PDF)

Hydrothermal Synthesis of Iridium-Substituted NaTaO3 Perovskites

“…Interestingly, for other iridium-substituted perovskites reported in the literature via other synthesis methods, different oxidation states of Ir may be found. For example, Calì et al observed Ir 3+ using X-ray photoelectron spectroscopy in 5 mol% Irsubstituted SrTiO 3 that had been prepared by solid-state synthesis at 1340 • C [31]. On the other hand, Kawasaki et al found Ir 4+ in samples of Ir-substituted SrTiO 3 prepared by solid-state synthesis at 1100 • C [32].…”

“…The Rh was found in the +3 oxidation state and on heating to higher temperatures the rhodium was lost, with phase transformation of the perovskite. 0.5% iridium-doped SrTiO 3 was prepared by a solid-state method, and in a second step under reducing conditions, exsolution of the Ir was found that resulted in supported Ir nanoparticles, embedded in the oxide support, showing little agglomeration when used in CO oxidation catalysis [31]. Ir-doped SrTiO 3 (1-5% Ir) has also been studied for photocatalysis, and the oxidation state of the Ir was found to dictate the properties towards photocatalytic water splitting, with Ir 4+ giving the most favourable properties [32].…”

Emergence and Future of Exsolved Materials

“…To the best of our knowledge, there are only three papers published on Ir exsolution, which focus on catalytic applications, one on the ability of this noble metal catalyst to efficiently convert CH 4 using CO 2 [ 155 ] and the other two on CO oxidation. [ 156,157 ] In all cases, it is demonstrated that the improved catalytic activity of the material was induced after the emergence of the Ir nanoparticles on the surface. In the former case this was shown to be due to the particles’ ability to catalyze the cleavage of the CH bond in CH 4 with high coking resistance.…”

A Robust Approach to In Situ Exsolve Highly Dispersed and Stable Electrocatalysts

“…[1,2] The in situ exsolution with perovskites emerges as time-saving and cost-effective way to fabricate NPs catalysts, [3][4][5][6] because of that this approach does not need multiple deposition steps or expensive precursors, while producing better-distributed and agglomeration-resistant metal NPs. [7][8][9][10] However, the exsolution of NPs from perovskites (ABO 3 ) still faces several challenges. First of all, this process works with limited elements, as only the A/B sites with similar size can accommodate the formation of perovskitetype structure through a substitution mechanism.…”