Nanocatalysts

Over the last decade there has been an increase in the interest on nanochemistry.

A variety of nanosized materials such as graphene sheets (2D), carbon nanotubes (1D) and, metal or metal oxide nanoparticles (0D) have been synthesized and proposed as potential building block of optical and electronic devices, as well as catalyst due to their enhanced surface chemistry. As a consequence, there is a necessity to fully understand and characterize from an atomistic point of view their structural and chemical properties.^104–106

Nanoparticles are used as catalysts in a wide range of chemical reactions. In particular, these systems are of high interest due to the presence of active sites involving shell structural defects or low-coordinated atoms. Furthermore, semiconductor nanoparticles have received great attention due to their unusual electronic and enhanced optical properties compared to bulk materials. Indeed, when the size of the nanocatalyst decreases from bulk size to the nanosize level, new unique both chemical and physical properties emerge. Generally in nanoscience, goals are the full control over the size, shape, optical, electronic, magnetic and structural behaviors. In this context, theoretical approaches may contribute to elucidate the reactivity of differential sites and these differential properties in comparison with macroscopic ones.

Nanosized electrocatalysts for OER are of considerable interest because their composition, size, shape and crystallinity are crucial and strictly related to macro-scopic surface properties, which may lead to huge changes into their chemical reactivity. As known, it is beneficial to reduce the particle size, because the effective surface is increased, forming more active sites for OER. In a similar way, core-shell combinations of two materials have displayed a superior electrocat-alytic activity, which is attributed to the formation of uncommon phases at the materials interface and the creation of other interfacing active sites.

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Metal Oxide Nanoparticles

Recently a wide number of approaches have been employed for the synthesis of nanoparticle catalysts including thermal evaporation in vacuum, electron-beam lithography and pulsed laser deposition, buffer-layer assisted growth, chemical va-por deposition, gas condensation, ionized cluster beam deposition, electrochemi-cal deposition methods, sol-gel or colloidal techniques, deposition-precipitation and impregnation methods, molecular cluster precursors and many more. The main requisite of these approaches is the production of robust materials that preserve their initial morphologies, or at least their narrow size distribution under reaction conditions with minimum coarsening.

RuO₂and IrO₂heterogeneous catalyst are the most active materials for the OER reported to date, in acidic conditions. Nobel metals have a high cost and low earth abundance, making nanoparticle catalysts more interesting because of their enhanced activity requiring lower amount of catalyst. For instance, iridium oxide nanoparticles have been synthesized in aqueous phase^107,108 or using dimethyl sulfoxide (DMSO)-metal adducts,¹⁰⁹and ruthenium oxide nanoparticles can be made via chemical vapor deposition,¹¹⁰ electrochemical deposition¹¹¹ and polyol methods.^112,113 However, the OER activities of these metal oxide nanoparticles are often not comparable, differences being attributed to different particle sizes, crystal structures (rutile or amorphous), and different degrees of surface hydration.

Interestingly, these metal oxide nanoparticles can be synthesized by first obtain-ing the correspondobtain-ing metal nanoparticle in an oleylamine-mediated solution both starting from the IrCl₄·xH₂O or RuCl₄·xH₂O as salt reduction approach, followed by thermal oxidation of the nanoparticles in an oxygen atmosphere. This method-ology needs the presence of a reducing agent such as NaBH₄or the classical sodium citrate for gold nanoparticles in the Turkevich method¹¹⁴which acts both as reducing agent and capping ligand. Capping agents are used in nanoparticle synthesis to stop the particle growth and aggregation. However, nanoparticle catalysts requires a clean surface like the ones obtained with the organometallic

1.4 - Water Oxidation Catalysts (WOC) 29 approach.^115,116This methodology uses an organometallic precursor as metal source, stabilized by the ligand double bonds as in the [Ru(COD)(COD)] complex and 3 bar pressure of H₂ to reduce the ligand double bonds and deliver nude Ru⁰atoms to the reaction media. Metallic nanoparticles start to grow until the stabilizing ligand surrounds the nanoparticle surface avoiding its growth and aggregation.

Characterization by X-ray diffraction (XRD) and high-resolution transmission microscopy (HRTEM) is required to determine the crystallinity of the synthesized materials, size distribution and aggregation.

Figure 1.9:HRTEM images of IrO₂(left) and RuO₂(right) with FFTs indexed to the rutile structures showing the corresponding (110) and (101) Miller index.

It is generally observed that the specific OER activities are higher in acid than in basic media for both RuO₂and IrO₂nanoparticles. Interestingly, the specific OER activity of IrO₂nanoparticles dispersion is slightly lower than that of RuO₂at low overpotentials in both acidic and alkaline reaction conditions. This tendency is in agreement with the computational study reported by Rossmeisl et al., that describes the optimum bonding strength to the OER intermediate species follow-ing the Sabatier principle statements discussed above. Another way to enhance their electrocatalytic stability is to synthesize a core-shell structure (IrO₂@RuO₂) and once subjected to OER electrocatalysis. It has been elucidated that this core-shell conformation can not only lower the overpotential but also increases the whole stability.¹¹⁷

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