Ir-based Catalysts Research Articles

The promoter effect of Nb species on the catalytic performance of Ir-based catalysts for VOCs total oxidation

Ir/TiO2 catalysts promoted by niobium have been synthesized, characterized by different complementary techniques, and tested for the total oxidation of a set of alkanes and their mixtures. The addition of appropriate amounts of niobium to Ir/TiO2 catalysts resulted in a remarkable increase in the catalytic activity compared to the Nb-free Ir/TiO2 catalysts. The promotion caused by the presence of niobium has been related to the massive presence of isolated IrOx surface species which, interestingly, present remarkable reducibility and, consequently, excellent catalytic activity. Conversely, the IrOx species formed in the catalyst in the absence of niobium also include IrO2 clusters with lower intrinsic reactivity. Similarly, Nb-loadings exceeding the theoretical monolayer tends to the formation of bulk Nb2O5 species on the titania surface that provokes the formation of IrO2 nanoclusters. A positive influence on the reactivity of non-stoichiometric surface Ti3+ species that generates oxygen vacancies is also observed.

Uncovering Activity-Stability Relationships in Mixed Ir-Based Catalysts Toward Improved Water Electrolysis

The role of proton exchange membrane water electrolysers (PEMWE) in providing clean and reliable green hydrogen is undeniable in a fully decarbonized energy system. Indeed, the ambitious “Hydrogen Shot” initiative by the U.S. DoE seeks to decrease in a decade green hydrogen pricing down to 1$/Kg H2.(1) Such endeavour will require a mass-scale implementation of PEMWEs, which is currently bottlenecked by the use of scarce precious metal catalysts, particularly at the anode where loadings of the state-of-the-art iridium (Ir) catalyst typically range between 1-3mgIr cm-2.(2) Thus, recent strategies in the literature have aimed to reduce Ir loading with several approaches such as higher catalyst dispersion and preparation of mixed metal oxides (IrxM1-xOy). The latter is particularly promising if the cation substituent M is an earth-abundant metal highly active to the oxygen evolution reaction (OER), which could lead to drastic catalyst cost reductions retaining high activities. The activity-stability relationships of various Ir phases are well-reported,(3) but hardly studied for IrxM1-xOy where any preferential dissolution of M under OER operation could result in misleading conclusions.The work presented here aims to bridge this knowledge gap by online quantification of catalyst dissolution products with our electrochemical flow cell coupled to inductively-coupled plasma mass spectrometry setup. First, we evaluated these relationships in cationic-substituted IrxM1-xO2 (100) epitaxial thin films, model systems which will allow us to clearly uncover any cation-dependent trends. It was observed that the Ir stability in such model systems directly correlated with the OER enhancement effect of the cation substituent: higher OER activities induced by the cation yielded lower Ir stabilities and vice versa. In addition, the IrO2 matrix does not seem to affect the thermodynamic dissolution trends of the cation M. In conjunction with ex-situ XPS characterization, we successfully identified two promising cation substituents considering the experimental activity-stability trends: Mo and W. Next, we moved to the industrially-promising IrxRu1-xO2 nanoparticle (NP) systems, with different relative compositions and degrees of crystallinity. Although Ru generally presents high dissolution rates under OER potentials, preliminary studies on sputtered thin films revealed a drastic stabilization of the RuO2 matrix upon incorporation of less than 20 at. % Ir.(4) The one-step synthetic approach used here, flame spray pyrolysis, allowed to easily tune the NPs Ir:Ru ratio by the relative content of the metal precursors in the pyrolized solution. Additionally, a post-calcination step enabled to convert the NPs phase from hydrous to rutile-type oxide. Using as start-up/shut-down OER protocol, we observed at 1000-fold lower stability for hydrous vs. rutile-type IrxRu1-xO2 regardless of the composition after selective Ru leaching. For rutile-type IrxRu1-xO2 nanoparticles, the sequential on/off OER operation revealed a ca. 10-fold decrease in Ru dissolution upon Ir incorporation, as well as the key role of surface Ru in OER activity.(5) Minimal Ru losses (<1 at. %) led to the formation of an Ir-rich protective shell, which for Ir0.2Ru0.8O2 resulted in an OER performance shift equivalent to that of pristine Ir0.8Ru0.2O2. These results showcase, contrary to previous reports, that Ru-rich compositions can indeed be implemented in PEMWE devices.

Iridium-copper oxide nanotubes catalyzing pH-universal oxygen evolution reaction with highly improved activity and durability via elemental substitution

Nanotubes of Ir-Cu binary metal oxide with various composition ratios (IrxCu1−xOy with x = 0.64, 0.48 and 0.33) were prepared using electrospinning followed by thermal annealing. IrxCu1−xOy nanotubes were investigated to be composed of rutile IrO2 phase where Cu substituted Ir sites. This elemental subsititution induced the amorphization and generation of oxygen vacancy defects in the rutile phase of IrxCu1−xOy nanotubes. Among IrxCu1−xOy series, Ir0.48Cu0.52Oy nanotubes, having a reduced noble Ir content by more than a half, showed the best activity for oxygen evolution reaction (OER) in pH-universal condition, and even outperformed the material containing only Ir, commercial Ir/C and the other Ir-based binary metal oxide catalysts found in the literature. Of importance, Ir0.48Cu0.52Oy presented an excellent stability during continuous OER for 20,000 s in all the acidic, neutral and alkaline media despite their amorphousness. The introduction of Cu into IrO2 was beneficial for improving not only the OER catalytic activity but the durability along with cost-effectiveness.

Supported Ir-Based Oxygen Evolution Catalysts for Polymer Electrolyte Membrane Water Electrolysis: A Minireview

Renewable energy sources are considered to play a key role in the future energy mix, with hydrogen being an important energy carrier as a long-term energy storage option. Polymer electrolyte membrane (PEM) water electrolysis (WE) allows the relatively quick and convenient production of pure hydrogen from only water and electricity; thus it is considered as one of the most practical ways to consume renewable energy for green hydrogen production. The efficiency of water splitting largely depends on the intrinsic activity, selectivity, and stability of the oxygen evolution electrocatalysts, currently based on noble metals (e.g., Ir or Ru), for PEM technology. The utilization of various support materials has been proposed for decreasing the noble metal loading, reducing costs of the technology, and enhancing the catalytic activity and durability. This minireview addresses recent advances and research prospects in the area of Ir-based supported oxygen evolution catalysts. Besides addressing the intrinsic activity and durability of the supported catalysts, consideration is given to their applications in PEM WE cells. It concludes with mention of challenges and future perspectives.

Structure and performance of supported iridium catalyst for the lean methane oxidation at low temperature

To clarify the relationship between the structure and performance for the Ir-based catalysts, a series of Ir/SiO2 catalysts were prepared by the colloid-impregnation method and evaluated in the lean methane oxidation at low temperature. The results indicate that the colloid-impregnation is effective to attain highly dispersed Ir nanoparticles on the SiO2 support. The catalytic activity of Ir/SiO2 is sensitive to the structure and state of loaded Ir species; the Ir/SiO2 catalyst with abundant defective Ir3+ species and mean Ir particle size below 2.0 nm exhibits extraordinarily high activity in methane oxidation. However, the activity of Ir/SiO2 decreases considerably with the increase of calcination temperature, due to the agglomeration of Ir particles induced by high temperature. Nevertheless, the thermal stability of supported Ir catalysts can be greatly improved by employing proper support with strengthened metal-support interaction, suggesting the enormous potential of supported Ir catalysts in the related applications.

Inter-relationships between Oxygen Evolution and Iridium Dissolution Mechanisms.

The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir‐based catalysts is, therefore, crucial for a scale‐up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state‐of‐the‐art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.

Recent developments of iridium-based catalysts for the oxygen evolution reaction in acidic water electrolysis

Advanced optimization strategies for Ir-based oxygen evolution catalysts include morphological regulation, heteroatomic doping, crystal phase control, and support introduction.

Deactivation of catalysts in simultaneous reversible and irreversible parahydrogen NMR signal enhancement, and the role of co-ligands in the stabilization of the reversible method.

Signal Amplification by Reversible Exchange (SABRE) and hydrogeneable Parahydrogen Induced Polarization (hPHIP) can enhance weak NMR signals, and thus increase the range of NMR applications. Here, using an N-heterocyclic carbene Ir-based catalyst, simultaneous SABRE and hPHIP was achieved for the compound with an N-donor site and an acetylene triple bond. It was demonstrated that the interplay between SABRE and hPHIP can be manipulated. Specifically, it was found that the hPHIP effect could be almost completely suppressed, while stable SABRE was observed in subsequent consecutive experiments. The presented results have the potential to increase the numbers of parahydrogen hyperpolarizable molecules.

Towards stable and highly active IrO2 catalysts supported on doped tin oxides for the oxygen evolution reaction in acidic media

Iridium oxide is the preferred catalyst for water oxidation but it is required to maximize its utilization to deploy Proton Exchange Membrane Water Electrolyzers (PEMWEs) into the large-scale applications panorama. A promising pathway for dispersing this precious catalyst is on an electric conductive and stable support. However, there is a lack of understanding how the support-catalyst interactions affect the stability/activity of the electrocatalyst under anodic conditions. This work discloses a modified, easy-scalable, polyol synthesis protocol to produce a highly active and stable iridium-based catalyst, supported on metal-doped tin oxides. The loading of Ir was reduced 30 wt.% compared to the reference IrO2, and dispersed on Sb-SnO2 (IrOx/ATO), In-SnO2 (IrOx/ITO) and SnO2 supports. All synthesized electrocatalysts not only surpassed the OER-mass activity of a commercial catalyst (IrO2) – reference – but also reached higher electrochemical active surface areas and enhanced stability under the OER conditions. The highest performance was achieved with Ir NPs supported on ITO (176 A/gIr vs. 15.5 A/gIr for the reference catalyst @ 1.51 V vs. RHE) and both IrOx/ITO and IrOx/SnO2 catalysts demonstrated remarkable stability after cycling the electrode and performing long-term chronopotentiometry. ITO is, therefore, an auspicious support to serve Ir-based catalysts as it favors a good bargain between activity and stability, while drastically reducing the amount of noble metal.

Inert metal induces the modulation of unsaturated aldehyde absorption mode for enhanced selective hydrogenation.

Selective hydrogenation of α,β-unsaturated aldehydes to obtain a high yield of unsaturated alcohols is important in industrial production. This is still a great challenge because it is thermally more favorable for the hydrogenation of CC than for the CO bond. Various strategies have been developed to optimize the catalysts for improving selectivity but are usually accompanied by the sacrifice of catalytic activity. Herein, we adopt the inert metal inducement strategy to synthesize a series of Ir-M alloy nanoparticle catalysts. The optimal catalyst IrCd5 exhibits impressive catalytic performance in the selective hydrogenation of cinnamaldehyde, achieving 96.7% conversion with 94.3% selectivity for cinnamal alcohol, which is far superior to that of the Ir counterpart. Furthermore, the H2 temperature-programmed desorption (H2-TPD) test, styrene-TPD test, surface valence band test and density functional theory calculations demonstrate that the adsorption mode of cinnamaldehyde shifted from parallel to vertical configurations after introducing an inert metal. Compared to Ir, the weaker adsorption of alkene and stronger adsorption of the substrate for IrCd5 lead to the prior adsorption and hydrogenation of the CO bond, thus elevating the selectivity of the cinnamal alcohol. This strategy disperses precious metal nanoparticles effectively, maximizes atomic utilization, and improves the selectivity, which provides a new avenue to design bimetal alloy catalysts for the selective hydrogenation of α,β-unsaturated aldehydes.

N,O-C Nanocage-mediated high-efficient hydrogen evolution reaction on IrNi@N,O-C electrocatalyst

Development of high-performance and stable electrocatalysts for hydrogen evolution reaction (HER) is crucial for hydrogen economy. Inspired by hydrogen evolution behavior on hydrogenase enzyme, in which the azadithiolate bridging ligand of active center facilitates fast proton shuttling and catalytic turnover, here we present an efficient approach to enhance both HER kinetics and stability realizing on an IrNi@N,O-C catalyst with individual IrNi nanoparticle separately confined in N and O co-doped carbon (N,O-C) nanocage. Experimental and theoretical investigations indicate that the N,O-C nanocage provides proton-adsorbing functionality on raising H* coverage over IrNi active centers, resulting in much accelerated HER kinetics. The IrNi@N,O-C presents superior HER performances compared to all reported Ir-based catalysts, with overpotential of 22 mV at 100 mA cm−2, ultrahigh mass activity of 9.82 A mgIr−1 (−0.050 V vs. RHE) and outstanding durability of 200 h at large current density. This work affords fresh insights/strategies for design of high-performance electrocatalysts

X-Ray Absorption Spectroscopy Studies of Ir-Oxide Based Oxygen Evolution Catalysts Revisited

Hydrogen plays a pivotal role in the future decarbonization of the energy sector [1], but its production in a sustainable way is crucial for it to have a positive environmental impact. Due to its high power density, quick start-up and response time as well as pressurized hydrogen delivery, polymer electrolyte membrane water electrolysis (PEMWE) is perfectly suited to facilitate H2 production from renewable electricity [2]. However, at large scale application, the use of scarce and expensive, Ir-based catalysts in PEM water electrolyser anodes limits the proliferation of this technology [3, 4]. For this reason, significant research activities are being conducted to better understand the oxygen evolution reaction (OER) mechanism on benchmark IrO2 based catalysts, as to further improve catalyst design and decrease the efficiency losses related to this reaction.When trying to elucidate a reaction mechanism, the use of operando and/or in situ techniques is often beneficial, and thus in last decades the OER mechanism on Ir-based materials has been extensively investigated through operando / in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [5-10]. Despite enormous efforts in this research area, there is still a lack of consensus regarding the Ir oxidation state under OER conditions and the nature of the OER-active sites. In line with this, in this study we employed operando XAS to investigate the Ir oxidation state in commercial IrO2 (Umicore®) catalyst under OER polarization curve conditions. These XAS measurements were performed both in transmission and fluorescence modes; for the former, electrodes with loadings ≈ 40 fold higher than those used in rotating disk electrode (RDE) measurements were required in order to achieve appropriate XAS data quality. In fluorescence mode, however, thinner electrodes with only four times higher loadings than in RDE could be investigated. Prior to the operando XAS measurement, the utilization of these IrO2 catalyst layers in the operando flow cell (FC) was assessed to ensure that the acquired spectroscopic data was representative of the processes occurring under real conditions. Achieving a good catalyst layer utilization was especially important in the case of the thick electrodes used in transmission XAS measurement, since their resistance is much higher compared to that in thin film RDE measurements. The so obtained results were additionally affected by beam damage of the Nafion ionomer used as the catalyst layer binder. All these factors can further lead to data misinterpretation. Regarding the beam damage, reducing the beam flux helped to prevent this effect and allowed us to obtain reliable results. Additionally, the use of thin catalyst layers in the operando FC and its combination of time-resolved XAS (so called “quickXAS”) in fluorescence mode led to novel insight on the effects of evolved O2 bubbles on the spectroscopic results and subsequent interpretation thereof.In summary, this contribution will provide an overview of common pitfalls and good practices encountered when conducting operando XAS studies of OER-electrocatalysts.

Sacrificial Cu Layer Mediated the Formation of an Active and Stable Supported Iridium Oxygen Evolution Reaction Electrocatalyst.

The production of hydrogen via a proton-exchange membrane water electrolyzer (PEM-WE) is directly dependent on the rational design of electrocatalysts for the anodic oxygen evolution reaction (OER), which is the bottleneck of the process. Here, we present a smart design strategy for enhancing Ir utilization and stabilization. We showcase it on a catalyst, where Ir nanoparticles are efficiently anchored on a conductive support titanium oxynitride (TiONx) dispersed over carbon-based Ketjen Black and covered by a thin layer of copper (Ir/CuTiONx/C), which gets removed in the preconditioning step. Electrochemical OER activity, stability, and structural changes were compared to the Ir-based catalyst, where Ir nanoparticles without Cu are deposited on the same support (Ir/TiONx/C). To study the effect of the sacrificial less-noble metal layer on the catalytic performance of the synthesized material, characterization methods, namely X-ray powder diffraction, X-ray photoemission spectroscopy, and identical location transmission electron microscopy were employed and complemented with scanning flow cell coupled to an inductively coupled plasma mass spectrometer, which allowed studying the online dissolution during the catalytic reaction. Utilization of these advanced methods revealed that the sacrificial Cu layer positively affects both Ir OER mass activity and its durability, which was assessed via S-number, a recently reported stability metric. Improved activity of Cu analogue was ascribed to the higher surface area of smaller Ir nanoparticles, which are better stabilized through a strong metal–support interaction (SMSI) effect.

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Self-supported amorphous iridium oxide catalysts for highly efficient and durable oxygen evolution reaction in acidic media

Developing highly active and stable electrocatalysts for acidic oxygen evolution reaction is still a great challenge. Herein, we developed a novel self-supported amorphous IrOx catalyst (IrOx/Ti) encapsulated with a small number of crystalline nanoparticles for efficient acidic OER. The resulting IrOx-50-AC/Ti electrode exhibits a superior OER performance with an overpotential of 250 mV at 10 mA cm−2 and Tafel slope of 50.4 mV dec−1 in 0.1 M HClO4. High contents of IrIII species and surface hydroxide groups, hierarchically porous structure and high defective density contribute to boost the OER activity. Importantly, the amorphous IrOx-50-AC/Ti also demonstrates a super long-term durability for 190 h due to the strong interaction between IrOx film and Ti. The unique amorphous-crystalline structure also enhances the stability and the activity by promoting the synergy between amorphous phase and crystalline phase. This work provides some valuable insights on developing efficient Ir-based catalysts for acidic water splitting.

High crystallinity design of Ir-based catalysts drives catalytic reversibility for water electrolysis and fuel cells

The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrOx at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrOx layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrOx layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.

Transition Metal Oxide-Modified Ir Nanoparticles Supported on SBA-15 Silica for Selective Hydrogenation of Substituted Nitroaromatics

Modification of active metals with metal oxide modifiers has attracted considerable attention in heterogeneous catalysis due to their synergistic effect. However, a controllable synthesis of highly reactive and stable metal–metal oxide hybrid nanocatalysts is difficult. To solve this problem, presynthesized IrM (M = Fe, Co, and Ni) bimetallic nanoparticles were initially confined in the mesopores of SBA-15 and were then in situ transformed to Ir-MOx hybrids. The obtained Ir-MOx/SBA-15 nanocatalysts show superior activity and selectivity in the hydrogenation of substituted nitroaromatics to corresponding aromatic amines compared to Ir/SBA-15. Among these Ir-based catalysts, Ir-FeOx/SBA-15 exhibits the highest activity and selectivity and has a wider substrate scope due to the interaction between Ir and FeOx. Therefore, our research provides a way to design reactive and stable hydrogenation catalysts.

A comparative study of CO catalytic oxidation on the single vacancy and di-vacancy graphene supported single-atom iridium catalysts: A DFT analysis

Engineering of high-performance catalysts is of great importance for reducing the greenhouse gas emission by the electrocatalytic oxidation of CO. Single-atom-catalysts (SACs) have gained substantial attention thanks to their superior catalytic activity for CO oxidation, and graphene has been considered as one most promising supporting material owing to its peculiar physicochemical properties. In this work, the mechanism of CO oxidation over iridium (Ir) embedded on both single vacancy graphene (Ir-GNSV) and di-vacancy graphene (Ir-GNDV) has been investigated with the aid of density functional theory (DFT). The structural properties of Ir-GNSV and Ir-GNDV were analyzed by Bader charge analysis and electron density difference map. The calculated adsorption energy values of CO and O2 molecules on both the Ir-GNSV and Ir-GNDV have validated that both molecules can be molecularly adsorbed on the surface of each catalyst at room temperature. The results put forth that the reaction mechanism of CO + O2 → OOCO → CO2 + O* prefers to Langmuir − Hinshelwood (LH) mechanism. The activation energy for the transition-state for Ir-GNSV has been calculated to be 0.31 eV, whereas the first transition state (TS1) and the second transition state (TS2) of Ir-GNDV have been determined as 0.30 eV and 0.26 eV, respectively. Moreover, the results have confirmed that Ir-GNSV and Ir-GNDV surfaces have high catalytic activity and selectivity towards CO oxidation. On the basis of these findings, the proposed Ir-GNSV and Ir-GNDV catalysts are considered to be promising SAC for CO oxidation at low-temperature. It can be speculated that this work paves the way for the engineering of boosted-performance Ir-based heterogeneous catalysts by providing deeper mechanistic insights.

Strong metal-support interaction assisted redispersion strategy for obtaining ultrafine and stable IrO2/Ir active sites with exceptional methane oxidation activity

Skillful arrangement of the active sites with stable dispersion and desired chemical states on the support surface is vital for their powerful application in catalysis, but remains a daunting challenge. Herein, we present a unique redispersion strategy for well dispersing and stabilizing IrO2/Ir active sites over TiO2 as an advanced catalyst for low temperature CH4 oxidation. The as-designed Ir/TiO2 with a 1 wt.% Ir loading amount exhibits a (>)99 % CH4 conversion and excellent durability at a temperature as low as 320 °C, which ranges among the best of state-of-the-art methane combustion catalysts. Various characterizations and DFT calculations were performed to reveal the redispersion mechanism and understand the reason for the enhanced performance. The superior performance is related to the proper chemical state of complex IrO2/Ir active sites and their ultrafine and stable dispersion, all relating to the strong metal-support interaction (SMSI) induced redispersion process. Such SMSI assisted active sites redispersion strategy provides useful guidance for the rational design of efficient Ir-based and other noble metal catalysts.