Surface Ir Atoms Research Articles

Advanced Nanostructured Textile Catalysts for Enhanced Hydrogen Production in PEM Water Electrolysis: An Overview and Recent Developments

As the global focus shifts towards a zero-emission society, hydrogen has emerged as a critical element in various industries, particularly in green hydrogen production. A promising method for this is Polymer Electrolyte Membrane Water Electrolysis (PEMWE), which uses pure water and electricity to produce high-purity hydrogen. However, the efficiency of PEMWE is highly dependent on the effectiveness of its catalysts, particularly in the context of the catalyst-coated membranes (CCMs) employed in these systems. Conventional preparation of these CCMs involves multiple steps, including synthesis and application of a catalyst ink, which often leads to catalyst agglomeration and requires high Ir (Iridium) loadings to maintain uniformity and performance.In our latest study, we have developed a novel approach using ultralight conductive IrO2 nanostructured textiles (NST) as a catalyst layer for PEMWE. This innovative method successfully circumvents the issue of electrically isolated inactive catalysts, a common problem with lower Ir amounts in traditional catalyst layers. By employing a process of sputtering IrO2 onto water-soluble polymer nanofibers, we have achieved facile CCM preparation and demonstrated unprecedented long-term hydrogen evolution with ultra-low Ir mass loadings at high rates. Our study revealed that the high performance of these CCMs is derived from their low resistivity and unique porous three-dimensional structure.The preparation of NST involves dissolving polyvinylpyrrolidone (PVP) in methanol, followed by electrospinning this solution onto a metal-foil collector. The resulting PVP nanofibers then undergo a sputtering process to deposit IrO2, forming the nanostructured textiles. This method eliminates the need for wet chemical routes and labor-intensive processes typically associated with conventional catalyst layer preparations, thus offering a more environmentally friendly and scalable alternative.Our performance evaluations of these NST CCMs, with varying Ir mass loadings, have shown significant advancements. The operational cell voltages of CCMs containing as little as 0.05 mg-Ir cm−2 were suppressed across all investigated current densities compared to conventional CCMs. Notably, operational voltages remained below 2 V even at the highest current density of 7 A cm−2. This level of performance is unprecedented for CCMs with such low catalyst amounts. Additionally, the long-term stability of these CCMs was remarkable, with operational times exceeding 1500 hours without significant increases in voltage, showcasing their potential for practical applications.An essential aspect of our study was the analysis of the electrical properties and the surface structure of the IrO2 NSTs. We observed that the electrical conductivity of these textiles was significantly higher than that of conventional catalyst layers. This high conductivity, coupled with the unique interconnected structure of the textiles, ensures efficient electrical contact between nano catalysts and the current collector, thus avoiding the formation of isolated catalysts. Our analyses revealed that the IrO2 NSTs have unique surface terminations and a high number of undercoordinated surface Ir atoms, contributing to their high intrinsic catalytic activity.Further insights were gained through operando X-ray absorption spectroscopy (XAS) measurements, which showed that the IrO2-nanostructured textiles possess a large number of D-band holes and are intrinsically highly active. This finding is crucial as it links the unique chemical state of the textiles to their high activity in oxygen evolution reactions (OER), a key process in PEMWE.Our recent developments in nanostructured textile catalysts for PEMWE mark a significant leap in hydrogen production technology. By demonstrating high-rate hydrogen production at low operational voltages and with significantly lower Ir mass loadings compared to conventional CCMs, we have addressed major challenges in the field. These advancements not only enhance the efficiency and sustainability of hydrogen production but also open avenues for applying this technology to other electrochemical conversion cells, paving the way for broader applications and further research. In this talk, we discuss the latest activity on NST, and highlight the challenges and opportunities for hydrogen production.

Grain Boundary Tailors the Local Chemical Environment on Iridium Surface for Alkaline Electrocatalytic Hydrogen Evolution

AbstractEven though grain boundaries (GBs) have been previously employed to increase the number of active catalytic sites or tune the binding energies of reaction intermediates for promoting electrocatalytic reactions, the effect of GBs on the tailoring of the local chemical environment on the catalyst surface has not been clarified thus far. In this study, a GBs‐enriched iridium (GB−Ir) was synthesized and examined for the alkaline hydrogen evolution reaction (HER). Operando Raman spectroscopy and density functional theory (DFT) calculations revealed that a local acid‐like environment with H3O+ intermediates was created in the GBs region owing to the electron‐enriched surface Ir atoms at the GBs. The H3O+ intermediates lowered the energy barrier for water dissociation and provided enough hydrogen proton to promote the generation of hydrogen spillover from the sites at the GBs to the sites away from the GBs, thus synergistically enhancing the hydrogen evolution activity. Notably, the GB−Ir catalyst exhibited a high alkaline HER activity (10 mV @ 10 mA cm−2, 20 mV dec−1). We believe that our findings will promote further research on GBs and the surface science of electrochemical reactions.

Grain Boundary Tailors the Local Chemical Environment on Iridium Surface for Alkaline Electrocatalytic Hydrogen Evolution.

Even though grain boundaries (GBs) have been previously employed to increase the number of active catalytic sites or tune the binding energies of reaction intermediates for promoting electrocatalytic reactions, the effect of GBs on the tailoring of the local chemical environment on the catalyst surface has not been clarified thus far. In this study, a GBs-enriched iridium (GB-Ir) was synthesized and examined for the alkaline hydrogen evolution reaction (HER). Operando Raman spectroscopy and density functional theory (DFT) calculations revealed that a local acid-like environment with H3 O+ intermediates was created in the GBs region owing to the electron-enriched surface Ir atoms at the GBs. The H3 O+ intermediates lowered the energy barrier for water dissociation and provided enough hydrogen proton to promote the generation of hydrogen spillover from the sites at the GBs to the sites away from the GBs, thus synergistically enhancing the hydrogen evolution activity. Notably, the GB-Ir catalyst exhibited a high alkaline HER activity (10 mV @ 10 mA cm-2 , 20 mV dec-1 ). We believe that our findings will promote further research on GBs and the surface science of electrochemical reactions.

Multiple Reaction Pathways for the Oxygen Evolution Reaction May Contribute to IrO2 (110)’s High Activity

Density functional theory calculations in conjunction with statistical mechanical arguments are performed on the rutile IrO2 (110) facet in order to characterize multiple reaction pathways on the surface at the highest active limit (the stoichiometric surface with all metal sites available) and at the lowest active limit (the oxygen-terminated surface). Alternative pathways to the oxygen evolution reaction (OER) are found, with multiple pathways determined at each step of the four proton-coupled electron transfer reaction. Of particular interest is the detailed characterization of a co-adsorption pathway utilizing neighboring, adsorbed O, OH species in order to evolve oxygen; activation energies of this pathway are <0.5 eV and therefore easily surmountable at the high operating potentials of OER. We also determined that surface Ir atoms can potentially participate in deprotonating an OOH* intermediate; the activation energy to this is 0.67 eV on the oxygen-terminated surface. These theoretical findings explain in part the high activity present in iridium oxide catalysts and also provide insight into the mechanistic pathways available on metal oxide catalysts, which may require the concerted interaction of nearest neighbor co-adsorbates to produce chemicals of interest.

The effects of MoOx decoration on the selective hydrogenation of crotonaldehyde over MoOx-promoted Ir/TUD-1 catalysts

A series of MoOx-promoted Ir/TUD-1 catalysts were tested for liquid phase selective hydrogenation of crotonaldehyde. The promoted catalysts showed higher activity and selectivity than the unpromoted one, with the best performance being obtained on an Ir-0.1Mo/TUD-1 (with a crotyl alcohol yield of 86.7% and a turnover number of 325.7 h−1). The large pore sizes of the TUD-1 is beneficial to the diffusion of the reactant and thus the supported catalyst is more active than that supported on the commercial SiO2. The characterization results revealed decoration of Ir particles by MoOx, which altered the morphologies and electronic properties of the catalysts. Such decoration resulted in the generation of Ir-MoOx interfacial sites, which altered the adsorption and activation of crotonaldehyde molecule and brought up enhanced activity and selectivity. However, at high MoOx content, the Ir surface was fully covered by MoOx, which results in strong adsorption of crotonaldehyde and fewer surface Ir atoms for H2 dissociation and thus declined activity.

Nucleation, coalescence, thermal evolution, and statistical probability of formation of Au/Ir/Pd nanoalloys in gas-phase condensation process

There has been little investigation of the formation and the mechanism of growth of trimetallic nanoclusters in the gas condensation technique using molecular dynamics (MD) simulation. In order to address this issue, we have simulated the formation of trimetallic Au/Ir/Pd nanoalloys in gas-phase condensation. We have analysed the effect of the Au mole fraction on the structural and thermodynamic properties of the nanoclusters formed during the condensation process using MD simulation. Our stability examinations have shown that an increase of Au concentration in the Pd-Ir nanoalloy is favourable and leads to the formation of a more stable cluster. Also, we observed that the Ir atoms prefer to lie at the nanoalloy cores, but the Au and Pd atoms lie at the nanoclusters surface. The surface energy of clusters also decreases as the Au concentration increases. Also, an increase of Au atoms in the nanoalloys can increase the cluster size. The results of the structural investigations using RDF showed that a core-shell morphology has not been formed during this condensation. Some ordered structures (including fcc-like and hcp-like structures) were formed in the nanoclusters in the condensation technique. The thermal evolution study showed that the nanoalloys become more spherical by increasing the temperature. Also, the surface Ir atoms decrease with increasing temperature whereas the number of surface Au and Pd atoms vary with the temperature. We have also derived an expression for the statistical probability of formation of a three-component nanocluster.

Adsorption of hydrogen on stable and metastable Ir(100) surfaces

Using the combination of high resolution core level spectroscopy and density functional theory we present a detailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complemented by an investigation of the hydrogen desorption process from various phases using temperature programmed desorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core level shifts match very well with those predicted by density functional theory based on established structural models. In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core level shift of surface Ir atoms is altered by +0.17eV for each Ir–H bond formed. In the submonolayer regime we find evidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identify four different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hex phase also four surface components are identified, which undergo a rather rigid core level shift of +0.15eV upon hydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms. Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogen independent of the total coverage. We conclude that the surface always separates into patches of fully covered deconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with the desorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts into the reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption for all phases to be θmax1×1−H=1.0ML, θmax5×1−H=0.8ML, and θmax5×1−hex−H≥1.0ML.

Size- and structure-controlled mono- and bimetallic Ir–Pd nanoparticles in selective ring opening of indan

Nearly monodispersed 1.6nm Ir, 2.3nm Pd nanoparticles, 2.7nm Pd(core)–Ir(shell) and 2.2nm Pd–Ir alloys with mixed surface atoms were synthesised in the presence of polyvinylpyrrolidone (PVP) and studied in the atmospheric ring opening of indan. The nanoparticles and supported catalysts were characterised by UV–vis spectroscopy, TGA, TEM, EDX, TPR, XPS, ISS and XANES. The nanoparticle size and structure control allowed insights into the bimetallic catalyst functioning. As soon as two contiguous surface Ir atoms exist on the nanoparticle surface, they display the same catalytic properties, most likely through the dicarbene ring-opening mechanism. Palladium serves only as a dispersing agent in Pd(core)–Ir(shell) structures providing 100% Ir dispersion, or as an inert surface diluting metal in Pd–Ir formulations with mixed nanoparticle surface. The Pd–Ir core–shell structure allows maximum selective ring-opening yield, which is 19% higher than that for the industrial Pt–Ir catalyst.

Spontaneous symmetry breaking of theIr(100)−(5×1)−hexsurface induced by hydrogen adsorption

A structural study by quantitative low-energy electron diffraction (LEED) and density functional theory (DFT) has been performed on the $\mathrm{Ir}(100)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}1)\text{\ensuremath{-}}\mathrm{hex}$ surface with $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ of hydrogen adsorbed at low temperature $(&lt;180\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The theory-experiment fit of LEED intensities based on calculations applying the two mirror symmetry planes of the uncovered surface is not satisfactory. Instead, the real structure has a unit cell with only one mirror plane yielding an excellent $R$ factor $({R}_{P}=0.16)$. The DFT investigation of the energetics also confirms this spontaneous symmetry breaking, yields quantitatively the same structure for the substrate as the LEED analysis and, moreover, retrieves the hydrogen positions. It was found that adsorption of hydrogen leads to an asymmetric increase of the buckling of the top hexagonal layer, with one of the previously protruding surface Ir atoms extruded even more by about $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. This may be regarded as a precursor of the temperature-activated phase transition above $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to a structure in which the most protruding atom of the $(5\ifmmode\times\else\texttimes\fi{}1)$ surface unit cell is ejected from the quasihexagonal top layer. These ejected atoms form single-atom-wide iridium wires on the remaining atoms, which rearrange to form a bulklike fcc(100) layer, leading to a complete lifting of the quasihexagonal reconstruction.

NO restructuring of surface Ir and bond formation to preadsorbed O on Ir{1 0 0} at 95 K

RAIRS experiments and density functional theory calculations have been performed to understand the adsorption behavior of nitric oxide on different phases of Ir{100} at 95K, namely, Ir{100}-(1×1), Ir{100}-(1×5), and O-precovered Ir{100}-(1×1) surfaces. Molecular NO adsorption is observed on all the phases and the adsorption structures of NO on the surfaces are determined. Mixed isotope experiments verify that no (NO)2 dimer is formed on the surfaces. Importantly, at an NO coverage of ∼0.3ML, a local lifting of reconstruction from the (1×5) to the (1×1) phase occurs, which is found to be driven by the preferential adsorption of NO on the (1×1) phase. On the O-precovered Ir{100} surface, an unusual NOδ+ species is identified, featuring a stretching frequency 30–60cm−1 higher than the NO gas phase frequency. This NOδ+ species is suggested to be NO bonded to preadsorbed O atoms rather than surface Ir atoms.

Kinetics of Electron Transfer and Oxygen Evolution in the Reaction of [Ru(bpy)3]3+ with Colloidal Iridium Oxide

The kinetics of electron transfer and oxygen evolution at citrate-stabilized IrOx·nH2O colloids were studied by time-resolved UV−visible spectrosopy and by steady-state photolysis of [Ru(bpy)3]2+ (bpy = 2,2‘-bipyridyl) and persulfate in a hexafluorosilicate/bicarbonate buffer. Time-resolved studies of the reaction of [Ru(bpy)3]3+ with these colloids show an initial fast electron transfer, corresponding to oxidation of Ir(III) to Ir(IV). Further oxidation of surface Ir atoms occurs concomitantly with oxygen evolution with a second-order rate constant of 1.3 × 106 M-1 s-1. Both the time-resolved reduction of [Ru(bpy)3]3+ by IrOx·nH2O and the photocatalytic oxygen evolution under non-light-limited photolysis conditions have a H/D kinetic isotope effect (KIE) of 1.0. This contrasts with significantly higher KIE values for oxygen evolution from molecular cis,cis-[(bpy)2Ru(OH2)]2O]4+ and [(terpy)(H2O)MnIII(O)2(OH2)terpy)]3+ water oxidation catalysts. This is consistent with the conclusion that, under the condit...

Hydrogenation of benzene and toluene over Ir particles supported on γ-Al 2O 3

Reactions of hydrogenation of the benzene and toluene, in the gas phase, were studied at 60 °C on Ir/γ-Al 2O 3 catalysts of wide dispersion. In the particle size range 3.0–2.0 nm, it was found that the turnover frequency (TOF) values of both reactions were nearly constant with average values of 101.0×10 −3 s −1 for benzene hydrogenation, and 19.3×10 −3 s −1 for toluene hydrogenation. These values were nearly the same as those obtained on Ir/SiO 2 catalysts (in the particle size range 7.0–1.0 nm). The agreement of these results obtained on different supports was an important evidence of the structure insensitivity of the two reactions. On the other hand, it was observed that the Ir/γ-Al 2O 3 catalysts with Ir particle size below 2.0 nm presented a decreasing in the TOF values of both two reactions (up to one order of magnitude) in comparison with the values obtained on Ir/SiO 2 catalysts. This decrease was attributed to the existence of an interaction between Ir atoms and oxygen ions of the alumina that increases the bond strength of chemisorbed unsaturated hydrocarbon, rendering inactive a fraction of surface Ir atoms. In the particle size range 2.0–1.0 nm, only about 50% of the accessible surface Ir atoms were considered as active sites, while for particle size less than or equal to 1.0 nm, about 20–10% of the accessible surface Ir atoms were considered as active sites. It was concluded that there are two types of adsorbed benzene and toluene: a non-reactive form which remains adsorbed on the catalyst without reaction and a reactive one that is hydrogenated.

Origin and consequences of aromatic back-bonding at a transition metal surface: Benzyne on Ir{100}

Recent low energy electron diffraction experiments have shown that partial dissociation of benzene at the Ir{100} surface yields an ordered overlayer of ortho-benzyne radicals (C6H4) with ring-planes inclined at 47.2° to the normal. The primary molecule-surface interaction may be attributed to σ bonds between radical C and surface Ir atoms, but this characteristic alone does not uniquely constrain the adsorbate to the observed orientation. Through first-principles density functional theory which gives good agreement with the experimental structure, we now demonstrate that the molecular tilt arises due to interaction of the aromatic π orbitals with the surface d orbitals, and not because of any dispersive intermolecular forces.

Physicochemical and catalytic properties of alumoiridium systems modified by indium in the dehydrogenation of normal higher paraffins

A study has been made of monometallic and In promoted alumoiridium catalysts for the dehydrogenation of the normal higher paraffins. It has been established that the introduction of In increases the catalytic activity and selectivity with respect to higher olefins. According to IR spectra of adsorbed CO it has been shown that introduction of In modifies the Ir surface possibly due to the formation of mixed clusters. In this process there is a decrease in the number of surface Ir atoms capable of adsorbing H2 at 300 K but a significant increase in the number of centers for high-temperature dissociative adsorption of H2 on the carrier. Analogous effects were observed earlier for alumoplatinum catalysts modified by In.

The effect of reduction conditions on the chloride content of [formula omitted] catalysts and their activity for benzene hydrogenation

Reduction of K 3IrCl 6, on TiO 2 occurs in two well-defined stages, the first of which occurs between 350 and 470 K and consumes an amount of H 2 independent of Ir content. No HCl is released during this stage. It is suggested that IrCl 6 3− ions interact with TiOH groups to form a monolayer of IrCl x (OTi) 6− x species which on reduction afford highly dispersed Ir particles (1–2.5 nm), and that HCl formed by elimination and during reduction chlorinates other TiOH groups. K 3IrCl 6 in excess of that required to form a monolayer exists as less reducible microcrystals leading to larger Ir particles on reduction. Complete removal of Cl − under H 2 cannot be achieved. The effect of precalcination, and of oxidation after reduction, on the Cl − content has been examined. Residual Cl − affects the O 2 titration of chemisorbed H, giving anomalously low results for the number of surface Ir atoms. Resulting turnover frequencies (TOF) are therefore unreliable, but Ir TiO 2 reduced at low temperature shows specific activity for benzene hydrogenation comparable to that for Ir SiO 2 . Low TOF's for Ir TiO 2 after high-temperature reduction are attributed to crowding of free sites by TiO 2− x , species. The role of Cl − in TiO 2-supported catalysts needs a complete reappraisal.

The chemisorption of carbon monoxide on the iridium (III) surface

The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.