Surface Oxygen Sites Research Articles

Magnetic adsorbents from co‐pyrolysis of non‐woody biomass and red mud for water decontamination

AbstractRed mud (RM) and non‐woody biomass are both underutilized resources for renewable composite materials, which could be used in environmental decontamination processes. This study aims to investigate the efficacy of co‐pyrolyzing non‐woody biomass with RM to produce a magnetic biochar composite. When pyrolyzed, RM is reduced to magnetic iron while the non‐woody biochar is responsible for the adsorption of organic compounds. Ibuprofen, acetaminophen, methyl orange, and methylene blue were used as test compounds to investigate the overall adsorptive capacity of the composite and to determine the possible adsorption mechanisms of biochar produced from RM pyrolyzed with switch grass, phragmites, rice husk, and miscanthus. The composite produced from a 1 to 1 mixture of RM and miscanthus showed the highest adsorption capacity with 13.8 and 8.34 mg/g of ibuprofen and acetaminophen adsorbed, respectively, which is attributed to its greater ‐interactions as a result of lower surface oxygen sites. Different ratios of RM to biomass were also tested for the production of the miscanthus composite, where it was found that the 1:2 ratio showed the best overall adsorption with 25.9 mg/g removal of acetaminophen, surpassing the miscanthus biochar's at 17.9 mg/g.

Graphitic nanoflakes modulate the structure and binding of human amylin.

Human amylin is an inherently disordered protein whose ability to form amyloid fibrils is linked to the onset of type II diabetes. Graphitic nanomaterials have potential in managing amyloid diseases as they can disrupt protein aggregation processes in biological settings, but optimising these materials to prevent fibrillation is challenging. Here, we employ bias-exchange molecular dynamics simulations to systematically study the structure and adsorption preferences of amylin on graphitic nanoflakes that vary in their physical dimensions and surface functionalisation. Our findings reveal that nanoflake size and surface oxidation both influence the structure and adsorption preferences of amylin. The purely hydrophobic substrate of pristine graphene (PG) nanoflakes encourages non-specific protein adsorption, leading to unrestricted lateral mobility once amylin adheres to the surface. Particularly on larger PG nanoflakes, this induces structural changes in amylin that may promote fibril formation, such as the loss of native helical content and an increase in β-sheet character. In contrast, oxidised graphene nanoflakes form hydrogen bonds between surface oxygen sites and amylin, and as such restricting protein mobility. Reduced graphene oxide (rGO) flakes, featuring lower amounts of surface oxidation, are amphiphilic and exhibit substantial regions of bare carbon which promote protein binding and reduced conformational flexibility, leading to conservation of the native structure of amylin. In comparison, graphene oxide (GO) nanoflakes, which are predominantly hydrophilic and have a high degree of surface oxidation, facilitate considerable protein structural variability, resulting in substantial contact area between the protein and GO, and subsequent protein unfolding. Our results indicate that tailoring the size, oxygen concentration and surface patterning of graphitic nanoflakes can lead to specific and robust protein binding, ultimately influencing the likelihood of fibril formation. These atomistic insights provide key design considerations for the development of graphitic nanoflakes that can modulate protein aggregation by sequestering protein monomers in the biological environment and inhibit conformational changes linked to amyloid fibril formation.

Pd-Promoted Bi2Zr2O7 Pyrochlore for Soot Particulate combustion: the Impact of Pd Distributing Location and Valence State on the Reactivity

In this study, Pd has been adopted to modify Bi2Zr2O7 pyrochlore on the surface (Pd/Bi2Zr2O7) or in the bulk (Pd-Bi2Zr2O7) by using different methods. Compared with the unmodified Bi2Zr2O7, the addition of Pd significantly improves the reactivity, following the sequence of Bi2Zr2O7 < Pd-Bi2Zr2O7 < Pd/Bi2Zr2O7. For Pd/Bi2Zr2O7, both PdO and metallic Pd0 are formed and located mainly on its surface, which is favorable for the activity. In contrast, for Pd-Bi2Zr2O7, the Pd species are majorly doped into the pyrochlore matrix at B-site as Pd2+ and Pd4+, which in fact restricts the activity. Furthermore, Pd modification can promote the generation of active O2− and O22- species, especially when Pd locates on the surface. As discovered, the distribution locations and valence states of Pd, together with the number of reactive surface oxygen sites, determines the activity.

Triggered lattice-oxygen oxidation with active-site generation and self-termination of surface reconstruction during water oxidation.

To master the activation law and mechanism of surface lattice oxygen for the oxygen evolution reaction (OER) is critical for the development of efficient water electrolysis. Herein, we propose a strategy for triggering lattice-oxygen oxidation and enabling non-concerted proton-electron transfers during OER conditions by substituting Al in La0.3Sr0.7CoO3-δ. According to our experimental data and density functional theory calculations, the substitution of Al can have a dual effect of promoting surface reconstruction into active Co oxyhydroxides and activating deprotonation on the reconstructed oxyhydroxide, inducing negatively charged oxygen as an active site. This leads to a significant improvement in the OER activity. Additionally, Al dopants facilitate the preoxidation of active cobalt metal, which introduces great structural flexibility due to elevated O 2p levels. As OER progresses, the accumulation of oxygen vacancies and lattice-oxygen oxidation on the catalyst surface leads to the termination of Al3+ leaching, thereby preventing further reconstruction. We have demonstrated a promising approach to achieving tunable electrochemical reconstruction by optimizing the electronic structure and gained a fundamental understanding of the activation mechanism of surface oxygen sites.

Significant effect of In doping on Ni/C catalyst for hydrogenation of dimethyl oxalate to ethanol: Synergistic effect of InNi3 alloy, surface oxygen, and defect sites

The Ni-In/C catalyst, prepared using a one-pot method, demonstrates superior catalytic performance compared to the Ni/C catalyst in the hydrogenation of dimethyl oxalate (DMO) to ethanol (EtOH). The introduction of doping results in the formation of defect sites and InNi3 alloy, enhancing both the adsorption of DMO and the activation of H2. Additionally, the presence of -OH groups (acting as Bronsted acid sites) on the carbon surface activates C-O, thereby effectively promoting the deep hydrogenation of intermediate products to produce EtOH. The catalyst exhibits remarkable industrial potential by achieving high ethanol yield through a straightforward preparation method, while maintaining low Ni content.

Enhanced Performance of Recovered Cathode Materials with Fluorine Impurity in Hydrometallurgy Recycling

With the growing lithium-ion battery (LIB) industry since its initial inception in the late 20th century, battery recycling is seen as an end-of-pipe process to resolve environmental and economic issues generated by the accumulation of waste batteries. In recent years, an increasing attention has been paid to the hydrometallurgy recycling due to its high recovery efficiency, low energy cost, as well as large-scale production capability. In order to achieve a high-quality recovered material, many resources have been poured into this field trying to develop better design and control of synthesis process. The impurities introduced in the recycling process is an important factor which could have unintended impacts on the final cathode product in a variety of aspects. The presence of fluoride ions in hydrometallurgy recycling, as one possible anion impurity which is released from the LiPF6 in electrolyte, aroused our research interest. In this work, the influence of the fluorine impurity on the recovered LiNi0.6Co0.2Mn0.2O2 (“NCM622”) cathode material via hydrometallurgy method is systematically investigated. The results show that the addition of fluorine impurity in the hydrometallurgy process improves the electrochemical properties of the recovered cathode materials.It is found that the fluorine impurity has lots of positive impacts on the synthesized cathode materials. First of all, the equilibrium of co-precipitation reaction is affected by the fluoride ions, resulting in the formation of holes in hydroxide precipitates. After sintering, cathode particles preserve a hollow structure which leads to an improved rate capability and cycle stability. In addition, surface oxygen sites of cathode materials are found to be substituted by impurity fluoride ions. As a result, an increased surface Ni2+ concentration contributes to a better cathode surface stability as well as a higher reversible capacity. The inclusion of fluoride (<1 at%) in cathode lattice is also beneficial to the bulk lithium-ion diffusion. Not only that, but the fluorine impurity has no clear detrimental effects on the particle morphology, crystal structure, transition metal composition, and element distribution of the cathode materials. Therefore, the electrochemical performance of synthesized cathodes has achieved different levels of improvement due to those positive influences from the fluorine impurity. Specifically, cathode with 0.2 at% impurity exhibits a discharge capacity of ~168 mAh/g with a remarkable retention of 98% at 1/3C after 100 cycles, about 8% higher than the capacity of virgin benchmark. Also, rate test outcomes of fluorine-affected samples are superior to that of virgin standard. In summary, the electrochemical properties of recovered cathode via hydrometallurgy process can be boosted by fluorine impurity. Unlike adverse impurities, fluoride’s positive role in hydro-recycling indicates that it is no need to strictly control them in pre-treatment steps. Figure 1

Engineering Active Surface Oxygen Sites of Cubic Perovskite Cobalt Oxides toward Catalytic Oxidation Reactions

Engineering Active Surface Oxygen Sites of Cubic Perovskite Cobalt Oxides toward Catalytic Oxidation Reactions

Interaction of Atomic Deuterium with Rutile TiO2(011)-(2×1)

Inspired by the significance of hydrogen–solid interaction in hydrogen energy and catalysis, adsorption, diffusion, and desorption behaviors of deuterium atoms in rutile TiO2(011) have been investigated by temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS). Upon exposure, a small portion of D atoms adsorb at surface oxygen sites, resulting in the band gap states at 1.35 eV below the Fermi level and desorbing as water at ∼400 K. Most of the D atoms will diffuse into the bulk due to the relatively low activation barrier and the huge capacity of the solid material. These bulk D species desorb as D2/HD between 500 and 800 K. While the desorbing D2O from surface hydroxyls saturates at ∼0.10 monolayer (ML), the yielding D2 is about 96 ML (equivalent coverage) at the largest atomic D exposure of 4.54 langmuir and no saturation trend has been observed in the present work. Detailed analysis indicates the bulk D will diffuse back to the surface and recombine as D2 at elevated temperatures. The differences between the behavior of H(D) in rutile TiO2(110) and TiO2(011) have been discussed by considering the presence of additional bridging oxygen atoms between the in-plane and topmost ones on the latter surface. The striking finding that most surface D atoms diffuse into the bulk of rutile TiO2(011) will not only broaden our understanding of the interaction of H/D with the prototypical metal oxide material but also provide clues to investigate the mechanism of H/D involving reactions over TiO2 catalysts, for example, hydrogen evolution and hydrogenation.

Three-in-one Ni doped porous SnO2 nanorods sensor: Controllable oxygen vacancies content, surface site activation and low power consumption for highly selective NO2 monitoring

Introducing oxygen vacancies and surface site activation is an effective method to enhance metal oxide gas sensing. However, the regulation of oxygen vacancy concentration within metal oxides and the controllable design of surface sites activation under mild conditions are critical but challenging techniques. Here, we demonstrate a simple and efficient solvothermal concomitant annealing method to obtain Ni doped SnO2 nanorods with controllable oxygen vacancies content, surface sites activation and unique morphology. The 10%Ni doped SnO2 nanorods sensor showed excellent trace NO2 sensitivity at low power consumption, and lowest detection limit is 20 ppb, which is nearly 4 times lower than that of pure SnO2. DFT calculation showed that the bridging oxygen vacancies as the main surface site activation and facilitate the adsorption of NO2, thereby significantly improved the NO2 sensitivity. This work undoubtedly opens up new avenue to exploring and designing advanced metal oxide sensitive materials with controllable oxygen vacancy concentration and surface sites activation.

Morphology effect of ceria support with hierarchical structure on the catalytic performance for nickel-based catalysts in dry reforming of methane

CeO2 is widely used as Ni-based catalyst support in methane dry reforming (DRM), and the hierarchical structure of the ceria behaves a favorable effect on the performance of the catalyst. Ceria with different morphologies, including dumbbell (CeO2-D), bouquet (CeO2-B) and shuttle (CeO2-S) shapes, are hydrothermally prepared, and the effect of ceria shape on the catalytic performance of Ni/CeO2 in DRM is investigated by a fixed-bed tubular reactor. The results indicate that Ni/CeO2-D demonstrates the best reaction conversions & TOF value, whose conversions of CH4 & CO2 can reach 92% & 87%, respectively. Furthermore, a series of characterization results (XPS, H2-TPR and Raman) confirm that the Ni/CeO2-D own the abundant oxygen vacancies and strong metal-support interaction, which promote the reactivity of CH4 and CO2, resulting in its most prominent catalytic performance. Additionally, Ni/CeO2-D possesses other distinct advantages due to its hierarchical dumbbell-like structure confirmed by high Ni dispersion (H2-TPD), abundant surface oxygen (H2-TPR) and sufficient basic sites (CO2-TPD).

Biowaste-Derived, Highly Efficient, Reusable Carbon Nanospheres for Speedy Removal of Organic Dyes from Aqueous Solutions.

The current work explores the adsorptive efficiency of carbon nanospheres (CNSs) derived from oil palm leaves (OPL) that are a source of biowaste. CNSs were synthesized at 400, 600, 800 and 1000 °C, and those obtained at 1000 °C demonstrated maximum removal efficiency of ~91% for malachite green (MG). Physicochemical and microscopic characteristics were analysed by FESEM, TEM, FTIR, Raman, TGA and XPS studies. The presence of surface oxygen sites and the porosity of CNSs synergistically influenced the speed of removal of MG, brilliant green (BG) and Congo red (CR) dyes. With a minimal adsorbent dosage (1 mg) and minimum contact time (10 min), and under different pH conditions, adsorption was efficient and cost-effective (nearly 99, 91 and 88% for BG, MG and CR, respectively). The maximum adsorption capacities of OPL-based CNSs for BG were 500 and 104.16 mg/g for MG and 25.77 mg/g for CR. Adsorption isotherms (Freundlich, Langmuir and Temkin) and kinetics models (pseudo-first-order, pseudo-second-order and Elovich) for the adsorption processes of all three dyes on the CNSs were explored in detail. BG and CR adsorption the Freundlich isotherm best, while MG showed a best fit to the Temkin model. Adsorption kinetics of all three dyes followed a pseudo-second-order model. A reusability study was conducted to evaluate the effectiveness of CNSs in removing the MG dye and showed ~92% efficiency even after several cycles. Highly efficient CNSs with surface oxygen groups and speedy removal of organic dyes within 10 min by CNSs are highlighted in this paper.

Transformation of chromium and its removal by Fe2O3 during the thermal disposal of municipal solid waste: A study based on density functional theory

Long-term exposure to chromium released during the thermal disposal of municipal solid waste (MSW) poses carcinogenic risks because of its volatility and biological toxicity. Experimental results showed that there was a strong interaction between γ-Fe2O3 and Cr species, but the specific mechanism of the interaction was unclear. In this study, reaction kinetics and density functional theory (DFT) were used to explore the homogeneous transformation pattern of Cr in flue gas and its adsorption on γ-Fe2O3 during the thermal disposal of MSW. It was found that H2O promoted the rapid transformation of CrOOH released from MSW into higher valence species such as CrO2(OH) and CrO2(OH)2. It was subsequently chlorinated by HCl to CrO2Cl and CrO2Cl2. During the heterogeneous reaction process, the adsorption energy of CrOOH, CrO2(OH), and CrO2Cl on the surface of γ-Fe2O3 (001) was CrOOH > CrO2(OH) > CrO2Cl. It mainly acted on surface oxygen sites. The adsorption energy and electron-loss capacity of CrOOH on the surface of γ-Fe2O3 (001) were enhanced with H2O. When HCl was present, although the adsorption energy of Cr on the surface of γ-Fe2O3 (001) was increased, the electron-losing ability was weakened and there was a tendency to be reduced. As for chlorination of CrOOH by HCl on the surface of γ-Fe2O3 (001), this process had two stages. In the first stage, CrOOH reacted with H2O to form CrO2(OH). In the second stage, CrO2(OH) was chlorinated into high-valence chlorides and chemically adsorbed on the surface of γ-Fe2O3 (001).

Effect of acidic components (SO42- and WO3) on the surface acidity, redox ability and NH3-SCR activity of new CeO2-TiO2 nanoporous aerogel catalysts: A comparative study

• New CeTiSO 4 2- and WO 3 -CeTi nanoporous aerogel catalysts for NH 3 -SCR reaction. • SO 4 2- modifies the nature of Ce species and increases the acidity of the CeTi aerogel. • WO 3 generates more reactive surface oxygen and active redox sites at CeTi surface. • WO 3 -CeTi displays the highest SCR activity at 320–400 °C (NO conversion > 90 %) • CeTiSO 4 2- exhibits superior SCR performance at 450–500 °C (NO conversion > 90 %) Selective catalytic reduction (SCR) of nitrogen oxide (NO) by ammonia (NH 3 ) was studied in this work over new sulfate (SO 4 2- ) or tungsta (WO 3 ) modified CeO 2 -TiO 2 aerogel catalysts. The catalytic systems were elaborated via sol gel method then characterized by: XRD, N 2 -Physisorption at 77 K, DRUV–vis, NH 3 -TPD and H 2 -TPR. It was revealed that the nature of acidic components influences differently the texture, surface oxygen concentration, acidity, reducibility and NH 3 -SCR activity of new Ce-based catalysts. Hence, the incorporation of sulfate modifies the nature of Ce species, improves their dispersion through the Ce-SO 4 2- interactions and, particularly, generates new strong acid sites which display superior catalytic performance at high temperature NO reduction (NO conversion into N 2 > 90 % between 450 and 500 °C over CeO 2 -TiO 2 -SO 4 2- catalyst). However, the addition of tungsta affects slightly the surface acidity of CeO 2 -TiO 2 catalyst but it induces the creation of more reactive surface oxygen and new redox sites at its surface (mainly due to the existence of W-Ce interactions) leading to highly active WO 3 -CeO 2 -TiO 2 system for the low temperature NH 3 -SCR reaction with above 90 % NO conversion into N 2 between 320 and 400 °C.

Catalytic oxidation of chlorinated aromatics over Fe-based oxide catalysts modified by Mn

FexMny mixed oxide catalysts prepared by co-precipitation method are applied in catalytic combustion of chlorobenzene (CB) and 1,2-dichlorobenzene (1,2-DCB). Characterization shows that Mn species of Fe9Mn1highly disperse into α-Fe2O3 as Fe-O-Mn structure. With the increase in Mn content, a part of Mn is separated as MnO2 and α-Mn2O3. Fe-O-Mn structure greatly increases the acidity (especially for strong acid) and reducibility of α-Fe2O3. In CB and 1,2-DCB oxidations, FexMny catalysts are active, of which Fe9Mn1 catalyst presents the highest activity. At 5% – 20% O2 and 500 – 2000 ppm 1,2-DCB at space velocity of 15,000 mL g-1h−1, the apparent rates at 300 °C over Fe9Mn1 are estimated to be 0.056 – 0.112 μmol min−1 m−2, and the temperatures needed for 50% and 90% conversions are 294 – 342 °C and 346 – 375 °C, respectively. High availabilities of surface oxygen and acid sites are related to effective Cl removal in aid of active hydroxyls produced during the reaction (confirmed by in situ FT-IR tests), which are responsible for high activity of Fe9Mn1 catalyst. For the catalysts with MnO2 and Mn2O3 domains on the surface, Cl strong adsorption results in their deactivation. Moreover, Fe9Mn1 catalyst shows high stability at 350 °C on feed stream containing 1000 ppm 1,2-DCB within 600 min duration or for five runs. The selectivity for CO/CO2 approaches 98% or higher and no chlorination products are detected within FID limitation (1 ppm) under the above mentioned broad reaction conditions, which are highly desirable for industrial application.

CO2 reduction on a nanostructured La0.5Ba0.5CoO3 perovskite: Electrochemical characterization and DFT calculations

A La0.5Ba0.5CoO3 perovskite was evaluated as a catalyst for the electrochemical reduction of CO2. Nanocrystalline structures of La0.5Ba0.5CoO3 were synthesized using polycarbonate porous templates by microwave irradiation. Electrochemical characterization was carried out by linear sweep voltammetry, cyclic voltammetry and chronoamperometry. The electroreduction of CO2 produced HCOOH with a faradaic efficiency of 30% at − 0.9 V vs SHE as determined by ionic chromatography. The rotating ring-disc electrode configuration was used to further study the electroreduction processes. A faradaic efficiency of 85% at − 1.2 V vs SHE attributed to CO, was obtained from the disk and ring currents. In parallel, the adsorption process of CO2 and plausible reduction pathways to HCOOH and CO were studied by DFT calculation on the (001) and (110) orientations of the perovskite. The CO2 molecule strongly adsorbs at the oxygen surface sites through the C atom in a bent configuration, determining the *COOH species as the only possible intermediate. The formation and release of HCOOH becomes feasible under an applied potential of − 0.8 V and − 0.7 V vs SHE for the (001) and (110) orientations, respectively. On the other hand, the calculation showed that the release of CO is only feasible at the (001) surface plane for potentials below − 0.9 V vs SHE. The calculated applied potentials are in good agreement, both in trend and magnitude, with the experimental observations despite the model approximation of neglecting solvent effects.

Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy

The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and thus the key to develop better materials for related applications. However, experimental methods to reveal this information remain scarce. Here we show that 17O solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify specific surface sites active for CO2 adsorption on MgO nanosheets. Two 3-coordinated bare surface oxygen sites, resonating at 39 and 42 ppm, are observed, but only the latter is involved in CO2 adsorption. Double resonance NMR and density functional theory (DFT) calculations results prove that the difference between the two species is the close proximity to H, and CO2 does not bind to the oxygen ions with a shorter O···H distance of approx. 3.0 Å. Extensions of this approach to explore adsorption processes on other oxide materials can be readily envisaged.

Reducing the high hydrogen binding strength of vanadium carbide MXene with atomic Pt confinement for high activity toward HER

To construct an efficient electrocatalyst for HER, the high binding energy of MXene must be mitigated through electronic modulation of active sites. Here, we propose atomic Pt substitution in V2CTx MXene to modulate the electronic structure and promote catalytic activity toward HER. Pt–V2CTx exhibits high HER performance with a low overpotential of 27 mV at −10 mA cm−2 in acidic media, comparable to the commercial Pt/C catalyst. X-ray absorption spectroscopy and DFT calculations indicate that the Pt atoms are efficiently confined to the V vacancy sites of V2CTx, accompanied by a unique electronic structure. The atomic substitution of Pt with higher occupied d states at the Fermi energy of Pt and surface oxygen sites can significantly indicate an optimum hydrogen-binding free energy (∆GH*), promoting HER performance. This work introduces further prospects for developing efficient electrocatalysts by feasible electronic regulation and highly improved catalytic activity through rational atomic engineering.

Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO2-Catalyzed CO2 Methanation Mechanism.

This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP–XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP–XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2–δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.

Role of surface steps in activation of surface oxygen sites on Ir nanocrystals for oxygen evolution reaction in acidic media

Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrOx formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts.

Acetic acid conversion to ketene on Cu2O(1 0 0): Reaction mechanism deduced from experimental observations and theoretical computations

Ketene, a versatile reagent in production of fine and specialty chemicals, is produced from acetic acid. We investigate the synthesis of ketene from acetic acid over the (3,0;1,1) surface of Cu2O(100) through analysis of the adsorption and desorption characteristics of formic and acetic acids. The results allow us to establish a reaction mechanism for ketene formation. Observations from x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy, and temperature programmed desorption (TPD), supported by a comparison with formic acid results, suggest that acetic acid reacts with Cu2O through deprotonation to form acetate species coordinated to copper sites and hydroxylation of nearby surface oxygen sites. For formic acid the decomposition of adsorbed formate species results in desorption of CO2 and CO while, for acetic acid, high yields of ketene are observed at temperature >500 K. Modeling by density functional theory (DFT) confirms the strong interaction of acetic acid with the (3,0;1,1) surface and the spontaneous dissociation into adsorbed acetate and hydrogen atom species, the latter forming an OH-group. In an identified reaction intermediate ketene binds via all C and O atoms to Cu surface sites, in agreement with interpretations from XPS. In the vicinity of the adsorbate the surface experiences a local reorganization into a c(2 × 2) reconstruction. The total computed energy barrier for ketene formation is 1.81 eV in good agreement with the 1.74 eV obtained from TPD analysis. Our experimental observations and mechanistic DFT studies suggests that Cu2O can operate as an efficient catalyst for the green generation of ketene from acetic acid.