Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation.

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO2 electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO2 surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO2 and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance.

Structural evolution of atomically dispersed Pt catalysts dictates reactivity.

The use of oxide-supported isolated Pt-group metal atoms as catalytic active sites is of interest due to their unique reactivity and efficient metal utilization. However, relationships between the structure of these active sites, their dynamic response to environments and catalytic functionality have proved difficult to experimentally establish. Here, sinter-resistant catalysts where Pt was deposited uniformly as isolated atoms in well-defined locations on anatase TiO2 nanoparticle supports were used to develop such relationships. Through a combination of in situ atomic-resolution microscopy- and spectroscopy-based characterization supported by first-principles calculations it was demonstrated that isolated Pt species can adopt a range of local coordination environments and oxidation states, which evolve in response to varied environmental conditions. The variation in local coordination showed a strong influence on the chemical reactivity and could be exploited to control the catalytic performance.

Insights into Copper Sulfide Formation from Cu and S K edge XAS and DFT studies.

An understanding of the fundamentals of the reaction between CuO with trace amounts of H2S to form CuS products is critical for the optimal utilization of this process in sulfur removal applications. Unfortunately, CuS is a complex material, featuring various Cu2-xS compounds (with 0 ≤ x ≤ 1), distorted crystal phases, and varying electronic structures and coordination environments of Cu and S ions. In this work, we combine ex situ and in situ X-ray absorption spectroscopy (XAS) at S and Cu K edges, fixed bed sorption experiments, DFT simulations, and other characterization techniques to speciate the CuS products formed at different temperatures (298-383 K) and from CuO sorbents with different crystallite sizes (2.8-40 nm). The results of our analysis identify the formation of a distorted CuS layer at the surface of CuO crystals with disulfide groups with shorter Cu-S bonds and higher delocalization of the positive charge of the Cu center into (S1-)2. This distorted CuS layer dominates the XAS signal at lower temperatures (298-323 K) and at the initial stages of sulfidation at higher temperatures (353 and 383 K) where conversion is low (<40%). First-principles atomistic simulations confirm the thermodynamic favorability of the formation of surface (S1-)2 on both CuO (111) and (1̅11) surfaces, providing further support for our experimental observations. Furthermore, these simulations reveal that the presence of disulfide bonds stabilized surface hydroxyl groups, leading to lower Gibbs Free Energies of their surface migration.

Dynamic Reorganization and Confinement of TiIV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes.

The effect of dynamic reorganization and confinement of isolated TiIV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene-TiIV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ∼7 Å, volume ∼185 Å3) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of TiIV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM TiIV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M-2 s-1; whereas within confining type-(ii) 12-MR pockets, there is a ∼5-fold enhancement to 48 ± 8 M-2 s-1. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M-2 s-1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are Δ H‡app = 43 ± 1 kJ mol-1 irrespective of active site location, confining environments exhibit diminished entropic barriers (Δ S‡app = -68 J mol-1 K-1 for unconfined type-(i) vs -56 J mol-1 K-1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ∼ 225 Å3). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.

Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal-Organic Framework.

Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N2O. Pair distribution function analysis, N2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C3H8 + N2O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)═O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mössbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N2 and C3H6 production rates were found to be first-order in N2O partial pressure and zero-order in C3H8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N2O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and  highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).

Understanding Structure-Property Relationships of MoO3-Promoted Rh Catalysts for Syngas Conversion to Alcohols.

Rh-based catalysts have shown promise for the direct conversion of syngas to higher oxygenates. Although improvements in higher oxygenate yield have been achieved by combining Rh with metal oxide promoters, details of the structure of the promoted catalyst and the role of the promoter in enhancing catalytic performance are not well understood. In this work, we show that MoO3-promoted Rh nanoparticles form a novel catalyst structure in which Mo substitutes into the Rh surface, leading to both a 66-fold increase in turnover frequency and an enhancement in oxygenate yield. By applying a combination of atomically controlled synthesis, in situ characterization, and theoretical calculations, we gain an understanding of the promoter-Rh interactions that govern catalytic performance for MoO3-promoted Rh. We use atomic layer deposition to modify Rh nanoparticles with monolayer-precise amounts of MoO3, with a high degree of control over the structure of the catalyst. Through in situ X-ray absorption spectroscopy, we find that the atomic structure of the catalytic surface under reaction conditions consists of Mo-OH species substituted into the surface of the Rh nanoparticles. Using density functional theory calculations, we identify two roles of MoO3: first, the presence of Mo-OH in the catalyst surface enhances CO dissociation and also stabilizes a methanol synthesis pathway not present in the unpromoted catalyst; and second, hydrogen spillover from Mo-OH sites to adsorbed species on the Rh surface enhances hydrogenation rates of reaction intermediates.

Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum.

Pd- and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH4 combustion to CO2, suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed PtxPd1-x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

Engineering of Ruthenium-Iron Oxide Colloidal Heterostructures: Improved Yields in CO2 Hydrogenation to Hydrocarbons.

Catalytic CO2 reduction to fuels and chemicals is a major pursuit in reducing greenhouse gas emissions. One approach utilizes the reverse water-gas shift reaction, followed by Fischer-Tropsch synthesis, and iron is a well-known candidate for this process. Some attempts have been made to modify and improve its reactivity, but resulted in limited success. Now, using ruthenium-iron oxide colloidal heterodimers, close contact between the two phases promotes the reduction of iron oxide via a proximal hydrogen spillover effect, leading to the formation of ruthenium-iron core-shell structures active for the reaction at significantly lower temperatures than in bare iron catalysts. Furthermore, by engineering the iron oxide shell thickness, a fourfold increase in hydrocarbon yield is achieved compared to the heterodimers. This work shows how rational design of colloidal heterostructures can result in materials with significantly improved catalytic performance in CO2 conversion processes.

Synthesis of Colloidal Pd/Au Dilute Alloy Nanocrystals and Their Potential for Selective Catalytic Oxidations.

Selective oxidations are crucial for the creation of valuable chemical building blocks but often require expensive and unstable stoichiometric oxidants such as hydroperoxides and peracids. To date, many catalysts that contain a single type of active site have not been able to attain the desired level of selectivity for partially oxidized products over total combustion. However, catalysts containing multiple types of active sites have proven to be successful for selective reactions. One category of such catalysts is bimetallic alloys, in which catalytic activity and selectivity can be tuned by modifying the surface composition. Traditional catalyst synthesis methods using impregnation struggle to create catalysts with sufficient control over surface chemistry to accurately tune the ensemble size of the desired active sites. Here we describe the synthesis of colloidal nanocrystals of dilute alloys of palladium and gold. We show that when supported on titania (TiO2), tuning the composition of the Pd/Au nanocrystal surface provides a synergistic effect in the selective oxidation of 2-propanol to acetone in the presence of H2 and O2. In particular, we show that certain Pd/Au surface ratios exhibit activity and selectivity far superior to Pd or Au individually. Through precise structural characterization we demonstrate that isolated atoms of Pd exist in the most active catalysts. The synergy between isolated Pd atoms and Au allows for the formation of reactive oxidizing species, likely hydroperoxide groups, responsible for selective oxidation while limiting oxygen dissociation and, thus, complete combustion. This work opens the way to more efficient utilization of scarce noble metals and new options for catalyzed selective oxidations.

Low-Temperature Restructuring of CeO2-Supported Ru Nanoparticles Determines Selectivity in CO2 Catalytic Reduction.

CO2 reduction to higher value products is a promising way to produce fuels and key chemical building blocks while reducing CO2 emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water-gas shift (RWGS) reaction. Describing catalyst features that control the selectivity of these two pathways is important to determine the formation of specific products. At the same time, identification of morphological changes occurring to catalysts under reaction conditions can be crucial to tune their catalytic performance. In this contribution we investigate the dependency of selectivity for CO2 reduction on the size of Ru nanoparticles (NPs) and on support. We find that even at rather low temperatures (210 °C), oxidative pretreatment induces redispersion of Ru NPs supported on CeO2 and leads to a complete switch in the performance of this material from a well-known selective methanation catalyst to an active and selective RWGS catalyst. By utilizing in situ X-ray absorption spectroscopy, we demonstrate that the low-temperature redispersion process occurs via decomposition of the metal oxide phase with size-dependent kinetics, producing stable single-site RuO x/CeO2 species strongly bound to the CeO2 support that are remarkably selective for CO production. These results show that reaction selectivity can be heavily dependent on catalyst structure and that structural changes of the catalyst can occur even at low temperatures and can go unseen in materials with less defined structures.

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis.

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Selective catalytic reduction of NO over Fe-ZSM-5: mechanistic insights by operando HERFD-XANES and valence-to-core X-ray emission spectroscopy.

An in-depth understanding of the active site requires advanced operando techniques and the preparation of defined catalysts. We elucidate here the mechanism of the selective catalytic reduction of NO by NH3 (NH3-SCR) over a Fe-ZSM-5 zeolite catalyst. 1.3 wt % Fe-ZSM-5 with low nuclearity Fe sites was synthesized, tested in the SCR reaction and characterized by UV-vis, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Next, this defined Fe-zeolite catalyst was studied by complementary high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (V2C XES) under different model in situ and realistic working (operando) conditions identical to the catalyst test bench including the presence of water vapor. HERFD-XANES uncovered that the coordination (between 4 and 5), geometry (tetrahedral, partly 5-fold), and oxidation state of the Fe centers (reduced in NH3, partly in SCR mixture, slight reduction in NO) strongly changed. V2C XES supported by DFT calculations provided important insight into the chemical nature of the species adsorbed on Fe sites. The unique combination of techniques applied under realistic reaction conditions and the corresponding catalytic data unraveled the adsorption of ammonia via oxygen on the iron site. The derived reaction model supports a mechanism where adsorbed NOx reacts with ammonia coordinated to the Fe(3+) site yielding Fe(2+) whose reoxidation is slow.

On the Cobalt Carbide Formation in a Co/TiO2 Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction.

Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO2 Fischer-Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO2 catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.

Directing reaction pathways via in situ control of active site geometries in PdAu single-atom alloy catalysts.

The atomic scale structure of the active sites in heterogeneous catalysts is central to their reactivity and selectivity. Therefore, understanding active site stability and evolution under different reaction conditions is key to the design of efficient and robust catalysts. Herein we describe theoretical calculations which predict that carbon monoxide can be used to stabilize different active site geometries in bimetallic alloys and then demonstrate experimentally that the same PdAu bimetallic catalyst can be transitioned between a single-atom alloy and a Pd cluster phase. Each state of the catalyst exhibits distinct selectivity for the dehydrogenation of ethanol reaction with the single-atom alloy phase exhibiting high selectivity to acetaldehyde and hydrogen versus a range of products from Pd clusters. First-principles based Monte Carlo calculations explain the origin of this active site ensemble size tuning effect, and this work serves as a demonstration of what should be a general phenomenon that enables in situ control over catalyst selectivity.

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction.

Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO2 to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu0.7Zn0.3 nanoparticles supported on ZnO/Al2O3, γ-Al2O3 and SiO2 to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al2O3 and CuZn/SiO2, but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO2. Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO2. In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity.

Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports.

Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.