Data-Centric Heterogeneous Catalysis: Identifying Rules and Materials Genes of Alkane Selective Oxidation.

Artificial intelligence (AI) can accelerate catalyst design by identifying key physicochemical descriptive parameters correlated with the underlying processes triggering, favoring, or hindering the performance. In analogy to genes in biology, these parameters might be called "materials genes" of heterogeneous catalysis. However, widely used AI methods require big data, and only the smallest part of the available data meets the quality requirement for data-efficient AI. Here, we use rigorous experimental procedures, designed to consistently take into account the kinetics of the catalyst active states formation, to measure 55 physicochemical parameters as well as the reactivity of 12 catalysts toward ethane, propane, and n-butane oxidation reactions. These materials are based on vanadium or manganese redox-active elements and present diverse phase compositions, crystallinities, and catalytic behaviors. By applying the sure-independence-screening-and-sparsifying-operator symbolic-regression approach to the consistent data set, we identify nonlinear property-function relationships depending on several key parameters and reflecting the intricate interplay of processes that govern the formation of olefins and oxygenates: local transport, site isolation, surface redox activity, adsorption, and the material dynamical restructuring under reaction conditions. These processes are captured by parameters derived from N2 adsorption, X-ray photoelectron spectroscopy (XPS), and near-ambient-pressure in situ XPS. The data-centric approach indicates the most relevant characterization techniques to be used for catalyst design and provides "rules" on how the catalyst properties may be tuned in order to achieve the desired performance.

Dynamics over a Cu-graphite electrode during the gas-phase CO2 reduction investigated by APXPS.

The electrocatalytic conversion of CO2 to fuels and chemicals using renewable energy is a key decarbonization technology. From a technological viewpoint, the realization of such process in the gas phase and at room temperature is considered advantageous as it allows one to circumvent the limited CO2 solubility in liquid electrolytes and CO2 transport across the electrical double layer. Yet, electrocatalysts' performances reported so far are promising but not satisfactory. To inform the design of new materials, in this study, we apply ambient pressure X-ray photoelectron and absorption spectroscopies coupled with on-line gas detection via mass spectrometry to investigate in situ performance and interface chemistry of an electrodeposited Cu on graphitic carbon support under conditions of CO2 reduction. We use the ISISS beamline at the synchrotron facility BESSY II of the HZB and the electrochemical cell based on polymeric electrolyte membrane previously developed. We show that under cathodic potential in which methanol is formed, a fraction of the electrode with a predominantly Cu(I) electronic structure undergoes reduction to metallic Cu. The C speciation is characterized by C-O and sp3 CH3 species whereas no atomic C was formed under this condition. We also show the important role of water in the formation of methanol from accumulated surface CH3 species.

The rise of electrochemical NAPXPS operated in the soft X-ray regime exemplified by the oxygen evolution reaction on IrOx electrocatalysts.

Photoelectron spectroscopy offers detailed information about the electronic structure and chemical composition of surfaces, owing to the short distance that the photoelectrons can escape from a dense medium. Unfortunately, photoelectron based spectroscopies are not directly compatible with the liquids required to investigate electrochemical processes, especially in the soft X-ray regime. To overcome this issue, different approaches based on photoelectron spectroscopy have been developed in our group over the last few years. The performance and the degree of information provided by these approaches are compared with those of the well established bulk sensitive spectroscopic approach of total fluorescence yield detection, where the surface information gained from this approach is enhanced using samples with large surface to bulk ratios. The operation of these approaches is exemplified and compared using the oxygen evolution reaction on IrOx catalysts. We found that all the approaches, if properly applied, provide similar information about surface oxygen speciation. However, using resonant photoemission spectroscopy, we were able to prove that speciation is more involved and complex than previously thought during the oxygen evolution reaction on IrOx based electrocatalysts. We found that the electrified solid-liquid interface is composed of different oxygen species, where the terminal oxygen atoms on iridium are the active species, yielding the formation of peroxo species and, finally, dioxygen as the reaction product. Thus, the oxygen-oxygen bond formation is dominated by peroxo species formation along the reaction pathway. Furthermore, the methodologies discussed here open up opportunities to investigate electrified solid-liquid interfaces in a multitude of electrochemical processes with unprecedented speciation capabilities, which are not accessible by one-dimensional X-ray spectroscopies.

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.

Materials genes of heterogeneous catalysis from clean experiments and artificial intelligence.

ABSTRACT: The performance in heterogeneous catalysis is an example of a complex materials function, governed by an intricate interplay of several processes (e.g., the different surface chemical reactions, and the dynamic restructuring of the catalyst material at reaction conditions). Modeling the full catalytic progression via first-principles statistical mechanics is impractical, if not impossible. Instead, we show here how a tailored artificial-intelligence approach can be applied, even to a small number of materials, to model catalysis and determine the key descriptive parameters ("materials genes") reflecting the processes that trigger, facilitate, or hinder catalyst performance. We start from a consistent experimental set of "clean data," containing nine vanadium-based oxidation catalysts. These materials were synthesized, fully characterized, and tested according to standardized protocols. By applying the symbolic-regression SISSO approach, we identify correlations between the few most relevant materials properties and their reactivity. This approach highlights the underlying physicochemical processes, and accelerates catalyst design. IMPACT STATEMENT: Artificial intelligence (AI) accepts that there are relationships or correlations that cannot be expressed in terms of a closed mathematical form or an easy-to-do numerical simulation. For the function of materials, for example, catalysis, AI may well capture the behavior better than the theory of the past. However, currently the flexibility of AI comes together with a lack of interpretability, and AI can only predict aspects that were included in the training. The approach proposed and demonstrated in this IMPACT article is interpretable. It combines detailed experimental data (called "clean data") and symbolic regression for the identification of the key descriptive parameters (called "materials genes") that are correlated with the materials function. The approach demonstrated here for the catalytic oxidation of propane will accelerate the discovery of improved or novel materials while also enhancing physical understanding. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1557/s43577-021-00165-6.

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ) thin film and bulk model systems during iron exsolution.

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.

Zirconium-Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming.

C-saturated Pd0 nanoparticles with an extended phase boundary to ZrO2 evolve from a Pd0 Zr0 precatalyst under CH4 dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO2 is efficiently activated at oxidic phase boundary sites and Pdx C provides fast supply of C-atoms toward the latter.

Chemical vapor deposition-prepared sub-nanometer Zr clusters on Pd surfaces: promotion of methane dry reforming.

An inverse Pd-Zr model catalyst was prepared by chemical vapor deposition (CVD) using zirconium-t-butoxide (ZTB) as an organometallic precursor. Pd-Zr interaction was then investigated with focus on the correlation of reforming performance with the oxidation state of Zr. As test reactions, dry reforming of methane (DRM) and methanol steam reforming (MSR) were chosen. Depending on treatments, either ZrOxHy or ZrO2 overlayers or Zr as sub-nanometer clusters could be obtained. Following the adsorption of ZTB on Pd(111), a partially hydroxylated Zr4+-containing layer was formed, which can be reduced to metallic Zr by thermal annealing in ultrahigh vacuum, leading to redox-active Zr0 sub-nanometer clusters. Complementary density functional theoretical (DFT) calculations showed that a single layer of ZrO2 on Pd(111) can be more easily reduced toward the metallic state than a double- and triple layer. Also, the initial and resulting layer compositions greatly depend on gas environment. The lower the water background partial pressure, the faster and more complete the reduction of Zr4+ species to Zr0 on Pd takes place. Under methanol steam reforming conditions, water activation by hydroxylation of Zr occurs. In excess of methanol, strong coking is induced by the Pd/ZrOxHy interface. In contrast, dry reforming of methane is effectively promoted if these initially metallic Zr species are present in the pre-catalyst, leading to a Pd/ZrOxHy phase boundary by oxidative activation under reaction conditions. These reaction-induced active sites for DRM are stable with respect to carbon blocking or coking. In essence, Zr doping of Pd opens specific CO2 activation channels, which are absent on pure metallic Pd.

The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase).

The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam significantly increases both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V(5+) species to the extent of Mo(6+), a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. Furthermore we could prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure are induced by the reaction mixture, which is however not dependent on the steam content.

The electronic factor in alkane oxidation catalysis.

This article addresses the fundamental question of whether concepts from semiconductor physics can be applied to describe the working mode of heterogeneous oxidation catalysts and whether they can be even used to discriminate between selective and unselective reaction pathways. Near-ambient-pressure X-ray photoelectron spectroscopy was applied to the oxidation of n-butane to maleic anhydride on the highly selective catalyst vanadyl pyrophosphate and the moderately selective MoVTeNbO(x) M1 phase. The catalysts were found to act like semiconducting gas sensors with a dynamic charge transfer between the bulk and the surface, as indicated by the gas-phase-dependent response of the work function, electron affinity, and the surface potential barrier. In contrast, only a minor influence of the gas phase on the semiconducting properties and hence no dynamic surface potential barrier was monitored for the total oxidation catalyst V2O5. The surface potential barrier is hence suggested as descriptor for selective catalysts.

Enhancing electrochemical water-splitting kinetics by polarization-driven formation of near-surface iron(0): an in†situ XPS study on perovskite-type electrodes.

In the search for optimized cathode materials for high-temperature electrolysis, mixed conducting oxides are highly promising candidates. This study deals with fundamentally novel insights into the relation between surface chemistry and electrocatalytic activity of lanthanum ferrite based electrolysis cathodes. For this means, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were performed simultaneously on electrochemically polarized La0.6 Sr0.4 FeO3-δ (LSF) thin film electrodes. Under cathodic polarization the formation of Fe(0) on the LSF surface could be observed, which was accompanied by a strong improvement of the electrochemical water splitting activity of the electrodes. This correlation suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may open novel paths in the search for electrodes with increased water splitting activity.

Photoelectron Spectroscopy at the Graphene-Liquid Interface Reveals the Electronic Structure of an Electrodeposited Cobalt/Graphene Electrocatalyst.

Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+), which is believed to be the active site of the catalyst.

A near ambient pressure XPS study of Au oxidation.

The surface of a gold foil under ozone oxidation was examined by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and scanning electron microscopy (SEM). Our in situ observations show that a surface oxide phase is formed during the exposure to ozone; however this phase decomposes under vacuum and even in the presence of ozone at temperatures higher than 300 °C. Assuming that an oxide overlayer completely covers the Au surface, the thickness of the oxide phase was estimated to be between 0.29 and 0.58 nm by energy-dependent XPS depth profiling. The surface oxidation led to structural modifications of the gold surface. These morphological changes do not disappear even under vacuum. In the Au 4f spectra, an additional component at low binding energy (83.3 eV), which appears during/after O3 treatment, is assigned to the presence of low-coordinated atoms which appear on the Au surface as a result of surface restructuring under oxidation. Ex situ SEM images demonstrate that only the region of the sample that was exposed to O3 shows the presence of ridges on the Au surface.

L-edge X-ray absorption study of mononuclear vanadium complexes and spectral predictions using a restricted open shell configuration interaction ansatz.

A series of mononuclear V((V)), V((IV)) and V((III)) complexes were investigated by V L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The spectra show significant sensitivity to the vanadium oxidation state and the coordination environment surrounding the vanadium center. The L-edge spectra are interpreted with the aid of the recently developed Density Functional Theory/Restricted Open Shell Configuration Interaction Singles (DFT/ROCIS) method. This method is calibrated for the prediction of vanadium L-edges with different hybrid density functionals and basis sets. For the B3LYP/def2-TZVP(-f) and BHLYP/def2-TZVP(-f) functional/basis-set combinations, good to excellent agreement between calculated and experimental spectra is obtained. A treatment of the spin-orbit coupling interaction to all orders is achieved by quasi-degenerate perturbation theory (QDPT), in conjunction with DFT/ROCIS for the calculation of the molecular multiplets while accounting for dynamic correlation and anisotropic covalency. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of spin multiplicities, magnetic sublevels and individual 2p to 3d core level excitations. This investigation is an important prerequisite for future applications of the DFT/ROCIS method to vanadium L-edge absorption spectroscopy and vanadium-based heterogeneous catalysts.

Reactivity of carbon in lithium-oxygen battery positive electrodes.

Unfortunately, the practical applications of Li-O2 batteries are impeded by poor rechargeability. Here, for the first time we show that superoxide radicals generated at the cathode during discharge react with carbon that contains activated double bonds or aromatics to form epoxy groups and carbonates, which limits the rechargeability of Li-O2 cells. Carbon materials with a low amount of functional groups and defects demonstrate better stability thus keeping the carbon will-o'-the-wisp lit for lithium-air batteries.

Importance of Metal-Support Interactions for CO2 Hydrogenation: An Operando Near-Ambient Pressure X-ray Photoelectron Spectroscopy Study on Gold-Loaded In2O3 and CeO2 Catalysts.

Metal-support interactions, which are essential for the design of supported metal catalysts, used, e.g., for CO2 activation, are still only partially understood. In this study of gold-loaded In2O3 and CeO2 catalysts during CO2 hydrogenation using near-ambient pressure X-ray photoelectron spectroscopy, supported by near edge X-ray absorption fine structure, we demonstrate that the role of the noble metal strongly depends upon the choice of the support material. Temperature-dependent analyses of X-ray photoelectron spectra under reaction conditions reveal that gold is reduced on CeO2, enabling direct H2 activation, but oxidized on In2O3, leading to decreased activity of Au/In2O3 compared to bare In2O3. At elevated temperatures, the catalytic activity of the In2O3 catalysts strongly increases as a result of facilitated CO2 and (In2O3-based) H2 activation, while the catalytic activity of Au/CeO2 is limited by reoxidation by CO2. Our results underline the importance of operando studies for understanding metal-support interactions to enable a rational support selection in the future.

First principles calculations of the structure and V L-edge X-ray absorption spectra of V2O5 using local pair natural orbital coupled cluster theory and spin-orbit coupled configuration interaction approaches.

A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal-ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.

The effect of Al-doping on ZnO nanoparticles applied as catalyst support.

A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al(2)O(3) methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO.

In situ study of the gas-phase electrolysis of water on platinum by NAP-XPS.

Chasing down the active state: Near-ambient-pressure X-ray photoelectron spectroscopy was used to study the surface of a Pt electrode during the oxygen evolution reaction (OER). A hydrated Pt metal phase with dissolved oxygen in the near-surface region is OER-active and considered to be the precursor of the analytically detected PtO2 , which is in fact the deactivation product of the electrode.

How strain affects the reactivity of surface metal oxide catalysts.

Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm(-2) ). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.