Revealing the Dynamics and Roles of Iron Incorporation in Nickel Hydroxide Water Oxidation Catalysts.

The surface of an electrocatalyst undergoes dynamic chemical and structural transformations under electrochemical operating conditions. There is a dynamic exchange of metal cations between the electrocatalyst and electrolyte. Understanding how iron in the electrolyte gets incorporated in the nickel hydroxide electrocatalyst is critical for pinpointing the roles of Fe during water oxidation. Here, we report that iron incorporation and oxygen evolution reaction (OER) are highly coupled, especially at high working potentials. The iron incorporation rate is much higher at OER potentials than that at the OER dormant state (low potentials). At OER potentials, iron incorporation favors electrochemically more reactive edge sites, as visualized by synchrotron X-ray fluorescence microscopy. Using X-ray absorption spectroscopy and density functional theory calculations, we show that Fe incorporation can suppress the oxidation of Ni and enhance the Ni reducibility, leading to improved OER catalytic activity. Our findings provide a holistic approach to understanding and tailoring Fe incorporation dynamics across the electrocatalyst-electrolyte interface, thus controlling catalytic processes.

Cubes to Cubes: Organization of MgO Particles into One-Dimensional and Two-Dimensional Nanostructures.

Developing simple, inexpensive, and environmentally benign approaches to integrate morphologically well-defined nanoscale building blocks into larger high surface area materials is a key challenge in materials design and processing. In this work, we investigate the fundamental surface phenomena between MgO and water (both adsorption and desorption) with particles prepared via a vapor-phase process (MgO nanocubes) and a modified aerogel process (MgO(111) nanosheets). Through these studies, we unravel a strategy to assemble individual MgO nanoparticles into extended faceted single-crystalline MgO nanosheets and nanorods with well-defined exposed surfaces and edges. This reorganization can be triggered by the presence of H2O vapor or bulk liquid water. Water adsorption and the progressive conversion of vapor-phase grown oxide particles into hydroxides give rise to either one-dimensional or two-dimensional (1D or 2D) structures of high dispersion and surface area. The resulting Mg(OH)2 lamella with a predominant (001) surface termination are well-suited precursor structures for their topotactic conversion into laterally extended and uniform MgO(111) grain surface configurations. To understand the potential of polar (111) surfaces for faceting and surface reconstruction effects associated with water desorption, we investigated the stability of MgO(111) nanosheets during vacuum annealing and electron beam exposure. The significant surface reconstruction of the MgO(111) surfaces observed shows that adsorbate-free (111)-terminated surfaces of unsupported MgO nanostructures reconstruct rather than remain as charged planes of either three-fold coordinated O2- ion or Mg2+ ions. Thus, here we demonstrate the role water can play in surface formation and reconstruction by bridging wet chemical and surface science inspired approaches.

A microwave assisted ionic liquid route to prepare bivalent Mn5O8 nanoplates for 5-hydroxymethylfurfural oxidation.

In order to develop highly active non-precious metal catalysts for the selective oxidation of the platform compound 5-hydroxymethylfurfural (HMF) to the value-added bio-chemical 2,5-diformylfuran (DFF), we prepared high purity bivalent Mn5O8 nanoplates by a microwave-assisted ionic liquid route. The precursor of bivalent Mn5O8 nanoplates was formed through π-π stacking between imidazolium rings of the ionic liquid 1-butyl-3-methyl-imidazolium chloride and extending hydrogen bonds between Cl anions and hydrohausmannite. An oriented aggregation growth occurred on the basis of the Ostwald ripening under microwave heating. The high purity bivalent Mn5O8 nanoplates obtained through calcination at 550 °C for 2 h exhibited high HMF conversion (51%) and DFF selectivity (94%) at 5 bar of oxygen pressure in 2 h. The high concentration of Mn4+ on the exterior surfaces of Mn5O8 nanoplates as active sites coupled with good crystallinity played key roles for desirable mass and heat transfer, and for fast desorption avoiding over-oxidation. The reaction process over the Mn5O8 nanoplates was proposed based on the understanding of Mn4+ active centers and lattice oxygen via a Mn4+/Mn2+ two-electron cycle to enhance their catalytic performance. Furthermore, the Mn5O8 nanoplates could be readily recovered and reused without loss of catalytic activity. Thus, the high purity Mn5O8 nanoplates with good catalytic performance raises the prospect of using the type of sole metal oxide for practical applications.

Direct high-resolution mapping of electrocatalytic activity of semi-two-dimensional catalysts with single-edge sensitivity.

The catalytic activity of low-dimensional electrocatalysts is highly dependent on their local atomic structures, particularly those less-coordinated sites found at edges and corners; therefore, a direct probe of the electrocatalytic current at specified local sites with true nanoscopic resolution has become critically important. Despite the growing availability of operando imaging tools, to date it has not been possible to measure the electrocatalytic activities from individual material edges and directly correlate those with the local structural defects. Herein, we show the possibility of using feedback and generation/collection modes of operation of the scanning electrochemical microscope (SECM) to independently image the topography and local electrocatalytic activity with 15-nm spatial resolution. We employed this operando microscopy technique to map out the oxygen evolution activity of a semi-2D nickel oxide nanosheet. The improved resolution and sensitivity enables us to distinguish the higher activities of the materials' edges from that of the fully coordinated surfaces in operando The combination of spatially resolved electrochemical information with state-of-the-art electron tomography, that unravels the 3D complexity of the edges, and ab initio calculations allows us to reveal the intricate coordination dependent activity along individual edges of the semi-2D material that is not achievable by other methods. The comparison of the simulated line scans to the experimental data suggests that the catalytic current density at the nanosheet edge is ∼200 times higher than that at the NiO basal plane.

Iodine-Catalyzed Isomerization of Dimethyl Muconate.

cis,cis-Muconic acid is a platform bio-based chemical that can be upgraded to drop-in commodity and novel monomers. Among the possible drop-in products, dimethyl terephthalate can be synthesized via esterification, isomerization, Diels-Alder cycloaddition, and dehydrogenation. The isomerization of cis,cis-dimethyl muconate (ccDMM) to the trans,trans-form (ttDMM) can be catalyzed by iodine; however, studies have yet to address (i) the mechanism and reaction barriers unique to DMM, and (ii) the influence of solvent, potential for catalyst recycle, and recovery of high-purity ttDMM. To address this gap, we apply a joint computational and experimental approach to investigate iodine-catalyzed isomerization of DMM. Density functional theory calculations identified unique regiochemical considerations owing to the large number of halogen-diene coordination schemes. Both transition state theory and experiments estimate significant barrier reductions with photodissociated iodine. Solvent selection was critical for rapid kinetics, likely because of solvent complexation with iodine. Under select conditions, ttDMM yields of 95 % were achieved in <1 h with methanol, followed by high purity recovery (>98 %) with crystallization. Lastly, post-reaction iodine can be recovered and recycled with minimal loss of activity. Overall, these findings provide new insight into the mechanism and conditions necessary for DMM isomerization with iodine to advance the state-of-the-art for bio-based chemicals.

Carbon Capture by Metal Oxides: Unleashing the Potential of the (111) Facet.

Solid metal oxides for carbon capture exhibit reduced adsorption capacity following high-temperature exposure, due to surface area reduction by sintering. Furthermore, only low-coordinate corner/edge sites on the thermodynamically stable (100) facet display favorable binding toward CO2, providing inherently low capacity. The (111) facet, however, exhibits a high concentration of low-coordinate sites. In this work, MgO(111) nanosheets displayed high capacity for CO2, as well as a ∼65% increase in capacity despite a ∼30% reduction in surface area following sintering (0.77 mmol g-1 @ 227 m2 g-1 vs 1.28 mmol g-1 @ 154 m2 g-1). These results, unique to MgO(111), suggest intrinsic differences in the effects of sintering on basic site retention. Spectroscopic and computational investigations provided a new structure-activity insight: the importance of high-temperature activation to unleash the capacity of the polar (111) facet of MgO. In summary, we present the first example of a faceted sorbent for carbon capture and challenge the assumption that sintering is necessarily a negative process; here we leverage high-temperature conditions for facet-dependent surface activation.

Palladium Intercalated into the Walls of Mesoporous Silica as Robust and Regenerable Catalysts for Hydrodeoxygenation of Phenolic Compounds.

Nanostructured noble-metal catalysts traditionally suffer from sintering under high operating temperatures, leading to durability issues and process limitations. The encapsulation of nanostructured catalysts to prevent loss of activity through thermal sintering, while maintaining accessibility of active sites, remains a great challenge in the catalysis community. Here, we report a robust and regenerable palladium-based catalyst, wherein palladium particles are intercalated into the three-dimensional framework of SBA-15-type mesoporous silica. The encapsulated Pd active sites remain catalytically active as demonstrated in high-temperature/pressure phenol hydrodeoxygenation reactions. The confinement of Pd particles in the walls of SBA-15 prevents particle sintering at high temperatures. Moreover, a partially deactivated catalyst containing intercalated particles is regenerated almost completely even after several reaction cycles. In contrast, Pd particles, which are not encapsulated within the SBA-15 framework, sinter and do not recover prior activity after a regeneration procedure.

Intermediate selectivity in the oxidation of phenols using plasmonic Au/ZnO photocatalysts.

Tunable reaction selectivity on a single catalyst is a continual goal in chemical syntheses. Herein, we report an unexpected light-directed intermediate selectivity using well-known plasmonic photocatalysts. We observed distinct intermediate selectivity behaviors between using UV and visible light irradiations. Chemical computations and quenching experiments suggest that the radicals generated by the plasmonic excitation govern the light-directed selectivity. The broader impact of this work ranges from selective yield of desirable intermediates for subsequent syntheses without tedious separation procedures, to arousing interest in examining new opportunities for plasmonic photocatalysts.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures.

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

Synthesis and Catalytic Performance of Gold Intercalated in the Walls of Mesoporous Silica.

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties of the materials were investigated. We used a one pot sol-gel approach to intercalate gold nano particles in the walls of mesoporous silica. To start with the synthesis, P123 was used as template to form micelles. Then TESPTS was used as a surface modification agent to intercalate gold nano particles. Following this process, TEOS was added in as a silica source which underwent a polymerization process in acid environment. After hydrothermal processing and calcination, the final product was acquired. Several techniques were utilized to characterize the porosity, morphology and structure of the gold intercalated mesoporous silica. The results showed a stable structure of mesoporous silica after gold intercalation. Through the oxidation of benzyl alcohol as a benchmark reaction, the GMS materials showed high selectivity and recyclability.

Sodiation Kinetics of Metal Oxide Conversion Electrodes: A Comparative Study with Lithiation.

The development of sodium ion batteries (NIBs) can provide an alternative to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. However, due to the larger size and higher m/e ratio of the sodium ion compared to lithium, sodiation reactions of candidate electrodes are expected to differ in significant ways from the corresponding lithium ones. In this work, we investigated the sodiation mechanism of a typical transition metal-oxide, NiO, through a set of correlated techniques, including electrochemical and synchrotron studies, real-time electron microscopy observation, and ab initio molecular dynamics (MD) simulations. We found that a crystalline Na2O reaction layer that was formed at the beginning of sodiation plays an important role in blocking the further transport of sodium ions. In addition, sodiation in NiO exhibits a "shrinking-core" mode that results from a layer-by-layer reaction, as identified by ab initio MD simulations. For lithiation, however, the formation of Li antisite defects significantly distorts the local NiO lattice that facilitates Li insertion, thus enhancing the overall reaction rate. These observations delineate the mechanistic difference between sodiation and lithiation in metal-oxide conversion materials. More importantly, our findings identify the importance of understanding the role of reaction layers on the functioning of electrodes and thus provide critical insights into further optimizing NIB materials through surface engineering.

Graphene as an efficient interfacial layer for electrochromic devices.

This study presents an interfacial modification strategy to improve the performance of electrochromic films that were fabricated by a magnetron sputtering technique. High-quality graphene sheets, synthesized by chemical vapor deposition, were used to modify fluorine-doped tin oxide substrates, followed by the deposition of high-performance nanocomposite nickel oxide electrochromic films. Electrochromic cycling results revealed that a near-complete monolayer graphene interfacial layer improves the electrochromic performance in terms of switching kinetics, activation period, coloration efficiency, and bleached-state transparency, while maintaining ∼100% charge reversibility. The present study offers an alternative route for improving the interfacial properties between electrochromic and transparent conducting oxide films without relying on conventional methods such as nanostructuring or thin film composition control.

Transitions from near-surface to interior redox upon lithiation in conversion electrode materials.

Nanoparticle electrodes in lithium-ion batteries have both near-surface and interior contributions to their redox capacity, each with distinct rate capabilities. Using combined electron microscopy, synchrotron X-ray methods and ab initio calculations, we have investigated the lithiation pathways that occur in NiO electrodes. We find that the near-surface electroactive (Ni(2+) → Ni(0)) sites saturated very quickly, and then encounter unexpected difficulty in propagating the phase transition into the electrode (referred to as a "shrinking-core" mode). However, the interior capacity for Ni(2+) → Ni(0) can be accessed efficiently following the nucleation of lithiation "fingers" that propagate into the sample bulk, but only after a certain incubation time. Our microstructural observations of the transition from a slow shrinking-core mode to a faster lithiation finger mode corroborate with synchrotron characterization of large-format batteries and can be rationalized by stress effects on transport at high-rate discharge. The finite incubation time of the lithiation fingers sets the intrinsic limitation for the rate capability (and thus the power) of NiO for electrochemical energy storage devices. The present work unravels the link between the nanoscale reaction pathways and the C-rate-dependent capacity loss and provides guidance for the further design of battery materials that favors high C-rate charging.

Shape-directional growth of Pt and Pd nanoparticles.

The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth. Herein, we develop and demonstrate the capabilities of a mathematical growth model for predicting metal nanoparticle shapes by studying a well known procedure that employs AgNO3 to produce {111} faceted Pt nanocrystals. The insight gained about the role of auxiliary species is then utilized to predict the shape of Pd nanocrystals and to corroborate other shape-directing syntheses reported in literature. The fundamental understanding obtained herein by combining modeling with experimentation is a step toward computationally guided syntheses and, in principle, applicable to predictive design of the growth of crystalline solids at all length scales (nano to bulk).

Phase evolution for conversion reaction electrodes in lithium-ion batteries.

The performance of battery materials is largely governed by structural and chemical evolutions during electrochemical reactions. Therefore, resolving spatially dependent reaction pathways could enlighten mechanistic understanding, and enable rational design for rechargeable battery materials. Here, we present a phase evolution panorama via spectroscopic and three-dimensional imaging at multiple states of charge for an anode material (that is, nickel oxide nanosheets) in lithium-ion batteries. We reconstruct the three-dimensional lithiation/delithiation fronts and find that, in a fully electrolyte immersion environment, phase conversion can nucleate from spatially distant locations on the same slab of material. In addition, the architecture of a lithiated nickel oxide is a bent porous metallic framework. Furthermore, anode-electrolyte interphase is found to be dynamically evolving upon charging and discharging. The present study has implications for resolving the inhomogeneity of the general electrochemically driven phase transition (for example, intercalation reactions) and for the origin of inhomogeneous charge distribution in large-format battery electrodes.

Origin of electrochromism in high-performing nanocomposite nickel oxide.

Electrochromic effects of transition metal oxides provide a great platform for studying lithium intercalation chemistry in solids. Herein, we report on an electronically modified nanocomposite nickel oxide (i.e., Li2.34NiZr0.28Ox) that exhibits significantly improved electrochromic performance relative to the state-of-the-art inorganic electrochromic metal oxides in terms of charge/discharge kinetics, bleached-state transparency, and optical modulation. The knowledge obtained from O K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that the internally grown lithium peroxide (i.e., Li2O2) species plays a major role in facilitating charge transfer thus enabling optimal electrochromic performance. This understanding is relevant to recent theoretical studies concerning conductivity in Li2O2 for lithium-air batteries (as cited in the main text). Furthermore, we elucidate the electrochromism in modified nickel oxide in lithium ion electrolyte with the aid of Ni K-edge XAS and Ni L-edge XAS studies. The electrochromism in the nickel oxide materials arises from the reversible formation of hole states on the NiO6 units, which then impacts the Ni oxidation state through the Ni3d-O2p hybridization states. This study sheds light on the lithium intercalation chemistry for general energy storage and semiconductor applications.

Polyoxotungstate stabilized palladium, gold, and silver nanoclusters: a study of cluster stability, catalysis, and effects of the stabilizing anions.

Polyoxometalates (POMs) have demonstrated the unusual ability to both stabilize nanoclusters and facilitate catalytic activity by allowing substrate access to sufficient metallic surface area. This paper presents a wet chemical synthetic method to prepare Pd, Ag, and Au metal nanoclusters stabilized by tetrabutylammonium salts of Phosphotungstates with the well-known Keggin [α-PW(12)O(40)](3-) (PW(12)), Wells-Dawson [P(2)W(18)O(62)](6-) (P(2)W(18)), and their lacunary derivatives [α-PW(11)O(39)](7-) (PW(11)) and [P(2)W(15)O(56)](12-) (P(2)W(15)) structures. The preparation of this series of nanoclusters has facilitated a comparison of the effect these anions have in nanocluster stabilization and catalysis. The nanocluster systems were found to be re-dispersible in acetonitrile and acetone. The effect of aging on stability was followed over 3 months and it was found that many systems were very stable for more than 70 days without precipitation, depending on type of polytungstate stabilizer and the presence of competing ions. The PW(12), P(2)W(18), PW(11), and P(2)W(15) stabilized Pd nanoclusters have been investigated for catalytic activity in the hydrogenation of 1-hexene and were found to be very active with the P(2)W(18) stabilized Pd nanoclusters demonstrating a best of 139,000 Total Turnovers, thus ranking this "soluble heterogeneous catalyst" among one of the longest lived reported in the literature. The nanocluster systems were characterized using instrumental techniques such as UV-Visible Spectroscopy, FTIR Spectroscopy, Powder X-ray Diffraction, Energy Dispersive Spectroscopy, X-ray Photoelectron Spectroscopy, and Transmission Electron Microscopy.

Hole doping in Al-containing nickel oxide materials to improve electrochromic performance.

Electrochromic materials exhibit switchable optical properties that can find applications in various fields, including smart windows, nonemissive displays, and semiconductors. High-performing nickel oxide electrochromic materials have been realized by controlling the material composition and tuning the nanostructural morphology. Post-treatment techniques could represent efficient and cost-effective approaches for performance enhancement. Herein, we report on a post-processing ozone technique that improves the electrochromic performance of an aluminum-containing nickel oxide material in lithium-ion electrolytes. The resulting materials were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy, and X-ray absorption spectroscopy (XAS). It was observed that ozone exposure increased the Ni oxidation state by introducing hole states in the NiO(6) octahedral unit. In addition, ozone exposure gives rise to higher-performing aluminum-containing nickel oxide films, relative to nickel oxide containing both Al and Li, in terms of switching kinetics, bleached-state transparency, and optical modulation. The improved performance is attributed to the decreased crystallinity and increased nickel oxidation state in aluminum-containing nickel oxide electrochromic films. The present study provides an alternative route to improve electrochromic performance for nickel oxide materials.

Analysis of lipids: metal oxide laser ionization mass spectrometry.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been used for lipid analysis; however, one of the drawbacks of this technique is matrix interference peaks at low masses. Metal oxide surfaces are described here for direct, matrix-free analysis of small (MW < 1000 Da) lipid compounds, without interferences in the resulting spectra from traditional matrix background peaks. Spectra from lipid standards produced protonated and sodiated molecular ions. More complex mixtures including vegetable oil shortening and lipid extracts from bacterial and algal sources provided similar results. Mechanistic insight into the mode of ionization from surface spectroscopy, negative ion mass spectrometry, and stable isotope studies is also presented. The metal oxide system is compared to other reported matrix-free systems.

Nanoscale gold intercalated into mesoporous silica as a highly active and robust catalyst.

Stable gold/mesoporous silica nanocomposites (with Au nanoparticles intercalated in the walls of mesoporous silica) were successfully synthesized by the hydrothermal method and applied as catalysts. A challenging issue associated with intercalation and the use of coordinating agents is the effect of the coordinating agent on the mesoporous silica structure and periodicity. This investigation is targeted at elaborating the effect of the coordinating agent on the resulting mesoporous structure. The amount of Au coordinating agent bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPTS) was systematically altered to synthesize a range of materials with varying Au loadings and morphologies. These materials were characterized by N(2) adsorption-desorption, x-ray diffraction, transmission electron microscopy and UV-visible spectroscopy. The structures of the catalysts were found to range from mesoporous to vesical- and foam-like upon varying the TESPTS/polymer template (P123) ratio. Additionally, the sizes of Au nanoparticles increased by increasing the amount of TESPTS. The catalytic properties of the resulting materials were examined using oxidation of benzyl alcohol and reduction of 4-nitrophenol as probe reactions. The intercalated systems demonstrated high activity and more importantly were robust and readily reusable. This approach to imparting stability to nanoscale materials may be much more broadly applicable and expand the types of environments in which they can be utilized.