Atomically Local Electric Field Induced Interface Water Reorientation for Alkaline Hydrogen Evolution Reaction.

The slow water dissociation process in alkaline electrolyte severely limits the kinetics of HER. The orientation of H2 O is well known to affect the dissociation process, but H2 O orientation is hard to control because of its random distribution. Herein, an atomically asymmetric local electric field was designed by IrRu dizygotic single-atom sites (IrRu DSACs) to tune the H2 O adsorption configuration and orientation, thus optimizing its dissociation process. The electric field intensity of IrRu DSACs is over 4.00×1010  N/C. The ab initio molecular dynamics simulations combined with in situ Raman spectroscopy analysis on the adsorption behavior of H2 O show that the M-H bond length (M=active site) is shortened at the interface due to the strong local electric field gradient and the optimized water orientation promotes the dissociation process of interfacial water. This work provides a new way to explore the role of single atomic sites in alkaline hydrogen evolution reaction.

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO.

Atom trapping leads to catalysts with atomically dispersed Ru1O5 sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru1/CeO2 is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru1/CeO2 shows very high NOx storage properties due to formation of stable Ru-NO complexes as well as a high spill-over rate of NOx onto CeO2. Only ∼0.05 wt % of Ru is required for excellent NOx storage. Ru1O5 sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO2 nanoparticles. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru1/CeO2 for NO reduction by CO at low temperatures: only 0.1-0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru1/CeO2 and its propensity to form oxygen vacancies/Ce+3 sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.

Phase-enabled metal-organic framework homojunction for highly selective CO2 photoreduction.

Conversion of clean solar energy to chemical fuels is one of the promising and up-and-coming applications of metal-organic frameworks. However, fast recombination of photogenerated charge carriers in these frameworks remains the most significant limitation for their photocatalytic application. Although the construction of homojunctions is a promising solution, it remains very challenging to synthesize them. Herein, we report a well-defined hierarchical homojunction based on metal-organic frameworks via a facile one-pot synthesis route directed by hollow transition metal nanoparticles. The homojunction is enabled by two concentric stacked nanoplates with slightly different crystal phases. The enhanced charge separation in the homojunction was visualized by in-situ surface photovoltage microscopy. Moreover, the as-prepared nanostacks displayed a visible-light-driven carbon dioxide reduction with very high carbon monooxide selectivity, and excellent stability. Our work provides a powerful platform to synthesize capable metal-organic framework complexes and sheds light on the hierarchical structure-function relationships of metal-organic frameworks.

A Lamellar MWW Zeolite With Silicon and Niobium Oxide Pillars: A Catalyst for the Oxidation of Volatile Organic Compounds.

In this work, an MWW-type zeolite with pillars containing silicon and niobium oxide was synthesized to obtain a hierarchical zeolite. The effect of niobium insertion in the pillaring process was determined by combining a controllable acidity and accessibility in the final material. All pillared materials had niobium occupying framework positions in pillars and extra-framework positions. The pillared material, Pil-Nb-4.5 with 4.5 wt % niobium, did not compromise the mesoporosity formed by pillaring, while the increase of niobium in the structure gradually decreased the mesoporosity and ordering of lamellar stacking. The morphology of the pillared zeolites and the niobium content were found to directly affect the catalytic activity. Specifically, we report on the activity of the MWW-type zeolites with niobium catalyzing the gas-phase oxidation of volatile organic compounds (VOCs), which is an important reaction for clean environmental. All produced MWW-type zeolites with niobium were catalytically active, even at low temperatures and low niobium loading, and provided excellent conversion efficiencies.

Phase-Contact Engineering in Mono- and Bimetallic Cu-Ni Co-catalysts for Hydrogen Photocatalytic Materials.

Understanding how a photocatalyst modulates its oxidation state, size, and structure during a photocatalytic reaction under operando conditions is strongly limited by the mismatch between (catalyst) volume sampled by light and, to date, the physicochemical techniques and probes employed to study them. A synchrotron micro-beam X-ray absorption spectroscopy study together with the computational simulation and analysis (at the X-ray cell) of the light-matter interaction occurring in powdered TiO2 -based monometallic Cu, Ni and bimetallic CuNi catalysts for hydrogen production from renewables was carried out. The combined information unveils an unexpected key catalytic role involving the phase contact between the reduced and oxidized non-noble metal phases in all catalysts and, additionally, reveals the source of the synergistic Cu-Ni interaction in the bimetallic material. The experimental method is applicable to operando studies of a wide variety of photocatalytic materials.

CO Hydrogenation on Cobalt-Based Catalysts: Tin Poisoning Unravels CO in Hollow Sites as a Main Surface Intermediate.

Site poisoning is a powerful method to unravel the nature of active sites or reaction intermediates. The nature of the intermediates involved in the hydrogenation of CO was unraveled by poisoning alumina-supported cobalt catalysts with various concentrations of tin. The rate of formation of the main reaction products (methane and propylene) was found to be proportional to the concentration of multi-bonded CO, likely located in hollow sites. The specific rate of decomposition of these species was sufficient to account for the formation of the main products. These hollow-CO are proposed to be main reaction intermediates in the hydrogenation of CO under the reaction conditions used here, while linear CO are mostly spectators.

Watching Kinetic Studies as Chemical Maps Using Open-Source Software.

A nonproprietary software package, "PyMca", primarily developed for X-ray fluorescence analysis offers an easy-to-use interface for calculating maps, by integrating intensity (of X-ray fluorescence, as well as any spectral data) over Regions Of Interest (ROI), by performing per pixel operations or by applying multivariate analysis. Here we show that, while initially developed to analyze hyperspectral two-dimensional (spatial) maps, this tool can be beneficial as well to anyone interested in measuring spectral variations over one or two dimensions, these dimensions being time, temperature, and so on. Different possibilities offered by the software (preprocessing, simultaneous analysis of replicas, of different conditions, ROI calculation, multivariate analysis, determination of reaction rate constant and of Arrhenius plot) are illustrated with two examples. The first example is the Fourier transform infrared spectroscopy (FTIR) follow-up of the saponification of oil by lead compounds. The disappearance of reagent (oil) and formation of products (lead carboxylates and glycerol) can be easily followed and quantified. The second example is a combined extended X-ray absorption fine structure (EXAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and mass spectroscopy (MS) analysis of RhAl2O3 catalyst under NO reduction by CO in the presence of O2. It is possible to appreciate, in a single shot, Rh particles' structure and surface changes and gas release and adsorption in the reaction conditions.