Atomically Ordered PdCu Electrocatalysts for Selective and Stable Electrochemical Nitrate Reduction

Electrochemical nitrate reduction (NO3 RR) has attracted attention as an emerging approach to mitigate nitrate pollution in groundwater. Here, we report that a highly ordered PdCu alloy-based electrocatalyst exhibits selective (91% N2), stable (480 h), and near complete (94%) removal of nitrate without loss of catalyst. In situ and ex situ XAS provide evidence that structural ordering between Pd and Cu improves long-term catalyst stability during NO3RR. In contrast, we also report that a disordered PdCu alloy-based electrocatalyst exhibits non-selective (44% N2 and 49% NH4+), unstable, and incomplete removal of nitrate. The copper within disordered PdCu alloy is vulnerable to accepting electrons from hydrogenated neighboring Pd atoms. This resulted in copper catalyst losses which were 10× greater than that of the ordered catalyst. The design of stable catalysts is imperative for water treatment because loss of the catalyst adds to the system cost and environmental impacts

Spectroscopic Characterization of Highly Active Fe–N–C Oxygen Reduction Catalysts and Discovery of Strong Interaction with Nafion Ionomer

Scaling up clean-energy applications necessitates the development of platinum group metal (PGM)-free fuel cell electrocatalysts with high activity, stability, and low cost. Here, X-ray absorption (XAS) at the Fe K-edge and Fe Kβ X-ray emission (XES) spectroscopies were used to study the electronic structure of Fe centers in highly active Fe–N–C oxygen reduction catalysts with significant commercial potential. X-ray absorption near-edge structure (XANES) analysis has shown that the majority (>95%) of Fe centers are in the Fe3+ oxidation state, while extended X-ray absorption fine structure (EXAFS) detected a mixture of single site Fe–N4 centers (>95%) and centers with short (∼2.5 Å) Fe–Fe interactions of Fe metal and/or Fe-carbide nanoparticles (0 oxidation state. Surprisingly, addition of Nafion, the most widely used ionomer, resulted in pronounced changes in the XAS spectra, consistent with a strong catalyst–ionomer interaction where long Fe–Fe interactions at ∼3.1 Å were shown to be a feature of Fe3+ ions bound with the Nafion. We conclude that exposure to Nafion during the device formulation has a different effect from the aggressive acid leaching typically used in the preparation of Fe–N–C catalysts. It was hypothesized that the polymer interacts with single sites’ Fe3+ centers, as well as with graphene layers protecting the Fe0 nanoparticles, and extracts some Fe ions into the Nafion matrix

Selective Water Oxidation to H₂O₂ on TiO₂ Surfaces with Redox-Active Allosteric Sites

Generation of hydrogen peroxide (H2O2) by electrocatalytic water oxidation is a promising approach for renewable energy utilization that motivates the development of selective catalytic materials. Here, we report a combined theoretical and experimental study, showing that alloyed TiO2 electrodes embedded with subsurface redox-active transition metals enable water oxidation to H2O2 at low overpotentials. Density functional theory calculations show that first-row transition metals (Cr, Mn, Fe, and Co) serve as reservoirs of oxidizing equivalents that couple to substrate binding sites on the surface of redox-inert metal oxides. The distinct sites for substrate binding and redox state transitions reduce the overpotential of the critical first step of water oxidation, the oxidization of H2O* to HO* (“*” = adsorbed), enhancing the selectivity for H2O2. Electrochemical analysis of alloyed TiO2 electrodes with subsurface Mn fabricated by atomic layer deposition confirms the theoretical predictions, showing enhanced selectivity for H2O2 generation (>90%) due to a significant shift of the onset potential (1.8 V vs reversible hydrogen electrode (RHE)), a 500 mV cathodic shift when compared to pristine TiO2 (2.3 V vs RHE). These findings show that otherwise inert metal oxides with subsurface redox-active sites represent a promising class of catalytic materials for a wide range of applications due to the uncoupling of substrate binding and catalytic redox-state transitions

Dynamic Full-Field Infrared Imaging with Multiple Synchrotron Beams

Microspectroscopic imaging in the infrared (IR) spectral region allows for the examination of spatially resolved chemical composition on the microscale. More than a decade ago, it was demonstrated that diffraction-limited spatial resolution can be achieved when an apertured, single-pixel IR microscope is coupled to the high brightness of a synchrotron light source. Nowadays, many IR microscopes are equipped with multipixel Focal Plane Array (FPA) detectors, which dramatically improve data acquisition times for imaging large areas. Recently, progress been made toward efficiently coupling synchrotron IR beamlines to multipixel detectors, but they utilize expensive and highly customized optical schemes. Here we demonstrate the development and application of a simple optical configuration that can be implemented on most existing synchrotron IR beamlines to achieve full-field IR imaging with diffraction-limited spatial resolution. Specifically, the synchrotron radiation fan is extracted from the bending magnet and split into four beams that are combined on the sample, allowing it to fill a large section of the FPA. With this optical configuration, we are able to oversample an image by more than a factor of 2, even at the shortest wavelengths, making image restoration through deconvolution algorithms possible. High chemical sensitivity, rapid acquisition times, and superior signal-to-noise characteristics of the instrument are demonstrated. The unique characteristics of this setup enabled the real-time study of heterogeneous chemical dynamics with diffraction-limited spatial resolution for the first time

Rapid Accumulation of Soil Inorganics on Plastics: Implications for Plastic Degradation and Contaminant Fate

As plastics degrade in the environment, chemical oxidation of the plastic surface enables inorganics to adsorb and form inorganic coatings, likely through a combination of adsorption of minerals and in situ mineral formation. The presence of inorganic coatings on aged plastics has negative implications for plastics fate, hindering our ability to recycle weathered plastics and increasing the potential for plastics to adsorb contaminants. Inorganic coatings formed on terrestrially weathered polyethylene were characterized using synchrotron spectroscopy and microscopy techniques across spatial scales including optical microscopy, nano-X-ray-fluorescence mapping (nano-XRF), nano-X-ray absorption near edge structure (nano-XANES), and high-energy resolution fluorescence detected-XANES (HERFD-XANES). Results indicate a heterogeneous elemental distribution and speciation which includes inorganics common to soil terrestrial environments including iron oxides and oxyhydroxides, aluminosilicates, and carbonates

Unraveling the CO Oxidation Mechanism over Highly Dispersed Pt Single Atom on Anatase TiO₂ (101)

Catalysts with noble metals deposited as single atoms on metal oxide supports have recently been studied extensively due to their maximized metal utilization and potential for performing difficult chemical conversions owing to their unique electronic properties. Understanding of the reaction mechanisms on supported single-metal atoms is still limited but is highly important for designing more efficient catalysts. In this study, we report the complexity of the CO oxidation reaction mechanism on Pt single atoms supported on anatase TiO2 (PtSA/a-TiO2) by coupling density functional theory (DFT) calculations and microkinetic analysis with kinetic measurements, in situ/operando infrared, and X-ray absorption spectroscopies. Starting from the adsorbed PtSA occupying an O vacancy induced by reductive pretreatment, we show that CO oxidation follows a complex mechanism consisting of initiation steps to reorganize the active site and multibranch reactive cycles, with the PtSA/a-TiO2 catalyst not returning to its initial configuration. The initiation step consists of CO and O2 adsorption healing the O vacancy, followed by CO oxidation using gas-phase CO to form Pt­(CO). The reactive cycle alternates O2 adsorption and dissociation to oxidize the catalyst to Pt­(O)­(O)­(CO) and branching pathways of competing Langmuir–Hinshelwood (LH)- or Eley–Rideal (ER)-type CO oxidation steps to reduce it again to Pt­(CO). In situ/operando infrared experiments, including cryogenic CO adsorption and isotopic CO exchange, confirm the combined involvement of strongly adsorbed CO and gas-phase CO in an Eley–Rideal step along the reaction cycle. Microkinetic modeling shows that Pt single atoms are present in a mixture of Pt­(CO), Pt­(CO)­(O2), Pt­(O)­(CO)­(O2), and Pt­(CO)­(CO3) structures as the main intermediates during steady-state CO oxidation, all having the C–O vibrational stretch close to the experimentally observed value of 2115 cm–1. Microkinetic modeling also shows that the fractional orders of CO and O2 measured experimentally originate from multiple steps with a high degree of rate control and not from a simple competitive adsorption. The results demonstrate the complex reaction pathways that even CO oxidation on a simple single-atom system can follow, providing mechanistic insights for designing efficient Pt-based single-atom catalysts. We further show that microkinetic modeling results are sensitive to changes in energies of intermediate and transition states within errors of density functional theory, which can ultimately lead to incorrect conclusions regarding the reaction pathways and most abundant reaction intermediates if not accounted for by experiments

Chemical Imaging of Catalyst Deactivation during the Conversion of Renewables at the Single Particle Level: Etherification of Biomass-Based Polyols with Alkenes over H-Beta Zeolites

The etherification of biomass-based alcohols with various linear α-olefins under solvent-free conditions was followed in a space- and time-resolved manner on 9 μm large H-Beta zeolite crystals by confocal fluorescence microscopy. This allowed us to visualize the interaction with the substrate and distribution of the coke products into the catalyst at the level of an individual zeolite crystal during the etherification process. The spectroscopic information obtained on the micrometer-scale zeolite was in line with the results obtained with bulk characterization techniques and further confirmed by the catalytic results obtained both for micrometer-scale and nanoscale zeolites. This allowed us to explain the influence of the substrate type (glycerol, glycols, and alkenes) and zeolite properties (Si/Al ratio and particle size) on the etherification activity. The etherification of the biomass-based alcohols takes place mainly on the external surface of the zeolite particles. The gradual blockage of the external surface of the zeolite results in a partial or total loss of etherification activity. The deactivation could be attributed to olefin oligomerization. The high conversions obtained in the etherification of 1,2-propylene glycol with long linear alkenes (up to 80%) and the pronounced deactivation of the zeolite observed in the etherification of glycerol with long linear alkenes (max. 20% conversion) were explained by the spectroscopic measurements and is due to differences in the adsorption, i.e., in the center of the zeolite particle for glycerol and on the external surface in the case of glycols

Reduction of Propionic Acid over a Pd-Promoted ReO_<i>x</i>/SiO₂ Catalyst Probed by X‑ray Absorption Spectroscopy and Transient Kinetic Analysis

A Pd-promoted Re/SiO₂ catalyst was prepared by sequential impregnation and compared to monometallic Pd/SiO₂ and Re/SiO₂. All samples were characterized by electron microscopy, H₂ and CO chemisorption, H₂ temperature-programmed reduction, and <i>in situ</i> X-ray absorption spectroscopy at the Re L_III and Pd K-edges. The samples were also tested in the reduction of propionic acid to 1-propanol and propionaldehyde at 433 K in 0.1–0.2 MPa H₂. Whereas monometallic Pd was inactive for carboxylic acid reduction, monometallic Re catalyzed aldehyde formation but only after high-temperature prereduction that produced metallic Re. When Pd was present with Re in a bimetallic catalyst, Pd facilitated the reduction of Re in H₂ to ∼+4 oxidation state at modest temperatures, producing an active catalyst for the conversion of propionic acid to 1-propanol. Under the conditions of this study, the orders of reaction in propionic acid and H₂ were approximately zero and one, respectively. Transient kinetic analysis of the carboxylic acid reduction to alcohols revealed that at least 50% of the Re in the bimetallic catalyst participated in the catalytic reaction. The Pd is proposed to enhance the catalytic activity of the bimetallic catalyst by spilling over hydrogen that can partially reduce Re and react with surface intermediates

Unraveling the CO Oxidation Mechanism over Highly Dispersed Pt Single Atom on Anatase TiO₂ (101)

Catalysts with noble metals deposited as single atoms on metal oxide supports have recently been studied extensively due to their maximized metal utilization and potential for performing difficult chemical conversions owing to their unique electronic properties. Understanding of the reaction mechanisms on supported single-metal atoms is still limited but is highly important for designing more efficient catalysts. In this study, we report the complexity of the CO oxidation reaction mechanism on Pt single atoms supported on anatase TiO2 (PtSA/a-TiO2) by coupling density functional theory (DFT) calculations and microkinetic analysis with kinetic measurements, in situ/operando infrared, and X-ray absorption spectroscopies. Starting from the adsorbed PtSA occupying an O vacancy induced by reductive pretreatment, we show that CO oxidation follows a complex mechanism consisting of initiation steps to reorganize the active site and multibranch reactive cycles, with the PtSA/a-TiO2 catalyst not returning to its initial configuration. The initiation step consists of CO and O2 adsorption healing the O vacancy, followed by CO oxidation using gas-phase CO to form Pt­(CO). The reactive cycle alternates O2 adsorption and dissociation to oxidize the catalyst to Pt­(O)­(O)­(CO) and branching pathways of competing Langmuir–Hinshelwood (LH)- or Eley–Rideal (ER)-type CO oxidation steps to reduce it again to Pt­(CO). In situ/operando infrared experiments, including cryogenic CO adsorption and isotopic CO exchange, confirm the combined involvement of strongly adsorbed CO and gas-phase CO in an Eley–Rideal step along the reaction cycle. Microkinetic modeling shows that Pt single atoms are present in a mixture of Pt­(CO), Pt­(CO)­(O2), Pt­(O)­(CO)­(O2), and Pt­(CO)­(CO3) structures as the main intermediates during steady-state CO oxidation, all having the C–O vibrational stretch close to the experimentally observed value of 2115 cm–1. Microkinetic modeling also shows that the fractional orders of CO and O2 measured experimentally originate from multiple steps with a high degree of rate control and not from a simple competitive adsorption. The results demonstrate the complex reaction pathways that even CO oxidation on a simple single-atom system can follow, providing mechanistic insights for designing efficient Pt-based single-atom catalysts. We further show that microkinetic modeling results are sensitive to changes in energies of intermediate and transition states within errors of density functional theory, which can ultimately lead to incorrect conclusions regarding the reaction pathways and most abundant reaction intermediates if not accounted for by experiments

Differences in the Nature of Active Sites for Methane Dry Reforming and Methane Steam Reforming over Nickel Aluminate Catalysts

The Pechini synthesis was used to prepare nickel aluminate catalysts with the compositions NiAl4O7, NiAl2O4, and Ni2Al2O5. The samples have been characterized by N2 physisorption, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Characterization results indicate unique structural properties and excellent regeneration potential of nickel aluminates. Prepared samples were tested when unreduced and reduced prior to reaction for methane dry reforming and methane steam reforming reactivity. NiAl2O4 in the reduced and unreduced state as well as NiAl4O7 in the reduced state are active and stable for methane dry reforming due to the presence of 4-fold coordinated oxidized nickel. The limited amount of metallic nickel in these samples minimizes carbon deposition. On the other hand, the presence of metallic nickel is required for methane steam reforming. Ni2Al2O5 in the reduced and unreduced states and NiAl2O4 in the reduced state are found to be active for methane steam reforming due to the presence of sufficiently small nickel nanoparticles that catalyze the reaction without accumulating carbonaceous deposits