Tuning Local Atomic Structures in MoS2 Based Catalysts for Electrochemical Nitrate Reduction.

In recent years, there has been a substantial surge in the investigation of transition-metal dichalcogenides such as MoS2 as a promising electrochemical catalyst. Inspired by denitrification enzymes such as nitrate reductase and nitrite reductase, the electrochemical nitrate reduction catalyzed by MoS2 with varying local atomic structures is reported. It is demonstrated that the hydrothermally synthesized MoS2 containing sulfur vacancies behaves as promising catalysts for electrochemical denitrification. With copper doping at less than 9% atomic ratio, the selectivity of denitrification to dinitrogen in the products can be effectively improved. X-ray absorption characterizations suggest that two sulfur vacancies are associated with one copper dopant in the MoS2 skeleton. DFT calculation confirms that copper dopants replace three adjacent Mo atoms to form a trigonal defect-enriched region, introducing an exposed Mo reaction center that coordinates with Cu atom to increase N2 selectivity. Apart from the higher activity and selectivity, the Cu-doped MoS2 also demonstrates remarkably improved tolerance toward oxygen poisoning at high oxygen concentration. Finally, Cu-doped MoS2 based catalysts exhibit very low specific energy consumption during the electrochemical denitrification process, paving the way for potential scale-up operations.

Functionalized Ferrocene Enables Selective Electrosorption of Arsenic Oxyanions over Phosphate─A DFT Examination of the Effects of Substitutional Moieties, pH, and Oxidation State.

Ferrocene (Fc)/ferrocenium (Fc+)-decorated carbon nanotube electrode materials have shown promise for selectively adsorbing arsenic (As) over dissimilar anions like Cl- and ClO4-, and isostructural transition-metal oxyanions for water remediation; however, the competition between same-group oxyanions (such as arsenate vs phosphate) is underexplored and poorly understood. We use ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to Fc/Fc+ with and without functional substitutions (OH, SH, NH2, COOH, CH3, C2H5, NO2, and Cl). This work aims to understand factors that induce the selective binding of toxic arsenic over phosphate. We find that neat Fc cannot distinguish the three oxyanions because physical forces (electrostatics and dispersion) dominate the Fc-oxyanion interactions. However, combined oxidation and substitution effects enable selectivity for As(V) over P(V). Oxidation of Fc to Fc+ allows the formation of Fc+-oxyanion covalent bonds with varying donor-acceptor character depending on the oxyanion. Additionally, NH2 and SH groups that donate charge to the base Fc+ molecule and H-bond to oxyanion induce an energetic preference for As(V) over P(V) by -0.23 and -0.13 eV, respectively. Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) over the other anions. Using the calculated energetics, we predict the pH-dependent binding selectivity of functionalized ferrocenium. These findings demonstrate the challenges of Fc/Fc+-oxyanion interaction for selective binding and provide a path for identifying other molecules and substituents for efficient metallocene adsorbent design.

Spatially separating redox centers on 2D carbon nitride with cobalt single atom for photocatalytic H2O2 production.

Redox cocatalysts play crucial roles in photosynthetic reactions, yet simultaneous loading of oxidative and reductive cocatalysts often leads to enhanced charge recombination that is detrimental to photosynthesis. This study introduces an approach to simultaneously load two redox cocatalysts, atomically dispersed cobalt for improving oxidation activity and anthraquinone for improving reduction selectivity, onto graphitic carbon nitride (C3N4) nanosheets for photocatalytic H2O2 production. Spatial separation of oxidative and reductive cocatalysts was achieved on a two-dimensional (2D) photocatalyst, by coordinating cobalt single atom above the void center of C3N4 and anchoring anthraquinone at the edges of C3N4 nanosheets. Such spatial separation, experimentally confirmed and computationally simulated, was found to be critical for enhancing surface charge separation and achieving efficient H2O2 production. This center/edge strategy for spatial separation of cocatalysts may be applied on other 2D photocatalysts that are increasingly studied in photosynthetic reactions.

Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption.

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly co-occurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six "target" oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five "competing" co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents.

Adatom surface diffusion of catalytic metals on the anatase TiO2(101) surface.

Titanium oxide is often decorated with metal nano-particles and either serves as a catalyst support or enables photocatalytic activity. The activity of these systems degrades over time due to catalytic particle agglomeration and growth by Ostwald ripening where adatoms dissociate from metal particles, diffuse across the surface and add to other metal particles. In this work, we use density functional theory calculations to study the diffusion mechanisms of select group VIII and 1B late-transition metal adatoms commonly used in catalysis and photocatalysis (Au, Ag, Cu, Pt, Rh, Ni, Co and Fe) on the anatase TiO2(101) surface. All metal adatoms preferentially occupy the bridge site between two 2-fold-coordinated oxygen anions (O2c). Surface migration was investigated by calculating the minimum energy pathway from one bridge site to another along three pathways: two in the [010] direction along a row of surface O2c anions and one in the [101[combining macron]] direction between two rows of surface O2c anions. For all adatoms, migration along the [010] direction is favored over migration along the [101[combining macron]] direction due to closer packing of the atoms in the [010] direction and therefore stronger adatom-surface interactions. As the adatom hops along the [010] direction, it preferentially moves through a metastable OTiO structure in which the adatom partially embeds itself within the surface, with the exception of Au, which remains above the surface. The adatoms migrate with relative activation energies of: Au (0.24 eV) < Ag (0.48 eV) < Rh (0.60 eV) < Co (0.78 eV) < Pt (0.84 eV) < Ni (0.86 eV) < Cu (1.23 eV) < Fe (1.79 eV) along the favored pathway. This preference arises from the strength of adatom-surface bonding and the electronegativity difference between the metal adatom and the TiO2 surface. We found a linear correlation between the binding energy/electronegativity and the activation energy for hopping where stronger binding energies and more oxidized adatoms have higher activation energies for adatom migration. The linear correlation developed in this work enables rapid estimations of the hopping rates of other transition metal adatoms across the TiO2 surface.

Mechanistic Insights into the Selectivity for Arsenic over Phosphate Adsorption by Fe3+-Cross-Linked Chitosan Using DFT.

Fe3+-cross-linked chitosan exhibits the potential for selectively adsorbing arsenic (As) over competing species, such as phosphate, for water remediation. However, the effective binding mechanisms, bond nature, and controlling factor(s) of the selectivity are poorly understood. This study employs ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to neat chitosan and Fe3+-chitosan. Neat chitosan fails to selectively bind As oxyanions, as all three oxyanions bind similarly via weak hydrogen bonds with preferences of P(V) = As(V) > As(III). Conversely, Fe3+-chitosan selectively binds As(V) over As(III) and P(V) with binding energies of -1.9, -1, and -1.8 eV for As(V), As(III), and P(V), respectively. The preferences are due to varying Fe3+-oxyanion donor-acceptor characteristics, forming covalent bonds with distinct strengths (Fe-O bond ICOHP values: - 4.9 eV/bond for As(V), - 4.7 eV/bond for P(V), and -3.5 eV/bond for As(III)). Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) under typical environmental pH conditions. Furthermore, our calculations suggest that the binding selectivity of Fe3+-chitosan exhibits a pH dependence. These findings enhance our understanding of the Fe3+-oxyanion interaction crucial for preferential oxyanion binding using Fe3+-chitosan and provide a lens for further exploration into alternative transition-metal-chitosan combinations and coordination chemistries for applications in selective separations.

Understanding the Effect of Single Atom Cationic Defect Sites in an Al2O3 (012) Surface on Altering Selenate and Sulfate Adsorption: An Ab Initio Study.

Adsorption is a promising under-the-sink selenate remediation technique for distributed water systems. Recently it was shown that adsorption induced water network re-arraignment control adsorption energetics on the α-Al2O3 (012) surface. Here, we aim to elucidate the relative importance of the water network effects and surface cation identity on controlling selenate and sulfate adsorption energy using density functional theory calculations. Density functional theory (DFT) calculations predicted the adsorption energies of selenate and sulfate on nine transition metal cations (Sc-Cu) and two alkali metal cations (Ga and In) in the α-Al2O3 (012) surface under simulated acidic and neutral pH conditions. We find that the water network effects had larger impact on the adsorption energy than the cationic identity. However, cation identity secondarily controlled adsorption. Most cations decreased the adsorption energy weakening the overall performance, the larger Sc and In cations enabled inner-sphere adsorption in acidic conditions because they relaxed outward from the surface providing more space for adsorption. Additionally, only Ti induced Se selectivity over S by reducing the adsorbing selenate to selenite but not reducing the sulfate. Overall, this study indicates that tuning water network structure will likely have a larger impact than tuning cation-selenate interactions for increasing adsorbate effectiveness.

Contrasting Capability of Single Atom Palladium for Thermocatalytic versus Electrocatalytic Nitrate Reduction Reaction.

The occurrence of high concentrations of nitrate in various water resources is a significant environmental and human health threat, demanding effective removal technologies. Single atom alloys (SAAs) have emerged as a promising bimetallic material architecture in various thermocatalytic and electrocatalytic schemes including nitrate reduction reaction (NRR). This study suggests that there exists a stark contrast between thermocatalytic (T-NRR) and electrocatalytic (E-NRR) pathways that resulted in dramatic differences in SAA performances. Among Pd/Cu nanoalloys with varying Pd-Cu ratios from 1:100 to 100:1, Pd/Cu(1:100) SAA exhibited the greatest activity (TOFPd = 2 min-1) and highest N2 selectivity (94%) for E-NRR, while the same SAA performed poorly for T-NRR as compared to other nanoalloy counterparts. DFT calculations demonstrate that the improved performance and N2 selectivity of Pd/Cu(1:100) in E-NRR compared to T-NRR originate from the higher stability of NO3* in electrocatalysis and a lower N2 formation barrier than NH due to localized pH effects and the ability to extract protons from water. This study establishes the performance and mechanistic differences of SAA and nanoalloys for T-NRR versus E-NRR.

Surface water H-bonding network is key controller of selenate adsorption on [012] α-alumina: An Ab-initio study.

Selenate adsorption onto metal oxide surfaces is a cost-effective method to remove the toxin from drinking water systems. However, the low selectivity of metal oxides requires frequent sorbent replacement. The design of selective adsorbents is stymied because the surface factors controlling selenate adsorption remain unknown. We calculate adsorption energies of selenate on the (012) α-Al2O3 surface using density functional theory to unravel the physics that controls adsorption. Our model is validated against experiment by correctly predicting selenate removal efficiency as a function pH. We find that the selenate adsorption energy on the anhydrous α-Al2O3 surface is surprisingly anti-correlated with the fully solvated adsorption energy; therefore, the direct interaction between adsorbate and sorbent is eliminated as the controlling mechanism. Rather, the change in number of surface hydrogen bonds after adsorption is the factor most correlated with the adsorption energy (R2 > 0.8); and is thus determined to be the factor controlling selenate adsorption. We find that pH affects adsorption by controlling the number of surface protons available for H-bonding to selenate. This work demonstrates that adsorption prediction should not be made based on gas phase sorption energies and suggests that surface engineering which increases surface protonation may be an effective strategy for increasing selenate sorption.

Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst.

Electrochemically converting nitrate ions, a widely distributed nitrogen source in industrial wastewater and polluted groundwater, into ammonia represents a sustainable route for both wastewater treatment and ammonia generation. However, it is currently hindered by low catalytic activities, especially under low nitrate concentrations. Here we report a high-performance Ru-dispersed Cu nanowire catalyst that delivers an industrial-relevant nitrate reduction current of 1 A cm-2 while maintaining a high NH3 Faradaic efficiency of 93%. More importantly, this high nitrate-reduction catalytic activity enables over a 99% nitrate conversion into ammonia, from an industrial wastewater level of 2,000 ppm to a drinkable water level <50 ppm, while still maintaining an over 90% Faradaic efficiency. Coupling the nitrate reduction effluent stream with an air stripping process, we successfully obtained high purity solid NH4Cl and liquid NH3 solution products, which suggests a practical approach to convert wastewater nitrate into valuable ammonia products. Density functional theory calculations reveal that the highly dispersed Ru atoms provide active nitrate reduction sites and the surrounding Cu sites can suppress the main side reaction, the hydrogen evolution reaction.

Neighboring Pd single atoms surpass isolated single atoms for selective hydrodehalogenation catalysis.

Single atom catalysts have been found to exhibit superior selectivity over nanoparticulate catalysts for catalytic reactions such as hydrogenation due to their single-site nature. However, improved selectively is often accompanied by loss of activity and slow kinetics. Here we demonstrate that neighboring Pd single atom catalysts retain the high selectivity merit of sparsely isolated single atom catalysts, while the cooperative interactions between neighboring atoms greatly enhance the activity for hydrogenation of carbon-halogen bonds. Experimental results and computational calculations suggest that neighboring Pd atoms work in synergy to lower the energy of key meta-stable reactions steps, i.e., initial water desorption and final hydrogenated product desorption. The placement of neighboring Pd atoms also contribute to nearly exclusive hydrogenation of carbon-chlorine bond without altering any other bonds in organohalogens. The promising hydrogenation performance achieved by neighboring single atoms sheds light on a new approach for manipulating the activity and selectivity of single atom catalysts that are increasingly studied in multiple applications.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides.

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

Preferential adsorption of selenium oxyanions onto {1 1 0} and {0 1 2} nano-hematite facets.

As the commercial use of nano metal oxides, including iron oxides, becomes more prevalent, there is a need to understand functionality as it relates to the inherent properties of the nanomaterial. Many applications of nanomaterials rely on adsorption, ranging from catalysis to aqueous remediation. In this paper, adsorption of selenium (Se), an aqueous contaminant, is used as a model sorbate to elucidate the relationships of structure, property, and (adsorptive) function of nano-hematite (nα-Fe2O3). As such, six nα-Fe2O3 particles were synthesized controlling for size, shape and surface area without capping agents. Sorbent characteristics of the six particles were then assessed for their impact on selenite (HSeO3-) and selenate (SeO42-) adsorption capacity and mechanism. Mechanism was assessed using in-situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine edge spectroscopy (EXAFS). Regression analyses were then performed to determine which characteristics best describe adsorption capacity and binding mechanisms of Se on nα-Fe2O3. The results demonstrate that crystal surface structure, specifically presence of the {0 1 2} facet promotes adsorption of Se and the presence of {0 1 2} facets promotes SeO42- sorption to a greater extent than HSeO3-. The data further indicates that {1 1 0} facets bind HSeO3- with binuclear complexes while {0 1 2} facets bind HSeO3- via mononuclear inner-sphere complexes. Specific nα-Fe2O3 facets also likely direct the ratio of inner to outer-sphere complexes in SeO42- adsorption.

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping.

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in nitrogen (N2) to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH3, and NH3 desorption. We found activation barriers (Ea) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al-N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal-nitrogen bonds and, thus, more-facile NH3 liberation.

Solvent Control of Surface Plasmon-Mediated Chemical Deposition of Au Nanoparticles from Alkylgold Phosphine Complexes.

Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuPPh3 onto the Ag surface because the PPh3 ligand is large compared to the size of the exposed surface between adsorbed benzenes. Instead, the Au-P bond of MeAuPPh3 dissociates in solution (Ea = 38.5 kcal/mol) in the plasmon heated near-surface region followed by the adsorption of the MeAu fragment on Ag in the interstitial space of the benzene monolayer. The adsorbed benzene forces the Au precursor to react through the higher energy path of dissociation in solution rather than dissociatively adsorbing onto the bare surface. This requires a higher temperature if the reaction is to proceed at a reasonable rate and enables the control of deposition by the light induced SPR heating of the surface and nearby solution.

Efficient generation of H2 by splitting water with an isothermal redox cycle.

Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.