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Encapsulated Pd@metal–oxide catalysts were investigated as alternative active structures to supported materials for CO oxidation; the effect of the metal oxide (TiO2, ZrO2, and CeO2) on activity was studied and Pd@ZrO2 exhibited the highest activity.

Efficient electrochemical transformations of molecular oxygen (oxygen reduction and evolution) for energy conversion/storage rely largely on the effective design of heterogeneous electrocatalysts. ...

Alkali metal–O2 batteries (i.e., Li/Na–O2) with high specific energies are promising alternatives to state-of-the-art metal-ion batteries. However, they are plagued by challenges arising from the u...

Electrochemical reduction of CO2 using solid oxide electrolysis cells (SOECs) has emerged as an attractive approach for converting CO2 to high energy molecules, such as CO, a key precursor for the ...

Shaping the energy landscape through development of more efficient electrochemical energy conversion and storage devices requires significant advancements in the catalysis of key electrochemical processes involving oxygen. This is especially the case for the oxygen evolution reaction (OER), which is largely challenged by the cost-ineffectiveness of the best performing electrocatalysts (i.e., Ru, Ir), along with their limited stability under acidic conditions. This presents a roadblock in the development of robust acid-based polymer exchange membrane electrochemical systems, currently the most advanced technologies for electrochemical energy conversion. Approaches such as dilution of Ru/Ir into flexible mixed metal oxide frameworks have been used as alternative strategies in designing robust OER electrocatalysts. Herein, we discuss the state of research in this area and detail the effect of the composition and structure of mixed metal oxides on their acidic OER activity and stability. Future directions for developing mixed metal oxide electrocatalysts suitable for acidic electrochemical environments are devised.

There is a critical need for sustainable routes to produce hydrogen peroxide, H2O2. A promising approach involves direct synthesis from molecular hydrogen and oxygen at (sub)ambient temperatures us...

Recent advances in the use of nonstoichiometric mixed metal oxides belonging to the perovskite family as cost-effective catalysts for various oxygen-related heterogeneous thermochemical and electro...

Metal–metal oxide interfaces can manipulate catalytic selectivity in multistep reactions, including hydrodeoxygenation (HDO) of biomass derivatives like furfuryl alcohol. These interfaces combine active sites of different functionalities towards hydrogen activation, HDO and subsequent hydrogenation. However, examining the interplay between these different sites is essential to achieve control over the interfacial properties, and thus over the reaction selectivity towards 2-methylfuran. Herein, through DFT calculations, we investigate the role of TiO2 encapsulated Pd interfacial sites towards dictating HDO product selectivity. A rutile TiO2 (1 1 0) nanowire over a Pd (1 1 1) surface is used as the interfacial model. TiO2/Pd sites are found to provide a bifunctional role at the interface. The results show that TiO2 generates reduced oxide sites such that C O bond of furfuryl alcohol is activated relative to Pd (1 1 1), with the alcohol group re-oxidizing the reduced site. The Pd surface activates H2 and enables hydrogenation to the final product. Consequently, deoxygenation is accelerated over a TiO2−x/Pd oxygen deficient interface, with an approximate kinetic analysis suggesting that reduced interfacial sites accelerate direct deoxygenation by ∼108 at 443 K, altering the Pd selectivity from the undesired furan product to the desired 2-methylfuran.

Tuning catalysis at solid–solid interfaces is critical for the development of next-generation energy storage devices such as Li–O2 batteries, where solid lithium–oxygen species are formed and dissociated on a solid catalyst. Herein, atomically controlled synthesis is combined with theoretical calculations, electrochemical studies, and detailed characterization measurements to show that the interface between an oxide catalyst and the solid products is key to selectively control discharge product distribution, consequently affecting charge overpotentials. A surface structure-dependent electrochemical performance for nonprecious metal-containing, nanostructured lanthanum nickelate oxide (La2NiO4+δ, LNO) electrocatalysts is demonstrated. LNO nanostructures with (001) NiO-terminated surfaces exhibit lower charge overpotentials, as opposed to irregularly terminated polyhedral-shaped oxides of the same composition. It is found that these LNO nanostructures, with controlled surface structure, enhance the performa...

Dwindling fossil fuel resources and substantial release of CO2 from their processing have increased the appeal to use biomass as a sustainable platform for synthesis of chemicals and fuels. Steps toward this will require selective upgrading of biomass to suitable intermediates. Traditionally, biomass upgrading has involved thermochemical processes that require excessive amounts of petrochemical-derived H2 and suffer from poor product selectivity. Electrochemical routes have emerged as promising alternatives because of ( a) the replacement of petrochemical-derived H2 by protons generated in situ, ( b) mild operating temperatures and pressures, and ( c) the use of electrode potential to tune reaction rates and product selectivity. In this review, we highlight the advances in the electrocatalytic hydrogenation and oxidation of biomass-derived platform molecules. The effects of important reaction parameters on electrochemical efficiency and catalytic activity/selectivity are thoroughly discussed. We conclude by summarizing current challenges and discussing future research directions.

High temperature electrochemical energy conversion and storage technologies, such as solid oxide electrochemical cells (SOCs), have emerged as promising alternatives to mitigate environmental issues associated with combustion-based technologies. There has been increased interest for nanoengineering SOC electrodes to enhance their efficiency. A major drive is the necessity for improved electrode kinetics via optimization of electrocatalysts for different key reactions in these devices. In this perspective, we discuss the requirements for SOC electrodes and nanoengineering strategies employed to achieve flexibility in electrode materials. We focus on identifying ways in which these nanoengineered materials foster advancements in the SOC electrocatalytic activity, selectivity, and stability. We conclude by proposing approaches that would lead to more stable electrocatalytic nanostructures with high degree of control over the number and nature of active sites. These nanostructures would enable systematic kinetic studies that could provide an in depth understanding of the reaction mechanisms that govern performance, leading to valuable knowledge for designing optimal electrode materials.

Inverted catalytic systems, in which metal oxide films are deposited on top of metallic substrates, present significant potential for catalysis, because of the ability to enhance the metal/metal oxide interface. Most of the reported methods for synthesizing these structures involve thermal treatment of the oxide film, which presents a significant challenge for reducible metal oxides, because of strong metal-oxide interactions, often leading to blocking of metal active sites. Herein, a solution-phase method is developed to synthesize Pd@TiO2 inverted catalytic structures where encapsulation is conducted at room temperature using a sol–gel reaction of Ti alkoxide precursors to circumvent thermal treatment. We show that key synthesis parameters can be used to control the desired physical properties of the inorganic encapsulating film to achieve enhanced catalytic performance. Significant effects on catalytic performance toward a probe reaction, hydroisomerization of 1-hexene, are reported as a function of th...

Solid oxide electrolysis cells (SOECs) are promising for the selective electrochemical conversion of CO2, or mixed streams of CO2 and H2O, into high energy products such as CO and H2. However, these systems are limited by the poor redox stability of the state-of-the-art Ni-based cathode electrocatalysts. Due to their favorable redox properties, mixed ionic-electronic conducting (MIEC) oxides have been considered as promising alternatives. However, improvement of the electrochemical performance of MIEC-based SOEC electrocatalysts is needed and requires an understanding of the factors that govern their activity. Herein, we investigate the effect of B-site 3d metal cations (Cr, Fe, Co, Ni) of LaBO3 perovskites on their CO2 electrochemical reduction activity in SOECs. We find that their electrochemical performance is highly dependent on the nature of the B-site cation and trends as LaFeO3 > LaCoO3 > LaNiO3 > LaCrO3. Among these perovskites, LaNiO3 is the least stable and decomposes under electrochemical conditions. In situ characterization and ab initio theoretical calculations suggest that both the nature of the B-site cation and the presence of oxygen surface vacancies impact the energetics of CO2 adsorption and reduction. These studies provide fundamental insights critical toward devising ways to improve the performance of MIEC-based SOEC cathodes for CO2 electroreduction.

Electrocatalysis of oxygen reduction and evolution (ORR and OER) have become of significant importance due to their critical role in the performance of electrochemical energy conversion and storage devices, such as fuel cells, electrolyzers, and metal air batteries. While efficient ORR and OER have been reported using noble-metal based catalysts, their commercialization is cost prohibitive. In this Perspective, we discuss the potential of nonprecious metal based, mixed electronic–ionic conducting oxides (i.e., perovskites, double perovskites, and Ruddlesden–Popper (R-P) oxides) for efficient oxygen electrocatalysis at high and low temperatures. The nonstoichiometry of oxygen in these materials provides key catalytic properties that facilitate efficient ORR/OER electrocatalysis. We discuss the importance of surface structure and composition as critical parameters to understand and tune the ORR/OER activity of these oxides. We argue that techniques facilitating controlled synthesis and characterization of t...

There has been a recent surge of interest in multicomponent catalysts that combine properties of chemically diverse materials. A major factor in this increased interest is the widespread recognition that the scaling relationships for adsorption and transition state energies of reactions place significant constraints on making step-change improvements in catalyst performance using monofunctional catalysts. In this perspective, we review the fundamental rationale for multicomponent materials and describe several classes of materials that offer promise for improving activity and selectivity in catalysis. Our focus is on illustrating how recent advances in the ability to prepare precisely controlled multicomponent nanostructures have the potential to enhance the capability to design highly active and selective catalysts.

Numerous important reactions consisting of combinations of steps (for example, hydrogenation and dehydration) have been found to require bifunctional catalysts with both a late-transition metal component and an acidic component. Here, we develop a method for preparing and controlling bifunctional sites by employing organic acid-functionalized monolayer films tethered to the support as an alternative to traditional ligand-on-metal strategies. This approach was used to create a reactive interface between the phosphonic acid monolayers and metal particles, where active-site properties such as acid strength were manipulated via tuning of the molecular structure of the organic ligands within the monolayer. After surface modification, the resultant catalysts exhibited markedly improved selectivity and activity towards hydrodeoxygenation of aromatic alcohols and phenolics. Moreover, by tuning the ligand of the acidic modifier, the rate of deactivation was significantly reduced. Bifunctional heterogeneous catalysts are usually prepared by dispersion of a metal on an acidic or basic support. Now a method has been developed to post-functionalize a catalyst and introduce tunable acidity by coating an organic acid layer on the support, resulting in improved performance as showcased for selected hydrodeoxygenation reactions.

Electrochemical high-temperature oxygen reduction and evolution play an important role in energy conversion and generation using solid oxide electrochemical cells. First-series Ruddlesden–Popper (R-P) oxides (A2BO4) have emerged as promising electrocatalysts for these reactions due to their suitable mixed ionic and electronic conductivities. However, a detailed understanding of the factors that govern their performance is still elusive, making their optimization challenging. In the present work, a systematic theoretical study is used to investigate the underlying factors that control the process of surface oxygen exchange, which governs oxygen reduction and evolution on these oxides. The effects of A- and B-site composition and surface termination of these oxides on their activities are elucidated. Among the different compositions, Co-based, B-site-terminated R-P oxides are predicted to exhibit the highest activity due to providing the best compromise between the energetics associated with oxygen dissocia...

Electrochemical reduction of CO2 using the electrical energy generated from renewable sources has attracted increasing interest as a potential route for producing high energy molecules from CO2. In this contribution, high temperature electrochemical reduction of CO2 to CO on a series of transition metal electrocatalysts is studied using DFT calculations combined with microkinetic modeling under solid oxide electrolysis cells (SOECs) operating conditions. We show that CO2 dissociation via a two-electron transfer process into adsorbed CO and O2– ions in the electrolyte is favorable on most of the metal electrocatalysts considered, with a dependence of the simulated electrolysis current density on the applied potential consistent with experimental observations. A “volcano”-type relation between the calculated electrolysis rates and the binding energies of atomic O is found, suggesting that the binding energy of O might be a good activity descriptor for high temperature CO2 electrolysis on transition metals. ...

Recent efforts to design selective catalysts for multi-step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO2 films of regulated porosity to gain a new level of control over catalyst performance, resulting in essentially 100 % HDO selectivity for two biomass-derived alcohols. This catalyst also showed exceptional reaction specificity in HDO of furfural and m-cresol. In addition to improving HDO activity by maximizing the interfacial contact between the metal and metal oxide sites, encapsulation by the nanoporous oxide film provided a significant selectivity boost by restricting the accessible conformations of aromatics on the surface.

In this short review, we highlight the recent advances in methane conversion processes at high and low temperatures. Methane conversion processes are of great importance in achieving a crude-oil independent supply of energy, fuels and chemicals for the future. Direct conversion of methane into chemicals and fuels has been often considered as the “holy grail” of current catalysis research due to the unreactive nature of methane, which makes targeted chemical transformations to fuels and chemicals very challenging. We discuss the progress in developing heterogeneous catalytic and electrocatalytic systems to overcome this challenge. We conclude by providing a perspective on the future of this area of research.

Kinetics of high temperature oxygen reduction reaction (ORR) on La2NiO4 + δ (LNO) nanostructures are investigated by means of electrochemical impedance spectroscopy, with the aim of determining (i) the critical steps that govern ORR in these catalysts, and (ii) ways to lower the overpotential losses associated with these steps. We have identified two main electrochemical processes that govern the polarization resistances during ORR: the electron transfer/oxygen vacancy healing ( O a d s + 2 e − + V O ⋅ ⋅ ⇔ O O ( L N O ) X ) , and the oxygen ion transfer through the electrocatalyst/electrolyte interface ( O O ( L N O ) X + V O ⋅ ⋅ ⇔ O O ( Y S Z ) X ) . We find that the nanostructure of LNO significantly effects the activation barriers associated with these processes with nanorod–structured LNO catalyst, highly terminated by [001] surface facets, exhibiting lower barriers compared to traditional, spherical-shaped catalysts. We also show that incorporation of the nanorod–structured LNO as cathode electrocatalysts in SOFCs leads to a significant improvement in the cell performance. These findings provide important insights on the electrochemical steps that govern ORR kinetics on LNO electrocatalyst, and ways to optimize these materials as cathode electrocatalysts for intermediate temperature SOFCs (IT-SOFCs).

First series Ruddlesden-Popper oxides (referred herein as R-P-1, with a formula An+1BnO3n+1 where n = 1) have been used in a number of electrochemical and thermochemical reactions. In this review, we examine in detail the effect of the synthesis methods and the composition of the A and B sites on their electrocatalytic/catalytic activity. Effects on important activity parameters, such as surface exchange coefficient (k), oxygen diffusion coefficient (D), hyperstoichiometry (δ), and electronic conductivity are discussed. We find that synthesis plays an important role in their final structure and hyperstoichiometric oxygen content, which significantly impact their activity. In addition, we show that the composition of the A and B sites has an effect on the catalytic/electrocatalytic activity parameters, such as D, k, δ, and electrical conductivity. The use of these oxides for thermal-catalysis is also discussed. We find that while R-P-1 oxides have been widely implemented for high temperature electrocatalysis, their potential for thermal-catalysis has not been fully explored. Limited reports on thermal-catalysis suggest that the redox properties of the B-site transition metal in these oxides, as well as the oxide’s ability to accept and release oxygen under reaction conditions play an important role in their catalytic activity. A perspective on catalysis by R-P-1 oxides is provided at the end of the review.

Advancing our understanding of solid–solid interfacial electrocatalysis is central for engineering the next generation energy storage technologies. Li-O2 batteries provide the highest energy density among all battery technologies, making them attractive for the widespread electrification of the transportation (>500 miles) and the aviation sectors.1 These regenerative systems rely on the reversible redox chemistry between metallic Li and molecular O2 leading to the formation and dissociation of solid Li x O2species (xLi+ + O2 + xe– -> Li x O2; E0 = 3.0 V vs. Li/Li+; 1=< x =< 2). Although promising, these systems suffer from large overpotential losses potentially stemming from challenges in electron transport through Li x O2 species, consequently resulting in reduced round-trip efficiencies (55-60%).1 Various catalysts have been used to overcome these losses. However, lack of a fundamental understanding on the interfacial factors that dictate selective formation of Li x O2 species on an electrocatalyst surface, has hindered systematic optimization of the energetics for these processes. Therefore, development of a framework to investigate the atomistic interactions between an electrocatalyst surface and the formed Li x O2 solid products is key to enhancing their overall performance. In this contribution, atomically-controlled synthesis, detailed electrochemical and characterization studies, along with periodic density functional theory (DFT) calculations are combined to showcase the importance of the surface structure in tailoring the solid–solid interfacial catalysis for enhanced cell performance.2 This is demonstrated through the incorporation of non-precious 3d metal-based mixed metal oxide cathode electrocatalysts belonging to the first-series Ruddlesden-Popper (R-P) phase of the general form (A = La, Ca, Sr; B = Mn, Fe, Co, Ni).3 The flexibility in the A- and B-site compositions, can be explored to tune the geometric and the electronic structure of the catalysts, thus making them appealing candidates.4-5 Initially, the experimental and theoretical calculations are benchmarked using La2NiO4 (LNO) as the catalyst. A significant enhancement in the overall performance (>0.7 V) is observed upon the incorporation of catalytically active (001) NiO terminated LNO. The discharge products formed on these surfaces are characterized using numerous techniques, including Raman spectroscopy, chemical titration and mass spectroscopy. These studies indicate that the enhanced performance of LNO stems from its ability to effectively stabilize electronically conductive lithium deficient LixO2 (x2O2. The developed combinatorial framework for LNO, is extended to various A- and B-site systems to identify the geometric and electronic factors that aid in selective perturbation of film formation energetics, that leads to enhanced performance. A framework for tuning solid–solid interfacial catalysis on these systems is devised; knowledge that is critical for enhancing the efficiency of next generation energy storage technologies. References (1) Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G., Nat. Energy 2016, 1, 16128. (2) Samira, S.†; Deshpande, S.†; Roberts, C. A.; Nacy, A. M.; Kubal, J.; Matesic, K.; Oesterling, O.; Greeley, J.; Nikolla, E., Chem. Mater. 2019, 31, 7300-7310. (3) Gu, X. K.†; Samira, S.†; Nikolla, E., Chem. Mater. 2018, 30, 2860-2872. (4) Gu, X. K.; Carneiro, J. S. A.; Samira, S.; Das, A.; Ariyasingha, N. M.; Nikolla, E., J. Am. Chem. Soc. 2018, 140, 8128-8137. (5) Samira, S.†, Gu, X. K.†, Nikolla, E., ACS Catal. 2019, 9, 10575-10586.

Hydrogen production from water electrolysis using solid oxide electrolysis cells (SOECs) has attracted considerable attention because of favorable kinetics and thermodynamics associated with operation at elevated temperatures. In the present work, we employ density functional theory calculations combined with microkinetic modeling to investigate the factors that govern this process on Ni and Ni-based alloy electrocatalysts. Our studies show that H2O dissociation is the rate-limiting step on Ni(111) and Ni(211), with Ni(211) exhibiting the lowest barrier for this step. The effect of alloying Ni with another metal on the energetics associated with this process is also investigated. Our studies show that the binding energies of the most abundant intermediates, OH and O, become gradually weaker, and the barriers for water dissociation become gradually higher as Ni is alloyed with metals from left to right in the periodic table. A volcano-type relationship between the calculated electrochemical rates and the b...

Synthetically tuning the surface properties of many oxide catalysts to optimize their catalytic activity has been appreciably challenging, given their complex crystal structure. Nickelate oxides (e.g., La2NiO4+δ) are among complex, layered oxides with great potential toward efficiently catalyzing chemical/electrochemical reactions involving oxygen (oxygen reduction, ammonia oxidation). Our theoretical calculations show that the surface structure of La2NiO4+δ plays a critical role in its activity, with the (001)-Ni oxide-terminated surface being the most active. This is demonstrated through the effect on the energetics associated with surface oxygen exchange, a key process in reactions involving oxygen on these oxides. Using a reverse microemulsion method, we have synthesized La2NiO4+δ nanorod-structured catalysts highly populated by (001)-Ni oxide-terminated surfaces. We show that these nanostructures exhibit superior catalytic activity toward oxygen exchange/reduction as compared with traditional catalys...

Li–O2 (Li–air) batteries are among the most promising energy storage technologies due to their high theoretical specific capacity and energy density. Key challenges with this technology include high overpotential losses associated with catalyzing the electrochemical reactions (i.e., oxygen reduction and evolution reactions) at the cathode of the battery. In this contribution, we report through the example of La2NiO4+δ that layered nickelate oxide materials with rod-shaped nanostructure exhibit promising electrochemical performance as cathode electrocatalysts for Li–O2 batteries. We demonstrate the ability to control the nanostructure of La2NiO4+δ electrocatalyst at the nanoscale level using a reverse-microemulsion synthesis approach. We show that Li–O2 batteries with cathodes containing rod-shaped La2NiO4+δ electrocatalyst exhibit lower charging potentials and higher reversible capacities when compared to batteries with carbon-only cathodes. Our studies indicate that the enhancement in the battery performance induced by the rod-shaped La2NiO4+δ electrocatalyst can be attributed to the fact that La2NiO4+δ nanorods (i) facilitate the formation of nanosized Li2O2 particles during discharge, and (ii) promote the electrocatalytic activity toward the oxygen evolution reaction during charging. These findings open up avenues for the utilization of (i) reverse-microemulsion method for controlling the nanostructure of layered oxide materials, and (ii) nanorod-structured nickelate oxides as efficient cathode electrocatalysts for Li–O2 batteries.

Recent efforts to design selective catalysts for multi-step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO

Extensive CO2 emissions from the processing of fossil fuels for energy generation have become a major contemporary challenge. An avenue to alleviate this problem is to electrochemically transform CO2 to high-energy molecules. In this chapter, we discuss promising heterogeneous electrocatalysts for low and high temperature electrochemical reduction of CO2 to valuable products, such as CO and hydrocarbons. Electrocatalyst size/composition/morphology effects on the activity, selectivity, and stability along with the proposed underlying mechanisms that govern low temperature electrochemical reduction of CO2 on promising electrocatalytic materials are discussed. Similarly, the performance and challenges of promising cathode electrocatalysts (i.e., Ni, bimetals, and mixed oxides) for high-temperature electrochemical reduction of CO2 using solid oxide electrolysis cells are evaluated. The chapter is concluded with a perspective on low- and high-temperature electrochemical reduction of CO2 by means of heterogeneous electrocatalysis.

Atomic layer deposition (ALD) has evolved as an important tool for the design and synthesis of well-defined nanostructures for catalysis. Its self-limiting nature makes possible the preparation of catalytic materials with well-defined subnanometer and nanometer features on high-surface-area supports with near-atomic precision. This allows for the establishment of the correlation between the active site structures and catalytic performance, thus providing insights for understanding the reaction mechanism and the rational design of improved catalysts. Important examples include ALD of metal oxides for the synthesis of catalytic sites and deposition of protecting layers on other materials, modification of high-surface-area supports, and synthesis of supported monometallic and bimetallic catalysts with controlled size, composition, and morphology, among others. This chapter provides the reader with detailed information on tailoring nanostructures for catalytic applications using ALD and a discussion about the challenges and opportunities for the field.

First series Ruddlesden-Popper oxides (referred herein as R-P-1, with a formula An+1BnO3n+1 where n = 1) have been used in a number of electrochemical and thermochemical reactions. In this review, we examine in detail the effect of the synthesis methods and the composition of the A and B sites on their electrocatalytic/catalytic activity. Effects on important activity parameters, such as surface exchange coefficient (k), oxygen diffusion coefficient (D), hyperstoichiometry (δ), and electronic conductivity are discussed. We find that synthesis plays an important role in their final structure and hyperstoichiometric oxygen content, which significantly impact their activity. In addition, we show that the composition of the A and B sites has an effect on the catalytic/electrocatalytic activity parameters, such as D, k, δ, and electrical conductivity. The use of these oxides for thermal-catalysis is also discussed. We find that while R-P-1 oxides have been widely implemented for high temperature electrocatalysis, their potential for thermal-catalysis has not been fully explored. Limited reports on thermal-catalysis suggest that the redox properties of the B-site transition metal in these oxides, as well as the oxide’s ability to accept and release oxygen under reaction conditions play an important role in their catalytic activity. A perspective on catalysis by R-P-1 oxides is provided at the end of the review.

The immense phase space of multimetallic materials spanned by structural and compositional degrees of freedom precludes thorough screening for efficient alloy catalysts, even with combinatorial high-throughput experiments or quantum-chemical calculations. Based on X-ray absorption spectroscopy measurements and density functional theory calculations, we have identified critical electronic structure descriptors that govern local chemical reactivity of different sites in metal alloys. These descriptors were used to develop a model that allows us to predict variations in the adsorption energy of various adsorbates on alloy surfaces based on easily accessible physical characteristics of the constituent elements in alloys, mainly their electronegativity, atomic radius, and the spatial extent of valence orbitals. We show that this model, which is grounded on validated theories of chemisorption on metal surfaces, can be used to rapidly screen through a large phase space of alloy catalysts and identify optimal alloys for targeted catalytic transformations. We underline the potential of the electronic structure engineering, relating alloy geometry to its catalytic performance using simple electronic structure descriptors, in catalysis.

While it is fairly straightforward to predict the relative chemical reactivity of pure metals, obtaining similar structure-performance relationships for alloys is more challenging. In this contribution we present experimental analysis supported with quantum chemical DFT calculations which allowed us to propose a simple, physically transparent model to predict the impact of alloying on the local electronic structure of different sites in alloys and on the local chemical reactivity. The model was developed through studies of a number of Pt alloys. The central feature of the model is that hybridization of d-orbitals in alloys does not lead to significant charge transfer between the constituent elements in the alloy, and therefore the width of the local density of d-states projected on a site, which is easily calculated from tabulated parameters, is an excellent descriptor of the chemical reactivity of the site.

We demonstrate that supported Sn/Ni alloy catalyst is more resistant to deactivation via carbon deposition than supported monometallic Ni catalyst in steam reforming of isooctane at moderate steam to carbon ratios, irrespective of the average size of metal particles and the metal loading. The experiments were performed for average diameters of catalytic particles ranging from 30 to 500 nm and for the loading of active material ranging from 15 to 44 wt% with respect to the total mass of catalyst. The steam reforming reactions were performed at conditions that are consistent with typical solid oxide fuel cell (SOFC) operating conditions. DFT calculations show that the reasons for the enhanced carbon-tolerance of Sn/Ni compared to monometallic Ni are high propensity of Sn/Ni to oxidize carbon and lower driving force to form carbon deposits on low-coordinated metal sites.

Carbon-induced catalyst deactivation is one of the main problems associated with the electrocatalytic and catalytic reforming of hydrocarbons over supported Ni catalysts. We have used DFT calculations to study various aspects of carbon chemistry on Ni surfaces. We demonstrate that the carbon tolerance of Ni can be improved by synthesizing Ni-containing surface alloys that, compared to monometallic Ni, preferentially oxidize C atoms rather than form C–C bonds and have a lower thermodynamic driving force, associated with the nucleation of carbon atoms on low-coordinated Ni sites. Using the molecular insights obtained in the DFT calculations, we have identified Sn/Ni surface alloy as a potential carbon-tolerant reforming catalyst. The predictions of the DFT calculations were supported by our reactor and catalyst characterization studies, which showed that Sn/Ni is much more resistant to carbon poisoning than monometallic Ni in the steam reforming of methane, propane, and isooctane at moderate steam-to-carbon ratios.

We have carried out equilibrium dissociation vapor pressure measurements on dimethonium bromide dihydrate and pentamethonium chloride and bromide dihydrates. None of these salts forms a monohydrate. Surprisingly the thermodynamic parameters for the two pentamethonium hydrates are nearly identical, although the properties of these hydrates are quite different. This is explained by a larger negative differential lattice enthalpy for the chloride dihydrate dissociation, which lowers the observed enthalpy of H 2 O removal, and greater stabilization of saturated solution by chloride ion which makes the chloride dihydrate deliquescent while the bromide dihydrate is efflorescent. Infrared comparison suggests that tetramethonium chloride dihydrate and pentamethonium chloride and bromide dihydrates have the ladder type halide–water structure determined by X-ray analysis in tetramethonium bromide dihydrate.

We have used HF/3-21G(*) geometry optimization to determine the relative energies, structures, symmetries, and nature of frontier orbitals for the seven isomeric [7.7.10 x , y ]ousenes in which two cationic tropyliumyl (C 7 H 6 + –) rings are substituted on the cage of the B 10 H 10 2− anion. The x , y =(2,7) isomer is the most stable. Energetically the remaining fall into two groups: (1,10), (2,4), and (1,6) which differ from (2,7) by less than 2 kcal mol −1 , and (2,6),(2,3), and (1,2) which differ by about 10 kcal mol −1 . The lower stability of the latter group is attributed to repulsive interactions between substituent rings rather than electronic effects. Three ousenes (1,2), (1,10), and (2,3) have lower symmetry than expected for a simple disubstituted B 10 H 10 2− cage as a result of steric effects. Examination of frontier orbitals demonstrates that all seven species should show charge transfer excitations from cage to both rings similar to that previously experimentally observed for the [7.10 2 ] hemiousenide ion.

We have used ab initio (HF/3-21G(∗) and HF/6-31+G ∗ ) single point energy calculations on model structures to investigate the nature of methyl C–H to anion interactions in tetramethylammonium tetrahydroborate. The preferred structure has four cations about the anion in a D 2 d arrangement, each of which forms a CHHB dihydrogen bond to anion. This arrangement is in good accordance with previous infrared spectral studies. The alternative D 2 d arrangement with cation C–H directed to faces of the BH 4 − tetrahedron is 6 kcal mol −1 higher in energy than the dihydrogen bonded model.

The immense phase space of multimetallic materials spanned by structural and compositional degrees of freedom precludes thorough screening for efficient alloy catalysts, even with combinatorial high-throughput experiments or quantum-chemical calculations. Based on X-ray absorption spectroscopy measurements and density functional theory calculations, we have identified critical electronic structure descriptors that govern local chemical reactivity of different sites in metal alloys. These descriptors were used to develop a model that allows us to predict variations in the adsorption energy of various adsorbates on alloy surfaces based on easily accessible physical characteristics of the constituent elements in alloys, mainly their electronegativity, atomic radius, and the spatial extent of valence orbitals. We show that this model, which is grounded on validated theories of chemisorption on metal surfaces, can be used to rapidly screen through a large phase space of alloy catalysts and identify optimal alloys for targeted catalytic transformations. We underline the potential of the electronic structure engineering, relating alloy geometry to its catalytic performance using simple electronic structure descriptors, in catalysis.

[EN] Conversion of carbohydrates to 5-(hydroxymethyl)furfural (HMF) may provide a step forward toward achieving a renewable biomass-based chemicals and fuels platform. Recently, we reported that a tin-containing, high-silica molecular sieve with the zeolite beta topology (Sn-Beta) can efficiently catalyze the isomerization of glucose to fructose in aqueous media at low pH. Herein, we describe the combination of Sn-Beta with acid catalysts in a one vessel, biphasic reactor system to synthesize HMF from carbohydrates such as glucose, cellobiose, and starch with high efficiency. HMF selectivities over 70% were obtained using this ¿one-pot¿ biphasic water/tetrahydrofuran (THF) reactor system. The key to successfully achieving the conversions/selectivities reported is that Sn-Beta is able to convert glucose to fructose at pH near 1 and in saturated aqueous salt solutions.