Eranda Nikolla

Measuring and relating the electronic structures of nonmodel supported catalytic materials to their performance.

Identifying structure-performance relationships is critical for the discovery and optimization of heterogeneous catalysts. Recent theoretical contributions have led to the development of d-band theory, which relates the calculated electronic structure of a metal to its chemical and catalytic activity. While there are many contributions where quantum-chemical calculations have been utilized to validate the d-band theory, experimental examples relating the electronic structures of commercially relevant nonmodel catalysts to their performance are lacking. We show that even small changes in the near-Fermi-level electronic structures of nonmodel supported catalysts, induced by the formation of surface alloys, can be measured and related to the chemical and catalytic performance of these materials. We demonstrate that critical shifts in the d-band center in alloys are related to the formation of new electronic states in response to alloying rather than to charge redistribution among constitutive alloy elements, i.e., the number of d holes and d electrons localized on the constitutive alloy elements is constant. On the basis of the presented results, we provide a simple, physically transparent framework for predicting shifts in the d-band center in response to alloying and relating these shifts to the chemical characteristics of the alloys.

Direct Electrochemical Oxidation of Hydrocarbon Fuels on SOFCs: Improved Carbon Tolerance of Ni Alloy Anodes

We show that Sn/Ni alloy anode electrocatalysts (0.5―1 wt % nominal loading of Sn with respect to Ni) exhibit improved tolerance to carbon-induced deactivation in the internal utilization of hydrocarbon fuels on solid oxide fuel cells (SOFCs) compared to conventional monometallic Ni electrocatalysts. The Sn/Ni electrocatalysts were tested in the direct electrochemical oxidation of methane and isooctane. It was observed that the electrochemical activity and electronic conductivity of Ni were not compromised significantly by introduction of Sn. We postulate that the improved carbon tolerance of Sn/Ni compared to monometallic Ni electrocatalysts is due to the formation of Sn/Ni surface alloys, which, unlike monometallic Ni, preferentially oxidize carbon atoms and fragments, removing these from the surface of the electrocatalyst rather than forming carbon―carbon bonds.

Electronic structure engineering in heterogeneous catalysis: Identifying novel alloy catalysts based on rapid screening for materials with desired electronic properties

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.

Synthesis of shape-controlled La2NiO4+δ nanostructures and their anisotropic properties for oxygen diffusion

This study highlights the synthesis of shape-controlled La2NiO(4+δ) nanostructures using a reverse microemulsion method. We report that surfactant to water mass ratio plays a key role in controlling the shape of the nanostructures. These nanostructures show a strong dependence of their oxygen transport properties on their geometries.

Communications: Developing relationships between the local chemical reactivity of alloy catalysts and physical characteristics of constituent metal elements

We have used X-ray absorption spectroscopy and quantum chemical density functional theory calculations to identify critical features in the electronic structure of different sites in alloys that govern the local chemical reactivity. The measurements led to a simple model relating local geometric features of a site in an alloy to its electronic structure and chemical reactivity. The central feature of the model is that the formation of alloys does not lead to significant charge transfer between the constituent metal elements in the alloys, and that the local electronic structure and chemical reactivity can be predicted based on physical characteristics of constituent metal elements in their unalloyed form.

Control of interfacial acid–metal catalysis with organic monolayers

AbstractNumerous 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.

From molecular insights to novel catalysts formulation

First-principles methods can be utilized to obtain elementary step mechanisms for chemical reactions on model systems. In this chapter, we will illustrate how this molecular information can be employed to motivate novel heterogeneous catalyst formulations. We will discuss a few examples where first-principles studies on idealized model systems were utilized, along with various experimental tools, to identify alloy catalysts that exhibit improved performance in a number of catalytic processes. We will emphasize the role of molecular approaches in the formulation of these catalysts.

Chapter 17 - Well-Defined Nanostructures for Catalysis by Atomic Layer Deposition

Abstract 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.

Heterogeneous electrocatalysts for CO2 reduction

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.

Nanostructured Nickelate Oxides as Efficient and Stable Cathode Electrocatalysts for Li–O 2 Batteries

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.