Compositionally versatile, nonstoichiometric, mixed ionic–electronic conducting metal oxides of the form A n+1B n O3n+1 (n = 1 → ∞; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal ...(TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design strategies for describing their OER activities are largely derived assuming a static, unchanged view of their surfaces, despite reports of dynamic structural changes to 3d TM-based perovskites during OER. Herein, through variations in the A- and B-site compositions of A n+1B n O3n+1 oxides (n = 1 (A2BO4) or n = ∞ (ABO3); A = La, Sr, Ca; B = Mn, Fe, Co, Ni), we show that, in the absence of electrolyte impurities, surface restructuring is universally the source of high OER activity in these oxides and is dependent on the initial oxide composition. Oxide surface restructuring is induced by irreversible A-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent oxide core that serves as the active surface for OER. The rate of surface restructuring is found to depend on (i) composition of A-site cations, with alkaline-earth-metal cations dominating lanthanide cation dissolution, (ii) oxide crystal phase, with n = 1 A2BO4 oxides exhibiting higher rates of A-site dissolution in comparison to n = ∞ ABO3 perovskites, (iii) lattice strain in the oxide induced by mixed rare-earth- and alkaline-earth-metal cations in the A-site, and (iv) oxide reducibility. Among the in situ generated 3d TM oxyhydroxide structures from A n+1B n O3n+1 oxides, Co-based structures are characterized by superior OER activity and stability, even in comparison to as-synthesized Co-oxyhydroxide, pointing to the generation of high active surface area structures through oxide restructuring. These insights are critical toward the development of revised design criteria to include surface dynamics for effectively describing the OER activity of nonstoichiometric mixed-metal oxides.
Hydrodeoxygenation chemistries play a key role in the upgrading of biomass‐derived feedstocks. Among these, the removal of targeted hydroxyl groups through selective C−O bond cleavage from molecules ...containing multiple functionalities over heterogeneous catalysts has shown to be a challenge. Herein, we report a highly selective and stable heterogeneous catalyst for hydrodeoxygenation of tartaric acid to succinic acid. The catalyst consists of reduced Mo5+ centers promoted by palladium, which facilitate selective C−O bond cleavage, while leaving intact carboxylic acid end groups. Stable catalytic performance over multiple cycles is demonstrated. This catalytic system opens up opportunities for selective processing of biomass‐derived sugar acids with a high degree of chemical functionality.
Selective, active and stable heterogenous MoOx−Pd/TiO2 catalyst for hydrodeoxygenation of biomass‐derived feedstocks. High yields of 92 % succinic acid production from tartaric acid are reported over multiple catalytic cycles. This results from in‐situ reduction of supported MoOx centers promoted by Pd on TiO2.
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•Methane conversion processes using heterogeneous catalysts.•Technologically advanced routes involve indirect conversion of methane.•Advantages and challenges with direct methanol ...synthesis from methane.•Recent advances in direct methane conversion to fuels and chemicals.•Reaction mechanisms and catalyst performance in methane conversion processes.
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.
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 the surface structures are key at achieving a correlation between structure and activity of these materials. We make the case for combinatorial approaches involving quantum chemical calculations combined with detailed characterization, controlled synthesis, and testing as effective ways for developing the fundamental knowledge at the molecular level required to guide the design of efficient nonstoichiometric, mixed metal oxides for oxygen electrocatalysis. We conclude by summarizing current advances and devising future directions in this area.
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•Oxygen evolution (OER) in acid is important for electrochemical energy conversion.•Current advances in OER catalysis by mixed metal oxides are summarized.•Ionic-electronic conducting ...mixed metal oxides are promising electrocatalysts.•Instability induced by metal leaching is a challenge for oxide catalyzed acid OER.•Future directions in developing robust mixed metal oxides for acid OER are devised.
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.
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•Review of catalysis/electrocatalysis by first series Ruddlesden-Popper oxides.•Effect of the A and B site composition on the catalytic properties of R-P-1 oxides.•Synthesis plays an ...important role in the electrocatalytic/catalytic properties.•Potential of R-P-1 oxides for thermal catalysis has not been fully explored.
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.
Sodium-containing batteries have the potential to address many of the challenges faced in the ongoing development of enhanced energy storage devices. Sodium is inexpensive and earth abundant, and ...aprotic Na–O2 batteries, in particular, have gravimetric energy densities significantly exceeding those of Li-ion devices. However, poor functional cell lifespans present a significant obstacle to the development of Na–O2 cells, with parasitic side reactions involving the NaO2 discharge products, leading to a rapid decline in cell performance. These parasitic reactions are hypothesized to occur through two main pathways: (i) deleterious dissolution of NaO2 into the electrolyte during periods of cell idling and (ii) disproportionation of NaO2 in the near-surface region to form Na-rich species (Na1+x O2) on the cathode. To formulate practical strategies to suppress these processes, in turn, the development of fundamental, molecular-level mechanistic understanding is essential. In this contribution, such mechanistic insights are elucidated by coupling density functional theory calculations with experimental observations to study the surface chemistry of the NaO2 discharge product. First, a series of ab initio surface phase diagrams are constructed to determine the structure of the NaO2 surfaces under realistic operating conditions, whereby an inverse relationship between surface coordination and surface energy is determined. Next, a molecular surface dissolution analysis is performed for the identified surface terminations, demonstrating a further inverse relationship between surface energy and the thermodynamic barrier for dissolution. Finally, a study of the thermodynamics of thin-film formation of sodium oxides over the NaO2 discharge product is carried out and suggests that an electrochemical reduction reaction, rather than an inherent chemical disproportionation, forms the observed Na-rich species in the near-surface region under high discharge overpotentials. From these insights, we suggest future studies that may yield practical design changes to improve stability and extend the lifespan of Na–O2 batteries.
In this contribution, we report on the high activity and selectivity of Keggin structure molybdenum-based polyoxometalates (POMs) in catalyzing the epimerization of aldoses. Near-equilibrium ...conversions and selectivities were obtained within the first hour of operation under aqueous conditions at relatively low temperatures and a wide range of pHs. Characterization of the molybdenum-based POM catalysts using X-ray diffraction and FTIR studies before and after the reaction showed no evidence of their decomposition. Our studies suggest that the active sites for the reaction are the molybdenum oxide octahedra on the surface of the Keggin structure of the molybdenum-based POMs (H3PMo12O40, Ag3PMo12O40, Sn0.75PMo12O40). Further characterization of the system using 31P NMR and X-ray photoelectron spectroscopy experiments showed that the interaction between the aldose (e.g., glucose) and the molybdenum oxide octahedra in the POM results in electron transfer from the aldose to molybdenum, leading to the formation of the reduced form of the POM (also known as heteropoly blue). Isotope labeling experiments demonstrated that the epimerization of glucose using molybdenum-based POMs proceeds via an intramolecular C1–C2 shift mechanism with an activation barrier of as low as ∼96 kJ/mol, obtained using controlled kinetic experiments. These findings open up avenues for the implementation of molybdenum-based POMs as single, selective, and stable catalytic systems for the efficient epimerization of aldoses under aqueous conditions at relatively low temperatures and a wide range of pHs.
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 ...synthesis of fuels and chemicals using the commercially established Fischer–Tropsch process. The in situ generation of syngas (CO and H2) has also been demonstrated in SOECs through the coelectrolysis of CO2 and H2O. However, conventional Ni-based SOEC cathodes exhibit high overpotential losses associated with CO2 activation, leading to the disproportional activation of CO2 and H2O during coelectrolysis, facilitating the equilibrium-limited thermochemical reverse water gas shift (RWGS) reaction. Thus, identification of factors that govern CO2 activation on transition metal electrocatalysts is important toward optimizing the performance of SOEC cathodes for modulated production of syngas. Herein, we experimentally assess the electrocatalytic performance of monometallic transition metal electrocatalysts (Fe, Ni, and Pd) toward electrochemical CO2 reduction in SOECs with the aim of understanding the electrocatalyst characteristics that govern this performance. We report that metal oxophilicity (a property correlated to the strength of metal–oxygen bonding) plays an important role in the energetics associated with electrochemical CO2 reduction and electrocatalyst deactivation via oxidation. We suggest that a compromise in the oxophilicity of the metal is required to achieve optimal electrochemical activity and stability because CO2 activation is facile on highly oxophilic transition metals to the left of Ni (i.e., Fe); however, strong oxygen binding on these metals leads to their deactivation via oxidation. Potential approaches that facilitate the electronic structure modulation of transitional metals to optimize their surface oxophilicity, such as alloying, are suggested.
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