Role of water in metal catalyst performance for ketone hydrogenation: a joint experimental and theoretical study on levulinic acid conversion into gamma-valerolactone
Iron-doped,
nickel oxyhydroxide (Ni(Fe)OOH) is one of the
best catalysts for the oxygen evolution reaction (OER) under alkaline
conditions. Due to Ni(Fe)OOH’s layered structure, electrolyte
species are able to easily intercalate between the octahedrally coordinated
sheets. Electrolyte cations have long been considered inert spectator
ions during electrocatalysis, but electrolytes that penetrate into
the catalyst may play a major role in the reaction process. In a joint
theoretical and experimental study, we report the role of electrolyte
counterions (K+, Na+, Mg2+, and Ca2+) on Ni(Fe)OOH catalytic activity in alkaline media.
We show that electrolytes containing alkali metal cations (Na+ and K+) yield dramatically lower overpotentials
than those with alkaline earth cations (Mg2+ and Ca2+). K+ and Na+ lower the overpotential
because they have an optimal acidity and size that allows them to
not bind too strongly or alter the stability of reaction intermediates.
These two features required for intercalated cation species provide
insight into selecting appropriate electrolytes for layered catalyst
materials, and enable understanding the role(s) of electrolytes in
the OER mechanism.
Carbon supported nanoparticles of monometallic Ni catalyst and binary Ni-Transition Metal (Ni-TM/C) electrocatalytic composites were synthesized via the chemical reduction method, where TM stands for the doping elements Fe, Co, and Cu. The chemical composition, structure and morphology of the Ni-TM/C materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS). The electrochemical properties towards hydrogen oxidation reaction in alkaline medium were studied using the rotating disc electrode and cycling voltammetry methods. A significant role of the TM dopants in the promotion of the hydrogen electrooxidation kinetics of the binary Ni-TM/C materials was revealed. A record-high in exchange current density value of 0.060 mA cm2Ni was measured for Ni3Fe1/C, whereas the monometallic Ni/C counterpart has only shown 0.039 mA cm2Ni. In order to predict the feasibility of the electrocatalysts for hydrogen chemisorption, density functional theory was applied to calculate the hydrogen binding energy and hydroxide binding energy values for bare Ni and Ni3TM1.
NiOOH has recently been used to catalyze water oxidation by way of electrochemical water splitting. Few experimental data are available to rationalize the successful catalytic capability of NiOOH. Thus, theory has a distinctive role for studying its properties. However, the unique layered structure of NiOOH is associated with the presence of essential dispersion forces within the lattice. Hence, the choice of an appropriate exchange-correlation functional within Density Functional Theory (DFT) is not straightforward. In this work, we will show that standard DFT is sufficient to evaluate the geometry, but DFT+U and hybrid functionals are required to calculate the oxidation states. Notably, the benefit of DFT with van der Waals correction is marginal. Furthermore, only hybrid functionals succeed in opening a bandgap, and such methods are necessary to study NiOOH electronic structure. In this work, we expect to give guidelines to theoreticians dealing with this material and to present a rational approach in the choice of the DFT method of calculation.
International audienceTo screen heterogeneous catalysts in silico, the linear energyrelationships derived from the Brønsted−Evans−Polanyi principle are extremelyuseful. They connect the reaction energy of a given elementary step to its activationenergy, hence providing data that can be fed to kinetics models at a minimal cost.However, to ensure reasonable predictions, it is essential to control the statisticalerror intrinsic to this approach. We derived several types of linear energy relationsfor a series of CH and OH bond scissions in simple alcohol molecules on compactfacets of seven transition metals (Co, Ni, Ru, Rh, Pd, Ir, and Pt) aiming at a singlebut accurate relation. The quality of the relation depends on its nature and/or on themanner the data are split: a single linear relation can be constructed for all metalstogether on the basis of the original Brønsted−Evans−Polanyi formulation with amean absolute error smaller than 0.1 eV, whereas the more recent transition statescaling approach requires considering each metal individually to reach an equivalentaccuracy. In addition, a close statistical analysis demonstrates that errors stemmingfrom such predictive models are not uniform along the set of metals and of chemical reactions that is considered opening the road to a better control of error propagation
International audienceMolecules extracted from biomass can be complex, and computing their reactivity on a catalyst is a real challenge for theoretical chemistry. We present herein a method to predict polyalcohol reactivity in heterogeneous catalysis. We start from a set of simple alcohol molecules, and we show that an accurate linear energy relationship can be constructed from DFT calculations for the O–H and C–H dehydrogenation reactions. We then show that this relation can then be used for a fast prediction of the reactivity of glycerol. Compared with pure DFT calculations, our method provides results of good accuracy with a systematic deviation of ∼0.1 eV. We were able to prove that this deviation is caused mainly by intramolecular effects occurring in glycerol and not in simpler molecules
Dry reforming of methane (DRM) is an important reaction in the actual environmental and energy crisis context. It enables the production of syngas from CO 2 and CH 4 reforming. While Ni catalyst presents a high activity regarding this process, it often suffers from deactivation. It was found that the Sn-doped Ni catalyst can avoid carbon deposition, but a decrease in DRM reactivity was also observed. In this work, we used density functional theory calculations in combination with microkinetic modeling first to understand how Sn doping affects the resistance to carbon deposition and the surface catalytic activities of Ni. Based on the understandings, we found that an ideal dopant should give rise to a proper adsorption energy of carbon such that (i) the C* formation process, e.g., CH 4 dissociation, is rate-controlling to improve the carbon resistance and (ii) relatively low dissociation barriers of CH 4 and CO 2 can be achieved to maintain a good activity. Therefore, the adsorption energy of carbon and the dissociation barriers of CH 4 and CO 2 can be utilized as descriptors for the stability and activity of Ni-based catalysts. Subsequently, we screened several metal dopants and found that the descriptors designed are capable of providing a consistent activity and stability trend with experiments reported in the literature. Therefore, our work could provide relevant guidelines to rationally design efficient catalysts for the DRM reaction.
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