Search

Efficient and inexpensive catalysts for the O2 reduction reaction (ORR) are needed for the advancement of renewable energy technologies. In this study, we designed a computational catalyst‐screening method to identify single and di‐atom metal dopants from first‐row transition elements supported on defect‐containing nitrogenated graphene surfaces for the ORR. Based on formation‐energy calculations and micro‐kinetic modelling of reaction pathways using intermediate binding free energies, we have identified four potentially interesting single‐atom catalysts (SACs) and fifteen di‐atom catalysts (DACs) with relatively high estimated catalytic activity at 0.8 V vs RHE. Among the best SACs, MnNC shows high stability in both acidic and alkaline media according to our model. For the DACs, we found four possible candidates, MnMn, FeFe, CoCo, and MnNi doped on quad‐atom vacancy sites having considerable stability over a wide pH range. The remaining SACs and DACs with high activity are either less stable or show a stability region at an alkaline pH. [ABSTRACT FROM AUTHOR]

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO4 > HNO3 > H2SO4, and the OER activity trend of HClO4 > HNO3 ∼ H2SO4. HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions Cl O 4 − binds the weakest followed by N O 3 − and S O 4 2 − . Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering. Microenvironment engineering through electrolyte optimization is a promising approach to mitigate catalyst poisoning effects in electrochemical systems, but the role of electrolyte anions is not fully understood. Here, in a combined experimental-theoretical evaluation, the authors study the effects of different acidic electrolytes (pH 1) on platinum for hydrogen (HER/HOR) and oxygen electrocatalysis (ORR/OER), finding that oxygen reduction performance can be improved 4-fold using nitric rather than sulfuric acid. [ABSTRACT FROM AUTHOR]

It has been a long‐standing goal to find alternative, cost effective catalysts for oxygen reduction reaction (ORR) in fuel cells. Molecular complexes capable of efficiently catalyzing ORR, when supported on a conducting solid surface could provide interesting hybrid materials combining the best of homogeneous and heterogeneous catalysis. We use density functional theory to study the ORR activity of molecular (unsupported) and gold supported 3d metal (II)‐porphycenes (MPc). All the 3d metal‐porphycenes adsorb strongly on the Au(111) support with negative free energy of formation ranging from 1.0–3.0 eV while solvation lowers the energy of molecular MPc by only 0.5 eV compared to the gas phase. These indicate that the 3d MPc are stable and anchored firmly to the gold surface under ORR conditions. Based on our analysis, the gold support enhances the ORR activity in some of the examined MPc systems. We find that the molecular MnPc and FePc are the best ORR electrocatalysts with theoretical overpotentials of 0.44 V and 0.36 V. This work highlights the important role of the support in preserving/increasing the activity of molecular catalyst. The role of conducting solid support in enhancing/preserving the ORR activity of molecular catalysts is investigated with DFT. Such combination makes use of the best of homogeneous and heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

The interaction between N2 and the surface results in a considerable rearrangement of the structure of the surface near the steps. The present work investigates the effect of this interaction on the rate of dissociative adsorption within the framework of harmonic transition state theory. Employing different models where selected Ru degrees of freedom are included, we show that the dominant effect of surface relaxation is a shift of the classical barrier height. Therefore, the “relaxed/N2” model, which takes relaxation of the surface into account but neglects vibrations of the surface atoms explicitly, can be used for reliable rate constant calculations. Sticking coefficients calculated employing the relaxed/N2 model based on different density-functional theory (DFT) calculations are then compared with experiment. Comparison with the experiment suggests that for the best DFT calculations the barrier height (0.49 eV) is about 0.15 eV too high. [ABSTRACT FROM AUTHOR]

This paper demonstrates a method for screening transition metal and metal alloy catalysts based on their predicted rates and stabilities for a given catalytic reaction. This method involves combining reaction and activation energies (available to the public via a web-based application ‘CatApp’) with a microkinetic modeling technique to predict the rates and selectivities of a prospective material. This paper illustrates this screening technique using the steam reforming of methane to carbon monoxide and hydrogen as a test reaction. While catalysts are already commercially available for this process, the method demonstrated in this paper is very general and could be applied to a wide range of catalytic reactions. Following the steps outlined herein, such an analysis could potentially enable researchers to understand reaction mechanisms on a fundamental level and, on this basis, develop leads for new metal alloy catalysts. [ABSTRACT FROM AUTHOR]

Trends in electrocatalytic activity of the oxygen evolution reaction (OER) are investigated on the basis of a large database of HO* and HOO* adsorption energies on oxide surfaces. The theoretical overpotential was calculated by applying standard density functional theory in combination with the computational standard hydrogen electrode (SHE) model. We showed that by the discovery of a universal scaling relation between the adsorption energies of HOO* vs HO*, it is possible to analyze the reaction free energy diagrams of all the oxides in a general way. This gave rise to an activity volcano that was the same for a wide variety of oxide catalyst materials and a universal descriptor for the oxygen evolution activity, which suggests a fundamental limitation on the maximum oxygen evolution activity of planar oxide catalysts. [ABSTRACT FROM AUTHOR]

Abstract: A general scheme for the screening of heterogeneous catalysts using density functional theory (DFT) calculations is presented, and the scheme is illustrated with a search for catalysts for the hydrogen evolution reaction. Simple techniques to estimate the activity of binary surface and bulk alloys for this reaction are described, and several computational tests for stability in reaction environments are presented. Careful application of these activities and stability criteria to a database of DFT calculations on ∼750 binary transition metal alloys leads to the identification of several surface and bulk alloys that are predicted to perform comparably to platinum, the canonical hydrogen evolution catalyst. This study marks the first use of full DFT calculations for high-throughput screening of transition metal catalysts. [Copyright &y& Elsevier]

The pace of materials discovery for heterogeneous catalysts and electrocatalysts could, in principle, be accelerated by the development of efficient computational screening methods. This would require an integrated approach, where the catalytic activity and stability of new materials are evaluated and where predictions are benchmarked by careful synthesis and experimental tests. In this contribution, we present a density functional theory-based, high-throughput screening scheme that successfully uses these strategies to identify a new electrocatalyst for the hydrogen evolution reaction (HER). The activity of over 700 binary surface alloys is evaluated theoretically; the stability of each alloy in electrochemical environments is also estimated. BiPt is found to have a predicted activity comparable to, or even better than, pure Pt, the archetypical HER catalyst. This alloy is synthesized and tested experimentally and shows improved HER performance compared with pure Pt, in agreement with the computational screening results. [ABSTRACT FROM AUTHOR]

The article offers an overview of the study conducted by Loffreda and colleagues regarding the order in which chemical bonds form in molecules bound to a surface of a catalyst. It mentions that researchers have shown that surface-bond energy is a descriptor of reaction selectivity. It mentions that they showed that the order in which various double bonds of unsaturated aldehydes are hydrogenated on a platinum surface can be understood by dividing potential reactions that could occur into groups.

• Density functional theory suggests new structure for ammonia catalyst active site. • K-promoter stabilizes the N-N transition state and destabilizes late intermediates. • 20 ppm water vapor is detrimental to ammonia production rate at low temperature. • Weak-binding catalysts are less susceptible to oxygen poisoning. • Mitigating oxygen poisoning is essential to low-temperature ammonia synthesis. The Haber-Bosch process has been studied extensively, yet a low-temperature, low-pressure process remains elusive. As has been shown many times, this stems in part from the difficulty of breaking the N-N triple bond. In this work, we highlight an additional reason for the lack of a low-temperature ammonia synthesis process: the effect of oxygen poisoning at low temperature. Using density functional theory (DFT), we have created a new model for the active site of industrial Haber-Bosch catalysts which explicitly includes the potassium promoter. Furthermore, we present a new micro-kinetic model for ammonia synthesis that includes the effect of oxygen poisoning due to trace water content in the input gas stream. Our model agrees well with previous experiments and shows that devising a strategy to avoid oxygen poisoning is crucial to creating a low-temperature Haber-Bosch process. Additionally, the model suggests that using a weaker-binding catalyst is one way to avoid oxygen poisoning if it is impractical to remove all water from the reactor. [ABSTRACT FROM AUTHOR]

Abstract: Scanning tunneling microscopy (STM) investigations have recently provided the first atom-resolved images of reaction intermediates in the key steps of the hydrogenation (HYD) and direct desulfurization (DDS) pathways in hydrodesulfurization over MoS2 nanoclusters. Surprisingly, special brim sites exhibiting a metallic character are observed to be involved in adsorption, hydrogenation and Ceavage. The insight is seen to provide a new framework for understanding the DDS and HYD pathways and the role of steric hindrance and poisons. Density functional theory (DFT) calculations have illustrated how support interactions may influence the activity of sulfided catalysts. The brim sites and the tendency to form vacancies are seen to differ in types I and II Coph name="sbnd" />S. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies show that the high activity Type II structures may be present as single sulfide sheets. Thus, stacking is not an essential feature of Type II catalysts. The article illustrates how the new scientific insight has aided the introduction of the new high activity BRIM™ type catalysts for FCC pre-treatment and production of ultra low sulfur diesel (ULSD). [Copyright &y& Elsevier]

Abstract: Developments in ammonia synthesis have also made it possible to take a more rational approach to research on ammonia decomposition. It has been found that the optimal catalyst for ammonia synthesis is never the optimal catalyst for ammonia decomposition, which is almost counter-intuitive in catalysis research. The approach taken for ammonia synthesis/decomposition could be useful for many other systems and lead to a more rational development of new and improved catalysts and catalytic processes. [Copyright &y& Elsevier]

We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions. [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of the steam reforming process from first-principles calculations, supported by insight from experimental investigations. In the present work we employ recently recognised scaling relationships for adsorption energies of simple molecules adsorbed at pure metal surfaces to develop an overview of the steam reforming process catalyzed by a range of transition metal surfaces. By combining scaling relationships with thermodynamic and kinetic analysis, we show that it is possible to determine the reactivity trends of the pure metals for methane steam reforming. The reaction is found to be kinetically controlled by a methane dissociation step and a CO formation step, where the latter step is found to be dominant at lower temperatures. The particle size of the metal catalysts particles have been determined by transmission electron microscopy (TEM) and the turn over frequency observed to be linearly dependent on the dispersion, supporting the theoretical notion that the active sites are most likely present as one dimensional edges. It has been found that determination of the correct particle size distribution of small (2–4 nm) Ru particles requires in situ TEM measurements under a hydrogen atmosphere. The overall agreement between theory and experiment (at 773 K, 1 bar pressure and 10% conversion) is found to be excellent with Ru and Rh being the most active pure transition metals for methane steam reforming, while Ni, Ir, Pt, and Pd are significantly less active at similar dispersion. [Copyright &y& Elsevier]

Steam reforming over nickel catalysts is widely used for industrial-scale production of hydrogen and synthesis gas. This work is a study of the effects of atmosphere and temperature on the rate of sintering of nickel steam-reforming catalysts. The relative nickel areas of Ni/MgAl2O4 and Ni/Al2O3 catalysts after sintering in H2O:H2 atmospheres at high (40 bar) and low (1 bar) pressures are reported. The data are discussed in terms of the recently proposed model for the sintering rate of supported nickel catalysts [J. Sehested, J. Catal. 217 (2003) 417] and density functional theory (DFT) calculations of the stability and diffusivity of transport species at the surface of nickel particles. OH-bonded nickel dimers are found to have a much lower energy of formation than nickel adatoms (ΔE=58 kJ mol−1). It is therefore concluded that in steam/hydrogen mixtures, OH-bonded nickel dimers are dominating the surface transport on nickel particles and consequently sintering via particle migration and coalescence. Expressions connecting the diffusion constant for nickel particles to the diffusion constant and energy of formation of nickel adatoms and of OH-bonded nickel dimers are given. These equations are used in a sintering model [J. Sehested, J. Catal. 217 (2003) 417] and good agreement between the model and the experimental data is obtained at moderate temperatures. Above temperatures of ca. 600 °C at 40 bar and approximately 700 °C at 1 bar total pressure, the rate and the activation energy of sintering increase considerably. The reason for this observation may be that sintering via Ostwald ripening dominates the sintering rate under these conditions. [Copyright &y& Elsevier]

For ammonia synthesis catalysts a volcano-type relationship has been found experimentally. We demonstrate that by combining density functional theory calculations with a microkinetic model the position of the maximum of the volcano curve is sensitive to the reaction conditions. The catalytic ammonia synthesis activity, to a first approximation, is a function only of the binding energy of nitrogen to the catalyst. Therefore, it is possible to evaluate which nitrogen binding energy is optimal under given reaction conditions. This leads to the concept of optimal catalyst curves, which illustrate the nitrogen binding energies of the optimal catalysts at different temperatures, pressures, and synthesis gas compositions. Using this concept together with the ability to prepare catalysts with desired binding energies it is possible to optimize the ammonia process. In this way a link between first-principle quantum mechanical calculations of gas–surface interactions, reactor design, and catalyst selection has been established for the first time. [Copyright &y& Elsevier]

Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies. This review discusses design strategies for state-of-the-art heterogeneous electrocatalysts and associated materials for several different electrochemical transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize catalyst performance. By examining the common principles that govern catalysis for different electrochemical reactions, we describe a systematic framework that clarifies trends in catalyzing these reactions, serving as a guide to new catalyst development while highlighting key gaps that need to be addressed. We conclude by extending this framework to emerging clean energy reactions such as hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, where the development of improved catalysts could allow for the sustainable production of a broad range of fuels and chemicals. [ABSTRACT FROM AUTHOR]

The article cites a study analyzing generation of nanopores during desorption of ammonia from metal ammine complexes. Hydrogen can be found by combining the metal ammines with an ammonia decomposition catalyst. Very high volumetric storage capacities similar to that of liquid ammonia can be determined and still the desorption process is facile. The metal ammine complexes have the complication as a hydrogen storage material that they need to be integrated with an ammonia decomposition catalyst in order to produce hydrogen.