Activity Descriptors for CO₂ Electroreduction to Methane on Transition-Metal Catalysts

The electrochemical reduction of CO₂ into hydrocarbons and alcohols would allow renewable energy sources to be converted into fuels and chemicals. However, no electrode catalysts have been developed that can perform this transformation with a low overpotential at reasonable current densities. In this work, we compare trends in binding energies for the intermediates in CO₂ electrochemical reduction and present an activity “volcano” based on this analysis. This analysis describes the experimentally observed variations in transition-metal catalysts, including why copper is the best-known metal electrocatalyst. The protonation of adsorbed CO is singled out as the most important step dictating the overpotential. New strategies are presented for the discovery of catalysts that can operate with a reduced overpotential

Methanol to Dimethyl Ether over ZSM-22: A Periodic Density Functional Theory Study

Methanol-to-DME conversion over ZSM-22 Brønsted acid sites is investigated on the basis of periodic density functional theory calculations. DME formation has been speculated to take place via the dissociative or associative pathway. It is shown that the dissociative pathway is the predominant pathway. We find that water lowers the activation energies of key reactions but that the lowering of the activation energies is insufficient to increase the rate because of the entropy loss associated with water adsorption. The consequence of acid strength on the methanol-to-DME conversion pathways is investigated on the basis of Al-, Ga-, or In-induced Brønsted acid sites. We show that linear correlations between activation energies and acid strength exist. It is found that weaker acidity leads to higher activation energies. We find that changes in acidity will not change the conclusion that the dissociative pathway is the predominant pathway

Theoretical Insights into Methane C–H Bond Activation on Alkaline Metal Oxides

In this work, we investigate the role of alkaline metal oxides (AMO) (MgO, CaO, and SrO) in activating the C–H bond in methane. We use Density Functional Theory (DFT) and microkinetic modeling to study the catalytic elementary steps in breaking the C–H bond in methane and creating the methyl radical, a precursor prior to creating C₂ products. We study the effects of surface geometry on the catalytic activity of AMO by examining terrace and step sites. We observe that the process of activating methane depends strongly on the structure of the AMO. When the AMO surface is doped with an alkali metal, the transition state (TS) structure has a methyl radical-like behavior, where the methyl radical interacts weakly with the AMO surface. In this case, the TS energy scales with the hydrogen binding energy. On pure AMO, the TS interacts with AMO surface oxygen as well as the metal atom on the surface, and consequently the TS energy scales with the binding energy of hydrogen and methyl. We study the activity of AMO using a mean-field microkinetic model. The results indicate that terrace sites have similar catalytic activity, with the exception of MgO(100). Step sites bind hydrogen more strongly, making them more active, and this confirms previously reported experimental results. We map the catalytic activity of AMO using a volcano plot with two descriptors: the methyl and the hydrogen binding energies, with the latter being a more significant descriptor. The microkinetic model results suggest that C–H bond dissociation is not always the rate-limiting step. At weak hydrogen binding, the reaction is limited by C–H bond activation. At strong hydrogen binding, the reaction is limited due to poisoning of the active site. We found an increase in activity of AMO as the basicity increased. Finally, the developed microkinetic model allows screening for improved catalysts using simple calculations of the hydrogen binding energy

A Theoretical Study of Methanol Oxidation on RuO₂(110): Bridging the Pressure Gap

Partial oxidation catalysis is often fraught with selectivity problems, largely because there is a tendency of oxidation products to be more reactive than the starting material. One industrial process that has successfully overcome this problem is partial oxidation of methanol to formaldehyde. This process has become a global success, with an annual production of 30 million tons. Although ruthenium catalysts have not shown activity as high as the current molybdena or silver-based industrial standards, the study of ruthenium systems has the potential to elucidate which catalyst properties facilitate the desired partial oxidation reaction as opposed to deep combustion due to a pressure-dependent selectivity “switch” that has been observed in ruthenium-based catalysts. In this work, we find that we are able to successfully rationalize this “pressure gap” using near-ab initio steady-state microkinetic modeling on RuO₂(110). We obtain molecular desorption prefactors from experiment and determine all other energetics using density functional theory. We show that, under ambient pressure conditions, formaldehyde production is favored on RuO₂(110), whereas under ultrahigh vacuum pressure conditions, full combustion to CO₂ takes place. We glean from our model several insights regarding how coverage effects, oxygen activity, and rate-determining steps influence selectivity and activity. We believe the understanding gained in this work might advise and inspire the greater partial oxidation community and be applied to other catalytic processes which have not yet found industrial success

Screened Hybrid Exact Exchange Correction Scheme for Adsorption Energies on Perovskite Oxides

The bond formation between an oxide surface and oxygen, which is of importance for numerous surface reactions including catalytic reactions, is investigated within the framework of hybrid density functional theory that includes nonlocal Fock exchange. We show that there exists a linear correlation between the adsorption energies of oxygen on LaMO₃ (M = Sc–Cu) surfaces obtained using a hybrid functional (e.g., Heyd–Scuseria–Ernzerhof) and those obtained using a semilocal density functional (e.g., Perdew–Burke–Ernzerhof) through the magnetic properties of the bulk phase as determined with a hybrid functional. The energetics of the spin-polarized surfaces follows the same trend as corresponding bulk systems, which can be treated at a much lower computational cost. The difference in adsorption energy due to magnetism is linearly correlated to the magnetization energy of bulk, that is, the energy difference between the spin-polarized and the non-spin-polarized solutions. Hence, one can estimate the correction to the adsorption energy as obtained from a semilocal functional directly from the bulk magnetization energy from a hybrid functional

Al–Air Batteries: Fundamental Thermodynamic Limitations from First-Principles Theory

The Al–air battery possesses high theoretical specific energy (4140 W h/kg) and is therefore an attractive candidate for vehicle propulsion. However, the experimentally observed open-circuit potential is much lower than what bulk thermodynamics predicts, and this potential loss is typically attributed to corrosion. Similarly, large Tafel slopes associated with the battery are assumed to be due to film formation. We present a detailed thermodynamic study of the Al–air battery using density functional theory. The results suggest that the maximum open-circuit potential of the Al anode is only −1.87 V versus the standard hydrogen electrode at pH 14.6 instead of the traditionally assumed −2.34 V and that large Tafel slopes are inherent in the electrochemistry. These deviations from the bulk thermodynamics are intrinsic to the electrochemical surface processes that define Al anodic dissolution. This has contributions from both asymmetry in multielectron transfers and, more importantly, a large chemical stabilization inherent to the formation of bulk Al­(OH)₃ from surface intermediates. These are fundamental limitations that cannot be improved even if corrosion and film effects are completely suppressed

Tuning the MoS₂ Edge-Site Activity for Hydrogen Evolution via Support Interactions

The hydrogen evolution reaction (HER) on supported MoS₂ catalysts is investigated using periodic density functional theory, employing the new BEEF-vdW functional that explicitly takes long-range van der Waals (vdW) forces into account. We find that the support interactions involving vdW forces leads to significant changes in the hydrogen binding energy, resulting in several orders of magnitude difference in HER activity. It is generally seen for the Mo-edge that strong adhesion of the catalyst onto the support leads to weakening in the hydrogen binding. This presents a way to optimally tune the hydrogen binding on MoS₂ and explains the lower than expected exchange current densities of supported MoS₂ in electrochemical H₂ evolution studies

Theoretical Limits to the Anode Potential in Aqueous Mg–Air Batteries

The aqueous Mg–air battery is an attractive candidate for some electric vehicle applications due to its high theoretical specific energy, environmentally and physiologically benign properties, and implied low cost from using earth-abundant materials. However, the experimentally observed potentials (1.6–1.2 V) are far from the thermodynamically predicted value of 3.09 V, based on the free energy of formation for the reaction Mg (s) + 1/2 O₂ (g) + H₂O (l) ⇌ Mg­(OH)₂ (s). It is generally believed that this large difference is principally due to the presence of Mg corrosion giving rise to a net corrosion potential, and that it would be possible to nearly obtain the full potential of 3.09 V if corrosion were completely suppressed. In this contribution, we present a density functional theory study of the hydroxide-assisted Mg anodic dissolution mechanism in the aqueous Mg–air battery. We show that the Mg surface is expected to be highly OH*-covered in the anodic dissolution process, and that the calculated intrinsic limiting potentials are in fact in reasonable agreement with experimentally observed potentials. These limiting potentials are dictated by sequential electrochemical adsorption of hydroxide to the Mg surface, and therefore, the bulk free energy of Mg­(OH)₂ (s) formation cannot be used to predict the intrinsic anode potential in the aqueous Mg–air battery. These intrinsic limits imply that completely suppressing Mg corrosion will not significantly increase the potential available for the Mg–air battery

Role of Subsurface Oxygen on Cu Surfaces for CO₂ Electrochemical Reduction

Under ambient conditions, copper with oxygen near the surface displays strengthened CO₂ and CO adsorption energies. This finding is often used to rationalize differences seen in product distributions between Cu-oxide and pure Cu electrodes during electrochemical CO₂ reduction. However, little evidence exists to confirm the presence of oxygen within first few layers of the Cu matrix under relevant experimental reducing conditions. Using density functional theory calculations, we discuss the stability of subsurface oxygen from thermodynamic and kinetic perspectives and show that under reducing potentials subsurface oxygen alone should have negligible effects on the activity of crystalline Cu

Naravne oblike gibanja kot sredstvo razvoja moči v mali odbojki (10-12 let)

RuO₂ has been reported to reduce CO₂ electrochemically to methanol at low overpotential. Herein, we have used density functional theory (DFT) to gain insight into the mechanism for CO₂ reduction on RuO₂(110). We have investigated the thermodynamic stability of various surface terminations in the electrochemical environment and found CO covered surfaces to be particularly stable, although their formation might be kinetically limited under mildly reducing conditions. We have identified the lowest free energy pathways for CO₂ reduction to formic acid (HCOOH), methanol (CH₃OH), and methane (CH₄) on partially reduced RuO₂(110) covered with 0.25 and 0.5 ML of CO*. We have found that CO₂ is reduced to formic acid, which is further reduced to methanol and methane. At 0.25 ML of CO*, the reduction of formate (OCHO*) to formic acid is the thermodynamically most difficult step and becomes exergonic at potentials below −0.43 V vs the reversible hydrogen electrode (RHE). On the other hand, at 0.5 ML of CO*, the reduction of formic acid to H₂COOH* is the thermodynamically most difficult step and becomes exergonic at potentials below −0.25 V vs RHE. We have found that CO₂ reduction activity on RuO₂ changes with CO coverage, which suggests that CO coverage can be used as a tool to tune the CO₂ reduction activity. We have shown the mechanism for CO₂ reduction on RuO₂ to be different from that on Cu. On Cu, hydrocarbons are formed at high Faradaic efficiency through reduction of CO* at ∼1 V overpotential, while on RuO₂, methanol and formate are formed through reduction of formic acid at lower overpotentials. Using our understanding of the CO₂ reduction mechanism on RuO₂, we suggest reduction of formic acid on RuO₂, which should lead to methanol and methane production at relatively low overpotentials