Nitrogen-doped carbon materials are promising metal-free catalysts for the electrochemical oxygen reduction reaction (ORR). A better theoretical understanding on the nature of the active site(s) ...would help further optimization of their ORR activity. Although quantum mechanical calculations have been widely employed to elucidate the active sites over various catalysts, these calculations are typically done assuming constantcharge conditions rather than the experimentally relevant constant-potential conditions. In this study, we employ the double-reference method to simulate the energetics of the ORR over pure and N-doped carbon materials under constant-potential conditions. We demonstrate that constant-potential calculations enable more accurate theoretical predictions, comparing well with existing experiments. Our key findings are (1) the zigzag edge of pure graphite is highly active for ORR, (2) the pyridinic N-doped armchair edge is highly active for ORR in alkaline media but not in acid, and (3) graphitic N can donate electrons to pyridinic N to enhance the ORR activity. Furthermore, these fundamental insights provide guidelines for the design of better carbonbased ORR catalysts.
Ethanol (EtOH) decomposition has been widely studied in recent years. However, the initial dehydrogenation selectivity on catalytic surfaces, which plays a crucial role in EtOH partial oxidation and ...steam reforming, is not well understood. Here, density functional theory (DFT) was used to calculate the initial dehydrogenation selectivities of EtOH on monometallic and X/Au (X = Pd and Rh) close-packed surfaces. The energy for the initial bond scissions of O–H and α- and β-C–H were calculated on each surface. The binding energy of EtOH is found to be a good reactivity descriptor for the scission of O–H and β-C–H bonds, while the binding energy of CH3CHOH is a good reaction descriptor for α-C–H bond scission. The scaling relationships between the activation energy barriers and binding energies on Pd/Au and Rh/Au surface alloys are significantly different from those of monometallic surfaces. Additionally, the specific atomic ensembles on the Pd/Au and Rh/Au surfaces have different initial dehydrogenation selectivities of EtOH. Our calculated scaling relationships were used to construct contour plots that provide predictive trends for the selectivity of the initial EtOH dehydrogenation. We conclude that the presence of specific atomic ensembles on the surface of alloy catalysts can efficiently control the reaction products of EtOH dehydrogenation.
Density functional theory was used to study the CO oxidation catalytic activity of CeO2-supported Au nanoparticles (NPs). Experimental observations on CeO2 show that the surface of CeO2 is enriched ...with oxygen vacancies. We compare CO oxidation by a Au13 NP supported on stoichiometric CeO2 (Au13@CeO2-STO) and partially reduced CeO2 with three vacancies (Au13@CeO2-3VAC). The structure of the Au13 NP was chosen to minimize structural rearrangement during CO oxidation. We suggest three CO oxidation mechanisms by Au13@CeO2: CO oxidation by coadsorbed O2, CO oxidation by a lattice oxygen in CeO2, and CO oxidation by O2 bound to a Au–Ce3+ anchoring site. Oxygen vacancies are shown to open a new CO oxidation pathway by O2 bound to a Au–Ce3+ anchoring site. Our results provide a design strategy for CO oxidation on supported Au catalysts. We suggest lowering the vacancy formation energy of the supporting oxide, and using an easily reducible oxide to increase the concentration of reduced metal ions, which act as anchoring sites for O2 molecules.
Even as a commercial cathode material, LiFePO4 remains of tremendous research interest for understanding Li intercalation dynamics. The partially lithiated material spontaneously separates into ...Li-poor and Li-rich phases at equilibrium. Phase segregation is a surprising property of LiFePO4 given its high measured rate capability. Previous theoretical studies, aiming to describe Li intercalation in LiFePO4, include both atomic-scale density functional theory (DFT) calculations of static Li distributions and entire-particle-scale phase field models, based upon empirical parameters, studying the dynamics of the phase separation. Little effort has been made to bridge the gap between these two scales. In this work, DFT calculations are used to fit a cluster expansion for the basis of kinetic Monte Carlo calculations, which enables long time scale simulations with accurate atomic interactions. This atomistic model shows how the phases evolve in Li x FePO4 without parameters from experiments. Our simulations reveal that an ordered Li0.5FePO4 phase with alternating Li-rich and Li-poor planes along the ac direction forms between the LiFePO4 and FePO4 phases, which is consistent with recent X-ray diffraction experiments showing peaks associated with an intermediate-Li phase. The calculations also help to explain a recent puzzling experiment showing that LiFePO4 particles with high aspect ratios that are narrower along the 100 direction, perpendicular to the 010 Li diffusion channels, actually have better rate capabilities. Our calculations show that lateral surfaces parallel to the Li diffusion channels, as well as other preexisting sites that bind Li weakly, are important for phase nucleation and rapid cycling performance.
The reaction mechanism of CO oxidation on Au/TiO2 catalysts remains elusive. Here, we employ density functional theory calculations to gain an understanding of several important aspects of the ...system, including CO adsorption, the active oxygen species, catalyst deactivation, and the promoting effect of moisture on catalytic activity. Distinct from previous theoretical studies, which tend to address these questions individually, here we construct a model of the catalytic system which can address all of the issues mentioned. For this, we have considered the complex interactions among reactants, products, and catalysts under reaction conditions. The main findings of our present study are (1) the Au/TiO2 interface boundary can be easily oxidized, (2) CO adsorption on oxidized Au results in the formation of O–Au–CO species, (3) surface lattice oxygen on a TiO2 support is the active oxygen species, (4) CO2 binds strongly on the Otop/Ti5c site, forming carbonate that blocks the active site, and finally (5) water can accelerate O2 dissociation and carbonate decomposition. The results of our theoretical model are compared with existing experimental observations and found to be largely consistent with them.
Pt dissolution under potential cycling has been identified as the dominant process that causes cathode losses in proton-exchange membrane fuel cells. In recent years, significant insights on the Pt ...dissolution process have been obtained from in situ Pt dissolution detection enabled by voltammetry coupled to inductively coupled plasma mass spectrometry. Despite extensive experimental research, theoretical studies continue to lag in the understanding of the atomic-scale mechanism of the Pt dissolution process due to the complicated subprocesses involved, including Pt oxidation, surface reconstruction, Pt oxide reduction, chemical corrosion, etc. Here, we employ global optimization and constant-potential density functional theory to simulate the complete process of Pt dissolution. We show that a two-dimensional Pt surface oxide consisting of interconnected square planar PtO4 units forms at applied potentials higher than 1.1 VRHE. The structural signatures and oxidation states of the Pt surface oxide are close to that of bulk Pt3O4 oxide. The PtO4 units can be reduced to PtOH(H2O)3+ in the cathodic scan and dissolve into the electrolyte. The dissolved PtOH(H2O)3+ species favorably accepts a proton and becomes Pt(H2O)42+. We also find that the dissolution of one Pt atom leads to the decomposition of the connected Pt(OH)4 units because of ligand losses, which then renders them susceptible to be reduced to Pt0. On the basis of our findings, we propose a cathodic Pt dissolution mechanism: Pt3O4 s + 8H+ + 6e– → Pt(H2O)42+ + 2Pt. An anodic Pt dissolution mechanism is also proposed. Our work provides a fundamental understanding of Pt dissolution under potential cycling, which is needed for the rational design of durable Pt-based cathodes for fuel cells.
DFT+U calculations of CO oxidation by Au12 nanoclusters supported on a stepped-CeO2(111) surface show that lattice oxygen at the step edge oxidizes CO bound to Au NCs by the Mars–van Krevelen (M-vK) ...mechanism. We found that CO2 desorption determines the rate of CO oxidation, and the vacancy formation energy is a reactivity descriptor for CO oxidation. Our results suggest that the M-vK mechanism contributes significantly to CO oxidation activity at Au particles supported on the nano- or meso-structured CeO2 found in industrial catalysts.
Density functional theory is used to determine the reaction mechanisms of CO oxidation and the active oxygen species on a Au/TiO2 model catalyst. The model consists of a Au rod supported along the ...TiO2 11̅0 direction of the TiO2(110) surface. An interfacial Au/Ti5c site at the interface boundary is identified to be particularly active toward O2 adsorption and dissociation. At this site, O2 dissociation has an energy barrier of 0.5 eV, which is facile at room temperature. The resulting adsorbed Au/O/Ti5c oxygen species are shown to be stable and active for CO oxidation under relevant reaction conditions with an activation energy of 0.24 eV. Furthermore, the adsorbed Au/O/Ti5c oxygen species functions as an electron reservoir, and it lowers the oxygen vacancy formation energy of a surface lattice oxygen (Obri), as well as the Ti interstitial formation energy, due to electron transfer from high-energy defect states to low-energy p-states of the adsorbed Au/O/Ti5c oxygen species. Hence, the Obri species is activated at the oxidized Au/TiO2 interface boundary and the energy barrier of CO oxidation with Obri is calculated to be 0.55 eV. Thus, the CO oxidation reaction can proceed at room temperature either via a Langmuir–Hinshelwood mechanism with an adsorbed Au/O/Ti5c oxygen species or via a Au-assisted Mars–van Krevelen mechanism with Obri.
Recently, we found that the atomic ensemble effect is the dominant effect influencing catalysis on surfaces alloyed with strong- and weak-binding elements, determining the activity and selectivity of ...many reactions on the alloy surface. In this study we design single-atom alloys that possess unique dehydrogenation selectivity towards ethanol (EtOH) partial oxidation, using knowledge of the alloying effects from density functional theory calculations. We found that doping of a strong-binding single-atom element (
e.g.
, Ir, Pd, Pt, and Rh) into weak-binding inert close-packed substrates (
e.g.
, Au, Ag, and Cu) leads to a highly active and selective initial dehydrogenation at the α-C-H site of adsorbed EtOH. We show that many of these stable single-atom alloy surfaces not only have tunable hydrogen binding, which allows for facile hydrogen desorption, but are also resistant to carbon coking. More importantly, we show that a rational design of the ensemble geometry can tune the selectivity of a catalytic reaction.
Doping of a strong-binding single-atom element into inert close-packed substrates leads to highly active and selective initial dehydrogenation at the α-C-H site of adsorbed ethanol.
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