NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Theoretical Study on the Electrochemical Catalytic Activity of Au-Doped Pt Electrode for Nitrogen Monoxide

In order to gradually reduce automobile exhaust pollution and improve fuel quality, the NOx sensor, which can be monitored in real time in an automobile engine’s electronic control system, has become an indispensable part of the automobile lean burn system. In these types of NOx sensors, Au-doped platinum electrodes have received great attention due to their selectivity towards NO. However, the reaction process of NO gas on the Au-doped platinum electrode in the sensor and the possible regulation mechanism is still unclear. In this paper, the density functional theory (DFT) was used to analyze the effect of Au-doped Pt electrodes on the performance of nitrogen oxide sensors in automobiles. Firstly, the adsorption energies of NO molecules on pure Pt and Au/Pt surfaces were compared. The adsorption and dissociation of NO on Pt substrates doped with Au monomers, dimers, and trimers were investigated. These results showed that Au can effectively weaken the adsorption energy of NO molecules on a Pt surface. It was noted that with the increase in the number of Au atoms on the surface of Pt(111), the adsorption capacity of NO molecules on the alloy surface becomes weaker. When observing the transition state of NO decomposition on three different alloy surfaces, the study showed that the activation energy and reaction heat of NO dissociation increased. It further showed that doping with Au increased the activation energy of NO decomposition, thereby effectively inhibiting the decomposition of NO.

Noble-Metal-Free Ni-W-O-Derived Catalysts for High-Capacity Hydrogen Production from Hydrazine Monohydrate

Development of active and earth-abundant catalysts is pivotal to render hydrazine monohydrate (N₂H₄·H₂O) viable as a hydrogen carrier. Herein, we report the synthesis of noble-metal-free Ni-W-O-derived catalysts using a hydrothermal method in combination with reductive annealing treatment. Interestingly, the thus-prepared Ni-based catalysts exhibit remarkably distinct catalytic properties toward N₂H₄·H₂O decomposition depending upon the annealing temperature. From a systematic phase/microstructure/chemical state characterization and the first-principles calculations, we found that the variation of the apparent catalytic properties of these Ni-based catalysts should stem from the formation of different Ni-W alloys with distinct intrinsic activity, selectivity, and distribution state. The thereby chosen Ni-W alloy nanocomposite catalyst prepared under an optimized condition showed high activity, nearly 100% selectivity, and excellent stability toward N₂H₄·H₂O decomposition for hydrogen production. Furthermore, this noble-metal-free catalyst enables rapid hydrogen production from commercially available N₂H₄·H₂O solution with an intriguingly high hydrogen capacity of 6.28 wt % and a satisfactory dynamic response property. These results are inspiring and momentous for promoting the use of the N₂H₄·H₂O-based H₂ source systems.

A density functional study on the electronic structure, nature of bonding and reactivity of NO adsorbing Rh0/±n (n = 2–8) clusters

Systematic investigations on lowest energy NO adsorbing neutral and ionic Rh_n (n = 2–8) clusters in the gas phase are executed with an all electron relativistic method using density functional theory (DFT) within the generalized gradient approximation.

The surface chemistry of NO(x) on mackinawite (FeS) surfaces: a DFT-D2 study

We present density functional theory calculations with a correction for the long-range interactions (DFT-D2) of the bulk and surfaces of mackinawite (FeS), and subsequent adsorption and dissociation of NO(x) gases (nitrogen monoxide (NO) and nitrogen dioxide (NO2)). Our results show that these environmentally important molecules interact very weakly with the energetically most stable (001) surface, but adsorb relatively strongly onto the FeS(011), (100) and (111) surfaces, preferentially at Fe sites via charge donation from these surface species. The NOx species exhibit a variety of adsorption geometries, with the most favourable for NO being the monodentate Fe-NO configuration, whereas NO2 is calculated to form a bidentate Fe-NOO-Fe configuration. From our calculated thermochemical energy and activation energy barriers for the direct dissociation of NO and NO2 on the FeS surfaces, we show that NO prefers molecular adsorption, while dissociative adsorption, i.e. NO2 (ads) → [NO(ads) + O(ads)] is preferred over molecular adsorption for NO2 onto the mackinawite surfaces. However, the calculated high activation barriers for the further dissociation of the second N-O bond to produce either [N(ads) and 2O(ads)] or [N(ads) and O2(ads)] suggest that complete dissociation of NO2 is unlikely to occur on the mackinawite surfaces.

Surface design of alloy protection against CO-poisoning from first principles

Pt-based alloy catalysts are of significant importance in fuel cells due to enhanced electrode reactivity and selectivity. Designing alloy surfaces suitable for catalyst via first-principles predictions has long played a central role in identifying promising candidates. We propose surface design for polymer electrolyte fuel cell (PEFC) based on the use of thermodynamically stable alloy surfaces. Using first-principles calculation, we have explored stable Pt alloy surfaces that possess superior catalytic properties for CO-despoisoning in hydrogen-related reactions of fuel cells. The stable Pt(25)M(75) (M = Rh, Cu) alloy surfaces both exhibited weaker CO and stronger H binding compared to the pure Pt surface, which yielded a significant increase in H coverage by around one order. These modifications of molecular adsorption are attributed to the deeper band centre of surface d electrons. Understanding the adsorption properties of the stable atomic structure at surfaces will help us to design suitable alloy surfaces with high-catalytic activity and prolonged actuation in fuel cells.

First principles investigation of NO2 and SO2 adsorption on γ-Al2O3 supported mono- and diatomic metal clusters

Atomistic simulations of NO₂ and SO₂ adsorption on γ-Al₂O₃ supported mono and diatomic clusters of Pt and Rh show that enhanced sulfur tolerance with respect to pristine metallic surfaces or extended nanoparticles, can be achieved.

Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO – the case of reconstruction driven growth?

We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick, low temperature coalescence of Pt in an NO atmosphere accompanied by surface reconstruction deduced from an apparent lattice parameter (ALP) evolution.

Theoretical study on adsorption and dissociation of NO2 molecule on Fe(111) surface

We applied periodic density-functional theory (DFT) to investigate the adsorption and dissociation of NO(2) on a Fe(111) surface. The most favorable adsorption configuration of NO(2)/Fe(111) is the FeNO(2)(S-mu(3)-N,O,O') configuration with NO(2) at the 3-fold-shallow site of the surface, which has an adsorption energy -64.59 kcal/mol. Of two geometries of NO(2)/Fe(111) for the stepwise NO(2) deoxygenation, one is the most stable structure, FeNO(2)(S-mu(3)-N,O,O'), with activation barriers 10.38 and 19.36 kcal/mol to break the first (ON-O bond activation) and second (N-O bond activation) nitrogen-oxygen bonds, respectively; another configuration FeNO(2)(B-mu(2)-N,O) has a smaller energy barrier (3.88 kcal/mol) to break the first ON-O bond. All these findings show that NO(2) can readily decompose on the Fe(111) surface. The rate constants for the two aforementioned processes were also predicted by VTST and RRKM theory, and the predicted total rate constants, k(total) (in units of cm(3) molecule(-1) s(-1)), can be represented by the equations k(total) = 5.61 x 10(-5)T(-2.060) exp(-0.639 kcal mol(-1)/RT) at T = 100-1000 K. To acquire insight into the great catalytic activity of the Fe(111) surface for the decomposition of NO(2), the nature of the interaction between the adsorbate and the substrate is subjected to a detailed electronic analysis.

Theory of nitride oxide adsorption on transition metal (111) surfaces: a first-principles investigation

In this work, we report a density functional theory study of nitric oxide (NO) adsorption on close-packed transition metal (TM) Rh(111), Ir(111), Pd(111) and Pt(111) surfaces in terms of adsorption sites, binding mechanism and charge transfer at a coverage of Theta(NO) = 0.25, 0.50, 0.75 monolayer (ML). Based on our study, an unified picture for the interaction between NO and TM(111) and site preference is established, and valuable insights are obtained. At low coverage (0.25 ML), we find that the interaction of NO/TM(111) is determined by an electron donation and back-donation process via the interplay between NO 5sigma/2pi* and TM d-bands. The extent of the donation and back-donation depends critically on the coordination number (adsorption sites) and TM d-band filling, and plays an essential role for NO adsorption on TM surfaces. DFT calculations shows that for TMs with high d-band filling such as Pd and Pt, hollow-site NO is energetically the most favorable, and top-site NO prefers to tilt away from the normal direction. While for TMs with low d-band filling (Rh and Ir), top-site NO perpendicular to the surfaces is energetically most favorable. Electronic structure analysis show that irrespective of the TM and adsorption site, there is a net charge transfer from the substrate to the adsorbate due to overwhelming back-donation from the TM substrate to the adsorbed NO molecules. The adsorption-induced change of the work function with respect to bare surfaces and dipole moment is however site dependent, and the work function increases for hollow-site NO, but decreases for top-site NO, because of differences in the charge redistribution. The interplay between the energetics, lateral interaction and charge transfer, which is element dependent, rationalizes the structural evolution of NO adsorption on TM(111) surfaces in the submonolayer regime.

First-principles study of phase equilibria in Cu-Pt-Rh disordered alloys

Phase stability of Cu-Pt-Rh ternary disordered alloys is examined by a combination of cluster expansion techniques and Monte Carlo statistical simulation based on first-principles calculation. The sign of pseudo-binary ECIs indicates that neighboring Cu and Pt strongly prefer unlike-atom pairs, Pt and Rh weakly prefer unlike-atom pairs, and Cu and Rh atoms prefer like-atom pairs, indicating that the ternary alloy retains the ordering tendency of the constituent binary alloys. The formation energy of a random alloy at T = 1200 K exhibits a negative sign for a wide range of Pt-rich compositions, while at Pt-poor compositions of x≤0.25, the formation energy has a positive value. Calculated affinities for the random alloy show the variety of energetically favored bonds for the alloy: Cu-Pt bonds in both first-and second-nearest neighbor (1-NN and 2-NN) are energetically preferred for all the composition range, the Pt-Rh bond in 1-NN is preferred at Pt-rich compositions, the Pt-Rh in 2-NN and Rh-Cu in 1-NN bonds are unfavored for all compositions, and the Rh-Cu bond in 2-NN is unfavored for Pt-poor compositions. We elucidate that the ordering tendency of 1-NN and 2-NN Cu-Pt, 2-NN Pt-Rh and 1-NN Cu-Rh atoms in constituent binary alloys is retained for the whole composition range of Cu-Pt-Rh ternary alloys, while that of 1-NN Pt-Rh and 2-NN Cu-Rh atoms significantly depends on composition.

Rational Design of Selective, Sulfur-Resistant Oxidation Emissions Catalysts

A new catalyst design strategy based on optimizing electronic structure has been proposed and then applied to a very important environmental application, the design of selective, sulfur-resistant oxidation emissions catalysts. The modified d-band center model developed by us in a previous study, together with an energy decomposition scheme, is used to correlate measures of reactivity with reaction barriers of SO2 + O --> SO3 and NO + O --> NO2 on surfaces. Our objective is to find a catalyst which is active in oxidizing NO to NO2 but relatively inactive in oxidizing SO2 to SO3. The Ir alloyed Pt(111) surface is found to have the highest selectivity for oxidation of NO over SO2 at 700 K. Unfortunately, there is a slope change in the correlation of the weighted d-band center with the adsorption of NO at the transition state, which narrows down the range of the theoretical selectivity. Our ongoing study aims at understanding the reason for this. The general importance of this study for surface catalysis is also discussed.