Gas-surface Interface Research Articles

Scattering of diatomic molecules from graphite

In the last years, state-to-state molecular dynamics simulations of some basic elementary processes, occurring at the gas–surface interface in a wide range of temperatures and collision energies, have been performed by adopting new potential energy surfaces. In this contribution, our attention is mostly addressed to the role of long-range forces, determining the physisorption of gaseous molecules on the surface. Such forces, formulated in terms of the improved Lennard–Jones interaction potential model, control the formation of precursor or pre-reactive state that plays a crucial role in the dynamical evolution of molecules impinging on the surface in the range of low–intermediate collision kinetic energies. The study focuses on the collisions of H2, O2, N2 and CO, initially in their ground and excited vibro-rotational levels, on a graphite surface. The resulting dispersion coefficients, which control the capture of impinging molecules, are compared and found in good agreement with those available in the literature. New selectivity and peculiarities of scattered molecules, crucial to control the kinetics of elementary chemical processes occurring at the gas–surface interfaces under thermal and sub-thermal conditions, of interest in different applied fields, are highlighted.Graphic abstract

Numerical study of gas–surface interface effects due to transpiration in a hypersonic flow over a blunt body

This paper investigates the impact of transpiration on a hypersonic flow over a cylinder, considering different degrees of rarefaction. The study analyzes the interaction between freestream argon gas flow at Mach 5 and transpiring argon gas at the fluid–solid interface at a velocity of 10 m/s. Freestream Knudsen numbers considered are 0.002, 0.01, 0.05, and 0.25, spanning from a continuum to rarefied regime. Flow simulations utilize the open-source direct simulation Monte Carlo solver, Stochastic PArallel Rarefied-gas Time-accurate Analyzer. The influence of transpiration on flow and surface properties is examined by comparing non-transpiration and transpiration cases. At all regimes, transpiration increases the normal shock stand-off distance, while a comparison of flow properties along the stagnation line reveals a reduction in the velocity and an increase in the post-shock temperature with transpiration. Surface heat flux comparison indicates that transpiring gas reduces heat flux on the cylinder's upstream-facing front surface at all Knudsen numbers. However, at Kn∞ = 0.25, a shift occurs, and surface heat flux starts increasing locally from the top/bottom point on the cylinder surface through the rear face of the cylinder. Furthermore, a test for the validity of the continuum-based blowing correction correlation function reveals the failure of the empirical model, even in the continuum regime at Kn∞ = 0.002, casting doubt on its applicability to vehicles with curvilinear blunt-body shapes. Additionally, a sensitivity analysis demonstrates that transpiring gas with a number density an order of magnitude higher than the freestream reduces stagnation peak heat flux by nearly 30%, while transpiring gas with a temperature two times higher than the freestream shows a ∼13% reduction.

Six-dimensional quantum dynamics of an Eley-Rideal reaction between gaseous and adsorbed hydrogen atoms on Cu(111).

In the form of direct abstraction of a surface adsorbate by a gaseous projectile, the Eley-Rideal (ER) reaction at the gas-surface interface manifests interesting dynamics. Unfortunately, high-dimensional quantum dynamical (QD) studies for ER reactions remain very challenging, which demands a large configuration space and the coordinate transformation of wavefunctions. Here, we report the first six-dimensional (6D) fully coupled quantum scattering method for studying the ER reaction between gas phase H(D) atoms and adsorbed D(H) atoms on a rigid Cu(111) surface. Reaction probabilities and product rovibrational state distributions obtained by this 6D model are found to be quite different from those obtained by reduced-dimensional QD models, demonstrating the high-dimensional nature of the ER reaction. Using two distinct potential energy surfaces (PESs), we further discuss the influence of the PES on the calculated product vibrational and rotational state distributions, in comparison with experimental results. The lateral corrugation and the exothermicity of the PES are found to play a critical role in controlling the energy disposal in the ER reaction.

Dynamics of Collision-induced Energy Transfer

Collisions are of fundamental importance in many relevant physico-chemical phenomena in the gas phase and at the gas-surface interface. Collision-induced energy transfer not only offers a sensitive probe of interatomic interactions, but also impacts an array of processes ranging from transport to reactivity. This perspective highlights some recent advances on collision-induced energy transfer dynamics. In the gas phase, cold collisions revealed remarkable quantum effects and stereodynamics of bimolecular inelastic scattering, which motivated development of new state-to-state quantum scattering methods. For surface processes, experiments of hydrogen atom scattering from various surfaces combined with first-principles molecular dynamics simulations shed valuable light on the relative importance of adiabatic energy transfer to surface phonons and nonadiabatic energy transfer to surface electrons. We outline further challenges in these fields and expect more fruitful interplay between experiment and theory.

Quantification of model-form uncertainties affecting the calibration of a carbon nitridation model by means of Bayesian Model Averaging

Severe epistemic uncertainties not only can affect the prescription of parameters within a given model but also the choice of models we make to interpret and infer from experimental data. In this work, we incorporate experimental, parametric and model-form uncertainties in the calibration of a carbon nitridation model. The model-form uncertainties considered stem from the different modeling choices that are taken as valid representations of a set of plasma wind tunnel experiments. To this end, we define a Bayesian Model Averaging strategy where the marginal posteriors of the nitridation reaction efficiencies are weighted by the marginalized likelihoods of the experimental data for each proposed model. First, Bayes factors are computed to possibly discard invalid models. The baseline model, a thermal equilibrium stagnation line flow with nitridation as only surface reaction, performs as well as all the alternative models proposed, which range from adding surface recombination reactions to considering thermal non-equilibrium in the gas and gas-surface interface. The presence of nitrogen recombination reactions is shown to broaden the support of the nitridation marginal posteriors considerably, allowing it to take on larger values. Lastly, a Bayesian model averaged Arrhenius law for the nitridation efficiencies is computed for a range of surface temperatures.

A pragmatic protocol for determining charge transfer states of molecules at metal surfaces by constrained density functional theory.

Electron transfer from a metal surface to a molecule is very important at the gas-surface interface, which can lead to electron-mediated energy transfer during molecular scattering from the surface, as evidenced by numerous state-to-state molecular beam experiments of NO and CO scattering from noble metal surfaces. However, it remains challenging to determine relevant charge-transfer states and their nonadiabatic couplings from first principles in such systems involving a continuum of metallic electronic states. In this work, we propose a pragmatic protocol for this purpose based on the constrained density functional theory (CDFT) approach. In particular, we discuss the influence of the charge partitioning algorithm used in CDFT to define the constraint property in molecule-metal systems. It is found that the widely used Bader charge analysis is adequate to define the physically sound CDFT diabatic states corresponding to a molecule with or without extra electron transferred from the metal. Numerical tests validate that the proposed CDFT scheme properly describes the electron transfer behaviors in several benchmark systems, namely, NO or CO interacting with Au(111) or Ag(111). The effects of the surface work function and the molecular electron affinity on electron transfer are discussed in detail by comparing the CDFT states of the four systems. This pragmatic CDFT protocol lays the foundation for constructing accurate global diabatic potential energy surfaces for these important systems and can be generalized to study other interfacial electron transfer related problems.

Stochastic calibration of a carbon nitridation model from plasma wind tunnel experiments using a Bayesian formulation

In this work, we calibrate a carbon nitridation model for a broad span of surface temperatures from existing plasma wind tunnel measurements by accounting for experimental and parametric uncertainties. A chemical non-equilibrium stagnation line model is proposed to simulate the experiments and obtain recession rates and CN densities, the measured model outputs. First, we establish the influence of the experimental boundary conditions and nitridation parameters on the simulated observations through a sensitivity analysis. Results show that such quantities are mostly affected by the efficiency of nitridation reactions at the gas-surface interface. We then perform model calibrations for each experimental condition and compare them based on the experimental data used. This allows us to check the consistency of the experimental dataset. Using only the trustworthy experimental data, we perform a calibration of Arrhenius law parameters for nitridation efficiencies considering all available experimental conditions jointly, allowing us to compute nitridation efficiencies even for surface temperatures for which there are no reliable experimental data available. The stochastic Arrhenius law agrees well with most of the data in the literature. This result constitutes the first nitridation model extracted from plasma wind tunnel experiments with accurate uncertainty estimates.

An Indirect Method for Determining the Local Heat Transfer Coefficient of Gas Flows in Pipelines.

An indirect method and procedure for determining the local heat transfer coefficient in experimental studies on the intensity of heat transfer at a gas–surface interface is described. The article provides an overview of modern approaches and technical devices for determining the heat flux or friction stresses on surfaces in the study of thermophysical processes. The proposed method uses a constant-temperature hot-wire anemometer and a sensor with a thread sensitive element fixed on the surface of a fluoroplastic substrate. A substrate with the sensor’s sensitive element was mounted flush with the wall of the investigated pipeline. This method is based on the Kutateladze–Leontiev approach (the laws of friction and heat transfer) and the hydrodynamic analogy of heat transfer (the Reynolds analogy): this is an assumption about the unity of momentum and heat transfer in a turbulent flow, which establishes a quantitative relationship between friction stresses on the heat exchange surface and heat transfer through this surface. The article presents a method for determining the speed of the developed measuring system. An example of a successful application of the proposed method in relation to the study of thermomechanical processes in the gas exchange systems of reciprocating internal combustion engines is described.

Influence of supercell size on Gas-Surface Scattering: A case study of CO scattering from Au(1 1 1)

We examine energy dissipation of impinging molecules (CO) on Au(1 1 1) using molecular dynamics on a machine learned high-dimensional potential energy surface (PES) describing both the molecular and surface degrees of freedom. The PES was trained using a neural network method from density functional theory energies and gradients obtained with a relatively small supercell size, but it is capable of providing an accurate description of the molecule-surface interaction for larger supercells. This property allowed us to investigate the dependence of the dissipation dynamics on the supercell size. Our simulations indicated that the supercell size has essentially no effect on the direct scattered molecules, but the energy dissipation of the trapping molecules is significantly influenced by the size of the simulation cell. This observation has important implications in understanding dissipation at the gas-surface interface and their effects on various dynamics processes.

Parabolic oxidation kinetics of a plutonium alloy at room temperature

Auger electron spectroscopy (AES) was used to characterize the oxide scale formed on an ∼0.5 wt. % Ga plutonium metal alloy. Analysis of changes to the characteristic Auger transitions for Pu and O during sputter depth profiles were used in combination with previously measured sputter yields to determine the thickness of the oxide scale and provide insight into its chemistry. From this data, the parabolic kinetics for the Pu-Ga alloy were determined as a function of relative humidity, RH = 14–92%, and oxygen partial pressure, p(O2) = 31–157 Torr, at 25 °C. For each relative humidity investigated, the parabolic rate constant was found to be independent of oxygen partial pressure. Power-law fits to the relative humidity effect on the rate constant was found to have a power of n = 1.265, indicating the diffusion species which rate-limits the reaction isn’t molecular water, which subsequent dissociates at the oxide-metal interface, or hydroxyl from dissociation at the gas-surface interface.

Mechanical Vibrational Relaxation of NO Scattering from Metal and Insulator Surfaces: When and Why They Are Different.

NO scattering from metallic and insulating surfaces represents contrasting benchmark systems for understanding energy transfer at gas-surface interface. Strikingly different behaviors of highly vibrationally excited NO scattered from Au(111) and LiF(001) were observed and attributed to disparate electronic structures between metals and insulators. Here, we reveal an alternative mechanical origin of this discrepancy by comparative molecular dynamics simulations with globally accurate adiabatic neural network potentials of both systems. We find that highly vibrating NO can reach for the high-dissociation barrier on Au(111), by which vibrational energy can largely transfer to translation or rotation and further dissipate into substrate phonons. This mechanical energy transfer channel is forbidden in the purely repulsive NO/LiF(001) system or for low-vibrating NO on Au(111), where molecular vibration is barely coupled to other degrees of freedom. Our results emphasize that the initial state and potential energy landscape concurrently influence the mechanical energy transfer dynamics of gas-surface scattering.

Employing Optical Emission Spectroscopy to Elucidate the Impact of Titanium Dioxide in Plasma-Assisted Catalysis

The roles of gas phase, gas–surface interface, and material properties of the catalyst must be understood to fully realize future applications of plasma-assisted catalysis (PAC) for pollution remediation. This requires understanding of fundamental processes contributing to plasma–catalyst synergy, including determination of molecular temperatures. Optical emission spectroscopy (OES) was employed for gas-phase processes in H2 and CH4 inductively coupled plasmas in the presence of catalytic TiO2. TiO2 introduction has a minimal effect on the rotational temperature of H2 [TR(H2)] in 100% H2 plasmas but reduces TR(H2) by ≤300 K in 100% CH4 plasmas. Time-resolved OES studies echo energy partitioning results and reveal further kinetic details on H2 formation and the impact of catalysts on gas-phase processes. Comprehensive analysis of the catalyst before and after plasma exposure reveals H2 plasmas act as etching systems whereas competing etching and deposition processes occur under some conditions in CH4 plasmas.

Tailoring the surface properties of porous zeolite constructs using plasma processing

Zeolites are widely used in adsorption, catalysis, and gas separations; however, the relative effectiveness of these applications is inherently impacted by material surface properties. Plasma modification presents an ideal methodology to address these issues, as it has an extensive parameter range and the potential to create tailored surface functionalities. Here, zeolite modification via plasma processing was investigated, along with the fabrication of different zeolite constructs (i.e., native zeolites, pellets and electrospun fibers). Water contact angle goniometry was employed to evaluate surface wettability, noting that untreated zeolites are nominally hydrophilic. We explored the use of fluorocarbon and H2O(v) inductively coupled plasmas to tune the resulting wettability of microporous NaX zeolites over a range of operating conditions. Specifically, after C3F8 plasma exposure, contact angles ranging from 123.9 to 138.2° were measured, depending on plasma treatment parameters. Formation of a fluorocarbon film was verified via X-ray photoelectron spectroscopy and scanning electron microscopy analyses. C2F6 and H2O(v) plasma exposure increased water absorption rates, as a consequence of surface etching and functionalization, respectively. Optical emission spectroscopy was used to probe gas-phase species, gleaning how the material intrinsically changes the plasma environment. Our studies revealed correlations between gas-phase spectroscopic analyses, the gas-surface interface, and the resulting plasma modified surface properties, ultimately leading to improved plasma processes.

Anharmonic calculations of vibrational spectra for molecular adsorbates: A divide-and-conquer semiclassical molecular dynamics approach.

The vibrational spectroscopy of adsorbates is becoming an important investigation tool for catalysis and material science. This paper presents a semiclassical molecular dynamics method able to reproduce the vibrational energy levels of systems composed by molecules adsorbed on solid surfaces. Specifically, we extend our divide-and-conquer semiclassical method for power spectra calculations to gas-surface systems and interface it with plane-wave electronic structure codes. The Born-Oppenheimer classical dynamics underlying the semiclassical calculation is full dimensional, and our method includes not only the motion of the adsorbate but also those of the surface and the bulk. The vibrational spectroscopic peaks related to the adsorbate are accounted together with the most coupled phonon modes to obtain spectra amenable to physical interpretations. We apply the method to the adsorption of CO, NO, and H2O on the anatase-TiO2 (101) surface. We compare our semiclassical results with the single-point harmonic estimates and the classical power spectra obtained from the same trajectory employed in the semiclassical calculation. We find that CO and NO anharmonic effects of fundamental vibrations are similarly reproduced by the classical and semiclassical dynamics and that H2O adsorption is fully and properly described in its overtone and combination band relevant components only by the semiclassical approach.

Eley Rideal recombination of hydrogen atoms on Cu(111): Quantitative role of electronic excitation in cross sections and product distributions.

Eley-Rideal (ER) reaction between a projectile and an adsorbate at a gas-surface interface represents many interesting dynamic features. Electron-hole pairs (EHPs) could play an important role in this process, given the interaction between the accelerated incident atom/molecule and the surface electrons. Here, we examine the EHP effects in a benchmark ER reaction of an impinging H/D atom and pre-covered D/H atoms on Cu(111) by ab initio molecular dynamics simulations. It is found that energy dissipation to EHPs lowers the reaction cross sections by ∼1/3, mainly due to the suppression of the hot atom mechanism, but has a minor effect on rotational distributions, leading to quantitative agreement with experimental data. More interestingly, electronic excitation is responsible for the isotopic effect in translational energy distributions, but probably not for that in angular distributions. Our results highlight the specific role of electronic excitation in detailed dynamic quantities in the ER process.

Activation of Reactions in the Complex Region Using Microwave Irradiation.

Many mode-specific behaviors in the gas phase and at the gas-surface interface have been reported in the past decades. Infrared activation of a reagent vibrational mode is often used to study these reactions. In this work, an inexpensive and easily applied scheme using microwave irradiation is proposed for activating complex-forming reactions by transferring populations between closely spaced resonances. The important combustion reaction of H + O2 ↔ O + OH is used as a model system to demonstrate the feasibility of the proposed approach. The existence of a nonzero transition dipole moment matrix element between two highly excited resonance states separated by a small energy gap in the model system may allow one to use microwave irradiation to intervene and control the model reaction. The high energy resonance states of the model reaction can also release their energy by photon emission, which is in agreement with the experimentally observed chemiluminescence process.

Hot-Atom-Mediated Dynamical Displacement of CO Adsorbed on Cu(111) by Incident H Atoms: An Ab Initio Molecular Dynamics Study

Nonthermal reactions between gaseous particles and adsorbates on a solid surface commonly exist at the gas–surface interface. Besides the well-known “Eley–Rideal” abstraction reaction, the adsorbate can also be exchanged by the gaseous species via dynamic displacement. We present here the first ab initio molecular dynamics study on the dynamical displacement of CO adsorbed on Cu(111) by incident H atoms. Our results agree reasonably well with the measured displacement cross section, product energy, and angular distributions. More importantly, we explicitly demonstrate that this displacement takes place via a hot atom mechanism. The energetic H atom, accelerated by its chemisorption energy near the surface, could cause CO desorption in various ways accompanied by a surprisingly large-energy dissipation to the surface phonons. As the CO–Cu binding is weakened, the metastable HCO intermediate is possibly formed. This scenario provides a common picture for similar processes with a strong repulsion between the...

Selectivity in the inelastic rotational scattering of D2 and HD molecules from graphite: Similarities and differences respect to the H2 case

The inelastic scattering of D2 and HD molecules impinging on a graphite surface in well-defined initial roto-vibrational states has been studied by using the computational setup recently developed to characterize important selectivities in the molecular dynamics occurring at the gas-surface interface. In order to make an immediate comparison of determined elastic and inelastic scattering probabilities, we considered for D2 and HD molecules the same initial states, as well as the same collision energy range, previously selected for the investigation of H2 behaviour. The analysis of the back-scattered molecules shows that, while low-lying initial vibrational states are preserved, the medium–high initial ones give rise to final states covering the complete ladder of vibrational levels, although with different probability for the various cases investigated. Moreover, propensities in the formation of the final rotational states are found to depend strongly on the initial ones, on the collision energy, and on the isotopologue species.

Flaw in Duncan’s Paradox

Sheehan recently discussed the possibility of existence of dynamically controlled pressure or temperature gradient in black body cavity. Duncan proposed that if this pressure gradient exist and can be measured in laboratory then it will violate the 2nd law of thermodynamics. Sheehan et al. performed black body cavity experiment by using tungsten and rhenium metals, H2 and as reactants in gas surface interface reaction and claim that they have indeed measured the temperature difference (equivalent to the pressure difference according to their definition). In this paper we will discuss about the flaw that exist in Duncan’s paradox and in Sheehan analysis. We will show that 2nd law of thermodynamics is still valid in their experiment.

Quantum State-Resolved Studies of Chemisorption Reactions.

Chemical reactions at the gas-surface interface are ubiquitous in the chemical industry as well as in nature. Investigating these processes at a microscopic, quantum state-resolved level helps develop a predictive understanding of this important class of reactions. In this review, we present an overview of the field of quantum state-resolved gas-surface reactivity measurements that explore the role of the initial quantum state on the dissociative chemisorption of a gas-phase reactant incident on a solid surface. Using molecular beams and either quantum state-specific reactant preparation or product detection by laser excitation, these studies have observed mode specificity and bond selectivity as well as steric effects in chemisorption reactions, highlighting the nonstatistical and complex nature of gas-surface reaction dynamics.