Gas Surface Reactions Research Articles

Synthesis of Co3O4/TiO2 composite by pyrolyzing ZIF-67 for detection of xylene

Co3O4/TiO2 composites with p–n heterojunction have been successfully prepared by pyrolyzing sacrificial template of Ti ion loaded Co-based Zeolitic imidazolate framework (ZIF-67). The structure and morphology of composite have been characterized by means of the analysis of XRD, FESEM, HRTEM and XPS spectra. The composite with a Co/Ti molar ratio of 4:1 exhibits the maximum sensing response of 6.17–50ppm xylene, which is 5 times higher than pristine Co3O4. Moreover, Co3O4/TiO2 composite also shows good selectivity, long–term stability and rapid response and recovery. Such excellent sensing performances are attributed to material porous structure, high specific surface and the formation of abundant p–n heterojunction that permits the gas adsorption, diffusion and surface reaction and then improve the gas sensing performance. This work develops a promising synthesized approach of metal oxide composites for broader MOFs application in gas sensor field.

Methane dissociation on Ni(111): A seven-dimensional to nine-dimensional quantum dynamics study.

As one benchmark system of CH4 dissociation on the Ni(111) surface, it is of great significance to explore the role of each degree of freedom (DOF) of reactant CH4 in its first C-H bond dissociation from quantum dynamics simulations. Here, the influence of the CH stretching DOF of methyl limited in C3v symmetry is quantitatively investigated as well as the important role of azimuth. We calculated the sticking probabilities, S0, of ground state (GS) CH4 dissociation on a rigid Ni(111) surface by performing some seven-dimensional to nine-dimensional (9D) quantum dynamics simulations based on one highly accurate and fifteen-dimensional (15D) ab initio potential energy surface which we recently developed. Our direct quantum dynamics results show that S0 of GS CH4 on four given surface impact sites are weakly enhanced by adding the CH stretching DOF of methyl but strongly weakened by the DOF of azimuth. Furthermore, using a 9D quantum dynamics model, we improve the post-treatment model for treating the influence of surface impact sites through a linear relationship between the effective potential barriers and the distances relative to that on the transition state site. These developed high-dimensional quantum dynamics models and improved post-treatments can be usefully extended for studying some complex polyatomic gas-surface reactions by other theoretical groups.

Dust in brown dwarfs and extrasolar planets

Context. Recent observations indicate potentially carbon-rich (C/O > 1) exoplanet atmospheres. Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/O ratio as additional parameter but carbon-rich cloud formation modeling is a challenge for the models applied. The determination of the habitable zone for exoplanets requires the treatment of cloud formation in chemically different regimes. Aims. We aim to model cloud formation processes for carbon-rich exoplanetary atmospheres. Disk models show that carbon-rich or near-carbon-rich niches may emerge and cool carbon planets may trace these particular stages of planetary evolution. Methods. We extended our kinetic cloud formation model by including carbon seed formation and the formation of C[s], TiC[s], SiC[s], KCl[s], and MgS[s] by gas-surface reactions. We solved a system of dust moment equations and element conservation for a prescribed Drift-Phoenixatmosphere structure to study how a cloud structure would change with changing initial C/O0 = 0.43...10.0. Results. The seed formation efficiency is lower in carbon-rich atmospheres than in oxygen-rich gases because carbon is a very effective growth species. The consequence is that fewer particles make up a cloud if C/O0 > 1. The cloud particles are smaller in size than in an oxygen-rich atmosphere. An increasing initial C/O ratio does not revert this trend because a much greater abundance of condensible gas species exists in a carbon-rich environment. Cloud particles are generally made of a mix of materials: carbon dominates if C/O0 > 1 and silicates dominate if C/O0 < 1. A carbon content of 80–90% carbon is reached only in extreme cases where C/O0 = 3.0 or 10.0. Conclusions. Carbon-rich atmospheres form clouds that are made of particles of height-dependent mixed compositions, sizes and numbers. The remaining gas phase is far less depleted than in an oxygen-rich atmosphere. Typical tracer molecules are HCN and C2H2 in combination with a featureless, smooth continuum due to a carbonaceous cloud cover, unless the cloud particles become crystalline.

Characterization of Carbon Ablation Models Including Effects of Gas-Phase Chemical Kinetics

The modeling of the oxidation and sublimation of carbon-based ablative thermal protection system materials remains an area of active research. In this paper, two gas–surface interaction models for carbon are studied at representative reentry conditions. One model is based on the widely used approach with an equilibrium saturated state assumption for the surface composition, and the second uses a detailed finite-rate chemical kinetics model for the gas–surface reactions. Key modeling parameters are varied in the finite-rate model to assess its sensitivity to modeling uncertainties. The gas-phase chemical kinetics models are also varied to characterize their effects on the ablation process. The models were evaluated using a generic sphere–cone geometry at four representative reentry conditions. It is found that there are notable differences in the predicted overall surface mass flux, and particularly in the details of the individual species mass fluxes to and from the surface. In addition, the gas-phase kinetic model is found to have important effects on the surface kinetics and the flux of individual species at the surface. From this study, it is clear that more detailed measurements of surface gas evolution under tightly controlled conditions are required to validate and improve the mechanism-based gas–surface interaction model for carbon. The approach does not capture the interaction of the nonequilibrium state of the gas interacting with the surface, making that approach highly suspect for many of the conditions studied.

Zinc oxide for gas sensing of formaldehyde: Density functional theory modelling of the effect of nanostructure morphology and gas concentration on the chemisorption reaction

The adsorption of formaldehyde (HCHO) gas on different one-dimensional ZnO nanostructures is examined using density functional theory calculations. The effects of nanostructure shape and gas concentration are investigated to determine how these factors may influence the surface reaction of HCHO for sensing applications. The binding energies, vibrational frequencies, charge transfer and density of states were determined. We show that HCHO associatively chemisorbs in multiple sites and orientations on the ZnO nanowire and facetted-nanotube. The facetted-nanotube offers a greater number of adsorption sites due to its larger surface area and hollow centre where HCHO can also adsorb. HCHO is able to form one or two bonds to the surface, where structures in the latter configuration show greater stability, with HCHO behaving as a charge acceptor. Conversely, HCHO acts as a charge donor in the singly coordinated structures. As the adsorption mechanism of HCHO gas on the surface of ZnO nanostructure is not well understood, this study provides useful insights into the gas-surface reaction that may assist future experimental development of ZnO nanostructures for gas sensing of formaldehyde.

Preface: Special Topic on Atomic and Molecular Layer Processing: Deposition, Patterning, and Etching.

Thin film processing technologies that promise atomic and molecular scale control have received increasing interest in the past several years, as traditional methods for fabrication begin to reach their fundamental limits. Many of these technologies involve at their heart phenomena occurring at or near surfaces, including adsorption, gas-surface reactions, diffusion, desorption, and re-organization of near-surface layers. Moreover many of these phenomena involve not just reactions occurring under conditions of local thermodynamic equilibrium but also the action of energetic species including electrons, ions, and hyperthermal neutrals. There is a rich landscape of atomic and molecular scale interactions occurring in these systems that is still not well understood. In this Special Topic Issue of The Journal of Chemical Physics, we have collected recent representative examples of work that is directed at unraveling the mechanistic details concerning atomic and molecular layer processing, which will provide an important framework from which these fields can continue to develop. These studies range from the application of theory and computation to these systems to the use of powerful experimental probes, such as X-ray synchrotron radiation, probe microscopies, and photoelectron and infrared spectroscopies. The work presented here helps in identifying some of the major challenges and direct future activities in this exciting area of research involving atomic and molecular layer manipulation and fabrication.

Hybrid modeling of integrated microchannel methane reformer for miniaturized GTL application using an effectiveness factor submodel based on complex surface chemistry

This paper proposes a methodology for efficient multiscale modeling and simulation of washcoated microchannel reactor by the use of an effectiveness factor submodel. The methodology is demonstrated in the context of modeling a microchannel methane reformer for miniaturized gas-to-liquids (GTL) application. The effectiveness factor submodel is derived specifically for a detailed multi-step reforming kinetics using the data provided by 2D CFD simulations with simplified reactor geometry and fully resolved washcoat. The generalized Thiele modulus approach is exploited to facilitate simple explicit estimations of the effectiveness factors of CH4 and H2O. The root mean squared error of the predictions is shown to be 0.015. On this basis, a hybrid model is formed including the CFD model of the reactor, 1D reaction-diffusion equations of the washcoat and the effectiveness factor submodel as the interface in-between. The hybrid model can accurately reproduce gas phase and intra-washcoat profiles of gas species and surface reaction intermediates for various washcoat properties along with time savings of more than an order of magnitude. The results also show that the reformer’s performance is sensitive to washcoat diffusion resistance.

Finite-Rate Oxidation Model for Carbon Surfaces from Molecular Beam Experiments

An oxidation model for carbon surfaces has been developed where the gas–surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high-energy molecular beam containing atomic and molecular oxygen (93% atoms and 7% molecules) was directed at a high-temperature carbon surface. The measurements revealed that carbon monoxide (CO) is the dominant reaction product and that its formation required a high surface coverage of oxygen atoms. As the temperature of the carbon sample was increased during the experiment, the surface coverage reduced and the production of CO was diminished. Most importantly, the measured time-of-flight distributions of surface reaction products indicated that CO and carbon dioxide () are predominately formed through thermal reaction mechanisms as opposed to direct abstraction mechanisms. These observations have enabled the formulation of a finite-rate oxidation model that includes surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics simulations. However, each reaction mechanism and all rate parameters of the new model are determined individually based on the molecular beam data. The new model is compared to existing models using both zero-dimensional gas–surface simulations and computational fluid dynamics simulations of hypersonic flow over an ablating surface. The new model predicts similar overall mass loss rates compared to existing models; however, the individual species production rates are completely different. The most notable difference is that the new model (based on molecular beam data) predicts CO as the oxidation reaction product with virtually no production, whereas existing models predict the exact opposite trend.

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.

Radical induced intermolecular linkage and energy level modifications of a porphyrin monolayer.

A new method to directly modify the surface structure and energy levels of a porphyrin monolayer was examined in the molecular scale using scanning tunneling microscopy and spectroscopy (STM and STS) and presented in this communication. The exposure to atomic oxygen has induced highly ordered surface cross-linking and changed the occupied and unoccupied orbital levels of a cobalt(ii) octaethyl porphyrin (CoOEP) monolayer, and as a result, the HOMO-LUMO gap was reduced by ∼10%. Counterintuitively, the STM/STS data indicated that the reactive central Co atoms did not participate in the gas-surface reactions. Reflection-absorption infrared spectroscopy (RAIRS) measurements further indicated that the STM observed intermolecular linkages are stabilized via hydrogen bonding. This CoOEP + O˙ system also illustrates an example that the six-fold surface packing symmetry predominates the four-fold molecular symmetry in producing a three-fold symmetric surface cross-linking structure.

Rotational and steric effects in water dissociative chemisorption on Ni(111).

Powerful laser techniques have recently enabled quantum-state resolved molecular beam experiments for investigating gas-surface reactions, which have unveiled intriguing vibrational, rotational, and also steric effects. For reactions involving polyatomic molecules, e.g., the dissociative chemisorption of methane and water, the rotational and related steric effects are far less understood despite a large body of theoretical work having been able to reproduce the observed vibrational mode specificity and related bond selectivity semi-quantitatively or even within chemical accuracy. Herein, we report a high dimensional quantum dynamics study of water dissociation on Ni(111) on a first-principles potential energy surface, focusing on the reactivities of D2O in various rotational quantum states with different spatial orientations. Through an accurate quantum mechanical description of this asymmetric top, remarkable dependence of the reactivity on the orientation is observed. This dependence is site specific and rotational state specific. These single site rotational and steric effects are partially justified by a sudden model on the basis of the overlap between the rotational wavefunctions and the angular potential near the transition state, but rotational steering also plays a significant role which complicates the dynamics. Although site averaging weakens the influence of initial rotational excitations and leads to minor effects to the reactivity, steric effects are predicted to be observable if the water molecule is selectively excited and aligned by a linearly polarized laser.

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

AbstractOxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La0.6Sr0.4Co0.2Fe0.8O3‐δ and (La0.8Sr0.2)0.95MnO3±δ, using gas‐phase isotope‐exchange with a 1:1 16O2:18O2 ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La0.6Sr0.4Co0.2Fe0.8O3‐δ is better at oxygen dissociation than (La0.8Sr0.2)0.95MnO3±δ. Heteroexchange, involving lattice oxygen, dominates on La0.6Sr0.4Co0.2Fe0.8O3‐δ. In contrast, (La0.8Sr0.2)0.95MnO3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.

Direct Observation of Oxygen Dissociation on Non-Stoichiometric Metal Oxide Catalysts.

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi-step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ and (La0.8 Sr0.2 )0.95 MnO3±δ , using gas-phase isotope-exchange with a 1:1 16 O2 :18 O2 ratio. Oxygen transport mechanisms between gas-surface reactions and surface-bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ is better at oxygen dissociation than (La0.8 Sr0.2 )0.95 MnO3±δ . Heteroexchange, involving lattice oxygen, dominates on La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ . In contrast, (La0.8 Sr0.2 )0.95 MnO3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.

Facile fabrication of chromium oxide micro/nanospheres and enhanced ethanol gas sensing performances

Chromium oxide (Cr2O3) micro/nanospheres are skillfully designed and successfully synthesized by a facile route without any templates or surfactants. Morphology and structure characterizations reveal that Cr2O3 micro/nanospheres are assembled by irregular nanoparticles as primary building blocks and present loose hierarchical architecture. The Cr2O3 micro/nanospheres sensors exhibit rapid response and recovery kinetics (5s and 6s), good selectivity and remarkable repeatability to ethanol gas at 220°C, and the sensitivity to 100ppm ethanol is about 4 times higher than that of the collapsed Cr2O3 micro/nanospheres based sensors. The enhanced gas sensing performances are predominantly attributed to the synergistic effect of the unique sphere-like architecture and highly loose structure, thus providing beneficial conditions to greatly improve gas diffusion, adsorption and surface reaction.

Investigation of reactive plasma species created in SO2 by an inductively coupled RF discharge in E- and H-mode

Optical emission spectroscopy (OES) and mass spectrometry were used to investigate the gas phase and surface reactions in inductively coupled SO2 plasma at various radiofrequency discharge powers up to 1000 W and gas pressures from 30 to 100 Pa. At such conditions, the plasma was created either in E- or in H-mode. In the E-mode, extensive radiation in the UV range was observed due to transitions of SO2 and SO molecules to the ground electronic states, whereas the other spectral features were marginal. At elevated powers, an abrupt transition to the H-mode occurred, where the total radiation increased for several orders of magnitude. Strong hysteresis was observed in the behaviour of all OES spectral features at the transitions between the E- and H-modes. In the H-mode, the atomic lines prevailed because of the relaxation of highly excited O and S atoms to the lower excited states, indicating high density of atoms. UV continuum was very weak and governed only by transitions of the SO radicals to the ground state. Thus, it was concluded that in the E-mode, predominantly SO and O radicals are formed during the partial dissociation of SO2 molecules, whereas in the H-mode, high dissociation to S and O atoms occurred, leading to the negligible concentration of SO2. However, in the flowing afterglow, the final gas composition was predominantly always SO2. The concentration of O2 was only approximately 3%, whereas the concentration of SO3 was marginal. This was explained by the recombination of the reactive plasma species formed in the plasma back to SO2 molecules on the surfaces of the remote plasma reactor.

Influence of the coating level on the heterogeneous ozonolysis kinetics and product yields of chlorpyrifos ethyl adsorbed on sand particles

Heterogeneous oxidation of chlorpyrifos ethyl (CLP) coated sand particles by gaseous ozone was studied. Mono-size sand was coated with CLP at different coating levels between 10 and 100 μg g−1 and exposed to ozone. Results were analyzed thanks to Gas Surface Reaction and Surface Layer Reaction Models. Kinetic parameters derived from these models were analyzed and led to several conclusions. The equilibrium constant of O3 between the gas phase and the CLP-coated sand was independent on the sand contamination level. Ozone seems to have similar affinity for coated or uncoated sand surface. Meanwhile, the kinetic parameters decreased with an increasing coating level. Chlorpyrifos Oxon, (CLPO) has been identified and quantified as an ozonolysis product. The product yield of CLPO remains constant (53 ± 10%) for the different coating level.The key parameter influencing the CLP reactivity towards ozone was the CLP-coating level. This dependence had a great influence on the lifetime of the CLP coated on sand particles, with respect to ozone, which could reach several years at high contamination level.

Eight-Dimensional Quantum Dynamics Study of CH4 and CD4 Dissociation on Ni(100) Surface

Methane dissociation on Ni(100) surface is one of the benchmark systems for exploring polyatomic molecular gas–surface reaction dynamics. We here present an eight-dimensional quantum dynamics study based on developing a highly accurate, 15-dimensional potential energy surface(PES). The CH4 interacting on a rigid Ni(100) surface is described with the methodology of neural network (NN) fit to plenty of ab initio configuration-energy points. With the best-fitting NN(PES), we obtain the sticking probability S0 of ground state (GS) CH4 and its isotopologue CD4 on a rigid Ni(100) from some direct quantum dynamics simulations. The promising azimuth-averaged and site-averaged approaches are employed to treat the influence of surface impact sites. The improved nonlinear lattice-sudden model is considered for the surface temperature effect. The final S0 of GS CH4 on Ni(100) surface at 475 K is in excellent agreement with two sets of available experiments. Furthermore, we calculate the S0 of GS CD4 on Ni(100) surfac...

Gas–surface reactions in the growth of dielectric films by chemical vapor deposition: The role of nonequilibrium surface diffusion of adsorbed precursors

A similarity between the diffusion equation and the Schrodinger equation is used to treat the problem of gas–surface reactions, with consideration given to the coupled processes of adsorption, surface chemical conversion, energy relaxation into the bulk, and surface diffusion over a regular one-dimensional chain of active surface sites. It is found that, in accordance with qualitative physical considerations, the nonequilibrium surface diffusion of the reaction precursor results, with an appreciable probability, in the formation of products not only at the initially attacked surface site, but also at neighboring sites.

Electron-hole pair effects in methane dissociative chemisorption on Ni(111).

The dissociative chemisorption of methane on metal surfaces has attracted much attention in recent years as a prototype of gas-surface reactions in understanding the mode specific and bond selective chemistry. In this work, we systematically investigate the influence of electron-hole pair excitations on the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111). The energy dissipation induced by surface electron-hole pair excitations is modeled as a friction force introduced in the generalized Langevin equation, in which the independent atomic friction coefficients are determined within the local-density friction approximation. Quasi-classical trajectory calculations for CH4/CH3D/CHD3 have been carried out on a recently developed twelve-dimensional potential energy surface. Comparing the dissociation probabilities obtained with and without friction, our results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity. Given similar observations in both water and methane dissociation processes, we conclude that electron-hole pair excitations would not play an important role as long as the reaction is direct and the interaction time between the molecule and metal electrons is relatively short.

Apparatus for testing gas-surface reactions for epicatalysis.

Recently, a new mode of gas-surface heterogeneous catalysis (epicatalysis) has been identified, having potential applications ranging from industrial and green chemistry to novel forms of power generation. This article describes an inexpensive, easily constructed, vacuum-compatible apparatus by which multiple candidate gas-surface combinations can be rapidly screened for epicatalytic activity. In exploratory experiments, candidate surfaces (teflon, kapton, glass, and gold) and gases (helium, argon, cyclohexane, water, methanol, formic acid, and acetic acid) were tested for epicatalytic activity. Kapton and teflon displayed small but reproducible differences in formic acid and methanol dimer desorption, thereby demonstrating the first examples of room-temperature epicatalysis. Other gas-surface combinations showed smaller or inconclusive evidence for epicatalysis.