Heterogeneous Surface Reactions Research Articles

In situ mass analysis of surface reactions using surface-enhanced Raman spectroscopy covering a wide range of frequencies

Both the structural change and mass change of adsorbates in heterogeneous surface reactions were simultaneously measured in situ using frequency-extended SERS spectroscopy.

Heterogeneous Surface Reaction Analysis of Non-Thermal-Plasma-Enhanced Spherical Mox/(Ce0.7ti0.3)O2 Catalyst Desulfurization: Effects of Plasma and Mox Loading on Surface Oxygen Defects

Heterogeneous Surface Reaction Analysis of Non-Thermal-Plasma-Enhanced Spherical Mox/(Ce0.7ti0.3)O2 Catalyst Desulfurization: Effects of Plasma and Mox Loading on Surface Oxygen Defects

Detailed modeling of aluminum particle combustion – From single particles to cloud combustion in Bunsen flames

A numerical model for aluminum cloud combustion which includes the effects of interphase heat transfer, phase change, heterogeneous surface reactions, homogeneous combustion, oxide cap growth and radiation within the Euler–Lagrange framework is proposed. The model is validated in single particle configurations with varying particle diameters. The combustion process of a single aluminum particle is analyzed in detail and the particle consumption rates as well as the heat release rates due to the various physical/chemical sub-models are presented. The combustion time of single aluminum particles predicted by the model are in very good agreement with empirical correlations for particles with diameters larger than 10 μm. The prediction error for smaller particles is noticeably reduced when using a heat transfer model that is capable of capturing the transition regime between continuum mechanics and molecular dynamics. The predictive capabilities of the proposed model framework are further evaluated by simulating the aluminum/air Bunsen flames of McGill University for the first time. Results show that the predicted temperature distribution of the flame is consistent with the experimental data and the double-front structure of the Bunsen flame is reproduced well. The burning rates of aluminum in both single particle and particle cloud configurations are calculated and compared with empirical correlations. Results show that the burning rates obtained from the present model are more reasonable, while the correlations, when embedded in the Euler–Lagrange context, tend to underestimate the burning rate in the combustion stage, particularly for the considered fuel-rich flames.

Nanoscale confinement in carbon nanotubes encapsulated zero-valent iron for phenolics degradation by heterogeneous Fenton: Spatial effect and structure–activity relationship

• Confinement effect was significant for 3–7 nm carbon nanotubes (CNTs) cavity encapsulated Fe. • The better reusability of Fe 0 @CNTs (3)-(7) was exhibited for five runs compared with other CNTs. • The active enhancement attributed to electron transfer and interaction between Fe and CNTs. • The confinement role reached 2.13-folds of homogeneous role for phenol removal by Fe 0 @CNTs (5). • Fe 0 @CNTs ( x ) with positive charges was conductive to adsorb the negatively charged phenols. In heterogeneous Fenton, nanostructured iron catalysts with confinement effect greatly improve the effectiveness for organic pollutants removal, however, this effect dependence on the space of substrate remains unclear and thereby limit the application of confined catalysis. Herein, Fe particles encapsulated in carbon nanotubes with inner diameter of 3–30 nm (Fe 0 @CNTs ( x )) were prepared and tested as catalysts to degrade model pollutant of phenol and four phenolic compounds ( p -diphenol ( p -DP), p -aminophenol ( p -AP), p -nitrophenol ( p -NP) and p -chlorophenol ( p -CP)). With the increasing of CNTs inner diameter from 3 to 30 nm, the encapsulated Fe particles linearly increased from 2.68 to 14.75 nm. The confinement effect was significant in the range of 3–7 nm of CNTs cavity, especially at 5 nm, which was critical to decrease iron leaching and enrich hydroxyl radical generation for efficient pollutants removal by surface heterogeneous reaction (2.13-folds compared with the homogeneous Fenton). In addition, the Fe 0 @CNTs (3) - (7) with positive charges was conductive to adsorb the negatively charged phenols ( p -DP and p -AP) compared with non-spatial confinement of Fe-CNTs with negative charge. According to the experiment and DFT calculations, the quantitative structure–activity relationship about electron transfer was established, indicating that the electron from highest occupied molecular orbital (HOMO) of p -AP could migrate to lowest unoccupied molecular orbital (LUMO) of Fe 0 @CNTs and further to HOMO of Fe 0 @CNTs. The electron in HOMO of Fe 0 @CNTs could migrate into H 2 O 2 to generate · OH. This finding suggested a new paradigm to fulfill selective oxidation of the pollutants with electron-withdrawing group by suitable heterogeneous catalysts having significant confinement effect with more effective utilization of · OH in Fenton reaction.

Mixed convection hybrid nanoliquid flow over an exponentially stretching rough (smooth) surface with the impacts of homogeneous–heterogeneous reactions

AbstractThis paper explores the influence of homogeneous and heterogeneous (surface) reactions in the mixed convection flow of hybrid nanoliquid (Cu–Fe3O4/water) over an exponentially stretching surface. In this analysis, we have considered chemical species of unequal diffusivity. In spite of this, the effects of surface roughness and surface mass transfer are also included in the analysis. The numerical solutions to the transformed governing equations of the physical problem have been obtained by employing the implicit finite difference method and the quasilinearization technique. The numerical results for drag coefficient and rate of transfer of heat, along with various profiles such as velocity, temperature, and species concentrations, for some appropriate parameters are displayed by graphs and tables. The rate of transfer of heat from rough (smooth) surfaces to the liquid in the case of hybrid nanoliquid is found to be greater as compared to the nanoliquid (Fe3O4/water) and base liquid (water). Where as, opposite trend is observed for skin friction co–efficient. The influence of heterogeneous reaction on the reactant (bulk liquid) is found to be significantly high as compared to the homogeneous reaction in the regime of the boundary layer. The obtained numerical outcomes are compared with those of earlier literature and are found to be in good agreement.

Data-Driven Analysis of Nonlinear Heterogeneous Reactions through Sparse Modeling and Bayesian Statistical Approaches.

Heterogeneous reactions are chemical reactions that occur at the interfaces of multiple phases, and often show a nonlinear dynamical behavior due to the effect of the time-variant surface area with complex reaction mechanisms. It is important to specify the kinetics of heterogeneous reactions in order to elucidate the microscopic elementary processes and predict the macroscopic future evolution of the system. In this study, we propose a data-driven method based on a sparse modeling algorithm and sequential Monte Carlo algorithm for simultaneously extracting substantial reaction terms and surface models from a number of candidates by using partial observation data. We introduce a sparse modeling approach with non-uniform sparsity levels in order to accurately estimate rate constants, and the sequential Monte Carlo algorithm is employed to estimate time courses of multi-dimensional hidden variables. The results estimated using the proposed method show that the rate constants of dissolution and precipitation reactions that are typical examples of surface heterogeneous reactions, necessary surface models, and reaction terms underlying observable data were successfully estimated from only observable temporal changes in the concentration of the dissolved intermediate products.

Photochemical reaction of superoxide radicals with 1-naphthol

The photochemical reactions between 1-naphthol (1-NP) and the superoxide anion radical (O2•–) were investigated in detail by using 365 nm UV irradiation. The results showed that the conversion rate of 1-NP decreased with the increase of the initial concentration of 1-NP, whereas by increasing the pH and riboflavin concentration, the photochemical reaction was accelerated. The second-order reaction rate constant was estimated to be (3.64 ± 0.17) × 108 L mol−1s−1. The major photolysis products identified by using gas chromatography – mass spectrometry (GC–MS) were 1,4-naphquinone and 2,3-epoxyresin-2,3-dihydro-1,4-naphquinone, and their reaction pathways were also discussed. An atmospheric model showed that both the bulk water reaction and the heterogeneous surface reaction deserve attention in atmospheric aqueous chemistry.

Ti-Modified LaFeO3/β-SiC Alveolar Foams as Immobilized Dual Catalysts with Combined Photo-Fenton and Photocatalytic Activity.

Ti-modified LaFeO3/β-SiC alveolar foams were used as immobilized, highly robust dual catalysts with combined photocatalytic wet peroxide oxidation and photocatalytic activity under UV-A light. They were prepared by incipient wetness impregnation of a β-SiC foam support, by implementing a sol-gel Pechini synthesis at the foam surface in the presence of dried amorphous sol-gel titania as a titanium source. The physicochemical and catalytic features suggest the stabilization at the foam surface of a substituted La1-xTixFeO3 catalyst analogous to its powdery counterpart. Taking 4-chlorophenol removal in water as a model reaction, its dual nature enables both high reaction rates and full total organic carbon (TOC) conversion because of a synergy effect, while its macroscopic structure overcomes the drawback of working with powdery catalysts. Further, it yields photonic efficiencies for degradation and mineralization of ca. 9.4 and 38%, respectively, that strongly outperform those obtained with a reference TiO2 P25/β-SiC foam photocatalyst. The enhancement of the catalyst robustness upon Ti modification prevents any Fe leaching to the solution, and therefore, the optimized macroscopic foam catalyst with 10 wt % catalyst loading operates through pure heterogeneous surface reactions, without any activity loss during reusability test cycles.

Experimental and Computational Analysis of NOx Photocatalytic Abatement Using Carbon-Modified TiO2 Materials

In the present study, two photocatalytic graphene oxide (GO) and carbon nanotubes (CNT) modified TiO2 materials thermally treated at 300 °C (T300_GO and T300_CNT, respectively) were tested and revealed their conversion efficiency of nitrogen oxides (NOx) under simulated solar light, showing slightly better results when compared with the commercial Degussa P25 material at the initial concentration of NOx of 200 ppb. A chemical kinetic model based on the Langmuir–Hinshelwood (L-H) mechanism was employed to simulate micropollutant abatement. Modeling of the fluid dynamics and photocatalytic oxidation (PCO) kinetics was accomplished with computational fluid dynamics (CFD) approach for modeling single-phase liquid fluid flow (air/NOx mixture) with an isothermal heterogeneous surface reaction. A tuning methodology based on an extensive CFD simulation procedure was applied to adjust the kinetic model parameters toward a better correspondence between simulated and experimentally obtained data. The kinetic simulations of heterogeneous photo-oxidation of NOx carried out with the optimized parameters demonstrated a high degree of matching with the experimentally obtained NOx conversion. T300_CNT is the most active photolytic material with a degradation rate of 62.1%, followed by P25-61.4% and T300_GO-60.4%, when irradiated, for 30 min, with emission spectra similar to solar light.

Homogenization approach to the upscaling of a reactive flow through particulate filters with wall integrated catalyst

Catalytic membranes can degrade gaseous pollutants to clean gas via a catalytic reaction to achieve green emissions. Further, a catalytic membrane is a three scale porous medium. Membranes used in catalytic filters usually have thicknesses of centimeters or millimeters, and consist of active (washcoat) particles, inert material and microscale, micron size, pores. The washcoat particles are porous material with nanoscale pores. The catalytic reactions are heterogeneous (surface reactions) and they occur on the surface of the nanopores. Obviously, simulations at fully resolved pore scale are not feasible, and upscaling techniques have to be applied. It is known that the same microscale problem can be upscaled to different macroscale equations depending on the characteristic numbers. In this paper we study the homogenization of reactive flow in the presence of strong absorption in the washcoat particles. Two reactive transport regimes are studied, where in both the reaction dominates over the convection and the diffusion. Péclet’s number in the first one is of order 1, and in the second one it is proportional to the ratio of the thickness of the catalytic membrane and the characteristic length of the microscale pores. Two different upscaled equations are obtained, respectively. Direct numerical simulation at microscale is used to validate these derived macroscale equations.

Loss of NO(g) to painted surfaces and its re-emission with indoor illumination.

Heterogeneous surface reactions play a key role in the chemistry of the indoor environment because of the large indoor surface‐to‐volume ratio. The presence of photocatalytic material in indoor paints may allow photochemical reactions to occur at wavelengths of light that are present indoors. One such potential reaction is the heterogeneous photooxidation of NO to HONO. NO(g) is commonly found indoors, originating from combustion sources, ventilation and infiltration of outdoor air. We studied the interaction of NO(g) with painted surfaces illuminated with indoor fluorescent and incandescent lighting. There is a loss of NO(g) to painted surfaces in the dark at both 0 and 50% RH. At 50% RH, there is a re‐release of some of that NO(g) under illumination. The same behavior is observed for illumination of different colored paints. This is in contrast to what is seen with TiO2 as the substrate, where photoenhanced uptake of NO(g) and formation of NO2(g) are observed. We hypothesize that the loss of NO(g) is due to adsorption and diffusion into the paint. The re‐release of NO under illumination is thought to be due to photooxidation of NO to HONO on the painted surface at higher relative humidities and subsequent HONO photolysis.

A multiscale study of density-driven flow with dissolution in porous media

In the present work, a suite of numerical experiments with linear stability analysis are conducted to study density-driven flow with chemical dissolution of two reactive fluids in synthetic porous media. Linear stability analysis at the Darcy scale is first performed to predict the interfacial phenomena and instability at the initial time. Pore-scale simulations using the lattice Boltzmann method (LBM) are further conducted to capture more mechanistic information and advance the understanding of the transport processes. Under different scenarios, it is demonstrated that the transport processes exhibit distinct behaviors, which are largely dominated by the interplay among density contrast, chemical reaction rate and evolution of the porosity/permeability. All the results indicate that the interfacial instability can be triggered by the density contrast between two miscible fluids, leading to the Rayleigh-Taylor (R-T) instability. The R-T instability can be suppressed by the heterogeneous surface reaction between the fluid and solid phases, which prevents the transport of the denser fluid. Over the long term, it is found that the interfacial instability is influenced by the evolution of the porosity/permeability due to dissolution, which potentially restarts the transport of the denser fluid.

The Vanadium Redox Reactions – Electrocatalysis or Not?

Catalytic effects of surface groups on porous carbon electrodes are claimed in literature for the redox reactions V(II)/V(III) and V(IV)/V(V). The literature is critically analysed also in relation to work of this group. A method on how to overcome the problem of assessing the electrochemically active surface area on porous electrodes is presented [1]. Applying this method, no catalytic effects for above reactions can be substantiated. It is further pointed out that the parameters electrochemical potential and temperature need to be used to assess electrocatalysis.The main control parameter determining the rate of an electrochemical reaction is the Gibbs free enthalpy of activation, ΔG≠ , in contrast to ΔH≠ for e.g. a heterogeneous surface reaction. This would suggest to clearly define electrocatalysis as the “lowering the activation barrier, ΔG≠ , in an electrochemical reaction” where, in fact, the lowering of ΔG≠ can result from a change in ΔH≠ and/or ΔS≠ (∂ ΔG≠ /∂η=∂ ΔH≠ /∂η−T∂ΔS≠/∂η , as in Figure1). Such a treatment of data of temperature dependent electrochemical reactions has been described in literatures [2,3]. An analysis according to the above-discussed procedures would pin down which quantity is responsible for the lowering of ΔG ≠ that may allow for a better interpretation of the electrocatalytic effect.Using the example of the redox reactions V(II)/V(III) and V(IV)/V(V) the literature was analysed regarding possible catalytic effects of functional groups on carbon electrodes, as in Figure2. The conclusion was that no electrocatalysis of either of these reactions can be confirmed. Observed enhancements of currents can rather be attributed to a change in the effective surface area of the, usually, porous electrode. A method is suggested how such effects can be distinguished from electrocatalysis by using electrochemical impedance spectroscopy (EIS). Figure2 gives an example how EIS data are used for evaluation. In more general terms, the criteria for electrocatalysis were described. Experimentally, an investigation should contain both, measurements of the potential dependence and the temperature dependence since the main parameter in electrocatalysis is the lowering of ΔG≠ . Only then, the respective influence of ΔH≠ and ΔS≠ can be assessed.AcknowledgmentsThis work was supported in part by Siemens AG through a research grant and by EPSRC through funding for the North East Centre of Energy Materials (NECEM) (EP/R021503/10).

Stagnation Flow of a SWCNT Nanofluid towards a Plane Surface with Heterogeneous-Homogeneous Reactions

The homogeneous-heterogeneous reaction in the boundary layer flow of a water-based nanofluid in the stagnation-point region of a plane surface is investigated. The type of small particles explored here is the single-walled carbon nanotubes. The homogeneous nanofluid model is employed for description of behaviours of nanofluids. Here, the homogeneous (bulk) reaction is isothermal cubic autocatalytic, while the heterogeneous (surface) reaction is single, isothermal, and first order. The steady state of this system is analysed in detail, with equal diffusion coefficients being considered for both reactants and autocatalysts. Multiple solutions of the reduced system are captured for some particular sets of physical parameters, which seem to be overlooked in all previous published works with regard to studies of homogeneous-heterogeneous reactions modeled by homogeneous nanofluid models. Besides, we discover the significant limitation of previous conclusion about that the solutions by homogeneous nanofluid flow models can be recovered from those by regular fluids.

Nucleation and Growth of a Nanobubble on Rough Surfaces.

We study the nucleation and growth of a nanobubble on rough surfaces using molecular dynamics simulations. A nanobubble nucleates and grows by virtue of a heterogeneous surface reaction which results in the production of gas molecules near the surface. We study the role of surface roughness in the nucleation and growth behavior of a nanobubble. We perform simulations at various reaction rates and surface morphology and quantified the growth dynamics of a nanobubble. Our simulations show that after the onset of nucleation, the nanobubble grows rapidly with radius following t1/3 behavior followed by a diffusive growth regime which is marked by t1/2 growth behavior. This growth behavior remains independent of surface roughness and reaction rates over the range considered in this study. We also quantified the oversaturation of gas required for nucleation of a nanobubble and demonstrated its dependence on the surface morphology.

Indoor air chemistry: Terpene reaction products and airway effects.

Reactive chemistry is ubiquitous indoors with a wealth of complex oxidation reactions; some of these are initiated by both homogeneous and heterogeneous reaction of ozone with unsaturated organic compounds and subsequent the hydroxyl radical, either in the gas-phase or on reactive surfaces. One major focus has been the reaction of common and abundant terpene-based fragrances in indoor air emitted from many wood-based materials, a variety of consumer products, and citrus fruits and flowers. Inhalation of the terpenes themselves are generally not considered a health concern (both acute and long-term) due to their low indoor air concentrations; however, their gas- and surface reactions with ozone and the hydroxyl radical produce a host of products, both gaseous, i. a. formaldehyde, and ultrafine particles formed by condensation/nucleation processes. These reaction products may be of health concern. Human cell bioassays with key reaction products from ozone-initiated terpene reactions have shown some inflammatory reactions, but results are difficult to interpret for human exposure and risk assessment. Acute effects like sensory irritation in eyes and airways are unlikely or present at very low intensity in real life conditions based on rodent and human exposure studies and known thresholds for sensory irritation in eyes and airways and derived human reference values for airflow limitation and pulmonary irritation. Some fragrances and their ozone-initiated reaction products may possess anti-inflammatory properties. However, long-term effects of the reaction products as ultrafine particles are poorly explored. Material and product surfaces with high ozone deposition velocities may significantly impact the perceived air quality by altered emissions from both homogeneous and heterogeneous surface reactions.

Local reactive boundary scheme for irregular geometries in lattice Boltzmann method

In this paper, a boundary scheme is proposed for the lattice Boltzmann method for convection-diffusion problems in irregular geometries with linear heterogeneous surface reaction. Compared with previous schemes, the physical picture of the proposed one is more clear, which reflects the consumption and production of the reaction at the boundary. Furthermore, as the unknown distribution functions at the boundary nodes are determined locally based on the kinetic flux of the incident ones, the present scheme can be easily applied to problems with complex geometric structures. The accuracy of the scheme is first tested by simulating the transient longitudinal mixing phenomenon and the convection-diffusion problems in inclined channels. The numerical results are in excellent agreement with the analytical solutions, and it is shown that the boundary scheme is of second-order accuracy in space for a straight wall in line with a link of the lattice. However, the order of accuracy will decrease for a general irregular wall. Finally, the dissolution process of a single calcite grain is simulated in both two-dimension (2D) and three-dimension (3D). Although a slight difference was observed between the results of 2D and 3D, all the results agree well with experimental measurements reported in previous study.

Oxidation and pyrolysis of ammonia mixtures in model reactors

The present work was devoted to an experimental characterization of NH3 oxidation and pyrolysis processes in model reactors. The oxidation tests were performed in a Jet Stirred Flow Reactor (JSFR) for stoichiometric ammonia/oxygen/argon and ammonia/oxygen/argon/water mixtures, as a function of mixture inlet temperature (Tin), at fixed pressure and residence time. Following, the behavior of ammonia/oxygen/argon mixture was analyzed in two tubular laminar flow reactors (LFRs) made of different materials (quartz and alumina), to evaluate potential surface heterogeneous effects.Pyrolysis tests were performed in both the reference model reactors, by substituting oxygen with argon.Experimental results realized in the JSFR allowed to identify three different kinetics regimes in ammonia oxidation: low (Tin < 1130 K), intermediate (1130 < Tin < 1250 K) and high temperatures (Tin > 1250 K). Surface heterogeneous reactions seemed to be negligible on ammonia reactivity and hydrogen profiles, but some effect could be observed on NOx concentration in the low-intermediate temperature range. Experiments realized in presence of water suggested that such species may passivate the surface, minimizing heterogeneous effects.Experimental tests realized in the LFRs confirmed these results, as hydrogen and oxygen profiles were independent of the reactor material, while NOx concentrations exhibited significant changes. Experimental results suggested that heterogeneous reactions were more relevant under pyrolytic conditions.Numerical simulations were performed with different kinetic mechanisms available in literature to value their capability to describe ammonia chemistry. None of them could accurately reproduce the experimental data for ammonia oxidation process, while they completely failed to predict ammonia pyrolysis features.

Highly robust La1-xTixFeO3 dual catalyst with combined photocatalytic and photo-CWPO activity under visible light for 4-chlorophenol removal in water

A highly robust Ti-substituted LaFeO3 dual catalyst was synthesized via a facile Pechini sol-gel route using a dried amorphous sol-gel titania as titanium source and a final calcination at 800 °C that enabled solid-solid diffusion of titanium atoms within the perovskite network. Using an amorphous precursor instead of crystallized titania nanocrystals increased the Ti → La atomic substitution degree in the crystalline network from 5 at% to 11 at%, together with a strong increase in the catalytic activity. The partially substituted La1-xTixFeO3 catalyst with a 11 at% substitution degree allowed pure heterogeneous surface reactions to take place under pure visible light (λ > 420 nm), with an increase in the catalyst robustness by more than two orders of magnitude compared to the unmodified LaFeO3 counterpart, evidenced by the absence of any Fe release, the structural stability of the substituted catalyst and the absence of any loss of catalytic activity with test cycles. Further, the strategy of combining both photocatalysis and H2O2-mediated heterogeneous photo-Fenton advanced oxidation processes within one single heterogeneous catalyst allowed the dual La1-xTixFeO3 catalyst to simultaneously take advantage from the higher reaction rate of photo-Fenton and from the higher mineralization yield of photocatalysis in water treatment. As a result, the dual 11 at% Ti-substituted La1-xTixFeO3 catalyst clearly outperformed its unmodified LaFeO3 counterpart and led to a complete mineralization of the 4-chlorophenol substrate under pure visible light, with strongly enhanced H2O2 decomposition, 4-chlorophenol and TOC conversion rates. Finally, the activity of the La1-xTixFeO3 catalyst under visible light contributed significantly to its overall activity obtained using the full solar spectra.

Distinguishing Plasma Contributions to Catalyst Performance in Plasma-Assisted Ammonia Synthesis

Plasma-assisted catalysis is the process of electrically activating gases in the plasma-phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasma phase reactions from the overall plasma-assisted catalytic kinetics by performing plasma-only experiments. By varying the gas composition, temperature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γ-alumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first-order in both N2 and H2. In ...