Surface Reaction Path Research Articles

Revealing the poisoning effect of potassium salts on MnOx catalysts for low-temperature simultaneous removal of NO and CO

Simultaneously elimination of NO and CO in the sintering flue gas is an urgent problem, in which the application of the bifunctional MnOx catalyst has demonstrated its effectiveness in removing NO and CO simultaneously. Potassium salts are frequently found in the sintering flue gas and exert a detrimental influence on the catalytic performance of catalysts. Herein, the primary focus of this study is to investigate different K+ species (K2SO4, KCl, KNO3) poisoning effect on bifunctional MnOx. The findings suggested a deactivation order in the sequence of KNO3 > KCl > K2SO4. Fresh Mn-AC catalyst had finer grains, and poisoning elements were evenly distributed. XPS results revealed an obvious decrease in Mn4+ and Oβ species across all catalysts after K+ species poisoning, hindering redox cycle activation and oxygen species mobility, thus impacting catalytic activity. The in situ DRIFTS results indicated the forbidden of E-R route over KNO3-poisoned catalysts and low reactivity of adsorbed species after poisoning for NO removal. KNO3 severely suppressed surface reaction paths on Mn-AC catalyst and declined the catalytic efficiency, especially in CO activation. Finally, a possible reaction mechanism model for the simultaneous removal of NO and CO was proposed. This study could provide reference for the development of bifunctional catalyst with high resistance to potassium salts.

Perovskite oxide based composite hollow fiber membrane for CO2 transport

Membrane technologies have been widely investigated for CO2 separation, especially inorganic membranes which could overcome the barrier of low melting point, and then been applied for combustion activities. Compared with molecular sieving membranes (MSMs), the mechanism during CO2 separation process on electrolyte membranes (EMs) is more complicated. Mixed ionic and electronic conductors, such as perovskite oxides, has been proved with superior activity than oxygen ion conductors because of the additional surface reaction path. To further demonstrate the applicability of perovskite oxides contained membrane for CO2 separation, in this work, CO2 transport via La0.6Sr0.4Co0.2Fe0.8O3-δ and carbonates composite membrane with hollow fiber microstructure was thoroughly investigated under various gas flow directions. This membrane presents the best permeability when CO2+N2 (feed gas) flows at shell side and He (sweep gas) flows at tube side in parallel model. In addition, it also presents excellent thermal shock resistance after six times rising and cooling operation with almost no performance degradation.

Pd-Catalyzed Reaction-Producing Intermediate S on a Pd/In2O3 Surface: A Key To Achieve the Enhanced CS2-Sensing Performances.

Although chemiresistive gas sensors, based on metal-oxide semiconductors, have exhibited particular promise for the monitoring of air pollution, they are often limited because of poor selectivity. In that case, to overcome this issue, according to the essence of the gas-sensing process, the method of reforming the surface reaction path on the surface of the sensing materials was used. Here, we report that Pd nanoparticles supported over the In2O3 composites, featured with a yolk-shell structure, enable the trace detection of carbon disulfide (CS2) gas molecules, which are immensely dangerous to humans and animals. Moreover, the prominent enhancement of the gas response and the ultraselective CS2-sensing characteristic were acquired in comparison with pristine In2O3 sensors. Significantly, density functional theory calculations revealed that the Pd supported on In2O3 greatly facilitates the adsorption capacity to CS2, and the intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In2O3 surface during the sensing process is a key to achieving a high CS2 gas response as well as ultraselectivity, which is well in agreement with the X-ray photoelectron spectroscopy analysis results. On the basis of these results, a new sensing mechanism model for the CS2-sensing process was put forward.

Two-site H2O2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

On the activity of bimetallic catalysts for ethanol steam reforming

The performance of CeO2-supported Pt–Ni and Pt–Co catalysts in the low temperature-Ethanol Steam Reforming (ESR) reaction has been evaluated studying the effect of the preparation method (impregnation/coprecipitation) and parameters such as dilution ratio, temperature, water-to-ethanol feed ratio and Gas Hourly Space Velocity (GHSV). The results show that impregnated samples perform better. In particular, the Pt/Ni/CeO2 catalyst starting from 350 °C leads to a products distribution very close to the equilibrium calculations, with a low CO content that is ideal for fuel cells devices. In addition, the Co-based catalysts appear attractive in terms of hydrogen yield and coking tendency.For the Pt/Ni sample at 370 °C, the analysis of products distributions vs contact time suggests a surface reaction path involving ethanol dissociative adsorption, dehydrogenation to acetaldehyde at short contact time (5–40 ms), followed by decomposition, reforming and CO-WGS reactions, to obtain H2, CH4, CO2 and CO. The last step is the methanation reaction, occurring at contact times higher than 100 ms, which drives the system to the equilibrium.

Water–oxygen interplay on tin dioxide surface: Implication on gas sensing

Resistance measurements and direct spectroscopic investigations were used to monitor the surface reaction path between oxygen and water at the surface of SnO 2. The experiments were carried out at high sensor operation temperature (330 and 400 °C) and at a constant background of water vapour. We found that there is a significant interaction between adsorbed oxygen ions and water vapour, which results in formation of terminal hydroxyl groups on tin dioxide surface. This observation is an evidence of water–oxygen interaction and so brings a new insight to the mechanistic modelling of the sensing with tin dioxide based sensors.

Si 1−y C y / Si (001) gas-source molecular beam epitaxy from Si2H6 and CH3SiH3: Surface reaction paths and growth kinetics

In situ surface probes and postdeposition analyses were used to follow surface reaction paths and growth kinetics of Si1−yCy alloys grown on Si(001) by gas-source molecular-beam epitaxy from Si2H6/CH3SiH3 mixtures as a function of C concentration y (0–2.6 at %) and temperature Ts (500–600 °C). High-resolution x-ray diffraction reciprocal lattice maps show that all layers are in tension and fully coherent with their substrates. Film growth rates R decrease with both y and Ts, and the rate of decrease in R as a function of y increases rapidly with Ts. In situ isotopically tagged D2 temperature-programmed desorption (TPD) measurements reveal that C segregation during steady-state Si1−yCy(001) growth results in charge transfer from Si surface dangling bonds to second-layer C atoms, which have a higher electronegativity than Si. From the TPD results, we obtain the coverage θSi*(y,Ts) of Si* surface sites with C backbonds as well as H2 desorption energies Ed from both Si and Si* surface sites. θSi* increases with increasing y and Ts in the kinetically limited segregation regime while Ed decreases from 2.52 eV for H2 desorption from Si surface sites with Si back bonds to 2.22 eV from Si* surface sites. This leads to an increase in the H2 desorption rate, and hence should yield higher film deposition rates, with increasing y and/or Ts during Si1−yCy(001) growth. The effect, however, is more than offset by the decrease in Si2H6 reactive sticking probabilities at Si* surface sites. Film growth rates R(Ts,JSi2H6,JCH3SiH3) calculated using a simple transition-state kinetic model, together with measured kinetic parameters, were found to be in excellent agreement with the experimental data.

Low pressure chemical vapor deposition of silicon carbide from dichlorosilane and acetylene

The deposition of silicon carbide thin film on Si (1 0 0) wafer, using low pressure CVD of SiH 2Cl 2/C 2H 2/H 2 reaction system, is investigated. The SiC film deposited at 1023 K is amorphous, 1073 K is microcrystalline, and 1173 K is (1 1 0) preferentially oriented. The deposition rate increases significantly with temperature. Deposition kinetics of surface and gas-phase paths is separated by varying the volume/surface ratio of deposition environment. The activation energy for gas-phase reaction path is estimated to be 76 kcal/mol, and that for surface reaction path is 71 kcal/mol. The experimental barrier for gas-phase reaction path is close to the values by ab initio calculation (Su and Schlegel), which range from 73 to 76 kcal/mol for the step of H 2 elimination from SiH 2Cl 2.

Electrically Active and Inactive B Lattice Sites in Ultrahighly B Doped Si(001): An X-Ray Near-Edge Absorption Fine-Structure and High-Resolution Diffraction Study

B lattice positions are determined as a function of B concentration ${C}_{\mathrm{B}}$ in ultrahighly doped Si(001):B layers grown by gas-source molecular beam epitaxy from ${\mathrm{B}}_{2}{\mathrm{H}}_{6}/{\mathrm{Si}}_{2}{\mathrm{H}}_{6}$. For ${C}_{\mathrm{B}}\ensuremath{\le}2.5\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3}$, all B atoms reside on tetrahedrally bonded electrically active substitutional Si sites. At higher ${C}_{\mathrm{B}}$, inactive B is incorporated as B pairs located on single Si sites and oriented primarily along in-plane [100] and [010] directions. The B pairs are ${\mathrm{sp}}^{2}$ bonded with trigonal coordination while substitutional single B atoms are ${\mathrm{sp}}^{3}$. A surface reaction path leading to inactive B incorporation is proposed.

Modeling Ignition of Catalytic Reactors with Detailed Surface Kinetics and Transport: Oxidation of H2/Air Mixtures over Platinum Surfaces

The catalytic ignition of H2/air mixtures over platinum is modeled using a stagnation-point flow model with detailed gas-phase, surface kinetics and transport using an arc-length continuation technique. Self-inhibition of the catalytic ignition of H2/air mixtures is observed in agreement with experiments. For compositions between ∼0.3 and ∼15% H2 in air at atmospheric pressure, hysteresis is created by site competition, while for mixtures with more than ∼15% H2 in air, thermal feedback is a prerequisite. It is found that the system shifts from a kinetics-limited regime on the extinguished branch to a transport-limited regime on the ignited branch. However, near ignition, the system tends toward a transport- and kinetics-limited regime. Sensitivity analysis on the reaction preexponentials shows that the competitive dissociative adsorption of H2 and O2 and the desorption of H* most affect the catalytic ignition temperature. Reaction path analysis reveals a change in dominant surface reaction paths as a function of feed composition. The effects of strain rate, pressure, and preheating on catalytic ignition are also discussed.

Homogeneous ignition of hydrogen-air mixtures over platinum

The homogeneous ignition of hydrogen-air mixtures over platinum is studied at atmospheric pressure using a stagnation-point flow model with detailed gas-phase kinetics, surface kinetics, and transport phenomena. The momentum, energy, and mass balances are discretized using a second-order finite difference scheme. The obtained set of algebraic equations is then solved using Newton's method. An are-length continuation is employed to determine ignitions and to perform parametric studies. Inhibition of the homogeneous ignition caused by the catalyst is observed in agreement with published experiments. A local maximum in the homogeneous ignition temperature is found at the surface stoichiometric point at ∼15% H2 in air, a composition determined by surface reactions and multicomponent transport effects. The gasphase inhibition is induced mainly by product formation through the termination reaction H+O2+M→HO2+M and by depletion of the fuel below ∼15% H2 in air or of the oxidant above ∼15% H2 in air. Sensitivity analysis shows that desorption of radicals has a minor effect on gas-phase ignition of H2 in air. On the other hand, the dissociative adsorption of O2 through O2+2°→2O°, (° denotes adsorbed species or an adsorption site) has a strong influence on the homogeneous ignition temperature. Reaction path analysis reveals a change in the major surface reaction path for formation of H2O° from 2OH°→H2O°+O° when H2 in air is less than ∼15% to OH°+H°→H2O°+° when this composition is greater than ∼15%. Implications for homogeneous ignition of hydrocarbons near catalysts are also discussed.

Photoemission studies of time-resolved surface reactions: Isothermal desorption of CO from Ni(111)

A new dynamic approach to spectroscopic studies of surface reactions is described. A fast-acting UHV gas dosing system is employed to prepare the adsorbed species in an initial chemisorption state under reaction conditions of elevated temperature and/or pressure (to ∼ 1 Torr). Individual reaction steps are then resolved in time using photoemission spectroscopy to probe the surface electronic structure, yielding information about the kinetics of individual reaction steps, the nature of chemisorbed species, and the surface reaction path. As a first application of this approach, the kinetics of isothermal desorption of CO from Ni(111) is studied for temperatures up to ∼230°C. The coverage-dependence of the rate constants is revealed directly in the shape of each desorption curve. Preexponential (frequency) factors as well as desorption activation energies show significant variation with coverage.