Surface Catalysis in Heterogeneous Reactions

Heterogeneous reaction characteristics and its effects on homogeneous combustion of methane/air mixture in microchannels II. Chemical analysis

High purity, carbon monoxide-free hydrogen and filamentous carbon can be produced by thermo-catalytic cracking of methane. Carbon filaments continue to grow until the catalyst deactivates because of carbon encapsulation. Regeneration of catalyst is important to maintain a continuous process. Our work on optimization of the partial regeneration method showed that activity of the catalyst can be sustained for longer times by gasifying not all but some extent of the deposited carbon. In the previous work, a kinetic model was developed to be able to understand the reaction mechanism of deactivation of fresh catalyst at the molecular level. The objective of this study was to develop a well-fitted kinetic model for deactivation of regenerated catalyst. A kinetic model that consists of surface reactions, filament formation, and deactivation was developed for the regenerated catalyst. It is assumed that reaction parameters at the molecular level do not change when the burn off degree is at a moderate extent but these parameters changed drastically when burn off degree is vastly increased or decreased. Rate constant for the encapsulation reaction is adjusted for the simulation results to be representative of the experimental results (typically specific weight of carbon (g C/g Ni) vs. time on stream). The system of differential algebraic equations consists of the steady-state equations for all surface intermediates, an algebraic equation of dissolution/segregation, diffusion equation, and the site balance equation. The system has been solved without assuming any rate-determining step or most abundant surface intermediates. Parameter estimation procedures were repeated for the deactivation cycles of regenerated 5 wt% Ni/\gamma-Al_2O_3 catalyst. The basic idea that underlies the model is that every carbon atom will diffuse through the nickel particle and participate in the formation of carbon filaments until the catalyst deactivates. Specific weight of carbon is calculated using the rate of carbon diffusion.

Interaction between heterogeneous and homogeneous reaction of premixed hydrogen–air mixture in a planar catalytic micro-combustor

Temperature-programmed plasma surface reaction: An approach to determine plasma-catalytic performance

An integrated theoretical/numerical framework is established and validated to study the chemical erosion of refractory-metal (tungsten, rhenium, and molybdenum) nozzle inserts in solid-rocket-motor environments, with a primary focus on tungsten. The formulation takes into account multicomponent thermofluid dynamics in the gas phase, heterogeneous reactions at the surface, energy transport in the solid phase, and nozzle material properties. Typical combustion species of nonmetallized ammonium-perchlorate/hydroxyl-terminated-polybutadiene propellants at practical motor operating conditions are considered. The erosion rates calculated by employing three different sets of chemical kinetics data available in the literature for the tungsten-steam reaction have been compared. The effect of considering either of two different tungsten oxides, WO2 or WO3, as the final product of surface reactions is also investigated. The predicted erosion rates compare well with experimental data. The oxidizing species of H2O proved more detrimental than CO2 in dictating the tungsten nozzle erosion. The material recessionrateiscontrolledbyheterogeneouschemicalkineticsbecausethediffusionlimitisnotreached.Theerosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. The tungsten nozzle erodes much more slowly than graphite, but at a rate comparable with that of rhenium. The molybdenum nozzle exhibits the least erosion for flame temperatures lower than 2860 K. Its low melting temperature (2896 K), however, restricts applications for propellants with high flame temperatures.

Methods for regenerating H3PW12O40 catalyst in the solvent-free direct preparation of dichloropropanol (DCP) from glycerol and hydrochloric acid gas were investigated. Regenerated H3PW12O40 catalyst was then applied to the solvent-free direct preparation of DCP. In the solvent-free direct preparation of DCP, selectivity for DCP over H3PW12O40 catalyst regenerated by method I (recovery of solid H3PW12O40 catalyst by evaporating homogeneous liquidphase product solution) significantly decreased with increasing recycling run, while that over H3PW12O40 catalyst regenerated by method II (regeneration of H3PW12O40 catalyst by oxidative calcination of solid product recovered by method I) was slightly decreased with no significant catalyst deactivation with respect to recycling run. On the other hand, selectivity for DCP over H3PW12O40 catalyst regenerated by method III (regeneration of H3PW12O40 catalyst by recrystallization and subsequent oxidative calcination of solid product recovered by method II) was the same as that over fresh catalyst without any catalyst deactivation with respect to recycling run. Thus, method III was found to be the most efficient method for the regeneration of H3PW12O40 catalyst.

La0.6Sr0.4CoO3-δ (LSC) perovskite, as a potential catalyst precursor for hydrogen (H2)-rich production by steam reforming of methanol (SRM) and oxidative steam reforming of methanol (OSRM), was investigated. For this purpose, LSC was synthesized by the citrate sol-gel method and characterized by complementary analytical techniques. The catalytic activity was studied for the as-prepared and prereduced LSC and compared with the undoped LaCoO3-δ (LCO) at several feed gas compositions. Furthermore, the degradation and regeneration of LSC under repeated redox cycles were studied. The results evidenced that the increase in the water/methanol ratio under SRM, and the O2 addition under OSRM, increased the CO2 formation and decreased both the H2 selectivity and catalyst deactivation caused by carbon deposition. Methanol conversion of the prereduced LSC was significantly enhanced at a lower temperature than that of as-prepared LSC and undoped LCO. This was attributed to the performance of metallic cobalt nanoparticles highly dispersed under reducing atmospheres. The reoxidation program in repetitive redox cycles played a crucial role in the regeneration of catalysts, which could be regenerated to the initial perovskite structure under a specific thermal treatment, minimizing the degradation of the catalytic activity and surface.

A stagnation-point flow burner facility has been developed to provide a canonical framework to study the catalytic surface reactions of premixed combustion systems. The configuration serves as an important platform to investigate the interaction between homogeneous and heterogeneous reactions with an independent control of the characteristic residence time scales. Methane/air, and propane/air mixtures were examined with or without the presence of a platinum catalyst located at the stagnation surface. The effects of oxidizer composition and nitrogen dilution were examined. Depending on the operating conditions, either a stable gas-phase flame is established between the nozzle and the stagnation surface or the unburned reactant mixture directly impinges on the heated surface. In the former case, the extinction limits of the gas-phase flame were measured for various equivalence ratios and flow conditions. In the latter case, the heat release resulting from the surface reaction was quantified by measuring the surface temperature of the stagnation plate. The results are discussed in terms of the relative contributions of the gas-phase and the surface reaction chemistry to the burner performance. Understanding gained from this study will provide insights into the role of catalytic reactions in extending the flammability of compact combustors subjected to excessive surface heat loss.

An experimental study was carried out for the surface reaction of methane on the platinum catalyst which is placed on the stagnation point of a circular cylinder. By paying much attention to the reproducibity of the catalyst activity, a stable and well activated catalytic surface was obtained. by measuring the methane concentration profile near the catalytic surface, diffusion flux to the surface and surface concentration of methane were obtained. From these results, the dependence of surface reaction on surface temperature and surface methane concentration was made clear. The rate of surface reaction on surface shows a linear dependence of surface reaction rate on temperature can be represented by the Arrhenius' equation. Activation energy for this surface reaction was determined to be 80 kJ/mol.

Heterogeneous reaction processes in metal combustion

Infrared laser generation of heterogeneous catalystsand laser- induced reactions at catalytic surfacesWayne C. DanenLos Alamos National Laboratory, Chemistry DivisionCHM -4, MS J567, Los Alamos, New Mexico 87545Sheng -San Cheng, Pradeep K. Iyer, Shan, -Jaw ChiouKansas State University, Department of ChemistryManhattan, Kansas 66506AbstractIt has been demonstrated that continuous wave infrared CO2 laser radiation can be util-ized to rapidly produce active catalysts from inert precursors. The activity and selectiv-ity of Ca0 produced from Ca(OH)2 for the isomerization of 1- butene to cis- and trans- 2 -but-ene is discussed. Variation of the laser irradiation time produces catalytic activity andselectivity qualitatively similar to that resulting from conventional calcination at differ-ent temperatures. Pulsed infrared laser- induced reactions at catalytic surfaces are alsodiscussed with emphasis on the dehydrobromination of 2- bromopropane and ethylene eliminationfrom glycine ethyl ester hydrochloride at BaSO4 , AUP04, and similar surfaces. Correlationsare made of the extent of reaction with various experimental parameters including nature ofthe catalyst, laser frequency, laser fluence, number of laser pulses, and reagent-catalystratioIntroductionThe use of infrared lasers to induce or augment chemical reactions has received consid-erable attention in recent years although the preponderance of such studies have been per-formed with homogeneous gas phase reactants.' We report herein the continuous wave (CW) in-frared laser generation of a heterogeneous catalyst as well as several pulsed infrared laserinduced reactions at solid catalytic surfaces. In the first case, a CW CO2 infrared laseris utilized to conveniently generate catalytically active Ca0 from Ca(OH)2. The techniqueis simple, fast, and gives reproducible catalytic behavior. Secondly, pulsed infrard laser -induced reactions at catalytic surfaces will be discussed with emphasis on the dehydrobrom-ination of glycine ethyl ester hydrochloride (GEEH).Experimental sectionLaser generation of catalytically active Ca0 from Ca(OH)2Experiments were conducted utilizing a 1 -m CW CO2 laser with an output power of 14 ± 1

The deactivation of catalysts and their regeneration are two very important challenges that need to be addressed for many industrial processes. The most quoted reasons for the deterioration of dimethyl ether synthesis (DME) concern the sintering and the hydrothermal leaching of copper particles, their migration to acid sites, the partial formation of copper and zinc hydroxycarbonates, the formation of carbon deposits, and surface contamination with undesirable compounds present in syngas. This review summarises recent findings in the field of DME catalyst deactivation and regeneration. The most-used catalysts, their modifications, along with a comparison of the basic parameters, deactivation approaches, and regeneration methods are presented.

Solid acids have become increasingly important for many liquid-phase industrial reactions these days. Montmorillonite clays (2:1 clay mineral) have been used as efficient solid acid catalysts for a number of organic and liquid phase reactions and offer several advantages over classic acids. Tailor made catalysts can be prepared from clays by suitably adjusting their acidity and surface area by acid activation. In the present work, preparation, characterization and performance of Pt (II) clays, Cu (II) clays, acid clay, and sol–gel hybrids of Cu (II) clays as solid catalysts in a test Friedel–Craft alkylation reaction of benzyl chloride with toluene using differential scanning calorimeter (DSC) are reported. Product formation has been analysed by FTIR spectroscopy. The main objective of this work is to show how clay as a solid catalyst affects reaction rates and activation energies. Acidity and dispersion of solid catalysts are two main factors which govern a catalysis reaction. Kinetic parameter analysis and XRD studies confirm that acid Pt (II) clay and Pt (II) clay dispersed by natural dispersants are more effective catalysts. In contrast to the reactions using AlCl3, the experimental conditions are non-polluting and the final work up does not require any aqueous treatment.

Optimization of the reactor–regenerator system with catalytic parallel–consecutive reactions

This paper summarises the result of previous experimental investigations of heterogeneous electrocatalytic reactions performed in flow cells which provide an environment with controlled parameters. Measurements of the oxygen reduction reaction in a flow cell with an electrode consisting of an array of Pt nanodisks on a glassy carbon substrate exhibited a decreasing fraction of the intermediate H2O2 in the overall reaction products with increasing density of the nanodisks. A similar result is true for the dependence on the catalyst loading in the case of a supported Pt/C catalyst thin‐film electrode, where the fraction of the intermediate decreases with increasing catalyst loading. Similar effects have been detected for the methanol oxidation.In order to give a possible explanation to the observed effect, we present a model of multistep heterogeneous electrocatalytic oxidation and reduction reactions based on an adsorption‐reaction‐desorption scheme using the Langmuir assumption and macroscopic transport equations. A continuum based model problem in a vertical cross‐section of a rectangular flow cell is proposed in order to explain basic principles of the experimental situation. It includes three model species A, B, C, which undergo adsorption and desorption at a catalyst surface, as well as adsorbate reactions from A to B to C. These surface reactions are coupled with diffusion and advection in the Hagen Poiseuille flow in the flow chamber of the cell. High velocity asymptotic theory and a finite volume numerical method are used to obtain approximate solutions to the model. Both approaches show a behaviour similar to the experimentally observed. Working in more general situations, the finite volume scheme was applied to a catalyst layer consisting of a number of small catalytically active areas corresponding to nanodisks. Good qualitative agreement with the experimental findings is established for this case as well.