Exothermic Catalytic Reactions Research Articles

High conductivity catalyst structures for applications in exothermic reactions

Highly exothermic catalytic reactions are problematic from a thermal management perspective and often dictate the type of reactor, heat exchanger, level of conversion/recycle and contacting scheme that are employed. To investigate the opportunity for enhanced heat transfer structures, 15wt% Co/Al2O3 catalyst particles (149–177μm dia.) were examined in both a packed bed configuration and after being entrapped within a 7.4vol.% network of sintered Cu fibers (12μm dia.). Fischer–Tropsch synthesis (FTS) at 225–255°C, 20bar, H2/CO of 2.0, was utilized as the probe reaction due to its exothermicity and temperature dependent selectivity. Both the hot spot and runaway state were prevented by utilizing metal microfibrous entrapped catalyst (MFEC) compared to the packed bed. In a 41mm ID reactor, the maximum temperature deviation from the centerline to the reactor wall was only 6.4°C for the copper MFEC. In contrast, the packed bed diluted to the same catalyst density and operated at an equivalent condition had a centerline temperature deviation of 460°C indicating ignition. The more isothermal temperature profile through the catalyst bed of the copper MFEC led to a higher selectivity of heavy products than that of the packed bed. Also, it enabled a larger reactor diameter to be used with more precise and robust temperature control.

Synthetic homeostatic materials with chemo-mechano-chemical self-regulation

Living organisms have unique homeostatic abilities, maintaining tight control of their local environment through interconversions of chemical and mechanical energy and self-regulating feedback loops organized hierarchically across many length scales. In contrast, most synthetic materials are incapable of continuous self-monitoring and self-regulating behaviour owing to their limited single-directional chemomechanical or mechanochemical modes. Applying the concept of homeostasis to the design of autonomous materials would have substantial impacts in areas ranging from medical implants that help stabilize bodily functions to 'smart' materials that regulate energy usage. Here we present a versatile strategy for creating self-regulating, self-powered, homeostatic materials capable of precisely tailored chemo-mechano-chemical feedback loops on the nano- or microscale. We design a bilayer system with hydrogel-supported, catalyst-bearing microstructures, which are separated from a reactant-containing 'nutrient' layer. Reconfiguration of the gel in response to a stimulus induces the reversible actuation of the microstructures into and out of the nutrient layer, and serves as a highly precise 'on/off' switch for chemical reactions. We apply this design to trigger organic, inorganic and biochemical reactions that undergo reversible, repeatable cycles synchronized with the motion of the microstructures and the driving external chemical stimulus. By exploiting a continuous feedback loop between various exothermic catalytic reactions in the nutrient layer and the mechanical action of the temperature-responsive gel, we then create exemplary autonomous, self-sustained homeostatic systems that maintain a user-defined parameter--temperature--in a narrow range. The experimental results are validated using computational modelling that qualitatively captures the essential features of the self-regulating behaviour and provides additional criteria for the optimization of the homeostatic function, subsequently confirmed experimentally. This design is highly customizable owing to the broad choice of chemistries, tunable mechanics and its physical simplicity, and may lead to a variety of applications in autonomous systems with chemo-mechano-chemical transduction at their core.

A simplified kinetic model for isothermal catalytic ignition

The ignition of catalytic combustion of the stoichiometric propane/air mixture on an isothermally heated platinum wire in different experimental conditions of total pressure and wire temperature is studied and discussed on the basis of a simplified kinetic model. The platinum wire is heated electrically with a specially designed power supply, which ensures a quasi-rectangular profile of its temperature. The ignition process is monitored by measuring the input power required to maintain a constant temperature of the wire during an exothermic catalytic reaction. The difference between the input powers recorded in air and in a fuel/air mixture, for the same wire temperature and gas total pressure, allows the elimination of the heat transferred to surroundings and conversion of the results into the catalytic reaction rate r R versus time curves of S-shaped form, illustrating the transition from kinetic to diffusion regime. The curve can be used to evaluate the ignition delay, as reported previously and also to fit different models to the data. The quasi-exponential increase of the isothermal reaction rate during the early stages of the process can be rationalized on the basis of a simplified kinetic model implying the multiplication of the adsorbed active intermediates. The adopted hypotheses allow the derivation of an analytical solution for the catalytic reaction rate before and during the ignition process, without diffusion limitations.

Autothermal reforming of biogas over a monolithic catalyst

This study focused on measurement of the autothermal reforming of biogas over a Ni based monolithic catalyst. The effects of the steam/CH 4 (S/C) ratio, O 2/CH 4 (O 2/C) ratio and temperature were investigated. The CH 4 conversions were higher under all examined temperatures than the equilibrium conversion calculated using the blank outlet temperature, because the catalyst layer was heated by the exothermic catalytic partial oxidation reaction. The CH 4 conversion increased with increasing O 2/C ratio. Moreover, the CH 4 conversion was higher than the equilibrium conversion calculated using the blank outlet temperature for O 2/C>0.42 and reached about 100% at O 2/C=0.55. However, the hydrogen concentration decreased for O 2/C>0.45 because hydrogen was combusted to steam in the presence of excess oxygen. On the other hand, the hydrogen and CO 2 concentrations increased and the CO concentration decreased with increasing S/C ratio. As a result, it was found that the highest hydrogen concentrations and CH 4 conversions were attained at the O 2/C ratios of 0.45-0.55 and the S/C ratios of 1.5-2.5. Moreover, the H 2/CO ratio could also be controlled in the range from about 2 to 3.5 to give at least 90% CH 4 conversion, by regulating the O 2/C or S/C ratios.

Stationary inverted Lyman populations and free-free and bound-free emission of lower-energy state hydride ion formed by an exothermic catalytic reaction of atomic hydrogen and certain group I catalysts

Abstract Rb+ to Rb2+ and 2K+ to K + K2+ each provide a reaction with a net enthalpy equal to the potential energy of atomic hydrogen. The presence of these gaseous ions with thermally dissociated hydrogen formed a plasma having strong VUV emission with a stationary inverted Lyman population. Significant Balmer α line broadening of 18 and 9 eV was observed from a rt-plasma of hydrogen with KNO3, and RbNO3, respectively, compared to 3 eV from a hydrogen microwave plasma. The reaction was exothermic since excess power of about 20 mW/cc was measured by Calvet calorimetry. We propose an energetic catalytic reaction involving a resonance energy transfer between hydrogen atoms and Rb+ or 2K+ to form a very stable novel hydride ion. Its predicted binding energy of 3.0471 eV with the fine structure was observed at 4071 Å, and its predicted bound-free hyperfine structure lines matched those observed for about 40 lines to within.01 percent. Characteristic emission from each catalyst was observed. This catalytic reaction may pump a CW HI laser.

Thermodynamically Coupled Transport in Simple Catalytic Reactions

Considerable work published on chemical reaction-diffusion systems investigates mainly mathematically coupled nonlinear differential equations. This study presents the modeling of a simple elementary chemical reaction with thermodynamically and mathematically coupled heat and mass transport with external mass and heat transfer resistances. The thermodynamic coupling refers that a flow occurs without or against its primary thermodynamic driving force, which may be a gradient of temperature or chemical potential. The modeling is based on the linear nonequilibrium thermodynamics approach and phenomenological equations by assuming that the system is in the vicinity of global equilibrium. This approach does not need detailed coupling mechanisms. The modeling equations contain the cross coefficients controlling the coupling between heat and mass flows in terms of transport coefficients and surface conditions. These coefficients need to be determined for rigorous analysis of chemical reaction systems with thermodynamically coupled transport phenomena. Some representative numerical solutions of the modeling equations are presented to display the effect of coupling on concentration and temperatures in time and space for simple exothermic catalytic reactions.

The genesis and importance of oxide–metal interface controlled heterogeneous catalysis; the catalytic nanodiode

Much of the research by Konrad Hayek’s group is focused on the influence of oxide–metal interfaces in heterogeneous catalysis. In this paper, we show that electronic excitations at the metal surface induced by exothermic catalytic reactions lead to the generation of energetic (hot) electron flows. We detected the flow of hot electrons during oxidation of carbon monoxide using Pt/TiO2 Schottky diodes. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. We found that heat generated by the reaction caused a negligible increase of temperature in our experimental range, suggesting that this thermal effect is not responsible for the generation of hot electron flow. We tested the stability and reversibility of chemicurrent generated during CO oxidation at 413~573 K. The activation energy calculated from the measurement of chemicurrent is quite close to that of the turnover rate of chemical reaction, which indicates that the generation mechanism of hot electrons is closely correlated with the chemical reaction. This correlation suggests that hot electron flow could be a new tool to probe the role of oxide–metal interfaces in heterogeneous catalysis.

Sorption-enhanced steam reforming of hydrocarbons with autothermal sorbent regeneration in a moving heat wave of a catalytic combustion reaction

A novel technological concept of sorption-enhanced steam reforming of hydrocarbons is suggested. The peculiarity of the concept is the autothermal regeneration of the carbon dioxide scavenger in the moving super-adiabatic heat wave of an exothermic catalytic combustion reaction performed directly inside the adsorption-catalytic bed. The capability and high efficiency of the proposed technological approach are confirmed by process simulation. The approach proposed is shown to open a way for the creation of an inexpensive, reliable and energy-saving adiabatic packed bed methane processors of unlimited processing capacity.

Hydrogen production from liquid hydrocarbon fuels for PEMFC application

Lack of efficient hydrogen storage intermediate has boosted the development of fuel processor or economic onsite hydrogen production techniques for application to proton exchange membrane fuel cell promptly. Aiming to develop onsite hydrogen production techniques for proton exchange membrane fuel cell application using nickel-based reforming catalysts and stainless steel reactors, in this paper, a novel process for H2 production from liquid hydrocarbon fuels was proposed and experimentally demonstrated on a lab scale. The main operations involved prereforming, autothermal reforming, high temperature water gas shift, low temperature water gas shift and H2 enrichment by Pd membrane. The results indicated that prereforming introduction prior to autothermal reforming suppressed undesired gas phase reactions efficiently and made reforming reactions perform catalytically and smoothly, which was confirmed by a stable 500 h time-on-stream test of both prereforming and autothermal reforming catalysts. The air distributed feed applied in autothermal reforming reactor coupled the endothermic steam reforming and exothermic catalytic combustion reactions over the catalyst closely, maintaining an appropriate temperature distribution curve for autothermal reforming catalyst bed. During the process of H2 enrichment by highly H2 permeable Pd composite membrane, concentration polarization played an important role.

Dynamics of Surface Catalyzed Reactions; the Roles of Surface Defects, Surface Diffusion, and Hot Electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.

Energy conversion from catalytic reaction to hot electron current with metal-semiconductor Schottky nanodiodes

Exothermic catalytic reactions induce electronic excitation at the metal surface, leading to the production of energetic hot electrons. We monitored the flow of hot electrons for over several hours using two types of metal-semiconductor Schottky diodes, Pt∕TiO2 or Pt∕GaN, during the platinum catalyzed oxidation of carbon monoxide. The thickness of Pt film used as the catalyst was 5nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. The chemicurrent was well correlated with the turnover rate of CO oxidation separately measured by gas chromatography, suggesting the possibility of application as chemical sensors with high sensitivity.

The Catalytic Nanodiode: Detecting Continous Electron Flow at Oxide–Metal Interfaces Generated by a Gas‐Phase Exothermic Reaction

Continuous flow of ballistic charge carriers is generated by an exothermic chemical reaction and detected using the catalytic metal-semiconductor Schottky diode. We obtained a hot electron current for several hours using two types of catalytic nanodiodes, Pt/TiO2 or Pt/GaN, during carbon monoxide oxidation at pressures of 100 Torr of O2 and 40 Torr of CO at 413-573 K. This result reveals that the chemical energy of an exothermic catalytic reaction is directly converted into hot electrons flux in the catalytic nanodiode. By heating the nanodiodes in He, we could measure the thermoelectric current which is in the opposite direction to the flow of the hot electron current. The chemicurrent is well correlated with the turnover rate of CO oxidation, which is separately measured with gas chromatography. The influence of the flow of hot charge carriers on the chemistry at the oxide-metal interface, and the turnover rate in the chemical reaction are discussed.

Innovative Reactors for Determining Kinetics of Highly Exothermic Heterogeneous Catalytic Reactions

For highly exothermic catalytic reactions particular measures have to be taken for obtaining reliable kinetic and mechanistic information, which has been preferably determined under the prevalence of isothermicity, and which is not disguised by transport processes. If this is true such kinetics can be reliably extrapolated to conditions outside the frame of experimental conditions. To this end various reactor set-ups are presented and their operation is illustrated by some case studies. The temperature-scanning reactor, which is not inherently isothermal, is included since non-isothermicity is not an obstacle to easy kinetic data assessment. Its performance is clarified by estimating kinetic parameters for the ammonia synthesis at elevated pressure using a commercial catalyst. Two types of novel micro-structured reactors are presented which have been applied for two highly exothermic catalytic reactions, i.e., the oxidative dehydrogenation of propane (ODP) and the ammonia oxidation to N2, N2O and NO. These studies served the purpose of getting mechanistic insights into the reaction mechanisms of both reactions, to validate a kinetic scheme for the ODP reaction and to derive the estimates of the respective kinetic parameters. Finally, a reactor for transient experiments in the peak pressure range of 1 to 10 Pa has been used for studying reaction steps related to the catalytic ammonia oxidation under isothermal conditions. Isothermal studies are possible due to the low concentrations of reactants by which any undesired hot spots on the catalyst surface were suppressed. The instrumental arsenal described is an asset for both fundamental as well as applied work in studying exothermic catalytic reactions.

Selective exothermic catalytic reactions in a reverse heat front

New classes of exact solutions of nonlinear sets of equations of the theory of heat and mass transfer in reactive media and mathematical biology are described. Various sets of the general form are considered, in which the chemical reaction rates depend on two or three arbitrary functions. Among the exact solutions obtained are solutions with ordinary, generalized, and functional separation of variables; time–periodic solutions; solutions that are periodic in the spatial coordinate; etc. A number of solutions contain arbitrary functions (they are expressed in terms of solutions of the linear heat equation and solutions of sets of ordinary differential equations). Sets describing the multicomponent reaction—diffusion and also some sets in several spatial variables are studied.

Selective exothermic catalytic reactions in a reverse heat front

Selective exothermic catalytic reactions in a reverse heat front

Selective exothermic catalytic reactions in a reverse heat front

A model system of exothermic reactions of selective and complete catalytic conversion is considered. The system comprises the reactions A + m[B] ⇒ C + [], C + n[B] ⇒ D + [], and B + [] ⇒ [B], where A and B are the initial reactants, C is the desired product (of the selective conversion), D is the undesired side product (of the complete conversion), [B] is the reactant B chemisorbed on the catalyst surface, and [ ] is a free catalytic active site. Numerical study of the system of reactions in an adiabatic catalyst bed shows that the yield of the desired product and the specific performance of the catalyst can be significantly increased if the reactions are performed not under steady-state conditions but in unsteady-state modes involving periodic time-separated introduction of the reactants A and B. Of greatest interest is to carry out the process in a reverse heat front propagating upward the flow of the gaseous reaction mixture, since, in this front, the maximal catalyst temperature is much lower and also the conversion selectivity is high.

Mathematical modelling of heat transfer in a catalytic reformer

In this work we analyse heat transfer in a system of channels connected by thin conducting walls. The channels are packed with catalytic pellets that promote exothermic catalytic combustion reactions and endothermic reforming reactions in adjacent channels. A model is developed in which the thermal conductivity and the thickness of the interconnecting wall can be used as control parameters characterizing the heat exchange between the neighbouring channels. The model is to be used as a mathematical tool to analyse design alternatives and develop accurate numerical techniques. Our objective is to study how the heat is transferred across the conducting walls and how this influences the temperature distribution in the channels. We use an asymptotic technique to do this. The structure of the walls is then examined in detail, focusing on the case when we have layered walls.

Three-dimensional stagnation point free convection on a reactive surface in a porous medium

The free convection, boundary-layer flow in a fluid-saturated porous medium near a general, three-dimensional stagnation point when heat is released by an exothermic catalytic reaction on the surface, converting a reactive component within the convective fluid to an inert product, is considered. The reaction is modelled by first-order kinetics with an Arrhenius temperature dependence. First, the flow behaviour for the initial, small time solution (unsteady flow) and certain ranges of parameter values is considered, and this is followed by the discussion of the possible steady-state solutions (large time) of the governing equations. The transition range between these two analytical solutions is performed numerically using a very efficient finite-difference scheme. It is shown that the analytical and numerical solutions are in very good agreement over appropriate time intervals and the influence of the reactant parameters λ, δ and e on the flow, heat transfer and concentration characteristics is discussed.

A high-speed method for obtaining kinetic data for exothermic or endothermic catalytic reactions under non-isothermal conditions illustrated for the ammonia synthesis

A detailed analysis of the method for fast acquisition of kinetic data by means of a polythermal-temperature-ramping reactor (PTR) as suggested by Wojciechowski in 1995 was performed. The ammonia synthesis reaction over a commercial Fe catalyst (BASF) was taken as an example. Kinetic data evaluation was based on a kinetic model as reported by Sehested et al. in 1999. As a result of data analysis, the experimental molar rate of change of ammonia could be described adequately over a wide range of N 2 and H 2 partial pressures as well as temperature. The pre-exponential factors, activation energies and adsorption enthalpies are in reasonable agreement with data for promoted iron catalysts reported in literature. Accordingly, the PTR method was evaluated as a efficient tool for kinetic data analysis. The number of experiments including different initial conditions can be significantly diminished since PTR experiments have not to follow the strict sequence of varying one experimental parameter after the other by keeping all others constant. The collected exit concentrations, exit temperature and contact time are correlated by spline-functions which allow the extraction of functional relationships required for derivation of rate expressions. If the steady-state of the catalyst is rapidly attained, temperature ramping allows fast continuous data acquisition. Thus, the PTR method has the potential of significantly reducing the time needed for determining reliable kinetics over a wide range of operating conditions.

Mathematical modelling of exothermic catalytic reaction in a single partially-wetted porous catalyst particle

The mathematical model for the multiphase steady-state processes on a partially-wetted porous catalyst slab in the conditions of the exothermic catalytic reaction, species diffusion, phase transitions, and capillary phenomena interaction is developed. Numerical experiment was carried out for the model reaction of α-methylstyrene hydrogenation. The impact of the external wetting efficiency on the steady-state regimes of the slab is investigated. The partially-wetted regime is characterised by studying the width of the dry zone inside the slab; the slab overheating, the area of the multiplicity.