scholarly journals On the optimum catalyst for structure sensitive heterogeneous catalytic reactions

2020 ◽  
Vol 131 (1) ◽  
pp. 5-17
Author(s):  
Dmitry Yu. Murzin
Keyword(s):  
Gibbs Energy ◽  
Cluster Size ◽  
Intermediate Formation ◽  
Reaction Rates ◽  
Structure Sensitivity ◽  
Heterogeneous Catalytic ◽  
Surface Intermediate ◽  
Structure Sensitive ◽  
Adsorbed Intermediate ◽  
Weak Structure

Abstract Reaction rates in a two-step catalytic sequence, when plotted vs adsorption energy of the key or the most abundant surface intermediate, result in volcano shaped curves. In the current work, the optimal catalyst is discussed for structure sensitive reactions, which display dependence of activity on the cluster size of the active catalytic phase. An expression is derived relating the Gibbs energy for formation of the intermediate with the Gibbs energy changes in the overall reaction, difference in adsorption thermodynamics on edges and terraces and the cluster size. The kinetic expressions display dependence of activity vs the Gibbs energy of the adsorbed intermediate formation. Numerical analysis demonstrates that when the overall equilibrium constant K is high and the reaction is thermodynamically very favorable, the maxima in the rates vs the adsorption constant for the optimal catalyst are much broader being less dependent on the cluster size. When structure sensitivity is pronounced, there are smaller differences in the rates for the optimum and less optimal catalysts in comparison with reactions showing weak structure sensitivity.

scholarly journals On the behaviour of structure-sensitive reactions on single atom and dilute alloy surfaces

2020 ◽  
Vol 10 (17) ◽  
pp. 5815-5828 ◽  
Author(s):  
Konstantinos G. Papanikolaou ◽  
Michail Stamatakis
Keyword(s):  
Structure Sensitivity ◽  
Single Atom ◽  
Structure Sensitive ◽  
Dilute Alloy ◽  
Structure Sensitive Reactions ◽  
Single Atom Alloy

Typically structure sensitive dissociation reactions exhibit reduced structure-sensitivity when taking place over low-index single atom alloy surfaces.

Catalytic Combustion Systems for Micro-Scale Gas Turbine Engines

2005 ◽  
Author(s):  
C. M. Spadaccini ◽  
J. Peck ◽  
I. A. Waitz
Keyword(s):  
Gas Turbine ◽  
Gas Phase ◽  
Surface Area ◽  
Power Density ◽  
Mass Flow ◽  
Catalytic Combustion ◽  
Reaction Rates ◽  
Design Parameters ◽  
Micro Scale ◽  
Heterogeneous Catalytic

As part of an ongoing effort to develop a micro-scale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, modeling, and experimental assessment of a catalytic combustion system. Previous work has indicated that homogenous gas-phase microcombustors are severely limited by chemical reaction time-scales. Storable hydrocarbon fuels, such as propane, have been shown to blowout well below the desired mass flow rate per unit volume. Heterogeneous catalytic combustion has been identified as a possible improvement. Surface catalysis can increase hydrocarbon-air reaction rates, improve ignition characteristics, and broaden stability limits. Several radial inflow combustors were micromachined from silicon wafers using Deep Reactive Ion Etching (DRIE) and aligned fusion wafer bonding. The 191 mm3 combustion chambers were filled with platinum coated foam materials of various porosity and surface area. For near stoichiometric propane-air mixtures, exit gas temperatures of 1100 K were achieved at mass flow rates in excess of 0.35 g/s. This corresponds to a power density of approximately 1200 MW/m3; an 8.5-fold increase over the maximum power density achieved for gas-phase propane-air combustion in a similar geometry. Low order models including time-scale analyses and a one-dimensional steady-state plug-flow reactor model, were developed to elucidate the underlying physics and to identify important design parameters. High power density catalytic microcombustors were found to be limited by the diffusion of fuel species to the active surface, while substrate porosity and surface area-to-volume ratio were the dominant design variables.

Reaction rates as virtual constraints in Gibbs energy minimization

2011 ◽  
Vol 83 (5) ◽  
pp. 1063-1074 ◽  
Author(s):  
Pertti Koukkari ◽  
Risto Pajarre ◽  
Peter Blomberg
Keyword(s):  
Gibbs Energy ◽  
Energy Minimization ◽  
Reaction Rate ◽  
Reaction Rates ◽  
Free Energy Calculations ◽  
Energy Calculation ◽  
Chemical Equilibria ◽  
Gibbs Energy Minimization ◽  
Physical Chemical ◽  
Work Factors

The constrained Gibbs energy method has been developed for the use of immaterial entities in the formula conservation matrix of the Gibbs energy minimization problem. The new method enables the association of the conservation matrix with structural, physical, chemical, and energetic properties, and thus the scope of free energy calculations can be extended beyond the conventional studies of global chemical equilibria and phase diagrams. The use of immaterial constraints enables thermochemical calculations in partial equilibrium systems as well as in systems controlled by work factors. In addition, they allow the introduction of mechanistic reaction kinetics to the Gibbsian multiphase analysis. The constrained advancements of reactions are incorporated into the Gibbs energy calculation by using additional virtual phases in the conservation matrix. The virtual components are then utilized to meet the incremental consumption of reactants or the formation of products in the kinetically slow reactions. The respective thermodynamic properties for the intermediate states can be used in reaction rate formulations, e.g., by applying the reaction quotients.

Catalytic Combustion Systems for Microscale Gas Turbine Engines

2005 ◽  
Vol 129 (1) ◽  
pp. 49-60 ◽  
Author(s):  
C. M. Spadaccini ◽  
J. Peck ◽  
I. A. Waitz
Keyword(s):  
Gas Turbine ◽  
Gas Phase ◽  
Surface Area ◽  
Power Density ◽  
Mass Flow ◽  
Catalytic Combustion ◽  
Flow Reactor ◽  
Reaction Rates ◽  
Design Parameters ◽  
Heterogeneous Catalytic

As part of an ongoing effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, modeling, and experimental assessment of a catalytic combustion system. Previous work has indicated that homogenous gas-phase microcombustors are severely limited by chemical reaction timescales. Storable hydrocarbon fuels, such as propane, have been shown to blow out well below the desired mass flow rate per unit volume. Heterogeneous catalytic combustion has been identified as a possible improvement. Surface catalysis can increase hydrocarbon-air reaction rates, improve ignition characteristics, and broaden stability limits. Several radial inflow combustors were micromachined from silicon wafers using deep reactive ion etching and aligned fusion wafer bonding. The 191mm3 combustion chambers were filled with platinum-coated foam materials of various porosity and surface area. For near stoichiometric propane-air mixtures, exit gas temperatures of 1100K were achieved at mass flow rates in excess of 0.35g∕s. This corresponds to a power density of ∼1200MW∕m3; an 8.5-fold increase over the maximum power density achieved for gas-phase propane-air combustion in a similar geometry. Low-order models, including time-scale analyses and a one-dimensional steady-state plug-flow reactor model, were developed to elucidate the underlying physics and to identify important design parameters. High power density catalytic microcombustors were found to be limited by the diffusion of fuel species to the active surface, while substrate porosity and surface area-to-volume ratio were the dominant design variables.

scholarly journals CO2 Hydrogenation to Methanol over La2O3-Promoted CuO/ZnO/Al2O3 Catalysts: A Kinetic and Mechanistic Study

Catalysts ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 183 ◽  
Author(s):  
Marios Kourtelesis ◽  
Kalliopi Kousi ◽  
Dimitris I. Kondarides
Keyword(s):  
Intermediate Formation ◽  
Reaction Rates ◽  
Catalytic Performance ◽  
High Specific Surface Area ◽  
Mechanistic Study ◽  
Partial Pressures ◽  
Surface Formate ◽  
Adsorption Of Co2 ◽  
In Situ Infrared Spectroscopy

The hydrogenation of CO2 to methanol has been investigated over CuO/ZnO/Al2O3 (CZA) catalysts, where a part of the Al2O3 (0, 25, 50, 75, or 100%) was substituted by La2O3. Results of catalytic performance tests obtained at atmospheric pressure showed that the addition of La2O3 generally resulted in a decrease of CO2 conversion and in an increase of methanol selectivity. Optimal results were obtained for the CZA-La50 catalyst, which exhibited a 30% higher yield of methanol, compared to the un-promoted sample. This was attributed to the relatively high specific surface area and porosity of this material, the creation of basic sites of moderate strength, which enhance adsorption of CO2 and intermediates that favor hydrogenation steps, and the ability of the catalyst to maintain a large part of the copper in its metallic form under reaction conditions. The reaction mechanism was studied with the use of in situ infrared spectroscopy (DRIFTS). It was found that the reaction proceeded with the intermediate formation of surface formate and methoxy species and that both methanol and CO were mainly produced via a common formate intermediate species. The kinetic behavior of the best performing CZA-La50 catalyst was investigated in the temperature range 190–230 °C as a function of the partial pressures of H2 (0.3–0.9 atm) and CO2 (0.05–0.20 atm), and a kinetic model was developed, which described the measured reaction rates satisfactorily.

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