35,745 research outputs found
Structure Sensitivity in Oxide Catalysis: First-Principles Kinetic Monte Carlo Simulations for CO Oxidation at RuO(111)
We present a density-functional theory based kinetic Monte Carlo study of CO
oxidation at the (111) facet of RuO. We compare the detailed insight into
elementary processes, steady-state surface coverages and catalytic activity to
equivalent published simulation data for the frequently studied RuO(110)
facet. Qualitative differences are identified in virtually every aspect ranging
from binding energetics over lateral interactions to the interplay of
elementary processes at the different active sites. Nevertheless, particularly
at technologically relevant elevated temperatures, near-ambient pressures and
near-stoichiometric feeds both facets exhibit almost identical catalytic
activity. These findings challenge the traditional definition of structure
sensitivity based on macroscopically observable turnover frequencies and allow
to scrutinize the applicability of structure sensitivity classifications
developed for metals to oxide catalysis.Comment: 15 pages, 5 figure
Influence of reaction products on the selective oxidation of ethene
The kinetics of the selective oxidation of ethene in air over an industrial silver on ¿-alumina catalyst were studied. Special attention was paid to the influence of the reaction products on the reaction rates of epoxidation and complete combustion. Kinetic data were obtained in two different types of internal recycle reactor and in a cooled tubular reactor, and were fitted separately to several reaction rate expressions based on different kinetic models. A Langmuir-Hinshelwood mechanism, in which adsorbed ethene reacts with adsorbed molecular oxygen, was chosen as the best kinetic model. The reaction products compete for adsorption on the active sites and reduce the rates of both reactions. Carbon dioxide enhances the selectivity towards ethene oxide, whereas water has almost no influence on the selectivity. The fitting of the three individual data sets obtained in the three reactors results in accurate, but different, reaction rate expressions, whereas the fitting of the three data sets simultaneously results in less accurate reaction rate expressions. The systematic deviations found may be explained, to some extent, by differences in the operating regimes in each reactor. The main reason for the deviations is probably the different catalyst activities in the three reactors caused by poisoning. The effect of the addition of products to the feed on the behaviour of the cooled tubular reactor can be described reasonably well by a mathematical model in which the kinetic equations obtained in the laboratory reactors are incorporated. The results of these simulations are sensitive to the kinetics used
CO Oxidation Catalysed by Pd-based Bimetallic Nanoalloys
Density functional theory based global geometry optimization has been used to
demonstrate the crucial influence of the geometry of the catalytic cluster on
the energy barriers for the CO oxidation reaction over Pd-based bimetallic
nanoalloys. We show that dramatic geometry change between the reaction
intermediates can lead to very high energy barriers and thus be prohibitive for
the whole process. This introduces challenges for both the design of new
catalysts, and theoretical methods employed. On the theory side, a careful
choice of geometric configurations of all reaction intermediates is crucial for
an adequate description of a possible reaction path. From the point of view of
the catalyst design, the cluster geometry can be controlled by adjusting the
level of interaction between the cluster and the dopant metal, as well as
between the adsorbate molecules and the catalyst cluster by mixing different
metals in a single nanoalloy particle. We show that substitution of a Pd atom
in the Pd cluster with a single Ag atom to form PdAg leads to
a potential improvement of the catalytic properties of the cluster for the CO
oxidation reaction. On the other hand, a single Au atom does not enhance the
properties of the catalyst, which is attributed to a weaker hybridization
between the cluster's constituent metals and the adsorbate molecules. Such
flexibility of properties of bimetallic nanoalloy clusters illustrates the
possibility of fine-tuning, which might be used for design of novel efficient
catalytic materials.Comment: 12 pages, 8 figure
Examination of the concept of degree of rate control by first-principles kinetic Monte Carlo simulations
The conceptual idea of degree of rate control (DRC) approaches is to identify
the "rate limiting step" in a complex reaction network by evaluating how the
overall rate of product formation changes when a small change is made in one of
the kinetic parameters. We examine two definitions of this concept by applying
it to first-principles kinetic Monte Carlo simulations of the CO oxidation at
RuO2(110). Instead of studying experimental data we examine simulations,
because in them we know the surface structure, reaction mechanism, the rate
constants, the coverage of the surface and the turn-over frequency at steady
state. We can test whether the insights provided by the DRC are in agreement
with the results of the simulations thus avoiding the uncertainties inherent in
a comparison with experiment. We find that the information provided by using
the DRC is non-trivial: It could not have been obtained from the knowledge of
the reaction mechanism and of the magnitude of the rate constants alone. For
the simulations the DRC provides furthermore guidance as to which aspects of
the reaction mechanism should be treated accurately and which can be studied by
less accurate and more efficient methods. We therefore conclude that a
sensitivity analysis based on the DRC is a useful tool for understanding the
propagation of errors from the electronic structure calculations to the
statistical simulations in first-principles kinetic Monte Carlo simulations.Comment: 27 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Chemical engineering design of CO oxidation catalysts
How a chemical reaction engineer would approach the challenge of designing a CO oxidation catalyst for pulsed CO2 lasers is described. CO oxidation catalysts have a long history of application, of course, so it is instructive to first consider the special requirements of the laser application and then to compare them to the characteristics of existing processes which utilize CO oxidation catalysts. All CO2 laser applications require a CO oxidation catalyst with the following characteristics: (1) active at stoichiometric ratios of O2 and CO, (2) no inhibition by CO2 or other components of the laser environment, (3) releases no particulates during vibration or thermal cycling, and (4) long lifetime with a stable activity. In all applications, low consumption of power is desirable, a characteristic especially critical in aerospace applications and, thus, catalyst activity at low temperatures is highly desirable. High power lasers with high pulse repetition rates inherently require circulation of the gas mixture and this forced circulation is available for moving gas past the catalyst. Low repetition rate lasers, however, do not inherently require gas circulation, so a catalyst that did not require such circulation would be favorable from the standpoint of minimum power consumption. Lasers designed for atmospheric penetration of their infrared radiation utilize CO2 formed from rare isotopes of oxygen and this application has the additional constraint that normal abundance oxygen isotopes in the catalyst must not exchange with rare isotopes in the gas mixture
Ion and mixed conducting oxides as catalysts
This paper gives a survey of the catalytic properties of solid oxides which display oxygen ion or mixed (i.e. ionic + electronic) conductivity. Particular consideration is given to the oxidation-reduction reactions of gas phase components, but attention is also devoted to oxygen exchange between gas and oxide. An attempt has been made to relate and explain the observed phenomena such as catalytic activity and selectivity in terms of the electrical conducting properties of the oxides, which depend on their crystal and defect structures.\ud
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In a number of cases possible applications of these materials in (electro)catalytic reactors, sensors, fuel cells, oxygen pumps, etc. are indicated
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(Photo)Electrocatalytic CO2 Reduction at the Defective Anatase TiO2 (101) Surface
Excessive carbon dioxide (CO2) emissions by combustion of fossil fuels are linked to global warming and rapid climate change. One promising route to lowering the concentration of CO2 in the atmosphere is to reduce it to useful small molecules via photoelectrocatalytic hydrogenation, which would enable solar energy storage with a zero-carbon emission cycle and perform a more efficient separation of the photogenerated electron and hole pair than pure photocatalysis. Indeed, photoelectrocatalytic CO2 reduction has been an intense focus of research. Using the density functional theory (DFT), we studied the CO2 reduction reaction on the defective anatase TiO2 (101) surface, at both the solvent/catalyst and the electrolyte/catalyst interfaces. The analysis of the electronic structure of the surface shows a contrast between the solvent/catalyst and the electrolyte/catalyst interfaces, which results in the two corresponding catalytic cycles being distinct. Our study explains at the electronic and mechanistic levels why methanol is the main product in the presence of the electrolyte and why the overpotential is not only controlled by the reaction process but also by the diffusion process
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CO2 Hydrogenation to Formate and Formic Acid by Bimetallic Palladium-Copper Hydride Clusters.
Mass spectrometric analysis of the anionic products of interaction between bimetallic palladium-copper tetrahydride anions, PdCuH4-, and carbon dioxide, CO2, in a reaction cell shows an efficient generation of the PdCuCO2H4- intermediate and formate/formic acid complexes. Multiple structures of PdCuH4- and PdCuCO2H4- are identified by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. The higher energy PdCuH4- isomer is shown to drive the catalytic hydrogenation of CO2, emphasizing the importance of accounting for higher energy isomers for cluster catalytic activity. This study represents the first example of CO2 hydrogenation by bimetallic hydride clusters
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