John Happel

Multiple isotope tracing of methanation over nickel catalyst

Abstract The methanation of mixtures of carbon monoxide and hydrogen over a supported nickel catalyst was studied by transient isotopic tracing with 13 C, 18 O, and D. A mechanism is proposed based on computer modeling which takes into account results from a wide variety of data. Evidence is presented that rate controlling steps involve hydrogenolysis of chemisorbed CH x species ( x = 0–3) rather than only the splitting of carbon monoxide or the formation of an “enolic” intermediate. Carbon dioxide formation appears to occur directly rather than through the water gas shift reaction. The computer program enables estimates to be made of concentrations of intermediates as well as velocities of individual steps in the mechanism. Under reaction conditions predominant adsorbed species appear to be carbidic carbon plus hydrogenated hydrocarbon intermediates.

Multiple isotope tracing of methanation over nickel catalyst: II. Deuteromethanes tracing

Abstract The methanation of mixtures of carbon monoxide and hydrogen was studied by a novel isotope transient tracing technique using deuterium. Concentrations of CHxads intermediates on the surface of the working catalyst are estimated. The most abundant species are Cads and CHads with CH2 ads and CH3 ads being present in smaller proportions. Evidence is presented that the mechanism involves sequential unidirectional hydrogenation of the intermediates by atomic adsorbed hydrogen.

Multiple isotope tracing of methanation over nickel catalyst. III: Completion of 13C and D tracing

Abstract The methanation of mixtures of carbon monoxide and hydrogen was studied by a transient superposition technique using terminal species marked by 13 C and D. Further evidence is presented that the most abundant reacting intermediate is the species CH ads . It appears that the hydrogenation of this species controls the rate of methanation. Carbidic carbon consists of relatively small pool of active C ads together with a larger pool of carbon which, although not active for methanation, is also not graphitic. A mechanism is presented which summarizes the findings of this and the previous two papers in this series.

Analysis of the Possible Mechanisms for a Catalytic Reaction System

Publisher Summary This chapter focuses on the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification is been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. The subject of reaction mechanism also has a bearing on other fundamental problems of physical chemistry. In carrying out the procedure for determining mechanisms that is presented in the chapter, one obtains a set of independent chemical reactions among the terminal species in addition to the set of reaction mechanism. Consideration of a chemical system in terms of unique direct reaction mechanisms required to produce observable rates of change of terminal species has distinct advantages, especially when multiple overall reactions are involved. The required necessary assumptions regarding possible elementary reaction steps are becoming increasingly accessible through modern tools for surface spectroscopy and fundamental theories of chemical kinetics of elementary reaction steps.

Estimation of non-uniquely identifiable parameters via exhaustive modeling and membership set theory

A new methodology for estimating parameters that are not structurally globally identifiable is presented. The set of all the models which have exactly the same input–output behavior is first generated. The influence of the measurement noise and structural error is then taken into account by assuming that upper and lower bounds of the acceptable output error are available. The set of all the models compatible with this hypothesis is characterized, and ranges for the possible values of the estimated parameters are provided. An example is treated involving two real–life model structures used to describe the behavior of an isotopic tracer in a reactor producing methane from carbon monoxide and hydrogen.

Modeling transient tracer studies in plug-flow reactors

Abstract The use of plug-flow reactors is common for kinetic studies in heterogeneous catalysis. Recently, transient tracing has been found to be advantageous in providing additional mechanistic information above that obtained by customary steady-state kinetics. Despite the advantages of transient tracing, it often suffers from incorrect data analysis in the case of plug flow because of failure to take into account the formalism needed to model plug-flow transient tracing as contrasted with that required to describe transient tracing in a gradientless recirculating reactor (CSTR). This paper presents a development of the appropriate differential equation system applying to plug-flow studies involving transient isotopic tracing superimposed on an overall steady-state kinetics. Closed mathematical expressions are presented for simple practical cases and used to illustrate important special characteristics of these systems.

The characterization of complex systems of chemical reactions

A method is presented for the enumeration of possible overall chemical reactions and mechanisms, based on an initial choice of elementary reactions connecting a set of chemical species. Such a procedure furnishes an important tool for the study of complex reaction systems. The method is presented in such a way as to be readily implemented by computer solution, which is indispensable for many systems.

Transient rate tracer studies in heterogeneous catalysis: Oxidation of carbon monoxide

Abstract Establishment of the mechanism of a catalytic reaction is important from the viewpoints of both catalyst development and construction of suitable rate equations for design purposes. The use of transient tracing employing isotopes is a useful tool for this purpose. In this paper, a new method is described in which the transfer of isotopic species is superposed on a heterogeneous catalytic reaction conducted under steady-state conditions. An efficient method of modeling the system of first-order differential equations describing tracer transfer is presented. The method is applied to the oxidation of carbon monoxide over hopcalite catalyst. A relatively simple model with one type of site for adsorbed carbon oxide species correlates the data well for both 13 CO 2 and 13 CO marking. The final step, CO 2 adsorption and desorption, is fast and reversible.

IDENTIFIABILITY AND DISTINGUISHABILITY TESTING FOR LINEAR MODELS IN HETEROGENEOUS CATALYSIS

If several values of the parameters of a model are associated with the same behavior, then the model is not identifiable and there is no hope of estimating a unique best value for the parameter vector from experimental data. Similarly, if several models with different structures correspond to the same behavior, then these models are not distinguishable and there is no hope of selecting a structure that best corresponds to the experimental data. Two methods for testing linear models for identifiability and distinguish ability are recalled and applied to types of catenary compartmental models encountered for instance when studying the isobutane-isobutene-hydrogen system by transient isotopic tracing.

Reversible reactions over non-metallic catalysts: A transient isotopic tracing of the isobutane-isobutene-hydrogen system over chromia

Abstract Whether a single rate-controlling step exists in complex reversible heterogeneous catalytic reactions is difficult to establish, especially with non-metallic catalysts, which often possess fewer active sites than supported metals. This problem is treated here using transient isotopic tracing with 13 C marking for the system isobutane-isobutene-hydrogen over a chromia catalyst. A statistical approach is employed for modeling the simultaneous occurrence of traced isobutane and isobutene in the product stream using the Horiuti-Polanyi, mechanism. We find that a single rate-controlling step that consists of the conversion of the half-hydrogenated chemisorbed species i -C 4 H 9 l to chemisorbed isobutene i -C 4 H 8 l and the reverse exists. Such information should be useful for catalyst design and prediction of performance.