The use of forced oscillations in heterogeneous catalysis - Summary

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The use of forced oscillations in heterogeneous catalysis

van Neer, F.J.R.

Publication date

1999

Link to publication

Citation for published version (APA):

van Neer, F. J. R. (1999). The use of forced oscillations in heterogeneous catalysis.

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The aim of the work presented in this thesis is both to get insight into the mechanism

underlying the interesting phenomena observed under forced oscillations on a catalytic system

and to demonstrate the use of forced oscillations in heterogeneous catalysis. Chapter 2-4 deal

with the first item. By means of analysing the results of simulations using microkinetics of

various catalytic reactions, the occurrence of resonance on a catalyst has been explained. Two

practical applications of forced concentration oscillations have been shown in chapter 5-6, in

which results of experimental work conducted on a simple catalytic reaction have been

presented.

Time averaged behaviour which is substantially different from steady state behaviour can be

obtained when reactant concentration oscillations or system temperature oscillations are

imposed on a catalyst. When the time averaged rate vs. oscillation frequency is not a

monotonically ascending or descending function going from low frequencies (quasi steady

state) to high frequencies (relaxed steady state), we speak of resonance. It follows from

chapter 2 that for a catalytic reaction in which first two components must adsorb molecularly

on the catalyst before they subsequently react, resonance can be brought about when the

sorption kinetics of the reactants are dissimilar. In case the concentration of one reactant is

forced, the requirements for the occurrence of resonance are 1) the forced component should

have faster sorption kinetics compared to the other component and 2) the catalytic surface

should be almost completely occupied. Resonance is induced by the inability of the catalytic

species of the non-forced reactant to follow the changes in the surface occupancy of the forced

reactant. This leads to a temporarily invariant level of the surface occupancy of the non-forced

component at a level which deviates from steady state occupancy levels. It has been

demonstrated that a thorough analysis of the dependence of steady state occupancies of all

components on the concentration of the forced reactant can fully explain this behaviour.

For reactions obeying molecular sorption mechanisms, reaction rate enhancement compared to

the highest steady state rate has not been observed in case of concentration forcing of one

component. In contrast, large rate enhancements have been obtained under imposed

temperature oscillations. Resonance reaction rates of at least 7 times the highest steady state

reaction rate have been found over a broad frequency range. Again, dissimilar dynamic

behaviour with respect to sorption kinetics of the reactants is a prerequisite to the occurrence

of resonance. In addition, for systems with low total surface occupancies the slowest

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component should occupy more sites with increasing temperature. Catalytic systems with high total surface occupancies of the slowest reactant should meet the opposite condition: less occupancy with increasing temperature. These latter conditions follow from a remarkable property of heterogeneous catalytic reactions which are subjected to temperature oscillations: the surface occupancy under fast temperature cycles appears to approach the steady state occupancy at the highest temperature used in the cycling. This has been proven both analytically and numerically. Again, analysis of steady state behaviour seems to be crucial in the prediction of resonance.

Analysis of microkinetic models towards their response on oscillations, demands an extensive exploration to the interesting resonance region. Numerical integration of the ODE's which describe the dynamics of a catalytic system is rather cumbersome. It requires a high computational effort by the successive integration steps that lead to convergence to the periodic steady state. Numerical integration does not easily allow interpretation with respect to dependence of the time averaged performance on forcing parameters, such as oscillation frequency and amplitude. In contrast, Carleman Linearisation (CL) provides analytical expressions for the dependent variables in the periodic steady state and therefore does not have the disadvantages as described before. The merits of CL have been emphasised in

chapter 3. However, the use of CL is limited. The original system of ODE's is not always

well approached by CL. In chapter 3 easily checkable requirements have been formulated for the use of CL. A sufficient condition appears to be the accurate CL prediction of the steady states at the extremes of the oscillations. Tools have been handed in to check CL estimations that does not meet this condition. Improvements of the steady state predictions leading to a better estimation of the response under forced oscillations, can be accomplished by either increasing the order of linearisation or shifting the point of linearisation closer to the erroneously predicted steady state.

In chapter 4 the CL technique has been used in the analysis of various microkinetic models with regard to their behaviour during forced concentration programming. The role of rate multiplicity, spillover and Eley-Rideal kinetics has been addressed. It has been shown that exchange in the model of molecular adsorption of the forced component by dissociative adsorption, does not lead to stronger resonance phenomena on the surface of the catalyst when the surface reaction rates are low. In case the non-forced component shows dissociative adsorption, stronger resonance has been observed. Dissociative adsorption combined with high surface reaction rates may lead to complex behaviour and multiplicity under concentration programming. In that case a response on imposed concentration oscillations can only be understood when the phase planes of the surface occupancies are investigated.

Spillover on the catalyst influences the response towards concentration programming in such a manner that the relaxed steady state is shifted to much lower oscillation frequencies. This is favourable compared to systems without spillover when the relaxed steady state rate is higher than the quasi steady state rate, except for cases showing positive resonance. Resonance peaks are dampened by inclusion of spillover terms in the model.

In chapter 4 it has further been proven that under forced concentration oscillations reaction rates higher than the optimal steady state rate can be obtained using adsorption/desorption models, in contrast to results presented before by others. However, improvements are rather marginal when single component cycling is used. This is caused by the fact that the occurrence of resonance, a prerequisite to achieve rate enhancement, relies on competition between components for adsorption on the catalyst surface. This implies that that the surface must be almost fully occupied. To get radical changes on the surface, another prerequisite for rate enhancement, the system should be forced using oscillations within a concentration window that also contains the optimal steady state. In the optimal steady state the coverage of both components is almost 50% in case of fully occupied surfaces. This means that there is not much left for improvement under periodic operation.

Chapter 5 addresses the use of forced concentration oscillations in order to unravel the

mechanism of a catalytic reaction; CO oxidation on oxidised CU/AI2O3. For the reoxidation of the catalyst by oxygen after reduction by CO, a three step mechanism has been proposed. Reaction of oxygen with three types of species has been found. First, oxygen can react with Cu sites containing both CO and O which results in production of CO2. The second step consists of oxidation of a site with adsorbed CO without the presence of in-plane oxygen. Subsequently, the newly formed species give CO2 via the first step. Finally also sites are oxidised without production of CO2. The use of forced oscillations in mechanistic studies has been proven to be a powerful tool by the inherent variation in time scales and the variation in initial conditions of the catalyst.

In addition it has been shown by isotopically labelling forced oscillation experiments that CO2 can adsorb on partly reduced CUO/AI2O3 and may subsequently decompose towards CO and an oxidised Cu species. This has been verified by transient FTIR in which the carboxylate species originating from CO2 adsorption could be monitored in time.

Self-oscillations during CO oxidation on EUROPT-3, a 0.3 wt% Pt/Al203 catalyst, has been

studied in chapter 6 in order to establish the mechanism for self-oscillations and to derive a mode of periodic operation able to suppress these instabilities. Steady state and transient FTIR

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experiments have been conducted aiming at the elucidation of the feedback mechanism

underlying self-oscillations. Under the applied conditions a net oxidation of Pt, which is

monitored by following CO-PtO using FTIR, leads the catalytic system into the region where

multiplicity occurs. It has been proven that oxidation and subsequent reduction of Pt on itself

can not be the driving force for the self-oscillations. Their dynamics are at least two orders of

magnitudes too slow. The existence of self-oscillations has been rationalised on the basis of

phase transitions. However, in the investigated system a change in the concentration of PtO

species is vital in the occurrence of self-oscillations as it brings the system into (or out of) the

multiplicity region. Self-oscillations may therefore be effectively prevented by keeping the

concentration of this species below a critical level. It has been shown that forced oscillations

using CO oxidation steps alternated by reduction steps in CO, is effective in avoiding the

self-oscillatory region as the concentration of PtO species is kept low.

In conclusion, this thesis demonstrates that forced oscillations can be very valuable in

heterogeneous catalysis, in particular for mechanistic studies and suppression of

self-oscillations. Using forced concentration oscillations to achieve rate enhancement is less

interesting as the improvements for the analysed models have been shown to be marginal. In

case applicable in practise, forced temperature oscillations may offer large improvements

compared to conventional steady state operation.