Reaction rate oscillations

When the development of dedicated i rared spectrometers for surface studies started some ten years ago, some of them were designed as more or less complete ellipsometers, which in principle are insensitive to the ambient gas phase molecules. Fedyk et al. detected CO adsorbed on an evaporated Cu film at 4 torr, while Golden et al. reported work at 100 torr. More recently, Burrows et used a Fourier transform spectrometer and the polarizer approach above to study the reaction-rate oscillations in the oxidation of CO on a large Pt polycrystalline foil at pressures up to one atmosphere. With this rapid FTIR spectrometer they obtained a time resolution of 0.6 s at a sensitivity of 5% of a full CO monolayer. 

Potentiometric techniques have been used to study autonomous reaction rate oscillations over catalysts and carbon monoxide oxidation on platinum has received a considerable amount of attention43,48,58 Possible explanations for reaction rate oscillations over platinum for carbon monoxide oxidation include, (i) strong dependence of activation energy or heat of adsorption on coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to adsorption or desorption of inert species, (iv) periodic oxidation or reduction of the surface. The work of Sales, Turner and Maple has indicated that the most... 

Reaction Rate Oscillations During the Carbon Monoxide Oxidation Reaction Over Pt/y-Al203 Catalysts An IR-Transmission Spectroscopy Study... 

Reaction rate oscillations have been observed during the oxidation reaction of CO over Pt/y-Al203 catalysts. The technique of IR transmission spectroscopy has been utilized to monitor the surface state of the catalyst under both steady-state and oscillatory conditions. The effect of hydrocarbon impurities and catalyst deactivation on the dynamic behavior of the system has also been investigated. 

Self-sustained reaction rate oscillations have been shown to occur in many heterogeneous catalytic systems Cl—8]. By now, several comprehensive review papers have been published which deal with different aspects of the problem [3, 9, 10]. An impressive volume of theoretical work has also been accumulated [3, 9, ll], which tries to discover, understand, and model the underlying principles and causative factors behind the phenomenon of oscillations. Most of the people working in this area seem to believe that intrinsic surface processes and rates rather than the interaction between physical and chemical processes are responsible for this unexpected and interesting behavior. However, the majority of the available experimental literature (with a few exceptions [7, 13]) does not contain any surface data and information which could help us to critically test and further Improve the hypotheses and ideas set forth in the literature to explain this type of behavior. 

Reaction rate oscillations have been observed during the CO oxidation reaction over Pt/Y-AI2O3 in an all Pyrex glass flow reactor. During our experimental study we have carefully tried to eliminate all known sources of Impurities and other possible extraneous interferences caused by temperature and flow controllers and by high volume recycle streams. The surface state of the catalyst has been monitored by the technique of IR transmission spectroscopy. During oscillations only the band at 2060 cm l has been found to oscillate. 

Of primary importance is the synchronization of the reaction rate oscil lations on various active microcenters on the crystal faces within the mac roscopic oscillation of the overall process rate. This synchronization is supposedly accounted for by diffusion of the gas phase components along the surface, among other factors. 

Reaction rate oscillations may be accompanied by temperature oscillations [temperature fluctuations of up to 500 K have been reported (24)] or they may be isothermal. Isothermality occurs either because the catalyst can conduct heat away much faster than the rate at which it is produced by the reaction, as is the case in UHV studies, or because isothermal conditions are forced on the system by anemometry, as described in the work of Luss and co-workers (757). Oscillation frequencies can range from more than 10 Hz (24) up to periods of several hours (217,219). Often there is evidence for several time scales in a single oscillating stem. Relatively regular high-frequency oscillations may be superimposed over relaxation oscillations (93,98), with the two types of oscillations caused by different changes on the catalyst surface. 

Rajagopalan et al. (1980) referring to Kurtanjek (1980) s work observed sustained oscillations of the reaction rate during the oxidation of hydrogen on a palladium wire. The temperature of the catalyst was maintained constant. It was concluded that the reaction rate oscillations observed during H2 oxidation over metal catalysts were associated with a transition between oxidized and reduced catalyst surface states as previously suggested by Kurtanjek (1980), see Section E.ll. Rajagopalan (1981) studied H2 oxidation, see Section E.l. 

IIIE) Kurtanjek, Z., Sheintuch, M., Luss, D. Reaction Rate Oscillations During the Oxidation 1980-2 of Hydrogen on Nickel. Ber. Bunsenges, Phys. Chem. 84(4) 374-377... 

Kurylko and Essenhigh (45, 46) subsequently examined this point in greater detail both experimentally and by extensive calculations and confirmed that Froberg s postulate was well based. During this work Kurylko also found that a change in location of the CO combustion region between inside and outside could generate both temperature and reaction rate oscillations. 

We have already hypothesised a reaction mechanism based on the redox chemistry of the Cu-ZSM5 catalyst [9]. According to that scheme, we proposed that N2O isothermal oscillations are related to the two possible roles assumed by N2O molecule in its interaction with catalyst copper sites. Based on the well known evidence that in Cu-ZSM5 the copper sites can exist in two different oxidation states (Cu and Cu") depending on the experimental conditions, this assumption expects that N2O could act both as an oxidant to transform Cu to Cu" and as a reductant to promote the reverse process and attributes reaction rate oscillations to the changes of the oxidation state of these sites, according to the following reactions ... 

The principal result of this work is that the nonlinear reaction kinetics is not restricted to macroscopic planes as (i) the planes 200A in diameter show the same nonlinear kinetics (ii) the regular waves appear under the reaction rate oscillations (iii) the propagation of reaction — dillusion waves includes the participation of the different crystal nanoplanes and indicates an effective coupling of adjacent planes. 

Fig. 8.9 (a) Stability diagram of metallic Pd(001) and PdO(OOl) during CO oxidation as a function of the CO partial pressure, Pco. and the Pd(OOl) surface roughness, measured as the FWHM of the (h,k,r) = (1,0,0.2) X-ray diffraction peak. The surface roughness (i.e., step density) shifts the metal-oxide phase transition to lower CO pressures, (b) Calculated cycle of a reaction rate oscillation. From Ref [72] (2010) by the Nature Publishing Group... 

A numerical calculation evidences the non stationary behaviour. Sustained oscillations of the surface values, of the limit—cycle type, propagate in the scale by getting damped. Consequently the reaction rate oscillates and the kinetics of growth of the oxidized... 

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