New insights into the Belousov-Zhabotinskii reaction derived from EQCM measurements at a gold electrode

Abstract

Electrochemical quartz crystal microbalance experiments were used to study the classical Belousov-Zhabotinskii (BZ) homogeneous oscillating system. This system involves 2 × 10(-3) M Ce(III), 0.28 M malonic acid and 0.063 M bromate as the main initial components in 1 M sulfuric acid solution. The gold-evaporated electrodes on a 10 MHz AT-cut quartz crystal were used for potentiometric and amperometric studies while the changes in crystal frequency provided mass data. Under open-circuit conditions, during the BZ reaction, oscillations of the gold electrode potential in the range ca. 0.8 to 1.07 V (SCE) with a period about 80 s occurred. They were accompanied by periodic 10-15 ng [i.e. ca. 45-70 ng cm(-2)] changes in the electrode mass. At more positive potentials a decrease in electrode mass occurred, while the mass increased at more negative potentials. At a constant applied electrode potential, corresponding to either the upper or the lower potential limit attained under open-circuit conditions, periodic pulses of cathodic current occurred and were accompanied by mass changes. A continuous decrease in the electrode mass occurred at 1.06 V. A detailed examination of the gold electrode behavior in the solutions containing individual components of the system using cyclic voltammetry and quartz crystal microgravimetry provided the information needed to interpret the mass changes that occur in the complete system. No significant changes in the electrode mass occurred in sulfuric acid solution in the potential range where current and mass oscillations take place in the full BZ reaction solution. The same result was found in sulfuric acid solutions containing either Ce(III) or malonic acid. Dissolution of gold occurred in a sulfuric acid solution containing bromate or bromide ions. Adsorption of bromide ions on gold electrode occurred in Br(-)-containing sulfuric acid solution at more negative potentials. In the BZ system, dissolution of gold in the presence of oxidizing (bromate) and complexing (bromide) species causes the decrease in the electrode mass that accompanies the positive potential jump under open-circuit conditions, or the current pulse that occurs at more negative applied constant potentials. Cathodic current pulses occurring at a constant electrode potential (either 0.8 or 1.06 V) are associated with the reduction of Ce(IV) formed as a result of periodic homogeneous oxidation of Ce(III) by bromate. Bromide ions formed in the course of the BZ reaction appear to play a significant role in electrode mass changes, causing a mass decrease at more positive potentials due to dissolution of gold, and a subsequent mass increase at more negative potentials due to adsorption processes.

Scanning Electrochemical Microscopy of Belousov–Zhabotinsky Reaction: How Confined Oscillations Reveal Short Lived Radicals and Auto-Catalytic Species

“…This detection mode allows selective control over the redox species generated electrochemically allowing for new mechanistic insights into the role of an electrode surface state in a BZ process. 44 It also parallels the specific case of inter-microreactors chemical communication as the consumption of ferroin at the tip may approximate the behavior of the efflux of chemicals during inter-reactor communication. Then, the tip−substrate confinement would approximate the shell−core behaviors within communicating microreactors.…”

“…Unlike potentiometry, amperometry perturbs the system as it enforces the ferroin concentration at the tip. This detection mode allows selective control over the redox species generated electrochemically allowing for new mechanistic insights into the role of an electrode surface state in a BZ process . It also parallels the specific case of inter-microreactors chemical communication as the consumption of ferroin at the tip may approximate the behavior of the efflux of chemicals during inter-reactor communication.…”

Measurement of a Bubble-Free Chemical Oscillator Using QCMs Treated with Self-Assembled Monolayers

“…However, it has been reported that the adsorption of bromine species and compounds on the QCM electrode dissolved gold. 24 On the other hand, a platinum electrode was sufficient for the oscillation measurement. 23 That is, the difference in the ΔF oscillation due to an electrode metal has not yet been clarified in detail.…”

“…It has been reported that the QCM could be used as a detection tool of the classic BZ oscillation. − The classic BZ reaction produces carbon dioxide bubbles as the result of chemical reaction of malonic acid . The bubbles disturb the QCM measurement due to adsorption on the electrode surface.…”

“…Gold is commonly used as an electrode metal of the QCM because of its chemical stability. However, it has been reported that the adsorption of bromine species and compounds on the QCM electrode dissolved gold . On the other hand, a platinum electrode was sufficient for the oscillation measurement .…”