Sonochemical Disproportionation of Carbon Monoxide in Water: Evidence for Treanor Effect during Multibubble Cavitation

Abstract

Eighty years after the discovery of ultrasound-induced chemical processes, [1] known as sonochemistry, it remains a subject of extensive research.[2] It is generally accepted that sonochemistry arises from acoustic cavitation, which is the nucleation, growth, and implosive collapse of microbubbles in liquids subjected to ultrasonic waves. Nevertheless, debate still continues over the origin of extreme conditions created by the bubble collapse. Usually the sonochemical reactions are interpreted according to Flynns "thermal" hypothesis presuming adiabatic or quasi-adiabatic transient heating of gases and vapours inside the cavitating bubble.[3] However, the recent observation of light emission from positively charged O 2 + species during single-bubble collapse in H 2 SO 4 provided evidence for nonthermal plasma formation inside the bubble.[4] The relationship between the mechanisms of single-bubble and multibubble cavitations is still not clear. Recent studies of multibubble sonoluminescence in H 2 SO 4 and water revealed some similarities in the origin of both kinds of processes. [5,6] In this view thermal equilibration during the cavitation event remains an important question that is not yet resolved. Herein, we report the first "chemical" evidence for nonequilibrium molecule vibrational excitation occurring during multibubble cavitation.We studied the ultrasonically driven disproportionation of carbon monoxide (DCO) in water saturated with a CO/Ar gas mixture. To our knowledge, this reaction had never been studied under ultrasonic irradiation. By contrast, the plasmachemical DCO reaction in the gas phase has been a very active topic of research as working media for CO lasers and as a promising method for carbon isotope separation.[7] According to published results, the strongly endothermic DCO reaction (DH = 5.5 eV mol À1 , E a = 6 eV mol À1 ) can be significantly accelerated by the vibrational excitation of CO molecules. The population of highly vibrational states CO*(n n ), n 40, occurs through an anharmonic vibrationto-vibration pumping mechanism (V-V), known as the Treanor effect. [8] This approach produces nonthermal plasma far from the thermodynamic equilibrium with a vibrational temperature (T v ) of CO molecules much higher than their translational (T 0 ) or rotational (T R ) temperatures. An experimental illustration of the Treanor V-V pumping is a kinetic isotope effect (KIE) during DCO in nonequilibrium plasma.[9] The coefficient of isotopic selectivity (a) for vibrationally excited isotopes can be expressed by Equation (1), whereis the relative defect of resonance.[7]In quasi-equilibrium kinetics, light isotopes react faster because of their higher "zero vibration level" 1 = 2 h w. In contrast, in nonequilibrium plasma, heavy isotopes react faster since they have higher vibrational temperature. Moreover, the nonequilibrium KIE is much stronger than the quasi-equilibrium effect. [7] We found that the sonochemical DCO reaction exhibits some similarities with that in nonequilibrium plasma. At the same t...

Nonequilibrium Vibrational Excitation of OH Radicals Generated During Multibubble Cavitation in Water

“…15 The overpopulation of higher vibrational states in a nonequilibrium plasma with Treanor behavior occurs through an anharmonic vibration−vibration (V−V) exchange. Recently, the Treanor effect has been observed for CO disproportionation in water treated with 20 kHz ultrasound 17 in full agreement with the results obtained in this work.…”

Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids

“…where the symbol “)))” indicates a process triggered by a cavitation event. It is generally recognized that the sites of mutual recombination for H atoms and OH • radicals are different: H atoms recombine inside the bubble, whereas the recombination of OH • radicals occurs mostly at bubble/liquid interface 22 . Carbon monoxide is known to be an effective intrabubble scavenger of OH • radicals leading to the increase of hydrogen yield 22 :…”

Plasma Formation during Acoustic Cavitation: Toward a New Paradigm for Sonochemistry

“…Instead of simplistic adiabatic heating model, the sonochemical processes can be considered in terms of more sophisticated plasma chemical approach allowing explaining the new effects in sonochemistry as well as in sonoluminescence. One of such processes is a carbon isotope effect during sonochemical carbon monoxide disproportionation in water [49]. The prolonged sonication of water with 20 kHz ultrasound in the presence of 20% CO/Ar gas mixture yields a tiny amount of solid carbon-containing product.…”