Superlocal chemical reaction equilibrium in low temperature plasma

Abstract

Low temperature plasmas (LTP) are a unique class of open-driven systems in which chemical reactions are unpredictable using established concepts. The terminal state of chemical reactions in LTP, termed the superlocal equilibrium state, is hypothesized to be defined by a proposed set of state variables. Using a LTP reactor wherein the state variables have been measured, it is shown that CO 2 spontaneously splits and the effluent speciation is independent of the influent speciation if the state variables are held constant and the residence time is long. CO 2 conversion at long residence times, which is expected to be nominally zero from equilibrium thermodynamics, can be as high as 70% in the LTP. The employed low pressure plasma reactor (P = 10 mbar) had a similar volume, productivity, and energy efficiency compared to an atmospheric pressure dielectric barrier discharge reactor, thanks to reaction rates that were three orders of magnitude faster. K E Y W O R D SCO 2 splitting, low-pressure reactor, low-temperature plasma, plasma chemistry, superlocal equilibrium 1 | INTRODUCTION A major challenge currently facing chemical engineering is the development of processes to transform CO 2 into platform chemicals from which valuable materials can be synthesized, with the goal of removing the greenhouse gas from the atmosphere. CO 2 is a relatively inert species that must be activated before it can react. Activation can be accomplished by photocatalysis, 1-3 electrochemistry, 4-7 raising the gas to high temperature, 8 or using low temperature plasma, [9][10][11] which is a partially ionized gas. Of these activation methods, plasma has a combination of desirable traits that stands out from the others: high-energy efficiency 12 (up to 90%), high reaction rates at the laboratory scale (approximately 1 t m −3 hr −1 ), and the possibility of low background gas temperature for compatibility with temperature-sensitive molecules. The synthesis of platform chemicals from CO 2 will likely require sophisticated reaction engineering. A major challenge preventing engineering of reactions between CO 2 and other molecules such as CH 4 or H 2 O in low temperature plasma is that one cannot make the local equilibrium assumption, 13 and consequently, practical methods to predict the direction and maximum extent of chemical reactions have not been forthcoming.The concept of thermodynamic equilibrium is generally used to predict the direction of chemical reactions and theoretical maxima for conversion and yield. In flow systems, which are intrinsically not at equilibrium, 14 the local equilibrium assumption 15 is often used for process modeling. One aspect of the local equilibrium assumption is that at given location in space, one can describe a single temperature, which is defined as T = ∂U ∂S À Á x , where U is internal energy, S is entropy, and the partial derivative is taken at constant work displacement x. Nonequilibrium systems can then be modeled using temperature gradients and the associated heat fluxes. 16 Such approaches for modeli...

Entropy production and chemical reactions in nonequilibrium plasma

“…The nonequilibrium environment is sustained by a flow of electrical work into the system, which can be partially stored as chemical potential in products, and partially rejected as heat to the ambient at low temperature 8 . Endergonic reactions can be promoted by nonequilibrium plasma, for example CO 2 splitting at low temperature, 7,8 and reasonable energy conversion efficiencies in the range from 10 to 90% have been demonstrated 10,11 . However, process development has been impeded by the lack of a thermodynamic framework that allows hypothetical reaction yields and theoretical limits on energy utilization to be calculated.…”

“…Unfortunately, equilibrium thermodynamics cannot be applied to nonequilibrium plasmas because different species have different temperatures at the same location in the reactor under steady operation 8 . There have been some attempts over the years to develop thermodynamic frameworks for chemical reactions in nonequilibrium plasma, 12‐15 but so far none have enjoyed widespread use, perhaps due to significant deviations of the predictions from observed experimental results 7 …”

“…Chemical reactions in nonequilibrium plasmas operate in the nonlinear regime. Several groups, including our own, have experimentally demonstrated that chemical reactions in nonequilibrium plasmas can evolve towards stationary states at which | A k |/ RT M ≫ 0.1, where T M is the temperature of the species participating in the reaction 7,35‐38 . The system is described as being governed by superlocal equilibrium, which means equilibrium is local in both space and species 7,8 .…”

“…Several groups, including our own, have experimentally demonstrated that chemical reactions in nonequilibrium plasmas can evolve towards stationary states at which | A k |/ RT M ≫ 0.1, where T M is the temperature of the species participating in the reaction 7,35‐38 . The system is described as being governed by superlocal equilibrium, which means equilibrium is local in both space and species 7,8 . The key idea is that at the same location in the reactor, extremely hot electrons that have kinetic energies on the order of bond dissociation energies (10,000 < T e < 100,000 K) are intermixed with heavy molecular gas species that have a much lower translational temperature in the range from 300 < T M < 1000 K (Figure 1).…”

“…Specifically, the idea is that the entropy production rate of the products of a reaction, compared to the entropy production rate of the reactants, determines the expected reaction extent based on the stationary and initial states. The idea is explored using our group's recently published experimental data on the CO 2 splitting reaction, which reaches a stationary state in nonequilibrium plasma independent of the initial speciation when a set of state variables are held constant 7 . The framework is furthermore applied to describe trends in stationary chemical composition in the nonequilibrium plasma as a function of the plasma state variables.…”