Epicatalysis

Epicatalysis is a newly identified class of gas-surface heterogeneous catalysis[1][2] in which specific gas-surface reactions shift gas phase species concentrations away from those normally associated with gas-phase equilibrium.[3] As a result, epicatalytically created gas phase concentrations (in a sealed, isothermal cavity) can remain in a stationary nonequilibrium state.

Epicatalysis is predicted by standard kinetic theory when two criteria are met: 1) gas-surface interactions are appreciable; and 2) the mean free path for gas phase reactions is long compared with the distance between gas-surface collisions, usually taken to be size scale of the confining vessel. When these conditions are met, the catalytic effects of the gas-surface interactions outweigh the gas-phase interactions, resulting in a gas-phase in a non-equilibrium state.

A traditional catalyst adheres to three general principles, namely: 1) it speeds up a chemical reaction; 2) it participates in, but is not consumed by, the reaction; and 3) it does not change the chemical equilibrium of the reaction. Epicatalysts overcome the third principle when gas-surface interactions are appreciable and gas-phase collisions are rare. Under these conditions the gas particles that desorb from an epicatalytic surface can retain characteristics of the gas-surface interactions far away from the surface because gas phase collisions are too infrequent to establish normal gas phase equilibrium. Because gas-surface interactions dominate over gas-phase interactions in determining the gas-phase concentrations, the gas phase can be held, continuously, far away from normal gas phase equilibrium.

Several well-studied examples of epicatalysis have been hiding in plain sight in scientific literature for nearly a century, including plasmas created by surface ionization,[4][5] notably in the Q-machine; [6][7] and hydrogen dissociation on high-temperature transition metals (e.g., W, Re, Mo, Re, Ir, and Ta) [8][9][10][11][12][13]

Epicatalysis could enable a number of new applications, including chemical streams enriched in high-energy or desirable reactants; lower operating temperatures for chemical reactions; novel forms of alternative energy, and options for green chemistry. [14]

Until recently, epicatalysis has been observed only at high temperatures (T > 1500 K) owing to the nature of their gas-surface reactions. (Covalent chemical bond energies and ionization energies for atoms are usually several electron volts.) However, as of 2015, epicatalysis has been observed at room temperature with molecules involving much weaker hydrogen-bonds (e.g., methanol, formic acid, water). Epicatalysis has been invoked in experimental tests of Duncan's paradox.[15]

References

  1. ↑ V. Parmon, Thermodynamics of Non-Equilibrium Processes for Chemists with a Particular Application to Catalysis (Elsevier, Amsterdam, 2010).
  2. ↑ K.W. Kolasinski, Surface Science: Foundations of Catalysis and Nanoscience, 2nd Ed. (John Wiley, England, 2009).
  3. ↑ Sheehan, D.P., Nonequilibrium heterogeneous catalysis in the long mean-free-path regime, Phys. Rev. E 88 032125 (2013).
  4. ↑ N.I. Ionov, J. Exp. Theor. Phys. 18 174 (1948), i.e, Zh. Eksp. Teor. Fiz. 18 174 (1948).
  5. ↑ S. Datz and E.H. Taylor, J. Chem. Phys. 25 389 (1956).
  6. ↑ R.W. Motley, Q-Machines (Academic Press, New York, 1975).
  7. ↑ N. Rynn and N. D'Angelo, Rev. Sci. Instrum. 31 1326 (1960).
  8. ↑ I. Langmuir, J. Am. Chem. Soc. 34 860 (1912); 34 1310 (1912).
  9. ↑ F. Jansen, I. Chen and M.A. Machonkin, J. Appl. Phys. 66 5749 (1989).
  10. ↑ D.P. Sheehan, Phys. Lett. A 280 185 (2001).
  11. ↑ T. Otsuka, M. Ihara and H. Komiyama, J. Appl. Phys. 77 893 (1995).
  12. ↑ X. Qi, Z. Chen and G. Wang, J. Mater. Sci. Technol. 19 235 (2003).
  13. ↑ L. Schäfer, C.-P. Klages, U. Meier and K. Kohse-Höinghaus, Appl. Phys. Lett. 58 571 (1991).
  14. ↑ V. Parmon, Thermodynamics of Non-Equilibrium Processes for Chemists with a Particular Application to Catalysis (Elsevier, Amsterdam, 2010).
  15. ↑ Sheehan, D.P., D.J. Mallin, J.T. Garamella, and W.F. Sheehan, Found. Phys. 44 235 (2014).
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