Nano-confined Nitrogen Hydrogenation on Ruthenium Surface: Prediction of Remarkable Entropic Shifts in the Reaction Equilibria

Micha Polak, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel
Leonid Rubinovich, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel

A new nanoscale phenomenon, "Nanoconfinement entropic Effect on Chemical Equilibrium" (NCECE), was predicted by us some years ago based on statistical-mechanics original formulation [1], and recently verified by reevaluation of published experimental data concerning DNA hybridization inside nano-chambers [2]. The NCECE generally increases the molecular fraction of the dominating constituent, namely the product/reactant in exothermic/endothermic reactions, and it depends on the reaction mixture size, stoichiometry and on the nanospace size [3].

Unlike some hypothetical reactions treated by us previously, the present work [4] for the first time evaluates NCECE effects for genuine surface reactions with practical implications, namely the three equilibrated consecutive hydrogenation steps, N+xH ↔ NH_x (x=1,2,3), in ammonia synthesis confined to the (0001) nano-dimensional surface of its probably best catalyst, Ru. Using reported DFT-based reaction energetics as input, remarkable variations in NH_x equilibrium molecular fractions are revealed by statistical-mechanical computations for a confined system comprised of only few adsorbed reactant atoms. For example, NH formation is enhanced up to ~ 50% compared to the thermodynamic limit (TL) of the macroscopic system. For concurrent ND and NH formation, including H-D exchange, temperature and coverage dependent superposition of the three NCECE contributions has significant effect on the ND yield and on the equilibrium constants of the nanosystems. The computed TL low fractions of NH₂ and NH₃ in the coupled three hydrogenation steps is further diminished by the NCECE, suggesting that from this aspect ammonia synthesis on smaller particles should be less effective.

[1] Micha Polak and Leonid Rubinovich, Nano Letters 8, 3543 (2008).

[2] Leonid Rubinovich and Micha Polak, Nano letters 13, 2247 (2013).

[3] Micha Polak and Leonid Rubinovich, Physical Chemistry Chemical Physics 13, 16728 (2011).

[4] Micha Polak and Leonid Rubinovich, Surface Science (submitted for publication).


Nano-confined Nitrogen Hydrogenation on Ruthenium Surface: Prediction of Remarkable Entropic Shifts in the Reaction Equilibria Micha Polak, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel Leonid Rubinovich, Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel A new nanoscale phenomenon, "Nanoconfinement entropic Effect on Chemical Equilibrium" (NCECE), was predicted by us some years ago based on statistical-mechanics original formulation [1], and recently verified by reevaluation of published experimental data concerning DNA hybridization inside nano-chambers [2]. The NCECE generally increases the molecular fraction of the dominating constituent, namely the product/reactant in exothermic/endothermic reactions, and it depends on the reaction mixture size, stoichiometry and on the nanospace size [3]. Unlike some hypothetical reactions treated by us previously, the present work [4] for the first time evaluates NCECE effects for genuine surface reactions with practical implications, namely the three equilibrated consecutive hydrogenation steps, N+xH ↔ NH_x (x=1,2,3), in ammonia synthesis confined to the (0001) nano-dimensional surface of its probably best catalyst, Ru. Using reported DFT-based reaction energetics as input, remarkable variations in NH_x equilibrium molecular fractions are revealed by statistical-mechanical computations for a confined system comprised of only few adsorbed reactant atoms. For example, NH formation is enhanced up to ~ 50% compared to the thermodynamic limit (TL) of the macroscopic system. For concurrent ND and NH formation, including H-D exchange, temperature and coverage dependent superposition of the three NCECE contributions has significant effect on the ND yield and on the equilibrium constants of the nanosystems. The computed TL low fractions of NH₂ and NH₃ in the coupled three hydrogenation steps is further diminished by the NCECE, suggesting that from this aspect ammonia synthesis on smaller particles should be less effective. [1] Micha Polak and Leonid Rubinovich, Nano Letters 8, 3543 (2008). [2] Leonid Rubinovich and Micha Polak, Nano letters 13, 2247 (2013). [3] Micha Polak and Leonid Rubinovich, Physical Chemistry Chemical Physics 13, 16728 (2011). [4] Micha Polak and Leonid Rubinovich, Surface Science (submitted for publication).

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