Wolkenstein’s Model of Size Effects in CO Oxidation by Gold Nanoparticles

Turaeva, N. N.; Krueger, Herman

doi:10.3390/catal10030288

Cited by 6 publications

(1 citation statement)

References 40 publications

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“…Thus, optimization of the reaction rate is achieved by manipulation of the Fermi level through alloying of metals with respect to the d-band center. Alternatively, other studies have described how the reaction rate of CO oxidation and reactivity of H 2 can be varied according to the size of nanoparticle domains , and the addition of alkali promoters. , …”

Section: Discussionmentioning

confidence: 99%

Kinetic Expression for Optimal Catalyst Electronic Configuration: The Case of Ammonia Decomposition

2020

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A new steady-state kinetic model of ammonia decomposition is presented and analyzed regarding the electronic properties of metal catalysts. The model is based on the classical Temkin-Ertl mechanism and modified in accordance with Wolkenstein's electronic theory by implementing participation of free electrons of the catalyst to change the chemical nature of adsorbed species. Wolkenstein's original theory only applied to semiconductors but by including the d-band model for splitting of adsorbate molecular orbitals into bonding/antibonding states, the electronic theory can be extended to metals. The relative population of charged versus neutral adsorbates is a function of the Fermi level of the catalyst and the d-band splitting of the adsorbate. Moreover, charged and neutral adsorbates will present different reactivity and the overall reaction rate can be described as a function of the Fermi level. For both simplified and full reaction mechanisms, including electronic steps, we present a steady-state rate equation where the dependence on the Fermi level of the metal creates a volcano-shaped dependence. According to the kinetic model, an increasing Fermi level of the catalyst, that approaching the antibonding state with adsorbed nitrogen molecules, will increase the fraction of neutral nitrogen molecules and enhance their the desorption. Concurrently, strong chemisorption of ammonia molecules proceeds easily through participation of additional free catalyst electrons in the adsorbate bond. As a result, the reaction rate is enhanced and reaches its maximum value. A further increasing Fermi level of the catalyst that approaches the antibonding state with ammonia molecules will result in a smaller fraction of negatively charged ammonia molecules and less dehydrogenation. Concurrently, the desorption of neutral nitrogen molecules occurs without impairment. As a result, the reaction rate decreases. The detailed kinetic model is compared to recent experimental measurements of ammonia decomposition on iron, cobalt and CoFe bimetallic catalysts. This result agrees with the classical qualitative Sabatier statement as well as the quantitative theoretical results of the optimal binding energy present by Nørskov and coworkers. In difference from the classical volcano-curves, considering a single reactant, the volcano-shaped reaction rate of the presented model is obtained via the interplay of both reactant adsorption and product desorption, which is achieved by considering the role of electronic subsystems.

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