3 research outputs found
DFT insights into the oxygen-assisted selective oxidation of benzyl alcohol on manganese dioxide catalysts
The reactivity pattern of the MnO2 catalyst in the selective aerobic oxidation of benzyl alcohol is assessed by density functional theory (DFT) analysis of adsorption energies and activation barriers on a model Mn4O8 cluster. DFT calculations predict high reactivity of defective Mn(IV) sites ruling a surface redox mechanism, L-H type, involving gas-phase oxygen. Bare and promoted (i.e., CeOx and FeOx) MnOx materials with high surface exposure of Mn(IV) sites were synthesized to assess kinetic and mechanistic issues of the selective aerobic oxidation of benzyl alcohol on real catalysts (T, 333- 363K). According to DFT predictions, the experimental study shows: i) comparable activity of bare and promoted catalysts due to surface Mn(IV) sites; ii) the catalytic role of oxygen-atoms in the neighboring of active Mn(IV) sites; and iii) a 0th-order dependence on alcohol concentration, diagnostic of remarkable influence of adsorption phenomena on the reactivity pattern. Evidences of catalyst deactivation due to the over-oxidation of benzyl alcohol to benzoic acid, acting as poison of the active sites, are discussed
DFT Study of Pt Particle Growth inside β‑Zeolite Cages
The preferred location and the corresponding energetics
of zeolite-embedded
single metal atoms and small metal particles are hot topics within
active site optimization and catalyst tuning, even as part of bifunctional
materials design. In this context, periodic density functional theory
was used to provide insights on the interactions of a platinum atom
with the microporous cages of a purely silicious β-zeolite (BEA)
framework. Cluster growth was subsequently addressed, up to Pt3@BEA systems, following a one-by-one platinum atom addition;
platinum migration between cages was taken into account as well. An
unbiased approach was employed, which allowed a wide panorama of structures
being considered in addition to a thorough analysis in terms of energetics,
cluster geometries, and cavity distortions. Calculations revealed
that the optimal interaction geometry for a single platinum atom is
realized where two strong Pt–O bonds in almost linear arrangement
can form, regardless of the cavity involved. This can cause distortions
or even breaking of the zeolite structure, a factor which however
is not decisive in determining the energetics of systems with two
and three platinum atoms. Platinum migration is associated with energy
barriers ranging from 100 to 200 kJ mol–1, depending
on the cages. Up to the dimensions considered here, preference for
clustering is observed, being the embedded Pt3 systems
in almost all cases energetically favored with respect to isolated
atoms within the BEA framework