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1.
Angew Chem Int Ed Engl ; 58(25): 8504-8509, 2019 06 17.
Artigo em Inglês | MEDLINE | ID: mdl-30985054

RESUMO

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3 O4 ⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Can Mn4-n O4 + (n=0-4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas-phase reactivity experiments with D2 O and H2 18 O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono-calcium cluster CaMn3 O4 + is observed to oxidize water via elimination of hydrogen peroxide. First-principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3 O4 + as well as the role of calcium.

2.
J Phys Chem A ; 118(37): 8572-82, 2014 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-24915185

RESUMO

Temperature-dependent gas phase ion trap experiments performed under multicollision conditions reveal a strongly size-dependent reactivity of Pd(x)(+) (x = 2-7) in the reaction with molecular oxygen. Yet, a particular stability and resistance to further oxidation is generally observed for reaction products with two oxygen molecules, Pd(x)O4(+). Complementary first-principles density functional theory simulations elucidate the details of the size-dependent bonding of oxygen to the small palladium clusters and are able to assign the pronounced occurrence of Pd(x)O4(+) complexes to a dissociatively chemisorbed bridging oxygen atomic structure which impedes the chemisorption of further oxygen molecules. The molecular physisorption of additional O2 is only observed at cryogenic temperatures. Additional experiments and simulations employing preoxidized clusters Pd(x)O(+) (x = 2-8) and Pd(x)O2(+) (x = 4-7) confirm the formation of the two different oxygen species.

3.
Nano Lett ; 13(11): 5549-55, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24164444

RESUMO

The interaction of ligand-free manganese oxide nanoclusters with water is investigated, aiming at uncovering phenomena which could aid the design of artificial water-splitting molecular catalysts. Gas phase measurements in an ion trap in conjunction with first-principles calculations provide new mechanistic insight into the water splitting process mediated by bi- and tetra-nuclear singly charged manganese oxide clusters, Mn2O2(+) and Mn4O4(+). In particular, a water-induced dimensionality change of Mn4O4(+) is predicted, entailing transformation from a two-dimensional ring-like ground state structure of the bare cluster to a cuboidal octa-hydroxy-complex for the hydrated one. It is further predicted that the water splitting process is facilitated by the cluster dimensionality crossover. The vibrational spectra calculated for species occurring along the predicted pathways of the reaction of Mn4O4(+) with water provide the impetus for future explorations, including vibrational spectroscopic experiments.


Assuntos
Compostos de Manganês/química , Óxidos/química , Oxigênio/química , Água/química , Catálise , Gases/química , Transição de Fase , Análise Espectral , Vibração
4.
J Am Chem Soc ; 134(51): 20654-9, 2012 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-23237307

RESUMO

The palladium oxide cluster Pd(6)O(4)(+) is formed as the sole product upon reaction of a bare palladium cluster Pd(6)(+) with molecular oxygen in an octopole ion trap under multicollision conditions. This oxide cluster is found to be resistant to further oxidation over a large temperature range, and further O(2) molecules merely physisorb on it at cryogenic temperatures. The particular stability of Pd(6)O(4)(+) is confirmed by the observation that the reaction of Pd(7)(+) with O(2) leads to fragmentation resulting in the formation of Pd(6)O(4)(+). However, in an oxygen-rich O(2)/CO mixture, Pd(6)O(4)(+) is identified as the catalytically active species that effectively facilitates the low-temperature oxidation of CO. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide detailed molecular level insight into the nano-oxide cluster chemistry and are able to reveal the full catalytic combustion reaction cycle.

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