The geometrical structure of the Au‐Fe2O3interfacial perimeter, which is generally considered as the active sites for low‐temperature oxidation of CO, was examined. It was found that the activity of the Au/Fe2O3catalysts not only depends on the number of the gold atoms at the interfacial perimeter but also strongly depends on the geometrical structure of these gold atoms, which is determined by the size of the gold particle. Aberration‐corrected scanning transmission electron microscopy images unambiguously suggested that the gold particles, transformed from a two‐dimensional flat shape to a well‐faceted truncated octahedron when the size slightly enlarged from 2.2 to 3.5 nm. Such a size‐induced shape evolution altered the chemical bonding environments of the gold atoms at the interfacial perimeters and consequently their catalytic activity. For Au particles with a mean size of 2.2 nm, the interfacial perimeter gold atoms possessed a higher degree of unsaturated coordination environment while for Au particles with a mean size of 3.5 nm the perimeter gold atoms mainly followed the atomic arrangements of Au {111} and {100} facets. Kinetic study, with respect to the reaction rate and the turnover frequency on the interfacial perimeter gold atom, found that the low‐coordinated perimeter gold atoms were intrinsically more active for CO oxidation.18O isotopic titration and Infrared spectroscopy experiments verified that CO oxidation at room temperature occurred at the Au‐Fe2O3interfacial perimeter, involving the participation of the lattice oxygen of Fe2O3for activating O2and the gold atoms for CO adsorption and activation.
Metastable phases of the photoswitchable molecular magnet K0.3Co[Fe(CN)6]0.77 ⋅ nH2O in sub-micrometer particles have been structurally investigated by synchrotron powder x-ray diffraction (PXRD) measurements. The K0.3Co[Fe(CN)6]0.77 ⋅ nH2O bulk compound (studied here with a sample having average particle size of 500 nm) undergoes a charge transfer coupled spin transition (CTCST), where spin configurations change between a paramagnetic CoII( S = 3/2) –FeIII( S = 1/2) high-temperature (HT) state and a diamagnetic CoIII( S = 0) –FeII( S = 0) low-temperature (LT) state. The bulk compound exhibits a unique intermediate (IM) phase, which corresponds to a mixture of HT and LT spin states that depend on the cooling rate. Several hidden metastable HT states emerge as a function of thermal and photo stimuli, namely: (1) a quench (Q) state generated from the HT state by flash cooling, (2) a LTPX state obtained by photoexcitation from the LT state derived by thermal relaxation from the Q state, and (3) an IMPX state accessed by photo-irradiation from the IM state. A sample with a smaller particle size, 135 nm, is investigated for which the particles are on the scale of the coherent LT domains in the IM phase within the larger 500 nm sample. PXRD studies under controlled thermal and/or optical excitations have clarified that the reduction of the particle size profoundly affects the structural changes associated with the CTCST. The unusual IM state is also observed as segregated domains in the 135 nm particle, but the collective structural transformations are more hindered in small particles. The volume change decreases to 2%–3%, almost half the value found for 500 nm particles (5%–8%), even though the linear thermal expansion coefficients are larger for the smaller particles. Furthermore, photoexcitation from the IM and LT states does not turn into single phases in the smaller particles, presumably because of the multiple interfaces and/or internal stress generated by the coexistence of small CoII–FeIIIand CoIII–FeIIdomains in the lattice. Since the reduced particle size limits cooperativity and domain growth in the lattice, CTCST in the small particle sample becomes less sensitive to external stimuli.
more » « less- Award ID(s):
- 1904596
- NSF-PAR ID:
- 10363309
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 131
- Issue:
- 8
- ISSN:
- 0021-8979
- Page Range / eLocation ID:
- Article No. 085110
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The geometrical structure of the Au‐Fe2O3interfacial perimeter, which is generally considered as the active sites for low‐temperature oxidation of CO, was examined. It was found that the activity of the Au/Fe2O3catalysts not only depends on the number of the gold atoms at the interfacial perimeter but also strongly depends on the geometrical structure of these gold atoms, which is determined by the size of the gold particle. Aberration‐corrected scanning transmission electron microscopy images unambiguously suggested that the gold particles, transformed from a two‐dimensional flat shape to a well‐faceted truncated octahedron when the size slightly enlarged from 2.2 to 3.5 nm. Such a size‐induced shape evolution altered the chemical bonding environments of the gold atoms at the interfacial perimeters and consequently their catalytic activity. For Au particles with a mean size of 2.2 nm, the interfacial perimeter gold atoms possessed a higher degree of unsaturated coordination environment while for Au particles with a mean size of 3.5 nm the perimeter gold atoms mainly followed the atomic arrangements of Au {111} and {100} facets. Kinetic study, with respect to the reaction rate and the turnover frequency on the interfacial perimeter gold atom, found that the low‐coordinated perimeter gold atoms were intrinsically more active for CO oxidation.18O isotopic titration and Infrared spectroscopy experiments verified that CO oxidation at room temperature occurred at the Au‐Fe2O3interfacial perimeter, involving the participation of the lattice oxygen of Fe2O3for activating O2and the gold atoms for CO adsorption and activation.
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Abstract A new nonheme iron(II) complex, FeII(Me3TACN)((OSiPh2)2O) (
1 ), is reported. Reaction of1 with NO(g)gives a stable mononitrosyl complex Fe(NO)(Me3TACN)((OSiPh2)2O) (2 ), which was characterized by Mössbauer (δ =0.52 mm s−1, |ΔE Q|=0.80 mm s−1), EPR (S =3/2), resonance Raman (RR) and Fe K‐edge X‐ray absorption spectroscopies. The data show that2 is an {FeNO}7complex with anS =3/2 spin ground state. The RR spectrum (λ exc=458 nm) of2 combined with isotopic labeling (15N,18O) reveals ν(N‐O)=1680 cm−1, which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm−1). Complex2 reacts rapidly with H2O in THF to produce the N‐N coupled product N2O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N2O in the absence of an exogenous reductant.
