Spectroscopic characterization of chromium (IV, V, VI) in Cr:Li2MSiO4 (M=Mg,Zn)

Abstract

To understand the exceptionally long fluorescence lifetime of Cr IV in Li 2 MSiO 4 (M ϭMg,Zn), several spectroscopic investigations are performed on Cr:Li 2 MSiO 4 (M ϭMg,Zn) crystals. X-ray absorption near-edge structure investigations attest that chromium is localized in tetrahedral sites and that Cr VI is the major species while Cr IV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of Cr V in elongated tetrahedral environment (d x 2 Ϫy 2 ground state͒, corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of Cr IV :Li 2 MSiO 4 are reported. The very long Cr IV excited state lifetime previously observed on powders samples is confirmed for the crystals ͑117 s at room temperature, 305 s at 30 K for Cr:Li 2 MgSiO 4 ) and is explained by combined contributions to the emission from both 1 E level and the lowest component of the 3 T 2 level, in thermal equilibrium. Contrary to the situation found in most Cr IV activated compounds, the 1 E level lies below the 3 T 2 one.

The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods

“…(ii) The energy position of the chromium K-edge, corresponding to the minimum energy required to promote a 1s electron to the continuum, is very sensitive to the chromium oxidation state, to the electronegativity of the nearest neighbor atoms, and to the coordination geometry. These results have been confirmed by recent studies, ,− which demonstrated that there is a direct correlation between the energy position of the edge and/or the preedge and the valence state of the chromium center. This means that in principle, it should be possible to use the position of the Cr−K edge and preedge transitions to determine the average chromium valence state and coordination geometry in unknown compounds, by using data collected for reference compounds.…”

Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

“…For these reasons, XAFS spectroscopies are, in principle, powerful techniques to understand the local structure and the oxidation state of Cr species on the Phillips catalyst. Nevertheless, in contrast to the large amount of XANES studies on various chromium compounds reported in the past, − only few XAFS works appeared on the Phillips catalyst or on related systems, ,,,,− and only one is focused on in situ C 2 H 4 polymerization . Three main reasons could be tentatively accounted for this lack.…”

The potential of spectroscopic methods applied to heterogeneous catalysts for olefinpolymerization

“…For the reasons outlined above, EXAFS spectroscopy is, in principle, a powerful technique to understand the local structure of grafted Cr species on the Phillips catalyst. Nevertheless, in contrast to the large amount of XAS studies on various chromium compounds reported in the past, [64][65][66][67][68][69] only a few works appeared about XAS applied on the Phillips catalyst or on related systems. 9,[70][71][72][73][74][75][76] This is mainly due to the low Cr loading (in the 0.5-1.0 wt% range) and its high reactivity towards oxygen and moisture.…”