The Nature of Surface CrOx Sites on SiO2 in Different Environments

Abstract

This perspective article critically reviews the catalysis literature on the nature of the surface CrO x sites present on SiO 2 in different environments. The recent application of in situ spectroscopic techniques that directly monitor the surface chromia sites on silica in different environments has significantly improved our fundamental understanding of supported CrO x /SiO 2 catalysts.

Tracking the reasons for the peculiarity of Cr/Al2O3 catalyst in ethylene polymerization

“…The spectroscopic properties of Cr 6+ on Al 2 O 3 are well known in the specialized literature. While for Cr 6+ on SiO 2 there is still a debate on the aggregation state of the Cr 6+ species (both mono-and dichromates have been proposed [5, 7-9, 20, 22-24, 26, 34, 35], or even mono-oxo CrO 5 [27][28][29][30][31][32][33]), Cr 6+ exist primarily as monochromate species on Al 2 O 3 [9,[19][20][21][22][23][24]36] (the corresponding DR UV-Vis-NIR spectrum is shown in Figure S1a). Temperature-programmed reduction measurements have demonstrated that Cr 6+ on Al 2 O 3 are more reducible than Cr 6+ on SiO 2 (i.e.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[9,246] Theinitial CrO 3 / SiO 2 contains predominantly Cr VI -dioxo surface complexes (SiO) 2 Cr(=O) 2 (Scheme 16), [247][248][249][250] although small amounts of EPR-active Cr sites are also present. [9,246] Theinitial CrO 3 / SiO 2 contains predominantly Cr VI -dioxo surface complexes (SiO) 2 Cr(=O) 2 (Scheme 16), [247][248][249][250] although small amounts of EPR-active Cr sites are also present.…”

“…[9,246] Theinitial CrO 3 / SiO 2 contains predominantly Cr VI -dioxo surface complexes (SiO) 2 Cr(=O) 2 (Scheme 16), [247][248][249][250] although small amounts of EPR-active Cr sites are also present. [250,256,257] Themetal sites can be reduced by reaction with CO. [258][259][260][261][262][263] Ther educed Cr sites react with ethylene immediately,w hich suggests that the active site is present after reduction by CO.D etailed studies showed that this reduced material contains mostly Cr II sites. [250,256,257] Themetal sites can be reduced by reaction with CO. [258][259][260][261][262][263] Ther educed Cr sites react with ethylene immediately,w hich suggests that the active site is present after reduction by CO.D etailed studies showed that this reduced material contains mostly Cr II sites.…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…Die aktiven Zentren des Phillips‐Katalysators waren bereits Thema zahlreicher Untersuchungen . Das Ausgangsmaterial, CrO 3 /SiO 2 , enthält vorwiegend Cr VI ‐Dioxo‐Oberflächenkomplexe (≡SiO) 2 Cr(=O) 2 (Schema ), aber auch geringe Mengen an EPR‐aktiven Cr‐Zentren . In Gegenwart von Ethylen werden die Cr VI ‐Dioxo‐Zentren zu Cr II ‐Zentren als wichtigste Oberflächenspezies und zu einer kleinen Menge Cr III ‐Zentren reduziert.…”

“…Das Ausgangsmaterial, CrO 3 /SiO 2 , enthält vorwiegend Cr VI ‐Dioxo‐Oberflächenkomplexe (≡SiO) 2 Cr(=O) 2 (Schema ), aber auch geringe Mengen an EPR‐aktiven Cr‐Zentren . In Gegenwart von Ethylen werden die Cr VI ‐Dioxo‐Zentren zu Cr II ‐Zentren als wichtigste Oberflächenspezies und zu einer kleinen Menge Cr III ‐Zentren reduziert. Die Metallzentren können auch durch Reaktion mit CO reduziert werden .…”