(112b) Elucidation of the Reaction Pathway and Mechanism of Cr6+ Species Formation for Development of Sustainable Cr (VI)-Free Crox/Al2O3 Catalysts.

Chromium-based catalysts are widely used in the industrial-scale. However, the presence of Hexavalent chromium (Cr⁶⁺) in Cr-based catalysts was a concern because Cr⁶⁺ is recognized as a hazardous substance and is regulated in various laws related to environmental protection. The active component of industrial catalysts is most often Cr³⁺ species that has a far less toxic profile than Cr⁶⁺ and is not classified and regulated as hazardous substances. It plays very important role in metabolism in human body and sold as a dietary supplement. This suggests that catalysts without Cr⁶⁺ can be sustainable products. Understanding of reaction pathway and mechanism of Cr⁶⁺ species formation would help the development of sustainable Cr⁶⁺-free CrOx/Al₂O₃ catalysts.

In this study authors investigated model catalysts with application of the TPD-MS, Raman spectroscopy, FTIR of pyridine adsorption, XRD, TPR and XAS. Based on this study the following conclusions were made:

1. Impregnation of Al₂O₃ carrier by H₂CrO₄ leads to formation of strong and weak aluminum-chromate on the surface. Only 2% of these species are stable up to 760ºC. Rest of them are decomposed during calcination to form Cr₂O₃.

2. The strongest stand-alone aluminum chromates species are produced by interaction of Cr⁶⁺ anions of impregnation solution with strongest Lewis’s acid sites of Al₂O₃.

3. 80% of the total Cr⁶⁺ species are formed during calcination in O₂ containing atmosphere by re-oxidation of Cr₂O₃ forming chromium-aluminum-chromates. Re-oxidation is possible only if Cr₂O₃ cluster is located on the interface of Al₂O₃ next to the strong Lewis’s acid sites.

4. This study provides directions for eliminating Cr⁶⁺ species in CrOx/Al₂O₃ catalyst to make this product sustainable.