Ambient-temperature NO oxidation over amorphous CrOx-ZrO2 mixed oxide catalysts: Significant promoting effect of ZrO2

• Novel amorphous CrOx-ZrO2 mixed oxide materials are synthesized.
• 100% NO conversion at ambient temperature for extended periods.
• Deactivated catalysts can be fully regenerated with a simple heat treatment.
• ZrO2 is found to stabilize Cr6+, the key redox sites in the mixed oxides.

A series of novel CrOx-ZrO2 mixed oxide catalysts are prepared via a sol-gel method. Within a range of Cr/Zr atomic ratios, the mixed oxides maintain high surface area, homogeneous amorphous phases. As compared to CrOx-only catalysts formed using the same method, the addition of zirconia greatly enhances the catalytic performance for ambient-temperature, low-concentration NO oxidation. X-ray Photoelectron Spectroscopy (XPS) and Electron Paramagnetic Resonance (EPR) analyses indicate an electronic effect of ZrO2 addition to the oxidation state of Cr. That is, ZrO2 addition induces an increase in surface concentrations of Cr6+. Rapid deactivation of a pre-reduced catalyst, coupled with the fact that a deactivated catalyst contains lower concentrations of surface Cr6+, provide rather strong evidence for a Mars-van Krevelen NO oxidation mechanism. Such a mechanism is also consistent with in situ DRIFTS observations.

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