PdPtVO_x/CeO₂-ZrO₂: Highly efficient catalysts with good sulfur dioxide-poisoning reversibility for the oxidative removal of ethylbenzene

The PdPtVO_x/CeO₂-ZrO₂ (PdPtVO_x/CZO) catalysts were obtained by using different approaches, and their physical and chemical properties were determined by various techniques. Catalytic activities of these materials in the presence of H₂O or SO₂ were evaluated for the oxidation of ethylbenzene (EB). The PdPtVO_x/CZO sample exhibited high catalytic activity, good hydrothermal stability, and reversible sulfur dioxide-poisoning performance, over which the specific reaction rate at 160°C, turnover frequency at 160°C (TOF_{Pd or Pt}), and apparent activation energy were 72.6 mmol/(g_Pt⋅sec) or 124.2 mmol/(g_Pd⋅sec), 14.2 sec^-1 (TOF_Pt) or 13.1 sec^-1 (TOF_Pd), and 58 kJ/mol, respectively. The large EB adsorption capacity, good reducibility, and strong acidity contributed to the good catalytic performance of PdPtVO_x/CZO. Catalytic activity of PdPtVO_x/CZO decreased when 50 ppm SO₂ or (1.0 vol.% H₂O + 50 ppm SO₂) was added to the feedstock, but was gradually restored to its initial level after the SO₂ was cut off. The good reversible sulfur dioxide-resistant performance of PdPtVO_x/CZO was associated with the facts: (i) the introduction of SO₂ leads to an increase in surface acidity; (ii) V can adsorb and activate SO₂, thus accelerating formation of the SO_x^2- (x = 3 or 4) species at the V and CZO sites, weakening the adsorption of sulfur species at the PdPt active sites, and hence protecting the PdPt active sites to be not poisoned by SO₂. EB oxidation over PdPtVO_x/CZO might take place via the route of EB → styrene → phenyl methyl ketone → benzaldehyde → benzoic acid → maleic anhydride → CO₂ and H₂O.