Enhancement in catalytic performance of birnessite-type MnO2-supported Pd nanoparticles by the promotional role of reduced graphene oxide for toluene oxidation

Abstract

Birnessite-type MnO₂ (δ-MnO₂)-supported reduced graphene oxide (rGO)-promoted palladium (xPd–yrGO/δ-MnO₂) samples were prepared using a polyvinyl alcohol-protected reduction method. The physicochemical properties of the catalysts were determined by means of numerous techniques, and their catalytic activities for the oxidation of toluene were evaluated. It was found that the introduction of rGO as an electron transfer bridge could significantly strengthen the metal–support interaction (SMSI) between Pd and δ-MnO₂, thereby enhancing the catalytic activity of the sample. Among all of the xPd–yrGO/δ-MnO₂ samples, 0.48Pd–1.0rGO/δ-MnO₂ performed the best: the toluene oxidation rates were 3.41 × 10⁻⁴ and 6.13 × 10⁻⁴ mol g⁻¹ s⁻¹ at 177 and 189 °C, respectively, with a space velocity of 60 000 mL g⁻¹ h⁻¹, which were much higher than that of 0.45Pd/δ-MnO₂ at similar temperatures. The doping of rGO led to an increase in the Mn³⁺/Mn⁴⁺ or O_ads/O_latt molar ratio, thus enhancing the catalytic activity of Pd/δ-MnO₂. The good performance of 0.48Pd–1.0rGO/δ-MnO₂ was associated with its high O_ads/O_latt molar ratio, good low-temperature reducibility, and strong interaction between Pd and δ-MnO₂. In situ DRIFTS results revealed that benzaldehyde and benzoate were the intermediate products of toluene oxidation over 0.48Pd–1.0rGO/δ-MnO₂. The physicochemical properties of the 0.48Pd–1.0rGO/δ-MnO₂ sample were not changed significantly after 70 h of stability test, but the activity of this catalyst decreased in the presence of CO₂, SO₂ or NH₃, which was due to the carbonate, sulfate and ammonia species formed on the surface of 0.48Pd–1.0rGO/δ-MnO₂ that covered the active sites and oxygen vacancies and thereby reduced the ability to bind with toluene.