Influence of the support on rhodium speciation and catalytic activity of rhodium-based catalysts for total oxidation of methane

Abstract

The role of the support in Rh-based catalysts for total oxidation of CH₄ was investigated using both amorphous SiO₂ and γ-Al₂O₃ as well as ZSM-5 and SSZ-13 zeolites with varying SiO₂/Al₂O₃ ratios. The methane oxidation activity was measured in both the presence and absence of 5 vol% H₂O and 20 ppm SO₂. The support had a strong impact on Rh speciation (Rh₂O₃ nanoparticles vs. single Rh atom sites), which was important for the activity, as the nanoparticle sites were found to be far more active than the single atom sites. A 2 wt% Rh/ZSM-5(SiO₂/Al₂O₃ = 280) catalyst with Rh exclusively as Rh₂O₃ nanoparticles was able to provide oxidation at a 75 °C lower temperature than an ion-exchanged 0.294 wt% Rh/ZSM-5(SiO₂/Al₂O₃ = 30) catalyst with Rh exclusively as single atoms despite a similar concentration of active sites in the two samples. All the catalysts were inhibited by the presence of water, but this inhibition was particularly strong for an amorphous SiO₂ support and for the most Al-rich zeolites and less severe for Si-rich zeolites. The inhibition from SO₂ was relatively stronger for the more Al-rich supports, which was attributed to an uptake of sulfur at Al sites that was detrimental to the performance of the active phase. In a realistic gas atmosphere containing both H₂O and SO₂, Si-rich ZSM-5(SiO₂/Al₂O₃ = 280) therefore emerged as the best support. This was because the low acidity ensured minimal loss of Rh to the less active single atom sites, avoidance of the detrimental sulfur uptake by the support seen for more Al-rich supports, and avoidance of the strong water-induced inhibition that occurs for amorphous SiO₂ and Al-rich zeolites.