In situ diffuse reflectance infrared (DRIFTS) identification of active sites in the CO + H2 reaction over lanthanide-modified Rh/Al2O3 catalysts

The CO/H2 reaction over a series of lanthanide-oxide (Ln2O3, Ln = La, Sm, Yb, Lu) modified Rh/Al2O3 catalysts is employed as a test reaction to show the capabilities of the DRIFTS technique in the identification of reaction sites. Methane, heavier hydrocarbons, methanol, ethanol and small amounts of formaldehyde are obtained in this process. Heavy hydrocarbons and methanol present a maximum in activity around 270°C while the maximum is located around 210°C for ethanol and HCOH. The addition of lanthanide oxides produces a decrease in the total activity of the Rh/Al2O3 catalyst. However, the selectivity towards heavy hydrocarbons and oxygenated products is increased. The DRIFTS spectra under reaction conditions have shown a relationship between the presence of a (CO) stretching band at 2030 cm−1 and the activity towards methanol. Ethanol production is however correlated with another band at 2010 cm−1. Rh+ ions on large metallic clusters are proposed as the active centres for methanol production. However, rhodium sites interacting with Ln3+ ions at the metal/support interface and in the vicinity of a partially hydrogenated carbon residue are suggested as the centres for the synthesis of ethanol.