Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate pyrochlores for higher alcohol synthesis

Two Rh-based lanthanum zirconate pyrochlores (La2Zr2O7; LZ) were prepared by the Pechini method and tested for the synthesis of higher alcohols via CO hydrogenation. In one, Rh was substituted into the pyrochlore lattice (LRZ, 1.7 wt%) while for the second, Rh was supported on an unsubstituted La2Zr2O7 (R/LZ, 1.8 wt%). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) results show that the surface reducibility depends on whether the Rh is in (or supported on) the LZ pyrochlore. The total hydrogen consumption in TPR for LRZ (0.28 mg H2/gcat) is much greater than R/LZ (0.07 mg H2/gcat), likely due to the presence of a perovskite phase in the LRZ (LaRhO3; identified by XRD), in which rhodium is more reducible than in R/LZ. The formation of the perovskite accompanies that of the pyrochlore in the synthesis process, which includes heat treatment up to 1000 °C. CO hydrogenation results show higher ethanol selectivity for R/LZ than LRZ, possibly due to the strong interaction between Rh and LZ on the R/LZ, forming atomically close Rh+/Rh0 sites, which have been suggested to favor ethanol production.

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► Rh-based lanthanum zirconate pyrochlore materials (LZ) were prepared, characterized, and used to convert syngas into alcohols.
► Rh-supported pyrochlore catalyst (R/LZ) shows more ethanol-to-methanol ratio compared to Rh-substituted (LRZ) counterpart.
► XPS and TPR show that alcohol selectivity is very sensitive to the location, stability and oxidation state of Rh in structure.