Precious metal catalysts in the clean-up of biomass gasification gas part 2: Performance and sulfur tolerance of rhodium based catalysts

Rh, Pt, and Pt-Rh catalysts on modified commercial zirconia support (m-ZrO2) were screened for the clean-up of gasification gas from tar, methane, and ammonia both in the absence and presence of H2S while varying the Rh metal content from 0.5 to 5 w-%. Our goal was to optimize the composition of the Rh/m-ZrO2 catalyst in view of the production of ultra clean gas applicable for liquid biofuels synthesis. In the presence of 100 ppm sulfur, increasing Rh concentration from 0.5 to 5 w-% did not greatly improve the activity of the catalyst. The bimetallic Pt/Rh/m-ZrO2 catalyst was also less active than the 0.5 w-% Rh/m-ZrO2 catalyst. Furthermore, the Rh/m-ZrO2 catalyst regained its performance at the set point of 800 °C when the sulfur feed was turned off even after exposures to 500–1000 ppm sulfur. Our data allow us to suggest that in the presence of sulfur, the active sites responsible for the reforming reactions are poisoned, but less impact occurs on sites responsible for oxidation reactions. Furthermore, the screening experiments allow to suggest that the Rh/m-ZrO2 catalyst could be applicable to hot gas cleaning in the presence of sulfur (> 50 ppm) at above 800 °C using a moderate gas hourly space velocity of approximately 3400 1/h. Since biomass gasification gas generally contains sulfur, the 0.5 w-% Rh/m-ZrO2 catalyst could be a promising option for gasification gas clean-up applications at temperatures above 800 °C where it reduces tar to very low levels.

Research highlights
► We studied Rh/m-ZrO2 and Rh/Pt/m-ZrO2 catalysts in clean-up of gasification gas.
► Increase in Rh concentration did not greatly improve activity with H2S in the feed.
► Bimetallic Pt/Rh/m-ZrO2 catalyst was less active than the 0.5 w-% Rh/m-ZrO2.
► Rh/m-ZrO2 regained its performance at 800°C even after 500 ppm H2S exposures.
► For high tar conversion with Rh/m-ZrO2, >800 °C and moderate GHSV are required.