Structural transformations of highly porous nickel catalysts during ethanol conversion towards hydrogen

Abstract

In this work, we report a liquid-phase reduction method to prepare porous non-supported amorphous nickel catalysts with high surface areas (65–250 m2/g). A highly crystalline face center cubic Ni (fcc-Ni) catalyst with 110 m2/g surface area was also prepared by frontal crystallization of the amorphous nickel catalyst. The catalytic activity and stability of these catalysts for ethanol decomposition was investigated at different time on stream (TOS) to understand structural transformations occurring at the early stages of catalyst activation-deactivation. Activity vs. TOS results obtained at 473 K show that on the amorphous catalysts the conversion increases from about 50% to 60–75% reaching a steady value at ∼30 h TOS, which remains constant during the observed 96 h of TOS. The fcc-Ni catalyst initially exhibits a higher conversion (∼85%), however, it quickly deactivates to a conversion in the similar range as the amorphous catalysts. It is also shown that BET surface areas of amorphous catalysts decreases during hydrogen pretreatment at 473 K due to crystallization, grain growth, and sintering. The structure of amorphous catalysts continuously refines to form a combination of fcc-Ni and hexagonal close-packed nickel (hpc-Ni) phases, as well as nickel carbide (Ni3C) and carbon layers that stabilize catalytic activity. The structure of the fcc-Ni catalyst remains unchanged during the 96 h TOS experiment indicating that carbon deposition might cause its initial deactivation. At 523 K, the amorphous catalyst shows 100% conversion, which remains constant during 96 h of TOS, while the fcc-Ni crystalline catalyst initially exhibits 95% conversion and then slowly deactivates to ∼80% at 96 h TOS. Thus at 523K the stabilized amorphous catalyst does not deactivate under the same TOS compared to the crystalline fcc-Ni catalyst, showing that the active sites on these catalysts are different. The findings of this work suggest that the liquid-phase reduction method can be used to prepare active and stable catalysts for reactions involving decomposition of alcohols and hydrocarbons to produce hydrogen.