New catalytica reports

Abstract

Nickel-iron alloy particles (Ni:Fe = 10:1) were supported on titania and alumina and studied under methanation reaction conditions: temperatures from 500 to 565 K, H2:CO ratios from 3 to 15, and pressures near atmospheric. In situ Mössbauer spectroscopy was used to monitor the state of iron in these catalysts after reduction in H2 at 713 and 773 K. For both supports, a significant fraction of the iron was alloyed with nickel after treatment in H2, while the remainder was associated with the support as Fe2+. The extent of alloy formation was greater for the titania-supported samples, indicating that iron is more reducible on titania than on alumina. Under methanation reaction conditions, some of the iron from the NiFe was oxidized to Fe2+. For titania-supported samples, this process was partially reversible upon switching to H2 at the same temperature, while the Fe2+ could not be re-reduced on alumina even after treatment in H2 at 773 K. The presence of Fe2+ under methanation reaction conditions suggests that titanium is present as Ti4+. Compared to NiFe on alumina, the titania-supported NiFe particles showed effects of “strong metal-support interactions” manifested by (i) higher methanation activity, (ii) higher activation energy and hydrogen partial pressure dependence of the methanation rate, (iii) slower deactivation under methanation reaction conditions, and (iv) greater selectivity toward higher hydrocarbons. These effects are explained by the presence of titania species (TiOx) on the surface of the titania-supported NiFe particles. These species decrease the extent of carbon deposition on the metal surface, thereby increasing the amount of “active” carbon at the expense of “inactive” carbon. The probability of carbon chain-growth on the titania-supported NiFe particles can also be increased by addition of water to the H2/CO feed.