Template-free routes to macroporous monoliths of nickel and iron oxides: Toward porous metals and conformally coated pore walls

In our continuing efforts to prepare macroporous monoliths of functional inorganic materials through novel template-free routes, we present here routes to porous NiO starting from Ni-Zn oxalates, and to porous ZnFe2O4 starting from Zn-Fe oxalates. Monolithic Ni has been prepared from porous NiO by hydrogen reduction, with retention of the macropore structure. Heating porous ZnFe2O4 under high hydrogen flow rates simultaneously reduces the metals and transports zinc in the vapor phase, leaving behind a macroporous Fe monolith. Robust monoliths with pore dimensions ranging from 1 to 5 mu m have been formed in all cases, with narrow pore diameter distributions. To further functionalize the porous NiO monolith, we have dip-coated a lanthanum source and fired it in air to obtain a conformal coating of La4Ni3O10 particles on the surfaces of the NiO pore walls. Unlike conventional dip-coating, the skeletal support is itself a reactant in this process, ensuring that the coating is well-dispersed on the pore walls.