Steering the methanol steam reforming reactivity of intermetallic Cu-In compounds by redox activation: stability vs. formation of an intermetallic compound-oxide interface

Abstract

To compare the inherent methanol steam reforming properties of intermetallic compounds and a corresponding intermetallic compound-oxide interface, we selected the Cu-In system as a model to correlate the stability limits, self-activation and redox activation properties with the catalytic performance. Three distinct intermetallic Cu-In compounds - Cu₇In₃, Cu₂In and Cu₁₁In₉ - were studied both in an untreated and redox-activated state resulting from alternating oxidation-reduction cycles. The stability of all studied intermetallic compounds during methanol steam reforming (MSR) operation is essentially independent of the initial stoichiometry and all accordingly resist substantial structural changes. The inherent activity under batch MSR conditions is highest for Cu₂In, corroborating the results of a Cu₂In/In₂O₃ sample accessed through reactive metal-support interaction. Under flow MSR operation, Cu₇In₃ displays considerable deactivation, while Cu₂In and Cu₁₁In₉ feature stable performance at simultaneously high CO₂ selectivity. The missing significant self-activation is most evident in the operando thermogravimetric experiments, where no oxidation is detected for any of the intermetallic compounds. In situ X-ray diffraction allowed us to monitor the partial decomposition and redox activation of the Cu-In intermetallic compounds into Cu0.9In0.1/In₂O₃ (from Cu₇In₃), Cu₇In₃/In₂O₃ (from Cu₂In) and Cu₇In₃/Cu0.9In0.1/In₂O₃ (from Cu₁₁In₉) interfaces with superior MSR performance compared to the untreated samples. Although the catalytic profiles appear surprisingly similar, the latter interface with the highest indium content exhibits the least deactivation, which we explain by formation of stabilizing In₂O₃ patches under MSR conditions.