Water-gas shift activity of Cu surfaces and Cu nanoparticles supported on metal oxides

Oxide supported Cu catalysts show significant activity for the water-gas shift reaction (WGS, CO + H₂O → H₂ + CO₂) but their performance is not fully understood and is highly dependent on the synthesis conditions or the nature of the oxide support. This article describes a series of new studies examining the water-gas shift activity of Cu/MgO(1 0 0) surfaces and compares it to the activities found for pure copper systems, Cu nanoparticles in contact with well-defined surfaces of TiO₂, ZnO, MoO₂ and CeO₂, and Cu cations present in mixed-metal oxides. Catalytic tests performed over CuFe₂O₄, Ce_1-xCu_xO₂ or CuMoO₄ show significant WGS activity only when the Cu cations in the mixed-metal oxide are reduced to metallic copper. Thus, Cu nanoparticles were deposited on different oxide surfaces and their WGS activity was measured in a batch reactor (P_CO = 20 Torr; P_{H2 O} = 10 Torr; T = 575-650 K). The WGS activity of the Cu nanoparticles supported on MgO(1 0 0) was 2-3 times larger than that of Cu(1 0 0). Even better WGS catalysts were obtained when Cu was deposited on CeO₂(1 1 1) or TiO₂(1 1 0). An apparent activation energy of 13.8 kcal/mol was found for the WGS on Cu/MgO(1 0 0). This is smaller than the value of 15.2 kcal/mol observed on Cu(1 0 0), and substantially larger than the values of 7-9 kcal/mol seen for the apparent activation energies of the Cu/CeO₂(1 1 1) and Cu/TiO₂(1 1 0) catalysts. Post-reaction surface characterization pointed to the lack of O vacancies in the Cu/MgO(1 0 0) catalysts. This is in contrast to results found for Cu/CeO₂(1 1 1) and Cu/TiO₂(1 1 0), where the oxide support exhibits a significant concentration of O vacancies as a consequence of the WGS reaction. The oxygen vacancies present in Cu/CeO₂(1 1 1) and Cu/TiO₂(1 1 0) help in the dissociation of the water molecule and reduce the apparent activation energy for the WGS process. Such a phenomenon cannot occur on the Cu/MgO(0 0 1) catalysts, and the main steps of the WGS probably take place on the Cu nanoparticles.