Role of surface reconstruction on Cu/TiO₂ nanotubes for CO₂ conversion

Carbon dioxide hydrogenation to CO via the reverse water gas shift (RWGS) reaction is one route to integrate CO₂ utilization into the chemical industry. TiO₂ supported Cu catalysts are known to be active for RWGS, but Cu is shown here to behave differently on TiO₂ nanotubes (TiNT) vs TiO₂ nanoparticles (TiNP). Whereas nanoparticle supports give low rates that are hardly changed by added Cu, the nanotube supports yield much higher activity and three distinct behaviors as the Cu surface density increases. At low surface densities (0.3 Cu/nm²), active Cu-O-Ti sites are created that have low apparent activation energies. At high surface densities (6 Cu/nm²), Cu nanoparticles on TiNT are formed, and reaction barriers are lowered when both Cu and TiNT surfaces are accessible. At intermediate surface densities, metallic Cu domains are engulfed by a TiOx overlayer formed during H₂ pretreatment, akin to those formed by classical strong metal support interactions (SMSI). These reduced layers are markedly more active for RWGS than the initial TiNT surfaces, but have similar activation barriers, which are higher than those for which both Cu and TiNP surfaces are exposed. These catalytic findings are supported by computational modeling, in situ IR, UV–vis, and X-ray absorption spectroscopies, and they provide insight into an important reaction for CO₂ utilization.