Metal–Support Interactions in Rhodium Catalysts Supported on Tetravalent Metal Pyrophosphates (MP2O7; M = Si, Ti, and Zr)

A series of tetravalent metal pyrophosphates (MP2O7; M = Si, Ti, and Zr) were studied as support materials for rhodium (Rh) catalysts to elucidate the effect of the M–O–P bond covalency on the interfacial interaction with the Rh. Based on the experimental and density functional theory calculation results, the bond covalency strongly depends on the type of M, with the Si–O–P bond exhibiting stronger covalency, whereas more ionic characters were found for M = Ti and Zr. The Rh K-edge extended X-ray absorption fine structure revealed that the supported Rh nanoparticles formed an interfacial Rh–O–P bond via the interaction with the surface-terminated [P2O7] unit with a corner-shared bitetrahedral structure. The electronic state of the Rh was therefore affected by the M–O–P bond. According to the X-ray photoelectron spectroscopy analysis, the Rh exhibited an electron deficiency due to an electron-withdrawing effect from the [P2O7] unit, which had a Lewis acid character. The extent of electron deficiency was significantly larger when the Rh nanoparticles were supported on SiP2O7, which has a more covalent bonding character compared with ZrP2O7 and TiP2O7, thus leading to the lack of chemisorption capacity for CO. Consequently, the Rh/SiP2O7 catalyst was far less active in a model reaction of a stoichiometric CO–NO–C3H6–O2 mixture, whereas a steep light-off of the reaction was achieved at lower temperatures for the Rh/ZrP2O7 and Rh/TiP2O7 catalysts.