(544j) Unraveling Surface State and Composition of Highly Selective Nanocrystalline Ni-Cu Alloy Catalysts for Hydrodeoxygenation of HMF

The selective hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) is an important step in cellulosic biomass upgrading to biofuels, where bimetallic oxophilic catalysts have shown promising performance. Well controlled, bimetallic NiCu and NiCu₃ nanocrystals (NCs) supported on carbon are shown to give high yields and selectivities to DMF. To shed light on the active phase, Near-Ambient-Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) was used to characterize the surface composition of these highly selective, base-metal catalysts under the reducing conditions relevant to the HDO reaction. Reactions were performed in a continuous flow reactor at 33 bar and 180 °C. The Ni alloys were significantly more selective for DMF compared to monometallic Ni or Cu catalysts. The nanocrystal NiCu₃/C catalyst exhibited a maximum DMF yield of 98.7% with well-controlled surface composition. NAP-XPS characterization showed that the Ni-Cu nanocrystals were completely reduced below 250 °C in H₂, implying that the catalysts are completely reduced under reaction conditions. The reducing of surface metal suggests that high DMF yields can be obtained even in the absence of oxidic overlayers. The NAP-XPS also indicated that the NiCu₃ nanocrystal structure consisted of a Cu-rich core and a 1:1 molar Ni:Cu shell.