In heterogeneous catalysis catalyst activation is often observed during the reaction process, which is mostly attributed to the induction by reactants. In this work we report that surface structure of molybdenum nitride (MoN_x) catalyst exhibits a high dependency on the partial pressure or concentration of reaction products i.e., CO and H₂O in reverse water gas-shift reaction (RWGS) (CO₂:H₂ = 1:3) but not reactants of CO₂ and H₂. Molybdenum oxide (MoO_x) overlayers formed by oxidation with H₂O are observed at reaction pressure below 10 mbar or with low partial pressure of CO/H₂O products, while CO-induced surface carbonization happens at reaction pressure above 100 mbar and with high partial pressure of CO/H₂O products. The reaction products induce restructuring of MoN_x surface into more active molybdenum carbide (MoC_x) to increase the reaction rate and make for higher partial pressure CO, which in turn promote further surface carbonization of MoN_x. We refer to this as the positive feedback between catalytic activity and catalyst activation in RWGS, which should be widely present in heterogeneous catalysis.

The production of fuel from the hydrodeoxygenation of vegetable oils has been extensively investigated on account of the decline of petroleum-based fuels and increase of ecological problems. The conversion of jatropha oil over Al-MCM-41-supported Ni, W, and Ni–W catalysts was studied at 3 MPa and 360 °C. Over the monometallic Ni and W catalysts, the biofuel yield was low, about 19.3 and 12.5 wt %, respectively, whereas the highest biofuel yield reached 63.5 wt % over the Ni–W bimetallic catalysts. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and high-resolution TEM results suggested that the proper amount of Ni and W would form a Ni₁₇W₃ active phase, the particle size of which varied with the content of Ni and W or preparation methods. The crystalline Ni₁₇W₃ phase formed when the content of both Ni and W reached 10%. With further increase of the content of W or Ni to 15%, the crystal size of Ni₁₇W₃ grew from 7 to 14 nm or to 20 nm, whereas the biofuel yield decreased with the increase of the Ni₁₇W₃ crystal size. The 10Ni–10W/Al-MCM-41 catalyst with the Ni₁₇W₃ crystal size of 7 nm showed the best performance for the transformation of jatropha oil into high-grade biofuel.