Single-phase mixed molybdenum-niobium carbides were synthesized to establish structure-activity relationships. Precursors for carburization were obtained by hydrothermal synthesis or, to achieve high molybdenum content (metal fraction chi(Mo) = 0.86), by flash-freezing of salt solutions and subsequent freeze-drying. Thermogravimetric and evolved gas analysis during carburization showed that bimetallic precursors were more easily reduced than monometallic ones; and as the niobium content increased, the removal of oxygen shifted to higher temperatures and from H2O to CO formation (from H-2 or CH4, respectively), and the final carburization temperature rose from 650 to 950 degrees C. Carbides crystallized in cubic NbC/MoC(cF8) structure for chi(Nb) >= 0.38 and hexagonal Mo2C(hP3) structure for chi(Nb) <= 0.14. After passivation, mixed metal carbides could be reduced at lower temperatures than Mo2C. With increasing molybdenum content of the carbides, CO uptake per gram increased, and turnover frequencies for hydrogenation of toluene to methylcyclohexane increased from 0 to 3.1 s(-1) (at a temperature of 250 degrees C, 21 bar pressure, and H-2/toluene = 36). At 400 degrees C, mixed carbides with x(Nb) >= 0.38 were more selective toward acid-catalyzed products and less selective toward hydrogenolysis products than carbides with lower niobium content. (C) 2017 Elsevier Inc. All rights reserved.