Rb-Promoted Molybdenum Carbide Nano-Catalysts for Higher Alcohol Synthesis from Syngas

The catalytic conversion of synthesis gas derived from coal, natural gas or renewable biomass is among the most promising routes to produce alternative energy carriers and chemical feedstocks. Although production of diesel fuel and methanol from syngas has been well-developed on a world scale, there is still a growing need for the direct synthesis of higher (C2+) alcohols for use as fuel additives or chemical precursors. When properly promoted by alkali metals, molybdenum carbide is considered to be a promising catalyst for this process. In this study, molybdenum carbide nanoclusters (1-3 nm) were synthesized on various supports using wet impregnation. The selectivity of catalytic syngas conversion toward C2+ alcohols was enhanced with added Rb2CO3 promoter. Although the alcohol selectivity decreased with CO conversion, the loss can be partially recovered with addition of a cobalt promoter. In situ X-ray absorption spectroscopy (XAS) was conducted in a self-built high-pressure reactor cell operating at syngas reaction conditions (573 K, 30 bar syngas), which revealed the existence of surface oxygen on Mo2C and the structural modification of Rb2CO3 promoter in CO hydrogenation. An air-free sample preparation method was developed for ex situ XAS analysis, which allowed direct evidence of surface oxidation of Mo2C caused by passivation to be observed. The reduction of highly dispersed cobalt domains (< 2 nm) in co-existence with Mo2C was also observed by XAS.
Diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) was used to characterize the surface of Mo2C nanoclusters at working conditions. With CO as a probe, DRIFTS unveiled the structure-function relationship between the Rb promoter and the catalyst for the first time. A strong correlation between the red-shifted band of adsorbed CO and the elevated alcohol selectivity was observed. More importantly, experimental results from probe reactions and IR spectroscopy, combined with the first-principle theoretical calculations (performed in collaboration with Georgia institute of Technology), suggested that the basic Rb2CO3 promoter likely interacted with the Mo2C catalyst by neutralizing the acidic hydroxyl groups on the catalyst surface.
Finally, an advanced multi-component system for steady-state isotopic transient kinetic analysis (SSITKA) was constructed to allow the measurement of fundamental kinetic parameters. The multi-product SSITKA revealed that the intrinsic turnover frequency (TOF) on the “slow” Mo2C catalysts was in fact comparable to other active catalysts for CO hydrogenation such as those based on Fe and Rh. Consequently, the very small coverage of the reactive intermediates on Mo2C surface was discovered to be the reason for its low apparent activity in CO hydrogenation.