Synthesis Gas Conversion over Rh-Mn-WxC/SiO2 Catalysts Prepared by Atomic Layer Deposition

Yifei Liu, Lifeng Zhang, Florian Göltl, Madelyn R. Ball, Ive Hermans, Thomas F. Kuech, Manos Mavrikakis, James A. Dumesic

Research output: Contribution to journalArticlepeer-review

16 Scopus citations

Abstract

The catalytic conversion of synthesis gas to value-added oxygenates and light hydrocarbons was studied on Rh and Rh-Mn clusters on tungsten carbide-overcoated silica (WxC/SiO2) at 523 K, 580 psi, and CO/H2 = 1/1. The WxC-modified SiO2 support was prepared by the overcoating of WOx on SiO2 using atomic layer deposition (ALD) followed by carburization. The reactivities of Rh/WxC/SiO2 catalysts with varying number of ALD cycles (number of cycles = 2, 5, 10, 20, and 30) were measured and showed that 5 ALD cycles of WxC suppressed the formation of methane. All WxC-modified catalysts showed enhancement in selectivity toward methanol and ethanol through CO hydrogenation and acetaldehyde hydrogenation, respectively. These catalysts also improved the overall turnover frequency (TOF). The addition of Mn species further promoted the activity and the selectivity toward ethanol and C2+ hydrocarbons, especially light alkenes. The best performing Rh-2Mn/5cycle-WxC/SiO2 catalyst (Rh:Mn molar ratio = 1:2) achieved 84.7% selectivity toward valuable oxygenates and C2+ hydrocarbons with a ratio of alkenes to alkanes equal to 1.7, compared to Rh/SiO2 which exhibited 80.4% selectivity toward the products aforementioned with a ratio of 0.5. The overall TOF was 20 times higher than that over Rh/SiO2 (i.e., 5.6 × 10-2 s-1 vs 2.9 × 10-3 s-1). X-ray diffraction revealed that the existence of W2C, which was the dominant phase in Rh/5cycle-WxC/SiO2, favored the suppression of methane and enhanced the production of alcohols and C2+ hydrocarbons compared to the WC support. Density functional theory calculations for Rh19, Rh31, and Rh37 clusters on various WC surfaces suggested that the shape of Rh clusters is condition dependent and subject to H2 pressure. The C- and O- binding energies on various sites for the clusters were used with scaling relations to probe their catalytic activity. With the use of this approach, the increase in the O binding energy when moving from the SiO2-supported Rh37 cluster to the WC-supported cluster leads to an increase in the activity of Rh/WxC/SiO2 at the expense of the reduction in the number of sites that are selective toward C2 oxygenates.

Original languageEnglish (US)
Pages (from-to)10707-10720
Number of pages14
JournalACS Catalysis
Volume8
Issue number11
DOIs
StatePublished - Nov 2 2018
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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