Abstract
A set of novel materials, denoted M/ZrO2@C(MIL) (M = Ru, Rh, Pd & In), were prepared by thermal transformation of MIL-140C containing 10% bipyridine linkers (MIL-140C-10), to provide sites for metal coordination within the framework. These materials were transformed into active catalysts for CO2 hydrogenation when heated in a gas mixture of H2 and CO2 (3:1), at 500 °C. The thermal treatment provided high surface area catalysts with high stability and high Ru or Rh metal dispersion which were very effective for the hydrogenation of CO2 to CH4, giving a CH4 production of 3.0–3.7 mol/g Ru/h or 4.2–4.3 mol/g Rh/h (at 400 °C, 33 bar and WHSV 23 L/h/g cat.). PXRD, XPS and TEM indicated that the effective catalysts consisted of nanoparticles of Ru0 (2–5 nm) or Rh0 (6 nm) associated with larger ZrO2 nanoparticles (10–20 nm), which were dispersed upon carbonaceous ribbons. Interestingly, at 250-350 °C, Pd/ZrO2@C(MIL) yielded mainly CO rather than CH4, with some CH3OH. The CH4 and CO production were not stable at 400 °C. TEM results for this catalyst indicated Pd0 and ZrO2 nanoparticles (initially 20 nm and 10–20 nm diameter, respectively). The lower, unstable activity compared to Ru and Rh could have been due to the initially larger Pd particles and their tendency to grow in size with reaction time. In/ZrO2 has mainly been used to catalyse CH3OH production, but In/ZrO2@C(MIL) gave less CH3OH than In/monoclinic ZrO2 and was less selective. At 400 °C In/ZrO2@C(MIL) was a stable, reverse-water-gas-shift catalyst (producing 0.9 mol CO/g In/h at WHSV 20 L/g cat/h). The In was well dispersed in the ZrO2–C and of small particle size. The poor selectivity for methanol may have been due to the tetragonal phase of the ZrO2 and the low surface In concentration.
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•Ru, Rh, Pd, In loaded MIL-140C-10 gave metal-ZrO2-C, M/ZrO2@C(MIL), in CO2/H2 at 500 °C.•Ru and Rh formed small dispersed particles in Ru, Rh/ZrO2@C(MIL).•Ru, Rh/ZrO2@C(MIL) had stable, similar CO2 methanation activity to literature catalysts.•Pd/ZrO2@C(MIL) included large Pd particles, increasing in size during a catalytic run.•Pd particle features can explain poor, unstable CO2 methanation for Pd/ZrO2@C(MIL).