CO2 conversion process efficiency comparison
Table 1 summarizes some of the best (to our knowledge) CO2 (from the air or pure) conversion processes under ambient conditions.11,33-36 Our Mg process produced 314 μmol g−1 h−1 total products when pure CO2 was used, which was significantly higher than those produced by the best-known catalysts, Pd7Cu1/TiO2 (19.6 μmol g−1 h−1),35 or Rb0.33WO3 (15 μmol g−1 h−1),33 both using high-power UV light (Table 1, S. N. 3 and 4). The Mg-assisted CO2 conversion efficiency was also better than that of Au-catalysts (0.063-0.13 μmol g−1 h−1, Table 1, S. N. 6-8).11,34,36 Even for CO2 directly from the air, Mg showed a productivity of 59 μmol products g−1 h−1, while the highest reported value so far was 2.25 μmol products g−1 h−1 using Rb0.33WO3, which requires high power UV light (Table 1, S. N. 5). Thus, Mg NPs are a very good alternative to photocatalysts for sustainable CO2 to fuel conversion under ambient conditions.
The comparison of a non-catalytic process with catalytic ones seems inappropriate. However, this is intentionally made to demonstrate that this Mg process, even being not catalytic, is still efficient and more sustainable than reported room temperature catalytic processes. For example, the Mg protocol yields 100 μmol g−1 methane in 0.4 h and to achieve the same methane yield, an Au catalyst needed 1587 h, i.e. 66 days (and solar energy for 66 days).34 None of the reported room temperature nano-catalysts showed stability for 1587 h and were often deactivated in just 12 to 24 h (Table 1). Thus, catalysts are not-consumed by definition, but in reality, most of them get deactivated in a few hours (“indirectly consumed”). The deactivated catalysts can be ideally regenerated/activated, but this has a substantial energy penalty and hydrogen gas consumption (and in the case of deactivation by nanoparticle sintering, no regeneration is possible). In contrast, Mg regeneration can be carried out by using a solar energy-driven laser without any hydrogen gas and for only 1 USD per kg.21
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To prove that CO2 was indeed the carbon source of products, a control experiment was conducted in argon gas (without CO2), in which the formation of methane, methanol and formic acid was not observed. For further confirmation, we used isotopically labeled 13CO2 and analyzed the products by GC (Fig. S2†) and 13C NMR spectroscopy (Fig. 2a). The 13C NMR spectrum (Fig. 2a) indicated methanol and formic acid. As a further control experiment, unlabeled CO2 was used. The formation of methanol and formic acid was evident from 1H NMR, whereas no 13C signal was obtained with the same NMR pulse sequence and the total number of scans (Fig. S1b†). This was due to the low natural abundance (1%) of 13C in unlabeled CO2 (Fig. S1b†). The reaction products were monitored after 0, 1, 5, 15, 30 and 60 min reaction time, using a GC with TCD, flame ionization detector (FID) and mass spectrometer (MS). The reduction of 13CO2 produced isotopic 13CH4 (m/z = 17, Fig. S1g†), which confirmed CO2 as a carbon source for methane, methanol, and formic acid.
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