Active site of copper-zinc methanol synthesis catalysts
In plain words
Most of the world's methanol is made from carbon oxides and hydrogen over copper particles mixed with zinc oxide. Whether the reaction runs on copper steps decorated with zinc, on a copper-zinc surface alloy, or at the boundary with zinc oxide is still argued.
Precise statement
For Cu/ZnO/Al2O3 under industrial conditions (about 500 K, $5e7\ \text{to}\ 1e8\ \mathrm{dyn}/\mathrm{cm}^{2}$ of CO/CO2/H2), identify the site responsible for most methanol turnover: Zn-decorated Cu steps (Behrens et al., Science 2012, https://doi.org/10.1126/science.1219831), a Cu-Zn surface alloy whose Zn coverage sets the rate (Kuld et al., Science 2016, https://doi.org/10.1126/science.aaf0718), or the Cu-$\mathrm{ZnO}_{x}$ interface, and whether the site differs for CO2 and CO hydrogenation. An answer is operando identification of site structure correlated with site-normalized rates and reproduced by first-principles kinetics.
What would settle it
Operando spectroscopy and microscopy counting candidate sites under reaction conditions, correlated with turnover and with computed kinetics.
Status in the literature
Competing assignments published in 2012, 2016 and 2020 (Laudenschleger, Ruland, Muhler, Nature Communications, https://doi.org/10.1038/s41467-020-17631-5) have not been reconciled.