The high consumption of fossil fuels causes the emission of greenhouse gases and thus the enhancement of the global warming. One possible alternative is methanol, which can be used in a wide range of applications as basic chemical to substitute hydrocarbons originating from fossil fuels. Nowadays, methanol is industrially produced by the heterogeneously catalyzed conversion of synthesis gas (CO, CO2, H2) over Cu/ZnO-based catalyst systems at elevated temperatures and pressures. For a methanol production with a low CO2 footprint, exhaust gases from steel production are promising resources containing huge amounts of H2, CO and CO2. However, the level of purity is low due to the presence of many known and unknown impurities in the ppm as well as in the ppb range. Until now, the effects of most impurities on the catalytic activity, selectivity and stability cannot be predicted concerning the lack of studies in literature. Therefore, we injected the impurities of interest into the syngas stream at high-pressure conditions (60 bar) with a self-constructed high-pressure pulse unit (HPPU) to investigate deactivation rates under industrially relevant conditions. In the case of N-compounds like NH3, we identified reversible poisoning of methanol synthesis by temporary blocking of the active sites. Applying ethylene hydrogenation in parallel to methanol synthesis and pure CO hydrogenation to methanol allowed us to identify the debated nature of ZnO as positively charged Znδ+ species: the hydrogenation of ethylene to ethane on metallic Cu0 was not influenced by NH3, which did not poison the much slower methanol synthesis from CO and H2, either. Consequently, both hypothesis concerning the active Cu0(Znδ+Oδ-) site of the Cu/ZnO/Al2O3 catalyst developed by Behrens et al. and Lunkenbein et al. describe the most active site for methanol formation properly. In addition, the performed high-pressure pulse experiments (HPPE) coupled with a probe molecule can be seen as the first published operando investigation of the surface reactivity of a low-temperature methanol synthesis catalyst.

On the identification of the active site of the Cu/ZnO/Al2O3 methanol synthesis catalyst

2019 Fall National Meeting

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