Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts

Abstract

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ ZnO/Al 2 O 3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu 0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al 2 O 3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO 2-containing syngas mixtures when pulsing NH 3 or methylamines as basic probe molecules.

Comparative study on the effect of different copper loading on catalytic behaviors and activity of Cu/ZnO/Al2O3 catalysts toward CO and CO2 hydrogenation

“…In both cases of CO and CO 2 hydrogenation, CZA-H catalyst exhibited lower coke deposition than CZA-L. According to the TGA analysis and catalytic activity results, it was suggested that the CZA-L catalyst with high ZnO content can migrate and generate the graphitic coke to block the active sites (Liang et al, 2019;Laudenschleger et al, 2020;Okemoto et al, 2020). Furthermore, large amounts of carbon deposition over CZA catalysts were formed by the decomposition of CO and CO 2 molecules with the reduction of ZnO and CuO, and Cu oxidation (Chiang et al, 2018), which was related to low catalytic activity as shown in Figure 6.…”

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction

“…Heterogeneous catalysts are dynamic entities and adapt their morphology and electronic structure to the chemical potential of the surrounding gaseous atmosphere 15,16 . Thus, for methanol synthesis catalysts, operando studies in pressure regimes that are industrially relevant (20-100 bar), as the ones discussed in the present work, are key to overcome the "pressure gap" 15,[17][18][19][20][21][22][23][24][25] , as they probe Cu and Zn under their actual working state, unraveling their active chemical and structural state. Investigations on Cu/ZnO systems revealed the presence of poorly crystalline ZnO interacting with Cu 15 , reversible changes of the Cu NP morphology depending on the reaction conditions 17,23 , CuZn bulk alloy formation under severe reduction conditions (600°C in CO/ H 2 ) 17,26 , and ZnO reduction and a stronger CuZn interaction for catalysts supported on SiO 2 [19][20][21] .…”

Methanol Synthesis by Hydrogenation of Hybrid CO2−CO Feeds

“…They show high activity, but their selectivity to methanol and stability are compromised by the competitive reverse water‐gas shift reaction (RWGS: CO 2 +H 2 ↔CO+H 2 O) and sintering mediated by the water byproduct, respectively. Replacing alumina by zirconia in this ternary system attained considerable gains on both fronts, since the latter increases the catalyst's CO 2 adsorption capability and hydrophobicity [1a,3b,4a–c,5] . Around the mid‐2010s, metal oxides with controlled vacancy chemistry were identified as extremely selective, though intrinsically less active, catalytic phases for CO 2 hydrogenation [1a,3b,5b] .…”