Insight into the Nature of the ZnO x Promoter during Methanol Synthesis

Abstract

Despite the great commercial relevance of zinc-promoted copper catalysts for methanol synthesis, the nature of the Cu-ZnO _x synergy and the nature of the active Zn-based promoter species under industrially relevant conditions are still a topic of vivid debate. Detailed characterization of the chemical speciation of any promoter under high-pressure working conditions is challenging but specifically hampered by the large fraction of Zn spectator species bound to the oxidic catalyst support. We present the use of weakly interacting graphitic carbon supports as a tool to study the active speciation of the Zn promoter phase that is in close contact with the Cu nanoparticles using time-resolved X-ray absorption spectroscopy under working conditions. Without an oxidic support, much fewer Zn species need to be added for maximum catalyst activity. A 5-15 min exposure to 1 bar H₂ at 543 K only slightly reduces the Zn(II), but exposure for several hours to 20 bar H₂/CO and/or H₂/CO/CO₂ leads to an average Zn oxidation number of +(0.5-0.6), only slightly increasing to +0.8 in a 20 bar H₂/CO₂ feed. This means that most of the added Zn is in a zerovalent oxidation state during methanol synthesis conditions. The Zn average coordination number is 8, showing that this phase is not at the surface but surrounded by other metal atoms (whether Zn or Cu), and indicating that the Zn diffuses into the Cu nanoparticles under reaction conditions. The time scale of this process corresponds to that of the generally observed activation period for these catalysts. These results reveal the speciation of the relevant Zn promoter species under methanol synthesis conditions and, more generally, present the use of weakly interacting graphitic supports as an important strategy to avoid excessive spectator species, thereby allowing us to study the nature of relevant promoter species.

Figure 1

Representative EM images of the (A–C) CuZn-15/SiO₂ and (D–F) CuZn-15/C catalysts. Frames A and D involve BF-TEM, and frames B–C and E–F involve HAADF-STEM with an elemental map overlay. Number-averaged Cu(Zn)O_x particle sizes are 3.4 ± 0.8 nm (frames B–C) and 4.2 ± 1.7 nm (frame D) for the fresh CuZn-15/SiO₂ and CuZn-15/C catalysts, respectively. The used catalysts (frames C and F) are after 150 and 100 h of catalysis in an H₂/CO/CO₂ feed, respectively. Please note that the pixel size in frame F is larger (521 pm) than in frames B, C, and E (368 pm). Corresponding zoomed-out images and EDX spectra are shown in Figures S5–S6.

Figure 2

Methanol formation rate of the CuZn-15/SiO₂ (red circles) and CuZn-15/C (black squares) catalysts in a CO₂-free (open symbols) or -enriched (filled symbols) syngas feed. The data points of the CuZn-15/C catalyst in H₂/CO and H₂/CO/CO₂ are the average over 4 and 2 separate runs, respectively. Conditions: 533 K, 40 bar(g), H₂/CO/He = 60/30/10 vol % or H₂/CO/CO₂/He = 60/27/3/10 vol %.

Figure 3

CO (+ CO₂) conversion of silica- and carbon-supported ZnO_x (10 wt %) in various syngas compositions. Label “+3% CO₂” in the total feed corresponds to a CO₂/(CO + CO₂) volume fraction of 0.10. Conditions: 533 K, 40 bar(g), H₂/(CO + CO₂)/He = 60/30/10 vol %, 21.9 mL min^–1 g_cat^–1.

Figure 4

Initial methanol turnover frequency (TOF_MeOH) of CuZnO_x/C (black squares) and metal oxide-supported (red circles) catalysts in an H₂/CO (open symbols) or an H₂/CO/CO₂ (filled symbols) feed (at t = 0). “SiO₂” = CuZn-15/SiO₂ catalyst. “com cat” = commercial, coprecipitated Cu/ZnO/Al₂O₃/MgO catalyst (58 wt % Cu, ca. 10 nm CuO particles). Conditions: 533 K, 40 bar(g), H₂/CO/He = 60/30/10 vol % or H₂/CO/CO₂/He = 60/23/7/10 vol %.

Figure 5

Ex situ reduction in 0.5 L min^–1 g_cat^–1 flow of 5 vol % H₂/Ar at 5 K min^–1 in 1 bar, mimicking the conditions used during in situ H₂ treatment monitored by XAS.

Figure 6

(A,B) Time-resolved, normalized absorption and (C,D) corresponding first derivatives of in situ XANES spectra at the Zn K-edge of the (A,C) CuZn-15/SiO₂ and (B,D) CuZn-15/C catalysts (solid lines). The spectra are depicted in the initial state at 298 K, during a treatment in 20 vol % H₂/He up to 543 K in 1 bar each ca. 5.7 min, and finally in an H₂ atmosphere at 453 K. Dashed lines show the first derivatives of macrocrystalline ZnO, Zn₂SiO₄, and Zn foil references at 298 K.

Figure 7

(A,B) Normalized absorption and (C,D) corresponding first derivatives of operando, normalized XANES spectra at the Zn K-edge of the (A,C) CuZn-15/SiO₂ and (B,D) CuZn-15/C catalysts (solid lines). Depicted during H₂/CO (and subsequent H₂/CO/CO₂) conversion at 20 bar and 533 K, each after 160 min. Gas compositions: H₂/CO/He = 60/30/10 vol % and H₂/CO/CO₂/He = 60/27/3/10 vol %. Dashed lines show the initial catalyst state (ZnO), macrocrystalline Zn₂SiO₄ and Zn₃₀Cu₇₀, and Zn foil at 298 K.

Figure 8

Fourier-transformed EXAFS spectra at the Zn K-edge of the (A,C) CuZn-15/SiO₂ and (B,D) CuZn-15/C catalysts (solid lines). (A,B) Depicted during in situ reduction in the initial state at 298 K and in an H₂ atmosphere at 453 K after an H₂ treatment at 1 bar (for conditions, see Figure 6). (C,D) Depicted during H₂/CO (and subsequent H₂/CO/CO₂) conversion at 533 K and 20 bar (for conditions, see Figure 7) and after catalysis. Dashed lines depict the macrocrystalline ZnO, Zn₂SiO₄, Zn₃₀Cu₇₀, and Zn foil references. The unlabeled arrows indicate the position of Zn–Zn or Zn–Cu bond formation.

Figure 9

Schematic representation of the ZnO_x speciation in the (A-C) CuZn-15/SiO₂ and (D-F) CuZn-15/C catalysts, depicted (A,D) in the initial state, (B,E) after reduction, and (C,F) under working conditions at 20 bar and 533 K. The various shades of between blue (Cu) and red (Zn) in the CuZn particles represent the relative extent of Zn⁰ incorporation into the Cu⁰ nanoparticles based on the estimated Zn ONs from the XANES analysis. For frames B and C, separate Cu⁰ nanoparticles may exist next to alloyed CuZn particles.