Photo-Chemically-Deposited and Industrial Cu/ZnO/Al2O3 Catalyst Material Surface Structures During CO2 Hydrogenation to Methanol: EXAFS, XANES and XPS Analyses of Phases After Oxidation, Reduction, and Reaction

Abstract

Industrial Cu/ZnO/Al2O3 or novel rate catalysts, prepared with a photochemical deposition method, were studied under functional CH3OH synthesis conditions at the set temperature (T) range of 240–350 °C, 20 bar pressure, and stoichiometric carbon dioxide/hydrogen composition. Analytical scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray adsorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methods were systematically utilized to investigate the interfaces, measured local geometry, and chemical state electronics around the structured active sites of commercially available Cu/ZnO/Al2O3 material or synthesized Cu/ZnO. Processed Cu K-edge EXAFS analysis suggested that various Cu atom species, clusters, metallic fcc Cu, Cu oxides (Cu2O or CuO) and the Cu0.7Zn2 alloy with hexagonal crystalline particles are contained after testing. It was proposed that in addition to the model’s Cu surface area, the amount, ratio and dispersion of the mentioned bonded Cu compounds significantly influenced activity. Additionally, XPS revealed that carbon may be deposited on the commercial Cu/ZnO/Al2O3, forming the inactive carbide coating with Cu or/and Zn, which may be the cause of basicity’s severe deactivation during reactions. The selectivity to methanol decreased with increasing T, whereas more Cu0.7Zn2 inhibited the CO formation through reverse water–gas shift (RWGS) CO2 reduction.

Graphic Abstract

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Acknowledgements

This work was supported by the Slovenian Research Agency (P1-0112, P1-0175, P2-152) and by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. XAS spectra measured at beamline P64 of PETRA III at DESY, Hamburg, a member of the Helmholtz Association (HGF) under project I-20160764 EC. We would like to thank Wolfgang Caliebe and Vadim Murzin for the expert advice on the beamline operation during the experiments.

Funding

The funding was provided by Horizon 2020 Framework Programme (Grant No. 730872) and Javna Agencija za Raziskovalno Dejavnost RS (Grant Nos. P1-0112, P1-0175, P2-152).

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Appendix

Appendix

See Figs. (10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21) and Tables (4, 5, 6, 7, 8, 9).

Fig. 10
figure10

High-temperature in-situ diffractograms of commercial catalyst CuO/ZnO/Al2O3 during the reduction. Commercial catalyst sample was scanned from 20° to 90°; the 1st scan was taken before reduction (20 °C), whereas thereafter, atmosphere was changed to H2/N2 (5 mol. %), increasing the temperature with 10 K min-1—after reaching 300 °C measurements were performed continuously (patterns 1–14)

Fig. 11
figure11

HRTEM image of a Cu grain with and oxidative surface layer

Fig. 12
figure12

Cu L3M4,5M4,5 auger transition spectras of the commercial Cu/ZnO/Al2O3 catalyst after reduction as well as after CH3OH synthesis

Fig. 13
figure13

Deconvolution of C1s XPS peaks of the reduced Cu/ZnO/Al2O3 catalysts

Fig. 14
figure14

XPS spectra of Cu/Al2O3 catalyst; a deconvolution of Cu 2p peaks, b deconvolution of O 1s peaks, c deconvolution of Al 2s peaks

Fig. 15
figure15

XPS spectra of Cu/ZnO; a deconvolution of Cu 2p peaks, b deconvolution of Zn 2p peaks, c deconvolution of O 1s peaks

Fig. 16
figure16

Cu K-edge XANES measured on catalyst A (dots: experiment; dashed line: best fit obtained with linear combination of XANES profiles of Cu fcc metal foil (31%) as references for metallic Cu, crystaline Cu2O (26%) as referemce for monovalent Cu, and crystalline CuO (43%) as reference for divalent Cu oxide. Fit components are plotted below)

Fig. 17
figure17

Cu K-edge XANES measured on catalyst B (dots: experiment; dashed line: best fit obtained with linear combination of XANES profiles of Cu fcc metal nanoparticles (69%) and Cu fcc metal foil (22%) as references for metallic Cu, crystaline Cu2O (6%) as referemce for monovalent Cu, and crystalline CuO (3%) as reference for divalent Cu oxide. Fit components are shown below)

Fig. 18
figure18

Cu K-edge XANES measured on the commercial Cu/ZnO/Al2O3 calcined catalyst (dots: experiment; dashed line: best fit obtained with linear combination of XANES profiles of crystalline CuO (23%) and CuO calcined (59%) as reference for divalent Cu oxide, and crystaline Cu2O (18%) as referemce for monovalent Cu. Fit components are shown below)

Fig. 19
figure19

Cu K-edge XANES measured on the commercial Cu/ZnO/Al2O3 catalyst after reduction (dots: experiment; dashed line: best fit obtained with linear combination of XANES profiles of Cu fcc metal nanoparticles (30%) and Cu fcc metal foil (32%) as references for metallic Cu, crystaline Cu2O (20%) as referemce for monovalent Cu, and crystalline CuO (18%) as reference for divalent Cu oxide. Fit components are shown below)

Fig. 20
figure20

Cu K-edge XANES measured on the commercial Cu/ZnO/Al2O3 catalyst after reaction (dots: experiment; dashed line: best fit obtained with linear combination of XANES profiles of Cu fcc metal nanoparticles (78%) as reference for metallic Cu, crystaline Cu2O (18%) as referemce for monovalent Cu, and crystalline CuO (4%) as reference for divalent Cu oxide. Fit components are shown below)

Fig. 21
figure21

SEM micrographs of used catalyst A (left) and catalyst B (right)

Table 4 Calculated parameters of the nearest coordination shells around Cu atoms present in the calcined CuO/ZnO/Al2O3 catalyst: average number of neighbours (N), distance (d) and Debye–Waller factor (DWF); uncertainty of the last digit is given in parentheses; the best fit is obtained with amplitude reduction factor (S02) of 0.90 and shift of energy origin of photoelectron, ΔEo of 3.2(7) eV
Table 5 Calculated parameters of the nearest coordination shells around Cu atoms in each of the three Cu EXAFS models, describing Cu compounds (Cu metal, Cu0.7Zn2 alloy and Cu oxide), present in Cu/ZnO/Al2O3 catalyst after reduction
Table 6 Calculated parameters of the nearest coordination shells around Cu atoms in each of the three Cu EXAFS models, describing Cu compounds (Cu metal, Cu0.7Zn2 alloy and Cu oxide), present in Cu/ZnO/Al2O3 catalyst after reaction
Table 7 Calculated parameters of the nearest coordination shells around Cu atoms in each of the three Cu EXAFS models, describing Cu compounds (Cu metal, Cu0.7Zn2 alloy and Cu oxide), present in catalyst A
Table 8 Calculated parameters of the nearest coordination shells around Cu atoms in each of the three Cu EXAFS models, describing Cu compounds (Cu metal, Cu0.7Zn2 alloy and Cu oxide) present in catalyst B
Table 9 CO2 hydrogenation profile over the prepared catalysts

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Pori, M., Arčon, I., Dasireddy, V.D.B.C. et al. Photo-Chemically-Deposited and Industrial Cu/ZnO/Al2O3 Catalyst Material Surface Structures During CO2 Hydrogenation to Methanol: EXAFS, XANES and XPS Analyses of Phases After Oxidation, Reduction, and Reaction. Catal Lett (2021). https://doi.org/10.1007/s10562-021-03556-1

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Keywords

  • CH3OH synthesis
  • Cu/ZnO-based catalysts
  • XPS
  • XANES
  • EXAFS analyses
  • Catalyst selectivity and activity