Catalysis Science & Engineering, Short talk
CE-026

Operando X-Ray Absorption Spectroscopy Identifies Monoclinic ZrO2:In Solid Solution as the Active Phase for the Hydrogenation of CO2 to Methanol

A. Tsoukalou1, A. Armutlulu1, E. Willinger1, C. R. Müller1, P. M. Abdala1*, A. Fedorov1*
1Department of Mechanical and Process Engineering, ETH Zürich, Switzerland

The direct hydrogenation of CO2 to methanol is a valuable yet underdeveloped reaction.[1] Recent studies have shown that In2O3-based catalysts are promising for this process.[2,3] The active phase of such catalysts is In2O3−x, i.e. partially reduced india with In–Vo–In active sites.[4] Using operando XAS-XRD studies on In2O3 nanocrystals (In K-edge), we have shown that these sites deactivate with time-on-stream (TOS) by over-reduction, forming metallic In.[5] ZrO2 support increases the activity and stability of In2O3 during CO2 hydrogenation, and monoclinic zirconia is superior to tetragonal zirconia phase.[3] However, it remains elusive how the phase of the ZrO2 support affects the oxidation state and the local structure of the In sites with TOS. Here, In2O3 nanocrystals with an average size of 7 nm were studied on amorphous, tetragonal and monoclinic zirconia (In2O3/m‑ZrO2, In2O3/t‑ZrO2, and In2O3/am‑ZrO2 catalysts, respectively). Using a capillary flow reactor interfaced with a gas chromatograph (GC) (Figure 1), combined operando XAS (In K-edge)-XRD data were recorded under CO2 hydrogenation conditions (300 °C, 20 bar, H2:CO2:N2 = 3:1:1).

Figure 1. a) Schematic of the operando setup for CO2 hydrogenation to methanol (BM31, ESRF, Grenoble, France), b) catalytic activity (gMeOH gIn2O3−1h−1) of In2O3 on m‑ZrO2, t‑ZrO2 and am‑ZrO2, c) structural evolution of In2O3/m‑ZrO2 during CO2 hydrogenation with selected d) XANES and e) EXAFS data. 

The catalytic activity directly correlates to the extent of In2O3 reduction and the evolution of the local structure of In sites with TOS, affected strongly by the phase of the ZrO2 support. Am‑ZrO2 promotes the rapid reduction of In2O3 to metallic In leading to an almost inactive amorphous catalyst. The tetragonal ZrO2 avoids the complete reduction of In2O3 to In0, but gives an average oxidation state of In below +2, which is associated with poor catalytic activity. In2O3 on m‑ZrO2 evolves into In2+/In3+ sites atomically dispersed in the lattice of m‑ZrO2, which leads to the activation of the catalyst with TOS. The formed ZrO2:In solid solution functions via active In–Vo–Zr sites (in contrast to less active In–Vo–In sites) that are stabilized by the m‑ZrO2 lattice against deactivation by over-reduction to In0.

[1] A. Goeppert et al., Chem. Soc. Rev., 2014, 43, 7995–8048.
[2] K. Sun et al., J. CO2 Util. 2015, 12, 1-6.
[3] O. Martin et al., Angew. Chem. Int. Ed., 2016, 55, 6261-6265.
[4] J. Y. Ye et al., J. Phys. Chem. C 2012, 116, 7817-7825.
[5] A. Tsoukalou et al., J. Am. Chem. Soc., 2019, 141, 13497-13505.