Zn Redistribution and Volatility in ZnZrO_x Catalysts for CO₂ Hydrogenation

Abstract

ZnO–ZrO 2 mixed oxide (ZnZrO x ) catalysts are widely studied as selective catalysts for CO 2 hydrogenation into methanol at high-temperature conditions (300–350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn 2+ species are mobile between the solid solution phase with ZrO 2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO 2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO 2 -to-hydrocarbon process optimization.

Deciphering Local Structural Complexity in Zn/Ga-ZrO2 CO2 Hydrogenation Catalysts

“…At 400 °C/H 2 , the Zr K-edge (Figure S13a) shows a slight decrease in white-line intensity, indicating a minor loss of oxygen atoms in its local environment, consistent with the formation of oxygen vacancies around Ga. Not surprisingly, a small amount of Hf is found in the catalyst: due to the ubiquitous presence of Hf in Zr ores, the former metal is a common impurity in commercially available Zr precursors. The Hf L 2 -edge XANES spectra of GaZr-21 (Figure S13b) showed no significant variations, suggesting that the Ga content does not change, indicating its higher stability in the ZrO 2 lattice with respect to Zn . Furthermore, the FT-EXAFS fit of the Ga-O first coordination shell shows an increase in its CN number, in parallel with the increase in the T d component (Figure S15).…”

“…It is noteworthy that the (102) Bragg reflection is not visible in GaZr-21, probably due to crystallite size-induced peak broadening; the tetragonal polymorph is instead confirmed by the presence of the 1s → 4d transition in Zr K-edge XANES spectra (Figure S7). Nevertheless, we have previously shown how accurate use of EXAFS fitting and XANES interpretation could confirm that the ZnZr-X catalyst exhibits a mixed cluster local structure, consisting of ZnO nanoclusters (ø < 50 Å) embedded and chemically bonded to a ZrO 2 matrix. ,, This structural scenario is reflected in the UV–vis spectra (Figure c), which can be qualitatively described as a combination of ZrO 2 and ZnO spectra. It can also be observed that the band gap of the ZnO component shifts to lower energies as the Zn content increases, in line with the reduction of the quantum confinement effect and the previously reported cluster growth …”

“… , GaZr-21 at RT presented a broad white line with nontrivial T d /O h identification (Figure e). During activation in H 2 , GaZr-21 in situ PXRD data supported the stability of the tetragonal polymorph, from the absence of extra phases (Figure S9a) and a linear unit cell volume expansion (Figure S9b), which was not observed in ZnZr-X catalysts by Redekop et al However, by monitoring the evolution of the XAS spectra, we observed a decrease in white-line intensity in parallel with a shift of the edge position to lower energies (Figure e). As is well known for the Ga K-edge, the edge position is not directly related to its oxidation state but rather to its coordination geometry.…”

“…On the contrary, the GaZr-21 spectrum (Figure d) displays electronic properties that are not identifiable in pure t-ZrO 2 and β-Ga 2 O 3 , suggesting the absence of Ga 2 O 3 clusters. Although initially not fully understood, , we recently reported how the ZnO cluster scenario in ZnZr-X can be observed from the fingerprints of Zn K-edge XANES spectra . Indeed, the 1s → 4p π and 1s → 4p σ transitions, which are well-defined in ZnO, are broadened in the ZnZr-X catalyst, highlighting the nanosized nature of ZnO.…”

“…This structural analysis method can then be applied to two recently studied catalysts, namely, ZnZr-15 and GaZr-21 (where the numbers indicate the wt % of Zn or Ga), both of which show high activity toward CO 2 conversion to methanol but with completely different stability, strongly related to their local structure. ,, Indeed, Zn/ZrO 2 catalysts presented the formation of ZnO clusters from both sol–gel and impregnation synthesis, while the same synthesis strategies lead to the formation of Ga/ZrO 2 reported as a solid solution. ,− The different atoms and their local structures correlate with different catalytic activity. Indeed, under similar conditions (space velocity 8000 cm 3 h –1 g catalyst –1 , 350 °C, and 30 bar) the former showed 10% CO 2 conversion with 60% selectivity toward methanol and the latter showed 15% CO 2 conversion with almost 80% selectivity toward methanol. , PXRD patterns (Figure a,b) and Zr K-edge XANES spectra (Figure S7) suggest solid solution formation from (i) tetragonal polymorph stabilization and (ii) a unit cell volume decrease indicating stoichiometric oxygen vacancy formation. It is noteworthy that the (102) Bragg reflection is not visible in GaZr-21, probably due to crystallite size-induced peak broadening; the tetragonal polymorph is instead confirmed by the presence of the 1s → 4d transition in Zr K-edge XANES spectra (Figure S7).…”