posted on 2023-11-27, 18:13authored byEric Prates da Costa, Xiaohui Huang, Christian Kübel, Xiaoyin Cheng, Katja Schladitz, Alexander Hofmann, Ulrich Göbel, Bernd M. Smarsly
The connectivity
and thermal stability of pores in heterogeneous,
mesoporous metal oxide catalysts are key properties controlling their
(long-term) efficacy. In this study, we investigate the influence
of pH and temperature during a common hydrothermal aftertreatment
step in the synthesis of mesoporous CexZr1–x–y–zYyLazO2−δ oxides obtained from molecular
precursors via hydrothermal synthesis. This study has a strong focus
on the methodological approach, elucidating whether and how even the
smallest changes in morphology and connectivity may be unraveled and
related to the underlying chemical processes to uncover key parameters
for the ongoing improvement of material properties. Deep insights
into the mesopore space were obtained by state-of-the-art physisorption
(including hysteresis scanning), electron tomography, and small-angle
X-ray scattering (SAXS) analysis. We also provide a simple tool to
simulate SAXS curves from electron tomography data that allow direct
comparison to experimentally obtained SAXS curves. Furthermore, the
impact on surface-bound nitrate groups and the development during
calcination were studied in detail by thermogravimetric analysis coupled
with mass spectrometry. The key observations indicate a significant
increase in thermal stability at temperatures as high as 1050 °C
and improved mesopore accessibility with an increase in pH of the
aftertreatment solution. The combined observations from the employed
methods suggest a pH-dependent removal of surface-bound nitrate groups
as well as a dissolution and reprecipitation-based fusing of the primary
particles that constitute the mesopore skeleton. This transformation
yields a mechanically and thermally stronger mesopore space with the
capability to endure high temperatures.