Functional materials based on metal oxides (Me_xO_y) are employed as catalysts and/or sorbents in processes undergoing in supercritical water (scH₂O), the green reaction medium used to obtain energy relevant or value added products [1,2]. The functional properties might be greatly affected by the synthesis method of the active Me_xO_y. Moreover, to maintain their performance is more practical and cost-effective to use supported Me_xO_y nanoparticles. Within this context, the present work deals with the preparation of carbon-supported ZnO and CuO composites by an innovative method based on impregnation of the active material under scH₂O conditions.  The method development make use of the recent knowledge on the transition between liquid-like (LL) and gas-like (GL) regimes of scH₂O [3], observable by neutron imaging when scH₂O interacts with porous carbon materials [4].
Metal oxides were impregnated on a carbon support with porous structure (C) made of activated carbon fibers. The continuous flow tubular reactor of the equipment denominated NISA (Neutron Imaging Supercritical-water Analysis) described in [4] was used. Three synthesis parameters were studied to understand their influence on the composites formulation in terms of crystallinity, size and morphology, as well as the amounts of loading and the concentration profile of the active element on the support: i) the impregnation temperature chosen in such a way to have scH₂O density corresponding either to LL or to GL states of water [3], ii) the flow rate determining both the residence time and the contact time between the metal precursor solution and the C support surface, and iii) the concentration of the metal nitrate aqueous solution used as precursor. The resulted composites were characterized by XRD, FT-IR spectroscopy, as well as by electron microscopies, SEM and TEM.
In isobaric supercritical conditions, increasing the synthesis temperature to values at which scH₂O is in the GL regime favors the impregnation of Me_xO_y on C support. At 250 bar and a reaction temperature of 668 K, it has been obtained  uniform impregnation of the support fibers with ZnO crystalline nanoparticles, of hexagonal symmetry, cubic morphology and ~60 nm in size. Under the same temperature and pressure conditions, impregnation of CuO on C is favored when starting from low concentrations of Cu²⁺ in the aqueous precursor solution and at low flow rates. Crystalline nanoparticles of cupric oxide, with monoclinic symmetry, cubic morphology, and dimensions of ~50 nm, uniformly cover the carbon fibers.
The results show that impregnation in scH₂O is a promising approach for the preparation of ZnO/C and CuO/C composite materials. This one-step synthesis method in a continuous flow uses neither seed layer nor mineralizer, and needs substantially lower preparation times than the conventional impregnation methods.
ACKNOWLEDGMENTS This research is funded by Romanian CNCS/CCCDI—UEFISCDI, grant number PN-III-P4-ID-PCE-2020-1241.
REFERENCES [1] Maxim, F., et al., Energies 2021, 14 (21), 7399. [2] Ates, A.; Hatipoglu, H. J. Inorg. Organomet. Polym. Mater. 2021, 31 (12), 4581–4593. [3] Maxim, F., et al., Adv. Sci. 2021, 8 (3), 2002312. [4] Maxim, F., et al., Nat. Commun. 2019, 10 (1), 4114.