Highly Cr-Substituted CeO₂Nanoparticles Synthesized Using a Non-equilibrium Supercritical Hydrothermal Process: High Oxygen Storage Capacity Materials Designed for a Low-Temperature Bitumen Upgrading Process

Abstract

As described herein, the synthesis of highly Crsubstituted CeO 2 nanoparticles (Cr-CeO 2 ) for low-temperature bitumen upgrading is demonstrated using a supercritical hydrothermal method including a subcritical region. A continuous flow reactor that can provide a non-equilibrium process was used to improve Cr substitution in the CeO 2 lattice. Consequently, a Cr-substitution concentration of 22.7 mol %, which was unobtainable using the equilibrium process (5.1 mol %), was achieved. As the Cr-substitution concentration increased, the Cr-CeO 2 morphology changed from a perfect octahedron to a cluster of small nanoparticles with high lattice strain. The increase in lattice strain strongly affects the rise in the oxygen storage capacity (OSC) of Cr-CeO 2 . Results show that high conversion of asphaltene (up to 47.9% at 350 °C) and high selectivity of syngas (H 2 and CO, up to 58.3% at 350 °C) were achieved through low-temperature catalytic bitumen upgrading in the presence of highly Cr-substituted CeO 2 nanoparticles. Furthermore, results show that the asphaltene conversion increased because of the rise in the OSC of Cr-CeO 2 , irrespective of the increase in the specific surface area, which indicates that the catalytic potential of Cr-CeO 2 is more dependent on OSC than their specific surface area.

Atomistic origin of high-concentration Ce3+ in {100}-faceted Cr-substituted CeO2 nanocrystals

“…The NC sizes was determined by performing a Rietveld analysis of the XRD pattern and was calculated to be 7.8 nm in average for Crsubstituted CeO 2 NCs, a value that is slightly larger than the pristine CeO 2 NCs (average size, 6.6 nm). A prolonged reaction time in the supercritical water was acquired control the morphology and particle size of Cr-substituted CeO 2 , which is usually smaller than the pristine CeO 2 under the same synthetic condition [38]. The difference in particle size is probably attributed to the difference in surface energy similar to the previous study [53].…”

“…In this regard, Singh et al reported a promising system of Cr-doped CeO 2 nanoparticles, which contained a surprisingly high Cr dopant amount of 33% exhibiting flexible-valence states of hexavalent and tetravalent, resulting in a higher OSC performance and enhanced catalytic reactivity of oxygen evolution and hydrogen generation [36,37]. Recently, highly Cr-substituted CeO 2 NCs was also fabricated using a non-equilibrium supercritical hydrothermal process [38]. Furthermore, Cr-doped CeO 2 nanostructures with a small amount (less than 10%) and a low valence state of Cr 3+ , other than Cr 6+ with high toxicity, are also investigated and exhibit high reactivity in diverse processes, such as soot combustion, photocatalytic CO 2 reduction, and electrocatalytic N 2 fixation to NH 3 [39][40][41].…”

One-Pot Hydrothermal Synthesis of Victoria Green (Ca3Cr2Si3O12) Nanoparticles in Alkaline Fluids and Its Colour Hue Characterisation

“…SEM observations revealed that monodispersed Ca 3 Cr 2 Si 3 O 12 agglomerates with a popcorn quasi-spherical shape were the dominant morphology under the alkaline hydrothermal conditions. These agglomerates are constituted of tiny anhedral crystals, which self-assembled to produce the 3D hierarchical “popcorn” architecture [ 20 , 21 ]. The anhedral particle size varies from 12 to 34 nm, as suggested by the crystallite sizes calculated from SEM micrographs ( Figure 5 a,b) and the results given in Table 1 .…”

Doping-Induced Combined Fano and Phonon Confinement Effect in La-Doped CeO2: Raman Spectroscopy Analysis

“…Doped cerium dioxide (CeO 2 ) is one of the popular materials used for many application purposes as oxygen-ion conductor in solid oxide fuel cells, oxygen storage, three-way catalysts (TWC) for exhaust gas treatments from vehicles, as UV-blocking material for sun protection factor (SPF) in cosmetics, and as electrode material for different types of sensors. − Cerium (Ce) atom reacts with oxygen and forms the two most stable structures that are CeO 2 (Ce 4+ ) and Ce 2 O 3 (Ce 3+ ), which exist in nature. For application purposes, the reduction of Ce 4+ to Ce 3+ and the creation of oxygen defects/vacancies in the host lattice play a vital role .…”