PhD Jingxia Yang

Synthesis of CeO2-Based Materials by Combination of Sol-Gel and Solvothermal Processing and their Application for Catalytic CO Oxidation

Abstract

Due to the good oxygen storage-release capacity of CeO2, it is widely used for CO oxidation as catalyst or catalyst support. To this aim, high surface area and fast formation of dissociated oxygen atoms are two important properties. In this research, CeO2 was synthesized by the sol-gel method using cerium tetra-butoxide as precursor, and the obtained gels were treated under different conditions in order to get materials with these properties. After optimizing the synthesis parameters for CeO2, Co3O4 and graphene were used to modify CeO2 to get composite materials with high catalytic activity.

First, CeO2 was synthesized by combined sol-gel and solvothermal processing of gels obtained from acetaldoximate-modified cerium(IV) t-butoxide (CeB) in the presence of the non-ionic surfactant Pluronic F127. The use of CeB as precursor contrasts very favorably with the often used ceric ammonium nitrate and results in more reliable and tailorable properties of the final materials. The kind of post-synthesis treatment of the gels and the composition of the precursor mixture proved to be crucial for obtaining high surface area ceria with a high Ce3+ proportion. Calcination in air or under nitrogen was compared with solvothermal treatment in ethanol or water and a combination of solvothermal treatment and calcination. The obtained materials were composed of 3.5 – 5.5 nm ceria nanoparticles. The highest specific surface area of 277 m2/g was obtained after solvothermal treatment, and 180 m2/g when solvothermal treatment was followed by calcination in air to remove residual organic groups. The highest Ce3+ proportion was 18% after solvothermal treatment in ethanol and additional calcination in air. CO oxidation on selected samples indicated that the catalytic activity scaled with the surface area and thus was largest for samples solvothermally treated in ethanol. The reaction rate of the best sample was about 75-times larger than that of commercial ceria.

As the combination of sol-gel and solvothermal (in ethanol) processing can yield CeO2 with high surface area and Ce3+ proportion, it was adopted to synthesize Co3O4-modified CeO2 (Co : Ce = 1:4) in order to lower the ignition temperature of CeO2 for CO oxidation. The distribution of Co was controlled by variations of introducing Co2+ ions into the gels. In route 1 (labeled as 1), a ceria gel was synthesized first, followed by Co(OAc)2 addition to the gel during solvothermal treatment. In route 2 (labelled as 2), a mixture of CeB and Co(OAc)2 was subjected to get the gel for solvothermal treatment. In route 3 (labelled as 3), CeB and Co2+ ions were interlinked by means of p-carboxybenzaldehyde oxime (POBC-H) to form a single-source precursor (SSP), which was transformed to a gel for solvothermal treatment. Three different morphologies were obtained, that is Co3O4 nanoparticles located on the surface of CeO2 particles (1), coexisting Co3O4 and CeO2 nanoparticles (2) or Co oxide structures homogeneously distributed within CeO2 (3). The effect of the different morphologies on the properties of Co3O4-CeO2 was investigated with regard to the crystallite phase(s), particle size, surface area and catalytic activity for CO oxidation. Material 1 with Co3O4 nanoparticles finely dispersed on the surface of CeO2 particles had the highest catalytic activity. Change of the Co proportion (10%-80%) of samples prepared by route 1 proved the importance of a finely dispersed Co3O4 phase.

Finally, graphene (rGO) was introduced into the CeO2-based materials as graphene-like organic residues were found during solvothermal treatment. This was beneficial for CO oxidation. rGO was synthesized by chemical reduction of graphene oxide (GO), which was prepared according to Hummer’s method. Both ethanol and L-ascorbic acid were used as reductive agents for rGO. The latter improved the quality of rGO (less defects), but resulted in more organic residues on the particles which inhibited the particle growth. The rGO composite improved the catalytic activity of CeO2, but not that of Co3O4-CeO2 prepared by route 1.