Co3O4−CeO2 Nanocomposites for Low‐Temperature CO Oxidation

Abstract In an effort to combine the favorable catalytic properties of Co3O4 and CeO2, nanocomposites with different phase distribution and Co3O4 loading were prepared and employed for CO oxidation. Synthesizing Co3O4‐modified CeO2 via three different sol‐gel based routes, each with 10.4 wt % Co3O4 loading, yielded three different nanocomposite morphologies: CeO2‐supported Co3O4 layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO‐TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co3O4 surface layer, reducing the light‐off temperature of CeO2 by about 200 °C. In contrast, intermixed oxides and Co‐doped CeO2 suffered from lower dispersion and organic residues, respectively. The performance of Co3O4‐CeO2 nanocomposites was optimized by varying the Co3O4 loading, characterized by X‐ray diffraction (XRD) and N2 sorption (BET). The 16–65 wt % Co3O4−CeO2 catalysts approached the conversion of 1 wt % Pt/CeO2, rendering them interesting candidates for low‐temperature CO oxidation.


Reference catalysts of CeO2, Co3O4 and Pt/CeO2
The reference CeO2 was synthesized using route 1 without adding Co(OAc)2 in the solvothermal step. Commercial Co3O4 from Fluka (purity 99.5%) was used as received. A reference catalyst of 1 wt.% Pt/CeO2 was prepared by impregnation. First, 0.02 g of Platinum(II) bis(acetylacetonate) were dissolved in 10 mL toluene and mixed with 0.99 g of commercial CeO2 dispersed in 40 mL toluene. After 2 h of stirring at room temperature, the entire solution was evaporated to dryness by gradually increasing temperature to 100 °C. The dried solid was subsequently calcined at 300 °C for 2 h in air. Pretreatment was carried out by oxidation in 20 vol.% O2 in He at 400 °C (30 min), followed by reduction in 5 vol. % H2 in He at 200 °C (30 min). This yielded Pt nanoparticles of 1.7 nm mean size, as revealed by CO chemisorption.

Characterization
Temperature programmed reduction CO temperature-programmed reduction (CO-TPR) was performed in a continuous-flow fixed-bed quartz reactor under atmospheric pressure. Catalyst samples (20 mg) were pre-treated in 50 ml/min synthetic air (200°C for 30 min by 10°C/ min ramping rate). After cooling to room temperature, the samples were exposed to 5 vol% CO in He (total flow 50 ml/min ), followed by heating to 800°C (10°C/min). The gas stream was analyzed by an online quadrupole mass spectrometer (QMS) (Prisma Plus QMG 220, Pfeiffer Vacuum) equipped with a Faraday detector.

Catalytic performance
The CO oxidation reaction was performed by using 10 mg of the samples or reference catalysts in the same type of quartz reactor. The samples were pretreated with synthetic air (50 ml/min) at 200°C for STE samples or at 400°C for STE-AC and oxide reference catalysts for 30 min (heating rate 10°C/min). For Pt/CeO2, after oxidation at 400°C, the sample was reduced at 200 °C for 30 min (heating rate 10°C/min) with 5 vol.% H2 in He. After cooling to 30°C, 5 vol% CO, 10 vol% O2 and 85 vol% He mixture (total flow 50 ml/min) were introduced, and the system was heated up to 400°C (ramping rate of 2.5°C/min). The concentrations of CO and CO2 in the outlet streams were monitored by gas chromatography with a HP-PLOT Q column and a flame ionization detector (FID).

Structure and surface characterization
XRD measurements were performed on a Philips X'Pert diffractometer using Cu-Kα radiation (λ=1.5406 Å; Xray tube at 40 kV and 40 mA) operating in Bragg-Brentano reflection geometry. The scanning range was 5-80° (2θ) in step scan mode of 0.05° (2θ), with 4.5 s per step.
Nitrogen sorption measurements were performed on an ASAP 2020 (Micromeritics). The samples were degassed in vacuum at 80°C for at least 5 h prior to measurement. The total surface area was calculated according to Brunauer, Emmett and Teller (BET) and the pore size distribution (from the desorption branch) according to Barrett, Joyner and Halenda (BJH).
The Pt dispersion and mean particle size were determined via CO-chemisorption, assuming a CO:Pt ratio of 1:1.