Porous Co-based spinel oxide prepared by soft-template method for ethanol oxidation

Highlights

• Porous Co-based spinel oxide can be synthesized by soft-template method.

• The physicochemical properties of Co-based spinel oxide can be effected by transition metal.

• Porous Co-based spinel oxide has high ethanol complete oxidation activity at low temperature.

• Catalytic activity is related to the pore structure and surface reactive oxygen species.

Abstract

The porous Co-based spinel composite oxides containing different transition metals (Cu, Fe, Ni, Mn, and Ce) were prepared by soft-template method for ethanol complete oxidation in air. The physicochemical properties of the prepared oxides were characterized by XRD, BET, H₂-TPR, and O₂-TPD. The results indicate that the transition metal ions can be dissolved into the Co₃O₄ lattice to form Co-based spinel oxide, which promote surface reactive oxygen species to be formed on the corresponding catalyst. The prepared Co-based spinel oxides have developed pore structure, and the specific surface area is larger than 80 m²·g⁻¹, even up to 132 m²·g⁻¹. The porous structure and high specific surface area can be traced back to the used soft-template reagent, and the pore size distribution and specific surface area can also be affected by the used transition metals. The synergistic effect from porous structure and surface reactive oxygen species can effectively promote ethanol oxidation to final product CO₂. The porous Co-based spinel oxide containing Ce exhibits superior ethanol catalytic oxidation activity and high stability, the ethanol catalytic oxidation conversion and oxidation final product CO₂ selectivity reached 99.7% and 99.1% at 200 °C, respectively. The results indicated that the porous Co-based spinel oxide catalyst has potential application value in ethanol oxidation elimination from air.

Introduction

Co₃O₄, which has an excellent catalytic oxidation performance, has attracted wide attention. And the Co₃O₄ is often used in catalytic oxidation of CO, CH₄, and small molecule organic compounds [[1], [2], [3], [4], [5]]. However, the monocomponent Co₃O₄ has some defects, such as, low catalytic activity, poor mechanical strength, and easy carbon deposition in the process of use [6,7]. The introduction of additive components to prepare Co-based composite oxide can effectively improve the properties of the components in the corresponding oxide catalyst, thereby improving the performances of the corresponding oxide catalyst [8,9]. The catalytic performances can be improved by introducing transition metal ions into improve Co₃O₄ lattice to form spinel oxide. The introduction of Mn into Co₃O₄ lattice could effectively promote the interaction between MnO_x and Co₃O₄, and improve the reactive oxygen to be produced and catalytic oxidation performances of the corresponding CoMn composite oxide [10]. The addition of Ni species can significantly improve the catalytic oxidation performance for the CoNi/TiO₂ composite oxides, which can be attributed to that the surface hydroxyl oxygen species and adsorbed oxygen species content can be effectively enhanced by the introduction of Ni species, and the Co³⁺ species content on the CoNi/TiO₂ oxide surface can also be enhanced [11]. Li et al. [12] found that doping rare earth La, Ce, Pr, and Nd into Co₃O₄ lattice could enhance the specific surface area and redox performance of the corresponding Co-based composite oxide, and the catalytic activity of the corresponding Co-based oxide can be improved.

Volatile organic compounds (VOCs) are a family of chemical species that encompass non-methane hydrocarbons (NMHCs), oxygenated VOCs (OVOCs), and other organic compounds, which are known as one of the major contributors to air pollution [13]. Ethanol has a high energy density, high polarity, adily oxidizable, low toxicity, and wide sources. It is regarded as a very promising substitutable fuel for internal-combustion engines, and also widely used as solvent and raw material in chemical production. Ethanol can directly discharge into the atmosphere by emissions and leaks from chemical production process. Ethanol molecules can be transformed into OVOCs (such as, acetaldehyde, acetic acid, and acetate, etc.) by photochemical reaction, partial oxidation reaction, or/and incomplete combustion of internal-combustion engines. The produced OVOC molecules are more difficult to be eliminated and more harmful than that of ethanol molecules, which can directly cause serious air pollution to harm human living environment and threaten human health [14]. Among all used VOCs elimination technologies, catalytic oxidation/combustion can convert VOCs into CO₂ and H₂O with non-toxic or little environmental hazards under far below the direct combustion temperature [15]. Catalytic oxidation technology has attracted more attentions due to its high purification efficiency, without secondary pollution, and low energy consumption. Therefore, the high efficiency catalytic elimination of OVOCs in the air, which represents by ethanol, has become a research hotspot in the environment and catalysis fields. And the core of related research is still the design and development of high performance catalytic materials.

In this paper, the porous Co-based spinel oxides containing Ce, Cu, Mn, Ni, or Fe were prepared by soft template method using CTAB as soft-template agent. The X-ray diffraction (XRD), specific surface area (BET), H₂-temperature programmed reduction (H₂-TPR), and O₂-temperature programmed desorption (O₂-TPD) tests were used to study their physicochemical properties, and their catalytic oxidation activity was evaluated by ethanol oxidation in air.