Co₃O₄-Based Materials as Catalysts for Catalytic Gas Sensors ^†

Abstract

The work deals with the development of Co₃O₄-based catalysts for application in catalytic gas sensors. Among the transition-metal oxide catalysts, cobalt oxide exhibits the highest activity in catalytic combustion. The catalytic activity of the catalysts was examined by Differential Scanning Calorimetry (DSC), evaluating the catalyst’s activity by measuring its thermal response to 1% methane (CH₄).

1. Introduction

Catalytic gas sensors are widely used to detect flammable gases to safely monitor the lower explosive limit (LEL). They were first described by Alan Baker in the early 1960s. Alumina-based catalysts with a high amount of Pd and Pt are used to ensure the proper detection of gases, especially of CH₄, for a required lifetime mainly due to the strong tendency of Pd-based catalysts to deactivate throughout operation. In view of the scarcity of precious metals, these catalysts should be substituted through metal-oxide catalysts containing none or only a low loading of noble metals.

Spinel-type Co₃O₄ is active for the catalytic combustion of CH₄ due its structure with variable valence states (Co²⁺/Co³⁺) [1]. Moreover, the doping of Co₃O₄ with further transition metals like Cr, Mn, Cu or Ni should improve further its catalytic ability. Especially the mixed Co₃O₄-based spinel NiCo₂O₄ with a Ni:Co molar ratio of 1:2 has been reported to have excellent catalytic properties towards CH₄ oxidation [2]. However, it was demonstrated by several studies that the morphology of Co₃O₄ plays a decisive role in catalytic activity due to variations in the crystal structure, formation of surface defects and surface-active species [3].

In the present work we investigated and analyzed the influence of the morphology and doping of Co₃O₄ with Ni on its catalytic activity using the DSC method which measures thermal response.

2. Materials and Methods

Pure (mCo₃O₄ and sCo₃O₄) and Ni-doped Co₃O₄ (NiCo₂O₄, Ni_0.5Co_2.5O₄) catalysts were synthesized by the same precipitating procedure as described in [2]. The difference between mCo₃O₄ and sCo₃O₄ catalysts is the atmosphere used during synthesis (N₂ or air).

Scanning transmission electron microscope (STEM) equipped with a secondary electron detector (SE) (Hitachi, Japan) was used to visualize the morphology of the catalysts. Differential Scanning Calorimetry (DSC, Netzsch, Selb, Germany) was used to examine thermal response of the catalysts to 1% CH₄ [4].

3. Discussion

In Figure 1 the different morphologies of the investigated Co₃O₄ catalysts are demonstrated.

Figure 2 shows the corresponding thermal responses at temperatures between 250 and 450 °C.

sCo₃O₄ reveals a significantly higher thermal response than the two Ni doped oxides and mCo₃O₄. The reason for this is the different morphology and the structure of the metal oxide. In our study, the effect of the catalyst morphology on their activity and thermal stability will be analyzed and discussed in detail.