Effect of Hydrazine Pretreatment on the Activity, Stability and Active Sites of Cobalt Species for Preferential Oxidation (PROX) of CO in H2-Rich Stream

Reineck Mhlaba,Takalani Magadzu,Thuto Mosuang

doi:10.3390/chemistry1010011

Abstract

The as-prepared (Co3O4) and hydrazine-treated (Co3O4(H)) cobalt catalysts were prepared using the precipitation method and evaluated at a temperature range of 40–220 °C for preferential oxidation (PROX) of CO in excess hydrogen. An improved surface reducibility with smaller crystallite size was noted on hydrazine-treated cobalt species (i.e., Co3O4(H) catalyst), which indicates some surface transformation. This finding correlates with the surface roughness formation (as depicted by scanning electron microscope (SEM) and transmission electron microscope (TEM) data), which was further confirmed by an increase in the Brunauer–Emmett–Teller (BET) surface area. The mesoporous structure of the Co3O4(H) catalyst remained intact, as compared to that of the Co3O4 catalyst. Interestingly, the in situ treatment of the standalone Co3O4(H) catalyst decreased the maximum CO conversion temperature (T100%) from 160 °C (over Co3O4) to 100 °C, with good selectivity. The Co3O4(H) catalyst showed good stability, with approximately 85% CO conversion at 100 °C for 21 h, as compared to a faster deactivation of the Co3O4 catalyst. However, the Co3O4(H) catalyst was unstable in both CO2 and the moisture environment. Based on the evaluation of spent hydrazine-treated (CoO(H)) cobalt catalyst, the high PROX activity is associated with the formation of Co3+ species as confirmed by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and temperature-programmed reduction (TPR) data.

Highlights

The adoption of hydrogen (H2) proton exchange membrane (PEM) fuel cells as source of energy of the future can mitigate the impact of carbon emission to the environment [1]
The structural compositions of the catalysts were confirmed by X-ray diffraction (XRD), FTIR, X-ray photoelectron spectra (XPS), temperature-programmed reduction (TPR), and TGA data, with the activity of the catalysts investigated under preferential oxidation (PROX) reaction
The XRD and TPR data of the Co3O4(H) catalyst indicated that a surface transformation occurred, as confirmed by an increase in the BET surface area

Summary

Introduction

The adoption of hydrogen (H2) proton exchange membrane (PEM) fuel cells as source of energy of the future can mitigate the impact of carbon emission to the environment [1]. Cobalt-based catalysts such as CoO and Co3O4 (as standalone catalysts) have recently attracted a significant amount of interest from the scientific community, due to their promising activities in the absence [18] and presence of hydrogen [19,20] The former (CoO) has exhibited good catalytic activities in PROX reaction (in excess H2) over a wide temperature range, as compared to other transition metal oxides [19]. A recent report indicated that the purity of the prepared Co3O4 phase plays a crucial role toward high CO oxidation [22] Such a variety of factors have never been considered on metal oxide catalysts such as Co3O4 alone, towards PROX reaction. The catalytic stability of the catalysts on PROX of CO in a dry, moisture, and CO2 atmosphere was evaluated for 21 h on stream at 100 ◦C

Preparation of Catalysts

Catalytic Activity Tests

Thermal and Structural Properties of Catalysts

Catalytic Performance of Various Cobalt Species on PROX of CO

Conclusions