Abstract
The effect of the presence of residual sodium (0.4 %wt) over a Co3O4 bulk catalyst for methane combustion was studied. Two samples, with and without residual sodium, were synthesized by precipitation and thoroughly characterised by X-ray diffraction (XRD), N2 physisorption, Wavelength Dispersive X-ray Fluorescence (WDXRF), temperature-programmed reduction with hydrogen followed by temperature-programmed reduction with oxygen (H2-TPR/O2-TPO), temperature-programmed reaction with methane (CH4-TPRe), ultraviolet–visible–near-infrared diffuse reflectance spectroscopy (UV-vis-NIR DRS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that during calcination, a fraction of the sodium atoms initially deposited on the surface diffused and migrated into the spinel lattice, inducing a distortion that improved its textural and structural properties. However, surface sodium had an overall negative impact on the catalytic activity. It led to a reduction of surface Co3+ ions in favour of Co2+, thus ultimately decreasing the Co3+/Co2+ molar ratio (from 1.96 to 1.20) and decreasing the amount and mobility of active lattice oxygen species. As a result, the catalyst with residual sodium (T90 = 545 °C) was notably less active than its clean counterpart (T90 = 500 °C). All of this outlined the significance of a proper washing when synthesizing Co3O4 catalyst using a sodium salt as the precipitating agent.
Highlights
Methane is a powerful greenhouse effect gas (25 times higher than that of CO2 ) that appears as a residue or flue gas in many different applications
The filtrates extracted from the filtration step were recovered and their pH was measured to to estimate the extent of the removal of residual sodium from the precursor
One of the catalysts was properly washed after the precipitation process while the other kept some residual sodium ions
Summary
Methane is a powerful greenhouse effect gas (25 times higher than that of CO2 ) that appears as a residue or flue gas in many different applications. The off-gas treatment technology must be efficient at low temperatures when operating with large gas flows with small amounts of methane. Under these conditions, the most suitable technique is catalytic oxidation. The most significant part of this research is focused on both single and mixed transition metal oxides Materials such as perovskites, hexaaluminates or spinels composed of different metals have been studied and proven active for the oxidation of methane [4,5,6,7]. Spinel oxides are mixed oxides with general formula AB2 O4, where A is a metal with oxidation state +2 and tetrahedral coordination and B is a metal with oxidation state +3 and octahedral coordination [8]
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