Abstract
The separation of Pd and CeO2 on the inner surface of controlled porous glass (CPG, obtained from phase-separated borosilicate glass after extraction) yields long-term stable and highly active methane combustion catalysts. However, the limited availability of the CPG makes such catalysts highly expensive and limits their applicability. In this work, porous silica obtained from acid leached rice husks after calcination (RHS) was used as a sustainable, cheap and broadly available substitute for the above mentioned CPG. RHS-supported Pd-CeO2 with separated CeO2 clusters and Pd nanoparticles was fabricated via subsequent impregnation/calcination of molten cerium nitrate and different amounts of palladium nitrate solution. The Pd/CeO2/RHS catalysts were employed for the catalytic methane combustion in the temperature range of 150–500 °C under methane lean conditions (1000 ppm) in a simulated off-gas consisting of 9.0 vol% O2, and 5.5 vol% CO2 balanced with N2. Additionally, tests with 10.5 vol% H2O as co-feed were carried out. The results revealed that the RHS-supported catalysts reached the performance of the cost intensive benchmark catalyst based on CPG. The incorporation of Pd-CeO2 into RHS additionally improved water-resistance compared to solely Pd/CeO2 lowering the required temperature for methane combustion in presence of 10.5 vol% H2O to values significantly below 500 °C (T90 = 425 °C).
Highlights
Methane possesses a global warming potential 15 to 34 times higher than that of carbon dioxide (CO2 ) [1]
We demonstrate how a waste product of rice production—rice husks—could serve as a starting material for a catalyst which can convert methane to the lower potent molecule for Catalysts 2019, 9, 26; doi:10.3390/catal9010026
Schliermann et al [27] investigated the effect of leaching on the metal content in the rice husks and were able to show that a significant reduction of the metal by leaching with citric acid is obtained
Summary
Methane possesses a global warming potential 15 to 34 times higher than that of carbon dioxide (CO2 ) [1]. It is of tremendous importance to reduce the release of methane to the atmosphere. The sources of methane in the air are of different origin. Natural sources of about 37%, mainly from wetlands, and from agricultural sources of about 45% have been identified as main contributors to the global methane emission [2]. As early as 1983, Seiler et al [3] estimated that 35 to 59 Tg/year of methane could be emitted from rice paddies worldwide. It is assumed that rice paddy fields add about 10% to the global methane release which is about 600 Tg/year [2]
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