Abstract
The catalytic reduction of nitrites over Pt-In catalysts supported on activated carbon has been studied in a semi-batch reactor, at room temperature and atmospheric pressure, and using hydrogen as the reducing agent. The influence of the indium content on the activity and selectivity was evaluated. Monometallic Pt catalysts are very active for nitrite reduction, but the addition of up to 1 wt% of indium significantly increases the nitrogen selectivity from 0 to 96%. The decrease in the accessible noble metal surface area reduces the amount of hydrogen available at the catalyst surface, this favoring the combination of nitrogen-containing intermediate molecules to promote the formation of N2 instead of being deeply hydrogenated into NH4+. Several activated carbon-supported Pt-In catalysts, activated under different calcination and reduction temperatures, have been also evaluated in nitrite reduction. The catalyst calcined and reduced at 400°C showed the best performance considering both the activity and the selectivity to nitrogen. This enhanced selectivity is ascribed to the formation of Pt-In alloy. The electronic properties of Pt change upon alloy formation, as it is demonstrated by XPS.
Highlights
Nitrate occurs naturally in some groundwater, its presence in drinking-water is frequently associated with contamination by excessive use of fertilizers, in combination with inappropriate farming practices and/or sewage disposal (World Health Organization, 2011)
In an attempt to avoid the risks derived from the presence of nitrates in drinking water, the World Health Organization (WHO) has established a maximum contaminant level of 50 mg L−1, whereas the U.S Environmental Protection Agency (USEPA) (World Health Organization, 2011) has lowered it up to 10 mg L−1
Pt-In bimetallic catalysts were studied in the catalytic reduction of nitrite
Summary
Nitrate occurs naturally in some groundwater, its presence in drinking-water is frequently associated with contamination by excessive use of fertilizers, in combination with inappropriate farming practices and/or sewage disposal (World Health Organization, 2011). As nitrite is the intermediate product in nitrate reduction, it is important to evaluate catalysts that allow for understanding the process of its selective reduction to nitrogen. Several activated carbon-supported Pt-In catalysts with different metal atomic ratios and activated under different calcination and reduction temperatures, have been tested in the reduction of nitrites in water with hydrogen. The reduction of nitrites was carried out in a semi-batch reactor, equipped with a magnetic stirrer and a thermostatic jacket, at room temperature and atmospheric pressure, and using hydrogen as reducing agent. 790 ml of deionised water and 400 mg of catalyst were fed into the reactor, the magnetic stirrer was adjusted to 700 rpm and the gas mixture of carbon dioxide and hydrogen (H2 + CO2 (1:1), flow rate 200 cm3/min) was passed through the reactor during 15 min to remove oxygen; CO2 acts as pH buffer (pH 5.5).
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