Abstract
In this work, the synthesis of Cu/SiO2 catalysts starting from pre-formed copper nanoparticle (CuNP) colloidal suspensions was carried out. Two different protocols for the CuNP synthesis were tested: (i) a green approach using water as solvent and ascorbic acid as reducer and stabilizing agent, and (ii) a second solvothermal method involving the use of diethylene glycol as solvent, sodium hypophosphite (NaH2PO2) as reducer, and polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as stabilizing agents. In addition, and for the sake of comparison, a third catalyst was prepared by solid state conventional grinding of CuO with SiO2. The catalysts were tested in the environmentally relevant catalytic reduction of NOX with H2, in a temperature range from 300 to 500 °C. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR) cycles, Raman spectroscopy, and N2 adsorption for specific surface BET measurements. From these techniques CuO and Cu(0) species were detected depending on the synthesis protocol. CuNP size and size distribution in the colloid suspensions were determined by transmission electronic microscopy (TEM). The catalyst prepared from the aqueous suspension (CuAsc/SiO2) exhibited higher NO conversion (100%) and selectivity (85%) toward N2 at the lower reaction evaluated temperature (300 °C). The CuCTAB/SiO2 catalyst obtained by the solvothermal approach showed activity at high reaction temperature (400 °C) preferentially. The metal–support mechanical mixture exhibited a negligible response at low temperature and low conversion (68%) and selectivity (88%) at 500 °C. Nanoparticle size and distribution on the support, together with the metal–support interaction, were postulated as the most plausible parameters governing the catalytic performance of the different Cu/SiO2 materials.
Highlights
The emission of nitrogen oxides (NOx, x = 1, 2) stemming from the combustion of fossil fuels in power plants or from the exhaust gases of mobile sources is an ever-present environmental concern
Because of the risks associated with handling and storage of NH3, NOx reduction with hydrocarbon (HC) technology (HC-selective catalytic reduction (SCR)) has been extensively studied [4]
Copper–silica catalysts synthesized from pre-formed Cu nanoparticles have shown a promising response towards the selective reduction of nitrogen oxides
Summary
The emission of nitrogen oxides (NOx , x = 1, 2) stemming from the combustion of fossil fuels in power plants or from the exhaust gases of mobile sources (i.e., road transport) is an ever-present environmental concern These gases contribute to air pollution and lead to multiple environmental problems such as acid rain, photochemical smog, the greenhouse effect, among others [1,2]. There exist numerous works in the literature reporting the use of different hydrocarbons as reducing agents and a wide variety of catalysts under these reaction conditions [4,5] While this approach looks very promising, it has not been extensively applied yet due to a series of drawbacks such as: (i) catalyst deactivation; (ii) limited selectivity of NOx to N2 ; (iii) high reaction temperatures, (iv) the need of excess HC that must be burnt, and (v) incremental air pollution (CO2 ) and energy consumption
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