Activating effect of cerium in hydrotalcite derived Cu–Mg–Al catalysts for selective ammonia oxidation and the selective reduction of NO with ammonia

Abstract

Hydrotalcite originated mixed metal Cu-Mg-Al oxide system was doped with various amounts of cerium (0.5 or 3.0 wt%) and tested in the role of catalysts for the selective catalytic oxidation of ammonia to dinitrogen (NH 3-SCO) and the selective catalytic reduction of NO with ammonia (NH 3-SCR). The activating effect of cerium was observed in both studied processes. However, the CeO 2 loading is a very important parameter determining catalytic performance of the studied samples. It was shown that an introduction of cerium into Cu-Mg-Al mixed oxide resulted in its significant activation in the low-temperature NH 3-SCR process, independently of the CeO 2 loading and a decrease in the efficiency of the NO reduction at higher temperatures, which was more significant for the catalyst with the lower cerium content. In the case of the NH 3-SCO process, the introduction of cerium into Cu-Mg-Al mixed oxide resulted in the activation of the low temperature reaction, which was more intensive for the catalyst with lower cerium content. These effects were related to the presence of cerium in the form of crystallites of various size and therefore their different reducibility.

Cu-Mg-Fe-O-(Ce) Complex Oxides as Catalysts of Selective Catalytic Oxidation of Ammonia to Dinitrogen (NH3-SCO)

“…It could be assumed that the formed Cu 2+ -O-Fe 3+ species in the Cu x Mg 1-x Fe 2 O 4 oxide are easier to reduce than Fe 3+ -O-Fe 3+ species in a pure MgFe 2 O 4 structure [49]. A similar effect has been reported for other Cu-Ce based catalysts [18,25,50]; however, this effect was not observed for the presented catalysts. CeO 2 reduction is expected to proceed at around 800 • C and cannot be differentiated from the complex Fe oxides reduction profile.…”

“…Characteristic diffraction lines occurring at approximately 21,35,41,43,50,63,67,74,78,85 and 88 °2θ were ascribed to spinel-like phases labelled as: (Cu,Mg)Fe2O3 (ICDD PDF-2 card No 01-076-9731). As expected [14,18,33], high temperature of calcination (800 °C) favors the formation of complex oxide structures; therefore, these patterns were observed for each sample irrespective of the copper loading. Apart from the complex spinel phases, characteristic diffraction lines for simple oxides like periclase (MgO) and tenorite (CuO) were also found.…”

“…According to the mechanisms of ammonia oxidation described in the literature the most probable mechanisms for Cu-Mg-Fe mixed metal oxides is the internal selective ammonia oxidation (i-SCR) that include oxidation of part of NH 3 to NO and further NO reduction by NH 3 to N 2 [13,16,17]. Because of that, possible improvement of Cu-Mg-Fe catalysts activity could be reached by the modification of their surface by other metals (e.g., noble metals, [16][17][18][19]59]) that are responsible for selective oxidation of ammonia to NO [60] and ensuring the accessibility of active species by structural promoters, e.g., aluminum [14,21,57,61].…”

“…Chemical composition of the samples was selected to determine the effect of Cu, Fe and Ce loading on NH 3 -SCO, while high calcination temperature was chosen to study the influence of spinel phase formation on catalytic activity and selectivity to dinitrogen. According to the literature [18,[22][23][24][25], iron and cerium-based materials show high efficiency in low-temperature catalytic reduction of NO to N 2 . Therefore, the addition of Fe and Ce should result in increasing the N 2 selectivity in NH 3 -SCO processes occurring over Cu-reach catalysts.…”

Ferrierite and Its Delaminated Forms Modified with Copper as Effective Catalysts for NH3-SCO Process

“…(2) transition metal oxides, typically active at higher temperatures but more selective to dinitrogen than noble metals [12][13][14]; and (3) zeolites modified with transition metals, which are catalytically active at temperatures higher than noble metals but are significantly more selective to dinitrogen [15][16][17].…”