Ammonia Adsorption and Oxidation on Cu/Mg/Al Mixed Oxide Catalysts Prepared via Hydrotalcite-Type Precursors

Abstract

Cu/Mg/Al catalysts obtained by controlled calcination of hydrotalcite-type (HT) anionic clays may be new interesting and cheap catalysts for the selective catalytic reduction (SCR) of NO by NH3. In this paper the ammonia adsorption and oxidation on Cu x Mg0.710 - x Al0.290 catalysts (x = 0.022, 0.046, and 0.072, as atomic ratio), obtained by calcination for 14 h at 923 K of HT precipitates, have been investigated and compared with those of the corresponding Mg0.710Al0.290 sample. The presence of copper strongly increases the SCR activity and the selectivity to nitrogen in ammonia oxidation, while the Mg/Al catalyst did not show SCR activity in these conditions and formed significant amounts of nitrogen oxides by ammonia oxidation. All samples adsorbed coordinatively ammonia on medium-week Lewis acid sites, while no Brønsted acidity was found, showing that protonic acidity is not necessary for both SCR and ammonia oxidation. With an increase in the copper content, the ammonia gave rise by oxidation to adsorbed hydrazine (likely via amide intermediates) and other adsorbed species, tentatively identified as imido or nitroxyl fragments and nitrogen anions. These surface species were probably involved in either selective or unselective ammonia oxidation, this last occurring via a Mars−van Krevelen-type mechanism. In order to have more information on the SCR activity of the Cu/Mg/Al catalysts, the NO adsorption also was investigated, showing that on the Mg/Al-mixed oxide free surface, NO disproportionates to nitrogen dioxide and to a species identified as hyponitrite anions. On the other hand, over the Cu-containing centers NO gave rise mainly to surface nitrosyl, being also oxidized to nitrates. On the basis of these data, it was hypothesized that on the Cu/Mg/Al catalysts the SCR took place between NO or nitrosyls and amide species, which were likely common intermediates in either SCR and ammonia oxidation.

Mechanism of selective catalytic oxidation of ammonia to nitrogen over Ag/Al2O3

“…Zawadzki [1] also suggested that -NH would react with molecular O 2 to produce NO at higher temperatures, but the author did not propose the in situ reaction between the NO and -NH x . Since the key intermediate (NO x ) of the SCR routes proposed by Ramis et al [8,14,[35][36][37][38] was detected in our experiment (Figs. 6 and 7), and the NH 3 could rapidly react with the in situ-formed NO x ad-species (Fig.…”

Thermal transformations of Cu–Mg (Zn)–Al(Fe) hydrotalcite-like materials into metal oxide systems and their catalytic activity in selective oxidation of ammonia to dinitrogen

“…Cu-Mg-Al hydrotalcite-like materials were used as catalysts for Bayer-Villiger oxidation of ketones [14]. Their calcined forms were used in the oxidative coupling of phenylethyne [15], adsorption and oxidation of ammonia [16], N 2 O decomposition [17] as well as in decomposition and reduction of nitrogen oxides and sulphur oxides [e.g. 6,[18][19][20][21].…”

The role of silver species on Ag/Al2O3 catalysts for the selective catalytic oxidation of ammonia to nitrogen

“…Previous studies showed that noble metals such as Pt, Ir and Au were active for the reaction of SCO of NH 3 at low temperature, however, the selectivity of N 2 was not satisfied for the reason that the main by-product at low temperature was N 2 O [4][5][6][7][8] [9][10][11][12][13][14][15][16][17], and many ion-exchanged zeolite catalysts [18][19][20][21][22], have also been investigated for the SCO of NH 3 , but NH 3 conversion levels of these catalysts at low temperature were not high enough to be of practical use.…”