A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

Abstract

For the first time, the standard and fast selective catalytic reduction (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable molecules only. The standard SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidation of the catalyst by NO + O2 and a reduction by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the reduction, and finally, oxidation by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown experimentally for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and Fourier transform infrared spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the standard SCR cycle. The activation energy calculated by density functional theory (DFT) indicates that the oxidation of an NO molecule by O2 to a bidentate nitrate ligand is rate-determining for standard SCR. Finally, the role of a nitrate/nitrite equilibrium and the possible influence of Cu dimers and Brønsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidation to NO2.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…Recent operando XAS studies by Lomachenko et al [15] demonstrate that, above 300 • C, mobile Cu components quickly diminish and four coordinated Cu(II) complexes convert to 3 coordinated ones that spectroscopically resemble a [Cu(II)-NO 3 − ]-Z species. This finding may be used to justify an SCR mechanism proposed by Janssens et al [13], who postulated that Cu(I) oxidation to Cu(II) is achieved by nitrate intermediate formation from NO+O 2 as the rate-limiting step:…”

“…Thus, it is necessary to generate intermediates with an N oxidation state of +3 for a viable mechanism. In the proposal by Janssens et al [13], this is achieved by the following reaction:…”

“…Note that in these two possible mechanisms, both the nitrate formation and N-H cleavage steps are energetically rather demanding, with DFT-calculated activation energies of~104 kJ/mol [13] and 119 kJ/mol [11], respectively. These values are in reasonable agreement with the apparent activation energy of~140 kJ/mol found experimentally (Figure 14), suggesting that the rate-limiting steps described above are rather plausible.…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”

Formation of Nitrogen Mono Oxide (NO) during Iron Ore Sintering Process

“…The total emission of NOx ultimately depends on the oxidation of fuel-N and the reduction of NO. [5][6][7][8] The research of NOx's generation and discharge in sintering process has drawn much attention in recent years. Studies have shown that sintering parameters such as water ratio, fuel ratio, quicklime ratio and the thickness of bed layer had important influences on the NOx concentration in flue gas.…”