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

Abstract

Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a molecular-level understanding of this catalyst are summarized: (1) The nature of the active sites and, in particular, their transformations under varying reaction conditions that include dehydration, the presence of the various SCR reactants and hydrothermal aging; (2) Discussions of standard and fast SCR reaction mechanisms. Considerable progress has been made, especially in the last couple of years, on standard SCR mechanisms. In contrast, mechanisms for fast SCR are much less understood. Possible reaction paths are hypothesized for this latter case to stimulate further investigations; (3) Discussions of rational catalyst design based on new knowledge obtained regarding catalyst stability, overall catalytic performance and mechanistic catalytic chemistry.

The H2O Effect on Cu Speciation in Cu-CHA-Catalysts for NH3-SCR Probed by NH3 Titration

“…Amongst state-of-the-art metal-promoted zeolite catalysts used in this process, Cu-exchanged chabazite (Cu-CHA) shows excellent activity and extraordinary hydrothermal stability over a broad temperature range under SCR conditions, triggering intensive research on the relationship between its unique structure and its DeNO x performance [3][4][5][6][7][8][9][10][11]. In this regard, the SiO 2 /Al 2 O 3 ratio (SAR) and the Cu loading content are important structure indexes, which could affect the nature of Cu cations existing in the catalyst framework and, based on their nature, location and redox properties, further impact the SCR activity [3][4][5][6][7][8][9]. Two Cu species are recognized to be present in the Cu-CHA catalyst: Z 2 Cu II (doubly coordinated to the framework) and ZCu II OH (balanced by one framework negative charge), whose relative ratio can be predicted based on Si/Al and Cu/Al ratios [5,6,11].…”

“…In this regard, the SiO 2 /Al 2 O 3 ratio (SAR) and the Cu loading content are important structure indexes, which could affect the nature of Cu cations existing in the catalyst framework and, based on their nature, location and redox properties, further impact the SCR activity [3][4][5][6][7][8][9]. Two Cu species are recognized to be present in the Cu-CHA catalyst: Z 2 Cu II (doubly coordinated to the framework) and ZCu II OH (balanced by one framework negative charge), whose relative ratio can be predicted based on Si/Al and Cu/Al ratios [5,6,11]. The proportion of such Cu species could also be affected by several factors, for example, hydrothermal aging [4,6,12] or hydrated/dehydrated conditions [5,6].…”

“…Two Cu species are recognized to be present in the Cu-CHA catalyst: Z 2 Cu II (doubly coordinated to the framework) and ZCu II OH (balanced by one framework negative charge), whose relative ratio can be predicted based on Si/Al and Cu/Al ratios [5,6,11]. The proportion of such Cu species could also be affected by several factors, for example, hydrothermal aging [4,6,12] or hydrated/dehydrated conditions [5,6]. It was also proposed, based on DFT calculations, DRIFTS, XAS and NH 3 -TPD tests [4,5,8,13], that these two species are able to coordinate a different number of NH 3 molecules, thus influencing, in turn, the NH 3 storage capacity, a relevant parameter for the SCR process.…”

Cu-IM-5 as the Catalyst for Selective Catalytic Reduction of NOx with NH3: Role of Cu Species and Reaction Mechanism

“…In addition to the zeolite framework, Cu species are also studied intensively. As commonly accepted, isolated Cu 2+ ions are the best catalytic active sites; CuAl 2 O 4 species are inactive in the reaction; other species such as [Cu(OH)] + -Z and [Cu-O-Cu] 2+ are still in debate [2,[15][16][17][18]. Gao et al [19] observed NH 3 oxidation followed [Cu-O-Cu] 2+ species formation under high levels of Cu ion-exchange degree.…”

Top 10 Cited Papers in the Section “Environmental Catalysis”

“…Moreover, the extent to which the NO conversion decreased after SO 2 introduction in the feed was relatively slight at 20 • C even if the catalyst did not regain its initial activity when SO 2 was removed, probably due to the formation of sulfates covering the active sites of the catalysts. The fourth article by Feng Gao et al (2018) [6] was devoted to the recent progress in atomic-level understanding of Cu/SSZ-13 selective catalytic reduction catalysts currently on the market for the control of NOx emissions from diesel vehicles. More specifically, the review focused on recent advances in the nature of catalytically active copper species, some limiting reactions involved in the SCR mechanism, and the effects of copper speciation on catalyst stability.…”