Contrasting Catalytic Functions of Metal Vanadates and Their Oxide Composite Analogues for NH3-Assisted, Selective NOx Transformation

V2O5 fuses with transition metals to create dozens of different metal vanadates, whose acidic/redox traits can be diverse yet optimized for selective catalytic NOx reduction (SCR) by changing the metals used or their metal:vanadium stoichiometry. However, no metal vanadate has been compared with its metal oxide composite analogue as an active phase for SCR, albeit a vanadate occasionally outperforms an oxide composite simulating a commercial catalyst (V2O5-WO3). Herein, Cu3V2O8 and CuO-VO2/V2O5 were rationally selected as model phases of metal vanadates and oxide composites and isolated using pH regulation of their synthetic mixture to <=similar to 5 (pH1/pH5) and similar to 11 (pH11), respectively. The pH1/pH5/pH11 samples were comparable with regard to morphological, textural, and compositional traits but not for crystallographic features. This thus provided the impetus to simulate the pH1/pH5/pH11 surfaces under a SO2-containing feed-gas stream, by which SOA2-/HSOA- functionalities (A = 3-4) were anchored on their (defective) Lewis acidic metals and/or labile oxygens (O-alpha). This could result in the formation of pH1-S/pH5-S/pH11-S, whose major surface species were Bronsted acidic bonds (SOA2-/HSOA-) and redox sites (O-alpha; mobile oxygen (O-M); oxygen vacancy (O-V)). pH1-S/pH5-S/pH11-S were similar in terms of NH3 binding energies and energy barriers in SCR yet escalated collision frequencies among the surface species involved in the sequence of pH11-S < pH5-S < pH1-S (via kinetic assessments), as was the case with the numbers of SOA2-/ HSOA- functionalities of the catalysts (via temperature-resolved Raman spectroscopy). These were coupled to elevate the efficiency of acidic cycling on the order of pH11-S < pH5-S < pH1-S. Meanwhile, the amounts of O-alpha and O-V (or O-M) innate to pH1-S/pH5-S were smaller than and comparable to those of pH11-S, respectively. Nonetheless, pH1-S/pH5-S provided greater O-M mobility than pH11-S, thereby proceeding better with redox cycling than pH11-S (via O-18-labeling O-2-on/off runs). Furthermore, pH1-S/pH5-S outperformed pH11-S in SCR under diffusion-limited domains, while enhancing the resistance to H2O, ammonium (bi)sulfate poisons, or hydrothermal aging over pH11-S by diversifying the selective N-2 production pathway other than SCR.

Automated summary

In this study, Cu3V2O8 and CuO-VO2/V2O5 were selected as model phases of metal vanadates and oxide composites, isolated by regulating the pH of their synthetic mixture. It was found that these materials exhibited differences in infrared spectroscopic properties.