Impacts of niobia loading on active sites and surface acidity in NbO /CeO2–ZrO2 NH3–SCR catalysts

Abstract

A series of NbO x /Ce 0.75 Zr 0.25 O 2 catalysts for the selective catalytic reduction of NO with ammonia (NH 3-SCR) were synthesized using a wetness impregnation method. The effect of niobia loading was studied in relation to the active sites and surface acidity. NH 3 /NO oxidation, X-ray diffraction, Infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, H 2 temperature-programmed reduction, O 2 /NH 3 temperature-programmed desorption, and diffuse reflectance infrared Fourier transformed spectroscopy experiments were performed to correlate the catalyst structure and surface properties to catalytic performance after Nb 2 O 5 modification. The catalyst with 15 wt.% Nb 2 O 5 loading showed high NH 3-SCR activity and nearly 100% N 2 selectivity within a broad operation temperature window (190-460 °C) at a high space velocity (300, 000 h −1). On this catalyst, Nb n+ was mainly distributed in the form of typical monomeric and polymeric NbO x species, and was partially incorporated into the Ce 0.75 Zr 0.25 O 2 lattice at the Nb n+-O-Ce n+ (Zr n+) interface. The electron redistribution effect arising from the occupation of cerium sites by Nb n+ ions promoted the formation of Ce 3+ ions, oxygen vacancies and active oxygen species. This interaction was closely associated with the distribution of NbO x species which varied with niobia loading. NbO x themselves were acid sites and by attracting electrons they enhanced Lewis acid sites on CZ surface, which promoted the adsorption of NH 3 and inhibited the unselective oxidation of NH 3 to NO x. The increased amounts of active oxygen species over NbCZ catalysts promoted the adsorptive oxidation of NH 3 to NH 2 and NO to NO 3 at low temperatures, and thus facilitated the reaction of ads-NH 3 and ads-NO 3-/NO 2 species. This effect as well as the increased amount of acid sites led to good NH 3-SCR performance of Nb15CZ in a wide temperature range.

Investigations on the Antimony Promotional Effect on CeO2–WO3/TiO2 for Selective Catalytic Reduction of NOx with NH3

“…The binding energies of the subpeak located at lower bind energy (529.4-530.3 eV) is attributed to lattice oxygen (denoted as O b ), and the one located at higher binding energy (531.3-532.0 eV) stands for active oxygen (denoted as O a ). [46] The active oxygen O a is thought to be more active in oxidation reactions than O b because of the higher mobility of the Relationship between the catalytic activity and reactant adsorption and desorption behavior A strong correlation has been established between the NH 3 SCR activity of catalysts and the adsorption status of NH 3 and NO x , especially in the low-temperature window in which the NH 3 SCR reaction is studied mainly on the basis of the Langmuir-Hinshelwood (L-H) mechanism between NH 3 and NO x adsorbed species. As a consequence, the relationship between the NH 3 SCR activity and reactant adsorption behavior over the catalysts surface was deeply studied by using in situ diffuse reflectance infrared Fourier transform (DRIFT) analysis and the TPD method to reveal the adsorption and desorption behavior over the different catalysts surfaces.…”

NH3-SCR reaction mechanisms of NbO /Ce0.75Zr0.25O2 catalyst: DRIFTS and kinetics studies

“…The acid-base properties, nontoxicity and lower cost of ceria also make them promising candidates for NH 3 -SCR catalysts. Recently, many solid-acid modified Ce based catalysts have been reported, such as CeO 2 -WO 3 [3][4][5][6][7][8][9], CeO 2 -Nb 2 O 5 [10][11][12][13][14][15], CeO 2 -MoO 3 [16][17][18][19][20][21][22], CeO 2 -PO 4 3- [23][24][25][26] and CeO 2 -SO 4 2- [27][28][29][30]. Among them, CeO 2 -Nb 2 O 5 based catalyst appears to be very promising due to its superior low-temperature activity, wide operation temperature window and acceptable resistances to SO 2 and alkali metals.…”

In situ DRIFTs investigation of the reaction mechanism over MnOx-MOy/Ce0.75Zr0.25O2 (M = Fe, Co, Ni, Cu) for the selective catalytic reduction of NOx with NH3

“…Previous works demonstrated that the best catalytic performance was achieved when the Ce:Zr molar ratio is 3:1 [9,13,14].…”