NOx storage and reduction pathways on zirconia and titania functionalized binary and ternary oxides as NOx storage and reduction (NSR) systems

Abstract

a b s t r a c tBinary and ternary oxide materials, ZrO 2 /TiO 2 (ZT) and Al 2 O 3 /ZrO 2 /TiO 2 (AZT), as well as their Ptfunctionalized counterparts were synthesized and characterized via XRD, Raman spectroscopy, BET, in situ FTIR and TPD techniques. In the ZT system, a strong interaction between TiO 2 and ZrO 2 domains at high temperatures (>973 K) resulted in the formation of a low specific surface area (i.e. 26 m 2 /g at 973 K) ZT material containing a highly ordered crystalline ZrTiO 4 phase. Incorporation of Al 2 O 3 in the AZT structure renders the material highly resilient toward crystallization and ordering. Alumina acts as a diffusion barrier in the AZT structure, preventing the formation of ZrTiO 4 and leading to a high specific surface area (i.e. 264 m 2 /g at 973 K). NO x adsorption on the AZT system was found to be significantly greater than that of ZT, due to almost ten-fold greater SSA of the former surface. While Pt incorporation did not alter the type of the adsorbed nitrate species, it significantly boosted the NO x adsorption on both Pt/ZT and Pt/AZT systems. Thermal stability of nitrates was higher on the AZT compared to ZT, most likely due to the defective structure and the presence of coordinatively unsaturated sites on the former surface. Pt sites also facilitate the decomposition of nitrates in the absence of an external reducing agent by shifting the decomposition temperatures to lower values. Presence of Pt also enhances partial/complete NO x reduction in the absence of an external reducing agent and the formation of N 2 and N 2 O. In the presence of H 2 (g), reduction of surface nitrates was completed at 623 K on ZT, while this was achieved at 723 K for AZT. Nitrate reduction over Pt/ZT and Pt/AZT via H 2 (g) under mild conditions initially leads to conversion of bridging nitrates into monodentate nitrates/nitrites and the formation of surface OH and NH x functionalities. N 2 O(g) was also continuously generated during the reduction process as an intermediate/byproduct.

Sulfur-tolerant BaO/ZrO2/TiO2/Al2O3 quaternary mixed oxides for deNOX catalysis

“…Detailed information regarding the synthesis procedures of Al 2 O 3 /ZrO 2 /TiO 2 (AZT, Al 2 O 3 : ZrO 2 : TiO 2 = 50 : 35 : 15, by mass) can be found in one of our recent reports. 18 The Pt/AZT material was prepared via a wetness impregnation method utilizing PtIJNH 3 ) 2 IJNO 2 ) 2 (Aldrich, diamminedinitritoplatinumIJII), 3.4 wt% solution in dilute NH 3 IJaq)). Before the incorporation of 1 wt% Pt, the Al 2 O 3 /ZrO 2 /TiO 2 support material was initially calcined in air at 973 K for 150 min in order to remove carbonaceous species and organic precursors.…”

“…Along these lines, in our former studies we have focused on the design, synthesis and characterization of a variety of binary and ternary mixed metal oxide systems which are promoted with reducible oxides such as Fe 2 O 3 /Fe 3 O 4 , 12 TiO 2 , 9,[13][14][15][16] CeO 2 , 8,17 and ZrO 2 . 18 For instance, we have demonstrated that CeO 2 is an effective promoter enabling a significant improvement in the NO x reduction and sulfur regeneration of NSR/LNT catalysts. 8,17 On the other hand, while TiO 2 promotion facilitates the sulfur release from a poisoned NSR/LNT catalyst, 9,16 due to the strong interaction between TiO 2 and BaO domains, BaO can diffuse into TiO 2 , forming complex mixed oxides leading to thermal aging and deactivation.…”

“…In one of our recent reports, structural evolution of ZrO 2 /TiO 2 and Al 2 O 3 /ZrO 2 /TiO 2 mixed oxides was followed as a function of temperature by means of X-ray Diffraction (XRD), Raman spectroscopy and Brunauer-Emmett-Teller (BET) specific surface area analysis techniques. 18 The amorphous ZrO 2 /TiO 2 material obtained after the synthesis readily crystallizes at temperatures higher than 773 K to form ordered and low surface area structures such as ZrTiO 4 and tetragonal zirconia (i.e. t-ZrO 2 ).…”

“…On the other hand, disordered (amorphous-like) Al 2 O 3 /ZrO 2 /TiO 2 persists even at 1173 K revealing a relatively higher NO x uptake than that of binary ZrO 2 /TiO 2 . 18 ZrO 2 /TiO 2 and Al 2 O 3 /ZrO 2 /TiO 2based materials were also investigated in terms of their composition, thermal stability, NO x storage capability and sulfur uptake/regeneration characteristics by the Toyota Motor Company. [27][28][29][30][31] ZrO 2 /TiO 2 with a mass ratio of 70 : 30 was claimed to have the highest tolerance against sulfur poisoning 30 and the influence of the Al 2 O 3 on NO x storage capacity, structural integrity, sintering resistance, and thermal deterioration has been demonstrated.…”

Sulfur Poisoning and Regeneration Behavior of Perovskite-Based NO Oxidation Catalysts

“…Lean burn engines have become very popular in recent years as they offer high fuel efficiencies and lower CO 2 emission. NO x storage and reduction (NSR) catalytic technology is one of the most popular solutions for lean-NO x aftertreatment applications [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. NO oxidation/reduction ability and resilience against sulfur/phosphorous [13,15,19,20] are among the most vital capabilities that are essential for such NO x abatement (i.e.…”