FT-IR and TPD Investigation of the NO_x Storage Properties of BaO/Al₂O₃ and Pt−BaO/Al₂O₃ Catalysts

Abstract

Al 2 O 3 catalysts has been investigated by IR spectroscopy and temperature-programmed desorption. Upon NO interaction, small amounts of nitrites, nitrates, and hyponitrite species were formed on the Ba-containing samples. The NO x storage capacity of the catalysts was highly enhanced upon adsorption of NO/O 2 mixtures and further upon NO 2 admission. Upon adsorption of NO/O 2 on Pt/Al 2 O 3 sample nitrites, nitrates and NO 2 δ+ species were mainly formed, showing a moderate thermal stability. Barium markedly increased the amount and stability of the stored NO x species, which were bidentate and monodentate nitrites and, in minor amounts, nitrates. Nitrites were removed below 750 K and/or transformed into ionic Ba nitrates, stable up to 800-900 K. Upon NO 2 adsorption, huge amounts of nitrates, but no nitrites, were formed on all the samples. Also in this case, Ba increased the amount and stability of the stored NO x species. The nature and the amounts of the stored NO x species formed upon adsorption of NO, NO/O 2 , or NO 2 were similar on Ba/Al 2 O 3 and Pt-Ba/Al 2 O 3 catalysts, whereas Pt slightly decreased their thermal stability. Bulky Ba nitrate was formed during the thermal desorption of NO 2 (and to a less extent of NO/O 2 ), inducing an extensive decomposition of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples.

Adsorption/Desorption Studies of NOx on Well-Mixed Oxides Derived from Co−Mg/Al Hydrotalcite-like Compounds

“…Similarly, for catalyst 1.5CoMAO (Co 1.5 Mg 1.5 Al oxides, Figure 8A), the peak at 1649-1636 cm -1 becomes stronger, indicating more bridging bidentate nitrates formed than for catalyst 0.0CoMAO. The initial peak at 1228-1240 cm -1 due to the adsorption of NO and assigned to bridging bidentate nitrite shifts to 1308 cm -1 as the strongest peak in the later adsorption stage that could be assigned to monodentate nitrate (also showing a peak at 1034 cm -1 ), 7,11,16 suggesting a redox conversion from bridging bidentate nitrite to monodentate nitrate in the presence of Co oxide. The other pronounced peak located at 1470 cm -1 belongs to the linear nitrite from adsorption of NO in the presence of Co oxide.…”

“…As shown in Figure 7A for catalyst 0.0CoMAO (Mg 3 Al oxide) as an example, the strong peak, initially located at 1246 cm -1 and then slightly shifted to 1263 cm -1 , is attributed to bridging bidentate nitrite, due to the adsorption of NO. 7,11,16,45 The other major band, initially located at 1709 cm -1 and later split into a broad band at 1641-1688 cm -1 , belongs to bridging bidentate nitrate, attributed to NO 2 adsorption. 7,11,16 Some other species, such as monodentate nitrite, linear nitrite, and monodentate nitrate, which exhibit IR peaks at 1300-1500 cm -1 , 7,11,16 Table 3).…”

“…7,11,16,45 The other major band, initially located at 1709 cm -1 and later split into a broad band at 1641-1688 cm -1 , belongs to bridging bidentate nitrate, attributed to NO 2 adsorption. 7,11,16 Some other species, such as monodentate nitrite, linear nitrite, and monodentate nitrate, which exhibit IR peaks at 1300-1500 cm -1 , 7,11,16 Table 3). The species adsorbed at 300°C ( Figure 7B) are quite similar, only with a lower magnitude.…”

“…The other pronounced peak located at 1470 cm -1 belongs to the linear nitrite from adsorption of NO in the presence of Co oxide. 16 For adsorption at 300°C on catalyst 1.5CoMAO ( Figure 8B), monodentate nitrate (1276, 1480, and 1048 cm -1 ) is the major species, with some other possible minor species such as bridging bidentate nitrite (1276 cm -1 ), 7,11,16 adsorbed N 2 O 4 (1726-1732 cm -1 ), 46 chelating bidentate nitrate (1553 and 1276 cm -1 ), and NO + (2343 cm -1 ). [46][47][48] The species adsorbed on catalyst 3.0CoMAO (Co 3 Al oxide) are mainly various nitrates.…”

Novel NO Trapping Catalysts Derived from Co−Mg/X−Al (X = Fe, Mn, Zr, La) Hydrotalcite-like Compounds

“…Currently, NOx reduction is conducted through exhaust gas recirculation (EGR), selective catalytic reduction (SCR) (3), direct decomposition of NO with Co/zeolite catalysts (4), and NOx storage/reduction (NSR) (5). Among these NOx reduction technologies, NSR catalytic treatment shows a higher potential (6) as NSR catalysts have been successfully used in stationary-power and natural-gas turbine posttreatment for several years (7).…”