Maria Cristina Campa

CATALYTIC ACTIVITY OF CO-ZSM-5 FOR THE ABATEMENT OF NOX WITH METHANE IN THE PRESENCE OF OXYGEN

Abstract The abatement of NO with CH4 in the presence of oxygen ([NO] = [CH4] = 1000 or 4000 ppm, [O2] = 0 to 2%, by volume) was studied on Co-ZSM-5 catalysts (Co content 0.29 to 4.1 wt.-%), prepared from H-ZSM-5 or Na-ZSM-5 by the ion-exchange method. On all samples, the amount of CO and NO adsorbed at room temperature was proportional to the cobalt content (CO/Co⋍ 0.5 and NO/Co⋍ 1.6), with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the adsorption was only slightly higher than that on Co-ZSM-5 with Co 2.0 wt.-%. Infrared spectroscopy (FTIR) showed the formation of carbonyls (one type only, on cobalt equivalent sites), cobalt mononitrosyls (two types) and dinitrosyls (two types). The intensity of bands from carbonyls and nitrosyls was about proportional to the cobalt content, with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the bands were roughly as intense as in the sample with Co 2.0 wt.-%. In the Co-ZSM-5 sample with Co 4.1 wt.-%, after heating with O2 at 773 K, or after its use in catalysis, diffuse reflectance spectroscopy (DRS) showed the presence of Co3O4, not detected by X-ray diffraction. In the presence of O2, the NO reduction rate was proportional to the Co content, except for the sample containing Co 4.1 wt.-%. The CH4 oxidation rate was proportional to the Co content, in the entire range of Co concentrations. The selectivity of catalysts for NO abatement (selective catalytic reduction, SCR), was nearly independent of Co content but was markedly lower on the sample with Co 4.1 wt.-%. The results suggest that only CoII ions exchanged in the framework of the ZSM-5 matrix are active in CO and NO adsorption and in the SCR reaction, whereas also the cobalt of the dispersed Co3O4 phase contributes to CH4 oxidation with O2.

The catalytic activity of Cu-ZSM-5 and Cu-Y zeolites in NO decomposition: dependence on copper concentration

NO decomposition was studied on Cu-ZSM-5 (Cu exchange extent from 23 to 210%) and Cu-Y (Cu exchange extent from 5 to 105%) catalysts at 773 K. The results show that the activity (NO molecules decomposed per gram of catalyst per second) increases by roughly 100-fold when the extent of exchange with copper in the ZSM-5 framework increases from 80 to 100%. This behaviour shows that not all Cu sites are equivalent in their decomposition activity. Cu-ZSM-5 samples prepared with either H-ZSM-5 or Na-ZSM-5 show the same activity pattern.

The selective catalytic reduction of NOx with CH4 on Mn-ZSM5: A comparison with Co-ZSM5 and Cu-ZSM5

The abatement of NO with CH4 in the presence of O2 was studied on Mn-ZSM5 (Mn percent exchange 2 to 75%), prepared from Na-ZSM5 by the ion-exchange method. Samples were characterised by FTIR, ESR and magnetic measurements. Both ESR and magnetic measurements showed that all manganese was present as MnII. FTIR showed the formation of carbonyls (one type only, on manganese equivalent sites). The intensity of bands from carbonyls was almost proportional to the manganese content. The selectivity for NO abatement, reaction orders in O2, NO or CH4, and apparent activation energy were independent of the manganese content, suggesting that the same surface complexes were formed on all Mn-ZSM5 catalysts. On Mn-ZSM5, turnover frequencies (molecules s−1 Mn-atom−1) for both NO abatement and CH4 reaction were nearly independent of the manganese content and close to the relevant values on Co-ZSM5 (previously investigated in our laboratory), but substantially higher than those on Cu-ZSM5 (Cu percent exchange 98%, studied for a comparison). On Mn-ZSM5, Co-ZSM5 and Cu-ZSM5, the adsorption at RT of NO+O2 or NO2 caused the formation of nitrates. The normalised intensity (per atom) of monodentate nitrate bands (∼1520 and ∼1320 cm−1) was (i) independent of the metal content, (ii) the same in Mn-ZSM5 and Co-ZSM5, and (iii) much higher in Mn-ZSM5 and Co-ZSM5 than in Cu-ZSM5. On all these catalysts, NO abatement rates were proportional to the concentration of these monodentate nitrates. On FTIR evidence, these species reacted with methane at the same temperature at which the various catalysts were active. These findings suggest a role of monodentate nitrates in the SCR reaction.

Cuo–ZnO–Al2O3 mixed oxides: preparation, bulk and surface characterization

CuO–ZnO–Al2O3 mixed oxides with different Cu : Zn : Al atomic ratios were obtained by thermal decomposition of hydroxycarbonate) precursors at 623 K in air for 24 h. Both precursors and mixed oxides were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS); surface area determination was also performed. The XRD patterns of the precursors show that, as a function of the Cu : Zn : Al atomic ratio, parent phases to hydrozincite, aurichalcite, malachite, gerhardtite, hydrotalcite and boehmite are present. In the calcined samples zincite, tenorite and spinel-like phases are present. The occurrence of Cu2+ species in C3v and in D4h symmetries has been evidenced by DRS in the most zinc-rich CuO–ZnO and in the most aluminum-rich CuO–ZnO–Al2O3 samples, respectively. A comparative study of the photoelectron and X-ray excited Auger transition of Cu and Zn species shows an increasing covalency of the Cu—O chemical bond in the CuO–ZnO samples with low copper loading and the formation of a copper–zinc ‘surface spinet’ in the oxide samples with high aluminum content. The quantitative XPS investigation has shown that the surface Cu : Zn atomic ratios are lower than those found by chemical analysis, thus pointing to a surface segregation of ZnO and/or of zinc ‘surface spinel’ phases. The surface segregation tends to be higher when the catalyst precursors are polyphasic.

Lean NOx reduction on Cu-NaY and Cu-HZSM5 zeolites at the spark ignition engine exhaust

Abstract The lean NO x reduction on the Cu-NaY and Cu-HZSM5 catalysts has been investigated at the spark ignition engine exhaust. Catalysts prepared by exchange method at different Cu loadings were characterized by XRD, XPS, ESR and redox treatments with CO and O 2 . The results show similar performance of Cu-HZSM5 and Cu-NaY. Reaction rates, normalized with respect to the total copper, resulted five times higher for Cu-HZSM5 compared to Cu-NaY. The nature of sites active for the lean NO x reduction is compared with that for the NO decomposition. It is concluded that different sites are operated for the two reactions and that a much higher fraction of copper is involved in the lean NO x reduction compared to the NO decomposition.

No abatement with CH4 in the presence of O2 on H-ZSM5 with Si/Al = 15-200: The dependence of activity on the proton concentration

AbstractThe abatement of NO with methane in the presence of oxygen was studied on commercial H-ZSM5 samples with Si/Al = 15–200 in a conventional flow apparatus. H-ZSM5 samples were used in the acid form or after exchanging protons with sodium ions to various extents. Their catalytic activity was compared with that of commercial H-mordenite and H-Y. On all H-ZSM5 catalysts, reaction rates RNO and

Reduction of nitric oxide with hydrogen on chromia / zirconia catalysts

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Propane dehydrogenation on chromia/silica and chromia/alumina catalysts

(molecules s−1 g−1) increased proportionally to the proton concentration, showing that either all protons or a constant fraction of them were equally active. On sodium-exchanged H-ZSM5 samples with Si/Al = 15–17, both RNO and