Hai-Ying Chen

Promoted Fe/ZSM-5 catalysts prepared by sublimation: de-NOx activity and durability in H2O-rich streams

Fe/ZSM-5 catalysts with an Fe/Al ratio 1:0, were prepared by sublimation of FeCl3 into H/ZSM-5. They display high activity and durability for the selective catalytic reduction of NOx to N2, both in dry and wet gas flows. These catalysts have now been modified by exchanging a second cation into the zeolite. Mere neutralization of zeolite protons by Na+ lowers the selectivity for NOx reduction to N2, but the cations Ce3+ and La3+ act as true catalyst promoters. With isobutane as the reductant in a simulated vehicular emission gas, almost 90% of NOx is reduced to N2 at 350°C over the La-promoted catalyst. The presence of 10% H2O in the feed does not impair the catalyst performance at high temperature; in the temperature region below 350°C it even increases the N2 yield. The beneficial effect of La is due to its lowering of the catalyst activity for the undesired combustion of the hydrocarbon. No signs of zeolite destruction are evident after 100 h TOS in a wet gas flow at 350°C. Carbonaceous deposits causing a slight deactivation are easily removed in an O2/He flow at 500°C; this in situ regeneration fully restores the original activity.

On the nature of active sites in Fe/ZSM-5 catalysts for NOx abatement

Abstract Fe/ZSM-5 catalysts with high Fe loading (Fe/Al∼1) have been prepared by sublimation of FeCl3 onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NOx reduction with hydrocarbons in an excess of O2 and H2O. TPR shows that the Fe in the zeolite cavities is different from Fe2O3 particles. Naked Fe3+ ions are absent; oxo-ions, which are equally well reducible by CO and H2, prevail. A minority of the Fe complexes lose oxygen upon mere heating to ∼500°C; some of the reduced sites are reoxidized only by N2O. The population of oxo-complexes that lose oxygen by heating depends on the Si/Al ratio, this dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO–Fe–O–Fe–OH]2+. Upon reacting with NO, the bridging O atom is transferred and NO2 is formed. This step is not rate limiting for active catalysts with high Al/Si ratio and high Fe loading, but it becomes critical with zeolites of low Al/Si ratio.

Reduction of NOx over Fe/ZSM-5 catalysts: mechanistic causes of activity differences between alkanes

Abstract Fe/ZSM-5 catalysts prepared by sublimation of FeCl 3 onto H/ZSM-5 catalyze the selective reduction of NO x by hydrocarbons to N 2 . The order of the relative rates and N 2 yields obtained with different alkanes reveals a non-trivial chemistry. The maximum yield is lower for propane than for n -butane but about the same for n - and iso -butane. However, at temperatures below this maximum, the N 2 yield is higher for propane and n- butane than for iso -butane. Deposits are formed on the catalyst that contain N atoms in a low-oxidation state which are able to react with NO 2 to form N 2 . TPO and FTIR results show that the amount and also the character of the deposits depend on the nature of alkanes. The change of the oxidation state of nitrogen from a high value in NO or NO 2 to a lower value in nitrile and amino groups of the deposit is rationalized by applying mechanistic concepts of organic chemistry, including the Beckmann rearrangement and fragmentation. FTIR spectra and the observed oxygen- and nitrogen-containing compounds by GC-MS are potential clues to the reaction mechanism.

Characterization and catalytic tests of Au/MFI prepared by sublimation of AuCl3 onto HMFI

Au/MFI was prepared by sublimation of AuCl3 onto HMFI. The oxidation state of the gold in the as-synthesized sample is Au3+. Upon heating in He or O2, it transforms into gold metal and electron-deficient gold particles which agglomerate to larger Au particles. Reduction of Au3+ with CO to Au+ is accompanied by formation of an Au+(CO) complex. The carbonyl ligand has a remarkable stabilization effect on Au+ ions in zeolite cages. Strong IR bands show the CO vibration, and also the strong perturbation of the T–O–T vibrations of the zeolite lattice. [Au+(CO)]/MFI has a characteristic XRD pattern and is stable up to 200 °C. Au/MFI catalyzes the decomposition of N2O to N2 even in the presence of 3% O2. With hydrocarbons or NH3 as the reductant, it has some NOx reduction activity, but these catalysts deactivate at higher temperature even in inert gas atmosphere.

Reduction of NOx over zeolite MFI supported iron catalysts: Nature of active sites

FTIR spectroscopy, catalytic reduction of NOx with iso-C4H10, and oxidation of C16O with 18O2 have been combined to characterize the Fe entities in Fe/MFI catalysts prepared by a variety of methods, including sublimation of FeCl3 vapor onto the H-form of MFI. Iron hydroxo and oxo ions in exchange positions, iron oxide clusters inside zeolite channels, and Fe2O3 particles at the external surface have been identified. Of special interest are IR bands in the 830–1000 cm−1 region. They are assigned to the perturbations by Fe ions of zeolite lattice vibrations and discriminate between hydroxo(oxo) ions and oxide clusters. Two bands in this region are assigned to Fe3+ and Fe2+; their respective intensities follow the changes in valency of the Fe ions. When MFI containing Fe ions are treated with aqueous NaOH, small iron oxide clusters are generated. This process is largely reversible: Fe ions are regenerated upon replacing the Na+ by H+ ions and initiating protonolysis at high temperature. The high CO/CO2 ratio in the product of NOx reduction with alkanes is consistent with a dominant role of hydroxo(oxo) ions, not oxide particles, as active sites. Oxidation of C16O with 18O2 over Fe/MFI results in a remarkably high concentration of C18O2 molecules. The superiority of the sublimation method in preparing Fe/MFI catalysts for NOx reduction is mainly due to the high dispersion of the Fe. Immediately after subliming FeCl3 onto H-MFI, the 3610 cm−1 band is completely wiped out, indicating that all protons have been replaced by [FeCl2]+ ions. Terminal OH groups and internal SiOH groups in MFI with high density of lattice effects are also destroyed, but these are completely regenerated in a subsequent hydrolysis step.

Characterization of Fe/ZSM-5 by isotopic exchange with 18O2

Abstract Exchange of 18 O 2 with O atoms in two groups of Fe containing ZSM-5 catalysts was studied. One class was prepared by sublimation of FeCl 3 onto H-ZSM-5 with various Si/Al ratios. These materials are highly active and selective catalysts in the reduction of NO x with alkanes. A second group, further called Fe 2 O 3 /ZSM-5, was prepared by impregnation. Significant differences in the exchange behavior were found between both groups; the exchange rate is higher for Fe 2 O 3 /ZSM-5 than for Fe/ZSM-5, in accordance with the higher selectivity for alkane combustion of Fe 2 O 3 /ZSM-5 and its low selectivity for NO x reduction. The data suggest that negligible amounts of iron oxide particles exist in the Fe/ZSM-5 catalyst. For Fe/ZSM-5 catalysts the exchange is not limited to the oxygen atoms directly attached to iron; some zeolite oxygen is also exchanged, although this does not occur for the metal-free zeolite. Kinetic analysis shows that with Fe 2 O 3 /ZSM-5 the R 2 exchange mechanism prevails in which 18 O 2 is exchanged in one step against two 16 O atoms. For the selective Fe/ZSM-5 catalyst R 1 exchange prevails at low temperature, i.e., 18 O 2 molecules exchange only one O atom against an atom of the solid. The qualitative difference between both types of catalyst indicates that Fe/ZSM-5 catalysts owe their selectivity in NO x reduction to Fe oxo-ions.