An in situ UV–vis and FTIR spectroscopy study of the effect of H and CO during the selective catalytic reduction of nitrogen oxides over a silver alumina catalyst (original) (raw)
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Journal of Catalysis, 2006
We examined the role of silver and alumina in Ag-alumina catalysts for the selective catalytic reduction (SCR) of NO x by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag-alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 • C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [Ag-O-Al] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV-vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [Ag-O-Al] species after leaching. NO-O 2 -TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NO x adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NO x species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH 4 to produce N 2 in the presence of oxygen, beginning at ∼300 • C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag-alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH 4 -SCR of NO x . These catalyze the direct oxidation of methane at temperatures as low as 300 • C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples. (M. Flytzani-Stephanopoulos). review by Burch et al. [13] discusses these types of catalysts, which include rare earth oxides (REO) and alumina-(or other oxides) supported Pt, Ga, In and Pd catalysts. The platinum group catalysts are active at low temperatures, but are limited by a narrow operating temperature window in which they display good selectivity. They are also more selective to undesired N 2 O, a potent greenhouse by-product. Ga and In catalysts are limited by loss of activity in the presence of water; and Pd catalysts are inhibited by excess of oxygen in the exhaust gas stream. Other reports in the literature point to the enhancement of both activity and stability using sulfated alumina or zirconia as supports for Pd , Mn [17] and Co for the SCR of NO with methane. The acidity of these supports was considered key to stabilizing the active state of the metal species.
Applied Catalysis B: Environmental, 2016
This work focuses on the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ethane, ethene, acetic acid, dimethyl ether (DME) and ethanol, respectively, over alumina-supported silver and indium catalysts. The impact of hydrogen and the nature of the reductant on the catalyst structure and SCR activity are studied. The ability to reduce NOx differs significantly among the studied reductants, but also between the two catalysts, of which Ag/Al2O3 is the overall most active catalyst. However, the In/Al2O3 catalyst exhibits high SCR activity with DME as reductant in contrast to the Ag/Al2O3 catalyst, which shows negligible activity. We attribute this difference to the higher number of acidic sites on the In/Al2O3 catalyst and the higher degree of DME combustion over Ag/Al2O3. Moreover, the number of sites that previously have been identified to promote hydrocarbon activation, i.e. silver cluster sites (Agn δ+) in Ag/Al2O3 catalysts and indium cluster sites (In 3+) in In/Al2O3 catalysts, are shown to increase upon addition of small amounts of hydrogen in the feed. We suggest that the increased SCR activity over In/Al2O3 originates from an increased number of sites for activation of hydrocarbons, analogous to previous proposals for Ag-based catalysts, in combination with a direct effect on the reaction mechanism.
The Journal of Physical Chemistry C, 2011
The performance of silver alumina catalysts and silver aluminate was studied in the selective catalytic reduction (SCR) of NO by propene. The use of boehmite during the impregnation step ensured a strong interaction between the silver species and the alumina surface in the final calcined catalyst. Thus, higher silver loadings (5-7 wt % silver) could be used without significant loss in selectivity to N 2 during SCR. The nature of the silver species and the formation of adsorbed surface species during SCR and hydrogen-assisted SCR (H 2 -SCR) was studied by activity measurements and by combined in situ IR and X-absorption spectroscopic measurements. The combination of these techniques in the same reaction cell allowed simultaneous monitoring of the state of silver and the formation of surface species under realistic reaction conditions. The active silver species on alumina support were concluded to be 2-dimensional oxidic Ag n δ+ species. Silver aluminate was ruled out as a possibility for an active phase for the SCR reaction. The oxidic Ag n δ+ species were present under both SCR and H 2 -SCR reaction conditions and even on a prereduced catalyst under the SCR reaction conditions. Hydrogen is proposed to enhance the formation of adsorbed surface species, especially nitrites, but not to change the nature of the active silver sites. † Part of the "Alfons Baiker Festschrift".
Journal of Catalysis, 2005
Decane-SCR-NO x , oxidation of NO and decane by molecular oxygen, and these reactions enhanced by hydrogen over Ag/alumina were investigated by monitoring of gaseous product composition, surface intermediates (in situ FTIR), and the state of silver (in situ UV-vis) under realistic reaction conditions in the steady-state and transient modes. It has been shown that oxidation of NO or decane by oxygen is greatly affected by the presence of decane or NO. Monodentate nitrates are formed preferentially and are more reactive compared with the bidentate species. Oxidation of decane mostly yields surface acetates, and the presence of NO x favors the formation of formates (acrylates). The reaction steps most enhanced by the addition of hydrogen to the SCR-NO x reaction are the transformations of the intermediate-CN species into-NCO and oxidation of the hydrocarbon to formates (acrylates). Although NO-NO 2 oxidation is also enhanced by hydrogen, this effect does not contribute to the increased rate of the SCR-NO x reaction. A small part of very reactive Ag + (estimated to be < 5%) is reduced to metallic charged Ag n δ+ clusters (n 8) during both the decane-and H 2 /decane-SCR-NO x reactions. The number of Ag n δ+ clusters formed depends mainly on the level of NO conversion to nitrogen, regardless of whether the conversion level is attained by the addition of hydrogen or by an increased concentration of decane or oxygen in the feed. The time-resolved responses of NO x-N 2 conversion and of the number of Ag n δ+ clusters to the addition/removal of hydrogen from the reactants indicate that the Ag n δ+ clusters are mainly formed because of the reducing effect of adsorbed CH x O-containing reaction intermediates. The active sites are suggested to be single Ag + ions or small Ag 2 O species. Hydrogen itself takes part in the SCR-NO x reaction. It is supposed that it dissociates, forms Ag-hydride, and enhances SCR-NO x by initiation radical reactions.
Catalysis Today, 2000
The distribution of gaseous products and the nature of the surface species generated during the selective catalytic reduction of NO with C 3 H 6 in the presence of excess O 2 (i.e. C 3 H 6 -SCR) were studied over both a 0.4% Co/␥-Al 2 O 3 catalyst and a sulphated 1.2% Ag/␥-Al 2 O 3 catalyst. The results were compared with those previously reported for the C 3 H 6 -SCR over 1.2% . High concentrations of NO 2 were observed in the product stream of the SCR reaction over the 0.4% Co/␥-Al 2 O 3 and sulphated 1.2% Ag/␥-Al 2 O 3 materials. The results show that (as in the case of the ␥-Al 2 O 3 and also probably that of the 1.2% Ag/␥-Al 2 O 3 ) the NO 2 was formed via an alternative route to the direct oxidation of NO with O 2 . The yields of NO 2 were higher over the Co/␥-Al 2 O 3 than over the other materials and in contrast to the other materials, no NH 3 was produced over the Co/␥-Al 2 O 3 catalyst. Based on these results and those of in situ DRIFTS experiments, a global reaction scheme incorporating organo-nitrogen species as key intermediates is proposed. In this scheme, NO, propene and oxygen react to form organo-nitro and/or organo-nitrito adsorbed species, the reaction products of which combine to yield N 2 . The results reported here suggest that Co preferentially promotes the formation of nitrito-compounds which can readily decompose to NO 2 , whereas Ag preferentially promotes the formation of nitro-compounds (from reaction of strongly bound ad-NO x species) which can decompose to isocyanates and ammonia. The sulphation of the 1.2% Ag/␥-Al 2 O 3 reduced the surface concentration of strongly bound ad-NO x species which were thought to react with the reductant or derived species to yield the organo-nitrogen species. : S 0 9 2 0 -5 8 6 1 ( 0 0 ) 0 0 2 9 5 -9
Applied Catalysis B: Environmental, 2000
A 1.2 wt.% Ag/␥-Al 2 O 3 catalyst for the selective catalytic reduction of NO with propene (C 3 H 6 -SCR) was rapidly and permanently deactivated following the addition of 100 ppm of SO 2 to the gas stream. DRIFTS studies performed on the deactivated silver-alumina material showed the formation of two different types of surface sulphate species. One of these surface species was a surface aluminium sulphate whereas the other corresponded to a sulphate associated with the silver phase. Treatment at higher temperatures in the deNO x feed did not regenerate the catalytic activity fully whereas reduction with H 2 was able to do so by removing only the sulphate species associated with the silver phase. When NO 2 was used as reactant, there was no need for a promoter for either the alumina or the sulphated alumina, both of those giving high conversions of the NO 2 to N 2 over a wide temperature window. The decrease in the SCR activity observed over the Ag/␥-Al 2 O 3 upon sulphation is thought to be associated with a decrease of the activity for the oxidation of NO to ad-NO x species (x > 1) promoted by the silver. It is suggested that the reduction of ad-NO x species and NO 2 by propene over the 1.2 wt.% Ag/␥-Al 2 O 3 catalyst (plain or sulphated) mostly occurred on the alumina (clean or sulphated).