Enhancement of decane-SCR-NO over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV–vis study (original) (raw)
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Mechanistic investigation of ethanol SCR of NO over Ag/Al2O3
Catalysis Today, 2012
A 2 wt.% Ag/␥-Al 2 O 3 catalyst was studied for the ethanol selective catalytic reduction of NO x from 200 to 550 • C and space velocities between 30,000 h −1 and 140,000 h −1. Peak NO x conversions reached 85% at 400 • C, and an activation energy was determined to be 57 kJ/mol with a feed of ethanol to NO x or HC 1 /NO x = 3. Up to 80% of the NO is oxidized to NO 2 at 250 • C, but overall NO x conversion is only 15%. Interestingly, ethanol oxidation occurs at much lower temperatures than NO x reduction; at 250 • C, ethanol oxidation is 80% when flowing ethanol + NO + O 2. This increased reactivity, compared to only 15% when flowing only ethanol + O 2 , combined with the observation that NO is not oxidized to NO 2 in the absence of ethanol illustrates a synergistic relationship between the reactants. To further investigate this chemistry, a series of DRIFTS experiments were performed. To form nitrates/nitrites on the catalysts it was necessary to include ethanol in the feed with NO. These nitrates/nitrites were readily formed when flowing NO 2 over the catalyst. It is proposed that ethanol adsorbs through an ethoxy-intermediate that results in atomic hydrogen on the surface. This hydrogen aids the release of NO 2 from Ag to the gas-phase which, can be subsequently adsorbed at ␥-Al 2 O 3 sites away from Ag. The disappearance of these nitrates/nitrites at higher temperatures proceeds in parallel with the increase in NO x reduction reactivity between 300 and 350 • C observed in the kinetic study. It is therefore proposed that the consumption of nitrates is involved in the rate determining step for this reaction.
Study of the “Fast SCR”-like mechanism of H2-assisted SCR of NOx with ammonia over Ag/Al2O3
Applied Catalysis B: Environmental, 2012
It is shown that Ag/Al 2 O 3 is a unique catalytic system for H 2-assisted selective catalytic reduction of NO x by NH 3 (NH 3-SCR) with both Ag and alumina being necessary components of the catalyst. The ability of Ag/Al 2 O 3 and pure Al 2 O 3 to catalyse SCR of mixtures of NO and NO 2 by ammonia is demonstrated, the surface species occurring discussed, and a "Fast SCR"-like mechanism of the process is proposed. The possibility of catalyst surface blocking by adsorbed NO x and the *Revised Manuscript Click here to view linked References 2 influence of hydrogen on desorption of NO x were evaluated by FTIR and DFT calculations.
Catalysts
Alumina-supported silver and indium catalysts are investigated for the hydrogen-assisted selective catalytic reduction (SCR) of NOx with ammonia. Particularly, we focus on the active phase of the catalyst and the formation of surface species, as a function of the gas environment. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy was used to follow the oxidation state of the silver and indium phases, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to elucidate the formation of surface species during SCR conditions. In addition, the NOx reduction efficiency of the materials was evaluated using H2-assisted NH3-SCR. The DRIFTS results show that the Ag/Al2O3 sample forms NO-containing surface species during SCR conditions to a higher extent compared to the In/Al2O3 sample. The silver sample also appears to be more reduced by H2 than the indium sample, as revealed by UV-vis spectroscopic experiments. Addition of H2, however, may promot...
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
Catal. Sci. Technol., 2014
Formation and stability of surface NO x species related to the promotional effect of H 2 over Ag-Al 2 O 3 as NO x reduction catalyst were investigated with temperature-programmed desorption and DRIFT spectroscopy. Formation of two groups of surface NO x species was found: a less thermally stable group of "low temperature (LT) species" and a more thermally stable group of "high temperature (HT) species". The LT NO x was attributable to the decomposition of surface NO x species formed on the active sites where its elimination by addition of H 2 or thermal decomposition correlated with higher NO oxidation and NO x reduction conversion. Under reaction conditions, these possibly inhibiting LT NO x species were stable up to about 300°C and their formation depended on donation of oxygen from surface oxides. Removal of LT nitrate species by H 2 accounted for only a fraction of the increased NO oxidation and NO x reduction conversion by co-feeding H 2 . Furthermore, it was also found that H 2 facilitates formation of HT NO x that primarily corresponded to the decomposition of spectator species on the Al 2 O 3 support identified as monodentate nitrate species. From TPD studies of C 3 H 6 -SCR, it was shown that H 2 not only eliminated LT NO x but also promoted formation of greater quantities of adsorbed hydrocarbons.
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.
Catalysis Letters, 2000
Addition of H 2 to a NO/NH 3 /O 2 /H 2 O feed for selective catalytic reduction of nitrogen oxide over Ag/Al 2 O 3 catalysts causes an unusual enhancement of activity, e.g., the marginal activity (< 10%) of 1 wt% Ag impregnated on c-Al 2 O 3 or mesoporous Al 2 O 3 modifications is boosted to nearly 100% over a broad temperature range from 200 to 550°C at a space velocity of 30,000 cm 3 g )1 h )1 ). Contrary, silver on SiO 2 or a-Al 2 O 3 shows no improvement of activity in the presence of H 2 . The effect is tentatively attributed to a higher percentage of intermediary nano-sized Ag clusters on high-surface area Al 2 O 3 in the presence of hydrogen. This promotes oxygen activation and hence NO oxidation to reactive intermediate nitrite species. The required dispersion of Ag cannot be stabilized on SiO 2 or a-Al 2 O 3 .
Journal of Catalysis, 1999
The selective catalytic reduction of NO with C 3 H 6 in the presence of a large excess of O 2 (i.e., C 3 H 6 -SCR (selective catalytic reduction)) was studied over γ -Al 2 O 3 , 1.2% Ag/γ -Al 2 O 3 , and 10% Ag/γ -Al 2 O 3 catalysts. The γ -Al 2 O 3 and the low-loading silver material exhibited high conversions to N 2 whereas the high-loading sample predominantly yielded N 2 O. Surprisingly, a comparison of actual NO 2 yields to thermodynamically predicted yields of NO 2 showed that the formation of NO 2 during the C 3 H 6 -SCR of NO over γ -Al 2 O 3 was not achieved through the direct oxidation of NO with O 2 . An alternative mechanism involving the formation of organonitrite species followed by their decomposition/oxidation was suggested to be the main route for the formation of NO 2 . The promoting role of low loadings of silver on alumina on the activity for N 2 production was attributed to the higher rate of formation of inorganic ad-NO x species (e.g., nitrates) as evidenced by in situ DRIFTS and thermogravimetric analyses. It was proposed that these inorganic ad-NO x species further react with the reductant or a derived species to form various organo-NO x compounds. In particular, organo-nitro and organo-nitroso compounds and/or their derivatives (e.g., isocyanate, cyanide, amines, and NH 3 ) were suggested to react with NO or the organo-nitrite and/or its derivative NO 2 to yield N 2 . When no reductant was present, the low-loading Ag/γ -Al 2 O 3 material was poisoned by strongly bound nitrates and its activity for NO 2 formation was similar to that observed over the alumina.
Journal of Catalysis, 2005
The effect of hydrogen and carbon monoxide on the selective catalytic reduction of NO x with decane over a 2 wt% Ag/Al 2 O 3 catalyst was investigated. Hydrogen promoted NO x conversion during the SCR reaction, in contrast to carbon monoxide, which had a slightly detrimental effect on catalyst activity. However, in situ UV-vis spectroscopy showed that both carbon monoxide and hydrogen promoted the formation of Ag clusters during the reaction. This, coupled with previous EXAFS experiments and UV-vis data on the rate of change of cluster formation, demonstrate that the enhanced activity from the addition of hydrogen to the NO x-SCR mix cannot be attributed exclusively to the formation of Ag clusters. It is proposed that hydrogen itself participates directly in the reaction mechanism.