Presence Of Excess Oxygen Research Articles

Enhancing the NO/C3H8/O2 Reaction by Using H2 over Ag/Al2O3 Catalysts under Lean-Exhaust Conditions

Abstract We examine the effect of H2 on the selective reduction of NO by C3H8 over Ag/Al2O3 catalysts under lean-exhaust conditions. The NO reduction activity caused by C3H8 at low temperature (590-760 K) is greatly increased by adding H2 in the presence of excess oxygen and water vapor.

Superior Pillared Clay Catalysts for Selective Catalytic Reduction of Nitrogen Oxides for Power Plant Emission Control

ABSTRACT Fe3+-, Cr3+-, Cu2+-, Mn2+-, Co2+-, and Ni2+-exchanged Al2O3-pillared interlayer clay (PILC) or TiO2-PILC catalysts are investigated for the selective catalytic reduction (SCR) of nitric oxide by ammonia in the presence of excess oxygen. Fe3+-exchanged pillared clay is found to be the most active. The catalytic activity of Fe-TiO2-PILC could be further improved by the addition of a small amount of cerium ions or cerium oxide. H2O and SO2 increase both the activity and the product selectivity to N2. The maximum activity on the Ce-Fe-TiO2-PILC is more than 3 times as active as that on a vanadium catalyst. Moreover, compared to the V2O5-WO3/TiO2 catalyst, the Fe-TiO2-PILC catalysts show higher N2/N2O product selectivities and substantially lower activities (by ~85%) for SO2 oxidation to SO3 under the same reaction conditions. A 100-hr run in the presence of H2O and SO2 for the CeO2/Fe-TiO2-PILC catalyst showed no decrease in activity.

Combinatorial computational chemistry approach to the design of deNO x catalysts

Combinatorial chemistry is an efficient technique for the synthesis and screening of a large number of compounds. Recently, we introduced the combinatorial approach to computational chemistry for catalyst design and proposed a new method called ‘combinatorial computational chemistry’. In the present study, we have applied this combinatorial computational chemistry approach to the design of deNO x catalysts. Various ion-exchanged ZSM-5 are candidates as catalysts for the removal of nitrogen oxides (NO x ) from exhaust gases in the presence of excess oxygen. Here, we describe the screening of the exchange cations in ion-exchanged ZSM-5 which are strong against poisons. We investigated the adsorption energies of NO and water on various ion-exchanged ZSM-5 catalysts. Cu +, Ag +, Au +, Fe 2+, Co 2+, Ni 2+, Pd 2+, Pt 2+, Cr 3+, Fe 3+, Ir 3+ and Tl 3+ were found to have high resistance to water molecules during the deNO x reaction.

Studies on the structural evolution of highly active Ir-based catalysts for the selective reduction of NO with reductants in oxidizing conditions

NOx reduction over Ir-based catalysts in the presence of excess oxygen with hydrocarbon as a reductant was investigated in the focus on observing microstructure of an Ir particle supported on various carriers and structural evolution of highly active Ir-based catalysts in the NOx reduction. Characterization of Ir-based catalysts using SEM, TEM, XRD, CO chemisorption and XPS, and reaction studies on various Ir-based catalysts have proved that the formation of a relatively large Ir metal particle with 40–60nm of nanocrystal size carried on inert supports has been a prerequisite for the evolution of high activities in the NOx reduction rather than the formation of Ir metal state on the catalyst. Furthermore, in inert gas conditions in a high temperature range of 850–950°C, Ir metal was easily formed by using the support such as TiO2 and ZrO2 that drastically decreased its specific surface area in the temperature range.

Reduction of NO by Propene Over Pt, Pd and Rh-Based ZSM-5 Under Lean-Burn Conditions

Selective catalytic reduction of NO by propene has been investigated over noble metal (Pt, Pd, Rh)-based ZSM-5 catalysts. These samples were tested in a gas mixture system in the presence of excess oxygen, simulating lean-burn exhaust gases. The sequence in activity for NO reduction was Pt > Rh ∼ Pd. Regarding the selectivity of the reaction to N2, an opposite trend was observed: Rh > Pd ∼ Pt. The catalytic systems have presented stable operation under isothermal conditions during time-on-stream experiments.

Microwave discharge-assisted catalytic conversion of NO to N2

By coupling microwave discharge with an Fe/HZSM-5 catalyst, novel effects have been observed for the conversion of NO to N2 in the presence of excess oxygen with high efficiency.

Electrochemical promotion of NO reduction by C3H6 and CO on Rh/YSZ catalyst — Electrodes

The selective catalytic reduction of NO by propylene or CO in the presence of excess oxygen is a system of great technological importance. The effect of Electrochemical Promotion (or Non-faradaic Electrochemical Modification of Catalytic Activity — NEMCA) was used to promote this reaction (C3H6 or CO/NO/O2) on Rh/YSZ catalyst-electrodes. It was found that both the catalytic activity and the selectivity of the Rh catalyst-electrode is affected dramatically upon varying its potential with respect to a Au pseudoreference electrode. Catalytic rate enhancements up to 15000% and 6000% were observed in the case of NO reduction by propylene, while the product selectivity to N2 production is affected significantly (up to 200%) upon positive potential application. Remarkable promotion of the catalytic activity was also observed in the case of NO reduction by CO, since up to 20-fold increases both in catalytic rates and in NO conversion were obtained under NEMCA conditions.

Effect of the Support in de-NOx HC-SCR Over Transition Metal Catalysts

Several catalysts based on transition metals (Cu, Co, Fe) and different supports (ZSM-5, activated carbon, Al2O3) have been tested by Temperature-Programmed Reaction (TPR) experiments for the selective catalytic reduction of NOx with propene in the presence of excess oxygen, simulating lean-burn conditions. The activity order with respect to the metal was Cu∼Fe>Co for all supports used. ZSM-5 catalysts have a superior behavior over Al2O3, as observed for noble metal catalysts. Application of activated carbon as a support is not practical due to its consumption at the reaction temperatures. The selectivity to N2 of the catalysts was also independent of the support, being higher than 95% in all the cases.

Enhanced activity of an In–Fe2O3/H-ZSM-5 catalyst for NO reduction with methane

Selective reduction of NO by CH4 on an In–Fe2O3/H-ZSM-5 catalyst was investigated in the presence of excess oxygen. Compared with In/H-ZSM-5, the In–Fe2O3/H-ZSM-5 catalyst with high Fe2O3 contents showed higher activity in a wide range of reaction temperatures. It was found that the addition of Fe2O3 yielded a promotion effect on CH4 activation. The influence of water vapor on NO conversion was also investigated. The activity of the In/H-ZSM-5 catalyst has been found to be strongly inhibited by water vapor, while the In–Fe2O3/H-ZSM-5 catalyst remained fairly active in the presence of 3.3% steam.

NO reduction by CH 4 in the presence of excess O 2 over Pd/sulfated zirconia catalysts

The selective catalytic reduction (SCR) of NO by methane in the presence of excess oxygen has been studied on a series of Pd catalysts supported on sulfated zirconia (SZ). This support is not as sensitive to structural damage by steaming as the acidic zeolites, such as H-ZSM-5 and H-Mor. In previous studies, it was shown that this type of acidic zeolites are able to stabilize Pd 2+ ions and promote high SCR activity and selectivity, which are typically not seen in Pd catalysts. In this contribution, it has been demonstrated that SZ is able to promote the NO reduction activity in a similar way to the acidic zeolites, by stabilizing Pd 2+ ions that is selective for NO reduction. As in the case of acidic zeolites, the stabilization of Pd 2+ ions can occur through a transfer of Pd species from particle to particle. One of the attractive features of Pd/SZ catalysts is that they are less sensitive to water and SO 2 poisoning than Pd/H-ZSM-5 catalyst and exhibit higher reversibility after removal of water or SO 2.

Perturbed Angular Correlation Characterization of Indium Species on In/H-ZSM5 Catalysts

Silicalite-supportedindium catalysts (In/H-ZSM5) were characterized by time differential perturbed angular correlation (PAC) and temperature-programmed reduction (TPR). The presence of different indium species was correlated with activity and selectivity during the NO selective catalytic reduction (SCR) with CH4 in the presence of excess oxygen. The main species identified on the In/H-ZSM5 surface were In2O3 (indium sesquioxide crystals); In+Z− and (InO)+Z− (different indium species exchanged in the zeolitic matrix); and highly dispersed noncrystalline In oxide species not bonded to the zeolitic matrix. Catalysts that were impregnated and then calcined at 500°C had low activity for the reaction under study, showing the presence of only In2O3 and noncrystalline In oxide species. Treatment at 750°C in O2 or at 500°C in H2 followed by reoxidation at the same temperature resulted in active catalysts showing an appreciable concentration of (InO)+Z− active species. The same active species were formed after indium ion exchange of NH4-ZSM5 was followed by calcination at 500°C. The PAC technique proved to be a powerful tool for the identification and quantification of indium species present on the surface of an H-ZSM5 support.

Selective catalytic reduction of NO by propene in excess oxygen on Pt- and Rh-supported alumina catalysts

A comparative study of Pt/alumina and Rh/alumina catalysts was performed for the selective catalytic reduction of NO with C 3H 6 in the presence of excess oxygen. Pt/alumina was more active for NO reduction at lower temperatures compared to Rh/alumina. However, the latter exhibited clearly superior performance in terms of selectivity to N 2. This makes Rh/alumina a more suitable catalyst for the selective catalytic reduction of NO under excess oxygen conditions. Detailed kinetic studies of the SCR of NO were performed on Pt/alumina and Rh/alumina to obtain low-temperature kinetic expressions for NO reduction and C 3H 6 oxidation in the presence and absence of NO. Qualitative similarities yet quantitative differences in these kinetic expressions appear to indicate the existence of two partially similar mechanistic schemes. One is based on the indirect participation of the reductant through reduction of the active sites, followed by dissociative adsorption of NO on reduced sites (applicable for Pt/alumina). The other is based on the direct participation of the reductant (apparently by its partial oxidation) in forming an activated intermediate species, followed by its interaction with activated NO (applicable for Rh/alumina).

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO2-pillared clay (TiO2-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V2O5/TiO2-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V2O5/TiO2 catalyst and was the same as the commercial V2O5+WO3/TiO2 catalyst. The V2O5/TiO2-PILC catalysts also had higher N2/N2O product selectivities as compared to V2O5 doped TiO2 catalysts. In addition, H2O+SO2 slightly increased the activities at high temperatures (>350°C) for the V2O5/TiO2-PILC catalysts. Addition of WO3 to V2O5 further increased the activities of the PILC catalysts. These results indicate that TiO2-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH3 on the V2O5/TiO2-PILC is similar to that on V2O5/TiO2 catalyst, i.e., N2 originates from the reaction between gaseous NO and NH3 adspecies.

States of Pd in Pd/H–ZSM-5 and Pd/Na–ZSM-5 catalysts and catalytic activity for the reduction of NO by CH 4 in the presence of O 2

Reduction of NO by methane in the presence of excess oxygen (NO–CH 4–O 2 reaction) catalyzed by four Pd/H–ZSM-5 catalysts having very different states of Pd (high or low dispersion ((A) or (B)); and preoxidized or prereduced (O or R)), was investigated, focusing on the relationships between the states of Pd and the catalytic performance. Pd/Na–ZSM-5 was also studied for comparison. The states of Pd (dispersion, distribution, and oxidation state) were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), TEM, and NaCl titration before the reaction and after 10 min, and 4 and 14 h of the NO–CH 4–O 2 reaction at 673 K and GHSV = 90,000 h −1. Except for the initial variation in 1–2 h, the differences in the catalytic performances as well as their variation with reaction time very well corresponded with the differences and variation of the state of Pd. For example, preoxidized Pd/H–ZSM-5(A), of which Pd atoms were mostly in the isolated Pd 2+ state and distributed almost uniformly in the zeolite particle, showed a high activity for NO reduction from the beginning and rapidly reached steady state. On prereduced Pd/H–ZSM-5(A), Pd was initially in highly dispersed metallic state but oxidized very quickly in the NO–CH 4–O 2 stream and after that the state of Pd and the catalytic performance exhibited a similar behavior as the preoxidized Pd/H–ZSM-5(A). Pd/H–ZSM-5(B), preoxidized and prereduced, had initially large PdO and Pd 0 particles (140–160 Å) on the external surface, respectively, which were dispersed gradually into the micropores as isolated Pd 2+. The oxidation of Pd 0 was very fast. The catalytic performance changed correspondingly; from low to high activity for NO reduction and from high to low activity for CH 4 oxidation. In the steady state at 673 K, all four catalysts showed the same high activity for NO reduction and modest activity for CH 4 oxidation, corresponding to the same state of Pd. On preoxidized Pd/Na–ZSM-5, which showed a low activity for NO reduction, Pd atoms were mainly in the form of PdO particles (170 Å) on the external surface, and they changed little during the reaction. These results clearly demonstrate that isolated Pd 2+ ions in the zeolite micropore are active and selective for NO reduction, and PdO particles on the external surface are active mainly for CH 4 oxidation. These conclusions are consistent with the results in the literature and our earlier work.

Selective Catalytic Reduction of Nitrogen Oxides by Ammonia over Fe3+-Exchanged TiO2-Pillared Clay Catalysts

Fe-exchanged TiO2-pillared clay (PILC) catalysts were prepared and used for selective catalytic reduction (SCR) of NOx by ammonia. They were also characterized for surface area, pore size distribution, and by XRD, H2-TPR, and FT-IR methods. The Fe–TiO2–PILC catalysts showed high activities in the reduction of NOx by NH3 in the presence of excess oxygen. SO2 further increased the catalytic activities at above 350°C, whereas H2O decreased the activity slightly. The catalysts were about twice as active as commercial-type V2O5–WO3/TiO2 catalyst in the presence of H2O and SO2. Moreover, compared to the commercial catalyst, the Fe–TiO2–PILC catalysts had higher N2/N2O product selectivities (e.g., 0–1% vs 9% N2O at 400°C) and substantially lower activities (by 74–88%) for SO2 oxidation to SO3 under the same reaction conditions. The activity was further increased to over three times that of the vanadia-based catalyst when Ce was added. The high activity and low N2O selectivity for the Fe–TiO2–PILC catalysts were attributed to their low activity in the oxidation of ammonia, as compared with vanadia catalysts. XRD patterns of Fe–TiO2–PILC were similar to those of TiO2–PILC, showing no peaks due to iron oxide, even when the iron content reached 20.1%. The TPR results indicated that iron in the Fe–TiO2–PILC catalysts with lower iron contents existed in the form of isolated Fe3+ ions. The activities of Fe–TiO2–PILC catalysts were consistent with their surface acidities, which were identified by FT-IR of the NH3-adsorbed samples. The enhancement of activities by H2O+SO2 was attributed to the increase of surface acidity resulting from the formation of surface sulfate species of iron.

Controlled regioselectivity of fatty acid oxidation by whole cells producing cytochrome P450BM-3 monooxygenase under varied dissolved oxygen concentrations.

Utilising whole cells of recombinant Escherichia coli K27 (pCYP102, pGEc47) containing active cytochrome P450BM-3 monooxygenase [E.C. 1. 14.14.1], multiple oxidations of saturated and unsaturated fatty acids were performed by the enzyme under conditions of excess oxygen. The amount of oxygen dissolved in the culture medium strongly influenced the regioselectivity of the reaction, as reflected in the distribution and amount of oxidised products. We have verified by gas chromatography/mass spectrometry that the products of in vivo biotransformation of pentadecanoic acid by cytochrome P450BM-3 are identical to those formed in cell-free extracts containing the enzyme. The formation of keto- and dihydroxy acids, side products which are characteristic for in vitro conversions with purified cytochrome P450BM-3 in the presence of excess oxygen, has been observed as well. Thus, by varying the oxygen concentration, we could control the regioselectivity of oxidation and the number of products made. Under oxygen limiting conditions, only monooxidised 12-, 13-, and 14-hydroxy-pentadecanoic acids were obtained. Consequently, unwanted side products could be excluded by modulating the amount of oxygen used in the bioconversion. Furthermore, whole cell oxidation of two unsaturated long-chain fatty acids, cis-pentadec-10-enoic and cis-hexadec-9-enoic acid, resulted in the production of epoxides, various subterminal hydroxyalkenoic acids and keto- and hydroxyalkanoic acids. Although we obtained higher activities of C15:0 conversion in vitro, the whole cell biocatalyst proved to be useful for specific oxidations of long-chain fatty acids since there is no need to add the costly cofactor NADPH. This biooxidation by E. coli K27 (pCYP102, pGEc47) under oxygen limitation has been demonstrated at the 2-L scale, showing that 12-, 13-, and 14-hydroxypentadecanoic acids can be produced in the g L-1 range.

Gas chromatographic kinetic study of carbon monoxide oxidation over platinum–rhodium alloy catalysts

The kinetics for the oxidation of carbon monoxide in the presence of excess oxygen over Pt–Rh alloy catalysts were studied by using the reversed-flow gas chromatography technique. Suitable mathematical analysis equations were derived by means of which the rate constants for the oxidation reaction of carbon monoxide, as well as for the adsorption and desorption of the reactant CO on the catalysts pure Pt, 75 atom% Pt+25 atom% Rh, 50 atom% Pt+50 atom% Rh, 25 atom% Pt+75 atom% Rh and pure Rh supported on SiO 2 were determined. All the catalysts show a maximum rate constant for the production of CO 2 at a characteristic temperature close to that found in the literature. The rate constants for the adsorption of CO increase generally with increasing temperature, while those for the desorption decrease with increasing temperature. From the variation of the rate constants with temperature activation energies for the oxidation reaction and adsorption of CO were determined, which are sensitive to the composition of the catalytic surface. The appearance of CO 2 and carbon, when introducing pure CO into the column with the catalysts, verified a partial dissociative adsorption (e.g., disproportionation) of CO on the catalysts used. The latter indicates a mechanism for the CO oxidation through a partial dissociative adsorption of CO followed by the reaction of adsorbed CO molecules with adsorbed O atoms.

Reduction of Nitric Oxide in Diesel Exhaust With the Addition of Methylamine

A Chemical gas-phase process capable to reduce the nitric oxide in diesel engine exhaust was studied. In this process, monomethylamine (CH3NH2) was added to the exhaust gas in a molar ration to NO varying between 1:1 and 4:1, Experiments were conducted in electrically heated quartz, reactors in a temperature range of 200°C to 600°C. Diesel exhaust gas and simulated exhaust gas were used in this experiment. The results showed thorough mixing of methylamine into the exhaust effectively breaks NO down into nitrogen and water, enabling more than 80 percent NO reduction in a reactor temperature range of 400°C to 540°C and at a molar ratio of 1. On the other hand, imperfect mixing between methylamine and exhaust gases results in excess ammonia and reduced NO decomposition. Consequently, it is suggested that the mixing is a very important factor in this technique. The results also show that the coexisting gases such as carbon monoxide, carbon dioxide, hydrocarbons, and water vapor in the diesel exhaust have no effect on NO reduction by methylamine. However, the presence of oxygen in excess of 10 percent in the exhaust is needed for an 80 percent NOx reduction. Furthermore, the mechanisms of the methylamine process were discussed.

A highly durable catalytic NOx reduction in the presence of SOx using periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions

Catalytic NOx reduction system in the presence of excess oxygen using periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions was investigated over various catalysts, comparing with HC-SCR in full lean conditions. Based on findings and studying the correlation between transient reaction profile from rich to lean excursions and reaction behaviors in both the NOx reductions, a highly effective and durable NOx reduction system in the presence of excess oxygen and sulfur oxides using the periodic two steps was designed. This system consisted of a short operation in oxidizing conditions and a far shorter operation in reducing conditions, and it effectively reduced NOx over Rh supported on alumina with no deterioration in the short-term durability test in the presence of 40 ppm SO 2.

The catalytic activity of CuO x/ZrO 2 for the abatement of NO with propene or ammonia in the presence of O 2

Samples CuO x /ZrO 2 (0.1–8.4 Cu atoms nm −2) were prepared by adsorption or impregnation methods. The characterisation of samples by means of XPS, XRD, DRS, ESR, IR and volumetric adsorption of CO, showed copper dispersion up to 2.5 atoms nm −2, and the presence of CuO in samples with higher Cu content. The selective catalytic reduction of NO with propene or ammonia in the presence of excess oxygen, was studied in a flow apparatus with reactant mixtures of various composition (NO : C 3H 6 : O 2 = 0–4000 ppm : 100–2000 ppm : 0–3.5%; and NO : NH 3 : O 2 = 0–700 ppm : 700 ppm : 3.6%). With both propene and ammonia, CuO x /ZrO 2 catalysts containing up to 2.5 Cu atoms nm −2 were active, selective and stable as a function of the time on stream. On all catalysts, with ammonia as reducing agents, up to 523 K, NO conversion equalled NH 3 conversion leading to 100% selectivity. With both propene and ammonia, NO molecules that were converted to N 2 per Cu atom and per second were nearly independent of the Cu content up to 2.5 Cu atoms nm −2.