Dongbai Liang

Unique properties of Ir/ZSM-5 catalyst for NO reduction with CO in the presence of excess oxygen

The reduction of NO with CO in the presence of excess oxygen was investigated over different noble metal catalysts for probing the relationship between catalytic properties and adsorption behaviors. Among the four precious metal catalysts investigated, Ir/ZSM-5 was found to be the only active one for NO reduction with CO under lean conditions. With the decreasing of the Ir content, higher NO conversion and CO selectivity was obtained. Temperature-programmed reaction (TPR) studies of NO/H-2/O-2 and NO/CO/O-2 showed that the Pt/ZSM-5 was active when H-2 was used as the reductant, whereas, the Ir/ZSM-5 was active when CO was the reducing agent. This difference is due to the different mechanisms of the two reactions. Temperature-programmed desorption (TPD) of NO, CO and O-2 showed that NO could dissociate more easily over the Ir/ZSM-5 than on the Pt/ZSM-5, while the oxidation of CO by O-2 proceeded more rapidly on the Pt/ZSM-5 than on the Ir/ZSM-5. The presence of excess O-2 inhibited drastically the dissociation of NO, which is considered as the key step for the NO-CO reaction. The high dissociation rate of NO over the Ir/ZSM-5 is visualized as the key factor for its superior high activity in NO reduction with CO under lean conditions

Performance and deactivation of Ir/γ-Al2O3 catalyst in the hydrogen peroxide monopropellant thruster

The performance of ir/gamma -al2o3 catalyst for the decomposition of high concentration hydrogen peroxide was investigated in a monopropellant thruster. the changes of ignition delay (t(0)), chamber pressure (p-c) and catalyst bed temperature (t-c) with the numbers of startup-shutdown cycles were proved to be effective indicators of catalyst bed efficiency. the fresh catalyst and the deactivated catalyst were characterized with h-2-tpr, xrd and xps. it was found that catalyst oxidation and surface sn poisoning are the major reasons of catalyst deactivation. (c) 2001 elsevier science b.v. all rights reserved.

The role of Mn and Li promoters in supported rhodium catalysts in the formation of acetic acid and acetaldehyde

Abstract RhMnLi/SiO 2 catalysts consisting of 1 wt% Rh for the synthesis of C 2 oxygenates from syngas were studied by CO + H 2 reaction and characterized by TPR, H 2 -TPD, ESR and IR techniques. The RhMnLi/SiO 2 catalysts, which showed a good selectivity for the formation of acetic acid and acetaldehyde, have been found to be quite different from the RhV/SiO 2 catalysts reported previously, which mainly promoted the formation of ethanol. Comparing with the RhV/SiO 2 catalysts, the RhMnLi/SiO 2 exhibited lower capacity of hydrogen adsorption, lower hydrogenation activity, more intense interaction of the manganese oxide with the Rh component, and more intense bands of twin-adsorbed CO (i.e. more Rh 1+ ion). The good selectivity of the RhMnLi/SiO 2 catalysts towards acetaldehyde and acetic acid formation were correlated to these features.

Characterization of Rh-based catalysts with EPR, TPR, IR and XPS

Rh-based catalysts to be used for the synthesis of c-2-oxygenates from syngas were characterized with epr, tpr, ir and xps methods. the chemical state of the mn component was extensively studied with epr after in situ reduction and treatment with various probe molecules. the results indicated that on the mn/sio2 catalyst, mn can exist as isolated mn2+ ions on the surface of sio2 through the formation of coordination compounds with surface hydroxyls and h2o molecules as ligands. thermal reduction of the mn/sio2 catalyst resulted in the migration and accumulation of the mn2+ ions. the results of tpr, ir, xps were consistent with those of epr, which indicated that on a rh-mn/sio2 catalyst, a rh-mn mixed oxide was formed, which stabilized the rh+ species. the formation of small clusters of the rh-mn mixed oxide inhibited deep reduction and accumulation of the mn component, while at the same time increased the dispersion of the rh component. as a promoter, mn acts as an electron acceptor, while li exhibits an electron-donation effect. the li component can inhibit the formation of rh-mn mixed oxide and increase the concentration of rh-0 on the surface of sio2. the existence of li may also cause the tilted-adsorption form of co on rh, as well as the spillover of h-2 from rh to the sio2 support. (c) 1999 elsevier science b.v. all rights reserved.

Effect of addition of Zn on the catalytic activity of a Co/HZSM-5 catalyst for the SCR of NOx with CH4

The selective catalytic reduction (SCR) of NOx by methane in the presence of excess oxygen was studied on a Zn-Co/HZSM-5 catalyst. It was found that the addition of Zn could improve effectively the selectivity of methane towards NOx reduction. When prepared by a coimpregnation method, the Zn-Co/HZSM-5 catalyst showed much higher catalytic activity than the two catalysts of a Zn/Co/HZSM-5 and Co/Zn/HZSM-5 prepared by the successive impregnation method. It is considered that there exists a cooperative effect among the Zn, Co and zeolite, which enhances the reduction of NO to NO2 reaction and the activation of methane

Direct decomposition of NO by microwave heating over Fe/NaZSM-5

Catalytic decomposition of NO was studied over Fe/NaZSM-5 catalyst. Novel results were observed with the microwave heating mode. The conversion of NO to N-2 increased remarkably with the increasing of Fe loading. The effects of a series of reaction parameters, including reaction temperature, O-2 concentration, NO concentration, gas flow rate and H2O addition, on the productivity of N-2 have been investigated. It is shown that the catalyst exhibited good endurance to excess O-2 in the microwave heating mode. Under all reaction conditions, NO converted predominantly to N-2. The highest conversion of NO to N-2 was up to 70%

Catalytic reduction of NO over in situ synthesized Ir/ZSM-5 monoliths

The catalytic performance of Ir-based catalysts was investigated for the reduction of NO under lean-burn conditions over binderless Ir/ZSM-5 monoliths, which were prepared by a vapor phase transport (VPT) technique. The catalytic activity was found to be dependent not only on the Ir content, but also on the ZSM-5 loading of the monolith. With the decreasing of the Ir content or the increasing of the ZSM-5 loading of the monolith, NO conversion increased. When the ZSM-5 loading on the cordierite monolith was raised up to ca. 11% and the metal Ir content was about 5 g/l, the NO conversion reached its maximum value of 73% at 533 K and SV of 20 000 h(-1). Furthermore, both the presence of 10% water vapor in the feed gas and the variation of space velocity of the reaction gases have little effect on the NO conversion. A comparative test between Ir/ZSM-5 and Cu/ZSM-5, as well as the variation of the feed gas compositions, revealed that Ir/ZSM-5 is very active for the reduction of NO by CO under lean conditions, although it is a poor catalyst for the C3H8-SCR process. This unique property of Ir/ZSM-5 makes it superior to the traditional three-way catalyst (TWC) for NO reduction under lean conditions

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. (c) 2000 elsevier science b.v. all rights reserved.

Interaction of methane with surfaces of silica, aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (η,γ and α) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactiveΝ1 band (2917 cm−1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm−1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, γ-alumina and HZSM-5 zeolite could exchange with CD4 at temperatures higher than 773K, while those on η-alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al2O3 (both η and γ) at temperatures higher than 473 K. The formate species would decompose to CO2, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, α-Al2O3 did not adsorb or react with methane in any case.

A study on reduction behaviors of the supported platinum-iron catalysts

Abstract The reduction behaviors of the supported platinum–iron catalysts and their comparison with supported iron catalysts were studied by TPR (temperature-programmed reduction)–in situ 57 Fe MBS (Mossbauer spectroscopy). The results indicated that the TPR processes of all Fe-containing catalysts were different from that of bulk α-Fe2O3. There were interactions between Pt, Fe and the γ-Al2O3 or SiO2 support for the Pt–Fe/γ-Al2O3 and Pt–Fe/SiO2 catalysts. All the iron-containing catalysts show that Fe3+ was highly dispersed on the support (γ-Al2O3 and SiO2) before reduction. No Fe0 was found in the reduction processes. The Fe3+ was reduced to Fe2+ in tetrahedral vacancy first for the reduction of the Pt–Fe/γ-Al2O3 catalyst. No Fe2+ in octahedral vacancy was found in the reduction of the Pt–Fe/SiO2 catalyst. Adding Pt to Fe/support (γ-Al2O3 or SiO2) could promote the reduction of the Fe species.