Characterization of Ag/γ-Al2O3 catalysts and their lean-NOx properties

Hoost, T. E.; Kudla, Robert; Collins, Kim M.; Chattha, M. S.

doi:10.1016/s0926-3373(96)00090-2

Cited by 83 publications

(57 citation statements)

References 31 publications

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“…This effect may be due to the presence of larger particles, which favor oxygen and propylene adsorption on the same active particle; hence, the propylene oxidation is more effective than the NO reduction. These results are in accord to other authors, 10,[22][23][24][25] that found a relationship between particle size, oxidation state and catalytic activities.…”

Section: Resultssupporting

confidence: 93%

Selective reduction of NOx by propylene over silver catalyst under oxidative conditions

Cónsul

Thiele

Baibich

et al. 2004

J. Braz. Chem. Soc.

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Neste trabalho foram estudados catalisadores Ag/Al 2 O 3 e Ag/Al 2 O 3 /Corderita, preparados por um método de impregnação convencional, na reação de redução de NO utilizando propeno como agente redutor. Os catalisadores foram caracterizados por redução a temperatura programada (RTP), quimissorção de oxigênio e área superficial, pelo método BET. A atividade catalítica foi determinada utilizando uma mistura reacional contendo 100 ppm de NO, 250 ppm de C 3 H 6 e 2% de oxigênio e uma velocidade espacial de 100000 h -1 . Os produtos da reação foram estudados por FTIR. Os resultados dos testes catalíticos mostraram 56% de conversão de NO para o catalisador 2,22%Ag/ Al 2 O 3 a 723 K. Os catalisadores de cordierita apresentaram um comportamento semelhante, mas a temperatura de máxima conversão de NO deslocou-se para temperaturas menores.In this study, a Ag/Al 2 O 3 and Ag/Al 2 O 3 /Cordierite catalyst was prepared by a conventional impregnation method and NOx reduction investigated using propylene as a reducing agent. The catalysts were characterized by Temperature Programmed Reduction (TPR), oxygen chemisorption and surface area, using the standard BET method. The catalytic activities were measured under the same reaction conditions, i.e., 100 ppm NO, 250 ppm C 3 H 6 , 2% of oxygen and 100000 h -1 space velocity. The reaction products were analyzed by FTIR. The results of the catalytic activity obtained for the 2.22%Ag/Al 2 O 3 catalyst showed 56% NO conversion at 723 K. The catalysts based on Cordierite presented an analogous behavior, but the temperature of maximum NO conversion was lower (620 K).Keywords: NOx reduction, silver/alumina catalyst, propylene, Cordierite IntroductionThe generation of NOx effluents from stationary and mobile sources poses an important environmental problem. [1][2][3][4] Gas effluent control in internal combustion engines, whose air/gasoline ratio is stoichiometric, can be achieved by using the so-called "three way catalysts" (TWC) typically containing Pt and Rh as the active metals. On the other hand, lean-burn engines, such as diesel engines and stationary sources for industrial use, operate with excess oxygen to ensure complete combustion. In these cases, TWCs are ineffective for NO elimination, for which reason selective catalytic reduction (SCR) is used. A typical commercial SCR catalyst uses V 2 O 5 -WO 3 -TiO 2 as the metal system and NH 3 as the reducing agent. This catalyst involves problems relating to the transport and storage of NH 3 and to vanadium toxicity. 5 In their studies of several catalysts prepared from zeolites substituted with H + , Na + , K + , Mg 2+ , Ca 2+ , Cr 3+ , Fe 3+ , Co 3+ , Ni 2+ , Cu 2+ , Zn 2+ and Ag, Iwamoto et al. 6,7 found that Cu-ZSM5 provided better NO reduction activity when hydrocarbons were used as reductants. However, this catalyst has a low thermal stability 6 and deactivates in the presence of water vapor and SO 2 . Moreover, large amounts of water vapor inhibit the catalyst's action, promoting the migration of Al +3 out of the zeolite struct...

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Section: Resultssupporting

confidence: 93%

Selective reduction of NOx by propylene over silver catalyst under oxidative conditions

Cónsul

Thiele

Baibich

et al. 2004

J. Braz. Chem. Soc.

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“…In the low Agloaded sample (2 wt% Ag) the Ag oxidation state was proposed, while in the 6 wt% Ag-sample metallic Ag was observed [38]. Similarly, Hoost et al [55] concluded that the oxidized rather than reduced form of Ag is the active phase for SCR of NO for Ag/ alumina catalysts. In the present study isolated Ag ions could not be unambiguously identi®ed by UV± Vis spectrometry.…”

Section: Discussionmentioning

confidence: 95%

Deep oxidation of methane over zirconia supported Ag catalysts

Kundakovic

Flytzani‐Stephanopoulos

1999

Applied Catalysis A: General

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Zirconia supported silver catalysts were studied for deep oxidation of methane. The catalyst structure was examined by XRD, XPS, STEM-EDX, HR-TEM and UV±Vis spectroscopy and related to the catalyst activity. Methane conversion strongly depends on Ag state and dispersion. At low Ag loading (<2 at%), small Ag particles (<5 nm) have low activity. The turnover frequency for methane oxidation increases as the Ag particle size increases from 5 to 10 nm as determined from HRTEM, while the activation energy remains the same. Ag ion-exchanged ZSM-5 zeolite materials, containing Ag in highly dispersed state as isolated Ag were studied for comparison. High conversion of methane was found only in zeolite catalysts containing a large amount of silver, some of which was in particle form. The reaction rate on zirconia-supported Ag particles is 0.77 order in methane and 0.37 order in oxygen. The reaction products, carbon dioxide and water, do not affect the methane oxidation rate in levels up to of 1.7 and 3.5 mol% in the feedgas, respectively. The active phase under reaction conditions for both low and high Ag loading is the oxygen covered metallic Ag surface, as was con®rmed by XPS and UV±Vis spectrometry. # 1999 Elsevier Science B.V. All rights reserved.

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“…When using low or moderate Ag loadings (from 1 to 3 wt%), silver oxide species and increased dispersion were observed, and higher conversions were also attained Hoost et al, 1997;Kung et al, 1997;Miyadera, 1997;Li and Flytzani-Stephanopoulos, 1999;Burch et al, 2002). In contrast to this, increasing silver loading was related to the formation of metallic Ag on the catalytic surface, which favored oxidation of the reductant by oxygen, and led to limited de-NO x activity .…”

Section: Introductionmentioning

confidence: 94%