Environmental Catalysis

Lox, E.S.; Engler, Bernd; Janssen, F.J.J.G.; Garten, R.L.; Betta, Ralph A. Dalla; Schlatter, James C.; Ainbinder, Zarah; Manzer, L. E.; Nappa, Mario J.; and, V. N. Parmon; Zamaraev, K. I.

doi:10.1002/9783527619474.ch12

Cited by 19 publications

(16 citation statements)

References 195 publications

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“…[2] Hence,t ailoring the particle size,also denoted "dispersion" in catalysis,is ap romising approach for adjusting the catalytic activity and selectivity.T his accounts especially for noble metals like Pt, Pd, and Rh, which are widely used in industrial processes such as hydrogenation reactions [3] and exhaust gas after treatment. [4] Mostly,anoptimum is found at acertain particle size, as,f or example,r eported for CO and NO oxidation over Ptbased diesel oxidation catalysts (DOC). [5] Hence,t here is as trong demand for tailoring catalysts specifically to their application and for designing them in ak nowledge-based manner.…”

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Tuning the Structure of Platinum Particles on Ceria In Situ for Enhancing the Catalytic Performance of Exhaust Gas Catalysts

et al. 2017

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A dynamic structural behavior of Pt nanoparticles on the ceria surface under reducing/oxidizing conditions was found at moderate temperatures (<500 °C) and exploited to enhance the catalytic activity of Pt/CeO -based exhaust gas catalysts. Redispersion of platinum in an oxidizing atmosphere already occurred at 400 °C. A protocol with reducing pulses at 250-400 °C was applied in a subsequent step for controlled Pt-particle formation. Operando X-ray absorption spectroscopy unraveled the different extent of reduction and sintering of Pt particles: The choice of the reductant allowed the tuning of the reduction degree/particle size and thus the catalytic activity (CO>H >C H ). This dynamic nature of Pt on ceria at such low temperatures (250-500 °C) was additionally confirmed by in situ environmental transmission electron microscopy. A general concept is proposed to adjust the noble metal dispersion (size, structure), for example, during operation of an exhaust gas catalyst.

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Tuning the Structure of Platinum Particles on Ceria In Situ for Enhancing the Catalytic Performance of Exhaust Gas Catalysts

et al. 2017

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“…Although lean-burn engines improve fuel efficiency, the exhaust gas temperature is significantly lower than from conventional gasoline powered engines, and consequently, catalysts with high oxidation activity at relatively low temperatures are needed for this application [5].…”

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“…Modern TWC converters used in gasoline ICEs contain Pt, Rh and Pd, dispersed on a washcoat applied to a cordierite ceramic monolith or metal monolith [3,5]. The monolith usually has a honeycomb structure with 1 mm square channels to accommodate the high gas throughputs from the exhaust with minimal pressure drop.…”

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confidence: 99%

“…The metal composition of the converter varies with application but typically contains 5-20:1 of Pt:Rh with a total metal loading of 0.9-2.2 g L −1 . Pd may be used to replace all or part of the Pt for cost savings [5]. Exhaust gas emissions from NGVs are difficult to control because low concentrations of CH4 (400-1500 ppmv) must be oxidized in the presence of high concentrations of H2O (10-15 vol.%) and CO2 (15 vol.%) at relatively low exhaust gas temperatures (450-550 °C).…”

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Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

2015

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Abstract:Effects of H2O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH4 present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH4 oxidation in a catalytic converter is limited by low exhaust gas temperatures (500-550 °C) and low concentrations of CH4 (400-1500 ppmv) that must be reacted in the presence of large quantities of H2O (10-15%) and CO2 (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH4 oxidation, water-induced sintering and reaction inhibition by H2O deactivate these catalysts. Recent studies have shown the reversible inhibition by H2O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H2O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H2O inhibition is postulated to occur by either formation of relatively stable Pd(OH)2 and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO2) are known to improve catalyst activity and stability. Kinetic studies of CH4 oxidation at conditions relevant to natural gas vehicles have OPEN ACCESSCatalysts 2015, 5 562 quantified the thermodynamics and kinetics of competitive H2O adsorption and Pd(OH)2 formation, but none have addressed effects of H2O on O mobility.

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“…Since there is a complex coupling of the physical and chemical processes in the monolith, improvements in the design of such reactors require a comprehensive understanding of the influence of various design and operating variables on the performance of the monolith. The various review articles [1][2][3][4][5][6][7] summarize the recent progress in catalytic monolith reactors.…”

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confidence: 99%

Light‐off location and front diffusion in a catalytic monolith reactor

Ramanathan

Gopinath

2008

AIChE Journal

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in Wiley InterScience (www.interscience.wiley.com).A one-dimensional solid-gas model has been used to study the light-off behavior in a reactor system. Techniques from nonlinear analysis are applied to first arrive at an analytical expression to estimate the light-off location in the reactor. The model is then extended to estimate the thickening of the light-off front in the vicinity of the light-off location due to heat diffusion effects in the solid substrate. Both these results are expressed in terms of all the important operating and design variables of the monolith reactor and also the reaction specific variables such as the kinetic parameters and heats of reaction. A specific case of a three-way catalyst (TWC) is numerically treated in detail to illustrate the application of this work. Finally, we focus on using the insight obtained from this study to suggest design guidelines to optimize the performance of automotive catalytic monolith reactors.

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Environmental Catalysis

Cited by 19 publications

References 195 publications

Tuning the Structure of Platinum Particles on Ceria In Situ for Enhancing the Catalytic Performance of Exhaust Gas Catalysts

Tuning the Structure of Platinum Particles on Ceria In Situ for Enhancing the Catalytic Performance of Exhaust Gas Catalysts

Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

Light‐off location and front diffusion in a catalytic monolith reactor

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