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Now showing items 1 - 16 of 5879

  • Passive NOx adsorption on Pd/H-ZSM-5: Experiments and modeling

    Ambast, Mugdha   Karinshak, Kyle   Rahman, Bhuiyan Md Mushfikur   Grabow, Lars C.   Harold, Michael P.  

    A combined experimental and modeling study of the passive NOx adsorber (PNA) is presented that advances the understanding and prediction of the effects of various operating parameters and material properties of H-ZSM-5 and Pd/H-ZSM-5. Experiments reveal the NOx uptake process to be kinetically limited. Two microkinetic schemes are developed to explain the NOx uptake and release and NO2 generation data. Both schemes are incorporated into a 1 + 1 D monolith model to predict and validate NOx uptake and temperature programmed desorption (TPD) data. Scheme I involves Z(-)[PdOH](+), Z(-)Pd(2+)Z(-)and Z(-)Pd(+) with Z(-)Pd(+) as the strongest NO binding site. Scheme II involves reduction of PdO2 to PdO and nitrate formation. Model tuning utilizes a combination of infrared spectroscopy measurements and density functional theory (DFT) estimates of energy barriers. The model is validated at different conditions including feed parameters and Pd loading.
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  • NOx reduction on Ceria: Impact of lean-rich cycling

    Zhou, Zhiyu   Harold, Michael P.   Luss, Dan  

    NOx storage and reduction (NSR) is a cyclic catalytic process that eliminates NOx from lean burn vehicles. Leanrich switching can achieve a higher conversion than steady state operation. There is an active debate about the NOx reduction mechanism for NSR catalysts containing CeO2 (ceria), particularly at high temperatures and fast cycle frequency. In order to isolate the role of ceria, we examine the performance of a ceria-washcoated monolith for a wide range of operating conditions, including temperature, cycling frequency, reductant type and feed concentration of oxidants (O-2, CO2 and H2O). The results reveal enhancement of NO conversion with faster cycling particularly at elevated temperatures ( > 550 degrees C). The data are consistent with a cyclic mechanism in which oxygen vacancies are created during the rich feed through reduction by H-2, CO, or C(3)H(6 )and filled during the lean feed through NO and/or O-2 oxidation. An excess of O-2 is detrimental to NO conversion, due to the competing oxidation of reduced ceria by O-2. The data reveal that at least two types of vacancy sites participate in the cyclic redox process. Surface vacancy sites provide rapid NO reduction, while utilization of bulk vacancy sites is slowed by solid-state diffusion limitations. For a fixed duty cycle, lean/rich switching operation is superior to steady-state operation and an optimal lean/rich switching frequency exists for different reaction conditions. Fast cycling is especially favorable for stoichiometric feeds or cycle-averaged rich feeds.
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  • A tribute to Frances Arnold

    Chen, Wilfred   Collins, Cynthia   Cirino, Patrick   Harold, Michael P.  

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  • A Tribute to James E. Bailey

    Chen, Wilfred   Harold, Michael P.   Clark, Douglas   Khosla, Chaitan  

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  • Preface to Special Issue

    Harold, Michael P.   Hickman, Dan   Bhan, Aditya   Dauenhauer, Paul  

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  • Coupled Hydrocarbon Desorption in Zeolite Beta-Containing Monolithic Catalyst

    Peng, Po-Yu   Zhou, Zhiyu   Harold, Michael P.   Luss, Dan  

    The 3-D, large-pore zeolite beta (BEA) is effective in trapping large hydrocarbons (HCs). We conducted temperature-programmed desorption (TPD) experiments of a BEA-containing monolithic catalyst prestored with various mixtures of propylene, hexane, and toluene. The addition of propylene to a hexane + toluene mixture results in an unusual interaction during TPD. A rather small concentration of propylene increases the desorption temperature of the hexane from a weakly held into a stronger bound component. A variation in the propylene concentration leads to the nonmonotonic dependence of a high temperature hexane desorption peak. The data suggest that the blockage of hexane desorption by propylene depends on the uptake time. The findings complicate the analysis and design of HC traps used in applications such as vehicle aftertreatment systems.
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  • A tribute to R. Byron Bird

    Harold, Michael P.   Mavrikakis, Manos   Grossmann, Ignacio E.  

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  • Ammonia Oxidation on Structured Composite Catalysts

    Shrestha, Sachi   Harold, Michael P.   Kamasamudram, Krishna   Yezerets, Aleksey  

    The NH3-based selective catalytic reduction of NO (x) on monolithic zeolite catalysts has emerged as the technology of choice for heavy-duty diesel vehicles. A class of Cu-exchanged zeolite catalysts has been developed that have very high ammonia sorption capacity and can achieve high NO (x) conversion to N-2 for a variety of transient conditions. In order to fully exploit the latest generation of SCR catalysts, an active, selective and robust post-SCR ammonia conversion system is needed to minimize the breakthrough of ammonia into the environment [1]. The goal of this study is to better understand the steady-state catalytic mechanism of post-SCR ammonia oxidative conversion and product selectivity on low-loading Pt-based catalysts and in so doing provide guidance in the development of a new class of ammonia slip catalysts.
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  • Editorial - a progress report

    Harold, Michael P.  

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  • Experimental and kinetic study of NO oxidation on model Pt catalysts

    Bhatia, Divesh   McCabe, Robert W.   Harold, Michael P.   Balakotaiah, Vemuri  

    Modeling and experimental studies on model Pt/Al(2)O(3) and Pt/BaO/Al(2)O(3) catalysts are performed to elucidate the kinetics of NO oxidation, which is a key step during the lean phase of NO, trap operation. Experiments show that a steady-state is never truly achieved during NO oxidation: a continuous decrease in the reaction rate with time is observed on both the catalysts. This decrease is distinct from and beyond the prompt inhibition of the NO oxidation reaction observed with NO(2) in the feed or product. NO oxidation carried out after catalyst pretreatments with H(2), O(2), and NO(2) indicates that NO(2) is responsible for the deactivation while NO(2) storage plays a negligible role. Experiments with NO(2) as the feed elucidate its role in the production of NO, either by storage or by decomposition, for a wide range of temperatures. The highly oxidizing nature of NO(2) suggests that the Pt surface could be covered with oxygen, either as chemisorbed O or as Pt oxides, which results in slow poisoning of the catalyst. Microkinetic analysis of the NO oxidation reaction shows O(2) adsorption as the rate-determining step and predominant surface species to be adsorbed NO and O. Based on the microkinetic studies, a global kinetic model is proposed which includes the inhibiting effect of NO(2) on the NO oxidation reaction. The importance of including coverage of NO in the global model at low temperatures is shown, which is neglected in the current literature global models. The model predicts the experimental observations for a wide range of temperatures within acceptable error limits. However, prediction of the transient data requires modeling of NO(2) storage, decomposition and the complex NO(2) inhibition chemistry in addition to other surface reactions. (C) 2009 Elsevier Inc. All rights reserved.
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  • Low-Dimensional Models for Real Time Simulations of Catalytic Monoliths

    Joshi, Saurabh Y.   Harold, Michael P.   Balakotaiah, Vemuri  

    We present accurate low-dimensional models for real time simulation, control, and optimization of monolithic catalytic converters used in automobile exhaust treatment. These are derived directly by averaging the governing equations and using the concepts of internal and external mass transfer coefficients. They are expressed in terms of three concentration and two temperature modes and include washcoat diffusional effects without using the concept of the effectiveness factor. The models reduce to the classical two-phase models in the limit of vanishingly thin washcoat. The models are validated by simulating the transient behavior of a three-way converter for various cases and comparing the predictions with detailed solutions. It is shown that these new models are robust and accurate with practically acceptable error, speed up the computations by orders of magnitude, and can be used with confidence for the real time simulation and control of monolithic and other catalytic reactors. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1771-1783, 2009
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  • Assessing intrusion by the capillary during spatially resolved mass spectrometry measurement

    Hoang Nguyen   Peng, Po Yu   Luss, Dan   Harold, Michael P.  

    The impact of the capillary probe of a spatially-resolved mass spectrometer system (SpaciMS) on the measured reactant conversion is reported using propylene oxidation over Pt/Al2O3 washcoated monoliths. The findings suggest that the invasive nature of SpaciMS depends on its configuration and application. Using monoliths with a range of cell densities (100-600 cells per square inch, CPSI), the concentration profiles of propylene sampled with probes of two different outer diameters (170 and 363 mu m) are compared with the temperature measured using coherent optical frequency domain reflectometry (c-OFDR). The comparison indicates that flow blockage has a negligible effect if the limiting propylene is depleted in the downstream reactor section. Suction by the probe compensates for the blockage for certain combinations of the channel diameter and probe size. In such cases the profile measured by a probe with the larger outer diameter (363 m) is similar to that measured by a smaller capillary (170 m). The experiments reveal that the axial position of the probe does not influence the flow profile in a 100 CPSI monolith channel, nor does it affect the amount of flow deflected to surrounding channels for a 600 CPSI monolith. Under some conditions the results are impacted by transverse concentration gradients. Their existence complicates the interpretation of the SpaciMS data. The difference between the location of propylene depletion and temperature maximum provides a useful metric for capturing the collective impacts of flow deflection and transverse gradients. The complexity of the flow, transport and reaction suggests that at least three dimensionless groups are needed to bound the operating regions in which the presence of the probe has minimal impact on the measured concentration. (C) 2016 Elsevier B.V. All rights reserved.
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  • Enhanced Selective Oxidation of Ammonia in a Pt/Al2O3@Cu/ZSM-5 Core-Shell Catalyst

    Ghosh, Rajat Subhra   Le, Thuy T.   Terlier, Tanguy   Rimer, Jeffrey D.   Harold, Michael P.   Wang, Di  

    The ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N-2. The state-of-the-art ASC has a dual-layer architecture composed of a Pt/Al2O3 (PGM) bottom layer and a metal (Fe, Cu)-exchanged zeolite (M-Z) top layer. The PGM layer provides high NH3 oxidation activity; however, the desired N-2 product is achieved over a narrow temperature range just above light-off, whereas the reaction byproducts N2O and NOx (i.e., NO and NO2) appear at intermediate and high temperatures, respectively. An advantage of the M-Z catalyst is the selective lean reduction of NO via conversion of NH3 to N-2 over a broad temperature range. Although recent studies demonstrate the effectiveness of the dual-layer design, further advances are needed to reduce the PGM loading and ASC volume while enhancing low-temperature activity. In this study, the dual-layer concept is scaled down to the level of a single core-shell (CS) catalyst particle, Pt/Al2O3@Cu/ZSM-5, composed of a PGM core and a M-Z shell, with the intent to meet the aforementioned challenges. The CS catalyst was realized by rational design of key synthesis steps, the most critical being the initial growth of an intermediate silicalite-1 layer to prevent Al leaching during the secondary growth of the ZSM-5 shell. Characterization of the CS spherical catalyst reveals a mesoporous PGM core (ca. 40 mu m diameter) that is active and a nearly dense zeolitic shell (ca. 1 mu m thick). Evaluation of the CS catalyst in a fixed-bed reactor shows excellent NH3 oxidation activity and N-2 selectivity. In addition, we obtained an unanticipated enhancement of the Pt/Al2O3 performance within the CS configuration that gives an exceptional light-off of the NH3 oxidation. Our findings reveal that the CS catalyst has an equivalent activity to that of a conventional Pt/Al2O3 catalyst containing 3 times higher Pt loading. Further, a dual-layer ASC composed of a bottom layer containing the seeded core Pt/Al2O3 and a Cu-SSZ-13 top layer achieves the same performance as a dual-layer ASC having 3 times higher Pt loading but with unmodified Pt/Al2O3. The enhanced activity of the Pt/Al2O3 catalyst is attributed to a modification of the reducibility of oxides of Pt crystallites owing to the overgrowth of silicalite-1 and ZSM-5 layers in the CS configuration. Finally, the separate impacts of H2O in the feed and of hydrothermal aging (HTA) on catalyst performance are reported. H2O in the feed is shown to have a negligible impact on conversion and product distribution. The silicalite-modified Pt/Al2O3 catalyst is more resilient to HTA treatment than conventional Pt/Al2O3.
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  • Autothermal reforming of methanol: Experiments and modeling

    Lattner, James R.   Harold, Michael P.  

    A bench-scale fixed-bed reactor for the autothermal reforming (ATR) of methanol under near-adiabatic conditions was constructed to experimentally demonstrate the conversion of methanol to hydrogen over a copper-based catalyst. Axial distribution of air through multiple porous ceramic membranes was employed to limit the peak temperature within the catalyst bed, which is critical for the stability of copper-based catalysts. Methanol conversion, product selectivities, and temperatures were measured at discrete axial positions as a function of H2O:C ratios, feed temperatures, pressures, and two different air distributor designs. The effect of space velocity was implicitly studied via the axial composition profile measurements while the O-2:C ratio was adjusted to achieve an overall methanol conversion exceeding 90%. The use of a copper-based catalyst with distributed air injection resulted in low CO effluent concentration of ca. 1.3% at a feed temperature of 200 degrees C, H2O:C ratio of 1.0, O-2: C ratio of 0.11, and total pressure between 2 and 5 bar. Distributed air injection limits the peak bed temperature to 280 degrees C while injection of air over a narrow front results in a peak temperature of ca. 575 degrees C. The CO composition was found to be primarily a function of temperature and H2O:C ratio, with CO yield minimized at low temperature and high H2O:C. The system was simulated using an adiabatic I D reactor model comprising kinetic rate expressions of Peppley et al. [B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F Mann, Appl. Catal. A: Gen. 179 (1999) 31-49]. Very good agreement between data and model was achieved by assuming the oxidation reaction to be instantaneous (limited by oxygen supply). The results support a phenomenological view that the exothermic oxidation reactions occur in a narrow zone in close proximity to the porous membranes, leaving the bulk of the catalyst between membrane tubes in the reduced state and therefore active for conducting the endothermic reforming reactions. (c) 2006 Elsevier B.V. All rights reserved.
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    A method for controlling NOx emissions, in certain instances from diesel or fixed position combustion engines. More specifically a method for forming emission control catalyst structures for fuel combustion, a method of producing the catalyst, and a method of operating the catalyst to control emissions.
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  • Modeling the effects of Pt loading on NOx storage on Pt/BaO/Al(2)O(3) catalysts

    Xu, Jin   Harold, Michael P.   Balakotaiah, Vemuri  

    Platinum plays an important, multi-functional role during NOx storage and reduction (NSR). Understanding and predicting the effect of Pt loading is essential to optimize the lean NOx trap. In this study, a microkinetic model is developed for NOx storage on a series of Pt/BaO/Al(2)O(3) catalysts with a range of Pt loadings (0-3.7 wt.%). A classification of the Ba sites into two storage site populations, proximal and non-proximal (bulk), is presented. A simple model for estimating the capacities of the two Ba site populations using the Pt loading is used to explain trends in the storage of NO/O(2) and NO(2)/O(2). The model integrates existing literature models for NO(2) storage on BaO/Al(2)O(3), Pt-catalyzed NO oxidation to NO(2), and spillover chemistry involving NO(2). Wherever possible, simplifications in the model are made based on sensitivity analyses. Literature estimates of kinetic parameters are adjusted if estimates of spillover rate constants are not sufficient to predict the storage data. The dual-site model comprises proximal storage sites that participate in the spillover chemistry and non-proximal sites that involve NO(2) that is generated by the Pt-catalyzed NO oxidation. The model shows reasonable agreement with the measured storage of NO and NO(2) in O(2) at 340 degrees C for a range of storage times and Pt loading. The model helps to elucidate the storage dynamics and the roles of Pt and should be useful for incorporation into a complete NOx storage and reduction model. Some further refinements to the model are discussed. (C) 2011 Elsevier B.V. All rights reserved.
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