Preeti Aghalayam

The objective of this work is to elucidate controlling mechanisms in NO x reduction, develop reduced-order reaction models, and analyze the reactor performance using the reduced-order reaction model for the NO-CO reaction. We start with the microkinetic model on platinum, which describes the mechanism of catalytic reduction of NO by CO. The formation of the main product N 2O and the competitive formation of the side product N 2 are accounted for in the microkinetic model. Sensitivity and reaction path analysis have been carried out to determine the rate-limiting steps as well as the most abundant reactive intermediates in the system. Owing to the differences between system performance at high and low temperatures, the model has been analyzed in detail in these temperature regimes. Two closed-form expressions, corresponding to the two global reactions involved, have been derived. The characteristic features of the microkinetic model such as the sharp increase in NO conversion and the selectivity to N 2O are captured well by the reduced model. The reduced-order model has been extended to the rhodium catalyst as well. © 2012 Wiley Periodicals, Inc.

The effect of inlet flow distribution on the performance characteristics of a catalytic converter is analyzed for two common inlet manifold designs. Three-dimensional steady-state computational fluid dynamics simulations are performed under isothermal and nonisothermal conditions for the entire converter comprising an inlet manifold, central monolith, and outlet section. The porous medium approximation is used for the catalytic monolith. The reaction mechanism for a diesel oxidation catalyst with five global reactions is employed. A parameter called the “flow distribution index” is defined in order to quantify the nonuniformity of the velocity profile within the monolith and hence analyze the effect of operating conditions for the two geometries. Under isothermal conditions, catalytic converters with straight and U-bend inlet manifolds behave similarly and closely match single channel predictions. However, under nonisothermal conditions, there is a considerable difference in performance between the two geometries because of temperature variations. The presence of heat effects in the nonisothermal case marginally improves the flow uniformity within the monolith. However, in the presence of heat effects, reactant conversion in the center of the monolith varies from the periphery and the single channel model slightly overestimates the conversion for both geometries.

Lean NOx traps (LNT) is an after-treatment technique that is used for NOx abatement in lean-burn engines. The objective of this work is to develop a generic kinetic model applicable to various LNT catalyst formulations and to apply it to the design and analysis of the LNT reactor. The kinetic model of 16 reactions is proposed using Langmuir-Hinshelwood kinetics. It is validated against experiments reported in the literature, performed for a family of catalyst formulations that use Pt-group metals as the active catalyst and barium-based NOx storage, and is capable of predicting the performance with minimal modification of parameters. A kinetic analysis highlights the contribution of each reaction to the overall kinetic scheme in order to determine the important steps and qualitatively validate the model. A reactor-level analysis is done by varying the operating conditions and the time scales of the lean and rich phases are optimized.

A computational fluid dynamics model, consisting of a mixing chamber coupled with a catalytic converter, is developed for the selective catalytic reduction (SCR) of NOx using urea. The NH3 required for the SCR reactions is produced from the injection and decomposition of urea in the mixing chamber. The conversion efficiency and concentration distribution of NH3 from the mixing chamber are analyzed for a range of operating conditions. The flow and species distribution profiles from the mixing chamber are incorporated as inlet boundary conditions at the entrance of the downstream SCR convertor. The SCR convertor comprises a central catalytic monolith and inlet and outlet diffuser sections. Variations in NOx concentration were observed within the monolith due to heat losses and nonuniformities in ammonia concentrations. While heat effects under non-isothermal conditions slightly improved the NOx conversion efficiency, nonuniformities in ammonia concentrations did not significantly influence the SCR performance. Thus, the radial variations in NH3 concentrations, owing to nonuniformity at the outlet of the mixing chamber, did not considerably impact the overall performance of the SCR. The effects of temperature, NO:NO2 ratio, and inlet velocity were investigated.

Selective catalytic reduction of nitric oxide using ammonia in the presence of excess oxygen remains a major challenge. In the present work, we developed mesoporous Al-SBA-15 supported NiRu catalysts with a high surface area and ordered mesoporous structure. The monometallic Ni catalyst were prepared by the modified beta-cyclodextrin impregnation method by maintaining 1/50 mol of β-CD/Ni and bimetallic NiRu catalyst was prepared by maintaining constant loading of 8 wt% Ni with different wt%. loading of xwt% Ru (x = 2, 4, 6 and 8). The developed catalysts were characterized by XRD, N2 sorption, FT-IR, Py-IR, UV-DRS, HRSEM, HRTEM and XPS. The NH3-SCR efficiency of both monometallic (Ni) and bimetallic (NiRu) catalysts are discussed. Additionally, monometallic 8 wt% Ni and Ru supported catalysts have been also synthesized by the conventional impregnation method for purposes of comparison. The remarkable low-temperature NH3-SCR conversion (90.2% NO conversion at 300 °C) of 8Ru8Ni/Al-SBA-15 catalyst under excess oxygen condition (7%) and superior % promotion (87%) of ruthenium addition was demonstrated with excellent textural properties, stability and corroborated with the physicochemical properties of the catalyst.

Cu/ZnO/ZrO2/Al2O3 catalysts are widely explored for CO2 conversion to methanol due to their higher activity and stability. However, mechanistic understanding of the performance of such catalysts is lacking due to ambiguity on the actual active sites. This study focuses on unraveling the nature of different interfaces on Cu/ZnO/ZrO2/Al2O3 catalyst by coupling experiments, Density Functional Theory (DFT) simulations and a DFT-based reactor scale multi-site microkinetic model. Although DFT calculations suggested the ZrO2/Cu interface to be the CO2 adsorption site, the validated microkinetic model predicted the ZnO/Cu interface to be the crucial reaction center. Reaction pathway analysis showed that methanol is produced through the formate pathway near the reactor entrance, whereas, the carboxyl pathway dominates in the latter zones, emphasizing the occurrence of both CO2 and CO hydrogenation. This deeper understanding of the reaction behavior of such multicomponent catalysts will aid in designing better catalysts and optimizing reaction conditions and systems.

Ag/γ-Al2O3 is an effective catalyst for the selective reduction of NOx (SCR) using propylene as a reducing agent. The catalyst performance is greatly influenced by the synthesis procedure. Various methods for synthesis of Ag/γ-Al2O3 are analyzed, and their performance is examined via packed bed reactor experiments in this work. An optimal one-pot synthesis method, single-step sol–gel (SSG) synthesis, is explored systematically. The SSG-synthesized catalyst shows better performance than those prepared via wet impregnation. The influence of synthesis conditions, specifically pH, on the textural and morphological properties of the SSG-synthesized Ag/γ-Al2O3, and therefore the activity for hydrocarbon-based SCR in a packed-bed reactor, are analyzed using experiments and simulations. The optimized catalyst demonstrates excellent performance (90% NOx conversion) for NOx reduction under nominal operating conditions with a wide activity temperature window (300–600°C). The catalyst shows good time-on-stream performance and is effective at higher inlet oxygen concentrations and space velocities. A global kinetic model, which uses synthesis-pH-dependent parameters, is proposed, and its ability to predict the activities of these catalysts is validated.