Steady-State and Transient Methods for Modeling Chemical Reactions on Supported Catalysts
<p>A systematic experimental strategy based on fluid-phase measurements is proposed for modeling dynamic behavior of heterogeneous catalytic reactions. The strategy utilizes steady-state rate, step-response, cycled-feedstream, and feedback-induced bifurcation techniques. Ethylene hydrogenation...
| Summary: | <p>A systematic experimental strategy based on fluid-phase measurements is proposed for modeling dynamic behavior of heterogeneous catalytic reactions. The strategy utilizes steady-state rate, step-response, cycled-feedstream, and feedback-induced bifurcation techniques. Ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub> was studied using this strategy. In addition, transmission infrared spectroscopy is applied to investigate support effects which accompany ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub>, and to the detailed study of CO adsorption, desorption and oxidation on Rh/Al<sub>2</sub>O<sub>3</sub>. The proposed experimental strategy combined with surface infrared spectroscopy provides a very powerful means for identification and validation of dynamic kinetic models.</p>
<p>Observed bifurcation behavior can be accurately attributed to a model for the catalytic reaction only if each dynamic element in the closed-loop experimental hardware is properly accounted for. Accordingly, time delay and feedback gain were the manipulated parameters in a feedback-induced bifurcation scheme aimed at validating a dynamic model for an experimental gas-phase reactor flow system without reaction. The apparatus consists of an isothermal, stirred, fixed-bed reactor, mass flow controllers, an infrared gas analysis system, and a computerized data acquisition and control system. Experimental bifurcations to sustained oscillations show that the stability of the reactor system is strongly influenced by delay. The relationships of time delay to Hopf bifurcation gains and frequencies provide a very sensitive basis for model comparisons.</p>
<p>Steady-state, step-response, feedback-induced Hopf bifurcation and forced concentration cycling experiments were applied to study ethylene hydrogenation over 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C. Step-response experiments indicate a time scale of 5000 s which is associated with chemisorbed hydrogen. Conversely, feedback-induced Hopf bifurcation data indicate this time scale to be on the order of 1 s in magnitude. In the overall strategy of dynamic modeling, the two techniques are complementary since each inherently focuses on an opposite region in the spectrum of time scales for the reactor system. Cycling the feedstream composition resulted in improvement of the time-average reaction rate for the ethylene hydrogenation reaction compared to steady-state reactor operation.</p>
<p>Steady-state, step-response and Hopf bifurcation data are also presented for 0.5% Pt/Al<sub>2</sub>O<sub>3</sub> at 30°C and compared with results for the 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst. A single value of 2.5 s for the surface time constant associated with chemisorbed hydrogen is sufficient for modeling behavior on 0.5% Pt/Al<sub>2</sub>O<sub>3</sub>, whereas the lower-loaded 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst requires two very different values. In addition, the 0.5% catalyst was used to demonstrate the general result that small discrepancies between the actual and chosen reference steady state give rise to imperfect, cusp-like bifurcations. Steady-state bifurcation data are also shown to be useful for discriminating among rival kinetic models.</p>
<p>Ethylene hydrogenation on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Transmission infrared spectroscopy was used to study hydrogen spillover dynamics on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C via hydroxyl/deuteroxyl exchange. Ethylene in the gas-phase markedly slows the rate of spillover. The presence of ethylene likely reduces the concentration of platinum-adsorbed hydrogen adatoms, the precursors of hydrogen spilled onto alumina, due to catalytic hydrogenation on the platinum. Surface transport of hydrogen atoms on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for ethylene hydrogenation on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Infrared spectra exhibit characteristics of both hydroxyl and deuteroxyl groups for reactor feed containing only D<sub>2</sub> and C<sub>2</sub>H<sub>4</sub>. This observation confirms the existence of a dissociative ethylene adsorption process.</p>
<p>A section of the thesis unrelated to ethylene hydrogenation investigates modeling applications of transmission infrared spectoscopy (TIR) by applying it to study adsorbed CO on Rh/Al<sub>2</sub>O<sub>3</sub> during CO chemisorption, steady-state, step-response, and forced-cycling oxidation experiments at 900 torr. At 300°C, the catalyst initially supported primarily a dicarbonyl CO species, but after use exhibited spectra characteristic of a surface mostly covered by linearly bound CO. A model that describes transient, diffusion-influenced CO adsorption and desorption for the supported catalyst is presented. It suggests that the CO desorption energy depends linearly on coverage, and that the magnitude of this dependence is a function of temperature. Observed rate dependence on bulk CO concentration for O<sub>2</sub> effluent levels of 0.5% and 0.25% is interpreted considering the effects of internal and external mass transport at 300°C. Step-response and forced-cycling oxidation experiments across stoichiometric conditions exhibit oxygen and CO storage effects characteristic of CO oxidation catalysts. Data indicating autonomous oscillation of CO coverage and CO<sub>2</sub> production are also presented.</p> |
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