Η επίδραση του πάχους και της μεθόδου εναπόθεσης του καταλυτικού υμενίου στο φαινόμενο της ηλεκτροχημικής ενίσχυσης και νέοι ηλεκτροχημικά ενισχυόμενοι αντιδραστήρες για τη μελέτη αντιδράσεων περιβαλλοντικού ενδιαφέροντος
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... ου υ Π Πα αν νε επ πι ισ στ τη ημ μί ίο ου υ Π Πα ατ τρ ρώ ών ν Π ΠΑ ΑΤ ΤΡ ΡΑ Α, , 2 20 00 08 8 Η ΕΠΙΔΡΑΣΗ ΤΟΥ ΠΑΧΟΥΣ ΚΑΙ ΤΗΣ ΜΕΘΟΔΟΥ ΕΝΑΠΟΘΕΣΗΣ ΤΟΥ ΚΑΤΑΛΥΤΙΚΟΥ ΥΜΕΝΙΟΥ ΣΤΟ ΦΑΙΝΟΜΕΝΟ ΤΗΣ ΗΛΕΚΤΡΟΧΗΜΙΚΗΣ ΕΝΙΣΧΥΣΗΣ ΚΑΙ ΝΕΟΙ ΗΛΕΚΤΡΟΧΗΜΙΚΑ ΕΝΙΣΧΥΟΜΕΝΟΙ ΑΝΤΙΔΡΑΣΤΗΡΕΣ ΓΙΑ ΤΗ ΜΕΛΕΤΗ ΑΝΤΙΔΡΑΣΕΩΝ ΠΕΡΙΒΑΛΛΟΝΤΙΚΟΥ ΕΝΔΙΑΦΕΡΟΝΤΟΣ Διδακτορική διατριβή Υποβληθείσα στο Τμήμα Χημικών Μηχανικών του Πανεπιστημίου Πατρών Υπό του Κωνσταντίνου Κουτσοδόντη του Γεωργίου Για την απόκτηση του τίτλου του Διδάκτορα του Πανεπιστημίου Πατρών ΠΑΤΡΑ, 2008 ABSTRACT The effect of Electrochemical Promotion of Catalysis (or Non-faradaic Electrochemical Modification of Catalytic Activity, NEMCA effect) is a phenomenon where application of small currents or potentials (±2V) alters the activity and selectivity of catalysts supported on ionic or mixed ionic-electronic conductors and modifies the electronic and thus catalytic properties in a controllable, reversible and to some extent predictable manner. As shown by numerous surface science and electrochemical techniques, including STM, electrochemical promotion is due to electrochemically controlled migration (backspillover or reversible spillover) of promoting or poisoning ionic species (O 2in the case of YSZ, TiO 2 and CeO 2 , Na + in the case of β″-Al 2 O 3 , protons in the case of Nafion) between the ionic or mixed ionic-electronic conductor and the gasexposed catalyst surface. The electropromoted catalytic rate can be up to 300 times larger than the unpromoted (open-circuit) catalytic rate (this work) and up to 3⋅10 5 times larger than the rate of ion backspillover from the ionic support to the catalyst surface. The effect of catalyst film thickness on the magnitude of electrochemical promotion (ρ and Λ values) has not been studied experimentally so far but a mathematical model has been developed, accounting for surface diffusion and reaction of the promoting species which predicts a strong variation of ρ and Λ with catalyst film thickness L. In this study we examine for the first time experimentally the effect of catalyst film thickness on the magnitude of the electrochemical promotion of catalysis, using porous Pt catalyst-electrodes prepared from Engelhard Pt paste with thicknesses in the range 0.2 to 1.4 μm. A detailed kinetic and electrokinetic investigation is presented, where the effect of temperature and film thickness is examined for two types of catalysts. It was found that increasing the thickness of porous catalyst films used in electrochemical promotion studies causes a decrease in the rate enhancement ratio ρ due to the gradual axial decrease from the three-phase-boundaries to the top of the film of the surface concentration of the promoting backspillover O 2species which diffuse and react on the porous catalyst surface. Increasing film thickness causes a moderate increase in the Faradaic efficiency Λ which can be predicted by the parameter 2Fr o /I 0 . The ρ and Λ behaviour is in good agreement with analytical model prediction and provides additional support for the O δpromoter reaction-diffusion model and for the sacrificial promoter mechanism of electrochemical promotion. Most electrochemical promotion studies have been carried out so far with thick (0.1 μm to 5 μm) porous metal catalyst films with a roughness factor of the order of 500 and small (typically less than 0.1%) metal dispersion, deposited on solid electrolytes using a variety of deposition techniques, including wet impregnation and deposition of organometallic pastes. Very recently, electropromotion studies have been extended to thin (40 nm) sputter coated porous metal catalysts with metal dispersion of the order of 10 to 30%. The effect of thickness with such thin (30 to 90 nm) sputtered Pt catalyst-electrodes on the magnitude of electrochemical promotion is discussed, as well as the effect of the catalyst deposition method (Sputtering, Pulsed Laser Deposition and Vapor Deposition) using the model reaction of ethylene oxidation. Rate enhancement ratio, ρ, values up to 440 and Λ values up to 10 3 where obtained for the 90 nm thick porous sputtered films, in agreement with the sacrificial promoter model and diffusion-reaction model of electrochemical promotion which predicts increase in ρ value with thinner films. Although there is a subsequent decrease in ρ when thicknesses of the order of 30 nm are reached due to the thermal migration of promoting ions and concomitant atomic similarity of EPOC and MSI, still, ρ values of the order of 10 are obtained. Although electrochemical promotion is not limited to any particular class of conductive catalyst, catalytic reaction or ionic support, there has been so far no successful commercial utilization of electrochemical promotion. The two main reasons for this are the use of expensive thick (typically 0.1-5 μm) catalyst films with metal dispersion below 0.01% and the lack of efficient and compact reactor designs allowing for the utilization of electrochemical promotion with a minimum of electrical connections to the external power supply. Both of these limitations can be overcome via the use of thin sputtered noble metal electrodes with metal dispersion exceeding 15% in monolithic electrochemically promoted reactors (MEPR) of the type used here. This is an important practical development because in these films the metal dispersion and utilization is comparable to that of state-of-the-art conventional supported catalysts. An environmental interest reaction, the reduction of NO by ethylene in the presence of excess oxygen, was investigated in a recently developed MEPR. In this novel dismantlable monolithic-type electrochemically promoted catalytic reactor, thin (~40 nm) porous catalyst films are sputter-deposited on thin (0.25 mm) parallel solid electrolyte plates supported in the grooves of a ceramic monolithic holder and serve as electropromoted catalyst elements. Using Pt-Rh(1:1)/YSZ/Au-type catalyst elements, the 8-plate reactor operated with apparent Faradaic efficiency exceeding unity achieving significant and reversible enhancement in the rates of C 2 H 4 and NO consumption in presence of up to 10% O 2 in the feed at gas flow rates up to 1000 cc/min. The Pt-Rh co-sputtered films exhibited very good performance in terms of stability and selectivity for N 2 formation, i.e. practically 100% under all reaction conditions. The reactor, which is a hybrid between a monolithic catalytic reactor and a flat-plate solid oxide fuel cell, permits easy practical utilization of the electrochemical promotion of catalysis.