WO2005072860A1

WO2005072860A1 - Method and apparatus for carrying out electrochemically promoted reactions

Info

Publication number: WO2005072860A1
Application number: PCT/GR2004/000006
Authority: WO
Inventors: Constantinos G. Vayenas; Stella Balomenou; Demetrios Tsiplakides; Alexandros Katsaounis; Susanne Brosda; Gyorgy Foti; Christos Comninellis; Sabine Thiemann-Handler; Berndt Cramer
Original assignee: University Of Patras
Priority date: 2004-01-28
Filing date: 2004-01-28
Publication date: 2005-08-11
Also published as: WO2005072860A8

Abstract

Disclosed herein is a new method and apparatus for carrying out electrochemically promoted catalytic reactions, i.e., catalytic reactions where the catalytic rate and selectivity is modified by applying an electrical current or potential between the porous catalyst film which is deposited on a solid electrolyte and a second catalytic material which is also coated on the other side of the solid electrolyte component. The new apparatus, termed monolithic electropromoted reactor, consists of an appropriately grooved ceramic monolithic casing and of flat or ribbed solid electrolyte plates coated with the appropriate catalysts and support and electrical connections in the parallel grooves carved in the inside walls of the ceramic monolithic reactor casing. The flat of ribbed solid electrolyte plates are parallel to each other and are coated on one side by one catalyst (Catalyst 1) and on the other side by a second catalyst (Catalyst 2). All the catalyst 1 coatings are brought in electronic contact with an electronically conductive coating on one of the inside wall surfaces of the ceramic reactor casing and all the catalyst 2 coatings are brought in electronic contact with a second conductive coating on the opposite wall surface of the ceramic reactor casing. In this way all the electrochemical cells formed one in each plate are electrically connected in parallel and the reactor can be electropromoted by electrical potential or current application at the two terminals. Series electrical connection is also possible via electrical connection of catalyst 2 in each plate with catalyst I in the next plate.

Description

METHOD AND APPARATUS FOR CARRYING OUT ELECTROCHEMICALLY PROMOTED REACTIONS

Disclosed herein is a new method and apparatus for carrying out electrochemically promoted catalytic reactions, i.e., catalytic reactions where the catalytic rate and selectivity is modified by applying an electrical current or potential between the porous catalyst film which is deposited on a solid electrolyte and a second catalytic material which is also coated on the other side of the solid electrolyte component. The new apparatus, termed monolithic electropromoted reactor, consists of an appropriately grooved ceramic monolithic casing and of flat or ribbed solid electrolyte plates coated with the appropriate catalysts and support and electrical connections in the parallel grooves carved in the inside walls of the ceramic monolithic reactor casing. The flat of ribbed solid electrolyte plates are parallel to each other and are coated on one side by one catalyst (Catalyst 1 ) and on the other side by a second catalyst (Catalyst 2). All the catalyst 1 coatings are brought in electronic contact with an electronically conductive coating on one of the inside wall surfaces of the ceramic reactor casing and all the catalyst 2 coatings are brought in electronic contact with a second conductive coating on the opposite wall surface of the ceramic reactor casing. In this way all the electrochemical cells formed one in each plate are electrically connected in parallel and the reactor can be electropromoted by electrical potential or current application at the two terminals. Series electrical connection is also possible via electrical connection of catalyst 2 in each plate with catalyst 1 in the next plate.

ELECTROCHEMICAL PROMOTION OF CATALYTIC REACTIONS

The phenomenon of Electrochemical Promotion of Catalysis (EPOC) or Non- Faradaic electrochemical promotion of catalytic activity (NEMCA effect) has been disclosed in several patents (Vayenas et al, European Patent 0480116 (1996), Frenzel et al. US Patent 6,194,623 B1 (2001), Stochniol et al, US Patent 6,210,557 B1 (2001)) and publications (Vayenas et al, Nature (1990), Vayenas et al. Modem Aspects of Electrochemistry (1995), Vayenas et al, "Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions" Kluwer/ Academic Publishers (2001)). The metal or conductive metal oxide catalyst is deposited on a solid electrolyte component and electrical current or potential is applied between the catalyst and a second electrode (termed counter electrode) also deposited on the solid electrolyte component. This electrical current or potential application causes pronounced and usually reversible changes in the catalytic activity and selectivity of the catalyst electrode. The induced change in catalytic rate is up to 6 orders of magnitude larger than the rate, 1/nF, of electrochemical supply or removal of ions (of charge n) to or from the catalyst-electrode through the solid electrolyte, where I is the applied current and F is the Faraday constant. The induced change in catalytic rate can be up to 150 times larger than the catalytic rate before current or potential application. The counter electrode may be exposed to a separate gas compartment (fuel cell-type design) or may by exposed to the same reactive gas mixture as the catalyst-electrode (single chamber design). The solid electrolyte component (e.g. plate or tube) may be either gas impervious, but can also be porous (Vayenas et al, European Patent 0480116 (1996)). The phenomenon of electrochemical promotion has been studied already for more than seventy catalytic reactions (Vayenas et al, "Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions" Kluwer/ Academic Publishers (2001)) but so far there has been no practical reactor configuration designed, constructed and tested in order to utilize it in industrial practice or in automotive exhaust catalysis. We disclose here a new reactor and method for utilizing the effect of electrochemical promotion of catalysis to enhance the rate and selectivity of catalytic reactions. The new reactor has all the geometric characteristics of a monolithic honeycomb reactor but, due to its special design disclosed herein, can be dismantled and assembled at will and can be used to electrochemically promote the catalyst- electrodes deposited on its plate components with only two external electrical connections. Therefore both electrical manifolding and also gas manifolding is extremely simplified in the disclosed monolithic electrochemically promoted reactor (MEPR). Furthermore one can use one or more of the plates of the new reactor as a gas sensor element and use the electrical signal generated by the electrochemical amperometric or potentiometric or constant-current potentiometric sensor element to adjust the electrical current or potential applied to the catalyst-electrode elements. This device is then an integrated chemical reactor-sensor (MEPRS) unit. We have designed, constructed and tested such units operated both as a MEPR and as a MEPRS. DESCRIPTION OF DRAWINGS

Figure 1 shows the monolithic electrochemical promoted reactor (MEPR) concept. The plate and reactor dimensions are quite flexible and those shown in Figure 1 are indicative and corresponding to those of the prototype units tested as described below. The ceramic reactor walls (casing) must be insulating and can be made, for example, by Machinable Ceramic (MACOR) material. It is enclosed in a suitably designed metal (or ceramic) gas manifolding casing with insulating material (e.g. vermiculite) placed, if desirable, between the ceramic reactor walls and the metal (or ceramic) gas manifolding casing in order to reduce mechanical stresses when the unit is used in an automotive exhaust. The internal side of the two opposing reactor walls have appropriately machined parallel grooves (typically 1-5 mm deep, thickness a few μm larger than the plate thickness (typically 500 μm). The distance between the parallel grooves dictates the reactor channel height and is typically 0.5 to 2 mm, depending on the desired reactor surface to volume ratio. The grooves can be made to terminate before the reactor exit in order to ensure that the plates cannot be entrained by the flowing gas stream. The solid electrolyte plates can be flat, in which case the resulting reactor channels are rectangular, or can be ribbed in which case the resulting reactor channels are rectangular or square. The rib height can be adjusted to either touch the next plate or to leave a small (e.g. 5 μm) margin between the rib top and the next plate. The plates can be gas-impervious or porous. The catalyst 1 is coated (e.g. via metal evaporation or sputtering or using organometallic metal pastes followed by sintering) on one side of the plates in such a way (e.g. Fig. 1) as to ensure electrical contact on one side of the plate with the electronic current collector deposited on one side of the inner reactor wall (current collector 1 , Fig. 1). The catalyst 2 is coated on the other side of the plate in such a way as (a) to avoid short-circuiting with current collector 1 (e.g. by leaving 4-6 mm of the plate surface uncoated, Fig. 1) and (b) to ensure electrical contact with the current collector 2 deposited on the opposite inner reactor wall (Fig. 1). The thickness of the catalysts 1 and 2 can be as low as 10 nm or as high as 10 μm. The catalysts can be porous. The current collectors 1 and 2 are connected via insulated metal sheets or wires to the external power supply or galvanostat or potentiostat. Figure 2 shows a prototype MEPR reactor with 22 8%Y₂0₃-stabilized-Zr0₂ (YSZ) plates and Rh (catalyst 1) and Pt (catalyst 2) catalyst-electrodes. Figure 3 shows a similar unit where one of the plates (top plate) has been replaced by a plate coated with Rh on one side and Au on the other, serving as a NO gas sensor, in order to obtain an integrated monolithic electrochemically promoted reactor sensor (MEPRS) unit.

EXAMPLE 1 The reactor shown in Figure 3 was used to electrochemically promote the complete oxidation of ethylene, chosen as a model reaction: C₂H₄ + 30₂ → 2C0₂ + 2H₂0 (1 ) Only two plates one coated on one side with Rh (Catalyst 1) and on the other side by Pt (catalyst 2) and one plate coated on one side with Rh (Catalyst 1) and on the other with Au (sensor plate). The reactor was operated at temperatures 100°C to 400°C at a total volumetric flow rate of 0.8 l/min. The feed composition was 0.9% C₂H₄ and 2% 0₂, the balance being He. It was first confirmed that the Rh/YSZ/Pt plate could be electropromoted by applying galvanostatically anodic and cathodic currents up to 6 mA and that the Rh/YSZ/Au plate was operating as a potentiometric sensor giving potentials between -0.2 and +0.2 V depending on temperature and gas composition. Subsequently we have short-circuited the Pt and Au electrodes and used them together as the counter electrode (Catalyst 2) and have also short-circuited the two Rh electrodes and used them together as the working electrode (Catalyst 1) and have carried out detailed electrochemical promotion experiments. Several examples are shown in Figures 4 to 6. Figure 4 shows the effect of applying galvanostatically a constant anodic current l=6mA at T=380°C to the rate, r, of ethylene oxidation, expressed in mol O/s, on the conversion of C₂H and on the potential, U, between catalyst 1 and Catalyst 2. The rate and conversion are enhanced by 17% and the rate increase, Δr, is 55 times larger than the rate, I/2F, of supply of O^2" ions to the Rh catalyst from the O^2" - conducting solid electrolyte. Since electrochemical promotion of catalysis is quantified by the parameters: P = r/r₀ (2) Λ = Δr(l/2F) (3) where r₀ is the open-circuit (unpromoted) rate and Δr(=r-r₀), is the electrochemically induced catalytic rate increase, one has in the experiment of Figure 4, p=1.17 and Λ=55. Figure 5 shows a similar experiment at 380°C with the same gas composition where the applied current is 1=12 mA. The rate increases by 45% and the Faradaic efficiency, A, is 77, i.e. p=1.45 and Λ=77. The sudden break in the r vs t transient is due to the electrochemically assisted surface Rh oxide decomposition. Figure 6 shows the steady state effect of applied anodic current, I, and concomitant rate, (I/2F) of supply of O^2" to the Rh catalyst (F is Faraday's constant) and the increase in catalytic rate. Also shown as broken lines are constant Faradaic efficiency, Λ, lines. It is obvious that Λ»1 , so the observed rate increase is caused by electrochemical promotion of catalysis (NEMCA effect) (Λ»1) and not by electrocatalysis (Λ<1).

EXAMPLE 2 The reactor shown in Figure 3 was used, exactly as shown in the figure, i.e., with 21 Rh/YSZ/Pt plates (Rh is catalyst 1 and Pt catalyst 2) and one Rh/YSZ/Au sensor plate. We used a total volumetric gas flowrate of 1.8 l/min and the same model reaction of complete C₂H oxidation. The feed composition was 0.9% C₂H₄ and 1.8% 0₂, the balance being He and carried out electrochemical promotion experiments at temperatures 250°C to 400°C. An example is shown in Figure 7 which depicts the effect of application of an anodic current l=80 mA on the rate of ethylene oxidation, ethylene conversion and Rh catalyst potential with respect to the counter Pt electrode. Also depicted is the transient behaviour of the Rh/YSZ/Au sensor pate potential. Application of 1=80 mA causes a 19.2% increase in the rate of ethylene oxidation and ethylene conversion (p=1.192). The rate increase is 11.1 time larger than the rate, (I/2F), of supply of O^2" to the Rh catalyst. Therefore at steady state the Faradaic efficiency Λ equals 11.1 and thus the electrochemical promotion behaviour of the reactor has been validated. References Cited

1. European Patent Appl. 90600021.1 "Metal-Solid Electrolyte Catalysts" CG. Vayenas, S. Bebelis, IN. Yentekakis and P. Tsiakaras (1990); European Patent 0480116; 24.7.1996 2. "The Dependence of Catalytic Activity on Catalyst Work Function", CG. Vayenas, S. Bebelis and S. Ladas, Nature 343, 625-627 (1990) 3. "The Electrochemical Activation of Catalysis», CG. Vayenas, M.M. Jaksic, S. Bebelis and S.G. Νeophytides in "Modern Aspects of Electrochemistry" 29, 57- 202 (1995) 4. "Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions" CG. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, and D. Tsiplakides, Kluwer Academic/Plenum Publishers, New York (2001).

5. U.S. Patent 6,194,623 B1 "Hydrogenation of organic compounds with the use of the NEMCA effect" A. Frenzel, CG. Vayenas, A. Giannikos, P. Petrolekas, C. Pliangos (2001).

6. U..S. Patent 6,210,557 B1 "Electrocatalytic selective oxidation of hydrocarbons" G. Stochniol, M. Duda, A. Kuehnle (2001)

Other publications

1. G. Foti, S. Wodiunig, Ch. Comninellis, Electrochemical promotion of catalysts for gas phase reactions, Current Topics in Electrochemistry, 7, 1-22 (2000).

2. R.M. Lambert, F. Williams, A. Palermo and M.S. Tikhov, Modelling alkali promotion in heterogeneous catalysis: in situ electrochemical control of catalytic reactions, Topics in Catalysis 13, 91-98 (2000).

3. "Electrochemical Enhancement of a Catalytic Reaction in Aqueous Solution", S. Neophytides, D. Tsiplakides, M. Jaksic, P. Stonehart and CG. Vayenas, Nature 370, 45-47, (1994).

4. U.S. Patent 6,531,704 B2 "Nanotechnology for engineering the performance of substances" T. Yadav, B.K. Miremadi (2003)

Claims

1. A catalytic reactor consisting of parallel porous or gas-impervious plates made of a solid electrolyte coated with the same or different catalytic material on their two sides, electrically connected in parallel or in series and supported in a ceramic casing, which also serves for establishing electrical contact between the catalyst coated on the plates.

2. A reactor as in claim 1 where the plates have parallel ribs on one or both of their surfaces.

3. A reactor as in claims 1 and 2 where the solid electrolyte is a mixed ionic- electronic conductor.

4. A method for enhancing or controlling the rates of catalytic reactions where the reactor of claims 1 , 2 and 3 is used and continuous electrical current or potential is applied between two end terminals.

5. A method as in claim 4 where the applied current or potential is varied in time.

6. A reactor as in claims 1 , 2 and 3 where one or more of the plates serves as a potentiometric, amperometric or constant current potentiometric element.

7. A reactor as in claims 1 , 2, 3 and 6 where the signal generated by the sensing element of claim 6 is used to control the applied potential or current to the reactor.

8. A method for sensing gas compositions and electrically controlling catalytic rates utilizing the reactor of claim 7.