Electrochemical Promotion of Catalysis

CHEMICAL REACTION ENGINEERING 137 CHIMIA 2002, 56, No. 4

Chimia 56 (2002) 137–142 © Schweizerische Chemische Gesellschaft ISSN 0009–4293

György Fóti, Ivan Bolzonella, Justyna Eaves, and Christos Comninellis*

Abstract: Recent progress made in our laboratory in the field of electrochemical promotion of heterogeneous catalytic gas reactions is presented. The phenomenon consists of non-Faradaic modification of the catalytic reaction rate as the result of electrochemical polarization of the interface between the catalyst and the solid electrolyte support. Two main aspects are addressed, the description of the phenomenon and the development of bipolar cell configurations suitable for practical applications. It is shown that the necessary condition to achieve electrochemical promotion is the formation of a double layer at the catalyst/gas interface by mechanism of ion backspillover from the solid electrolyte support. Electrochemical promotion is only feasible in an adequate temperature range, limited by the mobility and the lifetime of the promoting ion, and it is favored by high porosity and low film thickness of the catalyst. On the application side, two new cell designs have been developed, a ring-shaped and a multiple-channel configuration, both operated in bipolar polarization mode. The ring-shaped cell is shown to be almost free of current bypass. Feasibility of electrochemical promotion is successfully demonstrated with both configurations. Realization of efficient bipolar cell configurations for electrochemical promotion is very promising in view of future applications in dispersed catalytic systems. Keywords: Bipolar electrochemical cell · Catalysis · Current bypass · Electrochemical promotion · Gas-phase reactions

1. Introduction The catalyst, which is an electron ble rate enhancement due to electrochem- conductor, is deposited in form of a ical oxidation of ethylene. It is evident Electrochemical promotion (EP) means porous thin film on a solid electrolyte that electrochemical promotion is clearly the electrically controlled modification of support, which is an ion conductor at not a Faradaic electrochemical reaction. heterogeneous catalytic activity and/or the temperature of the catalytic reaction. This statement is further confirmed by selectivity. Discovered some fifteen years Application of an electric potential on the experimental fact that even a chemi- ago [1], the phenomenon has mainly been the catalyst/support interface or, which is cal reduction (e.g. reduction of NO with investigated in gas-phase reactions over equivalent, passing an electric current be- propylene over Rh/YSZ catalyst, see lat- metal and metal oxide catalysts [2][3]. tween catalyst and support causes a con- er) may be promoted by application of comitant change also in the properties of anodic (i.e. electrochemically oxidizing) the adjacent catalyst/gas interface, where current. Note that Fig. 1 illustrates the the catalytic reaction takes place. This re- general case when the steady-state open- sults in an alteration of the catalytic activ- circuit reaction rate after current interrup- ity controllable with the applied potential tion, r’, is significantly higher than its in- or current. itial value before current application, ro. Typical catalytic reaction rate tran- This irreversibility, first reported from sients due to electrochemical promotion our laboratory [4] and called permanent are shown in Fig. 1. The example is the promotion, is attributed to current-assist- combustion of ethylene with oxygen at ed chemical modification of the catalyst 375 °C over RuO2 catalyst interfaced surface. with yttria-stabilized zirconia (YSZ) sol- The experimental set-up used for ki- id electrolyte, which is an O2– conductor netic measurements is described in detail at this temperature. Application of an elsewhere [5]. The most important part is anodic current in this system means that the electrochemical cell placed in a tem- the solid electrolyte supplies O2– ions to perature-controlled reactor fed with the *Correspondence: Prof. Dr. C. Comninellis the RuO2/YSZ interface and the catalyst reactive gas mixture. The simplest elec- Ecole Polytechnique Fédérale de Lausanne serves as the collector of electrons. As trochemical cell for EP experiments is of Faculté des Sciences de Base seen in Fig. 1, a huge increase in reaction the single-pellet type where the working Institut de Sciences des Procédés chimiques et biologiques rate is observed during anodic current ap- electrode (catalyst) and the counter-elec- CH–1015 Lausanne plication. The effect is by orders of mag- trode (usually a porous gold film) are de- Tel.: +41 21 693 3674 nitude higher than the rate of O2– supply posited on the two sides of a thin solid Fax: +41 21 693 3190 E-Mail: [email protected] toward the catalyst which is, after Fara- electrolyte pellet. In this configuration, http://dcwww.epfl.ch/lgrc/ day’s law, equal to the maximum possi- widely used for fundamental studies of CHEMICAL REACTION ENGINEERING 138 CHIMIA 2002, 56, No. 4

Fig. 1. Electrochemical promotion of ethylene combustion over RuO2/YSZ catalyst. Sche- matic representation of evolution of the reaction rate due to anodic current application of 10 µA during 70 min. Maximal electrochemical rate increase from Faraday’s law: I /2F = 52 pmol O s–1. Experimental conditions: atmos-

pheric reactor, pC2H4 = 100 Pa, pO2 = 12 kPa, = 175 ml min–1STP, T = 380 °C.

the phenomenon, the catalyst itself is a lyst in a suitable electrochemical cell. feeder electrode hence it must be directly Two generations of advanced bipolar cell connected (e.g. with a fine gold wire) to configurations, a ring-shaped and a mul- the potentiostat. Such a configuration is tiple-channel cell, illustrated together obviously inadequate for practical appli- with a tubular reactor assembly in Fig. 2, cations with dispersed catalysts. The only have been developed in our laboratory viable way to achieve electrochemical using YSZ solid electrolyte; see section 3 promotion in dispersed systems implies for details. These bipolar cells may be re- indirect bipolar polarization of the cata- garded as the first successful steps to-

Fig. 2. Left: tubular reactor assembly and furnace used with bipolar cell configurations. 1: Pyrex tube (Ø = 25/18 mm, length: 400 mm, volume: 150 ml); 2: electro-chemical cell. Right: bipolar electrochemical cells. a: ring-shaped cell configuration; b: multiple-channel cell configuration. CHEMICAL REACTION ENGINEERING 139 CHIMIA 2002, 56, No. 4 wards achievement of electrochemical along, or near, the tpb in electrochemical ←→ promotion with highly dispersed cata- O2 +2D 2O · D (1a) reactions (oxygen evolution and/or oxi- lysts. A+O·D→ D+O·A (1b) dation of adsorbed reactants) obviously In this short paper, first a brief phe- O·A+ B→ C (1c) obeying to Faraday’s law, or they may nomenological description of electro- R+C→ P + B (1d) spill over the gas-exposed catalyst sur- chemical promotion will be given with face. The driving force of this migration special emphasis on its functional analo- Eqn. 1a describes the dissociative ad- is the difference in the electrochemical gy with classical promotion of catalysis, sorption and recombination of oxygen on potential of O2– ions between catalyst then results from our laboratory on the a donor D. The transfer between the do- and solid electrolyte due to polarization development of new bipolar cell configu- nor D and acceptor A is given by Eqn. 1b. of the catalyst [9]. The existence of back- rations in view of future industrial appli- The spillover oxygen (O) is a mobile spe- spillover oxygen ions at the gas-exposed cations will be presented. cies present on the acceptor surface with- catalyst surface was demonstrated by out being associated with a particular sur- XPS measurements [10]. No charged ox- face site. These mobile spillover species ygen species were detected under open- 2. The Phenomenon of may then interact with a, catalytically in- circuit conditions but they were found in Electrochemical Promotion active, surface site B (Eqn. 1c) forming a abundance after anodic polarization of new site C which may be active. Eqn. 1d the catalyst/solid electrolyte interface. In any electrochemical cell suitable represents the catalytic reaction at the ac- Charged backspillover oxygen and neu- for EP experiments several interfaces tive site C consuming a reactant R and tral chemisorbed oxygen may be clearly may be identified, such as electrode/elec- forming a product P. Kinetic models distinguished also by temperature-pro- trolyte interfaces, electrode/gas interfac- based on this mechanism are well sup- grammed desorption (TPD) [11]. It was es, and three-phase boundaries (tpb). Due ported by experiments [7]. shown that the oxygen adsorbed from the to the different mechanism of electric The most plausible model of electro- gas phase desorbs at lower temperature, conductivity in electrodes and electrochemical promotion, illustrated in Fig. hence it is more reactive than the electrolyte, polarization of the catalyst by cur- 3b, is based on analogous donor–accep- chemically generated oxygen specimen. rent application involves charge transfer tor interactions [2][3][8]. Consider the During migration over the gas-ex- reactions and a consequent charge sepa- example of anodic polarization of the cat- posed catalyst surface, the oxygen ions ration, all localized, at a first glance, at alyst. In this case, the donor is the solid are accompanied by their compensating the electrode/electrolyte interfaces and/ electrolyte, supplying O2– ions toward image charge in the catalyst, thus form- or at the three-phase boundaries. The re- the catalyst (acceptor). (This migration ing surface dipoles. Population of the sur- sulting promotion of catalytic activity re- from support to catalyst may be called face with dipoles, i.e. formation of an veals, however, modification also of an- backspillover in order to distinguish it electric double layer, increases the sur- other interface, namely the gas-exposed from the migration in opposite direction, face potential hence also the catalyst catalyst surface where the heterogeneous usual in catalysis and termed spillover.) work function, which is a measurable gas reactions occur. In fact, the key to The O2– ions are released from the solid quantity. In fact, work function measure- understanding electrochemical promo- electrolyte at the three-phase boundaries ments, performed also in our laboratory tion is to elucidate the mechanism of how (tpb). They may either be consumed [12], are a useful tool to investigate the the effect of polarization propagates from one interface to the other. It is believed that electrochemical promotion of catalysis, similar to usual (chemical) promotion and to metal–support interactions in heterogeneous catalysis, is related to spillover–backspillover phenomena. This latter can be described as the migration of adsorbed species from one phase on which they easily adsorb (donor) to another phase where they may not adsorb directly (acceptor). By this mechanism a seemingly inert material can acquire catalytic activity. A general reaction scheme of oxygen spillover in a purely (not electro-) catalytic system [6] may be formulated as follows, see also Fig. 3a.

Fig. 3. a: Schematic representation of the mechanism of oxygen spillover in a purely catalytic system; symbols as in Eqns 1. b: Schematic representation of the mechanism of electrochemical promotion under anodic current application via backspillover of charged promoting species (O2–). CHEMICAL REACTION ENGINEERING 140 CHIMIA 2002, 56, No. 4 phenomenon of EP. Any change in work configurations were of cylindrical shape, edge of the steady-state polarization (cur- function affects the binding strength of with two gold feeder electrodes on the rent–voltage) curves in three different chemisorbed reactants. Namely, the ad- outside jacket of the YSZ cylinder and polarization modes, shown in Fig. 4: di- sorption energy of electron acceptors with a catalyst film inside, i.e. on the in- rect cathodic (DC) polarization of the (e.g. atomic O) is weakened, while that of ner wall of the ring in the first case and on catalyst film relative to gold electrode 1, electron donors (e.g. C2H4) is strength- that of 37 parallel axial channels in the direct anodic (DA) polarization of the ened. Then, a consequent modification in second case. The feasibility of EP was catalyst film with respect to gold elec- catalytic reaction rates is evident. This successfully demonstrated with both con- trode 2, and indirect bipolar (IB) polari- picture of the electrochemical promotion figurations [5]. In this paper, the ring- zation between the two terminal gold is well supported by in situ cyclic voltam- shaped configuration allowing more ad- electrodes, gold 1 being the anode and etry [13] and by the transient behavior of vanced electrochemical characterization gold 2 the cathode. The method is based the promoted reaction rate [14]. of the cell is presented in detail. on the additivity of the potential at a giv- In summary, electrochemical promo- The example is a ring-shaped Rh/ en current in the linear (ohmic) region, so tion may be regarded as catalysis at an YSZ electrochemical cell prepared with a the current bypass is measured by the ex- electric double layer controlled by cur- cylindrical YSZ ring (OD 20 mm, cess of the sum of cell voltages in the two rent or potential application. The neces- ID 17 mm; height 10 mm). The rhodium monopolar polarization modes over the sary condition to achieve electrochemical catalyst film (nominal loading: 1.1 g m–2, cell voltage in the bipolar polarization promotion is the formation of a double film thickness: ~0.1 µm) and the gold mode. The fraction of current bypass, Ψ, layer at the catalyst/gas interface by electrodes were deposited by thermal de- at a given current is then calculated with mechanism of ion backspillover from the composition of suitable precursors, as de- the following expression: solid electrolyte support. This may occur scribed elsewhere [15]. The two symmet- over a wide range of conditions. The rical gold electrodes on the outer surface most important experimental parameters, were separated from each other by two (2) which favour electrochemical promotion, uncoated bands of 3 mm width. The po- are the following: long tpb (porous cata- larization of the catalyst film in usual bi- where V are cell potentials measured at lyst), moderate diffusion length (thin cat- polar operation is realized using the two the same current in the linear region, and alyst film), and adequate temperature gold films as feeder electrodes. For ad- Vo are overvoltages in the monopolar po- range, e.g. 250–500 °C for YSZ (a tem- vanced characterization of the cell, a larization modes extrapolated to zero cur- perature not too low to have enough mo- third electric connection may be added. rent. bility, and not too high to avoid high re- The latter, connected to the catalyst film The steady-state polarization curves activity of backspillover ions). itself, enables the performance of meas- of the cell exhibited an almost perfect ad- urements also in direct (monopolar) po- ditivity [5], see the example at 375 °C in larization modes. This finds use in deter- Fig. 5. Consequently, very low current 3. Development of Bipolar mination of the current bypass, which is bypass was obtained calculating with Electrochemical Cells an important parameter of a bipolar cell. Eqn. 2. It was found fairly independent of A model developed for electrode the current, but it rose slightly with tem- Two types of bipolar electrochemical stacks with liquid electrolytes [16] was perature (from 2–4% at 350 °C to 5–6% cell have been developed in our laborato- adapted for the estimation of current by- at 400 °C). Being a necessary condition ry, a ring-shaped and a multi-channel pass in the ring-shaped electrochemical to achieve efficient electrochemical pro- configuration. As shown in Fig. 2, both cell [17]. The method needs the knowl- motion in bipolar cells, realization of a

Fig. 4. Scheme of various modes of polarization using the ring-shaped electrochemical cell. CHEMICAL REACTION ENGINEERING 141 CHIMIA 2002, 56, No. 4

Fig. 5. Current–potential curves of the ring- shaped Rh/YSZ cell at different polarization modes. Ț: direct cathodic (DC); ̅: direct anodic (DA); ȣ: DC + DA; v: indirect bipolar (IB). Vcell: cell potential between the two feeder electrodes. T = 375 °C.

nearly bypass-free bipolar configuration is of great importance. The feasibility of electrochemical promotion with ring-shaped cells will be illustrated with the example of the reduction of NO by propylene in presence of oxygen over rhodium catalyst. Metallic rhodium is known to be an efficient catalyst for the reduction of NO [18][19]. The propylene–NO–oxygen system may serve as a model for the removal of nitro- gen oxides (NOx) and hydrocarbons (HC) from auto exhaust gases aiming to transform, under lean fuel conditions, both the reducing pollutants (HC) and the oxidizing pollutants (NOx) into harmless products such as N2, CO2 and H2O (Eqn. 3): thodically and the other half of it is anod- 18NO(g) + 2C H (g) → 9N (g) + 6CO (g) + 6H O(g) (3) ically polarized, gave rise to a promotion 3 6 2 2 2 twice as high as obtained in the direct polarization modes due to the nearly by- → 18NO(g) + C3H6(g) 9N2O(g) + 3CO2(g) + 3H2O(g) (4) pass-free cell configuration. The obtained promotion obviously 2C H (g) + 9O (g) → 6CO (g) + 6H O(g) (5) depends on the applied potential and on 3 6 2 2 2 the polarizing current. Fig. 7 shows the evolution of C3H6 and NO conversion In competition with the desired reac- seen that electrochemical promotion was under indirect bipolar (IB) polarization as tion, two reactions may prevent efficient successful in all polarization modes mul- a function of the cell potential (Vcell) and reduction of NOx: the partial reduction of tiplying the conversion, X, of C3H6 and of the total current (I) fed by the two gold NO producing N2O (Eqn. 4) and con- NO (both about 10% under open-circuit electrodes. The minimum cell potential sumption of HC via direct oxidation in conditions) with a factor of 2–2.5 in DC required to induce significant electro- the presence of excess O2 (typical for and DA modes and with a factor of 3–4 in chemical promotion was 2–3 V and the exhaust gases from lean-burn engines) IB mode. Interestingly, polarization in promotion leveled off at 6–7 V (~1 mA) (Eqn. 5). both monopolar modes was efficient and where the current–voltage curve of the The aim is now to induce modifica- resulted in an almost identical promotion, cell enters the region of purely ohmic be- tion of the catalytic activity of rhodium in independently of the sign of polarization. havior, see Fig. 5. this complex system by potential or cur- This is a further argument to affirm that Achievement of a significant electro- rent application. electrochemical promotion is not a chemical promotion effect in bipolar The catalytic performance of the ring- Faradaic effect. In fact, the reaction ex- cells is very promising in view of future shaped Rh/YSZ electrode was tested at hibits inverted volcano (minimum rate) applications of the phenomenon with dis- 375 °C at varying feed compositions type behavior [3] because the kinetics is persed catalysts. and polarization potentials. Fig. 6 shows positive order in both the electron accep- the results obtained with feed composi- tor (NO) and electron donor (C3H6) reaction of C3H6 (0.25 kPa)/NO (0.5 kPa)/O2 tants. In these experiments, not only the 4. Conclusions (0.5 kPa). Polarization of the rhodium feasibility of EP with bipolar polarization film was made by applying constant cell was demonstrated but also this mode of The results clearly demonstrate the potentials: 5 V in the direct polarization polarization was found to be the most ef- feasibility of electrochemical promotion modes (DC and DA) and 10 V in the indi- ficient. The indirect bipolar polarization, with bipolar electrochemical cells. In fu- rect bipolar polarization mode (IB). It is when one half of the catalyst film is ca- ture work, the development of dispersed CHEMICAL REACTION ENGINEERING 142 CHIMIA 2002, 56, No. 4

Fig. 6. Steady-state conversion, X, of C3H6 ( left) and NO (right) in the ring-shaped Rh/YSZ cell at open-circuit condition (OC) and at three different potentiostatic polarization modes: direct cathodic (DC; –5V), direct anodic (DA; +5V), and indirect bipolar (IB; 10V). The correspond- ing current is also given. Feed composition and temperature as indicated. = 200 ml min–1 STP.

Fig. 7. Effect of the cell potential, Vcell (left), and of the total current, I (right), on the conversion, X, of C3H6 and NO under indirect bipolar (IB) polarization in the ring-shaped Rh/YSZ cell. Feed composition, gas flow and temperature as in Fig. 6.

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