ELECTROCHEMICAL THEORY | Non Faraday Electrochemical Modification of Catalysts Activity

Non Faraday Electrochemical Modification of Catalysts Activity CG Vayenas and S Brosda, University of Patras, Patras, Greece & 2009 Elsevier B.V. All rights reserved.

Introduction The term NEMCA refers to nonfaradaic electrochemical modification of catalytic activity. The NEMCA effect is also known as electrochemical promotion or electrochemical promotion of catalysis (EPOC) or electropromotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on electronically conductive catalysts deposited on ionic (or mixed ionic–electronic) supports upon application of electric current or potential (typically 72 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The NEMCA effect was first reported in 1981 for the case of the oxidation of ethylene (C2H4) by gaseous oxygen on porous silver catalyst films deposited on Y2O3stabilized ZrO2 (YSZ), an O2 conductor, and has been described in the literature for more than 80 catalytic systems on a variety of catalysts (platinum, palladium, rhodium, silver, iridium(IV) oxide (IrO2), ruthenium(IV) oxide (RuO2), nickel, copper, gold) using numerous anionic (YSZ, calcium fluoride (CaF2)), cationic (b00 Al2O3, Na3Zr2Si2PO12, K2YZr(PO4)3, CaZr0.9In0.1O3–a, CsHSO4, Nafion), or mixed electronic–ionic (titanium di-oxide (TiO2), cerium(IV) oxide (CeO2), YZrTi) supports, and also aqueous and molten salt electrolytes. The observed change in catalytic rate is typically 5–105 times larger than the electrochemical reaction rate (i.e., the rate of ionic transport in the support – the rate of ion supply to or ion removal from the catalyst); thus, the effect is strongly nonfaradaic. The electropromoted catalytic reaction rate is typically 2–500 times larger than the open-circuit (i.e., unpromoted) catalytic rate.

Basic Phenomenology The basic phenomenology of this effect when using O2, Na þ , and H þ conducting solid electrolytes is shown in Figures 1–3. The (usually porous) metal catalyst electrode, typically 0.5–5 mm thick, is deposited on the solid electrolyte and under open circuit (I ¼ 0, no electrochemical rate) produces a catalytic rate r0 for ethylene oxidation (Figures 1 and 3) or carbon monoxide oxidation (Figure 2). Application of an electrical current, I, or potential (72 V) between the catalyst and a counterelectrode causes very pronounced and nonfaradaic (i.e., DrcI/2F ) alterations to the catalytic rate, r, and, quite often, to the product selectivity (Figure 4).

64

The rate of the catalytic reaction, r, can become up to 500 times larger than the open-circuit rate, r0 , and up to 3  105 times larger than the faradaic rate (I/2F for O2,  I/F for Na þ and H þ ) of ion supply (or removal) to (or from) the catalyst electrode. Up to 2007, more than 70 different catalytic reactions (oxidations, hydrogenations, dehydrogenations, isomerizations, decompositions) have been electrochemically promoted on platinum, palladium, rhodium, silver, gold, nickel, iridium(IV) oxide, and ruthenium(IV) oxide catalysts deposited on O2 (YSZ), Na þ (b00 -Al2O3), H þ (CaZr0.9In0.1O3  a, Nafion), F (calcium fluoride), aqueous, molten salt, and mixed ionic–electronic (titanium dioxide, cerium(IV) oxide) conductors. Clearly, EPOC is not limited to any particular class of conductive catalyst, catalytic reaction, or ionic support.

Definitions and Some General Characteristics The magnitude of electrochemical promotion for a given catalytic reaction is described by three parameters: 1. The faradaic efficiency, L, is expressed as L ¼ ðr  r0 Þ=ðI =nF Þ

½1

where r is the electrochemically promoted catalytic rate, r0 the unpromoted (open-circuit) catalytic rate, I the applied current, n the charge of the promoting ion, and F (96 460 C mol1) Faraday’s constant. A reaction is electrochemically promoted when |L|>1. For |L|r1 electrocatalysis occurs. Values of L as high as 3  105 or as low as  3  104 have been measured (Table 1). For oxidation reactions, L>1 implies electrophobic behavior ð@r =@UWR > 0Þ and Lo–1 implies electrophilic behavior ð@r =@UWR o0Þ where UWR is the catalyst (working electrode, ‘W’) potential with respect to the reference (‘R’) electrode. For oxidation reactions on metals supported on O2 conductors, L also expresses the ratio of the lifetimes of the promoting O2 species and of normally chemisorbed O on the catalyst surface. 2. The rate enhancement ratio, r, is expressed as r ¼ r =r0

½2

Values of r as high as 500 or as low as zero (complete catalyst poisoning) have been measured.

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

I U

Electrochemical oxidation rate r e  I/nF

Catalytic oxidation rate r Current-induced rate change r >> I/nF

C2H4  6O2

C2H4  3O2

12e  2CO2  2H2O e

2CO2  2H2O UWR

e

2

2

O

O

O

2

65

Catalyst electrode Solid electrolyte Counter and reference electrodes

O2(g) (a) 50 I0

I0

I  1 μA

800

100 600 r (× 108 mol Os1)

TOF (s1)

80

60

40

30

400

20

200

10

20

200 0

(b)

0

r 0  (I/2F )104

r0 0

0

Catalyst potential, UWR (mV)

40

τ

2FNG /I 20 t (min)

40

110

130

Figure 1 (a) Basic experimental setup and operating principle of electrochemical promotion with O2 conducting supports. Reproduced from Brosda S and Vayenas CG (2002) Rules and mathematical modeling of electrochemical and classical promotion: 2. Modeling. Journal of Catalysis 208: 38–53. (b) Catalytic rate (r) and turnover frequency (TOF) response of C2H4 oxidation on Pt deposited on Y2O3-stabilized ZrO2 (YSZ), an O2 conductor, upon step changes in applied current. T ¼ 370 1C, pO2 ¼ 4:6 kPa, pC2 H4 ¼ 0:36 kPa. Also shown (dashed line) is the catalyst-electrode potential (UWR) response with respect to the reference (R) electrode. The catalytic rate increase (Dr) is 25 times larger than the rate before current application (r0) and 74 000 times larger than the rate of O2 supply to the catalyst electrode (I/2F). NG is the Pt–gas interface surface area (in mol Pt) and TOF is the catalytic turnover frequency (moles of oxygen reacting per surface Pt mole per second). Reproduced from Bebelis S and Vayenas CG (1989) Nonfaradaic electrochemical modification of catalytic activity: 1. The case of ethylene oxidation on Pt. Journal of Catalysis 118: 125–146. Copyright (1989), with permission from Elsevier.

3. The promotional index PIi of promoting species, i, (e.g., Od, Nad þ ) is expressed as PIi ¼

D r =r0 D yi

½3

where Dr is the promotionally induced rate enhancement (r  r0) and yi the coverage of the promoting species on the catalyst surface; PIi40 corresponds to promotion, whereas PIio0 corresponds to poisoning. Values of PINaþ up to 6000 and of PIO2 up to 100 have been measured (Table 1). It should be noted that although L is a parameter relevant to electrochemical promotion (and to metal–

support interactions (MSIs)), the other two parameters, r and PIi , are relevant to both electrochemical and classical promotion. Since the early days of electrochemical promotion, it has been shown both experimentally and theoretically that the order of magnitude of |L|, that is of the absolute value of L, for a given catalytic reaction, metal and solid electrolytes can be estimated from jLjE2Fr0 =I0

½4

where I0 is the exchange current of the metal–solid electrolyte interface, measurable using ln I  DUWR

66

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity Electrocatalytic reaction Na(  AI2O3)  e Na(Pt) I U

CO 

e e

Potential and Work Function

Catalytic reaction O2

CO2

Pt Na

Na

Na

U WR

Au

Au

(a)

0.5

0.02 0.04 0.06

0

Na

eDUWR ¼ DF

0.5 1

10 7.5 5

1

(b)

2.5 0

1

2 3 t (min)

4

5

r (× 107 mol s1)

U WR (mV)

l0

l  20 A

0

A key parameter in electrochemical promotion studies is the catalyst potential UWR. The subscript ‘WR’ denotes the potential of the working (W) electrode (which also serves as the catalyst) with respect to a reference (R) electrode. In situ work function measurements and theoretical studies have shown that there exists over wide temperature ranges a one-to-one correlation between the change in UWR and the concomitant change in the work function, F, of the gas-exposed, that is catalytically active, catalyst surface:

6

Figure 2 (a) Basic experimental setup and operating principle of electrochemical promotion with Na þ conducting supports. Reproduced from Vayenas CG, Brosda S, and Pliangos C (2002). The double layer approach to promotion, electrocatalysis, electrochemical promotion and metal-support interactions. Journal of Catalysis 216:487–504, with permission from elsevier. (b) Catalytic rate (r) response of CO oxidation on Pt deposited on b00 -Al2O3, a Na þ conductor upon step changes in applied current. Also shown is catalyst potential (UWR) response. T ¼ 350 1C, pCO ¼ 2 kPa, pO2 ¼ 2 kPa. Note that the rate passes through a maximum at yNa ¼ 0.015, as the reaction rate of CO oxidation on Pt exhibits volcano-type behavior with respect to the catalyst potential and work function. Upon current interruption (I ¼ 0), the rate (r) and potential (UWR) do not return to their initial values. This is accomplished only by imposing potentiostatically the initial UWR value. In this experiment, the potentiostat, previously used to control UWR, is disconnected at t ¼  1 min and then at t ¼ 0 the galvanostat is used to apply a constant current. Dashed curves correspond to rate and UWR transients obtained with different previously imposed UWR values. Note that the Na coverage (inset axis) always determines the r and UWR values during the transients. Reproduced from Yentekakis IV, Moggridge G, Vayenas CG, and Lambert RM (1994) In situ controlled promotion of catalyst surfaces via NEMCA: The effect of Na on the Ptcatalyzed CO oxidation. Journal of Catalysis 146(1): 292–305. Copyright (1994), with permission from Elsevier.

(Tafel) plots. However, until recently, no predictions whatsoever could be made about the sign of L, or about the magnitude of r and PIi. This is because eqn [4] basically reflects that electrochemical promotion is due to electrochemical introduction of promoters on a catalyst surface, although the sign of L and the magnitude of r and PIi refer to the purely fundamental aspects of promotion itself.

½5

Thus upon varying the catalyst potential, UWR, in electrochemical promotion studies, the catalyst work function, F, also varies. Increasing coverage of electron acceptor (A) (electronegative) promotion species increases the catalyst potential and work function, whereas increasing coverage of electron donor (D) (electropositive) promoting species decreases the catalyst potential and work function. Upon varying the catalyst potential UWR and work function F of a given catalyst at constant temperature and gas composition, large variations are obtained in the catalytic rate, r, from its value, r0, under open-circuit (unpromoted) conditions. As the electrochemical promotion literature shows, there exist four main types of r versus UWR (or r vs F) dependence (Figure 5): a. electrophobic reactions, where r increases monotonically with UWR and F, b. electrophilic reactions, where r decreases monotonically with increasing UWR and F, c. volcano-type reactions, where r passes through a maximum with UWR and F, and d. inverted volcano-type reactions, where r passes through a minimum with UWR and F As also shown in Figure 5, a dimensionless catalyst potential or work function P can be expressed as P ¼ FUWR =RT ¼ DF=kB T

½6

where F and kB are Faraday’s and Boltzmann’s constants, and DF denotes the change in catalyst work function from its value on the catalyst material under the conditions of the reference electrode (in modeling studies DF usually denotes the change in F from its value at the potential of zero charge (pzc), but this change in catalyst reference state is not important, as it only adds a constant to P). Consequently, the above four main types of r versus UWR (or r vs DF) dependence can also be equivalently presented on the r/r0 versus P plane (Figure 5).

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

67

Electrochemical reaction Catalytic reaction OH(Pt) C2H4  3O2 H(CaZr0.9In0.1O3α)  O(Pt)  e 2CO2  2H2O I U

e

Pt H

e

H

H

Au

(a)

U WR Au

50

1 I0

I0

I 3 A

12

T  405 C

0.5

pO  5.05 kPa

40

2

pC

2H4

10

 1.05 kPa

6

30 0.5

20

U WR (V)

8

r (× 108 mol Os1)

TOF (s1)

0

1

4 r0  14.7  108 (mol s1)

10

r  27.5  108 mol s1 (I/2F )  1.55  1011 mol s1   2.87  17 700

2

0

0

0 2

(b)

20 t (min)

1.5

2

40

Figure 3 (a) Basic experimental setup and operating principle of electrochemical promotion using a H þ conductor during C2H4 oxidation on Pt deposited on CaZr0.9In0.1O3  a. Reproduced with permission from Vayenas CG, Brosda S, and Pliangos C (2002). The double layer approach to promotion, electrocatalysis, electrochemical promotion and metal-support interactions. Journal of Catalysis 216: 487–504. (b) Catalytic rate (r ), catalytic turnover frequency (TOF), and catalyst potential response to step changes in applied current (UWR). The increase in O consumption (Dr) is 17 700 times larger than that anticipated from Faraday’s law and corresponding rate of proton transfer to the Pt catalyst (  I/F). Reprinted from Makri M, Buekenhoudt A, Luyten J, and Vayenas CG (1996) Nonfaradaic electrochemical modification of the catalytic activity of Pt using a CaZr0.9In0.1O3–a proton conductor. Ionics 2: 282–288, with kind permission of Springer Science and Business Media.

Reaction Classification Table 1 classifies practically all hitherto published electrochemical promotion studies on the basis of the solid electrolyte support and corresponding promoting ion used. The table also provides the catalyst used, the operating temperature range and the measured maximum (L41) or minimum (Lo  1) L value, as well as the measured maximum (r41) or minimum (ro1)r and the maximum measured PIi value. Note that for the case of nonreacting promoters (e.g., Na þ ), no L values are given in many cases as they are not very useful. They tend, in principle, to infinity at steady state. In practice,

|L| is finite because of some unavoidable promoter evaporation and is usually larger than 105. Table 2 classifies again the published electrochemical promotion studies of Table 1, which include complete and partial oxidations, catalytic hydrogenations, dehydrogenations, and isomerizations. This table also focuses on the purely catalytic (chemical) aspects of electrochemical promotion and provides three additional important parameters: (i) The range of pA/pD values used in the investigation, where pA and pD are the partial pressures of the electron acceptor (e.g., oxygen) and the electron donor (e.g., hydrocarbon) reactant used.

68

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity 1.4

125 30 wt% Pd/C

1.2

rcis rtrans rbutane I (mA)

100

75

0.8

I (mA)

r (× 106 mol s1)

1

0.6

50

25 0.2

0

0.1

(a)

0.2 0.3 Cell voltage (V)

 @F  o0 @yj yiaj

100 50

Selectivity (%)

60 30 40

20

20

0 (b)

0

0.05 0.1 0.15 Cell voltage (V)

0.2

V

40

Faradaic efficiency,

80

cis-butene trans-butene butane

½7

and as an electron donor (D) when it decreases the work function of the surface:

0

0.4

A reactant, j, is defined as electron acceptor (A) when it increases the work function of the surface:  @F  >0 @yj yiaj

0.4

0

purely electrophilic (qr/qF)o0, volcano type, and inverted volcano type (The term ‘global’ denotes the behavior observed over the entire UWR and thus F range investigated (typically from  1.5 to þ 1.5 eV versus open-circuit voltage) as opposed to ‘local’, which denotes the r versus F behavior at a specific UWR and F value).

½8

Table 2 shows that the global r versus F behavior can often change from one type to the other even for the same reaction and metal catalyst upon changing the solid electrolyte support or the gaseous composition. On classifying the reactions in this way (Table 2), several clear patterns and regularities appear, which have led to the identification of the rules of electrochemical promotion.

10

Promotional Rules

0

In recent years, simple and rigorous rules have been found for electrochemical promotion that allows one to predict the type of catalytic rate dependence on catalyst potential (electrophobic, electrophilic, volcano, and inverted volcano, Figure 5) on the basis of the unpromoted kinetics, that is on the basis of the catalytic rate dependence on the reactants’ partial pressures. This was done by classifying all electrochemical promotion studies published by 2001 (72 reactions, Table 2) in the above four categories and by making the following four observations:

Figure 4 (a) Electrochemical promotion of an isomerization reaction. Steady-state effect of cell potential on the cell current (I) and on the rates of formation of cis-2-butene, trans-2-butene, and butane produced from 1-butene supplied over a dispersed Pd/C catalyst electrode deposited on Nafion, a H þ conductor at room temperature. Reprinted in part with permission from Ploense L, Salazar M, Gurau B, and Smotkin ES (1997) Proton spillover promoted isomerization of n-butylenes on Pd-black cathodes/ Nafion 117. Journal of the American Chemical Society 119: 11550–11551. Copyright (1997) American Chemical Society. (b) Corresponding effect of cell potential on the selectivities to cis-2butene, trans-2-butene, and butane and on the apparent faradaic efficiency (L) defined as Drtotal /(I/F). Thus, each proton catalyzes the isomerization of roughly 50 molecules of 1-butene to cis-2butene and trans-2-butene. Reproduced from Vayenas CG, Brosda S, and Pliangos C (2002). The double layer approach to promotion, electrocatalysis, electrochemical promotion and metal-support interactions. Journal of Catalysis 216:487–504, with permission from Elsevier.

(ii) The observed kinetic order (positive order, negative order, or zero order) of the catalytic reaction with respect to the electron donor (D) and the electron acceptor (A). (iii) The observed global r versus work function F behavior, that is purely electrophobic (qr/qF)40,

a. That all electrophobic reactions (qr/qUWR 40) are positive order in the electron donor (D) reactant (qUWR/qyD or qF/qyDo0, e.g., ethylene, benzene (C6H6)) and zero or negative order in the electron acceptor (A) reactant (qUWR/qyA or qF/qyA 40, e.g., oxygen, nitric oxide (NO)). b. That all electrophilic reactions are negative or zero order in the electron donor reactant and positive order in the electron acceptor reactant. c. That in all volcano reactions, the rate versus pA (partial pressure of electron acceptor) and rate versus pD (partial pressure of electron donor) curves also pass through a maximum. d. That all inverted volcano reactions are positive order in both reactants.

O2 O2

O2 O2

O2 O2 O2

C2H4 C2H6

CH4 CO

CO CH3OH C3H6 CH3OH C2H4 C2H4 H2 C3H6 CO CO H2 H2S CH4 H2 CO CO C2H4 C3H6 CH4 CO CH3OH CH3OH CH4 CO CH4 CO CH4 C2H4 C2H4

O2 O2 O2 H2O O2 O2 O2 O2

O2 CO2 NO N2O O2 O2 O2 O2

NO O2 CO2 NO,O2 NO,O2 O2 CO

Electron acceptor (A)

Electron donor (D)

Reactants

CO2 H2CO, CO2 CO2 H2CO, CO, CH4 CO, CO2, N2, N2O CO2 CH4, CO N2, N2O, CO2 N2, N2O, CO2 CO2 CxHy, CxHyOz Sx, H2 CO2 CO CO2, N2, N2O CO2, N2 C2H4O, CO2 C3H6O, CO2 CO2, C2H4, C2H6 CO2 H2CO, CO, CH4 H2CO, CO2 C2H4, C2H6, CO2 CO2 CO, CO2 CO2 CO2 CO2 CO2

CO2 CO2

CO2 CO2

Products

Pt Pt Pt Pt Pt Rh Rh Rh Rh Pd Pd Pt Pd Pd Pd Pd Ag Ag Ag Ag Ag Ag Ag Ag–Pd Ni Au Au IrO2 RuO2

Pt Pt

Pt Pt

Catalyst

468–558 300–500 350–480 400–500 380–500 250–400 300–450 250–450 250–450 400–550 300–370 600–750 380–440 500–590 320–480 440 320–470 320–420 650–850 350–450 550–750 500 700–750 450–500 600–900 450–600 700–750 350–400 240–500

600–750 300–550

260–450 270–500

T (1C)

Classification of electrochemical promotion (EP) studies based on the type of solid electroltyte

1. EP studies utilizing YSZ. Promoting ion:O2

Table 1

rmax (>1) or rmin (o1) 55 20 7 70 3 6 5 4, 15a 6 3a 7 90 3a 150a 20a 2 3a 11 90 10 3 2 30a 2a 30a 15 6a 2 8a 5 2a 3 3a 6 115

Lmax (>0) or Lmin(o0) 3  105 300  100 5 2  103  500 1000 1  104  3  103  10  50 5  104 200 1  103 20 1  103 10 – 2  103  50  700  20 300 300 5 20  25  95  1.2 30 12  60 3 200 4  103

(Continued )

55 20 – 70 2 – 5 3 – – – 90 2 150 20 1 2 10 90 – – – 30 1 30 15 – – – 4 – – – 5 115

PIo 2

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity 69

O2

CO

O2

O2

O2 O2

C2H4

C2H4

C2H4 C3H6

O2 O2

C6H6 C2H4 NO NO NO NO C2H2 C2H4 O2 O2 O2

C2H4 CO

H2 H2 C2H4 CO C3H6 H2

NH3

Electron donor (D)

Reactants

Electron acceptor (A)

5. EP studies utilizing K þ conductors

H2 C2H4 CO C2H4

Electron acceptor (A)

Electron donor (D)

Reactants

4. EP studies utilizing Na þ conductors

Electron acceptor (A)

Electron donor (D)

Reactants

3. EP studies utilizing mixed conductors

Electron acceptor (A)

Electron donor (D)

Reactants

2. EP studies utilizing F  conductors

Table 1 Continued

N2, H2

Products

C2H4, C2H6 C2H4O, CO2 CO2 CO2

C6H12 C2H4, C2H6 CO2, N2, N2O CO2, N2, N2O CO2, N2, N2O N2, N2O

CO2 CO2

Products

CO2 CO2

CO2

CO2

Products

CO2

Products

Pd Ag Ag Pt

Pt Pt Pt Pt Pt Pt

Pt Pt

Catalyst

Fe

Catalyst

Pt Pt

Pt

Pt

Catalyst

Pt

Catalyst

100–150 100–300 280–400 320–400 375 360–400 70–100 240–280 360–420 430

b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 b00 -Al2O3 Na3Zr2Si2PO12

K2YZr(PO4)3

500–700

T (1C)

180–300 300–450

Solid electrolyte

T (1C)

b00 -Al2O3 b00 -Al2O3

400–475 400–500

500

450–600

T (1C)

500–700

T (1C)

Solid electrolyte

TiO2 (TiO þ x, O2) CeO2 (CeO þ x, O2) YZTi 10b YZTi 10b

Solid electrolyte

CaF2

Solid electrolyte

–

Lmax (>0) or Lmin(o0)

4.5

rmax (>1) or rmin (o1)

0.13 – 2 10

025 0.3 8 B0 a N 13a 10 30 – – – –

rmax (>1) or rmin (o1)

Lmax (>0) or Lmin(o0)

3 2 2.4

20

rmax (>1) or rmin (o1)

2.5

rmax (>1) or rmin (o1)

5  104 1  105  1  105 – – – – – –

 105  250 1000  1000

5  10

3

Lmax (>0) or Lmin(o0)

200

Lmax (>0) or Lmin(o0)

–

PIK þ

– 40 – 300

 30  30 250  10 – 500 200 – 6000

PINaþ

– – –

20

PIF 

1.5

PIF 

70 Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

O2 N2

C2H4 H2 NH3 CH4 H2 H2 1-C4H8

O2

H2

b

Change in product selectivity observed. 4.5 mol% Y2O3–10 mol% TiO2–85.5 mol% ZrO2.

SO3

O2

SO2

a

Products

Electron acceptor (A)

H2O

Products Pt

Pt Fe Fe Ag Ni Pt Pd

Pt

V2O5–K2SO4

Solid electrolyte

H2O–0.1M KOH

25–50

 3  10 6 150 – 300 20  28

4

Lmax (>0) or Lmin(o0)

350–450

T (1C)

20

6

 100

20

PIOH

– 6 – 10 12 5 –

PIH þ

rmax (>1) or rmin (o1)

6

rmax (>1) or rmin (o1)

5 12 3.6 8a 2 6 40a

rmax (>1) or rmin (o1)

Lmax (>0) or Lmin(o0)

Lmax (>0) or Lmin(o0)

385–470 440 530–600 750 150–170 25 70

T (1C)

T (1C)

CaZr0.9In0.1O3  CaZr0.9In0.1O3  CaZr0.9In0.1O3  SrCe0.95Yb0.05O3 CsHSO4 Nafion Nafion

Solid electrolyte

Solid electrolyte

Catalyst

Catalyst

Catalyst

CO2 NH3 N2, H2 C2H6, C2H4 C2H6 H2O C4H10, 2-C4H8 (cis, trans)

Products

Electron donor (D)

Reactants

8. EP studies utilizing molten salts

Electron acceptor (A)

Electron donor (D)

Reactants

7. EP studies utilizing aqueous alkaline solutions

C2H4 O2

Electron acceptor (A)

Electron donor (D)

Reactants

6. EP studies utilizing H þ conductors

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity 71

72

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity U WR (V) 0.8

80

0.4

U WR (V) 0

1.5

0.4

1.0

pNO  1.3 kPa pC H  3.7 kPa 2 4

r /r 0

r /r 0

5 10 pO  0.25 kPa 2 pCH  10 kPa

5

0

T  450 C

10

T  700 C

0.5

4

pO  0.50 kPa 2 pCH  10 kPa

1

4

1

10

5

0

  FU WR /RT   /k BT

(a)

0.6

5

25 20 15 10 5

0.1

0

0.2

5

  FU WR /RT   /k BT

(b)

U WR (V) 0.1 8

0

U WR (V) 0.8 0.4

0.3

0

0.4

0.8

10

5

r /r 0

r /r 0

5

T  560 C pO2  0.2 kPa pCO  11 kPa

1 2

0

2

2

pCH

3OH

 0.9 kPa

1 15 10 5

4

  FU WR /RT   /k BT

(c)

T  425 C pO  19 kPa

(d)

0

5

10

15

  FU WR /RT   /k BT

Figure 5 Examples of the four types of global electrochemical promotion behavior: (a) electrophobic, (b) electrophilic, (c) volcano type, and (d) inverted volcano type. (a) Effect of catalyst potential and work function change (vs I ¼ 0) for low (20:1) and high (40:1) CH4 to O2 feed ratios, Pt/YSZ. (b) Effect of catalyst potential on the rate enhancement ratio for the rate of NO reduction by C2H4 consumption on Pt/YSZ. (c) NEMCA-generated volcano plots during CO oxidation on Pt/YSZ. (d) Effect of dimensionless catalyst potential on the rate constant of H2CO formation, Pt/YSZ. P ¼ FUWR/RT( ¼ DF/kBT). YSZ, Y2O3-stabilized ZrO2. Reproduced from Vayenas CG, Brosda S, and Pliangos C (2001) Rules and mathematical modeling of electrochemical and chemical promotion: 1. Reaction classification and promotional rules. Journal of Catalysis 203: 329–350. Copyright (2001), with permission from Elsevier.

½10

G1. When the electron acceptor reactant is strongly adsorbed on the catalyst surface, the reaction is electrophobic. G2. When the electron donor reactant is strongly adsorbed on the catalyst surface, the reaction is electrophilic. G3. When both reactants are strongly adsorbed on the catalyst surface, the reaction is volcano type. G4. When both reactants are weakly adsorbed on the catalyst surface, the reaction is inverted volcano type.

Equivalently, one can express the above four electrochemical promotion rules in terms of the chemisorptive propensity of the electron acceptor and electron donor reactants. These rules, termed global electropromotion rules G1–G4 (Table 3), are expressed as follows:

The above rules are shown schematically in Figure 6(b) in terms of the magnitude of the adsorption equilibrium constants kD and kA, multiplied by the corresponding reactant partial pressures pD and pA. The above four rules are supplemented by three additional ones (G5–G7, Table 3), which address the case of

These observations form the rules of electrochemical promotion in Table 3, and are summarized schematically in Figure 6(a), in terms of the catalytic reaction rate orders with respect to the electron acceptor (A) reactant (aA) and with respect to the electron donor (D) reactant (aD). The reaction orders aA and aD are expressed as 

aA ¼

@ ln r @ ln pA

 aD ¼



@ ln r @ ln pD

½9 pD

 pA

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

73

Table 2 Classification of electrochemical promotion (EP) studies based on global r vs F behavior A. Purely electrophobic reactions Reactants (D) (A)

Solid electrolyte

pA/pD

T (1C)

Kinetics in D @r/ @pD|F

Kinetics in A @r/ @pA|F

Rule

Catalyst

C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 CO CO CH4 C3H6 CH4 C6H6 C2H2 H2 H2 H2S

O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 H2 H2 CO2 C2H2,C2H4 –

Pt Pt Pt Rh Ag IrO2 RuO2 Pt Pd Pd Ag Ag Pt Pt Rh Pd Pt

ZrO2(Y2O3) b00 -Al2O3 TiO2 ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) CaF2 ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) b00 -Al2O3 b00 -Al2O3 ZrO2(Y2O3) b00 -Al2O3 ZrO2(Y2O3)

12–16 238 3.5–12 0.05–2.6 0.2–1.1 300 155 11–17 500 0.2–4.8 20–120 0.02–2 0.02–0.12 1.7–9 0.03–0.7 0.1–5.9 –

260–450 180–300 450–600 250–400 320–470 350–400 240–500 500–700 400–550 380–440 320–420 650–850 100–150 100–300 300–450 70–100 600–750

þ þ þ þ þ þ þ þ ? þ þ þ X0 ? þ X0 ?

0 0 0 0 0 0 p0 0 ? 0 p0 0 B0 ? 0 0

G1 G1 G1 G1 G1 G1 G1 G1 ? G1 G1 G1 G1 ? G1 G1 ?

CH4 NH3 NH3 CH4

– – – H2O

Ag Fe Fe Ni

SrCe0.95Yb0.05O3 CaZr0.9In0.1O3  a K2Yzr(PO4) ZrO2(Y2O3)

– 4–12 kPa 4–12 kPa 0.05–3.5

750 530–600 500–700 600–900

? þ þ þ

? G1 G1 G1

p0

B. Purely electrophilic reactions Reactants (D) C2H4 C2H4 C2H4 C2H4 CO C3H6 CH3OH CH4 H2 H2

C2H4 C2H4 CO CO CO

(A)

Catalyst

Solid electrolyte

pA/pD

T (1C)

O2 O2 O2 O2 O2 O2 O2 O2 N2 C2H4 CH3OH CH3OH NO NO NO NO N2O 1-C4H8

Pt Pt Pt Ag Ag Pt Ag Au Fe Ni Pt Ag Pt Pt Pt Pd Pd Pd

CaZr0.9In0.1O3  CeO2 YZTi10 b00 -Al2O3 b00 -Al2O3 ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) CaZr0.9In0.1O3  CsHSO4 ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) b00 -Al2O3 b00 -Al2O3 ZrO2(Y2O3) ZrO2(Y2O3) Nafion

4.8 1.6–3.7 3 0.3–0.4 0.1–10 0.9–55 0–2 0.1–0.7 0–3 1 – 0-6 kPa 0.2–10 0.1–1.1 0.3–5 0.5–6.5 2–50 –

385–470 500 400–475 240–280 360–420 350–480 500 700–750 440 150–170 400–500 550–750 380–500 280–400 320–400 320–480 440 70

Kinetics in D @r/ @pD|F

Kinetics in A @r/ @pA|F

Rule

– – ? – 0 p0 ? 0 ? ?

þ þ ? þ þ þ þ þ ? ? ? þ þ ? þ þ þ ?

G2 G2 ? G2 G2 G2 G2 G2 ? ? ? G2 G2 ? G2 G2 G2 G2

0 ? p0 B0 –

C. Volcano-type reactions Reactants (D) (A)

Solid electrolyte

pA/pD

T (1C)

Kinetics in D @r/ @pD|F

Kinetics in A @r/ @pA|F

Rule

Catalyst

C2H4 CO CO H2 H2 SO2 C3H6 H2

Pt Pt Pt Pt Pt Pt Pt Pt

Na3Zr2Si2PO12 ZrO2(Y2O3) b00 -Al2O3 H2O–0.1N KOH Nafion V2O5-K2S2O7 b00 -Al2O3 b00 -Al2O3

1.3–3.8 0.2–55 0.5–20 0.3–3 0.2–5 1.8 2–70 0.3–6

430 468–558 300–450 25–50 25 350–450 375 360–400

– þ – þ þ ? – –

þ – þ – – ? þ þ

G3 G3 G3 G3 G3 ? G3 G3

O2 O2 O2 O2 O2 O2 NO NO

(Continued )

74

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

Table 2 Continued D. Inverted volcano reactions Reactants (D)

Catalyst

Solid electrolyte

pA/pD

T (1C)

Kinetics in D @r/ @pD|F

Kinetics in A @r/ @pA|F

Rule

(A)

C2H4 C3H6 CO CO CO C2H6 CH4 CH3OH H2 C2H6 CO

O2 O2 O2 O2 O2 O2 O2 O2 CO2 NO, O2 NO, O2

Pt Pt Ag Ag–Pd alloy Au Pt Pt Pt Pd Rh Rh

TiO2 YZTi10 ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3) ZrO2(Y2O3)

0.2–0.3b 5 0.6–14 3.5–12.5 3–53 0.06–7 0.02–7 3–45 0.2–1.1 0.08–8c 0.33c

450–600 400–500 350–450 450–500 450–600 270–500 600–750 300–500 500–590 250–450 250–450

þ ? þ þ þ þ þ þ þ þ þ

þ ? þ þ X0 þ þ ? þ NO: þ O2: 0 NO: þ O2: 0

G4 ? G4 G4 G4 G4 G4 ? G4 G4 G4

a

pD ¼ pC2 H2 þ pC2 H4 . low pA, pD region. c CA/pD is the ratio pNO =pC3 H6 and pNO/pCO. The pO2 range is between 0 and 16 kPa. ?, no data available. b

monomolecular reactions, as well as the effect of electron donicity of the reactants on the magnitude of the observed promotional effect. All these rules can be derived from the two fundamental rules F1 and F2 of Table 3, which simply state that at constant gas composition, the coverage of an electron acceptor (or donor) reactant decreases (or increases) with increasing work function F. It must be noted that, depending on the experimental conditions (temperature, gas composition), the same catalytic reaction (e.g., ethylene oxidation on platinum, or ammonia (NH3) synthesis on iron) can switch between electrophobic, electrophilic, volcano, and inverted volcano behavior. This is also clear from Figure 6(a) by noting that the reaction orders vary with temperature and gas composition, or from Figure 6(b) by noting that kA and kD are temperature dependent and, obviously, the products kApA and kDpD are both temperature and gas composition dependent. Thus, catalytic oxidations on metals are typically electrophobic under fuel-lean conditions, electrophilic under fuel-rich conditions, volcano type at low temperatures and intermediate oxygen to fuel ratios, and inverted volcano type at high temperatures where both kA and kD are small (Figure 6(b)). It should also be noted that, obviously, all the promotional rules discussed here are applicable under conditions of kinetic control and cannot be used under conditions of mass transfer or equilibrium limitations. The rules, in their present form, are also not applicable to cases where the promoter coverage is so high (e.g., more than B0.2) that site blocking of the catalyst surface becomes dominant.

Origin of Electrochemical Promotion It took several years to fully understand the origin of electrochemical promotion, by using (1) a large number of

surface science techniques, including X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, temperature-programmed desorption (Figure 7), photoelectron emission microscopy, and scanning tunneling microscopy (Figure 8); (2) more conventional catalytic techniques, including rate transient analysis and work function measurements; (3) electrochemical techniques, including cyclic voltammetry (Figure 7) and AC impedance spectroscopy; and (4) theoretical ab initio quantum mechanical calculations. All these techniques have provided a unanimous answer to the problem: Electrochemical promotion is due to the current or potential-controlled electrocatalytic (faradaic) introduction of promoting species (e.g., Od, Nad þ ) from the solid electrolyte to the catalyst–gas interface where an overall neutral double layer is formed. The density of this double layer (and the field strength in it) varies as the applied potential is varied and this affects both the work function of the surface and the chemisorptive bond strength of reactants and intermediates, thus causing dramatic and reversible alterations in catalytic rate (Figure 9).

Double-Layer Isotherms and Kinetics The experimentally proven existence of an overall neutral effective double layer (EDL) at the metal–gas interface has been utilized recently to derive, starting from simple and rigorous thermodynamic and electrostatic principles, adsorption isotherms that account explicitly for the electrostatic interactions between the adsorbates and the double layer (Figure 9). One starts from the equilibrium adsorption condition: m¯ j ðgÞ ¼ m¯ j ðadÞ ¼ mj ðadÞ þ P˜j  E˜  NAV

½11

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

Table 3

75

Local and global promotional rules

Type of reaction Local rules Donicity of reactants Kinetics

Predicted promotional behavior

D þ A-products D: Electron donor @F/@yDo0 Rate positive order Rate negative order in D @r/@pD40 in A @r/@pAo0 RULE L1 Electrophobic behavior @r/@F40, L41

A: Electron acceptor @F/ @yA40 Rate negative Rate positive order order in D in A @r/@pDo0 @r/@pA40 RULE L2 Electrophilic behavior @r/@Fo0, Lo  1

Global rules G1 and G2 Donicity of reactants Open-circuit kinetics and strength of adsorption

D: Electron donor @F/@yDo0 Rate positive order in D Rate zeroth or negative @r/@pD40 order in A @r/@pAp0 D weakly odsorbed

Predicted promotional behavior Global rule G3 Donicity of reactants Open-circuit kinetics and strength of adsorption

A strongly adsorbed kDpD{kApA and 1{kApA Purely electrophobic behavior @r/@F40

Electron donor @F/@yDo0 Strong adsorption Strong adsorption Rate postitive order in D @r/@pD40

Predicted promotional behavior Global rule G4

Rate negative order in A @r/@pAp0

D and A strongly adsorbed kApA4kDpD41 Volcano-type behavior

Donicity of reactants Open-circuit kinetics and strength of adsorption

A: Electron acceptor @F/@yA40 Rate zeroth or Rate positive order negative order in A @r/@pA40 in D @r/@pDp0 D strongly adsorbed A weakly adsorbed kDpDckApA and kDpDc1 Purely electrophilic behavior @r/@Fo0

Electron acceptor @F/@yA40 Strong Strong adsorption adsorption Rate postitive Rate negative order in A @r/@pA40 order in D @r/@pDo0 kDpD4kApA41

Electron donor @F/@yDo0 Weak adsorption Rate positive order in A @r/@pD40

Electron acceptor @F/@yA40 Weak adsorption Rate positive order in A @r/@pA40

Predicted promotional Inverted volcano-type behavior behavior Rule G5: The above rules G1–G4 apply also when D and A are both electron acceptors or electron donors. In this case D is always the stronger electron donor or weaker electron acceptor and A is always the weaker electron donor or stronger electron acceptor. Rule G6: A monomolecular reaction is electrophobic for an electron donor adsorbate and electrophilic for an electron acceptor adsorbate. Rule G7: The maximum rate modification obtained under electrochemical promotion conditions increases for any fixed overpotential, or fixed promoter coverage, with increasing difference in the electron acceptor–electron donor character of the two reactants. Fundamental rules F1 and F2   @yD X0 @F pA ; pD   @yA p0 @F pA ; pD Practical rules P1: yA-1 ) Electronegative promoter recommended P2: yD-1 ) Electropositive promoter recommended P3: yA,yD{1 ) Electropositive electronegative promoter recommended

76

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity

with

1



G4

P ¼ DF

r

r

Reaction order of acceptor, A

G1

G3





kj ¼ exp

r

0

G2

1

 1

(a)

0 Reaction order of donor, D

1

103 G1 r

r

102

G3

k Ap A

101 

0

Had ¼ T 2



ðm0j ðgÞÞ  m0j ðadÞ

2

 102

101

(b)

101

102

yj ð1  yÞ

 expðlj PÞ

lj c DF 2d

½15

where DHad;0 j is the heat of adsorption for DF ¼ 0. Assuming cEd, one obtains

103

where m¯ j is the electrochemical potential of adsorbed species j, mj is its chemical potential, P˜j taken as a vector is its dipole moment in the adsorbed state, E˜ is the local field strength in the double layer, assumed uniform, and NAV is Avogadro’s constant. The equilibrium condition leads to the following EDL isotherm: kj pj ¼

½14

  @ðm¯ j ðadÞ Þ pj ; yj ; T

 0 k Dp D

Figure 6 (a) Effect of reaction orders aA and aD with respect to the electron acceptor (A) and electron donor (D) reactant on the observed rate dependence on changing catalyst work function F (electrophobic (/), electrophilic (/), volcano (-) and inverted volcano (,)), and range of the validity of the corresponding promotional rules G1, G2, G3, and G4. (b) Effect of the magnitude of adsorption equilibrium constants kD and kA and corresponding partial pressures pD and pA, of the electron donor and electron acceptor reactant, respectively, on the observed rate dependence on changing catalyst work function F (electrophobic (/), electrophilic (-), and inverted volcano (,)), and range of the validity of the corresponding promotional rules G1, G2, G3, and G4. Reproduced from Brosda S, Vayenas CG, and Wei J (2006) Rules of chemical promotion. Applied Catalysis B: Environmental 68: 109–124. Copyright (2006), with permission from Elsevier.



!

RT

DHad; j ¼ DHad;0 j þ

10

103 3 10

½13

r

r

G4

kB T

one can derive that

G2

101

,

where DF is the deviation of the work function, F, from its value at the pzc of the double layer, c is the dipole length, d is the double-layer thickness (Figure 9), o is the angle formed between the adsorbate dipole and the field strength, and lj is the partial charge transfer parameter. This parameter is zero for a truly covalent chemisorptive bond, positive for an electron donor adsorbate, and negative for an electron acceptor adsorbate (e.g., equal to  1 for an adsorbed anion of charge  1). Using eqn [13] with cos o ¼ 1 and the definition of the isosteric enthalpy of adsorption

r



c cos o 2d

½12

DHad; j ¼ DHad;0 j þ

  lj DF 2

½16

Thus for an electron acceptor adsorbate (ljo0), eqns [15] and [16] predict a linear decrease in DHad with increasing DF, whereas for electron donor adsorbates (lj 40) they predict a linear decrease in DHad with decreasing DF. Both predictions are in good agreement with experiment and with rigorous quantum mechanical calculations. The effective double-layer isotherm (eqn [12]) can be used to derive analytical mathematical expressions for catalytic promotional kinetics. For the case of surface reaction rate control, the corresponding expression is r¼

kR kA kD pA pD exp½ðlD þ lA PÞ ½1 þ kD pD expðlD PÞ þ kA pA expðlA PÞ2

½17

where kR ¼ kR0 expðlR PÞ and lR is the partial charge transfer parameter of the transition state. In the limit of very weak adsorption (kApA,kDpD{1), repulsive interactions may be neglected and only the attractive ones are considered. In this case, eqn [17]

77

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity 50 I0

I  1 A

I0

40

1/2F  5.2  1012 mol O/s max  26 max  74000 N G  4.2  109 mol Pt

r (× 108 mol O s1)

TOF (s1)

80 30

2FNG /I  800 s

200

Strongly bonded backspillover state gets populated at a rate I/2F, catalytic rate is 99% due to weakly bonded state

10

0

400

Strongly bonded O backspillover state consumed over a period TOFmax /

20

40

20

600

0

0

0

200

2FNG/I 1200

(a)

Catalyst potential UWR (mV)

100

60

800

r 0  1.5  108 mol O/s r  38.5  108 mol O/s

2400

6600

78 000

Time (s) T (K)

600

20

700

800

900

40

3900 s 30 2030 s

Strongly bonded O backspillover state

12 700 s

2FNG/I  2500 s

10 0 0s 20 s

10

340 s 8

t

t

20

125 s

30 1

0s

4

2FNG/I  1200 s

20 I ( A)

dN/dT (1011 mol s1)

16

Weakly bonded highly reactive state

Strongly bonded O backspillover state

0.7

0.4

60 s 40 s 160 s 100 s 400 s 260 s 600 s

0.1 UWR (V)

0 (b)

300

400

500

600

0.2

0.5

0.8

Weakly bonded highly reactive O state

(c)

T ( C)

Figure 7 Nonfaradaic electrochemical modification of catalytic activity (NEMCA) and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate (r ) of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient) and (b) the O2 TPD spectrum on Pt/YSZ after current (I ¼ 15 mA) application at various times t. (c) The cyclic voltammogram of Pt/YSZ after holding the potential at UWR ¼ 0.8 V at various times t. Reprinted from Vayenas CG, Bebelis S, Pliangos C, Brosda S, and Tsiplakides D (2001) Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal–Support Interactions. New York: Kluwer Academic/Plenum Publishers: with kind permission of Springer Science and Business Media.

becomes r¼

kR kD kA pD pA exp½maxð0; lD P þ maxð0; lA PÞÞ ½1 þ kD pD exp½maxð0; lD PÞ þ kA pA exp½maxð0; lA PÞ2 ½18

where max(x, y) denotes x when x>y, y when xoy, and x or y when x ¼ y.

The success of eqns [17] and [18] to describe the above recently derived promotional rules can be appreciated from Figure 10, which shows the transition from electrophobic to electrophilic to volcano-type and to inverted volcano-type behavior by simply changing the values of the adsorption equilibrium constants kD and kA.

78

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity STM tip

Pt film Au connector

Pt(111) G/P β″Al2O3 I

Pt counter electrode

Pt reference electrode

U WR (a)

25 Å

25 Å

(b)

500 Å (c)

Figure 8 (a) Schematic of the experimental setup for using STM to investigate the Pt(1 1 1) surface of a Pt single crystal interfaced with b00 -Al2O3. (b) Low scanning area scanning tunneling microscopy (STM) images (unfiltered) of the (left ) sodium-cleaned and (right) ˚ . Reproduced from the Electrochemical Society. (c) Larger scanning sodium-dosed Pt(1 1 1)-(2  2)-O adlattice. Total scan size is 159 A area STM image (unfiltered) of a Pt single crystal surface consisting mainly of Pt(1 1 1) terraces and covered by a Pt(1 1 1)-(12  12)-Na adlattice formed via electrochemical Na þ supply on the Pt(1 1 1)-(2  2)-O adlattice. Each sphere on the image corresponds to a Na atom. From Tsiplakides D and Vayenas CG (2001) Electrode work function and absolute potential scale in solid state electrochemistry. Journal of the Electrochemical Society 148(5): E189–E202. Reproduced by permission of The Electrochemical Society.

Practical Considerations Similar to catalysis and promotion, electrochemical promotion is applicable only to reactions with a negative DG( ¼  nFUrev) where Urev is the reversible potential. Denoted by DU, the potential applied between the catalyst and the counterelectrode, then the energy consumption per mole of product is

EC ¼

  I DUt nFI DU nF DU DU ¼ ¼ DU ¼ Drt nF Dr L L Urev

½19

Thus for DUEUrev, the energy consumption is L times smaller than the negative of the free energy of the reaction. Therefore, for typical L values (10–105) the energy consumption and concomitant electricity cost per mole of product is negligible. Despite the negli-

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity O Gas reactants (e.g., O2, CO)

O

O

O





Effective double layer

C O 

O 



O O   

Metal









2

2

2

2

O (a)

Effective double layer

O

O

79

Double layer

O

Gas reactants (e.g., O2, CO) C



Na



O Na 



 O





Na 

Metal 





O

Double layer

Na



Na

Na



O



Na

Na

Solid electrolyte (β-Al2O3)

Solid electrolyte (YSZ)

q j d

~ E

 ~ Pj

艎

~ n

qj

Metal (b)

Figure 9 (a) Schematic representation of a metal electrode deposited on a O2-conducting (left) and on a Na þ -conducting (right) solid electrolyte, showing the location of the metal–electrolyte double layer and of the effective double layer created at the metal–gas interface due to potential-controlled ion migration (backspillover). Reproduced from Vayenas CG, Brosda S, and Pliangos C (2001) Rules and mathematical modeling of electrochemical and chemical promotion: 1. Reaction classification and promotional rules. Journal of Catalysis 203: 329–350. Copyright (2001), with permission from Elsevier. (b) Schematic of an adsorbate, modeled as a dipole, in the presence of the double layer at the metal–gas interface. Reproduced from Brosda S and Vayenas CG (2002) Rules and mathematical modeling of electrochemical and classical promotion: 2. Modeling. Journal of Catalysis 208: 38–53. Copyright (2002), with permission from Elsevier.

gible electricity consumption, two main factors have hindered so far the practical utilization of electrochemical promotion: a. The low dispersion and thus the poor metal utilization of the thick porous metal films used in most electrochemical promotion studies. b. The lack of compact reactor designs allowing for the utilization of EPOC with a minimum of electrical connections. Very recently, there has been spectacular progress in both directions, which is briefly discussed here.

Effect of Film Thickness Electrochemical promotion of catalysis has been studied very recently with film thicknesses as low as 30 nm prepared by a variety of techniques including sputtering, pulsed laser deposition, and impregnation. The metal dispersion of some of these films is up to 30%, comparable with that of commercial supported catalysts.

Furthermore, it was recently shown that r increases in general with decreasing film thickness, as expected from theoretical sacrificial promoter reaction-diffusion models. Although there is a subsequent decrease in r, when thicknesses of the order of 30 nm are reached because of the thermal migration of promoting ions and concomitant atomic similarity of EPOC and MSIs, still, r values of the order of 10 are obtained. This important finding is further corroborated by recent work showing that the TOFs on electropromoted platinum or rhodium films are a factor of 5–10 higher for C2H4 oxidation or nitric oxide reduction than on commercial supported catalysts under identical gas composition and temperature conditions. Thus, electrochemical promotion of such thin films leads to better noble metal utilization than that obtained with state-of-the-art commercial nanodispersed catalysts.

Monolithic Reactor Designs The recent development of monolithic YSZ-based electropromoted reactors, already tested under real car exhaust conditions, seems to provide an effective solution

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity U WR (V),  (eV)

0.8 10 1

0.2

10

A

10

0 20

20

D A 0.1

(c)

 D, A

0.8

r /r0

 D, A

0 20

0

10

20

kD 0.01, kA 0.01 pA1 pD1, D 0.15, A 0.15

0.6

10

D A

0.4

kD  100, kA  100 pD  1 pA  1 D  0.15 A  0.15 10

10



1

0.4

0

0.01

U WR (V),  (eV) 1.0 0.5 0 0.5 1.0 1.0

0.8 0.6

10

(b)

U WR (V),  (eV) 1.0 0.5 0 0.5 1.0 1.0

0.2

0.1

0.01 0

1.0

D A

0.2



(a)

0.5

1

0.1

D 0 20

0.6 0.4

D A

0

kD  100, pD  1 100 kA  0.01, pA  1 D  0.15 10 A  0.15

100

kD  0.01, pD1 kA  100, pA1 D  0.15 A  0.15

0.4

1.0 0.5 1.0

1.0

 D, A

0.6

0.5

r /r0

 D, A

0.8

0

r /r0

U WR (V),  (eV) 1.0 0.5 1.0

r /r0

80

0.2 1 20

0.01



0 20

10

0

10

20



(d)

Figure 10 Effective double-layer model predicted electrochemical or classical promotion behavior: (a) electrophobic, (b) electrophilic, (c) volcano type, and (d) inverted volcano type. Reproduced from Brosda S and Vayenas CG (2002) Rules and mathematical modeling of electrochemical and classical promotion: 2. Modeling. Journal of Catalysis 208: 38–53. Copyright (2002), with permission from Elsevier.

to the reactor design problem inhibiting so far the practical utilization of EPOC. The use of EPOC in practical polymer electrolyte membrane fuel cell units, operating, for example, as preoxidation reactors, is also currently under study.

appears quite feasible. In retrospect, the successful use of ionically conducting or mixed conducting supports (zirconium dioxide, Y2O3-stabilized ZrO2, cerium(IV)oxide, titanium dioxide) in many commercial catalysts during the last few decades can be attributed to self-driven electrochemical promotion of the nanodispersed catalyst particles.

Conclusions The NEMCA effect is a phenomenon at the interface of heterogeneous catalysis and electrochemistry. It is closely related to classical promotion, where the promoter is added ex situ during catalyst preparation and to the phenomenon of MSIs with oxidic supports. The MSIs can be viewed as a self-driven NEMCA microsystem where the promoting O2 ions are thermally migrating from the support to the dispersed catalyst nanoparticles and replenished in the support by gaseous oxygen. The molecular mechanism of NEMCA is now well understood and its direct application to practical systems

Nomenclature Symbols and Units d e Ec E˜ F Had I

double-layer thickness Electrocatalytic reaction energy consumption per mole of product local field strength in the double layer Faraday constant isosteric enthalpy of adsorption electrical current

Electrochemical Theory | Non Faraday Electrochemical Modification of Catalysts Activity d e Ec E˜ F Had I I0 kA kB kD kR k0R c n NAV PA PD PIi P˜j r r0 re R t T Urev UWR aA aD DG DH 0ad;j Dr DU hj !A !D kA kD kR

double-layer thickness Electrocatalytic reaction energy consumption per mole of product local field strength in the double layer Faraday constant isosteric enthalpy of adsorption electrical current exchange current adsorption equilibrium constants of electron acceptor (A) Boltzmann constant adsorption equilibrium constants of electron donor (D) adsorption equilibrium constants of the transition state adsorption equilibrium constants at the potential of zero charge dipole length charge of the promoting ion Avogadro constant partial pressure of electron acceptor (A) partial pressure of electron donor (D) promotional index vector of the dipole moment in the adsorbed state rate of the catalytic reaction open-circuit rate rate of the electrocatalytic reaction reference electrode time temperature reversible potential catalyst potential catalytic reaction rate order with respect to the electron acceptor (A) catalytic reaction rate order and with respect to the electron donor (D) change in Gibbs free enthalpy heat of adsorption promotionally induced rate enhancement potential applied between the catalyst and the counterelectrode coverage of the promoting species j on the catalyst surface coverage of electron acceptor (A) coverage of electron donor (D) partial charge transfer parameter of electron acceptor (A) partial charge transfer parameter of electron donor (D) partial charge transfer parameter of the transition state

K lj l¯ j P q s U x

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faradaic efficiency chemical potential of adsorbed species j electrochemical potential of adsorbed species j dimensionless catalyst potential (P = FUWR/RT = DF/kBT) rate enhancement ratio NEMCA time constant, tE2FNG/I work function angle formed between the adsorbate dipole and the field strength

Abbreviations and Acronyms EDL EP EPOC MSI NEMCA pzc STM TOF YSZ

effective double layer electrochemical promotion electrochemical promotion of catalysis metal–support interaction nonfaradaic electrochemical modification of catalytic activity potential of zero charge scanning tunneling microscopy turnover frequency Y2O3-stabilized ZrO2

See also: Electrochemical Theory: Double Layer; Electrokinetics; Kinetics; Electrolytes: Solid: Oxygen Ions; Solid: Protons.Fuel Cells – Solid Oxide Fuel Cells: Overview.

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