Electrochemical promotion of rhodium-catalyzed NO reduction by CO and by propene in the presence of oxygen

Article abstract of DOI:10.1021/jp004131y

The serious environmental implications of NO_x emission from automotive sources has stimulated increased interests directed at catalytic abatement of such emission. Electrochemical promotion was used to study the effect of added oxygen on alkali

Full text of DOI:10.1021/jp004131y

2800
J. Phys. Chem. B 2001, 105, 2800-2808
Electrochemical Promotion of Rhodium-Catalyzed NO Reduction by CO and by Propene in
the Presence of Oxygen
Federico J. Williams, Mintcho S. Tikhov, Alejandra Palermo, Norman Macleod, and
Richard M. Lambert*
Department of Chemistry, UniVersity of Cambridge, Cambridge CB2 1EW, U.K.
ReceiVed: NoVember 9, 2000; In Final Form: January 22, 2001
The catalytic performance of a rhodium thin film in contact with the solid electrolyte Na-â′′ alumina can be
greatly enhanced by the reversible electrochemically controlled transport of sodium from the electrolyte to
the metal surface. By this means, the reduction of nitric oxide by carbon monoxide or by propene can be
promoted, even in the presence of oxygen. The effect is due to the Na-enhanced dissociation of adsorbed
NO, the key reaction-initiating step. XP and Auger spectroscopies show that, under promoted conditions, the
alkali-metal surface phase consists of carbonate, nitrate, or both, depending on the gas composition. To a
first approximation, the chemical identity of the counterion appears not to play a significant role. With increasing
oxygen partial pressure the promotional effects of sodium are progressively decreased, and the markedly
different behavior of CO and propene as reductants is due to the opposite effects of coadsorbed alkali metal
on the electronegative or electropositive adsorbate, respectively. At the highest oxygen partial pressures and
alkali-metal coverages, drastic poisoning by sodium is due to strong alkali-metal inhibition of propene
2 2 3
adsorption, excessive formation of Na O, and oxidation of Rh to Rh O .
5
1
. Introduction
at the surface of a working metal catalyst. The technique is
+
implemented by pumping promoter species (Na in the present
The serious environmental implications of NOx emission
case) from a suitable solid electrolyte (Na-â′′ alumina in the
present case) to a porous metal film catalyst with which it is in
contact. By varying the catalyst potential (measured with respect
to a reference electrode), one may control reversibly the alkali-
metal coverage and consequently the catalytic behavior of the
metal surface.
Earlier work showed that, in the absence of gaseous oxygen,
EP by Na of Rh thin film catalysts greatly enhanced both activity
and selectivity toward N2 production in the reduction of NO
by CO and by propene. The resulting insight into the reaction
mechanism led to the development of Rh/γ-Al2O3 dispersed
catalysts, classically promoted by Na, which proved highly
efficient in the catalytic reduction of NO by propene. This
illustrates the value of EP as a tool for elucidating and exploiting
promoter effects in heterogeneous catalysis.
from automotive sources has stimulated a huge amount of pure
and applied research directed at catalytic abatement of such
emission. Although so-called three-way catalytic converters are
very effective in oxidizing CO and hydrocarbons, they are
substantially less effective in reducing NO to nitrogen, producing
1
N2O in addition. Although nitrous oxide is not yet a regulated
pollutant, it is a powerful greenhouse gas, and future legislation
is likely to include restrictions on N2O emission. In this
connection, note that over the warm-up period of the catalyst
N2O emission accounts for 60-80% of the NO converted by
6
7
2
current three-way converters. Another important limitation of
three-way converters is that they are ineffective for NO reduction
in the presence of excess gaseous oxygen, because O2 reacts
faster with the available reductants than NO itself. This prevents
the implementation of fuel-efficient, lean-burn spark ignition
engines which would otherwise make a very significant
contribution to reduced CO2 emission. Two important funda-
mental questions arise. First, how can we improve the selectivity
toward N2 formation? Second, to what extent can any such
improvements be sustained in the presence of gaseous oxygen?
We have already shown that alkali-metal additives substan-
tially improve the catalytic activity and nitrogen selectivity of
dispersed palladium and platinum catalysts under simulated
8
Here, for the first time, we use EP to investigate the effect
of added oxygen on alkali-metal promotion of NO reduction.
The effects of oxygen partial pressure and catalyst potential
(
sodium coverage) on the NO + CO + O2 and NO + C3H6 +
O2 reactions have been investigated. Complementary postreac-
tion XPS data were acquired to monitor changes in the amount
and chemical identity of the Na compounds present under a
variety of conditions, and to determine changes in the oxidation
state of the rhodium film.
3,4
three-way conditions.
It is found that sodium does promote both the activity and
nitrogen selectivity of rhodium in catalytic reduction of NO by
CO or propene, even in the presence of oxygen. Increasing the
oxygen partial pressure beyond a certain point decreases the
effectiveness of alkali-metal promotion, eventually resulting in
a transition to a regime of alkali-metal-induced poisoning. The
chemical identity of the alkali-metal surface compounds in the
promoted and poisoned regimes is established; the mode of
promoter action, the origin of poisoning, and the reaction
In a related effort, we have used electrochemical promotion
EP) to study systematically the effects of alkali-metal modifiers
on the catalytic performance of Pt, Cu, and Rh metal film
catalysts. EP provides an effective means of studying the effects
of promoters and the associated catalytic reaction mechanisms,
because it provides in situ control of the promoter concentration
(
*
To whom correspondence should be addressed. Phone: 44 1223
3
36467. Fax: 44 1223 336362. E-mail: rml1@cam.ac.uk.
1
0.1021/jp004131y CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/14/2001

Electrochemical Promotion of NO Reduction
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2801
mechanism are also discussed. The results indicate poisoning
due to alkali-metal inhibition of propene adsorption, excessive
formation of Na2O, and oxidation of Rh to Rh2O3.
3. Results
.1. Effect of Catalyst Potential and Oxygen Concentration
on Reaction Rates. CO as Reductant. Figure 1 shows typical
steady-state (potentiostatic) rate data obtained for the reduction
of nitric oxide by carbon monoxide in the presence of oxygen.
Turnover frequencies (TOFs) are expressed as molecules of
product per Rh surface atom per second. The rates of production
of CO2 (Figure 1a), N2O (Figure 1b), and N2 (Figure 1c) at
3
2
. Experimental Methods
The EP samples for catalytic testing and spectroscopic
analysis were prepared by depositing a rhodium metal film on
one face of a Na-â′′ alumina wafer: this constituted the catalyst
5
95 K are shown as a function of catalyst potential (VWR) for
(working electrode). Gold counter and reference electrodes were
constant inlet pressures of NO and CO (P(CO) ) P(NO) ) 0.9
kPa) for a range of oxygen concentration (0, 0.2, 0.4, and 2
kPa). Figure 1d shows the effect of catalyst potential and oxygen
concentration on the nitrogen selectivity, where this quantity is
defined as the ratio of the nitrogen rate to the sum of the nitrogen
deposited on the other face, all three electrodes being deposited
by dc sputtering of Rh or Au in argon. Methods used for
characterizing the Rh film by XRD, XPS, and surface area
6
measurements have been described in detail elsewhere. The
true metal area of our rhodium sample was measured by the
CO methanation technique described in detail in ref 6.
+
nitrous oxide rates. It is apparent that the CO2, N2O, and N2
reaction rates are dependent on catalyst potential and on the
Reactor measurements were performed in a CST reactor
operated at atmospheric pressure. The EP sample was suspended
in the reactor with all electrodes exposed to the reactant gas
mixture. Inlet and exit gas analysis was carried out by a
combination of on-line gas chromatography (Shimadzu-14B;
molecular sieves 13X and Haysep-N columns), on-line mass
spectrometry (Balzers QMG 064), and on-line nondispersive
infrared analyzers (dual channel Siemens Ultramat 6 analyzers
calibrated for CO/CO2 and NO/N2O). N2, N2O, CO, CO2, O2,
and C3H6 were separated and their concentrations determined
by gas chromatography; additionally, N2O, CO, CO2, and NO
were also monitored continuously using the IR detectors after
the necessary calibrations were performed. Reaction gases (5%
NO/He, 5% CO/He, 5% C3H6/He, 20% O2/He, and He) were
of ultrahigh purity and were fed to the reactor by mass flow
controllers (Brooks 5850 TR). The total flow rate was kept
oxygen partial pressure. Decreasing the catalyst potential (thus
increasing the sodium coVerage ) in the absence of gaseous
oxygen (black circles) increased the rates of CO2 and N2
5
production. In contrast, the rate of formation of N O decreased
2
with increasing sodium coverage. As a result, the selectivity
toward nitrogen formation increased. As the oxygen partial
pressure increased in the interval 0-0.4 kPa, qualitatively similar
results were obtained, but with progressive attenuation of the
promoting effect of Na. However, at P(O2) ) 2 kPa (open
circles), the N2 and N2O rates (and hence the selectivity) became
invariant with Na loading whereas the CO2 rate showed a small
increase at the highest Na loadings. Table 1 summarizes relative
rate changes (ratios of maximally promoted/unpromoted reaction
rates, F) and selectivity variations for the various oxygen
concentrations. It should be noted that increasing the oxygen
partial pressure causes changes in both the unpromoted (VWR
) +400 mV) and promoted (VWR ) -300 mV) conditions.
For the unpromoted catalyst, progressively increasing P(O2)
caused (i) increased CO2 rate, (ii) decreased N2 rate, (iii) a
maximum in the N2O rate, and (iv) a decrease in the nitrogen
selectivity. With the promoted catalyst, increasing P(O2) caused
a progressive decrease in the effect of sodium on (i) reaction
rates (all the F values tend to unity) and (ii) nitrogen selectivity.
Propene as Reductant. Figure 2 illustrates the effect of sodium
and oxygen on the reduction of nitric oxide by propene. The
data were obtained at 623 K with P(C3H6) ) P(NO) ) 1 kPa
and P(O2) ranging from 0 to 2 kPa. A 1:1 propene:NO ratio
was chosen to ensure that the system started out in a regime
where the effect of alkali metal is to promote the reaction.6
Under these conditions of partial pressures and temperature,
CO2, N2, N2O, and H2O were the only products. Parts a-c of
Figure 2 show steady-state rate data for CO2 (a), N2O (b), and
N2 (c) production as a function of VWR for different (constant)
values of oxygen pressure. Figure 2d shows the corresponding
nitrogen selectivity data. Decreasing the catalyst potential
-
5
-1
3
constant in all experiments at 34 × 10 mol s (500 cm -
(STP)/min). Reactant conversion was restricted to <15% to
avoid mass-transfer limitations. Control experiments were
carried out in which the total flow was varied by a factor of 5
to verify that the observed changes in activity were indeed due
to changes in actual surface reaction rates, unaffected by mass-
transfer limitations. Nitrogen and carbon mass balances always
closed to within 5%. Reactant concentrations are expressed in
kPa, where 1 kPa is equivalent to a concentration of 1% or
1
0000 ppm.
A galvanostat-potentiostat (Ionic Systems) was used to
maintain a given potential difference between the working and
reference electrodes (potentiostatic mode). All experiments were
carried out in potentiostatic mode by following the effect of
catalyst potential (VWR, measured with respect to the reference
electrode) on the reaction rates.
XPS experiments were performed under UHV conditions
-
10
(base pressure <10
Torr) in a VG ADES 400 UHV
spectrometer system equipped with a reaction cell. The EP
sample was mounted onto a manipulator that allowed translation
between the reaction cell and the spectrometer chamber. Full
details regarding sample mounting, manipulation, and data
(
increasing the sodium coVerage) in the absence of gaseous
oxygen (filled circles) resulted in the following: (i) increased
rates of CO and N formation, (ii) decreased N O production,
and (iii) a substantial increase in the selectivity toward nitrogen
formation. In this respect the behavior is qualitatively similar
to that found for CO in the absence of oxygen.
In the presence of gaseous oxygen, electropumping of Na to
the catalyst surface in the oxygen pressure range up to 1 kPa
again resulted in activity and selectivity promotion. That is, with
increased Na loading, the CO and N rates increased, whereas
2
2
2
9
acquisition are given in an earlier publication. Quoted binding
energies are referred to the Au 4f7/2 emission at 83.8 eV from
the grounded Au wire that formed the electrical connection to
the Rh working electrode. Reference spectra for sodium
carbonate and sodium nitrate were obtained by pressing small
quantities of the pure compounds into the surface of a pure
aluminum disk. This technique gave samples that did not exhibit
electrostatic charging effects and were free from carbon
contamination.
2
2
the N O rate decreased, resulting in increased nitrogen selectiv-
ity. The effects of Na promotion diminished with increasing
oxygen partial pressure. However, at P(O2) ) 2 kPa, all rates
2

2802 J. Phys. Chem. B, Vol. 105, No. 14, 2001
Williams et al.
Figure 1. Effect of catalyst potential (VWR) on CO
2
(a), N
2
O (b), and N
2
(c) formation rates and on nitrogen selectivity (d) for different partial
pressures of oxygen. Conditions: T ) 595 K, P(NO) ) P(CO) ) 0.9 kPa.
TABLE 1: Effect of Increasing the Oxygen Partial Pressure
on the Na-Induced Enhancement of Reaction Rates and
Nitrogen Selectivities for the CO + NO Reactiona
passed through a maximum. The general trend is that the
promotional effects of sodium on the reduction of NO by CO
or propene become less significant as the oxygen partial pressure
increases. This abatement of Na promotion is most pronounced
when propene is the reductant: in this case the system eventually
exhibits total poisoning of catalytic activity when the oxygen
exceeds a certain threshold.
P(O
2
)
Sp(N
(%)
2
)
(kPa)
F(CO
2
)
F(N
2
)
2
F(N O)
0
0
0
2
1.2
1.1
1.06
1.03
1.5
0.38
0.78
0.88
1
54 f 82
37 f 50
30 f 36
24 f 24
.2
.4
1.35
1.2
1
To understand these effects of oxygen on catalytic perfor-
mance, postreaction XPS analyses of the catalyst surface were
carried out for the relevant range of oxygen partial pressures
and for different catalyst potentials. The results presented refer
to the reduction of NO by propene, the more complex of the
two systems, because this is the critically important process for
practical implementation of catalytic NO reduction in oxygen-
a
u (p) stands for unpromoted (optimally promoted). Conditions:
P(CO) ) P(NO) ) 0.9 kPa, T ) 595 K.
decreased over the accessible range of Na loading, the effect
being especially marked in the case of the major products (CO2,
N2), which no longer exhibited any rate enhancement. In other
words, as the oxygen pressure increased the system underwent
a switch in behavior. At lower oxygen pressures both reaction
rates and nitrogen selectivity were enhanced by Na. However,
at 2 kPa of oxygen, pumping Na to the surface resulted in
marked activity loss, especially for the nitrogen-containing
products, accompanied by a small loss in selectivity. Since this
activity loss was so dramatic, the experiment was repeated twice
to check the reproducibility. The relevant data are shown as
times signs in Figure 2, from which it is evident that (i) the
quenching behavior was reproducible and (ii) the electrochemi-
cal response was also reversible.
1
0
containing environments.
3.2. Postreaction XPS Analysis. XP spectra were obtained
immediately after the appropriately biased catalyst film was
exposed to the conditions of temperature and reactant partial
pressures typical of those encountered in the reactor to detect
the various surface species present under reaction conditions.
During exposure of the catalyst to the reaction mixture, the VWR
values were such that the Rh film was either (i) electrochemi-
cally clean (unpromoted) or (ii) sodium promoted. A full
description of the procedure used to obtain the postreaction XP
1
1
spectra can be found in an earlier publication.
Table 2 shows the effect of sodium on the reaction at different
oxygen partial pressures in terms of relative rate changes (F)
and the corresponding nitrogen selectivity variations. Note that,
under unpromoted conditions, progressively increasing the
oxygen concentration resulted in a steady increase in the CO2
rate, whereas the N2O and N2 rates and the nitrogen selectivity
XP spectra were taken after the sample was exposed to a
mixture of 1 kPa of propene + 1 kPa of NO + x kPa of O2 at
623 K and 1 bar of total pressure (He carrier gas). Here, x ) 0
kPa, 0.5 kPa (i.e., conditions under which sodium promotes the
reaction), and 2 kPa (where sodium poisons the reaction). The
three different reaction mixtures are designated 1, 2, and 3 for

Electrochemical Promotion of NO Reduction
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2803
Figure 2. Effect of catalyst potential (VWR) on CO
pressures of oxygen. Conditions: T ) 623 K, P(NO) ) P(C H
3 6
2
(a), N
2
O (b), and N
) ) 1 kPa.
2
(c) formation rates and on nitrogen selectivity (d) for different partial
TABLE 2: Effect of Increasing the Oxygen Partial Pressure
on the Na-Induced Enhancement of Reaction Rates and
sodium was detectable after the unpromoted sample (VWR )
+
600 mV) was exposed to the different reaction mixtures
3 6
H
+ NO Reactiona
Nitrogen Selectivities for the C
P(O
kPa)
(
spectra a, c, and e). We have shown that this residual emission
Su(N
) f S (N
(%)
p
5,12
2
)
2
2
)
is from Na in the underlying Na-â′′ alumina spectroscopically
visible through cracks and imperfections present in the porous
rhodium film. In every case, electropumping Na to the catalyst
(
F(CO
2
)
F(N
2
)
F(N
2
O)
0
0
0
1
2
1.52
1.47
1.41
1.17
0.08
1.89
1.76
1.57
1.23
0.07
0.47
0.5
0.47
0.6
49 f 78
49 f 77
51 f 77
61 f 76
55 f 52
.1
.5
(VWR ) -100 mV) resulted in a substantial increase in Na
coverage of the Rh surface (spectra b, d, and f). Note that the
binding energy (BE) of the Na 1s XP signal in spectrum f is
shifted ∼1 eV toward higher BE with respect to spectra b and
d. This reflects a change in the nature of the sodium compounds
formed in reaction mixture 3.
0.1
a
u (p) stands for unpromoted (optimally promoted). Conditions:
) ) P(NO) ) 1 kPa, T ) 623 K.
3 6
P(C H
The top panel in Figure 3 shows Na KLL XAES spectra
corresponding to the promoted cases of reaction mixtures 2 and
x ) 0, 0.5, and 2.0 kPa of oxygen, respectively. Each reaction
gas exposure was carried out for two different values of catalyst
potential: (i) VWR ) +600 mV (unpromoted catalyst) and (ii)
VWR ) -100 mV (promoted catalyst). XP spectra were thus
acquired for the following six sets of conditions:
3
, i.e., to XP spectra d and f in the lower panel. (The dashed
lines are reference Na KLL spectra for sodium carbonate and
sodium nitrate.) These Auger results confirm that the sodium
compound present under reaction mixture 3 (spectrum f) is
distinct from that present under reaction mixture 2. In addition,
they show that the former (spectrum f) is neither a carbonate
nor a nitrate, whereas the latter (spectrum d) could be either.
(1) reaction mixture 1 (without oxygen), unpromoted (spec-
trum a);
(2) reaction mixture 1 (without oxygen), Na promoted
(
(
(
(
(
spectrum b);
(
3) reaction mixture 2 (P(O2) ) 0.5 kPa), unpromoted
spectrum c);
4) reaction mixture 2 (P(O2) ) 0.5kPa), Na promoted
spectrum d);
5) reaction mixture 3 (P(O2) ) 2 kPa), unpromoted
spectrum e);
Although the Na 1s BE does depend on the chemical identity
1
3
of the particular sodium compound (e.g., Na2CO3 (1071.5 eV),
13
14
(
NaHCO3 (1071.3 eV), NaNO2 (1071.6 eV), NaNO3 (1071.4
14
15
eV), Na2O (1072.5 eV) ), this does not provide unambiguous
chemical fingerprinting. We therefore identify the various Na
compounds by making use of the corresponding C 1s, N 1s,
and O 1s spectra. As shown below, the sodium compounds
formed may be identified as sodium carbonate (spectrum b), a
mixture of sodium carbonate and sodium nitrate (spectrum d),
and sodium oxide (spectrum f).
(
(
6) reaction mixture 3 (P(O2) ) 2 kPa), Na promoted
spectrum f).
Figure 3 shows Na 1s XP spectra for the six different cases
described above. It is apparent that a very small amount of

2804 J. Phys. Chem. B, Vol. 105, No. 14, 2001
Williams et al.
TABLE 3: Effect of Increasing Oxygen Partial Pressure and
Na Coverage on the Oxidation of the Rh Catalyst and on the
Attenuation of the Rh 3d XPS Signal for the C
2 3 6
3
H
6
+ NO +
a
O Reaction Conditions: P(C H ) ) P(NO) ) 1KPa,
T)623K
P(O
2
)
V
WR
oxidation attenuation film thickness
(kPa)
(mV)
(%)
(%)
(Å)
0
0
0
0
2
2
(spectrum a)
(spectrum b)
+600
-100
0
0
14
21
23
30
13
53
38
58
36
42
1.3
7
4.6
8.3
4.3
5.2
.5 (spectrum c) +600
.5 (spectrum d) -100
(spectrum e)
(spectrum f)
+600
-100
a
3 6
Conditions: P(C H ) ) P(NO) ) 1 kPa, T ) 623 K.
Figure 3. Sodium 1s XP spectra acquired after the catalyst was exposed
to 1 kPa of propene, 1 kPa of NO, and varying partial pressures of
oxygen at T ) 623 K: P(O
and VWR ) -100 mV (spectrum b); P(O
mV (spectrum c) and VWR ) -100 mV (spectrum d); P(O
WR ) +600 mV (spectrum e) and VWR ) -100 mV (spectrum f). The
top part shows Na KLL XAES spectra corresponding to P(O ) ) 0.5
kPa and VWR ) -100 mV (spectrum d) and P(O ) ) 2 kPa and VWR
100 mV (spectrum f). Also shown in dashed lines are the Na KLL
2
) ) 0 kPa, VWR ) +600 mV (spectrum a)
) ) 0.5 kPa, VWR ) +600
) ) 2 kPa,
2
2
V
2
Figure 5. Carbon 1s XP spectra acquired after the catalyst was exposed
to 1 kPa of propene, 1 kPa of NO, and varying partial pressures of
2
)
-
oxygen at T ) 623 K: P(O
and VWR ) -100 mV (spectrum b); P(O
mV (spectrum c) and VWR ) -100 mV (spectrum d); P(O
WR ) +600 mV (spectrum e) and VWR ) -100 mV (spectrum f).
2
) ) 0 kPa, VWR ) +600 mV (spectrum a)
) ) 0.5 kPa, VWR ) +600
) ) 2 kPa,
reference spectra corresponding to bulk sodium nitrate and carbonate
samples.
2
2
V
the higher BE doublet at 308.6 and 313.3 eV is due to Rh2O3.17
Within the XPS sampling depth, the fraction of the catalyst that
had undergone oxidation under the reaction conditions can be
estimated from the ratio of the integrated intensity corresponding
to the Rh2O3 to the total integrated intensity. The results of this
procedure are given in Table 3, which interprets the attenuation
of Rh 3d emission in terms of the effective thickness of a
hypothetical continuous film of overlying material. The first
entry (1.3 Å) is of course physically unrealistic and is best
thought of as equivalent to ∼1/3 of a monolayer of adsorbate
on an otherwise bare Rh surface. It is evident from Table 3
that coverage of the postreaction Rh catalyst by surface species
is greater under promoted than unpromoted conditions. The
results in Table 3 also show that the rhodium film undergoes
oxidation only when there is O2 present in the gas phase. It is
also evident that the degree of oxidation of the rhodium film
increased with (i) the oxygen partial pressure and (ii) Na loading.
Figure 5 shows carbon 1s XP spectra for the six different
conditions described above. These provide some information
about the chemical identity of the Na compounds that are formed
under reaction conditions. The C 1s region exhibits two main
contributions, one at lower binding energy (∼284-285 eV) and
the other at higher binding energy (∼288-290 eV). The former
Figure 4. Rhodium 3d XP spectra acquired after the catalyst was
exposed to 1 kPa of propene, 1 kPa of NO, and varying partial pressures
of oxygen at T ) 623 K: P(O
a) and VWR ) -100 mV (spectrum b); P(O
mV (spectrum c) and VWR ) -100 mV (spectrum d); P(O
WR ) +600 mV (spectrum e) and VWR ) -100 mV (spectrum f).
2
) ) 0 kPa, VWR ) +600 mV (spectrum
) ) 0.5 kPa, VWR ) +600
) ) 2 kPa,
2
2
V
Rh 3d XP spectra are shown in Figure 4. Note that under
our conditions the inelastic mean free path of the Rh photo-
16
electrons (kinetic energy 946 eV) was ∼11 Å, which provides
the required surface sensitivity. These data illustrate two
important points. First, the BE shifts indicate changes in the
oxidation state of the rhodium film. Second, in any given case,
the intensity attenuation provides a measure of the coverage of
the metal by surface species. Two main contributions may be
distinguished in the Rh 3d spectra: the lower BE doublet at
18
is due to elemental carbon (284 eV ) and to propene fragments
(284-285 eV), whereas the latter is due to sodium carbonate
1
9
(289 eV ). The small feature at ∼287 eV could be due to
1
8
partially oxidized carbonaceous species. After the catalysts
were exposed to reaction mixture 1 under unpromoted conditions
(spectrum a), at least two peaks were apparent due to (i)
elemental carbon and (ii) propene fragments. Exposing the
3
07.3 and 312 eV corresponds to metallic rhodium, whereas

Electrochemical Promotion of NO Reduction
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2805
Figure 6. Nitrogen 1s XP spectra acquired after the catalyst was
exposed to 1 kPa of propene, 1 kPa of NO, and varying partial pressures
Figure 7. Oxygen 1s XP spectra acquired after the catalyst was
exposed to 1 kPa of propene, 1 kPa of NO, and varying partial pressures
of oxygen at T ) 623 K: P(O
a) and VWR ) -100 mV (spectrum b); P(O
mV (spectrum c) and VWR ) -100 mV (spectrum d); P(O
WR ) +600 mV (spectrum e) and VWR ) -100 mV (spectrum f).
2
) ) 0 kPa, VWR ) +600 mV (spectrum
) ) 0.5 kPa, VWR ) +600
) ) 2 kPa,
of oxygen at T ) 623 K: P(O
a) and VWR ) -100 mV (spectrum b); P(O
mV (spectrum c) and VWR ) -100 mV (spectrum d); P(O
WR ) +600 mV (spectrum e) and VWR ) -100 mV (spectrum f).
2
) ) 0 kPa, VWR ) +600 mV (spectrum
) ) 0.5 kPa, VWR ) +600
) ) 2 kPa,
2
2
2
2
V
V
BE.17 Exposure of the unpromoted surface to oxygen-free
reaction gas (mixture 1) under unpromoted conditions generates
chemisorbed oxygen and subsurface oxygen (spectrum a).
Alkali-metal promotion in the same reaction gas (mixture 1)
produces chemisorbed oxygen, sodium carbonate (cf. the C 1s
spectrum b), and subsurface oxygen. Addition of oxygen
(mixtures 2 and 3) results in the appearance of an emission at
530 eV that correlates with the oxidation of the rhodium film
detected in the Rh 3d spectra (Figure 4). Accordingly, this O
1s emission is assigned to Rh2O3. Specifically, exposing the
unpromoted surface to the oxygen-containing reaction mixture
2 produced rhodium oxide and chemisorbed oxygen (spectrum
c); Na promotion in the same reaction gas produced Na nitrate
in addition to Rh2O3 and O(a) (spectrum d). At the highest
oxygen pressure (reaction mixture 3) rhodium oxide and
chemisorbed oxygen are again present in the unpromoted
conditions (spectrum e). This time, however, Na pumping to
the catalyst resulted in poisoning, and the appearance of Na2O
catalyst to reaction mixture 1 under promoted conditions
(
(
spectrum b) generated at least four C 1s contributions, namely,
i) elemental carbon, (ii) propene fragments, (iii) a shoulder at
287.4 eV, possibly due to partially oxidized carbonaceous
species, as noted above, and (iv) a peak at 289 eV due to sodium
carbonate. Progressive addition of oxygen to the reaction gas
resulted in a corresponding decrease in the amount of elemental
carbon and carbonaceous species (spectra c-f). The emission
at 289-290 eV due to sodium carbonate persists in spectrum d
(reaction mixture 2, promoted conditions) but is eventually
quenched (spectra f corresponding to reaction mixture 3,
promoted conditions). Note that the peak ascribed to carbonate
is upshifted by ∼1 eV between spectrum b and spectrum d. In
summary, (i) the presence of gaseous oxygen decreases coverage
by carbon and carbonaceous species, (ii) carbonate is only
present when the catalyst exhibits promotion by Na, and (iii)
carbonate is absent when the catalyst exhibits poisoning by Na.
Figure 6 shows corresponding N 1s XP spectra in which two
2
0
rather than NaNO (spectrum f).
contributions are readily apparent: chemisorbed N at 397.4 eV
3
and nitrate at 407.3 eV.21 In addition, spectrum a exhibits a
It is worth noting that in every case application of VWR
)
+
600 mV at reaction temperature (pumping Na away from the
weak feature at 400 eV that may be ascribed to chemisorbed
NO.22 Thus, exposure to reaction mixture 1 under unpromoted
catalyst) resulted in complete disappearance of the sodium
compounds (variously carbonate, nitrate, and oxide), leaving
only a small amount of chemisorbed oxygen on the Rh surface.
This provides confirmation of the overall validity of the spectral
interpretations offered above, and the conclusions drawn on the
catalytic behavior, proposed below.
conditions results in the presence of both chemisorbed N and
NO (spectrum a), whereas under promoted conditions only
chemisorbed N is detected (spectrum b). When the reaction gas
contains some oxygen (reaction mixture 2), only chemisorbed
N is detected on the unpromoted surface (spectrum c). However,
Na promotion in reaction mixture 2 generates both N adatoms
and nitrate (spectrum d). At the highest oxygen pressure
4
. Discussion
(
reaction mixture 3) no nitrogen-containing adsorbates are
Interpretation of our reactor data requires the explanation of
detectable on both the unpromoted and promoted surfaces
two principal phenomena. First, there is a regime in which
increasing the sodium coverage causes an increase in reaction
rates and nitrogen selectivity. Second, as the oxygen content in
the gas phase is increased the effects of sodium promotion on
the nitrogen rate and selectivity progressively decrease. The
effect is most pronounced in the case of the NO + propene
reaction, where all promotional effects are eventually suppressed
and the system actually exhibits total poisoning of all catalytic
activity. However, before the effects of electrochemically
supplied sodium are discussed, it is necessary to consider the
chemical state of the promoter phase under reaction conditions.
4.1. Chemical State of the Alkali-Metal Additive under
Reaction Conditions. The postreaction XPS demonstrates that
(spectra e and f).
The O 1s XP spectra shown in Figure 7 contain three principal
contributions. The lowest BE peak (529-530 eV) is due to
rhodium oxide (530 eV ) and Na2O (529.6 eV ). The 531.4
eV BE peak is due to chemisorbed oxygen and carbonate
(
associated with a sodium compound since it disappears upon
imposing a positive bias (thus pumping Na away from the Rh
surface). We therefore assign it to sodium nitrate, in good
agreement with the reported BE for this compound2 and the
corresponding N 1s spectrum shown in Figure 6. It is possible
that subsurface oxygen may make a small contribution at this
17
15
17
2
3
531.6 eV ). The highest BE peak (532.5 eV) must be
4

2806 J. Phys. Chem. B, Vol. 105, No. 14, 2001
Williams et al.
the chemical identity of the sodium-containing promoting (or
poisoning) phases is dependent on the gas atmosphere. When
the gas phase consisted of equal amounts of NO and propene
NO(a) + N(a) f N O(g) + 2S
(3)
(4)
2
N(a) + N(a) f N (g) + 2S
2
(spectra b), sodium was present as the carbonate. Addition of
some oxygen (spectra d) resulted in a mixture of sodium
carbonate and nitrate being formed. Ultimately, at the highest
oxygen pressure (spectra f) sodium oxide was formed. Our
results indicate that the identity of the adsorbed counterion(s)
O(a) + CO (or propene) f CO (+H O)
(5)
2
2
produced in the critical reaction-initiating step (2) react with
adsorbed CO (or with hydrocarbon fragments, in the case of
propene) to form CO2. In this scheme, (2) is rate limiting and
(nitrate, carbonate) is not critically important in determining
31
the promoting effect of Na. This effect has been discussed
is promoted by Na as argued above. Nitrogen selectivity depends
on the relative rates of (3) and (4). Na promotes activity and
nitrogen selectivity as a result of accelerating (2), increasing
N(a) and O(a) coverages and decreasing that of NO(a). As a
result, there is an increase in the rates of N2 and CO2 production
in (4) and (5), respectively, and a decrease in the rate of (3),
yielding the observed increase in overall activity and nitrogen
selectivity (Figures 1 and 2).
4
elsewhere within the theoretical framework developed by Lang
et al.27
The Rh 3d XP spectra (Figure 4) are revealing in regard to
the extent of loading and morphology of the Na surface
compounds. Under promoted conditions the Rh XP signal is
attenuated by an overlayer consisting principally of sodium
compounds. Since this surface is catalytically active, at least
some of the surface must expose promoter-modified Rh sites
while the rest is covered by the Na compound. Therefore, the
nominal film thickness values given in Table 3 (7, 8.3, and 5.2
Å) imply that under promoted conditions the sodium compounds
are present, at least in part, as three-dimensional crystallites.
The peaks at 289 and 290 eV in the C 1s XP spectra (Figure 5,
spectra b and d) have been assigned to sodium carbonate. This
assignment is based on our own reference spectrum (BE ) 289.5
eV) and on values reported in the literature. These latter range
from 289.4 eV for bulk sodium carbonate samples25 to 290.4
Consider now the effects of added oxygen. Figure 1 shows
that, in the case of the NO + CO reaction, increasing the oxygen
partial pressure in the absence of Na promotion causes (i) an
increase in the CO2 production rate, (ii) a decrease in the rate
of N2 production, (iii) a maximum in the rate of N2O production,
and (iii) a decrease in nitrogen selectivity. These effects can be
understood in terms of the competitive adsorption of oxygen
and NO. Increasing the oxygen partial pressure causes an
increase in the oxygen coverage, resulting in an increase in the
CO2 rate as observed. At the same time NO adsorption and
dissociation are inhibited, resulting in a decrease in both the
nitrogen rate and nitrogen selectivity. The maximum in the
unpromoted N2O rate as a function of increasing oxygen content
may be understood as follows. When P(O2) ) 0, the N2O rate
is low (see Figure 1b) because NO dissociation is efficient.
However, as the O2 pressure increases to 0.2 kPa, N2O
production increases because increased O(a) coverage inhibits
NO dissociation, thus increasing θNO and favoring N2O forma-
tion. A further increase in P(O2) decreases N2O production due
to decreased NO adsorption resulting from increased competition
from O2 for vacant sites.
The behavior observed with propene as reductant is similar,
though not indentical. As shown in Figure 2, increasing the
oxygen pressure in the absence of Na results in (i) an increase
in the CO2 rate and (ii) the rate of nitrogen production and the
nitrogen selectivity passing through a maximum. These effects
may be rationalized as follows. Spectrum a in Figure 5 shows
that in the absence of gaseous oxygen the surface of the catalyst
is covered mainly by hydrocarbon fragments. Increasing the
oxygen partial pressure results in an increase in oxygen
coverage, “cleaning off” hydrocarbonaceous fragments (also
shown in Figure 5), and thus increasing the rate of CO2
production. As a result, the number of free sites available for
NO adsorption and dissociation increases, resulting in an
increase in the rate of nitrogen production and in nitrogen
selectivity. However, a further increase in oxygen partial
pressure eventually leads to a decrease in site availability for
NO dissociation as the surface becomes overpopulated by O(a).
Therefore, the nitrogen rate and nitrogen selectivity should both
fall, so that the maximum exhibited by both these quantities is
understandable.
2
6
eV for submonolayer quantities of sodium carbonate. There-
fore, we propose that the BE variations we observe reflect a
change in the state of aggregation of the sodium carbonate from
largely three-dimensional to mainly two-dimensional.
4
.2. Mode of Alkali-Metal Promotion. The reactor data
clearly demonstrate the reversible dependence of reaction rates
on catalyst potential. This is due to the reversible, potential-
controlled, electropumping of sodium ions from the solid
electrolyte to the rhodium catalyst. Equally, the postreaction
XPS data (Figure 3) show the occurrence of reversible spillover/
back-spillover of Na to/from the surface of the Rh film.
Decreasing the catalyst potential delivers Na to the surface,
which, in the presence of a reactive gas atmosphere, results in
the formation of sodium compounds whose chemical identity
depends on the composition of the gas phase. The resulting
promotional effects of sodium on the reduction of NO by CO
or propene may be understood in terms of the enhanced NO
+
adsorption and dissociation induced by the coadsorbed Na . The
dissociation of adsorbed diatomic molecules in the electric field
of adjacent alkali-metal cations has received a detailed theoreti-
27
cal analysis and may be regarded as well understood. The field
lowers the energy of the NO antibonding orbital with respect
to the Fermi level, thus increasing charge transfer from Rh to
*
the NO π orbital, resulting in increased metal-N bond strength
and decreased N-O bond strength. Clear experimental evidence
for alkali-metal-induced NO dissociation comes from single-
crystal studies on Rh(111)/K2
8,29
30
and on Rh(100)/Na.
On the above basis, the following reaction mechanism permits
a rationalization of promotion by Na of the Rh-catalyzed
reduction of NO by CO or propene in the absence of oxygen.
The mechanism is also consistent with the single-crystal studies
3
1
of Belton et al.
4
.2. Oxygen-Induced Inhibition of Alkali-Metal Promo-
NO(g) + S f NO(a)
(1)
tion. Here we discuss the consequences of increasing the oxygen
partial pressure under promoted conditions. As shown in Figures
1 and 2, the promotional effects of sodium on the reduction of
NO by both CO and propene decrease as the oxygen partial
pressure is increased. This occurs to the extent that they
S represents a vacant site for adsorption. Oxygen adatoms
NO(a) + S f N(a) + O(a)
(2)

Electrochemical Promotion of NO Reduction
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2807
8
disappear (when CO is used as the reductant) or they result in
a decrease in reaction rates (when propene is used as the
reductant) when oxygen is increased above a certain threshold.
This behavior is explicable in terms of the effect of alkali-metal
modifiers on the competitive adsorption of the reactants. Sodium
is an electropositive adsorbate and should therefore decrease
the strength of chemisorption of electron-donating adsorbates
alumina catalysts, it actually poisons the system under three-
3
3
way conditions, i.e., in the presence of substantial amounts
of gaseous oxygen. This provides further support for the view
that EP studies on model thin film catalysts provide insight into
the behavior of the corresponding practical dispersed catalysts.
5. Conclusions
(
propene); conversely, it should strengthen the chemisorption
(
1) Electrochemically supplied sodium can promote both the
of electron acceptors (CO, NO, and O(a)). The postreaction XP
data are in accord with this view: spectra c, d, e, and f in Figures
5
of oxygen-containing species at the expense of adsorbed
hydrocarbon fragments.
catalytic activity and nitrogen selectivity in the rhodium-
catalyzed reduction of NO by CO or propene, even in the
presence of oxygen. This is understandable in terms of alkali-
metal-enhanced NO chemisorption and dissociation.
and 7 show that sodium increases the steady-state coverage
(2) Under promoted conditions, the alkali-metal surface phase
Thus, progressive suppression of the promoting effect of
sodium promotion by addition of gaseous oxygen is due to
alkali-metal-enhanced adsorption of oxygen at the expense of
propene: the system self-poisons by excessive adsorption of
one of the reactants. The other consequence of alkali-metal-
enhanced oxygen adsorption is oxidation of the rhodium catalyst
itself (Figure 4). Alkali-metal-enhanced oxidation of silicon is
consists of carbonate, nitrate, or both, depending on the gas
composition. To a first approximation the chemical identity of
the counteranion appears not to play a significant role. Some
of this material is present as three-dimensional crystallites.
(3) With increasing oxygen partial pressure the promotional
effects of sodium are progressively decreased. This is under-
standable in terms of alkali-metal-enhanced adsorption of
oxygen and subsequent suppression of NO adsorption and
dissociation.
3
2
well-known and has recently been reported in the case of
copper.11
It is known that sodium enhances the adsorption and
dissociation of CO, NO, and O2 when coadsorbed separately
with these species on rhodium.30 However, the effect of sodium
on the competitive adsorption of CO, NO, and O2 is not known.
Our data show that the unpromoted rate of nitrogen production
decreases as the partial pressure of oxygen increases. As
discussed above, this implies decreased NO adsorption and
dissociation resulting from increased competition from O2 for
vacant sites. In the Na-promoted case the nitrogen rate also
decreases as P(O2) increases. This suggests that sodium
enhances oxygen adsorption more strongly than that of nitric
oxide. As a consequence, increasing P(O2) under promoted
conditions results in a relative increase of the surface coverage
of oxygen adatoms at the expense of both free sites and NO
molecules, thus resulting in a decrease in the level of promotion.
That is, alkali-metal-enhanced NO dissociation diminishes as
P(O2) increases because the probability of NO(a) interacting
with a Na-promoted Rh site decreases as O(a) increases.
(4) With propene as reductant, actual poisoning by sodium
occurs at the highest oxygen partial pressure and is due to three
additional effects, namely, strong alkali-metal inhibition of
propene adsorption, excessive formation of Na O, and oxidation
2
of Rh to Rh O .
2
3
(5) The markedly different behavior of CO and propene as
reductants is due to the opposite effects of coadsorbed alkali
metal on the electronegative or electropositive adsorbate,
respectively.
Acknowledgment. F.J.W. acknowledges financial support
from the Fundaci o´ n YPF, Fundaci o´ n Antorchas, British Council
Argentina, and King’s College Cambridge. This work was
supported by the U.K. Engineering and Physical Sciences
Research Council and by the European Union under Grants GR/
M76706 and BRPR-CT97-0460, respectively.
References and Notes
Why is oxygen inhibition of alkali-metal promotion more
pronounced in the case of propene (where actual poisoning is
observed) than in the case of CO? With CO as reductant, adding
O2 decreased the promotional effects of sodium. As discussed
above, this is due to the decreased probability of NO(a)
interacting with a Na-promoted Rh site as O(a) increases.
However, with propene as the reductant, at the maximum
oxygen pressure, adding sodium actually poisoned all reactions.
This difference in behavior is understandable in terms of the
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reaction is strongly inhibited by two effects that work in the
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