Reaction of Br2 with adsorbed CO on Pt, studied by the surface interrogation mode of scanning electrochemical microscopy

Article abstract of DOI:10.1021/ja907626t

(Chemical Equation Presented) Scanning electrochemical microscopy surface interrogation (SI-SECM) in the cyclic voltammetry mode was successfully used to detect and quantify adsorbed CO on a Pt electrode by reaction with electrogenerated Br₂. T

Full text of DOI:10.1021/ja907626t

Published on Web 11/11/2009
Reaction of Br with Adsorbed CO on Pt, Studied by the Surface Interrogation
2
Mode of Scanning Electrochemical Microscopy
Qian Wang, Joaqu ´ı n Rodr ´ı guez-L o´ pez, and Allen J. Bard*
Center for Electrochemistry and Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin,
Austin, Texas 78712
Received September 8, 2009; E-mail: ajbard@mail.utexas.edu
1
We studied the reaction of bromine (Br
2
) with adsorbed carbon
tip as long as there is CO(ads) with which to react. This transient
positive feedback loop produces an increase in the current at
monoxide (CO(ads)) on polycrystalline platinum in aqueous 0.5 M
-
-
H
2
SO
4
, to yield CO
2
and Br , by the recently introduced surface
the interrogator tip due to an enhanced flux of Br because of
the proximity of the Pt substrate to the tip. Figure 1B shows a
SI result in the cyclic voltammetry (CV) mode and its compari-
son to the negative feedback (NF) mode, i.e., a blank experiment
with the Pt substrate at open circuit, and to the electrochemical
positive feedback (PF) mode, i.e., a blank experiment with the
1
2
interrogation mode of scanning electrochemical microscopy
SI-SECM). In this technique, a “titrant” is electrogenerated at a
2
,3
(
tip in close proximity to the surface under investigation, thus
allowing the in situ detection and quantification of adsorbed
intermediates at the solid-liquid interface. In the present Com-
munication, we demonstrate its utility for the evaluation of a new
reaction, where Pt acts not only as a support for CO(ads) but also as
a heterogeneous catalyst that promotes the oxidation process.
The adsorption and electrochemical oxidation of CO on Pt, on
both single-crystal and polycrystalline surfaces, have been widely
2
Pt substrate set at a potential where it can reduce Br . The theory
1
of the SI mode shows that, during chemical reaction at the
substrate, the current of the interrogator electrode is within the
electrochemical window shown in the inset of Figure 1B.
Consumption of the adsorbate causes the response of the
4
5-8
studied by electrochemical techniques
and their combination
9
-11
with Raman and IR spectroscopy.
Part of the interest in these
studies is driven by the identification of CO(ads) as an important
blocking intermediate (i.e., a poison) in the oxidation of short-chain
1
2
3,13
alcohols and organic acids for their use in fuel cells.
Studies
of the electrochemical oxidation of CO(ads) on Pt have traditionally
been carried out by scanning or stepping an electrode into the
•
oxidizing potential regime, where the formed OH (ads) radical is
5
,9,14
proposed to transform CO(ads) into CO
2
.
With the SI-SECM
technique at hand, we sought to find a tip-generated reactive
substance that could achieve this transformation, in the hope of
providing new strategies for quantification of CO(ads) in catalytic
systems and perhaps for the removal of this strongly adsorbed
Figure 1. SI-SECM detection of CO(ads) on a Pt UME substrate. (A-)
Production of reactive Br2 from electrochemical oxidation of Br
•
intermediate. The reactive Br radical is of particular interest since
(initially present in solution) at the interrogator tip. Br
2
diffuses and
1
5,16
reacts with CO(ads) at the Pt substrate, which is kept at open circuit, to
2
its precursor, Br , can be electrogenerated at a SECM setup
-
•
17
2
yield CO and regenerate Br , which is recycled to the interrogator tip,
and its oxidizing properties are comparable to those of OH (ads)
.
giving a transient positive feedback. (B) Comparison of negative
feedback (NF), positive feedback (PF), and SI; all scans ν ) 20 mV/s,
2 4
all solutions 0.5 M H SO with 1 mM KBr, Ar-purged, and kept under
Figure 1A shows schematically the SI-SECM mode for the
-
production of Br
2
from bromide (Br , initially present in solution) and
an Ar blanket. Interrogator tip held for 20 s at 0.75 V and then scanned
from 0.75 to 1.15 V vs Ag/AgCl in all runs. Pt substrate conditions: SI,
held at open circuit and predosed with CO (5 min at 0.1 V vs Ag/AgCl);
NF, held at open circuit, no predosing; PF, held at 0.75 V vs Ag/AgCl,
no predosing. Inset shows comparison of NF and PF.
its reaction with CO(ads) at the Pt substrate. Here two ultramicroelec-
trodes (UMEs), one acting as an interrogator tip and the other as a
substrate, were concentrically aligned and separated by a gap of ca.
1
-2 µm. The substrate was first dosed with CO and maintained
afterward at open circuit. The solution was then changed to 1 mM
KBr in 0.5 M H SO (without disturbing the aligned electrodes). As
shown in Figure 1A, after stripping any CO(ads) on the interrogator tip
rest ) 0.75 V vs Ag/AgCl), the tip was scanned across an appropriate
potential range (0.75-1.156 V vs Ag/AgCl) to produce Br , as shown
2
4
interrogator tip to decay to NF. The SI scans (Figure 1B) revealed
a response that consisted of two peaks, often merging into a
single peak with a shoulder (e.g., see Figure S1, Supporting
Information). Independent of shape, the quantification of the
amount of adsorbate was consistent in all runs. This was done
by integration of the SI scans to yield the reacted charge of the
adsorbate, which we compared to the charge for the underpo-
(E
2
in eq 1; we assume the overall reaction of Br
2
with CO(ads) at the Pt
surface proceeds according to eq 2.
Pt tip
Br-
-
2
8 Br + 2e
(1)
2
19
tential deposition of hydrogen on Pt for a series of experiments,
as shown in Table S1 (Supporting Information). We found the
Pt substrate
-
+
Br + CO
+ H O
8 2Br + CO + 2H
(2)
CO coverage on polycrystalline Pt, θCO, to be ∼0.5, which is
2
(ads)
2
2
6
lower than the result recently reported for Pt (111) (∼0.68).
Reaction 2 regenerates the initial bromide anion, which
produces a transient positive feedback loop at the interrogator
2
A possible route for the oxidation of CO(ads) by Br is shown in
1
8
eqs 3-5:
1
7046 9 J. AM. CHEM. SOC. 2009, 131, 17046–17047
10.1021/ja907626t  2009 American Chemical Society

C O M M U N I C A T I O N S
Pt substrate
otherwise be difficult to prove with conventional electrochemistry
on a single electrode.
•
Br2
8 2Br
(3)
(ads)
Acknowledgment. Q.W. is on leave from Chemistry Depart-
ment, School of Science, Beijing Institute of Technology, Beijing,
China, and thanks the National Scholarship Fund of the China
Scholarship Council for support and research advisor Dr. Huibo
Shao from the Beijing Institute of Technology. Helpful discussions
with Fu-Ren F. Fan and Cheng-Lan Lin are also gratefully
acknowledged. J.R.-L. thanks the William S. Livingston Endowment
Fund for a research fellowship and the complementary scholarship
support provided by Secretar ´ı a de Educaci o´ n P u´ blica of M e´ xico
and the Mexican Government. We acknowledge support from the
Robert A. Welch Foundation (F-0021) and the National Science
Foundation (CHE-0808927).
Pt substrate
2
Br •( + CO(ads)
8 OCBr2
(4)
(5)
ads)
-
+
OCBr + H O f 2Br + CO + 2H
2
2
2
In fact, the product of reaction 4, OCBr
analogue of phosgene), has been reported (but also debated) to
be the product of a photosensitized low-yield reaction of Br and
2
or “bromophosgene” (an
2
0
2
CO in the gas phase in the absence of catalyst, where the radical-
forming reaction 3 proceeds through photochemical excitation. In
our experiment, the Pt substrate provides a catalytic surface where
reaction 3 happens more readily. The hydrolysis process in reaction
is thermodynamically allowed and provides the Br necessary
for the transient positive feedback as well as a source of oxygen
2
1
-
Supporting Information Available: Additional experimental details,
surface interrogation scan, and figures. This material is available free
of charge via the Internet at http://pubs.acs.org.
5
for the production of CO
dissolved in solution, Br
2
. If CO is not supported on Pt and only
is unable to oxidize it, as shown by CV
References
2
on a macroelectrode (Figure S2,Supporting Information).
(1) Rodr ´ı guez-L o´ pez, J.; Alpuche-Aviles, M. A.; Bard, A. J. J. Am. Chem.
Soc. 2008, 130, 16985–16995.
A second SI experiment involved the co-adsorption of CO and
(2) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.;
Marcel Dekker: New York, 2001.
-
22
cyanide (CN ) on the Pt UME surface. A cyanide-covered Pt
UME was used to limit the amount of CO(ads) at Pt in order to verify
(
3) For applications of SECM in catalysis see: Pust, S. E.; Maier, W.; Wittstock,
G. Z. Phys. Chem. 2008, 222, 1463–1517.
(
4) Vielstich, W. CO, Formic Acid, and Methanol Oxidation in Acid
Electrolytes - Mechanisms and Electrocatalysis. In Encyclopedia of
Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH: Weinheim,
Germany, 2003; Vol. 2, pp 466-511. Iwasita, T. In Handbook of Fuel
Cells; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; Wiley: Chichester,
UK, 2003; Vol. 2, pp 603-624.
its reaction with Br
tion). The SI-SECM results show that, with CN , the adsorption
2
, as shown in Figure S3 (Supporting Informa-
-
of CO is blocked and furthermore that, despite the isoelectronic
-
nature of CN and CO, their reactivity toward Br
2
is different. This
is an important step in the
reaction scheme or that CN- blocks sites for the dissociation of
Br
suggests that the formation of OCBr
2
(5) Siwek, H.; Lukaszewski, M.; Czerwinski, A. Phys. Chem. Chem. Phys.
2008, 10, 3752–3765.
(
6) Inkaew, P.; Korzeniewski, C. Phys. Chem. Chem. Phys. 2008, 10, 3655–
3661.
7) Offer, G. J.; Kucernak, A. R. J. Electroanal. Chem. 2008, 613, 171–185.
2
.
(
Finally, a bulk experiment was carried out to identify CO
the end product of the reaction. A beaker with 10 mmol of Br
00 mL of 0.5 M H SO
2
as
in
(8) Czerwinski, A.; Sobkowski, J. J. Electroanal. Chem. 1978, 91, 47–53.
(
9) Lopez-Cudero, A.; Cuesta, A.; Glaudio, G. J. Electroanal. Chem. 2006,
86, 184–195.
10) Batista, E. A.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 2004, 108, 14216–
4222.
11) Samjeske, G.; Komatsu, K.-I.; Osawa, M. J. Phys. Chem. C 2009, 113,
0222–10228.
2
5
1
2
4
was bubbled with CO through an inlet
(
(
1
where Ar was also continuously passed; Pt black was added as a
catalyst. The outlet of the beaker was connected to a container with
1
1
00 mL of a saturated and Ar-purged solution of Ba(OH)
transparent). A control experiment, in which no Pt black was added,
showed only cloudiness in the Ba(OH) solution after 5 min of
CO bubbling. This is attributed to small impurities of CO in the
CO feed. When Pt black was added to the reaction mixture, an
abundant precipitate of BaCO was observed in the Ba(OH)
solution. The precipitate was identified as the carbonate through
the “carbonate-acid test”. The addition of AgClO solution to the
2
(12) Lu, G.-Q.; Crown, A.; Wieckowski, A. J. Phys. Chem. B 1999, 103, 9700–
9
711. Lu, G.-Q.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. B 2000,
(
1
04, 5566–5572. Chen, Y.-X.; Shen, Y.; Heinen, M.; Jusys, Z.; Osawa,
M.; Behm, R. J. J. Phys. Chem. B 2006, 110, 9534–9544. Lai, S. C. S.;
Kleyn, S. E. F.; Rosca, V.; Koper, M. T. M. J. Phys. Chem. C 2008, 112,
2
2
1
9080–19087. Cao, D.; Lu, G.-Q.; Wiekcowski, A.; Wasileski, S. A.;
Neurock, M. J. Phys. Chem. B 2005, 109, 11622–11633.
(
13) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003,
3
2
1
15, 229–235.
(14) Koper, M. T. M.; Jansen, A. P. J.; van Santen, R. A.; Lukkien, J. J.; Hilbers,
P. A. J. J. Chem. Phys. 1998, 109, 6051–6062.
4
(
15) Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. J. Chem. Soc., Faraday
residue of the reaction mixture showed the precipitation of whitish-
yellow AgBr, which also supports Br- as an end product. This
Trans. 1996, 92, 3799–3805.
(16) Sheffer, M.; Mandler, D. J. Electrochem. Soc. 2008, 155, D203–D208.
17) Nakanishi, S.; Sakai, S.-I.; Hatou, M.; Fukami, K.; Nakato, Y. J.
(
2 2
experiment confirms that CO can be oxidized to CO by Br through
Electrochem. Soc. 2003, 150, E47-E51.
2
3
(18) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. O. Acc. Chem.
Res. 1990, 23, 357–363.
(19) Woods, R. Chemisorption at Electrodes. In Electroanalytical Chemistry;
Bard, A. J., Ed.; Marcel Dekker: New York, 1976; pp 1-162.
the catalytic action of Pt.
In conclusion, the SI-SECM technique allowed the production
of Br
2
on an interrogator tip, which reported transient positive
(
20) Lissi, E.; Simonaitis, R.; Heicklen, J. J. Phys. Chem. 1972, 76, 1416–1419.
Piva, A. Gazz. Chim.Ital. 1915, 45 (1), 219–237. Lenher, S.; Schumacher,
H.-J. Z. Phys. Chem. 1928, 135, 85–101. Schumacher, H.-J.; Lenher, S.
Ber. Deutsch. Chem. Ges. B: Abhandlungen 1928, 61, 1671–1675.
Livingston, R. J. Phys. Chem. 1930, 34, 2121–2122.
-
feedback (due to Br regeneration) as an indication of the reactivity
of this halogen with CO(ads) on a Pt substrate. CO(ads) under saturation
conditions was reproducibly quantified at the polycrystalline Pt
surface with θCO ≈ 0.5 The reaction is shown to be blocked by the
action of pre-adsorbed cyanide, which demonstrates the surface
(
(
21) Francisco, J. S. Chem. Phys. Lett. 2002, 363, 275–282.
22) Huerta, F.; Morallon, E.; Vazquez, J. L. Electrochem. Commun. 2002, 4,
251–254.
(
23) Caution! Because of the possibility of forming bromophosgene (eq 4), these
2
character of the process. The formation of CO as an end product
experiments were performed in a hood at all times, and extensive Ar purging
was further tested in a bulk experiment. The use of SI-SECM
provided access to the study of a reaction that has not been
previously reported to be catalyzed by Pt, a finding that would
of the reaction products was done after the conclusion. OCBr
2
is unstable
2
in basic solution, so the Ba(OH) traps it in addition to the CO .
2
JA907626T
J. AM. CHEM. SOC. 9 VOL. 131, NO. 47, 2009 17047