Estudio De Trabajo

pubs.acs.org/JPCL

Visible Light Photoreduction of CO2 Using CdSe/Pt/TiO2
Heterostructured Catalysts
Congjun Wang,*,†,‡,§ Robert L. Thompson,†,‡,§ John Baltrus,† and Christopher Matranga†
†

National Energy Technology Laboratory, U.S. Department of Energy, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236,
and ‡Parsons Project Services, Inc., P.O. Box 618, South Park, Pennsylvania15219

ABSTRACT A series of CdSe quantum dot (QD)-sensitized TiO2 heterostructures
have been synthesized, characterized, and tested for the photocatalytic reduction
of CO2 in the presence of H2O. Our results show that these heterostructured
materials are capable of catalyzing the photoreduction of CO2 using visible light
illumination (λ > 420 nm) only. The effect of removing surfactant capsfrom the
CdSe QDs by annealing and using a hydrazine chemical treatment have also been
investigated. The photocatalytic reduction process is followed using infrared
spectroscopy to probe the gas-phase reactants and gas chromatography to detect
the products. Gas chromatographic analysis shows that the primary reaction
product is CH4, with CH3OH, H2, and CO observed as secondary products. Typicalyields of the gas-phase products after visible light illumination (λ > 420 nm) were
48 ppm g-1 h-1 of CH4, 3.3 ppm g-1 h-1 of CH3OH (vapor), and trace amounts of
CO and H2.
SECTION Surfaces, Interfaces, Catalysis

O

ne quarter of the world's coal reserves are located in
the United States. Coal is also the workhorse of our
power industry, responsible for more than half of theelectricity consumed by Americans. Even though coal is a
national treasure, managing CO2 emissions from its utilization is perhaps the largest technical challenge currently faced
by the fossil energy industry. Although CO2 is routinely
captured from industrial processes such as ammonia production and limestone calcination, existing capture technologies
are not cost-effective for use in power plants. Assuch,
transformational new technologies are needed to help address the CO2 challenge.
The photocatalytic reduction of CO2 is a promising technical
solution since it uses readily available sunlight to convert CO2
into valuable chemicals, such as methanol or methane, in a
carbon friendly manner.1 TiO2 is a popular catalyst for this
photoreduction because it is inexpensive, has reasonableactivity, and is abundant. Despite these attributes, the efficiency of TiO2 for photovoltaic and photocatalytic applications
is severely limited by its large band gap (∼3.2 eV) and rapid
charge carrier recombination dynamics.2 For photovoltaics
applications, the band gap sensitization of TiO2 by dye molecules has improved activity and demonstrated conversion
efficiencies of over 11%.3 Enhancementof activity has also
been achieved by introducing impurity atoms such as N and S
into TiO2.4 Despite the tremendous progress in developing
more robust and efficient dyes for photovoltaic cells,3b one
disadvantage of dye molecules for photoreduction applications
is the potential for photo- and thermal degradation. Likewise,
impurity doping has the disadvantage of creating defect states,which may inherently limit the effectiveness of this strategy.5

r 2009 American Chemical Society

Recently, there has been a growing interest in using
semiconductor nanocrystal QDs to sensitize TiO2 for photovoltaic6 and photocatalytic7 applications. These QDs are
thermally stable, have large absorption cross sections, and
do not easily photodegrade. QDs such as CdSe form a type II
bandalignment with TiO2. As a result, any photoexcited
electrons in the CdSe QDs can be injected into TiO2 and
subsequently collected by an electrode or used to initiate a
photocatalytic reaction. This type of heterostructured material
can make use of visible or near-IR photons as well as facilitate
charge separation to prevent recombination. Using this approach, the photocatalytic decomposition of...