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8/7/2019 reaction of benzaldehyde on titanium oxide
http://slidepdf.com/reader/full/reaction-of-benzaldehyde-on-titanium-oxide 1/2
page 42 2008 NNIN REU Research Accomplishme
Investigation of the Reaction of Benzaldehyde on Titanium Dioxide
Christina Elias
Chemical Engineering, Ohio State University
NNIN REU Site: Center for Nanoscale Systems, Harvard University, Cambridge, MA
NNIN REU Principal Investigator(s): Professor Cynthia M. Friend
NNIN REU Mentor(s): Dr. Lauren Benz, Dr. Jan Haubrich, Department of Chemistry and Biochemistry, Harvard University Contact: [email protected], [email protected], [email protected], [email protected]
Abstract/Introduction:
The interaction between oxide surfaces and organic
compounds is of general industrial importance as many
catalytic processes involve reactions between these
species. Furthermore, volatile organic compounds
are a class of environmental pollutants [1,2] and
the interaction of such molecules with natural and
engineered oxide surfaces is not well understood [3]. It
is thought that processes such as adsorption, reaction,
and/or decomposition over such oxides play a crucial
role in the remediation of the pollutants. Additionally,
organic compounds can alter the surface properties
of oxide coatings, aerosols particles and catalysts
signicantly.
Therefore we have chosen to study the interaction of an organic molecule containing an aldehydic function with
the surface an oxide using a simple model system: benzaldehyde on rutile titanium dioxide (TiO2) (110). Previously
we observed carbon coupling reactions via loss of oxygen for such systems employing temperature programmed
reaction spectroscopy (TPRS) [5]. Conversion of benzaldehyde to stilbene occurs for approximately 70 percent of
the adhered benzaldehyde molecules. Previous studies attribute the reactivity to defects such as oxygen vacancies
that leave active ensembles of reduced Ti3+ sites on the oxide substrate [6].
In the present study, scanning tunneling microscopy (STM) experiments were performed under ultra-high vacuum
(UHV) conditions to identify the adsorption sites of benzaldehyde and search for reaction intermediates. Although
benzaldehyde appeared mobile at room temperature, a preferred adsorption on ve coordinated Ti4+ sites was
observed; however, the movement of benzaldehyde molecules during the time scale of our experiment prevented a
better resolution of the features. Future work will require cooling to low temperatures to immobilize the molecules,
allowing for the precise identication of the active sites and possible reaction intermediates.
Experimental Procedure:
A UHV Omicron STM set up (base pressure ~ 3 × 10-10 torr)
equipped with low energy electron diffraction (LEED), two
mass spectrometers and an ion gun for argon sputtering,
was used in this study. The sample surface was cleaned by
a series of sputtering (10-6 mbar Ar, 1 keV, lament current10 mA, 20 min.) and annealing (~ 850-900°K, 5 min.) cycles
prior to imaging. Temperatures were calibrated using a type
K thermocouple. Benzaldehyde (Aldrich 99.5% plus purity)
was further puried by freeze-pump-thaw cycles and dosed
at room temperature exposures measured in Langmuir: 1L
= 1 torr•µs = 1.33 10-6 mbar•s) on the surface from a doser
positioned 1 cm above the surface. Constant current images
were taken with typical scanning parameters of 0.1-0.3 nA
and 1-3 V. Both electrochemically etched and platinum/
iridium (Pt/Ir) W tips were used while scanning.
Results:
Figure 2 shows the TiO2
(110) surface after cleaning. The
spatial resolution achieved with STM clearly allows for
the observation of Ti4+ (a, bright) and bridging oxygen (b,
dark) rows. Inherent defects included bridging oxygen atom
vacancies seen as bright breaks in the O rows (c), and stepedges (d). These locations are regions of high interest for
potential binding sites.
In Figure 3 one can see that the same area of the sample
following dosing of ca 0.5 L benzaldehyde at room temp-
erature. This coating is depicted clearly in the image and is
represented by the bright round features which were observed
bound to the titanium atom rows (g). Hence the preferred
binding site of benzaldehyde must be over ve coordinated
Ti4+ ions. In contrast to our expectations, benzaldehyde did
Figure 1: Reaction scheme developed after TPRS data analysis.
8/7/2019 reaction of benzaldehyde on titanium oxide
http://slidepdf.com/reader/full/reaction-of-benzaldehyde-on-titanium-oxide 2/2
Chemistry page 43
not bind to bridging oxygen vaciences or step edges, which are
usually strong adsorption sites.
Figure 4 that this is again the same region scanned after a ve
minute time elapse. This image illustrates how benzaldehyde
molecules are mobile at room temperature. While some molecules
remain in their original binding locations, others diffuse away to
new locations. Still no accumulation at step edges occurs.
Conclusions:
Points to take away from the work completed this summer
include the following: benzaldehyde molecules bind weakly to
ve coordinated Ti4+ sites at room temperature. Notably they
did not adsorb preferentially to oxygen vacancies or steps. At
room temperature, they diffuse slowly across the surface. Future
low temperature scanning to immobilize the molecules while
investigating the reaction of benzaldehyde is required.
Acknowledgments:
I would like to thank my principal investigator, Cynthia Friend,and mentors, Lauren Benz and Jan Haubrich, and organizations
including National Science Foundation, National Nanotechnology
Infrastructure Network Research Experience for Undergraduates
Program, and the Harvard Center for Nanoscale Systems.
References:
[1] Usher, C.; Michel, A.; Grassian, V.; Chem. Rev. 2003, 103, 4883.
[2] E.P.A, U. S. The original list of hazardous air pollutants, 2007.
[3] Diebold, U. Surface Science Reports 2003, 48, 53.
[4] Agency for Toxic Substances and Disease Registry, Toxicological
Prole for Acrolein, Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA, 1990.
[5] Benz, L.; Haubrich, J.;Quiller, R. G.; " Reactions of Benzaldehyde
and Acrolein on TiO2(110)and the Inuence of Defects and
Coadsorbed Species", in preparation.
[6] (a) Sherrill, A.B.; Idriss, H.; Barteau, M.A.; Chen, J.G. Catalysis
Today 2003, 85, 321; (b) Idriss, H.; Pierce, K.G.; Barteau, M.A.
Journal of the American Chemical Society 1994, 116, 3063.
Figure 2, top: STM image of clean TiO2
surface (imaging parameters:
30 nm × 30 nm, 300K). Features identifed are (a) bright Ti atom rows;
(b) dark O atom rows; (c) bridging O vacancy; (d) step edge; (e) unknown
contaminant.
Figure 3, middle: STM image of TiO2
immediately after a 0.5L benz-
aldehyde exposure (imaging parameters: 30 nm × 30 nm, 300K).
Figure 4, bottom: STM image taken after 5 min. Filled circles are previous
molecule locations, while new molecules are circled with hatched lines.
(imaging parameters: 30 nm × 30 nm, 300K).