UV-enhanced exchange of O2 with H2O adsorbed on TiO2

Article abstract of DOI:10.1006/jcat.1998.2292

Ultraviolet light dramatically increases the rate of isotope exchange between gas-phase O₂ and water adsorbed on TiO₂ at room temperature, but it does not affect the rate of CO₂-water exchange. Both ethanol and acetaldehyde, when coadsorbed with H₂¹⁸O, dramatically decrease the rate of O₂ exchange, but not CO₂ exchange, with adsorbed H₂¹⁸O. This decrease is attributed to a combination of competition for adsorbed oxygen between exchange and photocatalytic oxidation of the adsorbed organic and blocking of the oxygen adsorption sites by the organic. The same oxygen species participate in O₂-H₂¹⁸O exchange and photocatalytic oxidation.

Full text of DOI:10.1006/jcat.1998.2292

Journal of Catalysis 181, 155–159 (1999)
Article ID jcat.1998.2292, available online at http://www.idealibrary.com on
RESEARCH NOTE
UV-Enhanced Exchange of O2 with H2O Adsorbed on TiO2
Darrin S. Muggli and John L. Falconer
Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Received April 16, 1998; revised September 24, 1998; accepted October 1, 1998
exchange with absorbed H218O on TiO2 and the effect of
co-adsorbed organics on the exchange rate.
Ultraviolet light dramatically increases the rate of isotope ex-
change between gas-phase O2 and water adsorbed on TiO2 at
room temperature, but it does not affect the rate of CO2–water
exchange. Both ethanol and acetaldehyde, when coadsorbed with
H218O, dramatically decrease the rate of O2 exchange, but not CO2
exchange, with adsorbed H218O. This decrease is attributed to a
combination of competition for adsorbed oxygen between exchange
and photocatalytic oxidation of the adsorbed organic and blocking
of the oxygen adsorption sites by the organic. The same oxygen
species participate in O2-H218O exchange and photocatalytic oxid-
c
Ultraviolet light has been reported to increase the ex-
change between gas-phase O2 and the lattice oxygen ofTiO2
at room temperature (10, 11). Courbon et al. (11) deter-
mined that both UV light and TiO2 are required for oxygen
isotope exchange and proposed an exchange mechanism
that involved one surface oxygen atom at a time. They cor-
related the oxygen isotope exchange rate with the PCO
rate of isobutane to acetone. During PCO of isobutane in
a batch reactor with excess 18O2, oxygen isotope exchange
took place only after the all the isobutane had reacted, even
though the isobutane charge corresponded to a coverage of
ation.
1999 Academic Press
2
only 10 isobutane molecules/nm2. They concluded that
INTRODUCTION
the same surface oxygen species participates in PCO and
oxygen exchange.
Heterogeneous photocatalytic oxidation (PCO) has po-
tential applications for complete oxidation of organic pol-
lutants in dilute systems. Organics can be oxidized to CO2
and H2O at room temperature on TiO2 catalysts in the
presence of UV or near-UV illumination. The UV light
excites electrons from the valence to the conduction band
of the semiconductor catalyst, leaving holes behind. The
electron–hole pairs can initiate redox reactions with surface
species.
The roles of O2 and water during PCO are not well under-
stood. Some studies have reported that photocatalytic ac-
tivity decreases in the absence of water in the feed stream
(1–4), whereas higher water concentrations can have an in-
hibitory effect (1, 3). In other studies (5–8) with various re-
actants, PCO took place in a dry feed stream without deacti-
In contrast, when Yanagisawa and Ota (12) adsorbed
18O2 at 90 K on rutile TiO2, oxygen did not exchange with
the surface when it was irradiated by UV at room tem-
perature, although they observed 18O2 photodesorption.
Yanagisawa et al. (13) used TPD to determine the extent of
isotope exchange between adsorbed O2 and a MgO surface
that had been previously irradiated by UV light. They
exposed the MgO surface to UV light for 10 min at room
temperature before cooling to 80 K and adsorbing 18O2.
During the subsequent TPD, they observed 16O18O peaks.
EXPERIMENTAL METHODS
The apparatusused for photocatalyticexchange and TPD
vation. We showed previously (5) that, although high water was described previously (14). Approximately 30 mg of
concentrations reduce the surface coverages of ethanol and Degussa P-25 TiO2 catalyst was coated as a thin layer (av-
its intermediates, the rate of CO2 formation did not change. erage thickness <0.5 m) on the inside of an annular Pyrex
Lu et al. (9), using isotopically labeled O2 and water, ob- reactor so that all the TiO2 was exposed to UV light. The
served that oxygen from adsorbed H218O was incorporated annular reactor had a 1-mm annular spacing so that high
into some reaction products during PCO of CH3Cl. They gas flow rates could be maintained across the catalyst to
concluded that water participates in PCO when adsorbed minimize mass transfer effects and rapidly flush gas phase
oxygen is present, and thus the rate of exchange between products from the reactor. The outside diameter of the reac-
O2 and H2O is of interest for better understanding PCO. tor was 2 cm and the reactor was 13 cm high so that sufficient
We studied the effect of UV irradiation on O2 and CO2 catalyst mass was present to allow detection of the reaction
155
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156
MUGGLI AND FALCONER
products by the mass spectrometer. Six UV lamps (GE, used. After PCO or photocatalytic exchange, TPD or TPO
4 W) surrounded the photocatalytic reactor and the light was performed by heating the catalyst at 1 K/s to 723 K and
intensity at the catalyst surface location, measured with a holding at this temperature until no desorption products
radiometer, was approximately 0.3 mW/cm2. The radiome- were detected. A He flow gas was used for TPD, whereas a
ter could only measure light from one direction, but the 20% O2 in He gas mixture was used for TPO.
light that penetrated through the thin catalyst layer would
RESULTS AND DISCUSSION
strike the layer on the other side of the reactor and, thus,
the intensity was greater than 0.3 mW/m2. This made the
intensity variation at different locations around the reactor
less, but the light was more intense at the midpoint of the
reactor than at the ends. The maximum light intensity was
near 360 nm (15).
Before each experiment, heating the reactor for 30 min
in approximately 20% O2 in He to 723 K and then cooling
to room temperature created a reproducible surface. Metal
shields were placed between the reactor and the UV lights,
and the shieldswere removed to illuminate the catalyst after
the lights attained a steady-state output. Two 1- L pulses of
H218O (Isotec, 99% 18O enrichment) saturated the catalyst
in the dark prior to exchange. Exchange was studied by
injecting 16O2 (560 mol/g catalyst) and C16O2 (220 mol/g
catalyst) pulses or by continuously flowing 0.2, or 3% O2
in He over a monolayer of H218O adsorbed on the catalyst.
The exchange was studied both in the dark and under UV
illumination.
Pulsed O2 Exchange
Figure 1 shows the rates of formation of 16O18O and 18O2
when 0.5-ml pulses of 16O2 were injected over illuminated
and dark TiO2 that was saturated with H218O. A 0.5-ml
pulse of 16O2 was also injected downstream of the cata-
lyst for calibration. No isotopic exchange was observed in
the dark since the fraction of the oxygen pulse that was
16O18O (0.39% ) was equal to that of the reference calibra-
tion (0.40% ) and corresponds to the natural abundance of
18O, which is 0.2% of total oxygen atoms. Note that the
16O18O signal during calibration had a greater maximum
than that when O2 was pulsed over the dark catalyst. Since
the calibration injection port was located closer to the mass
spectrometer, less peak broadening due to axial dispersion
and backmixing occurred. The areas under the curves were
identical, however. In contrast, when 16O2 was pulsed over
illuminated TiO2 that was saturated with H218O, the oxy-
gen atoms in 16O2 exchanged with those in the adsorbed
H218O. Approximately 2.6% of the O2 at the reactor exit
was 16O18O. Approximately 4.7% of the 16O18O exchanged
further with the adsorbed water to produce 18O2 (0.13% of
the total O2 pulse) during UV illumination. The amount of
18O exchanged during the experiment corresponded to 7%
of the total 18O in the adsorbed H218O. Thus, UV light in-
creased the rate of oxygen exchange dramatically at room
temperature; no measurable exchange took place in the
dark.
The exchange of O2 and CO2 with H218O that was coad-
sorbed with ethanol or acetaldehyde was also studied. Dur-
ing the O2 exchange, the adsorbed organic was oxidized
photocatalytically. Either a monolayer of organic (ethanol
or acetaldehyde) was adsorbed and then two pulses of
H218O (560 mol/g catalyst) were injected upstream of the
reactor or a monolayer of water was adsorbed and then
two pulses of organic (560 mol/g catalyst) were injected
before UV illumination. Changing the adsorption order
changed the relative coverages (5). These adsorbed species
were then exposed to 16O2 pulses (560 mol/g catalyst).
For some experiments, a 150 mol/g catalyst pulse of either
acetaldehyde or ethanol was injected into the O2 flow dur-
ing exchange of a monolayer of H218O. The duration of the
pulse was approximately 2 s for acetaldehyde and 30 s for
ethanol. The duration of the ethanol pulse was longer since
it was injected as a liquid and more time was needed for it
to evaporate.
A Balzers QMA 125 quadrupole mass spectrometer
monitored the reactor effluent immediately downstream
of the reactor. The mass spectrometer was interfaced to
a computer to record multiple mass peaks simultaneously.
The mass spectrometer signals were calibrated by injecting
known volumes of gases into the flow stream downstream
of the reactor and signals were corrected for cracking in the
mass spectrometer. Plots of rates versus time were gener-
ated using frequent calibration of the gas-phase products
by injecting known quantities into the flow stream. To cal-
FIG. 1. Oxygen exchange between 16O2 pulses and a monolayer of
ibrate the 18O species, the corresponding 16O species were H218O on TiO2 in the dark and during UV illumination.

UV-ENHANCED EXCHANGE OF O2
157
TPD of H218O
To determine if water and TiO2 exchange oxygen dur-
ing TPD, successive TPDs were performed by readsorbing
H218O after each temperature ramp. The percentage of wa-
ter that desorbed as H218O was 34% during the first TPD,
55% in the second, and 63% in the third. This suggests that a
portion of the adsorbed water exchanges oxygen with TiO2
either during TPD or at room temperature prior to TPD.
The total amount of water (H216O + H218O) that desorbed
during the first TPD (710 mol/g catalyst) was similar to
that reported previously (16). The amount of H218O that
desorbed during the first TPD (250 mol/g catalyst) was sig-
nificantly less than the amount of 18O detected during pho-
tocatalytic exchange and subsequent TPD of H218O, even
though the same adsorption procedure was used. This indi-
cates that oxygen atoms in some adsorbed water molecules
most likely exchange with the surface during TPD. The
oxygen atoms in H218O could have exchanged at room tem-
perature with lattice oxygens, which then exchanged with
the O2.
FIG. 2. Rates of formation of 16O18O and 18O2 in 0.2% 16O2 flow for
TiO2 under UV irradiation. The TiO2 was saturated with H218O at the start
of exchange.
Continuous Flow Exchange
Pulse Exchange: Coadsorbed H218O and Organic
Figure 2 shows the UV-enhanced oxygen exchange of ad-
sorbed H218O with gas-phase O2 while flowing 0.2% O2 in
He. The rates of 16O18O and 18O2 formation quickly reach
maxima and then decreased. The rate of 16O18O formation
was 25 times that of 18O2 throughout the experiment, indi-
cating that an O4 intermediate is not probable. This indi-
cates that 18O2 most likely forms from further exchange of
16O18O with adsorbed 18H2O. In contrast, Yanagisawa et al.
(13) presented evidence for both O3 and O4 intermediates
for the exchange of O2 with the lattice oxygen of MgO.
Saturation coverage of water on TiO2 was determined
previously to be approximately 750 mol/g catalyst (16).
Approximately 65% of the adsorbed H218O exchanged with
gas-phase O2 to form 16O18O and 18O2 during the 3.5-h
experiment, and 20% desorbed during the subsequent
TPD. Water desorption was not complete when TPD was
stopped, and thus approximately 15% of the adsorbed
H218O either had not desorbed when the TPD was stopped,
or it had exchanged oxygen with the surface (perhaps dur-
ing TPD) and desorbed as H216O. Exchange of oxygen be-
tween water and TiO2 is discussed below. The rate of 16O18O
formation after 3.5 h of illumination was one-tenth the ini-
tial rate, even though only 65% of the oxygen in the ad-
sorbed H218O had exchanged. This may be because the light
intensity was not uniform or because the TiO2 surface is
heterogeneous, with H2O on some sites exchanging more
rapidly. The lights were turned off twice for 6 min during the
experiment to check for baseline drift, which was insignifi-
cant. Note that the 16O18O formation rate quickly dropped
to zero when the lights were switched off, indicating that
the appearance of 16O18O in the gas phase is not desorption
limited.
When H218O was coadsorbed with ethanol, pulses of 16O2
over the UV-irradiated TiO2 did not produce a detectable
amount of 16O18O or 18O2, even though a significant amount
of H218O was on the surface. In contrast, the CO2 produced
by PCO readily exchanged with adsorbed H218O. For each
experiment, approximately 70% of a monolayer of organic
was adsorbed. When ethanol was adsorbed first and then
H218O, 85 mol/g catalyst of H218O desorbed during TPO.
When the adsorption procedure was reversed, 165 mol/g
catalyst of H218O desorbed during TPO, compared to ap-
proximately 250 mol/g catalyst of H218O that desorbed
during the first TPD of a monolayer of H218O. Water des-
orption was not complete when the TPD was stopped, and
some adsorbed H218O exchanged oxygen with the surface
during TPD. Similarly, when H218O was coadsorbed with
acetaldehyde, no exchange took place when O2 was pulsed
over the UV-irradiated TiO2 surface, and the amounts of
H218O that desorbed during TPO were approximately the
same (120–125 mol/g catalyst) regardless of adsorption
order. This result is similar to that reported by Courbon
et al. (11); during PCO of isobutane (excess 18O2) in a batch
reactor, oxygen exchange with latticeoxygen took place only
after the all the isobutane had reacted. Furthermore, since
PCO of isobutane produces water, the absence of gas phase
16O18O or 18O2 did not exchange with adsorbed water, al-
though the small initial charge of isobutane would produce
a small amount of water.
Continuous Flow: Organic Injection
The exchange between a monolayer of adsorbed H218O
and flowing 0.2% O2 immediately stopped when a

158
MUGGLI AND FALCONER
change. This may be because the CO2 coverage is higher
than the O2 coverage on a water-saturated TiO2 surface.
Exchange in 3% O2
The exchange of adsorbed H218O with flowing 3% O2 in
He was similar to that shown in Fig. 2. Increasing the O2
concentration from 0.2 to 3% quadrupled the maximum
rate of16O18O formation (0.42 mol/gcatalyst), but the 18O2
maximum rate (0.01 mol/g catalyst) was only 2.5 times that
in 0.2% O2 flow. Thus, the exchange rate was less than first
order in O2 concentration between 0.2 and 3% O2. When
the exchange was stopped after 1 h of illumination, 55% of
the H218O monolayer had exchanged, but the rate of 16O18O
formation was one-twentieth of the initial rate, due to either
light nonuniformity or surface heterogeneity.
Figure 4 shows the effect of injecting ethanol (150 mol/g
catalyst) during exchange of adsorbed H218O in 3% 16O2 in
He flow. When ethanol was pulsed over the catalyst after
300 s of illumination, the rates of 16O18O and 18O2 formation
drasticallydecreased. The rate of16O18O decreased from 0.3
to 0.02 mol/g catalyst/s, whereas the 18O2 rate decreased
from 0.005 mol/g catalyst/s to a rate that was approxi-
mately zero. In contrast to the results in 0.2% O2, exchange
still took place when half a monolayer of ethanol was ad-
sorbed. This may be attributed to the change in relative
rates of exchange and PCO with oxygen concentration. In-
creasing the O2 concentration by a factor of 14 quadrupled
the 16O18O formation rate, whereas the maximum rate of
CO2 formation in 3% O2 (Fig. 4) only doubled. Since higher
oxygen concentrations increase the rate of exchange more
than the rate of PCO, exchange competes more effectively
for the adsorbed oxygen.
FIG. 3. Rates of formation of 16O and 18O-containing species on UV-
illuminated TiO2 in 0.2% O2 flow. The TiO2 was saturated with H218O at
the start of the experiment. After 600 s, a pulse of ethanol (150 mol/g
catalyst) was injected.
150 mol/g catalyst pulse of organic was injected (Fig. 3).
For experiments in which either acetaldehyde or ethanol
was injected, the rate of 16O18O formation immediately de-
creased to zero after the injection, but gradually increased
after some of the organic oxidized to CO2. The rate of 18O2
formation also stopped when the organic was injected, but
any subsequent increase in the 18O2 rate after the organic
injection could not be detected due to its low rate. Mass bal-
ances indicate that the absence of O2 exchange was not due
to complete consumption of O2 by PCO. Approximately
15–25% of a monolayer of organic was still on the surface
when the exchange between O2 and adsorbed H218O re-
sumed. These organic coverages are 50 to 100 times greater
than those estimated by Courbon et al. (11) for their exper-
iments with isobutane and exchange with the lattice oxy-
gen. However, exchange with lattice oxygen may be much
more sensitive to the presence of adsorbed organics, and the
amount of water formed during PCO of isobutane would
be small since only a small amount of isobutane was admit-
ted into the reactor. The difference between our results and
those of Courbon et al. suggests that 16O2 exchange with ad-
sorbed H218O, and H218O did not first exchange with lattice
oxygen, which then exchanged with16O2.
Note in Fig. 3 that a significant amount of the CO2 is la-
beled with 18O because CO2 that formed during PCO read-
ily exchanges with adsorbed water. Oxygen atoms readily
exchanged during repeated pulses of CO2 over adsorbed
H218O on both illuminated and dark TiO2. The CO2–H218O
exchange takes place during PCO, even though the TiO2
has high coverages of organics (17). Illumination did not in-
crease the rate of exchange, and thus, the exchange process
for O2 and CO2 appear quite different. The CO2 exchange
FIG. 4. Rates of formation of 16O and 18O-containing species on UV-
illuminated TiO2 in 3% O2 flow. The TiO2 was saturated with H218O at
the start of the experiment. After 300 s, a pulse of ethanol (150 mol/g
rate was significantly greater than the UV-enhanced O2 ex- catalyst) was injected.

UV-ENHANCED EXCHANGE OF O2
159
Comparisons to Literature
tween gas-phase O2 and adsorbed water. No exchange took
place when the TiO2 was pretreated with UV light before
pulsing O2. The O2-H218O exchange may be the result of a
higher concentration of surface O2 through photoadsorp-
tion (although the amount of photoadsorbed O2 is expected
to be small), or more likely, UV light increases the exchange
reaction rate.
Courbon et al. (11) observed that O2 did not exchange
with lattice oxygen in TiO2 when isobutane was in the gas
phase. They reasoned that PCO and exchange compete
for the same oxygen intermediate, and the rate of PCO is
much faster. Our results also indicate that the same oxygen
species participates in PCO and O2-H218O exchange. How-
ever, our initial rate of 16O18O formation due to exchange is
comparable to the rate of CO2 formation in Fig. 3. The PCO
rate per adsorbed molecule is greater than the oxygen ex-
change rate in the absence of organics because the organic
coverage is less than the water coverage and because some
intermediates formed but did not desorb. Thus, the dras-
tic drop in exchange rate is not solely due to competition
with PCO for oxygen since the CO2 formation rate during
PCO is not drastically greater than the 16O18O formation
rate in the absence of organics. Furthermore, Fig. 2 shows
that nearly all the adsorbed H218O can exchange, indicating
that the organic does not just displace the H218O that can ex-
change. Since the adsorbed organic readily oxidizes to CO2,
as shown in Fig. 3, the organics do not completely block oxy-
gen adsorption sites. However, if organics block a portion
of sites available for oxygen adsorption, the PCO and ex-
change rates cannot be directly compared. The amount of
adsorbed oxygen could be much smaller in the presence of
the organic, but the PCO rate is still significant at this lower
O2 concentration, whereas the exchange rate is not. There-
fore the intrinsic PCO rate may be much greater than the
O2-H218O exchange rate but blocking of oxygen adsorption
sites by adsorbed organics may mask the difference.
ACKNOWLEDGMENT
We gratefullyacknowledge support bythe NationalScience Foundation
Grant CTS-9714403.
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