Photocatalytic decomposition of aliphatic alcohols, acids, and esters

Article abstract of DOI:10.1006/jcat.2000.3093

The photocatalytic decomposition (PCD) of aliphatic alcohols, acids, and esters in an inert atmosphere on TiO₂and Pt/TiO₂ at room temperature was studied using transient reaction techniques. The addition of Pt to TiO₂ increased the rate of reaction and changed the selectivity significantly, even though Pt is not a photocatalyst and does not have a measurable rate of thermal catalytic activity at room temperature. Although none of the reactants decomposed to form H₂ on TiO₂, significant amounts of H₂ formed for some reactants during on Pt/TiO₂, presumably because H atoms spill over onto Pt and recombine to form H₂. Hydrogen formed on Pt/TiO₂ for all the organics with H bonded to the carbon in a -C-O- group. When only alkyl groups were bonded to the carbon in a -C-O- group, alkanes formed but not H₂. Abstraction of a H atom appears to be the first step in PCD of alcohols, acids, and esters. Alcohols react to form the corresponding aldehyde and either H₂2 or alkanes. Acids and esters react similarly through two parallel pathways, and one pathway extracted lattice oxygen.

Full text of DOI:10.1006/jcat.2000.3093

Journal of Catalysis 197, 303–314 (2001)
doi:10.1006/jcat.2000.3093, available online at http://www.idealibrary.com on
Photocatalytic Decomposition of Aliphatic Alcohols, Acids, and Esters
M. Catherine Blount, Jason A. Buchholz, and John L. Falconer1
Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Received June 26, 2000; revised October 16, 2000; accepted October 17, 2000; published online December 21, 2000
various organics used helped distinguish which functional
groups are most reactive on Pt/TiO2.
The photocatalytic decomposition (PCD) of aliphatic alcohols,
acids, and esters in an inert atmosphere on TiO2 and Pt/TiO2 at room
temperature was studied using transient reaction techniques. The
addition of Pt to TiO2 increased the rate of reaction and changed the
selectivity significantly, even though Pt is not a photocatalyst and
does not have a measurable rate of thermal catalytic activity at room
temperature. Although none of the reactants decomposed to form
H2 on TiO2, significant amounts of H2 formed for some reactants
during on Pt/TiO2, presumably because H atoms spill over onto Pt
and recombine to form H2. Hydrogen formed on Pt/TiO2 for all the
organics with H bonded to the carbon in a –C–O– group. When
only alkyl groups were bonded to the carbon in a –C–O– group,
alkanes formed but not H2. Abstraction of a H atom appears to be
the first step in PCD of alcohols, acids, and esters. Alcohols react to
form the corresponding aldehyde and either H2 or alkanes. Acids
and esters react similarly through two parallel pathways, and one
c
Photocatalytic decomposition has been studied mainly in
the liquid phase on TiO2 and Pt/TiO2, and acetic acid de-
composition was used in several studies (7–14). Kraetler
and Bard (7) used both acetic acid and sodium acetate
solutions, and found that acetic acid decomposed to CH4
and CO2 along with small amounts of C2H6 and H2. When
deuterated acetic acid (CH3COOD) was reacted, mon-
odeuterated methane and deuterated hydrogen gas were
observed, but C2H6 was not deuterated. They proposed that
acetic acid decomposed to CO2, CH3(ads), and H(ads). The
CH4 then formed by combination of CH3(ads) and H(ads)
and C2H6 by the combination of two CH3(ads). They sug-
gested that CH4 may also form by reaction of CH3(ads)
with H2O(ads). In addition to acetic acid, Kraetler and Bard
(7) studied the PCD of propionic, n-butyric, n-valeric and
pivalic acids. They found that these acids decomposed to
their corresponding alkane and CO2: RCOOH → RH +
CO2. The dimer product, R–R, was only observed for acetic
acid.
pathway extracted lattice oxygen.
2001 Academic Press
INTRODUCTION
Heterogeneous photocatalytic oxidation (PCO) is a
promising technique for the complete oxidation of dilute
organic pollutants in waste gas streams. Many organics can
be oxidized to CO2 and H2O at room temperature on TiO2
catalysts in air when illuminated with UV or near-UV light.
The UV light excites electrons from the valence band into
the conduction band. The resulting electron/hole pairs can
then migrate to the surface and initiate redox reactions with
adsorbed organics. The rate of PCO can be increased for
many organics by adding low concentrations of Pt as small
particles supported on the TiO2 surface (1–6).
In the absence of gas phase O2, photocatalytic reactions
also take place on TiO2 and Pt/TiO2, and these reactions
are of interest because they may provide insight into the
reaction processes that take place during PCO. Moreover,
Pt appears to have a large effect on the rate and selectivity
for PCD. Thus, the photocatalytic decomposition (PCD) of
several aliphatic alcohols, acids, esters, and aldehydes was
studied in the absence of O2 on two Pt/TiO2 catalysts. The
Yoneyama et al. (8) also observed CO2, CH4, C2H6, and
H2 formation during PCD on Pt/TiO2 of aqueous solu-
tions of acetic acid and sodium acetate. They proposed
a mechanism similar to that of Kraetler and Bard (7);
however, their mechanism had an additional pathway:
CH3 + CH3COOH → CH4 + CH2COOH. The authors
attributed large amounts of CO2 to the oxidation of ethanol
and acetaldehyde intermediates. Nosaka et al. (10, 11) de-
tected methyl radicals during PCD of acetic acid in water
using ESR. They suggested that photo-induced holes react
with acetic acid to form CO2, CH3, and H, and that methyl
radicals mainly form CH4 by reacting with H.
Sclafani et al. (15) and Muggli and Falconer (16) observed
that gas-phase acetic acid decomposes to CH4, CO2, and
small amounts of C2H6 during PCD. Using labeled acetic
acid (CH133COOH), Muggliand Falconer (16) proposed two
parallel pathways for acetic acid PCD on TiO2:
CH133COOH(ads)
→
13CO2(g) + CH4(g)
[1]
2CH133COOH(ads) + O(lattice)
1 To whom correspondence should be addressed, Fax: (303)492-4341.
E-mail: john.falconer@colorado.edu.
→ C2H6(g) + 213CO2(g) + H2O(ads)
.
[2]
303
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

304
BLOUNT, BUCHHOLZ, AND FALCONER
Their results indicated that the first step is dissociation
of the O–H bond and PCD then proceeds through the
resulting acetate species. They found that lattice oxygen
consumed during C2H6 formation was slowly replenished
by diffusion from the TiO2 bulk, or more rapidly by
injecting gas-phase O2.
EXPERIMENTAL METHODS
The apparatus used for PCD, temperature-programmed
desorption (TPD), and temperature-programmed oxida-
tion (TPO) was described previously (28). Approximately
30 mg of catalyst was coated as a thin layer on the inside of
an annular Pyrex reactor. The catalysts used were Degussa
P25 TiO2 (75% anatase/25% rutile with a BET surface area
of 50 m2/g), 0.2 wt% Pt/TiO2, and 2 wt% Pt/TiO2. The pla-
tinized TiO2 catalysts were prepared by combining Degussa
P25 TiO2 and a H2PtCl6 solution with HCl. Acetic acid and
Na2CO3 were added to obtain a pH of 4. Nitrogen was then
bubbled through the solution, which was illuminated for
6 h. The catalyst was then washed in distilled H2O and
dried at 373 K for 24 h. The annular reactor had a 1-mm
annular spacing so that high gas flow rates could be main-
tained to minimize mass transfer effects and rapidly flush
gas phase products from the reactor. Twelve near-UV lights
(8 W, type F8T5BLB), positioned evenly around the reac-
tor and approximately 2.5 cm from the catalyst film, illumi-
nated the photoreactor. The light intensity at the catalyst
surface was measured as 2.5 mW/cm2, which is almost an
order of magnitude higher than that used by Muggli and
Falconer (16, 29–31, 37–39) during transient PCD studies of
formicand aceticacid on TiO2. The reactor effluent wasana-
lyzed with a quadrupole mass spectrometer (Balzers QMA
125). Computer-controlled data acquisition simultaneously
monitored and recorded selected masssignals, temperature,
and elapsed time. The mass spectrometer signals were cali-
brated by injecting known quantities of gases or liquids into
the appropriate gas stream composition, and the mass sig-
nals were corrected for cracking in the mass spectrometer.
Before each experiment, the catalyst was heated to 723 K
in a flowing O2/He mixture and then cooled to room tem-
perature to create a reproducible surface. Organics were
injected upstream of the reactor and allowed to evaporate
into the flowing gas and adsorb onto the catalyst. All exper-
iments started with the catalyst saturated with organic. The
organics studied were methanol (Aldrich, 99% ), ethanol
(McCormick Distilling Co.), t-butanol (Fisher Scientific,
certified), acetaldehyde (Aldrich, 99% ), acetone (Fisher
Scientific, certified), formic acid (Sigma, 99% ), acetic acid
(Aldrich, 99.99% ), propionic acid (Aldrich, 99.5% ), methyl
formate (Aldrich, 99% ), methyl acetate (Aldrich, 99% ),
t-butylacetate (Aldrich, 99% ), and phenylacetate (Aldrich,
99% ). After exposure to an organic, the reactor was flushed
for at least an hour to remove gas-phase organic, so that
only reaction of the adsorbed monolayer was studied. Pho-
tocatalytic decomposition was studied by illuminating the
catalyst in He (100 standard cm3/min). The He was purified
by flowing through a molecular sieve immersed in liquid N2,
and the O2 concentration in the He stream was below the
mass spectrometer detection limit. The O2 concentration
during PCD is estimated to be less than 0.3 ppm (16).
Though 2-propanol does not decompose photocatalyti-
cally in the absence of O2 on TiO2 (17, 18), it decomposes
to acetone and H2 when a low loading of Pt is added to
TiO2 (17–22). Ait-Ichou et al. (18) proposed that the initial
step in the PCD of gas-phase 2-propanol was the reaction
of adsorbed 2-propanol with a photogenerated hole to form
the adsorbed alkoxy radical (CH3)2CHO(ads). They investi-
gated the influence of Pt loading on the steady-state PCD
activity. The H2 production rate increased with Pt loading
up to 1.5 wt% Pt and was constant at higher loadings for
the range studied. They suggested that this behavior was ex-
plained by reverse spillover of hydrogen atoms from TiO2
to the Pt particles where the hydrogen atoms combined to
form gas-phase H2. For Pt loadings below 1.5% , the migra-
tion of the hydrogen to the Pt particles was proposed to be
rate determining.
Pichat et al. (23, 24) reported that gas-phase ethanol
decomposed to accetaldehyde and H2 during PCD. Only
undeuterated hydrogen evolved during PCD of labeled
gas-phase ethanol (CD3CH2OH) (23). The lack of HD
or D2 indicates that the -hydrogen atoms of simple
aliphatic alcohols are not involved in the primary steps of
PCD.
The PCO of aliphatic alcohols (22, 25–33) and acids (25,
30, 34, 35) has been studied extensively, but relatively lit-
tle work has been done on the PCO of esters (25, 36).
Aliphatic alcohols and acids oxidize readily during PCO,
and no catalyst deactivation was observed during the PCO
of various aliphatic alcohols, acids, and methyl formate
(25).
In the current study, transient reaction techniques were
used to investigate the surface processes involved in PCD
and the role of Pt in PCD for gas phase reactants. A mono-
layer of organic was adsorbed on oxidized Pt/TiO2, and ex-
cess organic was flushed from the gas phase. The catalyst
was then illuminated with near-UV light in the absence of
gas-phase O2, and reaction products were detected with a
mass spectrometer. The role of lattice oxygen was studied
by interrupting the PCD and observing how various dark
times affected the PCD behavior. Strongly bound species
that remained on the surface after PCD were removed
and analyzed using temperature-programmed desorption
(TPD) and oxidation (TPO). Alcohols, acids, and esters
were used. The organics were chosen to provide insight into
the reaction pathways and the roles of the acidic and alco-
holic oxygen groups. Two Pt loadings on TiO2 were used
for PCD of selected organics to investigate the role of Pt
during PCD.

PHOTOCATALYTIC DECOMPOSITION OF ALCOHOLS
305
TABLE 1
The lights were turned on and off periodically for varying
lengths of time during PCD to investigate the role of lattice
oxygen and surface diffusion on the PCD of the various or-
ganics. After PCD, TPD and TPO were performed by heat-
ing the catalyst to 723 K and holding at that temperature
until all mass signals returned to their original baseline val-
ues. Flowing He (100 standard cm3/min) was used for TPD,
and 20% O2/He (100 standard cm3/min) was used for TPO.
Desorption Amounts during PCD
Desorption amount ( mol/g catalyst)
Reaction
time
Adsorbed
molecule
Other
organics
Total
C
(min)
H2
CH4
CO2
2 wt% Pt/TiO2
Methanol
t-butanol
Formic acid
Acetic acid
Propionic acid
Methyl formate
Methyl acetate
t-Butyl acetate
Acetaldehyde
Acetone
15
15
15
30
40
50
15
15
10
15
224
173
105
237
15
31
15
337
158
520
967
206
182
99
70
RESULTS
158
257
335
206
92
68
17
9
13
316
Photocatalytic Decomposition of Alcohols
313
A monolayer of methanol readily decomposed on 2%
Pt/TiO2 when illuminated with UV light, and the main reac-
tion was dehydrogenation to form H2 and a surface species,
but CO2 also formed at a low rate. As shown in Fig. 1, H2
formed almost immediately upon illumination, and its rate
was as much as 50 times higher than the rate of CO2 forma-
tion. Although the rate of H2 formation quickly reached a
maximum, the rate of CO2 increased much more slowly
92
90
69
31
17
51
31
22
0.2 wt% Pt/TiO2
Methanol
Ethanol
Formic acid
Acetic acid
10
10
10
10
15
35
106
82
80
21
Trace
87
71
231
100
21
Trace
87
147
944
100
68
4
236
and was still increasing when the lights were turned off Propionic acid
after 10 min. The amounts of CO2 and H2 that desorbed
Methyl formate
243
during PCD are given in Table 1. The TPD spectra fol-
lowing methanol PCD were similar to the TPD spectra of
illuminated after 30 min in the dark, the H2 formation rate
doubled, and the CO2 rate was approximately 1.7 times
higher than the rate before the dark time. The CO2 rate
increased with time before the dark time, but it decreased
when PCD was restarted after the dark time (Fig. 1).
Ethanol PCD was similar to that for methanol; only
gas-phase H2 and CO2 formed, and the H2 rate was 90
times higher than the CO2 rate. The maximum H2 for-
mation rate during ethanol PCD was 0.5 mol/g cata-
lyst/s. As with methanol, no PCD reaction was detected
on unplatinized TiO2. During TPD after ethanol PCD on
Pt/TiO2, unreacted ethanol and its decomposition products,
unreacted methanol, and the amounts desorbed are given
in Table 2. Methanol PCD on 0.2% Pt/TiO2 was similar to
PCD on 2% Pt/TiO2, but the areal H2 formation rate (nor-
malized by saturation methanol coverage) was about 1.4
times higher on the 2% catalyst. The CO2 formation rate,
however, was higher by a factor of 1.6 on the 0.2% Pt/TiO2
(Table 3). Methanol did not react to a measurable extent
during PCD on unplatinized TiO2.
The effect of dark time on PCD was studied by turning
the UV lights off and then re-illuminating the catalyst after
a period of dark time. In Fig. 1 when the catalyst was re-
TABLE 2
Desorption Amounts during TPD and TPO
Amount desorbed during TPD
(
mol/g catalyst)
Adsorbed
molecule
Other
TPO
Total
C
H2 CO2 CO CH4 organics (CO2)
2 wt% Pt/TiO2
Methanol
t-butanol
250
49
128
71
94
44
162
97
132
118
48
196
63
109
120
292
93
7
49
36
48
4
98
<1
24
28
106
126
337
427
271
313
694
397
387
Formic acid
Acetic acid
Methyl acetate
Methyl formate 100
t-Butyl acetate 82
72
119
265
41
40
37
83
6
0.2 wt% Pt/TiO2
152 214
Methyl formate 251
29
11
435
FIG. 1. Photocatalytic decomposition of methanol on 2% Pt/TiO2.

306
BLOUNT, BUCHHOLZ, AND FALCONER
TABLE 3
Maximum Areal Rates for 0.2 and 2% Pt/TiO2
Maximum rates 104
(
mol/g catalyst/s)/( mol/g)
0.2 wt% Pt/TiO2 2 wt% Pt/TiO2
CO2
Adsorbed
molecule
H2
CO2
H2
Methanol
Formic acid
Acetic acid
19
14
—
13
1.4
14
21
5
26
13
—
15
0.89
18
39
9
Methyl formate
acetaldehyde, ethylene, CO2, CO, and H2O, desorbed. The
PCD of acetaldehyde was performed because acetaldehyde
may be a decomposition product during ethanol PCD (23,
24). No H2 was detected during PCD of a monolayer of
acetaldehyde, but as shown in Fig. 2, gas-phase CO2 and
acetaldehyde desorbed.
FIG. 3. Photocatalytic decomposition of t-butanol on 2% Pt/TiO2.
Photocatalytic decomposition of t-butanol was per-
formed because, unlike the primary alcohols, no hydrogens
are attached to the alkoxy carbon (–C–OH). During PCD
of t-butanol, CH4 was the main product, as shown in Fig. 3,
but CO, CO2, and butane were also detected. Acetone des-
orbed during TPD after t-butanol PCD, as did unreacted
t-butanol and its decomposition products. No acetone des-
orbed during TPD of t-butanol when PCD had not been
performed prior to TPD. Acetone, like acetaldehyde, was
relatively unreactive during PCD on 2% Pt/TiO2 (Fig. 4);
only small amounts of CH4 and CO2 formed. The rate of
CH4 formation during acetone PCD was less than 1/10th of
the CH4 formation rate during t-butanol PCD.
Photocatalytic Decomposition of Carboxylic Acids
Formic acid decomposed to form H2 and CO2 during
PCD on 2% Pt/TiO2 (Fig. 5). The CO2 and H2 peak shapes
FIG. 4. Photocatalytic decomposition of acetone on 2% Pt/TiO2.
FIG. 2. Photocatalytic decomposition of acetaldehyde on 2% Pt/TiO2.
FIG. 5. Photocatalytic decomposition of formic acid on 2% Pt/TiO2.

PHOTOCATALYTIC DECOMPOSITION OF ALCOHOLS
307
were similar, but the CO2 rate was 1.4 times the H2 rate. The
remainder of the hydrogen in the formic acid formed H2O,
which was strongly adsorbed on TiO2 and desorbed during
the subsequent TPD. The TPD after formic acid PCD was
characteristic of unreacted formic acid and H2O, and the
carbon and hydrogen mass balance for the PCD and TPD
were within 10% . In Fig. 5, after 30 min of PCD, the lights
were turned off. When the catalyst was re-illuminated after
30 min in the dark, the CO2 and H2 formation rates were 1.8
and 1.3 times higher, respectively, than immediately before
the dark time. On the 0.2% Pt/TiO2, PCD was similar to that
on 2% Pt/TiO2, but the CO2 was approximately equal to the
H2 rate on the 0.2% Pt/TiO2 and the maximum formation
CO2 formation rate was 0.45 mol/g catalyst/s.
On TiO2 without Pt, only CO2 desorbed during formic
acid PCD, and no H2 formed. The maximum CO2 rate on
TiO2 was only 20% of the rate on 2% Pt/TiO2. Water that
formed during PCD was observed during TPD after PCD
on TiO2, and the TPD spectra were typical of unreacted
formic acid and H2O.
Gas-phase CO2, CH4, and ethane formed immediately
upon illumination during PCD of acetic acid on 2% Pt/TiO2
(Fig. 6), but no H2 was observed. After 25 min of PCD
and a 30-min dark time, the CO2 and CH4 formation rates
were 4 and 3.3 times higher, respectively, when the catalyst
was re-illuminated than immediately before the dark time.
On 0.2% Pt/TiO2, the CO2 rate was one-third and the CH4
rate was one-half of the corresponding rates on 2% Pt/TiO2
(Table 3). The CO2/CH4 ratio was 1.4 for the 2% Pt/TiO2.
The remainder of the carbon formed ethane, and the carbon
mass balance was within 5% for the PCD on 2% Pt/TiO2.
The CO2/CH4 ratio was roughly 1 for the 0.2% Pt/TiO2, and
less ethane formed than on the 2% catalyst. The carbon
mass balance on 0.2% Pt/TiO2 was within 10% .
FIG. 7. Photocatalytic decomposition of propionic acid on 2% Pt/TiO2.
tane formed, but no H2 formed during PCD on 2% Pt/TiO2
(Fig. 7). Though the CO2 rate was approximately the same
for both 0.2 and 2% Pt/TiO2, the C2H6 rate was 1.5 times
higher on the 0.2% Pt/TiO2 catalyst. As with acetic acid,
the CO2/C2H6 ratio was 1.4 for the 2% Pt/TiO2 and 1 for
the 0.2% Pt/TiO2. The remainder of the carbon formed bu-
tane, and the carbon mass balance for both catalysts was
within 6% .
Photocatalytic Decomposition of Esters
During PCD of methyl formate (HCOOCH3) on 2%
Pt/TiO2, H2 and CO2 formed immediately upon illumina-
tion, as shown in Fig. 8, but no CH4 or C2H6 was observed.
The H2 rate was approximately 1.7 times the CO2 rate. After
a 30-min dark time, the CO2 and H2 formation rates were
3.2 and 3.8 times greater than immediately before the dark
Propionic acid (CH3CH2COOH) reacted through simi-
lar pathways to those of acetic acid; CO2, ethane, and bu-
FIG. 8. Photocatalytic decomposition of methyl formate on 2%
Pt/TiO2.
FIG. 6. Photocatalytic decomposition of acetic acid on 2% Pt/TiO2.

308
BLOUNT, BUCHHOLZ, AND FALCONER
FIG. 9. Temperature-programmed desorption after methyl formate
FIG. 11. Photocatalytic decomposition of methyl acetate on 2%
PCD on 2% Pt/TiO2.
Pt/TiO2.
CO2, and CH4 desorbed. No measurable reaction occurred
during methyl acetate PCD on TiO2 (16, 35, 37).
time, respectively. During TPD after PCD, CO, CO2, and
H2 desorbed at high temperatures, and a small amount
of ethanol and ethane (not shown for clarity) desorbed
(Fig. 9). These spectra, with the exception of the ethane
and ethanol formation, are similar to TPD of formic acid
(Fig. 10). The desorbing species may also be from the de-
composition of unreacted methyl formate or from the de-
composition of other unidentified intermediates remaining
on the catalyst surface.
During PCD of methyl formate on 0.2% Pt/TiO2, the CO2
formation rate was 55% and the H2 rate was 85% of the
corresponding rates on the 2% Pt/TiO2 catalyst (Table 3).
Methyl acetate (CH3COOCH3) decomposed on 2%
Pt/TiO2 to form H2, CH4, and CO2 at similar rates (Fig. 11),
and a smallamount ofethane (not shown) also formed. Dur-
ing the TPD after PCD, unreacted methyl acetate, H2, CO,
Photocatalytic decomposition of t-butyl acetate
(CH3COOC(CH3)3) formed CH4, isobutylene, acetone,
and CO2, as shown in Fig. 12. The CH4 and isobutylene
rates immediately reached maxima, whereas acetone and
CO2 reached maximum rates after a few minutes of PCD.
The CH4 and isobutylene rates also decreased faster than
the acetone and CO2 rates. After a 30-min dark period,
the CH4 and acetone rates increased only slightly, and no
increase was observed for isobutylene and CO2.
During TPD after PCD of t-butyl acetate (Fig. 13), ace-
tone desorbed in both low and high temperature peaks
typical of acetone adsorbed on TiO2 (28). In addition,
isobutylene, CO, CO2, H2, and CH4 desorbed. Because CO,
CO2, H2, and CH4 are all weakly adsorbed on TiO2, their
FIG. 10. Temperature-programmed desorption of adsorbed formic
FIG. 12. Photocatalytic decomposition of t-butyl acetate on 2%
acid on 0.2% Pt/TiO2.
Pt/TiO2.

PHOTOCATALYTIC DECOMPOSITION OF ALCOHOLS
309
Moreover, liquid 2-propanol was reported to decompose
photocatalytically on Pt/TiO2 to form H2, acetone, and a
small amount of CO2. Nishimoto et al. (17) observed the
formation of H2 and formaldehyde during PCD of aqueous
methanol on Pt/TiO2. Formaldehyde on TiO2 decomposes
during TPD to form CO and H2O (31), and it is expected to
decompose faster on Pt/TiO2, probably forming CO and H2.
Thus, formaldehyde was not likely to desorb during TPD
after PCD of methanol.
The aldehydes oxidized photocatalytically to form CO2,
H2O, and other products that remain on the surface, but
they are much less reactive than the alcohols. Thus much
less CO2 formed than H2 during methanol and ethanol
PCD; indeed, the initial ratios of H2 to CO2 were 50 to 90,
respectively. The CO2 rate was higher during acetaldehyde
PCD (Fig. 2) than during ethanol PCD because ethanol
first decomposes to acetaldehyde. Likewise, approximately
6 times more CH4 than CO2 formed during t-butanol PCD
(Fig. 3), and most of the CH4 was from t-butanol and not
acetone decomposition, since only small amounts of CH4
formed during acetone PCD (Fig. 4). Similarly, all of the H2
formed during ethanol PCD was from ethanol, since H2 did
not form during acetaldehyde PCD (Fig. 2).
To form CO2 from the aldehydes, additional oxygen was
required, and it was most likely extracted from the TiO2
lattice to form a reduced surface (16, 35, 39). Indeed, when
the Pt/TiO2 was held in the dark after PCD, the rate of CO2
formation from methanol PCD was almost twice as high
when the lights were turned back on (Fig. 1), as expected if
lattice oxygen diffused from the TiO2 bulk during the dark
time (35). The rate of H2 formation also increased after the
dark time even though H2 formation does not need lattice
oxygen; apparently alcohol PCD is faster on oxidized TiO2.
In addition to the alcohols decomposing to form aldehy-
des, which then more slowly oxidized to CO2, t-butanol had
a parallel reaction pathway during PCD to form isobutane
and CO. Much less t-butanol reacted through this pathway
than through reaction [3].
FIG. 13. Temperature-programmed desorption after t-butyl acetate
PCD on 2% Pt/TiO2.
appearance at high temperatures during TPD was due to
the decomposition of unreacted t-butyl acetate or another
intermediate remaining on the catalyst surface. Acetone is
strongly adsorbed on TiO2 (28), so it was probably displaced
from the surface during PCD by either water or another in-
termediate that formed. Isobutylene is not expected to ad-
sorb strongly on TiO2. Thus, the isobutylene that desorbed
during TPD may be from the decomposition of unreacted
t-butyl acetate or another intermediate.
Photocatalytic decomposition of phenyl acetate
(CH3COO–C6H5) was performed because the hydrogens
in the phenyl group are expected to be too strongly bound
to participate in reaction. During PCD of phenyl acetate
on 2% Pt/TiO2, only a trace amount of CO2 formed, and
no other gas-phase species formed.
DISCUSSION
Reactions during Photocatalytic Decomposition
During the first 10 min of methanol PCD on 2% Pt/TiO2
(Fig. 1), the CO2 rate increased with time. Since the CO2
formed from formaldehyde oxidation, the CO2 rate ap-
parently increased as the formaldehyde concentration in-
creased. In addition, the first CO2 that formed may have
The alcohols studied appear to decompose photocatalyt-
ically on Pt/TiO2 to their corresponding aldehydes:
CH3OH → H2 + CH2O(ads)
CH3CH2OH → H2 + CH3CHO(ads)
(CH3)3COH → CH4 + (CH3)2CO(ads)
[1]
[2] adsorbed on TiO2, since a small amount of CO2 adsorbs on
TiO2; as the sites for CO2 adsorption were saturated, CO2
.
[3]
appeared in the gas phase.
Note that for reactions [1]–[3] and also for PCD of
2-propanol reported in the literature (17–22), H2 was the
primary reaction product if the alcohol possessed a hydro-
gen bonded to the -carbon. Since t-butanol does not, it did
not form H2, but instead split off a CH3 group, which was
hydrogenated. The general reactions are
The aldehydes are concluded to form because acetone was
detected during TPD following PCD of t-butyl alcohol, and
acetone was not detected during TPD of adsorbed t-butanol
when no PCD had been performed. In addition, previous
PCD studies of gas- and liquid-phase alcohols on Pt/TiO2
have observed formation of the corresponding aldehydes
(17–19, 23, 24). Pichat et al. (23, 24) observed acetalde-
hyde formation during PCD of liquid ethanol on Pt/TiO2.
Rx CH yOH → H2 + aldehyde if y = 1–3
[4]

310
BLOUNT, BUCHHOLZ, AND FALCONER
oxygen was consumed during PCD on Pt/TiO2, and thus the
Rx COH → RH + aldehyde where R is not H. [5]
effect of the reduced surface on reaction [9] was visible.
The esters followed similar reaction pathways during
PCD on Pt/TiO2:
The same trend was observed for the organic acids; H2
only formed if H atoms were bonded to the -carbon, and
otherwise alkanes formed
HCOOCH3 → H2 + CO2 + intermediate
CH3COOCH3 → H2 + CO2 + CH4 + intermediate. [15]
[14]
HCOOH → H2 + CO2
CH3COOH → CH4 + CO2
CH3CH2COOH → C2H6 + 2CO2.
[6]
[7]
[8]
Some adsorbed intermediate, perhaps an aldehyde such
as formaldehyde, also formed during methyl acetate and
methyl formate PCD. Two reaction pathways were ob-
served for t-butyl acetate:
Thus, the general reaction for the acids is
RCOOH → RH + CO2.
[9]
CH3COOC(CH3)3 + O(l) → 2CH4 + (CH3)2CO + CO2
[16]
In addition, the acids all exhibited a parallel pathway
through which a small fraction of the acid reacted. This
pathway was the same as that reported previously for PCD
of formic and acetic acid on TiO2 in which lattice oxygen is
extracted to form H2O (16, 35, 37):
CH3COOC(CH3)3 → CH4 + CH2C(CH3)2 + CO2. [17]
Reaction [16] requires additional hydrogen from either hy-
drogen radicals or water to balance the chemical equation.
As observed for the alcohols and acids, H2 only formed
from esters during PCD when H atoms were bonded to
one of the -carbons. For methyl formate, the H bound to
the acid carbon and H in the methyl group both formed
H2, so H2 formed at a higher rate during PCD of methyl
formate than methyl acetate. The maximum areal rate for
H2 formation during methyl formate PCD was 1.2 and 1.6
times, respectively, the maximum H2 rate for formic acid
and methyl acetate PCD on 2% Pt/TiO2.
HCOOH + O(l) → H2O + CO2
2CH3COOH + O(l) → H2O + C2H6 + 2CO2
2CH3CH2COOH + O(l) → H2O + C4H10 + 2CO2. [12]
[10]
[11]
On Pt/TiO2, this secondary pathway has the general form:
2RCOOH + O(l) → R2 + 2CO2 + H2O.
[13]
The general reaction for esters is
Note that for reactions[9]and [13], the -carbon formsCO2.
Reactions [7], [10], and [11] were also observed for PCD of
formic and acetic acid on TiO2 (16, 35, 37), but H2 was only
observed when Pt was on the surface. On TiO2, isotope
labeling showed that only the -carbon formed CO2 during
PCD. The mass balances for acetic acid and propionic acid
products on Pt/TiO2 are also consistent with the -carbon
forming CO2 and the R groups forming alkanes.
The acetic acid PCD rates reported on TiO2 by Muggli
et al. (16, 37) were an order of magnitude lower than those
observed in this study on Pt/TiO2. Note, however, that the
light intensity used in these previous studies was approxi-
mately an order of magnitude lower than for this study. Af-
ter a 10-min PCD of acetic acid on TiO2 and a 7-min dark
period, the CH4 rate was the same upon re-illumination as
it was immediately before the dark period, but the CO2 and
C2H6 rates had increased (16). In our study, all the rates in-
creased after a dark period following acetic acid PCD on
2% Pt/TiO2. Additionally, all of the rates increased after a
dark period during acetic acid PCD on TiO2 with the higher
light intensity (40). Apparently, reaction [7] was slowed on
the reduced surface, even though it does not require lattice
oxygen. Because the rates were significantly higher and the
reaction time was longer on the Pt/TiO2 catalyst in the cur-
rent study than on TiO2 in the previous studies, more lattice
´
RCOOCHx R y + O(l) → RH + CO2 + H2 + intermediate
if x = 1–3 [18]
RCOOCR y + O(l) → RH + CO2 + aldehyde + alkene
where R is not H. [19]
In addition, the esters appear to react through a parallel
pathway similar to that observed for acids, but only a small
fraction of the ester reacted through this pathway:
´
2RCOOCHx R y + O(l)
→ R2 + CO2 + H2O + intermediate if x = 1–3. [20]
Reaction Mechanisms
Ait-Itchou et al. (18) and Pichat et al. (24) in their studies
of gas-phase isopropanol PCD on Pt/TiO2 proposed that
the reaction is initiated by the reaction of a generated hole
with an adsorbed isopropanol molecule:
TiO2 + hv → h+ + e
(CH3)2CHOH(ads) + h+ → (CH3)2CHO(ads) + H(+ads)
.

PHOTOCATALYTIC DECOMPOSITION OF ALCOHOLS
311
SCHEME 3
esters. Because phenyl acetate was unreactive, this suggests
that a H attached to –C–O– or a H attached to –R–C–O–,
where R isan alkylgroup, isnecessaryfor PCD to occur, and
Schemes 2 and 3 are not significant pathways. In addition,
because an alkene did not form during PCD of ethanol or
t-butanol, abstraction of a H from the R groups attached to
the -carbon does not appear to be a significant pathway
(Scheme 2).
For t-butyl acetate, however, Scheme 1 does not fully
explain all the reaction products observed. By Scheme 1,
t-butyl acetate would react to isobutylene, CO2, and CH3,
which could then scavenge a H to form CH4:
SCHEME 1
In their mechanisms, the H+ combine on electron-rich Pt
particles to form H2, and (CH3)2CHO. reacts to acetone and
H2. The decomposition products observed during PCD are
consistent with initiation by abstraction of a H (Schemes 1
and 2) or the addition of a nucleophile to the organic
molecule (Scheme 3). Although the catalyst was heated to
730 K in an O2/He flow before each experiment, the TiO2
could still be partially covered with hydroxyl groups (41,
42). Hydrogen abstraction could occur from reaction with
a hydroxyl radical, direct reaction with an electron or hole,
or reaction with some other nucleophile. Several pathways
are possible depending on which H is abstracted.
Phenyl acetate was unreactive during PCD on Pt/TiO2.
Since the hydrogenson the phenylgroup are stronglybound
(43), these hydrogens are unavailable for abstraction. Thus,
phenyl acetate does not decompose through Scheme 1 for
In addition to these products, acetone also formed during
t-butyl acetate PCD. Both Schemes 2 and 3 explain acetone
formation. Acetone would form by Scheme 2:
And Scheme 3 would form acetone by
Thus, though PCD initiation may require H attached to
–R–C–O–, where R is an alkyl group, Schemes 2 and/or 3
appear to also play a role during PCD for t-butyl acetate.
Because the C–H bond is weaker than the C–C bond,
H2 forms when H is bound to the -carbon, and alkanes
are not formed. However, when no H is attached to the
-carbon, as is the case for t-butanol, acetic and propionic
acids, methylacetate, and t-butylacetate, the onlydecompo-
sition pathway available is alkane formation. For example,
SCHEME 2

312
BLOUNT, BUCHHOLZ, AND FALCONER
during ethanol PCD, H2 forms but no methane is observed be explained by spillover of hydrogen atoms from the TiO2
because the C–H bond is easier to break than the C–C bond: surface onto the Pt particles where the hydrogen combined
to form H2 (44). When the Pt loading was below 1.5% , mi-
gration ofhydrogen to the Pt particleswasrate determining.
Table 3 compares the areal formation rates on 0.2 and 2%
Pt/TiO2 for methanol, formic acid, acetic acid, and methyl
formate. DuringmethanolPCD, the maximum arealH2 rate
on 2% Pt/TiO2 was almost 1.5 times that on 0.2% Pt/TiO2,
By Scheme 1, methyl formate is expected to decompose to
formaldehyde, H., and CO:
but the maximum CO2 areal rate was only 60% of that
on the 0.2% Pt/TiO2. Increasing the Pt loading increased
methanol dehydrogenation to formaldehyde by providing
more sites for H atom combination. The formaldehyde ox-
idation rate decreased, however, with Pt loading, perhaps
because Pt blocks lattice oxygen sites. Linsbigler et al. (45)
observed that Pt, when deposited on TiO2, titrated oxygen
vacancy sites and thus the CO oxidation rates were lower
at low temperature. The oxygen vacancy sites were regen-
erated at high temperatures as the Pt clustered. With more
Pt on the surface, more of the sites responsible for aldehyde
oxidation may be covered, thus lowering the oxidation rate.
For formic acid PCD, an increase in the Pt loading in-
creased the areal CO2 rate but had essentially no effect
on the H2 rate. The CO2 and H2 signals are similar during
formic acid PCD, and the only other species presumably
formed is H2O. Thus, the overall rate of formic acid decom-
position increased with the higher Pt loading, and appar-
ently the rate of H2O formation also increased. For methyl
formate, the higher Pt loading increased the CO2 forma-
tion rate by 80% , but only increased the H2 rate by 20% .
As with formic acid, the overall rate of methyl formate in-
creased with the higher loading of Pt, and the overall rate
of H2O formation is presumed to increase also.
The higher loading of Pt increased the rate of acetic acid
decomposition, and the CO2/CH4 ratio also increased. Be-
cause the results on both 2 and 0.2% Pt/TiO2 are consistent
with the alkoxy carbon forming CO2 and the methyl carbon
forming CH4 and ethane, the increase in the CO2/CH4 ratio
suggests that the rate of pathway 2 proposed by Muggli and
Falconer (16) increased. Thus, more ethane was formed at
the expense of CH4. A similar effect was observed for pro-
pionic acid. Thus, for all acids the PCD rates were higher
for higher Pt loadings.
As with the other aldehydes, formaldehyde is expected to
react slowly during PCD to CO2 and H2O. No CO was de-
tected during methyl formate PCD; however, any CO that
forms may extract lattice oxygen and oxidize to CO2, which
was detected during PCD.
Effect of Platinum
Platinum has a dramatic effect on the rate of PCD. For
example, methanol and ethanol show almost no reactiv-
ity on TiO2 in UV light in the absence of O2. On Pt/TiO2,
however, H2 formed at high rates as the alcohols dehydro-
genated to the corresponding aldehydes. The rate is higher
apparently because H atoms spill over from the TiO2 sur-
face and combine on the Pt surface and H2 desorbs. On
TiO2, the rate of H atom recombination is low; since TiO2
is not a good H2 dissociation catalyst, it is not effective at
recombining H atoms.
Muggli and Falconer (35) reported that formic acid de-
composed on TiO2 to form CO2 and H2O during PCD
through the overall reaction:
HCOOH(ads) + O(lattice) → CO2(g) + H2O(ads)
With the addition of Pt, a significant amount of H2 formed
during PCD, and the areal CO2 formation rate increased
by a factor of 4 (Fig. 5). Muggli and Falconer (16) pro-
posed two parallel pathways for acetic acid PCD on TiO2:
CH3COOH(ads) → CO2(g) + CH4(g)
2CH3COOH(ads) + O(lattice)
[1]
Effect of Dark Time
Muggli and Falconer (16, 35) concluded that lattice oxy-
gen was consumed during acetic acid and formic acid PCD.
→ C2H6(g) + 2CO2(g) + H2O(ads)
.
[2]
They proposed that dissociation of the O–H bond was the They showed that following PCD, oxygen diffused from
first step in PCD. Hydrogen abstraction also appears to be bulk TiO2 to the surface in the dark and replenished the
the first step during PCD on Pt/TiO2.
lattice oxygen. Thus, the PCD rate was higher after an ex-
Ait-Ichou et al. (18) found that larger loadings of Pt tended dark time. This same behavior was observed on
dispersed on the TiO2 surface increased the PCD rate of Pt/TiO2 for formic and acetic acid PCD, indicating that sim-
2-propanol until a maximum was reached. Past this loading ilar reaction pathways take place on Pt/TiO2.
of 1.5 wt% Pt, the H2 formation rate decreased with in-
The PCD rates on Pt/TiO2 also increased after an ex-
creased Pt loading. They proposed that the behavior could tended dark time for propionic acid, alcohols, methyl

PHOTOCATALYTIC DECOMPOSITION OF ALCOHOLS
313
formate, and methyl acetate. Note that the PCD rate of Esters also react through two parallel pathways. The first
the alcohols was much lower in the absence of Pt. The mass pathway is
32 signal did not change during or after PCD, indicating
that an O2 impurity did not affect the PCD rates. Presum-
ably, the only PCD product from the acids that required
RCOOCHx R y + O(l) → RH + H2 + CO2
if x = 1–3 [5]
additional oxygen was H2O, but both CO2 and H2O pro-
duced during alcohol PCD required additional oxygen. The
formation of H2O and CO2 consumed lattice oxygen and
reduced the TiO2. During the dark time, lattice oxygen was
RCOOCR y + O(l) → RH + CO2 + aldehyde + alkene
where R is not H [6]
replenished, and the surface was reoxidized so that the rates
were higher when the lights were turned back on. The PCD
and the second pathway is
rate after the dark period was not as high as the initial PCD
rate because less organic was on the surface and because
2RCOOCHx R y + O(l) → R2 + 2CO2 + H2O. [7]
the dark time was not long enough for the complete oxida-
tion of the catalyst surface. The PCD rates of pathways that
did not consume lattice oxygen also increased following a
dark time, apparently because the PCD rates were higher
on oxidized TiO2 than on reduced TiO2. During PCD of
t-butyl acetate, only the CH4 and acetone rates increased
after a 30-min dark period, and the CO2 and isobutylene
rates were the same as immediately before the dark time.
In the mechanism proposed for t-butyl acetate PCD, lattice
oxygen is only required for the pathway that produces ace-
tone; thus, the isobutylene rate is not expected to increase
after a dark period.
For methyl formate and methanol, the relative ratios of
the desorbing products were slightly different immediately
before the dark time and when the PCD was restarted. Im-
mediately before the dark time, the H2/CO2 ratio was 1.5
for methyl formate, and the ratio changed to 1.7 after the
catalyst was re-illuminated. A similar change was observed
for the CO2/H2 ratio during methanol PCD. After 30 min
of dark time, the catalyst surface was not completely reox-
idized. With this partial surface reduction, the rate of one
step of the reaction mechanism, such as dehydrogenation,
may increase more than the rate of another step of the reac-
tion mechanism resulting in a change in the product ratios.
The PCD pathways are consistent with a reaction mecha-
nism whose first step is hydrogen atom abstraction. Plat-
inum increased the PCD rates dramatically for some reac-
tants, and H2 formed by spillover of H atoms from TiO2 to
Pt. Hydrogen formed during PCD only when H was bound
to an alkoxy carbon. Lattice oxygen was consumed during
PCD, and diffusion of bulk oxygen in the dark replenished
the lattice oxygen.
ACKNOWLEDGMENTS
We gratefully acknowledge support by the National Science Founda-
tion, Grant CTS-9714403, and J.A.B. acknowledges support by the NSF-
REU Site Grant EEC-9820477. We also thank Professor G. Barney Ellison
and Dr. Steve Blanksby of the Chemistry Department at the University
of Colorado for valuable discussions.
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