Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

Article abstract of DOI:10.1021/jacs.7b05184

The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe₂O₃ photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe₂O₃ to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO₂ photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe₂O₃. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe₂O₃ is retarded ～20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.

Full text of DOI:10.1021/jacs.7b05184

Subscriber access provided by University of Florida | Smathers Libraries
Article
Kinetics of Photoelectrochemical Oxidation
of Methanol on Hematite Photoanodes
Camilo A. Mesa, Andreas Kafizas, Laia Francàs, Stephanie R. Pendlebury, Ernest Pastor,
Yimeng Ma, Florian Le Formal, Matthew Mayer, Michael Grätzel, and James R. Durrant
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05184 • Publication Date (Web): 22 Jul 2017
Downloaded from http://pubs.acs.org on July 22, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Current/potential response of the measured photoanodes under simulated 1 sun
illumination, a) αꢀFe2O3 under dark (black dashed line) and front illumination conditions
(electrode/electrolyte), measured in 0.1M NaOH aqueous solution (black) and 0.1M NaOH in
95% methanol (red) and b) αꢀFe2O3 (red) and anatase TiO2 (blue) photoanodes measured in
0.1M NaOH in 95% methanol under chopped illumination.
130x184mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Formaldehyde evolution calculated with Faradaic efficiency of unity from the bulk
electrolysis (black line) and quantified from the calibration curve (violet circles).
180x130mm (96 x 96 DPI)
ACS Paragon Plus Environment

Page 3 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Oxidation of 95% methanol in 0.1 M aqueous NaOH on hematite during 5s on/5 s off
pulsed 365 nm illumination conditions at 0.55 VAg/AgCl, a) photoinduced absorption of excited
species (h+) probed at 650 nm and b) transient photocurrent measured simultaneously.
136x183mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Rate law analysis, photocurrent density, Jsur
and surface hole density, dps, of the
holes
oxidation of methanol on αꢀFe2O3 at 0.55 V (dark red) and 0.00 V (light red), and TiO2 (blue) at ꢀ
0.80 V applied potentials.
194x138mm (96 x 96 DPI)
ACS Paragon Plus Environment

Page 5 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Kinetic isotopic effect (KIE) of the oxidation of methanol on αꢀFe2O3. Rate law analysis,
photocurrent density, Jsur
and surface hole density, dps, of the oxidation of CH3OH at 0.00 V
holes
(light red) and at 0.55 V (dark red) and CD3OD at 0.55 V (purple).
145x107mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Plausible mechanism of methanol oxidation on αꢀFe2O3 where the R.L.S. requires
oxidations by two valence band holes and involves CꢀH bond breaking, leading to reꢀhybridization
of the carbon to produce formaldehyde with a Faradaic efficiency of unity; h+ refers to surface
valence band holes (Fe(IV)=O species not adjacent to adsorbed methanol).
157x189mm (96 x 96 DPI)
ACS Paragon Plus Environment

Page 7 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Kinetics of Photoelectrochemical Oxidation of Methanol on Hema-
tite Photoanodes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Camilo A. Mesa,† Andreas Kafizas,† Laia Francàs,† Stephanie R. Pendlebury,† Ernest Pastor,†,⊥
Yimeng Ma,† Florian Le Formal,†, § Matthew Mayer,§ Michael Grätzel§ and James R. Durrant*,†
†Department of Chemistry, Imperial College London, South Kensington Campus, London, SW7 2 AZ, UK
§Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, Station 6, CH-1015 Lau-
sanne, Switzerland
KEYWORDS: Semiconductors, Organic Substrates Oxidation, Rate Law Analysis, Kinetic Isotopic Effect
ABSTRACT: The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3
photoanodes are studied using photo-induced absorption spectroscopy and transient photocurrent measurements. Meth-
anol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady
state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes
on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar se-
cond order kinetics for methanol oxidation with a second order rate constant two orders of magnitude higher than on α-
Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ~ 20
fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under one sun irradia-
tion on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde
and concomitant hydrogen evolution.
INTRODUCTION
used to scavenge holes in metal oxide photoanodes such
as TiO28,9 and α-Fe2O3.10 (Photo)electrochemical oxidation
of methanol to formaldehyde has been reported, although
with only a low Faradaic efficiency,11,12 with commercial
formaldehyde synthesis from methanol primarily being
achieved at high temperatures on iron molybdate cata-
lysts.13 However, very little consideration has been given
to the key kinetic and thermodynamic parameters con-
trolling the rate limiting step (R.L.S.) of PEC oxidation of
methanol and their implications in the system efficiency,
i.e. rate and yield of reaction.
Electrochemical and photoelectrochemical (PEC) pro-
cesses are widely used to drive organic oxidation reac-
tions, with applications including molecular syntheses,
photocatalytic pollutant destruction and photoelectro-
chemical hydrogen generation (i.e. water splitting). For
example, in PEC hydrogen generation, organic substrate
oxidation can replace water oxidation as a source of elec-
trons for proton reduction. In such systems, oxidation of
sacrificial organic molecules has been shown to increase
hydrogen generation yields, avoiding the kinetic limita-
tions of water oxidation.1,2 Additionally, selective PEC
oxidation of industrial by-products can be used to synthe-
size higher value products. Different cases include the
selective oxidation of glycerol to produce dihydroxyace-
tone,3 the synthesis of hydrogen and aldehydes or ketones
from biomass4 and epoxides from alkenes.5 For example,
Yao and co-workers6 have recently reported a highly se-
lective oxidation of various benzyl alcohols on H-titanate
nanotubes. However, using photogenerated charge carri-
ers to drive these processes can be challenging in terms of
production yields and selectivity, often with only limited
understanding of reaction mechanisms.7
This paper focuses on methanol oxidation, as a model
oxidation reaction, on a widely studied photoanode mate-
rials, α-Fe2O3, and a comparison of the kinetics of this
reaction on an alternative photoanode material, TiO2. The
use of hole scavengers such as methanol has been shown
to be an effective strategy to reduce electron/hole recom-
bination losses in such metal oxides, as an alternative to
the application of anodic potentials.10,14 Methanol oxida-
tion studies on semiconductors such ZnO and TiO2 have
indicated a methanol adsorption process followed by the
formation of a CH3O· radical and its subsequent oxidation
with a valence band hole. Other studies have provided
evidence for a photocurrent doubling mechanism where
methanol scavenging results in the formation of two long-
In the particular case of PEC hydrogen evolution, or-
ganic substrates such as methanol or ethanol have been
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 15
lived conduction band electrons per scavenged hole. Linear Sweep Voltammograms were measured in the
1
2
3
4
5
6
7
8
However, kinetic and mechanistic studies under operat-
ing conditions have received relatively little attention to
date, and are the subject of this paper.
dark and under electrode-electrolyte (EE; front side) illu-
mination conditions with a photon flux equivalent to ap-
proximately 100 mW.cm-2 (1 sun) provided by two 365 nm
LEDs (LZ1−10U600, LedEngin Inc). The scan speed was 20
mV.s-1 and the light was chopped at a frequency of 0.4 Hz.
In this study, we employ a rate law analysis to deter-
mine the key factors involved in the R.L.S of the methanol
oxidation reaction (MOR). Such factors include different
surfaces (metal oxides), the density of surface holes and
the kinetic isotope effect of deuterium. The approach,
following that recently employed by Le Formal et. al. for
water oxidation,15 employs photoinduced absorption
(PIA) spectroscopy to determine the density of surface
holes, and correlates this density with transient photocur-
rent (TPC) measurements under quasi-steady state condi-
tions of PEC methanol oxidation using long (5 s) pulsed
irradiation. We provide evidence, from rate law analyses,
that the kinetics of the oxidation of methanol on α-Fe2O3
and TiO2 are independent of the band bending at the
semiconductor-liquid junction, but are instead sensitive
to the choice of semiconductor and to the density of sur-
face holes. The results presented herein allow us to pro-
pose a kinetic model and a plausible mechanism for the
MOR on α-Fe2O3 that serves as a model for this oxidation
reaction on such metal oxide photoanodes.
Opto-electronic setup
Photo-induced absorption (PIA) spectroscopy allows
long-lived photogenerated species to be monitored under
pseudo steady-state conditions. The PIA signal is propor-
tional to the density of holes accumulated at the surface.
Simultaneously, the transient photocurrent (TPC) signal
is measured in the PEC cell, by converting the potential
difference between the photoanode and the counter elec-
trode (as measured across a 98.7 Ohm resistor) into cur-
rent using Ohm’s law. Therefore, the TPC signal provides
information on the extraction of electrons through the
external circuit. The PIA and TPC signals were measured
simultaneously for a 10 s period, with a 5 s on/5 s off 365
nm LED pulse. The PIA signal was measured by register-
ing the change in the optical density (absorption) of the
hematite after excitation by the UV light. The light inten-
sity of the LEDs was varied between 0.5 and 70 mW.cm-2
by applying a fixed current (from 0.05 to 0.70 A). This is
equivalent to a photon flux of 0.06-2.7 and 0.55-5.4 suns
for hematite and anatase, respectively, as calculated by
Ma et. al.,18 by integrating the solar power flux from the
lowest limit of the measured solar spectrum to the typical
absorption edge of hematite (600nm) and anatase
(380nm). Detailed information about the PIAS and TPC
measurement systems can be found in Le Formal et. al.15
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EXPERIMENTAL SECTION
Preparation of the semiconductor films
Silicon doped α-Fe2O3 photoanodes were prepared by
atmospheric pressure chemical vapor deposition
(APCVD), by a procedure detailed elsewhere.16 These
nanostructured 400 nm thick α-Fe2O3 films show a den-
dritic structure with a feature size of 5-10 nm at the sur-
face and roughness factor of 21.
Formaldehyde quantification
Formaldehyde, as methanol oxidation product, was
quantified by spectrophotometric measurements. A violet
colour is developed by reaction between formaldehyde
and 4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole, 4-
Amino-5-hydrazino-1,2,4- triazole-3-thiol (Purpald)
≥99%, Sigma-Aldrich. A calibration curve was prepared
from a concentration of 0 to 5 ppm of formaldehyde, ACS
reagent, 37% W, Sigma-Aldrich, following a method de-
veloped by Jacobsen et. al.19 The quantification was car-
ried out in a Perkin-Elmer Lambda 25 spectrophotometer,
measuring at 549 nm. The method is sensitive also to ac-
etaldehyde, propionaldehyde, butyraldehyde and benzal-
dehyde at different wavelengths; however, the only alde-
hyde expected from the oxidation of methanol is formal-
dehyde.
Mesoporous TiO2 photoanodes were grown from a col-
loidal anatase paste, according to the method developed
by Xiao-e et. al.17 Mesoporous films were produced on
FTO glass by doctor-blading the anatase using a k-bar
followed by heat treatment at 450 °C in air for 30 min.
These films were approximately 1 ꢀm thick with crystal-
lites of anatase ~40 nm wide and roughness factor of 120.14
Photo-electrochemical setup
A 3-electrode cell was used for photoelectrochemical
(PEC), photo-induced absorption (PIA) spectroscopy and
transient photocurrent (TPC) measurements. The electro-
lyte solution contained typically 0.1 M NaOH and 4% (for
TiO2) and 95% (for α-Fe2O3) volume methanol in deion-
ized water. For the kinetic isotopic effect (KIE) study, the
electrolyte solution contained 95% CD3OD in 0.1 M NaOD
in D2O. No concentrations higher than 98% methanol
were tested due to insolubility of NaOH in such high
methanol concentrations at room temperature. A Pt mesh
was used as the counter electrode and a silver/silver chlo-
ride (Ag/AgCl) saturated with KCl (E° = +0.197 V vs NHE)
as the reference electrode. All potentials are reported
against this Ag/AgCl electrode since the conversion of the
potentials from Ag/AgCl to the reversible hydrogen elec-
trode (RHE) in highly concentrated organic-aqueous solu-
tions might not be accurate.
RESULTS
Figure 1A shows a typical current/potential (J-V) re-
sponse of a nanostructured Si-doped hematite (APCVD α-
Fe2O3) in 0.1 M NaOH and in 95% methanol in 0.1 M
NaOH (see supporting information (SI), Figure S1, for the
photocurrent response on both α-Fe2O3 and TiO2 pho-
toanodes, as a function of methanol concentration). Un-
der simulated 1 sun illumination conditions, the photo-
current onset for the 95% methanol electrolyte, assigned
below to the methanol oxidation reaction, requires ap-
proximately 270 mV less oxidative potential than that for
ACS Paragon Plus Environment

Page 9 of 15
Journal of the American Chemical Society
water oxidation, consistent with previous studies.10,11,20
Additionally, the oxidation of methanol produces 3.9
1
2
3
4
5
6
7
mA.cm-2 photocurrent at strong anodic potential (0.55
VAg/AgCl) compared to 2.6 mA.cm-2 obtained from water
oxidation at the same applied potential. Both, the shift in
the onset potential and enhancement in the plateau pho-
tocurrent are likely due to the more facile oxidation of
methanol.10
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Formaldehyde evolution calculated with Fara-
daic efficiency of unity from the bulk electrolysis (black
line) and quantified from the calibration curve (violet
circles).
The product of methanol oxidation on hematite was de-
termined by spectrophotometric titration. Formaldehyde
was formed with a 96 % Faradaic efficiency as shown in
Figure 2 (see Figures S2 and S3 for further details on the
bulk electrolysis and the calibration curve). This high Far-
adaic efficiency indicates that formaldehyde is not further
oxidized to formic acid or carbon dioxide, under these
experimental conditions. A general equation correspond-
ing to the oxidation can be written as follows:
CH3OH + 2h+ → CH2O + 2H+
(1)
where h+ represents a surface hole on the photoanode.
The strikingly high Faradaic efficiency of this reaction on
hematite compared to an electrochemical route on Pt
(81%)23 and a photochemical process on TiO2, in methanol
concentrations under 1%, (30%)11 makes this an appealing
route for PEC formaldehyde synthesis.
In order to analyze the kinetics of methanol oxidation
on our hematite and titania photoanodes, PIA spectros-
copy and TPC measurements were conducted employing
variable intensities for a duration of 5 s at 365 nm, as de-
tailed in the experimental section. In these studies, the
PIA signal is employed to monitor the absorbance, and
therefore the density of long-lived photogenerated holes,
whilst the photocurrent density monitors the net flux of
holes transferred to the electrolyte in quasi-steady state
conditions (i.e. the rate of methanol oxidation). This ap-
proach follows that previously reported by Le Formal et.
al., where we have demonstrated that this PIA approach
allows us specifically to probe the accumulation of long
lived holes within the depletion region at the photoanode
surface, and therefore assay the surface density of holes
driving surface electrochemistry.15 For the PIA data, probe
wavelengths of 650 and 500 nm were employed for hema-
tite and titania photoanodes respectively, corresponding
to their valence band hole photoinduced absorption max-
ima.9,15
Figure 1. Current/potential response of the measured
photoanodes under simulated 1 sun illumination, a) α-
Fe2O3 under dark (black dashed line) and front illumi-
nation conditions (electrode/electrolyte), measured in
0.1M NaOH aqueous solution (black) and 0.1M NaOH in
95% methanol (red) and b) α-Fe2O3 (red) and anatase
TiO2 (blue) photoanodes measured in 0.1M NaOH in
95% methanol under chopped illumination.
Figure 1B presents comparative J-V curves under
chopped illumination for the oxidation of methanol on
nanocrystallne α-Fe2O3 versus that on mesoporous ana-
tase TiO2. At high applied potentials, methanol oxidation
on hematite produces an order of magnitude more photo-
current than on anatase, most likely due to better light
absorption by hematite relative to titania (bandgaps of
2.1eV and 3.1 eV respectively). On the other hand, titania
shows a photocurrent onset approximately 500 mV ca-
thodic of that for hematite, in accordance with their dif-
ference in the valence band edge (2.6 and 2.1 VNHE at pH
14, respectively).21,22
Figure 3 shows the PIA (Figure 3A) and TPC (Figure 3B)
responses for the oxidation of 95% methanol in 0.1 M
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 15
NaOH on α-Fe2O3 with the photoanode held at 0.55
particle size in these APCVD α–Fe2O3 films).27 As ex-
1
VAg/AgCl. This strongly anodic applied potential minimizes
recombination in the photoanode.24,25 The PIA signal
measured at 650 nm presented in Figure 3A shows a char-
acteristic slow rise and plateau when the LED light is
turned on and a decay when the LED light is turned off.
The rise and plateau are assigned to the accumulation and
reaching of a steady state hole flux. The decay, when the
light is turned off, is assigned to the dissipative reaction
of long-lived hematite surface holes. These PIA rise and
fall kinetics are faster than those we have reported previ-
ously for water oxidation in the absence of methanol,15
consistent with the expected faster kinetics of methanol
oxidation. The steady state is reached when the flux of
holes towards the surface is the same as the rate of their
reaction rate with the methanol. The photocurrent signal
presented in Figure 3B exhibits faster rises and decays but
similar steady-state behavior compared to the PIA signal,
with the faster kinetics being assigned to fast electron
extraction from the hematite film. The TPC signal drops
rapidly to zero with no cathodic current spikes when the
light is switched off, confirming the absence of any signif-
icant back electron/hole (or ‘surface’) recombination un-
der these strongly anodic conditions, in agreement with
previous studies.15,24–26
pected, due to the resulting more severe recombination
losses, higher light intensities were required to generate
comparable PIA and photocurrent signals to those ob-
tained at 0.55 VAg/AgCl. Confirming this, Figure S5 shows
that at low applied potentials, back electron/hole recom-
bination is not completely turned off and accelerates the
decay kinetics of the holes accumulated at the surface of
the photoanode. Furthermore, an analagous study was
conducted using TiO2 as the photoanode in 4% methanol
in 0.1 M NaOH electrolyte (see SI Figures S6A and S6B), a
lower methanol concentration was used as the photocur-
rent increases following methanol addition saturated at
this concentration (see Figure S1). This enabled us also to
monitor the titania surface hole accumulation under con-
ditions of quasi-steady state methanol oxidation. Before
undertaking a quantitative comparison of these data, we
present first the kinetic model used for their analysis.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Based on Le Formal et. al.,15 we turn now to a simple ki-
netic model for the PEC oxidation of methanol on the
photoanodes studied herein. The model is considered
under steady state conditions, when the change in surface
hole density, dps, with the time, dt, is zero (see eq 2) and
the flux of photogenerated holes to the surface Jhsoulres , is
equivalent to the photocurrent, JV (see eq 3).
dps
= 0 = J
− k
⋅ pαs
(2)
(3)
sur
obs
holes
MeOH
dt
Obs
log
(
JV
)
=
α
⋅log
(
ps
)
+ log
(
k
)
MeOH
Obs
where
k
is the observed rate constant for metha-
MeOH
nol oxidation and α is the order of the methanol oxida-
tion reaction with respect to the density of surface accu-
mulated holes, ps. As such, a plot of log(JV) versus log(ps)
will have a gradient equivalent to the reaction order α. JV
can be determined from the current densities (e.g. Figure
3B) measured at 5 s after light on (i.e. quasi-steady state
conditions). The surface density of holes, ps can be de-
termined at the same time from the PIA (e.g. Figure 3A)
using the Beer-Lambert Law from measured hole extinc-
tion coefficients at the probe wavelengths used (640 M-1
cm-1 for α-Fe2O315 and 2000 M-1 cm-1 for TiO228).
Figure 3. Oxidation of 95% methanol in 0.1 M aqueous
NaOH on hematite during 5s on/5 s off pulsed 365 nm
illumination conditions at 0.55 VAg/AgCl, a) photoinduced
absorption of excited species (h+) probed at 650 nm and
b) transient photocurrent measured simultaneously.
Further data analogous to those shown in Figure 3,
were collected at 0.00 VAg/AgCl (see SI Figures S4A and
S4B). Under these modest potential conditions, the at-
tenuated space charge layer width results in less band-
bending (as the space charge layer is smaller than the
ACS Paragon Plus Environment

Page 11 of 15
Journal of the American Chemical Society
D2O as electrolyte. Figure 5 shows the resulting rate law
sur
1
2
3
4
5
6
7
8
analysis comparing the oxidation of CH3OH versus
CD3OD on α-Fe2O3. For the d4-methanol electrolyte the
gradient of log(JV) versus log(ps) is also ~ 2, but showed a
twenty fold reduction in current, compared with CH3OH,
at equivalent surface hole densities. The corresponding
second order rate constant for CD3OD oxidation (1.35
holes-1nm2s-1) gives a KIE of ~20, decreasing the ‘turn over
frequency’ per hole from ~ 20 s-1 to ~ 1 s-1 under conditions
of approximately one sun irradiation. These slower kinet-
ics for CD3OD oxidation indicate that the rate limiting
step of the reaction is a chemical step and involves the
breaking of a C-H bond, as we discuss further below.
Figure 4. Rate law analysis, photocurrent density,
J
holes
and surface hole density, dps, of the oxidation of meth-
anol on α-Fe2O3 at 0.55 V (dark red) and 0.00 V (light
red), and TiO2 (blue) at -0.80 V applied potentials.
Figure 4 shows plots of the photocurrent density, JV, as
a function of the surface hole density, ps, for the oxidation
of methanol on TiO2 and α-Fe2O3, employing the PIA and
TPC data shown in Figures 3, S4 and S6. The data are
plotted in units of nm-2, correcting for surface roughness
of the two electrodes. For all data sets, the gradients of
log(JV) versus log(ps) are ~ 2 (within the range 1.88 to 2.13,
see Figure 4), indicating that in all cases the oxidation of
methanol is second order with respect to surface accumu-
lated holes. This second order behavior is further sup-
ported by an initial rates law analysis (see Figure S7) of
the PIA decay kinetics for methanol oxidation on hema-
tite at 0.55 VAg/AgCl. From equation 3, we obtain second
order rate constants of 15000 and 33 holes-1nm2s-1 for TiO2
and α-Fe2O3 respectively, independent of the applied po-
tential. For α-Fe2O3 at 0.55 VAg/AgCl under conditions of
approximately one sun irradiation (~ 4 mA cm-2 ), ps is
~0.5 holes nm-2, leading to a hole flux to the electrolyte of
~ 10 holes nm-2 s-1, corresponding to a ‘turn over frequen-
cy’ per hole of ~ 20 s-1. We further note that we obtain
indistinguishable rate constants, and rate laws, for meth-
anol oxidation on hematite at 0.00 and 0.55 VAg/AgCl, de-
spite the large difference in band bending and recombina-
tion losses between these two conditions; a point we dis-
cuss in further detail below.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
DISCUSSION
We have shown that, under operating photoelectro-
chemical oxidation conditions, methanol is fully oxidized
to formaldehyde on both TiO2 and α-Fe2O3 with a rate
that depends on the square of the density of surface ac-
cumulated holes, by quasi steady-state kinetic analysis of
the reaction. We note this result differs from our analysis
of water oxidation on identical α-Fe2O3 (see Figure S9),
where we observed first order behavior with respect to ps
at low surface hole densities transitioning to third order
behavior at high hole densities.15 Moreover, the kinetics of
the methanol oxidation reaction depend upon the metal
oxide surface chemistry, as well as the presence of deuter-
ium in the electrolyte, but are independent of the applied
potential.
We first focus on the hematite data collected at two dif-
ferent applied potentials. It is striking that our plots of JV
versus ps collected at 0.00 and 0.55 VAg/AgCl overlay each
other, showing the same reaction order and rate constant
with respect to surface hole density. These reaction con-
ditions are very different, with severe surface elec-
24
tron/hole recombination losses at 0.00 VAg/AgCl but no
surface recombination at 0.55 VAg/AgCl. Although water
oxidation does not occur at 0.00 VAg/AgCl, it is possible that
it becomes competitive with methanol oxidation at high
applied potentials, however this is ruled out by SI Figure
S9. The agreement between 0.00 and 0.55 VAg/AgCl data
confirms the validity of our experimental protocol and
that our analysis is indeed addressing the kinetics of
methanol oxidation at the semiconductor/electrolyte in-
terface. We observe that at equivalent surface hole densi-
ties, the kinetics of methanol oxidation are independent
of applied potential, clearly demonstrating that the kinet-
ics of the reaction are not determined by the electrode
Fermi level or band bending. Rather this observation in-
dicates that these kinetics are simply determined by the
density of holes accumulated at the electrode surface,
with an energy determined by the valence band edge.
This situation is consistent with the semiconducting na-
ture of hematite, and contrasts to the behavior of metal
electrodes, where changing the applied potential changes
the free energy driving the reaction.21,29
Figure 5. Kinetic isotopic effect (KIE) of the oxidation of
methanol on α-Fe2O3. Rate law analysis, photocurrent
sur
density,
J
and surface hole density, dps, of the oxi-
holes
dation of CH3OH at 0.00 V (light red) and at 0.55 V (dark
red) and CD3OD at 0.55 V (purple).
We undertook a kinetic isotope effect study to further
analyze the kinetics and the second order dependence of
the reaction with respect to the density of accumulated
holes at the surface. Therefore, we collected analogous
data to that shown in Figure 3 (see SI Figures S8A and
S8B) using deuterated 95% d4-methanol in 0.1M NaOD in
Turning now to the comparison of titania and hematite
shown in Figure 4, it is apparent that titania shows ~ 500
fold faster methanol oxidation kinetics than hematite,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 15
despite the lower photocurrent ‘saturating’ methanol We note that this would be an unusually high KIE value
1
2
3
4
5
6
7
8
concentration (4% in titania compared to 95% in hema-
tite, as shown in Figure S1). This difference in the concen-
tration of methanol needed to reach the maximum pho-
tocurrent densities has been suggested to depend on a
competitive mechanism of adsorption between water and
methanol on anatase,30 compared to hematite where a
strong chemisorption of methanol (as methoxide species)
has been reported.30 This difference in concentration de-
pendence may also be related to the lower Faradaic effi-
ciencies reported for methanol oxidation on TiO2 (e.g.,
30% reported by Wahl et. al.11 for the oxidation of 0.4%
methanol in 0.1M NaOH). Despite these differences, it is
striking that methanol oxidation on titania also exhibits
second order behavior as a function of surface hole densi-
ty, suggesting some similarity in the reaction mechanism.
A full analysis of methanol oxidation on titania, and its
dependence on, for example, methanol concentration, is
beyond the scope of this study. Nevertheless, it is clear
that methanol oxidation on titania is at least two orders of
magnitude faster than on hematite. These faster kinetics
can be most obviously assigned to the deeper valence
band edge of TiO2 relative to α-Fe2O3 (2.6 and 2.1 VNHE at
pH 14, respectively),21,22 providing a larger energy offset to
drive the methanol oxidation reaction, although we note
that differences in methanol surface adsorption may also
be important.
for a C-H bond breaking considering only the stretching
mode of this bond (KIEExpected ~ 7).35 However, C-H bond
breaking of surface bound CH3O. species will be associat-
ed with significant structural changes, and specifically a
change in the carbon hybridization from sp3 to sp2. This
re-hybridzation can be expected to lead to considerable
differences in the zero-point energy of the transition state
due to the loss of stretching as well as bending modes.35–37
Therefore, we conclude that the rate limiting step in the
oxidation of methanol under conditions of ~ 1 sun illumi-
nation and alkaline electrolyte involves the breaking of a
C-H bond and an associated re-hybrization of the carbon.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A two-step methanol oxidation mechanism on oxide
surfaces has been proposed previously in the context of
observations of photocurrent doubling for similar sys-
tems.31,32 This pathway of reaction begins with the adsorp-
tion of a molecule of methanol on the metal center, re-
leasing a proton. Subsequently, a surface hole is trans-
ferred to the adsorbed methoxide, forming a methoxy
radical, which then undergoes a second oxidation step by
injection of an electron into the conduction band of the
photoanode, producing formaldehyde.11,23,31 This photo-
current doubling mechanism (where one photon gener-
ates two conduction band electrons) has been observed
previously under low light intensity conditions on TiO2
and ZnO.11,32,33 However, we note that Schoenmakers et.
al.33 have reported a reduction in the quantum efficiency
of methanol oxidation on ZnO from 2 to 1 when increas-
ing the light intensity from ~0.005 to 5 suns. The results
we report here provide an explanation for Schoenmakers
et. al.’s observation, and indicate that under ~1 sun irradi-
ation conditions, where there is significant hole accumu-
lation at the oxide surface, both steps of methanol oxida-
tion are driven by photogenerated valence band holes.
This results, in the observed second order dependence on
surface hole density and no significant current doubling
effect, are in agreement with previous reports at operating
~ 1 sun conditions.12,34
Figure 6. Plausible mechanism of methanol oxidation
on α-Fe2O3 where the R.L.S. requires oxidations by two
valence band holes and involves C-H bond breaking,
leading to re-hybridization of the carbon to produce
formaldehyde with a Faradaic efficiency of unity; h+ re-
fers to surface valence band holes (Fe(IV)=O species not
adjacent to adsorbed methanol).
Based upon these data, and previous literature
studies,11,12,20,23,31–34,38 we propose a mechanism for metha-
nol oxidation on hematite and titania photoanodes, as
shown in Figure 6. We note that Grassian and co-
workers39 have determined the ‘saturating’ methanol ad-
sorbed surface density on α-Fe2O3 to be ~ 2 x 1013 mole-
cules.cm-2, corresponding to one molecule of methanol
adsorbed every 5 nm2 on α-Fe2O3, which is comparable to
our measured surface hole densities (~ 5 x 1013 h+.cm-2 un-
der one sun irradiation). This reported methanol coverage
allows us to rule out direct interactions between adsorbed
methanol molecules. As such, we assign the R.L.S. in our
observed second order methanol oxidation to be the oxi-
dation of a surface adsorbed methoxy radical with a hem-
atite surface valence band hole, as illustrated in Figure 6.
We finally focus on the kinetic isotopic effect (KIE)
shown in Figure 5. The aforementioned methanol adsorp-
tion on the photoanode surface leads to the loss of the H
atom from the O-H bond. Therefore our observation of a
KIE of ~ 20 is most obviously assigned to a C-H bond
breaking in the rate limiting step of methanol oxidation.
ACS Paragon Plus Environment

Page 13 of 15
Journal of the American Chemical Society
We note that our observation of second order methanol
Corresponding Author
1
2
3
4
5
6
7
8
oxidation indicates that the concentration of methoxy
radicals scales with the surface hole density, implying that
under steady state conditions an equilibrium is formed
between these species. This mechanism is also consistent
with the observation of methoxy radicals during methanol
oxidation on oxide surfaces that has been reported previ-
ously.11,23,31,38 Surface hematite holes have been assigned
previously to Fe(IV)=O species;40 methanol oxidation re-
quires two of these species to diffuse together to form the
reactive species as indicated in Figure 6. This mechanism
is in accordance with our observed second order behavior
under technologically relevant conditions reported herein
of 1 sun illumination. Our observation of a similar, second
order, rate law for methanol oxidation on TiO2 suggests
that the same reaction mechanism is also likely to operate
on this metal oxide, with the higher rate constant result-
ing from the deeper valence band in titania compared to
hematite.
j.durrant@imperial.ac.uk
Present address
⊥ Physical Biosciences Division, Lawrence Berkeley National
Laboratory, Berkeley, California 94720, USA.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We acknowledge Dr. Robert Godin for helpful discussions
and financial support from the European Research Council
(project Intersolar 291482), Swiss National Science Founda-
tion (project: 140709) and Swiss Federal Office for Energy
(project: PECHouse 3, contract number SI/500090–03).
C.A.M thanks COLCIENCIAS for funding, L.F. thanks the EU
for a Marie Curie fellowship (658270) and E.P. thanks the
EPRSC for a DTP scholarship.
REFERENCES
(1)
(2)
Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253.
Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. J. Mater.
Chem. A 2015, 3, 2485.
CONCLUSIONS
The use of organic oxidation substrates can substantial-
ly reduce the requirement of strong anodic potentials for
PEC hydrogen evolution, as well as potentially enable the
synthesis of useful organic compounds. As a study case,
we have reported kinetic and mechanistic analyses for the
selective oxidation of a model substrate, methanol to
formaldehyde, on titania and hematite photoanodes un-
der PEC working conditions. The methanol oxidation
reaction was found to be second order with respect to the
density of surface holes, indicating a reaction mechanism
where both steps of methanol oxidation are driven by
valence band holes. The second oxidation, involving a C-
H bond breaking, is the rate limiting step. Remarkably,
this oxidation is observed to proceed with near unity Far-
adaic efficiency, suggesting a potentially attractive route
to formaldehyde synthesis from methanol. Our observa-
tion of second order behavior also has important implica-
tions for technological applications, as it implies that the
kinetics, and therefore potentially the efficiency, of this
widely used reaction will be super-linearly dependent
upon the surface density of accumulated holes, with im-
plications for photoanode design (e.g. surface area) and
optimum operational light intensities.
(3)
(4)
Brink, G.-J. ten; Arends, I. W. C. E.; Sheldon, R. A. Science
2000, 287, 1636.
Palmisano, G.; Garcia-Lopez, E.; Marci, G.; Loddo, V.;
Yurdakal, S.; Augugliaro, V.; Palmisano, L. Chem. Commun.
2010, 46, 7074.
(5)
Kasap, H.; Caputo, C. A.; Martindale, B. C. M.; Godin, R.;
Lau, V. W.; Lotsch, B. V.; Durrant, J. R.; Reisner, E. J. Am.
Chem. Soc. 2016, 138, 9183.
(6)
(7)
(8)
(9)
Yang, J.; Shen, X.; Wei, J.; Zhang, L.; Zhao, D.; Yao, B. Catal.
Sci. Technol. 2016, 6, 7604.
Purnima, T.; Ruberu, A.; Nelson, N. C.; Slowing, I. I.; Vela, J.
J. Phys. Chem. Lett. 2012, 3, 2798.
Antoniadou, M.; Lianos, P. Appl. Catal. B Environ. 2010, 99,
307.
Kafizas, A.; Wang, X.; Pendlebury, S. R.; Barnes, P.; Ling,
M.; Sotelo-Vazquez, C.; Quesada-Cabrera, R.; Li, C.; Parkin,
I. P.; Durrant, J. R. J. Phys. Chem. A 2016, 120, 715.
Pendlebury, S. R.; Barroso, M.; Cowan, A. J.; Sivula, K.;
Tang, J.; Grätzel, M.; Klug, D.; Durrant, J. R. Chem.
Commun. 2011, 47, 716.
Wahl, A.; Ulmann, M.; Carroy, A.; Jermann, B.; Dolata, M.;
Kedzierzawski, P.; Chatelain, C.; Monnier, A.; Augustynski,
J. J. Electroanal. Chem. 1995, 396, 41.
(10)
(11)
(12)
(13)
Micka, K.; Gerischer, H. J. Electroanal. Chem. 1972, 38, 397.
Soares, A. P. V.; Portela, M. F.; Kiennemann, A. Catal. Rev.
2005, 47, 125.
(14)
(15)
Wang, X.; Kafizas, A.; Li, X.; Moniz, S. J. A.; Reardon, P. J.
T.; Tang, J.; Parkin, I. P.; Durrant, J. R. J. Phys. Chem. C
2015, 119, 10439.
Le Formal, F.; Pastor, E.; Tilley, S. D.; Mesa, C. A.;
Pendlebury, S. R.; Grätzel, M.; Durrant, J. R. J. Am. Chem.
Soc. 2015, 137, 6629.
ASSOCIATED CONTENT
Supporting Information.
Current-voltage curves for different concentrations of meth-
anol in 0.1M NaOH on α-Fe2O3 and TiO2, formaldehyde
quantification details are presented. PIA and TPC signals
measured at 0.00 V for hematite and -0.80 V for anatase, as
well as, PIA and TPC signals for the oxidation of CD3OD on
hematite at 0.55 V are shown. The effect of recombination on
PIA decays, an initial rates analysis for methanol oxidation
and a comparison of rate law analysis for water and methanol
oxidation are also presented. This material is available free of
charge via the Internet at http://pubs.acs.org.
(16)
(17)
(18)
Kay, A.; Cesar, I.; Grätzel, M. J. Am. Chem. Soc. 2006, 128,
15714.
Xiao-e, L.; Green, A. N. M.; Haque, S. A.; Mills, A.; Durrant,
J. R. J. Photochem. Photobiol. A Chem. 2004, 162, 253.
Ma, Y.; Pendlebury, S. R.; Reynal, A.; Le Formal, F.; Durrant,
J. R. Chem. Sci. 2014, 5, 2964.
(19)
(20)
Jacobsen, N. W.; Dickinson, R. G. Anal. Chem. 1974, 46, 298.
Klahr, B.; Gimenez, S.; Zandi, O.; Fabregat-Santiago, F.;
Hamann, T. ACS Appl. Mater. Interfaces 2015, 7, 7653.
Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061.
Chen, S.; Wang, L.-W. Chem. Mater. 2012, 24, 3659.
Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T. J.
(21)
(22)
(23)
AUTHOR INFORMATION
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 15
Electroanal. Chem. 2004, 571, 273.
of the Symposium on Photoelectrochemistry; Rajeshwar, K.,
(24)
Le Formal, F.; Pendlebury, S. R.; Cornuz, M.; Tilley, S. D.;
Grätzel, M.; Durrant, J. R. J. Am. Chem. Soc. 2014, 136, 2564.
Peter, L. J. Solid State Electrochem. 2013, 17, 315.
Peter, L. M. Chem. Rev. 1990, 90, 753.
van de Krol, R.; Grätzel, M. Photoelectrochemical Hydrogen
Production; van de Krol, R., Grätzel, M., Eds.; Springer: New
York, 2012.
Cowan, A. J.; Leng, W.; Barnes, P. R. F.; Klug, D. R.;
Durrant, J. R. Phys. Chem. Chem. Phys. 2013, 15, 8772.
Bard, A. J.; Faulkner, L. R. Electrochemical methods :
fundamentals and applications, 2nd ed.; John Wiley & Sons,
Inc.: New York, 2001.
Ed.; The Electrochemical Society: Pennington, Nuw Jersey,
1997; Vol. 97.
H. Schoenmakers, G.; Vanmaekelbergh, D.; J. Kelly, J. J.
Chem. Soc. Faraday Trans. 1997, 93, 1127.
Nomikos, G. N.; Panagiotopoulou, P.; Kondarides, D. I.;
Verykios, X. E. Appl. Catal. B Environ. 2014, 146, 249.
Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic
Chemistry; University Science Books: Sausalito, CA, 2006.
Stewart, R.; Van der Linden, R. Tetrahedron Lett. 1960, 1, 28.
Westheimer, F. H. Chem. Rev. 1961, 61 (3), 267.
Shen, M.; Henderson, M. A. J. Phys. Chem. Lett. 2011, 2,
2707.
1
2
3
4
5
6
7
8
(25)
(26)
(27)
(33)
(34)
(35)
(28)
(29)
(36)
(37)
(38)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(30)
(31)
Wang, C.-Y.; Groenzin, H.; Shultz, M. J. J. Am. Chem. Soc.
2005, 127, 9736.
Zhou, C.; Ren, Z.; Tan, S.; Ma, Z.; Mao, X.; Dai, D.; Fan, H.;
Yang, X.; LaRue, J.; Cooper, R.; Wodtke, A. M.; Wang, Z.; Li,
Z.; Wang, B.; Yang, J.; Hou, J. Chem. Sci. 2010, 1, 575.
Fermín, D. J.; Ponomarev, E. A.; Peter, L. M. In Proceedings
(39)
(40)
Carlos-Cuellar, S.; Li, P.; Christensen, A. P.; Krueger, B. J.;
Burrichter, C.; Grassian, V. H. J. Phys. Chem. A 2003, 107,
4250.
Zandi, O.; Hamann, T. W. Nat. Chem. 2016, 8.
(32)
ACS Paragon Plus Environment

Page 15 of 15
Journal of the American Chemical Society
Insert Table of Contents artwork here
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
ACS Paragon Plus Environment