Photo-thermal reactions of ethanol over Ag/TiO2 catalysts. the role of silver plasmon resonance in the reaction kinetics

Article abstract of DOI:10.1039/c8cc01814f

Photo-thermal catalytic reactions of ethanol over Ag/TiO₂ were conducted in order to probe into the role of plasmonic resonance response in the reaction kinetics. In the 300-500 K temperature domain the increase in reaction rate is found to be mainly due to changes in the activation energy while above this temperature range the increase was due to the pre-exponential factor. These results might be linked to the role of plasmonic Ag particles in polarising the reaction intermediates and therefore increasing the reaction products at temperatures up to about 500 K.

Full text of DOI:10.1039/c8cc01814f

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Photo-thermal reactions of ethanol over Ag/TiO2 catalysts. The role
of silver plasmon resonance in the reaction kinetics.
M.A. Nadeema and H. Idrissa,b*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Photo-thermal catalytic reactions of ethanol over Ag/TiO2 were enhancement of CO2 production (70% at 100 oC) appeared to be
conducted in order to probe into the role of plasmonic resonance caused primarily by increased levels of acetaldehyde produced by
response in the reaction kinetics. In the 300-500K temperature photo-oxidation over TiO2 and its subsequent migration to Pt metal
domain the increase in reaction rates is found to be mainly due to leading to CO2 production by thermal reaction.3 Chanmanee and co-
changes in the activation energy while above this temperature the workers studied alkane reverse combustion under photo-thermal
increase was due to the pre-exponential factor. These results might conditions.4 They observed an increase in productivity of liquid
be linked to the role of plasmonic Ag particles to polarise the hydrocarbons with temperature but with a very weak incident
reaction intermediates and therefore increase the reaction photon quantum yield (IPQY) of 0.02-0.05% on a per electron
products at temperatures up to about 500K.
stored basis. Hu and co-workers studied the temperature-induced
visible light photocatalytic hydrogen production from water and
While elevated temperatures are not needed in photo-catalytic methanol using black TiO2.5 They found an enhancement in the
reactions because of their low activation energy recent reports reaction rate that was attributed to visible light photo-catalysis due
indicate an enhancement in chemical reaction rates when both light to the presence of Ti3+ states in TiO2. At elevated temperatures, the
and heat are used.1-5 To date, there is a lack of understanding and injection of electron, into Ti3+ states, was proposed to be behind the
probably consensus on how or even whether photo-excited increase in the relative population of adsorbed methanol molecules
electron transfer rates can be enhanced by thermal reactions over a in “vibrationally” excited states, which lead to improved reaction
semiconductor. Several explanations were provided depending on rates.
the type of the catalyst and reaction studied. For example, Upadhye In this work, we have found that ethanol photoreaction rate
and co-workers demonstrated that the localized surface plasmon increases with increasing reaction temperature. The experiments
resonance (LSPR) increases the reverse water gas shift reaction were conducted in an UHV chamber with TiO2 and 3 wt. % Ag/ TiO2
(RWGS) rate (CO2 + H2 ꢀ H2O + CO) on Au catalysts.1 A light to (anatase-88% + rutile-12%) catalysts. Details of the experimental
chemical efficiency of up to 5% was computed. The authors set-up and procedure are provided in the supplementary
proposed that the LSPR changes the intrinsic reaction kinetics on information. UV-VIS, XRD and XPS analyses of 3 wt. % Ag/TiO2
the catalyst surface via either “hot” electron generation mechanism catalyst are presented in Fig. 1. In the case of UV-Vis spectrum,
or LSPR mediated adsorbate polarization mechanism. Tan and co- relatively sharp absorption edges at 415 nm and 387 nm are
workers observed that, despite being a weak thermal ethanol consistent with the intrinsic band gap absorption of TiO2 rutile and
oxidation catalyst, Au/TiO2 displayed considerable increases of its anatase phases, respectively. The rise in the background in the
catalytic performance upon excitation with visible and UV light.2 visible region that extends into UV region is attributed to the
The enhancement under UV illumination was attributed to surface Plasmon absorption of Ag particles.7, 8 XRD patterns show
congruent roles of the photo- and thermal- catalysis, while that Ag2O (111) diffraction in addition to diffraction peaks at 25.3 and
under visible light illumination was due to plasmonic-mediated 27.5 due to TiO2 anatase (101) and rutile (110), respectively. The
electron charge transfer from Au particles to the TiO2 support (the average crystallite sizes calculated using Scherer equation were 31,
so-called “hot” electrons). Kennedy and co-workers investigated the 44 and less than 10 nm (mean particle size) for anatase, rutile and
photo-thermal catalytic oxidation of ethanol over non-plasmonic Ag particles, respectively.
catalysts: TiO2 and
1
wt.
%
Pt/TiO2. The photo-thermal The chemical nature and atomic % of Ag in the near–surface region
determined from XPS Ag3d core–level peak areas are given in Fig. 1
and Table 1.
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products.12 Ethanol desorption profile (310–700 K) is de-convoluted
into two peaks. The low temperature peak centred at 380 K is
attributed to molecular and re-combinative desorption (of
ethoxides with OH(a)) adsorbed on non-defected surfaces. The high
temperature peak centred at about 540 K is attributed to re-
combinative desorption of ethoxides on oxygen-defect sites.13
On both catalysts, ethylene and acetaldehyde are the main
products, desorbed with a combined carbon selectivity of ca. 30%.
CH3CH2O(a)+O(s) → CH3CHO+OH(s)
(on 5 fold Ti atoms)14 (2)
CH3CH2O(a) → CH2CH2(↑) + OH(a)
(on oxygen defects) (3)
Ethylene selectivity is higher in the case of Ag/TiO2 when compared
to that of TiO2 alone. This is opposite to our previous work on
Au/TiO2. Earlier, we have noticed that ethylene is the dominant
product desorbed from TiO2 followed by acetaldehyde but the
15, 16
ratios decreased with an increase in Au metal loading.12,
Because Ag is far easier to oxidize when compared to Au and
because its work function is similar to that of TiO2; it may contribute
in C-O bond dissociation at its vicinity which in turn may increase
the dehydration reaction of ethanol.
Figure 1. XRD, UV-Vis, TEM (dark field) and XPS Ag 3d of 3 wt. %
Ag/TiO2
Both Ag metal and Ag+ were present as evidenced by the Ag3d5/2
binding energy values of 368.8 eV and 367.4 eV corresponding to
Ag0 and Ag+, respectively.10 An increase in Ag/Ag+ ratio is noticed
and attributed to in situ reduction of Ag+ during photoreaction. XPS
core levels of Ti and O were also collected and found consistent
with typical TiO2 (not shown).10, 11
Photoreactions of ethanol on metal supported on TiO2 has been
studied in some detail over the last decade and it has been found
that ethanol is mainly converted to H2 and acetaldehyde. The later
further reacts to produce CO, CO2, and CH4.9,17 Ethanol
photoreaction on metal supported on TiO2 under UV radiation
11
occurs via either two photons – two electrons mechanism10, or
through a one photon – two electron transfer mechanism (current
doubling effect).18-20
Table 1. XPS results of
photoreaction.
3
wt.
%
Ag/TiO2 before and after
The effect of temperature on ethanol photoreaction rate is
presented in Fig.3. Each line represents a separate run as explained
in experimental section.
at. %
Core Peak position
∆E
Chemical
FWHM (eV)
level
(eV)
368.8
374.9
367.3
373.2
(eV) Before After
Ag3d
2.1
2.2
2.3
2.4
5/2
Ag
6.1
5.9
0.6
1.0
0.6
Ag3d
Ag3d
Ag3d
3/2
5/2
Ag2O
0.7
3/2
Figure 2. Ethanol-TPD of the main carbon containing products after
adsorption at 300K on TiO2 and Ag/TiO2 catalysts. CF stands for
mass spectrometer correction factor.
Figure 3. Production of acetaldehyde from ethanol pre-dosed TiO2
and Ag/TiO2 catalysts upon UV irradiation as a function of time at
the indicated temperatures. Insets indicate the exponential fitting
of acetaldehyde photo-production at 628K.
Ethanol is dissociatively adsorbed on TiO2 at 300K, equation 1.
CH3H2OH + O(s)→ CH3CH2O(a) + OH(s)
(1)
These set of experiments were all done at the same initial surface
coverage as indicated in the experimental section and therefor one
can directly relate the product desorption to catalytic activity. As
shown in figure 4 other products were also observed. Concurrent
hydrogen desorption is shown in Fig. S1. Because the UHV chamber
A fraction of ethanol starts to desorb at 310 K followed by a second
desorption together with other products from ca. 500 K (Fig.2). The
desorption profiles are multiplied with their appropriate mass
spectrometer correction factor to reflect the accurate amounts of
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pressure was ca. 5 × 10−10 Torr before reaction, O2 and water
background would be in the low 10−11 to high 10−12 Torr range. An
increase in acetaldehyde desorption (Fig. 3), at all investigated
temperatures, together with other products with increasing the
surface temperature give evidences of a synergism between heat
and radiations. All experiments were performed several times and
were found to be reproducible. At all investigated temperature Ag
/TiO2 was more active than TiO2 alone. The wider peak in the case
of Ag/TiO2 upon UV excitation, at all investigated temperatures, is
linked to the presence of Ag, making more heterogeneous
additional sites.
Figure 5. Ln reaction rate for acetaldehyde production from ethanol
in photo-thermal conditions over TiO2 and Ag//TiO2 catalysts.
Second, the very low activation energy, at low temperatures is
typical of a photo-catalytic reaction. Yet the difference between
TiO2 and Ag/TiO2 in the 300-500K range is beyond experimental
errors (5 to 14 kJ mol-1) resulting in 4-fold increase of the reaction
rate; the decrease of the pre-factor with decreasing the activation is
to be noted, a phenomenon known as the compensation effect.26
Third, extrapolation of the reaction rate to lower temperature
shows
a crossing at about 270K. In other words, at this
Figure 4. Ethanol photoreaction products over TiO2 and Ag/TiO2
upon UV radiations as a function of temperature. Surfaces were
flashed to 628 K before each run to ensure constant coverage, then
cooled to the indicated temperature. The complete process of
correction factor (CF) and product yield calculation from area under
mass spectrometer response is explained in ref.12
temperature the addition of Ag does not enhance the reaction rate.
The addition of a metal to a semiconductor may enhance the
photoreaction rate based on a work function difference (∆WF). The
work function of Ag and TiO2 (anatase or rutile) are actually very
close27, 28 and therefore the difference in reaction rates may not be
linked to ∆WF. Scheme 1 presents the initial steps of the reaction
with two electrons injected into the semiconductor as follows.
Figure 4 presents products desorbed from TiO2 and Ag/TiO2
catalysts upon UV radiation as a function of temperature. The total Upon light excitation followed by electron transfer from the VB to
product yield increased with increasing temperature. The decrease
at 663 K is due to decrease in the initial surface coverage as seen in the VB, and are transformed to an oxy-radical. We have recently
the CB (step 1), ethoxide species (step 2) inject one electron into
15,
figure 2 and in refs.
16. Methyl radicals are also produced and
indicated using DFT+D computation that ethoxides are more prone
follow acetaldehyde production patterns. This has been noticed to making this reaction than ethanol29 in line with other’s finding30
.
21
22
earlier from acetone as well as acetaldehyde and proposed to These radicals have high energy and can directly inject the second
involve thermal reaction of acetaldehyde with adsorbed oxygen to electron into the CB31 (step 3), a phenomenon called “current
form
a
photoactive acetaldehyde–oxygen complex.22 This is doubling effect”.18-20 These two electrons in the absence of
followed by a substrate mediated photodecomposition mechanism molecular oxygen, as it is this work, are taken by the two protons of
that induces the fragmentation of the acetaldehyde–oxygen surface hydroxyls to make one molecule of hydrogen. In the
complex into gas phase methyl radical and surface adsorbed
scheme a Ag particle is added close to the CB of TiO2. Ag absorbs
formates.22 In a recent work on TiO2(110) rutile single crystal we light and as a result its free electrons oscillate making the so termed
have found similar results (CH3 radical production from ethanol23).
“plasmon resonance (PR)”. PR can increase the reaction rate of a
This was not the case for anatase (101) single crystal.24 The TiO2 P25 semiconductor via either electronic or photonic effects.32 Photonic
used in this work contains both phases; it is possible that CH3
radical formation is predominantly linked to the rutile phase.
effect of PR is either long range (light scattering) or short range to
increase the local electric field (step 5). Other works have also gave
Figure 5 presents the Arrhenius plots for ethanol reactions on evidences for the enhancement of the reaction rates due to LSPR of
TiO2 and Ag/TiO2. The activation energies extracted from the Ag when deposited on semiconductors.33,34 Although at 300K this
plots are indicated together with the pre-factors. There are a enhancement is found to be small when compared to noble metals
such as Pd.10 One would expect that long-range effect would be
few points that are worth pointing out in the figure. First, both
catalysts have a discontinuing reaction rate with temperature (at independent from temperature, yet the short-range effect that
ca. 500K = 0.002 K-1) and this is more pronounced in the case of
results in decrease of the e-h recombination may indeed increase
TiO2. It is worth indicating that the reaction is purely function of the reaction rate if it provides enough electric field strength to
the incident photons at the indicated temperatures, in other
words in the absence of photons no reaction takes place.
change the conformation of an adsorbate in its transition state. The
deviation between the reaction rate between Ag/TiO2 and TiO2 up
to about 500K (1/T = 0.002 K-1, figure 5) is explained by invoking this
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possibility. It is important to indicate that excitation with visible
light alone (> 400nm) did not results in any noticeable reaction
products indicating that both TiO2 and Ag need to be excited.
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Scheme 1. Schematic representation of Ag/TiO2 energetics upon
light excitation. A Ag particle (blue sphere) is shown at the contact
with the surface of TiO2. Also shown are the oxidation steps of
ethoxide to acetaldehyde upon light excitation. 1. e-h formation
upon light absorption, 2. One electron transfer to the VB (to trap
one hole). 3. One electron transfer for the oxy-radical to the CB
(because it has higher potential energy level than the CB). 4.
Possible excited electron transfer from the CB of TiO2 to the Ag
particle. 4’. Possible “hot electron” transfer from Ag to the CB of
TiO2. 5. Electric field generated upon light absorption by a Ag
particle at the interface with TiO2. 5 may have enough electric field
strength to polarise an intermediate and increase the reaction rate.
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In summary, the addition of Ag to TiO2 resulted in the enhancement
of the photo-thermal catalytic reaction of ethanol when compared
to TiO2 at all investigated temperatures but in two different
temperature regimes. In the [300-500K] temperature-range, the
increase seems to be mainly due to changes in the activation energy
while in the [500-625K], the activation energy is found to be the
same for TiO2 and Ag/TiO2 and the main increase is due to the pre-
factor. It is thus possible that the surface plasmon resonance (SPR)
of Ag induces the polarisation of reaction intermediates involved in
the oxidation and/or in the desorption of final products at
temperatures below 500K. Above this temperature, there is enough
thermal energy and Ag nanoparticles are not needed for their SPR.
25.
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