Engineering rGO nanosheets-adsorption layer supported Pt nanoparticles to enhance photo-thermal catalytic activity under light irradiation

Article abstract of DOI:10.1039/c9ta00148d

The direct conversion of renewable clean solar energy into heat to drive various catalytic reactions, e.g., oxidative degradation of volatile organic compounds, is highly desirable but remains challenging because of its low efficiency. In this work, a Pt-decorated-rGO nanosheet adsorption layer is rationally fabricated to achieve highly selective conversion of gaseous toluene to carbon dioxide under light irradiation. Specifically, its maximum photo-thermal conversion efficiency reaches 17.6% with a significant toluene conversion of 98% and CO₂ yield of 96% under 146 mW cm^-2 infrared light irradiation, together with excellent stability of nearly 45 h, much superior to that of the previous reported catalyst of Pt-rGO-TiO₂. The comparison characterizations evidence that this excellent performance is predominantly attributed to the synergistic effects of ultrabroadband strong IR light absorption, efficient light-to-heat conversion, well-dispersed active Pt nanoparticles and strong reactant adsorption capacity with light irradiation. This work highlights that rGO nanosheets-adsorption layer directs light harnessing, Pt nanoparticle dispersion and reactant adsorption, showing great promise for redox reactions through photo-thermal effect. It is anticipated that this study will provide insight into the design of more energy-efficient catalysts with significant utilization of solar energy.

Full text of DOI:10.1039/c9ta00148d

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Engineering rGO nanosheets-adsorption layer supporte
D
d
OI: 10.1
P
039/C9TA00148D
t
nanoparticles to enhance photo-thermal catalytic activity under light
irradiation†
‡
a
‡ab
ab
ab
bc
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Juan-Juan Li, Song-Cai Cai, Xi Chen, Dong-Xu Yan, Jing Chen, Hong-Peng
Jia*ab
a
CAS Center for Excellence in Regional Atmospheric Environment, and Key
Laboratory of Urban Pollutant Conversion Institute of Urban Environment, Chinese
Academy of Sciences, Xiamen 361021, P.R. China. E-mail: hpjia@iue.ac.cn
b
University of Chinese Academy of Sciences, Beijing, 100049, P.R. China
c
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Xiamen Institute of Rare-earth Materials, Haixi Institutes, Chinese Academy of
Sciences, Xiamen, Fujian 361021, P.R. China
† Electronic supplementary information (ESI) available.
‡ These authors contributed equally to this work.
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Abstract
The direct converting renewable clean solar energy into heat to drive various
catalytic reactions, e.g., oxidative degradation of volatile organic compounds is highly
desirable but remains challenging because of its low efficiency. In this work,
Pt-decorated rGO nanosheets-adsorption layer is rationally fabricated to achieve
highly selective conversion of gaseous toluene to carbon dioxide under light
irradiation. Specifically, its maximum photo-thermal conversion efficiency is able to
reach 17.6% with a significant toluene conversion of 98% and CO yield of 96%
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2
under 146 mW/cm infrared light irradiation, together with excellent stability of
nearly 45 h, much superior to that of the previous reported catalyst of Pt-rGO-TiO2.
The comparison characterizations evidence that such excellent performance is
1

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predominantly attributed to synergistic effects of ultrabroadband strong IR light
DOI: 10.1039/C9TA00148D
absorption, efficient light-to-heat conversion, well dispersed active Pt nanoparticles
and strong reactant adsorption capacity with light irradiation. This work highlights
that rGO nanosheets-adsorption layer directs light harnessing, Pt nanoparticles
dispersion and reactant adsorption, which shows great promise for redox reactions
through photo-thermal effect. It is anticipated that this study would provide insight
into the design of more energy-efficient catalysts with a significant utilization of solar
energy.
Keywords: Solar energy; Pt-rGO hybrid; Adsorption layer; Photo-thermal effect;
Catalytic oxidation
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1. Introduction
Catalysis is an extremely efficient approach for atmospheric degradation of
volatile organic compounds. However, in the commercial heterogeneous catalytic
reactions, a large amount of thermal energy involved in this process is generally
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required by fossil-derived fuels and is characterized by low energy efficiency, and
combusting of nonrenewable crude oil associated with natural resource depletion is
causing imminent energy crisis and irreversible environmental damage. The direct
converting renewable clean solar energy into heat has been of tremendous research
interest recently for a number of potential applications in solar power generation or
seawater desalination, as well as in environmental catalysis.2-5 Fundamentally,
solar-to-heat energy conversion requires materials that have strong absorption
capability spanning a broad solar spectrum and enable forceful coupling of the
harvested photons into thermal energy accessible to catalytic active site for chemical
reactions. Currently, photothermal phenomenon powered by localized
plasmon-mediated heating on the surfaces of noble metal nanoparticles (NPs) can be
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realized.6-8 Despite these advancements, the metallic nanomaterials that have beenView Article Online
DOI: 10.1039/C9TA00148D
reported only respond to a limited range of solar spectrum and usually have a rather
low energy utilization efficiency, since the optical properties across the entire solar
spectrum of these plasmonic nanomaterials critically depend on size, shape and
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composition. It is still an important, as well as challenging issue to improve the
solar-to-heat efficiency, especially the infrared (IR) wavelength light that constitutes
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53% of the solar energy reaching earth’s surface. The exploitation of catalysts that
extending the light absorption from the UV-Visible to IR region always remains a
high-priority target, and this can be done by narrowing the bandgap of the materials
based on the principle of the full utilization of solar light.11
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Inspired by the observation that a surprisingly giant infrared absorption band in
reduced graphene oxide (rGO) originating from the coupling of electronic states to the
asymmetric stretch mode,12 there has been a resurgence of great attraction in an
exploration of graphene-based photo-thermal responsive materials. Our recent study
has demonstrated that the Pt-rGO-TiO composite featuring a broad IR light
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wavelength absorption possesses the capability for showing remarkable photo-thermal
catalytic activity under IR light irradiation, with its performance intensively correlated
to the advisable introduction of few layered rGO nanosheets which could enhance
light-to-heat conversion, improve reactant adsorption capacity and enable well
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dispersed supported Pt-TiO catalyst on the 2D rGO nanosheets. Notably, such
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photo-thermal catalyst suffered from a moderate selective conversion of gaseous
toluene to carbon dioxide under light irradiation, accompanied by generated
recalcitrant carbonaceous intermediates possibly depositing/blocking/poisoning
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adsorption sites and catalytic active sites. Accordingly, it remains a challenge but an
opportunity, to further explore a material that can thoroughly decompose organic
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pollutants into end-product CO , especially in the IR-light region.
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In order to get a more active catalyst, one needs to modulate the molecule-scale
surface properties to fundamentally improve reactant adsorption, enhance
light-to-heat temperature and expose a greater fraction of catalytical active sites.
Specifically, the adsorption of the reactant onto the catalyst particularly at the
light-induced-temperature needs to be carefully considered. Herein, we show that the
rGO nanosheets-adsorption layer supported Pt NPs, characterized by excellent IR
light absorption, efficient light-to-heat conversion, high reactant adsorption capacity
and well dispersed active Pt NPs, can convert solar light especially a low-intensity IR
photon flux into heat to enhance catalytic activity. In particular, Pt-rGO hybrid
displays high catalytic performance with 98% of toluene conversion and 96% of CO2
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yield under IR irradiation intensity of 146 mW/cm , as well as excellent stability. It is
anticipated that this work would open new avenues towards the design of more
energy-efficient catalyst with a significant utilization of inexhaustible solar energy.
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2. Experimental
2.1. Preparation of graphene oxide
Graphene oxide (GO) was synthesized from natural graphite powder by a
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modified Hummers method. In detail, 2 g of graphite powder (Alfa Aesar) was put
into a mixture of 50 mL of concentrated H SO and 1 g of NaNO in an
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ice water bath and 8 g of KMnO was added gradually for 120 min under vigorous
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stirring. Then, the solution was heated to 35 C in an oil-bath and kept stirring for 2 h.
Successively, 50 mL of deionized (DI) water was added into the above mixture,
o
followed by stirring at 95 C for 0.5 h. Then, to the resulting mixture were added 50
mL of DI water and 20 mL of 30 % H O to remove unreacted KMnO . When cooled
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to room temperature, the supernatant then underwent two more high-speed
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centrifugation steps at 8000 rpm for 10 min, then washed with 5% HCl aqueous
DOI: 10.1039/C9TA00148D
solution to remove metal ions followed by DI water to remove the acid. The product
was ultrasonicated for 1.5 h, then stirred under ambient condition overnight. The
mixture was then carefully diluted with deionized (DI) water, filtered, and washed
until the pH of rinse water became neutral. Finally, a homogeneous GO aqueous
solution (4 mg/mL) was obtained.
2.2. Preparation of Pt-rGO composite
To synthesize the Pt-reduced graphene oxide (Pt-rGO) composite, a simple
solvothermal method using ethanol-water as solvent was used as the following.16,17 40
mL of GO aqueous solution (4 mg/mL) was ultrasonicated in a 80 mL of deionized
water and 60 mL of anhydrous ethanol solution to disperse it well; after that, 0.437
mL of H PtCl (3.8 mg/mL) aqueous solution was then rapidly injected into the above
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GO solution by pipet to prepare Pt-rGO nanocomposite catalyst. The mixing solution
was aged with vigorous stirring for 2 h to obtain a homogeneous suspension. Then,
this suspension was transferred to a Teflon-sealed autoclave and maintained at 120 °C
for 24 h. Under such a solvothermal condition, the solvent of ethanol-water has the
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strong power to simultaneously reduce Pt and GO into Pt and rGO. The resulting
composites were filtered and washed with deionized water, and fully dried at 60 °C in
a vacuum oven to get the final Pt-rGO composite.
For comparison, the preparation procedure of rGO was similar to that of Pt-rGO
without addition of Pt solution.
2.3. Catalyst characterization
Powder X-ray diffraction (XRD) characterization was carried out on a X’Pert
Pro automatic powder diffractometer operated at 40 kV and 40 mA using Cu K
monochromatized radiation. Raman spectroscopy was carried out on LabRAM
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Aramis using a green laser of 532 nm wavelength. The actual loading content of noble
DOI: 10.1039/C9TA00148D
-metal was determined by inductively coupled plasma atomic emission spectroscopy
(ICP-AES, Varian, VISTAMPX-ICP). Nitrogen adsorption-desorption measurements
of the catalysts were performed on a Quantachrome autosorb iQ2 adsorption
automatic instrument at liquid nitrogen temperature. All samples were degassed under
o
vacuum at 60 C for 24 h before the measurement. Diffuse reflectance spectra were
recorded using a Varian Cary 5000 UV-Vis spectrophotometer at room temperature
using BaSO as a reference. XPS analysis was conducted on a Thermo Scientific
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ESCALAB 250 with a monochromatised microfocused Al X-ray source.
Transmission electron microscopy (TEM) was analyzed using a JEOL model JEM
2100 EX instrument at an acceleration voltage of 200 kV. Temperature-programmed
desorption (TPD) measurement was performed on a Quantachrome Chemstar
instrument. 50 mg catalyst sample was filled in an adsorption vessel and pretreated in
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a He flow (50 mL/min) at 150 C for 60 min then cooled/heated to specified
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temperature (25, 120, 150 and 180 C) for 120 min in a flow of gaseous toluene (30
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mL/min). Then, the sample was swept with He for 60 min and heated to 600 C at a
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rate of 10 C/min in a He flow of 30 mL/min. The desorbed products were monitored
simultaneously by mass spectroscopy equipment. The specific heat capacity of the
sample was obtained by using differential scanning calorimeter (Netzsch STA 449 C,
o
Thermal Analysis Corporation, Germany) from 30 to 300 C at a heating rate of 5
oC/min in an air flow of 50 mL/min. In situ DRIFTS was performed on
a FTIR spectrometer (Nicolet Nexus 670)
equipped with
a smart
collector
and a MCT/A detector. Prior to experiment, the sample was pretreated at 150 °C for
60 min in a He flow to remove adsorbed impurities. The background spectrum was
collected under He and automatically deducted from the sample spectra. Afterward,
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gaseous toluene/He was introduced to the cell at a flow rate of 50 mL/min at elevated
DOI: 10.1039/C9TA00148D
temperature, and then DRIFTS spectra were recorded.
2.4. Photo-thermocatalytic activity
The photo-thermocatalytic activity of the samples for toluene oxidation was
evaluated in a cylindrical stainless steel reactor with a quartz window under IR light
irradiation (375 W, Philips) with a 800 nm cut-off filter. In the experiments, an
ethanol suspension containing 0.1 g of the catalyst was prepared, and then coated on a
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fiberglass membrane of 50 mm diameter, dried at 60 C and finally placed on the
photoreactor. After IR irradiation for 10 min, the simulated air stream (21% O /N )
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containing 930 mg/L of toluene passed through the catalyst layer (0.1 g) with the total
flow rate of 51.8 mL/min, giving a gas hourly space velocity (GHSV) of 31080 mL/(g
h). The concentration of toluene and the evolution of CO were monitored by an
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on-line gas chromatograph equipped with two flame ionization detectors (FID) and a
nickel-based methanizer. The toluene conversion and CO yield for the photocatalytic
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reaction were calculated by using by using Eqs (1) and (2), respectively.
Toluene conversion (%) = 100% × ([Toluene] -[Toluene] )/[Toluene]
in
(1)
(2)
in
out
CO yield (%) = 100% × [CO ]
/[CO ]
2 theoretical
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2.5. Photocatalytic activity
The procedure of measuring photocatalytic activity of the catalyst for toluene
oxidation with the irradiation of IR lamp is the same as that of photo-thermocatalytic
activity described above besides the following procedure: A 0.1 g catalyst powder
was coated on the bottom of a cylindrical quartz reactor (5 cm×5 cm×1 cm). A
thermocouple was put on the catalyst to measure its temperature. The photo-reactor
was placed in a glycerol-water bath to keep the reaction temperature at near room
◦
temperature (18 C) under IR lamp irradiation.
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2.6. Toluene adsorption experiment
The N stream with 51.8 mL/min containing 930 mg/L of toluene passed through
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the catalyst layer (0.1 g) at various temperatures. The concentration of toluene before
and after the adsorption process was measured by an on-line gas chromatograph
equipped with FID.
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3. Results and discussion
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Fig. 1 (a) XRD patterns; (b) XPS spectra of C 1s and Pt 4f; (c) Raman spectra; (d) SEM image (Pt
NPs is marked by red cycle) and (e) HR-TEM images of as-prepared Pt-rGO hybrid (the right side
of (e) is the Pt NPs size distribution).
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Crucially, the insight gained from theoretical and experimental works suggests
that transition to rGO through chemical or thermal removal of oxygen in GO allows it
to be reduced incrementally so that the infrared light absorption can be significantly
increased by several fold.12,18,19 Our one-step strategy for the synthesis of Pt-rGO
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hybrid via solvothermal reaction according to the previous procedure, simultaneous
reduction of GO and in-situ loading of Pt NPs on the rGO nanosheets. The XRD peak
at around 39.7° of as-prepared Pt-rGO hybrid in Fig. 1(a) can be well indexed to the
(111) diffraction peak of Pt crystalline state with face-center-cubic structure. The
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typical peak of GO located at around 10.2° is not observed, but the broad lump peak
DOI: 10.1039/C9TA00148D
at around 24.5° corresponding to characteristic rGO is clearly observed. Moreover,
the reduction of GO to rGO could be further characterized by XPS. As displayed in
Fig. 1(b), the deconvoluted C1s peaks located at 284.6, 286.0, 288.2 and 290.2 eV are
characteristic of C-C, C-O, C-O-C/C=O and COOH,15 respectively. Notably, the
peaks associated with C-C (284.6 eV) became predominant, while the peaks
associated with the oxidized carbon species were greatly weakened, indicating a
significant deoxygenation of oxygen-containing functional groups of GO. The
characteristic peaks of Pt 4f and Pt 4f located at 71.7 eV and 75.0 eV are assigned
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to the core levels of Pt . Raman spectroscopy has been applied to distinguish the
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sp /sp hybridization and ordered/disordered carbon structures in carbonaceous
materials.21 As shown in Fig. 1(c), the Raman spectra of Pt-rGO hybrid exhibits
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prominent D band (∼1350 cm ) and G band (∼1590 cm ) of rGO, corresponding to
breathing mode of k-point phonons of A1g symmetry adjacent to adefect/edge and E2g
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phonon of C sp atoms in a planar conjugated structure, respectively. The Pt-rGO
hybrid has an increased D/G intensity ratio relative to rGO, suggesting a decrease in
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the size of in-plane sp domains and a partially ordered crystal structure of Pt-rGO
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hybrid due to a potential interaction between Pt NPs and rGO nanosheets. The SEM
and HR-TEM images of Pt-rGO hybrid in Fig. 1(d) and (e) clearly show that rGO is a
sheet-like structure with scrolling and corrugation, meanwhile Pt NPs are well
dispersed with a mean particle size of about 2.46 nm on the external surface of rGO
nanosheets. Therefore, XRD, XPS, Raman and HR-TEM characterizations together
demonstrate that GO has been effectively deoxygenated to form rGO nanosheets and
uniform Pt NPs have been successfully assembled on the surface of rGO nanosheets
by means of solvothermal treatment. Impressively, Pt-rGO hybrid appears in black
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(Fig. S1) and is stable under outdoor condition. Moreover, the chemical stability of
DOI: 10.1039/C9TA00148D
the Pt-rGO hybrid nanostructure in an air atmosphere at various temperatures was
further investigated by MS and TG. Under air atmosphere, the Pt-rGO hybrid is stable
at temperature up to 180 °C because (1) the signal of mass spectra presents that the
carbon skeletons of rGO starts to be broken down by combustion and enables
the production of CO at temperature above 180 °C (Fig. S2(a)); (2) TG displays that
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a slight weight loss occurs from room temperature to 400 °C (Fig. S2(b)).
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Fig. 2 DRS of rGO, Pt-rGO and Pt-rGO-TiO composite.
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To experimentally evaluate the light absorption capacity, we further measured
the diffuse reflectance of Pt-rGO hybrid. As shown in Fig. 2 and Table 1, the Pt-rGO
hybrid exhibits prominent absorption feature over the whole range of the solar
spectrum with a total absorptance of 96.87% involving 93.38% of UV, 97.01% of
visible, and 96.63% of infrared irradiation. Additionally, the Pt-rGO-TiO presents a
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similar absorption feature with the Pt-rGO, but a slightly decreased absorptance
owing to the inefficient absorption in the Vis-IR region for TiO . Importantly, Pt-rGO
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hybrid shows a comparable absorption with rGO in the whole solar spectrum and a
similar zero-band-gap (Fig. S3), indicating that Pt-decorated rGO nanosheets well
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inherits the light absorption feature of rGO nanosheets since the light absorption
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ability of Pt NPs is much weaker than that of rGO nanosheets. Significantly, Pt-rGO
hybrid with such ultra-narrow band edge energy located in the infrared range would
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give rise to infinite heat generation through the vibration of crystal lattice, which is
expected to be a promising material for the development of solar–thermal conversion.
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Table 1 Physical structural parameters of different samples.
Sample
SBET (m ∙g )
pore volume (cm³∙g )
pore size (nm)
Pt-rGO
440.08
0.36
Pt-rGO-TiO2
65.62
0.46
2
-1
-1
1.47
3.93
band gap (eV)
Pt loading (wt%)
0
0.89
0, 2.94, 3.02
0.91
a
solar
UV
Vis
96.87
93.38
97.01
96.63
193.54
50.14
36.65
27.61
94.72
97.33
93.93
95.92
54.14
0
absorptance (%)b
IR
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5 C
o
dynamic adsorption capacity of 120 C
c
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toluene (g/L)
150 C
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80 C
a
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The value is obtained from ICP analysis.
b
24,25
Absorptance (a=∫(1−R(λ))*E)/∫E,
IR: 800-2500 nm.
solar: 300-2500 nm, UV: 300-400 nm, Vis: 400-800 nm,
c
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Dynamic adsorption capacity is integrated from breakthrough curves in Fig 6(a).
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Encouraged by the strong ultrabroad absorption band of Pt-rGO hybrid, IR light
irradiation was employed to inspect its photothermal catalysis reaction such as toluene
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oxidation. For IR illumination at 88, 115 and 146 mW·cm , the catalytic toluene
conversion over Pt-rGO hybrid are as high as 21.36%, 86.18% and 98.13% (Fig. 3(a)),
and the catalytic mineralization are 17.66%, 80.84% and 96.13% (Fig. 3(b)),
respectively, ~9.28%, ~16.85% and ~8.71% higher than that of Pt-rGO-TiO2
composite. The apparent activation energies (E ) based on Pt-rGO and Pt-rGO-TiO
app
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are calculated to be 106.42 kJ/mol and 123.90 kJ/mol (Fig. 3(c)), respectively.
Strikingly, the trend observed for catalytic performance is positively correlated with
light intensity, which greatly increases with light intensity. Specifically, the reaction
temperature spontaneously increases from room temperature to 122 °C, 154 °C and
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180 °C for Pt-rGO, and to 116 °C, 147 °C and 176 °C for Pt-rGO-TiO composite
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under IR irradiation intensity of 88, 115 and 146 mW/cm (Fig. 3(d)), respectively,
evidently higher than the resulting temperature (74 °C, 89 °C, 108 °C) of the reaction
system with fiberglass membrane but without catalyst coating. Importantly, the
chemical stability measurement (Pt-rGO hybrid is highly stable in an air
atmosphere at temperature up to 180 °C, Fig. S2), together with the durability
experiment (the amount of evolved CO under catalytic reaction condition is
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continuous and stable, Fig. S4) rule out the possibility of carbon skeletons of rGO,
confirming that the carbon source of reaction product CO indeed originated from the
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photothermal catalytic oxidation of C H . Likewise, as revealed by on-line GC and
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MS analyses in Fig. S4, toluene catalysis experiments with oxygen atmosphere
yielded the dominant product of CO , but under oxygen free atmosphere, neither
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distinct degradation of toluene nor the discernable generation of CO can be clearly
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observed, which ascertained that the oxygen source of reaction product CO certainly
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deriving from the extraction of oxygen from oxygen atmosphere for toluene oxidation
reaction. Additionally, recycling experiment for Pt-rGO hybrid exhibits that no
noticeable change occurs on the conversion and mineralization of toluene during 15
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catalytic runs for 45 h under IR irradiation of 146 mW·cm (Fig. 3(e)). Delightedly,
as shown in Fig. S5 and Table S1, Pt-rGO hybrid displays well retained morphology,
crystallinity and structural integrity from the XRD, SEM, HR-TEM and Raman
measurements, and no any oxidation of Pt and rGO nanosheets from XPS analysis
after reaction, demonstrating its excellent recyclability and stability under light
irradiation conditions. Furthermore, Pt-rGO hybrid has been shown to be active for
the total oxidation of other common VOCs, such as isopropanol, acetone, n-hexane,
benzene, o-xylene. As displayed in Fig. S6, high conversion activity for this series of
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VOCs can be achieved following the sequence: benzene ≈ o-xylene > isopropanol >
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acetone > n-hexane > 74%, and the mineralization activity order was as follows:
isopropanol > acetone > n-hexane > benzene > o-xylene, indicating that Pt-rGO
hybrid is a potential promising material available for most emitted VOCs oxidation.
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Fig. 3 (a) Time course of toluene conversion, (b) CO yield (c) linear-fitting Arrhenius plots of
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Pt-rGO and Pt-rGO-TiO catalysts and (d) the corresponding temperature curves over Pt-rGO,
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Pt-rGO-TiO and blank fiberglass membrane under IR irradiation with various light intensities (88,
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115 and 146 mW/cm ); (e) the durability of Pt-rGO composite for toluene oxidation with IR
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irradiation of 146 mW/cm under steady state condition.
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To differentiate the contributions of photoinduced hot holes driven
photocatalysis or/and radiative heat transfer driven photo-thermocatalysis to the high
catalytic activity, the steady-state temperature on the photocatalyst surface for toluene
oxidation was measured in situ with a thermocouple. The reaction temperature was
maintained at near room temperature by which the reactor was placed in a
glycerol-water bath of ultralow temperature to eliminate the potential interference of
photoinduced heat. After adsorption reaches saturation in 260 min at the temperature
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of 18 C, neither distinct decrease in toluene concentration nor discernable increase in
DOI: 10.1039/C9TA00148D
CO concentration was clearly observed in the control experiments within 60 min
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under IR irradiation with 146 mW/cm (Fig. S7). This result indicates that the Pt-rGO
hybrid is photocatalytic inactive toward toluene oxidation reaction at room
temperature under light illumination, which can be ascribed mainly to (1) the zero
bandgap of rGO endowed the holes in the hybrid with inadequate oxidation capacity
stemming from ungratified position matching between valence-band of semiconductor
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catalyst and redox potentials of toluene species/active oxygen (•O and •OH radicals)
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species, resulting in a quick depletion of excitation energy via thermal radiative
transfer, and (2) inexistence of energetic hot holes injection from the excitation of Pt
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NPs with broad light absorption, but without recognizable peak. As proved by the
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ESR spectra in Fig. S8, neither •O nor •OH radicals signal was detected in the IR
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light illuminated Pt-rGO suspension, indicating no reactive oxygen species generated
from electronic excitation effect of Pt plasmon resonance and rGO. Previous works
have established that the strong and anisotropic bonding and the low mass of the
carbon atoms give graphene and graphene-based composite strong light absorption
capability covering the full solar spectrum range,13,26-28 where the strongly absorbed
light radiation on Pt-rGO composite is efficiently converted into heat via
electron-phonon and phonon-phonon processes. Thereby, a rapid photothermal
heating occurs on Pt-rGO hybrid upon light irradiation, resulting in a remarkable
increase environment temperature of material.
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Fig. 4 (a) Time course of toluene conversion and (b) CO yield over Pt-rGO composite modulated
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at an identical temperature under IR, UV-Vis, full spectrum irradiation with various light
intensities.
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Based on the above observations, we infer that photo-induced thermocatalysis is
the decisive effect that catalyzes toluene oxidation. To further verify temperature
dependence of reaction under various illumination conditions, the catalytic activity for
toluene oxidation by irradiation of Pt-rGO hybrid with full spectrum, UV-Vis or IR
light was carried out. As is displayed in Fig. 4, at identical temperature conditions
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illuminated by light with different wavelength, the toluene conversion and CO yield
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over Pt-rGO hybrid within 150 min of full spectrum and UV-Vis irradiation, is almost
equivalent to those achieved of IR illumination. Significantly, Pt-rGO hybrid can
effectively channel the flux of UV, Vis and IR photons into heat to drive redox
catalysis application, which refers to the photo-assisted catalytic process dominated
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solely by the photothermal effect of Pt-rGO hybrid without an additional
DOI: 10.1039/C9TA00148D
photoactivated charge transfer and injection. The photothermal conversion efficiency
(η) is defined by the ratio of the converted thermal energy versus the irradiation
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energy of photon flux, where maximum solar–heat conversion efficiency is able to
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17.6% for Pt-rGO composite under IR irradiation of 146 mW/cm (Fig. S9).
Expectantly, Pt-rGO composite material shows great promise for redox reaction
through photo-thermal effect in view of the efficient utilization of solar energy, which
opens the alternative possibility of emerging light-driven photo-thermocatalysis in
comparison with conventional electricity-driven thermal-based catalysis. To
determine the origin of enhanced catalytic activity especially mineralization on
Pt-rGO hybrid, we conduct a series of control experiments for performance
comparison with Pt-rGO hybrid. As illustrated in Fig. S10, rGO material initially
exhibits almost complete toluene degradation in the first 5 minutes, but gradually
decreases with irradiation time, and finally reduces to zero at 70 min or above under
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IR irradiation intensity of 146 mW/cm , this is because toluene adsorption is a fast
process, and equilibria could be reached in about 70 min for rGO material.
Significantly, after adsorption of toluene to saturation, neither observable toluene
degradation nor discernable CO generation occur under IR illumination even if the
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rGO itself displays strong IR light absorption and remarkable photothermal effect,
suggesting that no catalytic oxidative degradation of toluene could be achieved on
rGO material, whose role would be not in the catalysis of this reaction but in proper
positioning of adsorption layer. Importantly, Pt-decorated rGO nanosheets-adsorption
layer exhibits significantly improved activity, highlighting that the photoinduced heat
supplied to Pt active sites holds the key to the catalytic performance. Since the energy
intensity of black rGO nanosheets was very high (absorptance of 97 %), essentially
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independent of Pt NPs with limited absorptance (67 %) in this system. Herein, the Pt
DOI: 10.1039/C9TA00148D
catalytic sites coordinated on rGO nanosheets can serve as a highly active and durable
heterogeneous catalyst for toluene oxidation, wherein the rGO bridges homogeneous
light absorbers with Pt active sites for the efficient heat transfer.
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Fig. 5 (a) Nitrogen adsorption/desorption isotherms and (b) corresponding pore size distribution
curves of Pt-rGO and Pt-rGO-TiO2.
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As described above, the Pt-rGO hybrid shows the superior photo-thermal
catalytic activity with even higher toluene conversion and mineralization efficiency
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than that of Pt-rGO-TiO composite under light irradiation. To get a deeper insight
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into the performance enhancement of the Pt-rGO hybrid, multifarious combined
characterizations such as BET, toluene adsorption/desorption were conducted to
investigate the physicochemical properties of the material. The textural structure of
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Pt-rGO hybrid is performed by nitrogen physisorption measurement. As shown in Fig.
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5(a), the adsorption–desorption isotherm shows a type IV isotherm with pronounced
adsorptions at low (<0.1 P/P ) and medium (0.2–0.9 P/P ) relative pressures, which
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indicates the existence of micropores and mesopores in rGO nanosheets. Moreover,
a very small hysteresis loop between the adsorption and desorption branches in the
pressure range of 0.4–0.9 P/P further reveals a highly uniform pore size in porous
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rGO nanosheets. The pore size distribution curve (DFT model) derived from the
adsorption branch in Fig. 5(b) clearly exhibits a dominant peak at 0.82 nm and several
small peaks at 1.25, 3.11, 3.89, 6.32 and 9.33 nm. While for Pt-rGO-TiO sample, the
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adsorption curve distinctly shows the initial uptake in the low pressure range of near
0.02, the gradual increase of the volume adsorbed in the medium pressure range and
no saturation in the high pressure range, which indicates the existence of extremely
small number of micropores and various sizes of mesopores in wide size
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distribution. The hysteresis loop shifts to a higher and wider P/P pressure range
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(0.4–1.0 P/P ), which is related to the presence of a wider range of mesopores, along
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with larger diameters. The pore size distribution curve (BJH model) derived from
the desorption branch clearly displays two narrow distributions of pores centered
around 1.97 and 3.94 nm as well as one broad distribution centered approximately
34.10 nm, suggesting Pt-rGO-TiO mainly consist of mesoporous structure. The pore
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parameters and BET surface areas are summarized in Table 1. Significantly, the
adsorption data of Pt-rGO hybrid indicate a very high specific surface area of 440.08
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m ∙g . In most cases, the support with a large specific surface area helps to increase
the exposed active surface sites of metal loading for gas absorption and catalytic
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surface reactions, and thus promote the injection of activated oxygen atoms to the
adsorbed toluene for improving the catalyst efficiency. The size distribution
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histogram of the Pt NPs in Pt-rGO hybrid by randomly picking out 100 black dots
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from HR-TEM images in Fig. 1(e) exhibits that the average size of the well dispersed
Pt NPs is 2.46 nm, smaller than that in Pt-rGO-TiO2.
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Fig. 6 (a) The breakthrough curve for adsorption adsorption (toluene concentration of 930 mg/L,
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gas flow rate of 51.8 mL/min, catalyst of 0.10g) and (b) Toluene-TPD over Pt-rGO and
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Pt-rGO-TiO at various temperatures.
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In principle, both adsorption and catalytic oxidation could decrease toluene
concentration. Before the catalytic oxidation, dynamic adsorption behavior of Pt-rGO
and Pt-rGO-TiO samples for gaseous toluene with a concentration of 930 mg/L in a
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N stream was measured by an on-line gas chromatograph equipped with FID. Fig.
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6(a) presents the evolution of toluene outlet concentration as a function of time at
different temperatures. At the beginning of breakthrough curve for Pt-rGO, the
toluene outlet concentration is nil, which means that all gaseous toluene is completely
adsorbed, indicating good adsorption capacity at saturation. One can also observe that
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breakthrough occurs more rapidly on Pt-rGO-TiO in couples of minutes where
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toluene outlet concentration rises to initial concentration. Adsorption is closely related
to temperature where almost no obvious toluene adsorption occurs on Pt-rGO-TiO at
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the temperature of 120 C or above, but toluene adsorption capacity over Pt-rGO
hybrid are as high as 193.54, 50.14, 36.65 and 27.61 g/L at the temperature of 25, 120,
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150 and 180 C, much higher than those obtained with the Pt-rGO-TiO composite.
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Under identical conditions, Pt-rGO hybrid shows superior adsorption ability
compared with that of Pt-rGO-TiO composite. Namely, gaseous toluene is retained
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longer on Pt-rGO than on the Pt-rGO-TiO , which is consistent with toluene-TPD in
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Fig. 6(b). Dramatically, the toluene adsorption capacity of the catalysts follows the
same trend of SBET. Previous reports have also shown the surface area-determining
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adsorption capacity for either microporous or mesoporous materials. However, even
eliminating the direct effect of surface area by normalization, the adsorption capacity
of Pt-rGO hybrid is still much higher than that of Pt-rGO-TiO composite, revealing
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strong superiority of microporous Pt-rGO. This observation is specifically associated
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with different adsorption potentials of microporous and mesoporous in catalysts
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where the adsorption potential from adjacent micropore walls can be stacked overlap
filling and thus promote the adsorption of the gas toluene on the catalyst surface,
while the adsorption potential in mesopores is significantly shallow due to the large
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pore size. Given the highly strong adsorption potential in micropores, either the
reactant adsorbed or mediates mineralization formed in micropores during catalytic
oxidation are relatively difficult to release and stay a considerable long time at the
catalyst surface.36 Unsurprisingly, rGO possesses a big delocalized sp carbon
structure, endowing it with good adsorption efficiency towards reactant with a
conjugate aromatic structure mainly via π–π cooperative interactions. Moreover, the
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effective site of the sp carbon of rGO has abundant free-flowing π electrons and can
be activated by conjugating with the long pair electrons from the loaded Pt atoms,
which simultaneously increases the π−π interaction. As a result, the adsorption
capacity of Pt-rGO hybrid toward toluene is increased with the π−π interaction, which
contributes to increasing the number of activated toluene molecules. The analysis
above reveals that such conspicuous adsorption capacity of Pt-rGO hybrid, is
predominantly ascribed to the strong affinity between the toluene molecules and the
surface of Pt-rGO hybrid, deriving from the extremely large surface area, abundant
micropore structure and highly strong π−π interaction. As presented in Fig 1, the
photo-thermal catalytic activity of Pt-rGO was greatly improved with higher rGO
content in comparison to that of Pt-rGO-TiO , achieving approximate 8-17%
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enhancement of mineralization efficiency. This highlighted that a low rGO content in
the Pt-rGO-TiO sample did not provide adequate adsorption capacity to suppress the
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accumulation of carbonaceous intermediates, resulting in a decreased toluene
mineralization. However, upon increasing the rGO content, the intimate contact area
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and resultant prolonged interfacial interaction between catalyst and reactant enable the
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adsorbed toluene/intermediates to sufficiently react with oxygen to generate end
product of CO and H O.
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Fig. 7 In situ DRIFTS spectra of adsorbed species on Pt-rGO hybrid in a continuous toluene/air
feed gas upon heating at the elevated temperature.
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To reveal more details of toluene catalytic oxidation over Pt-rGO hybrid, in situ
DRIFTS spectroscopy was performed to explore the evolved surface chemical species.
Prior to toluene adsorption and reaction experiments, the Pt-rGO catalyst was flushed
with high-purity He at 150 °C for 1 h to remove adsorbed impurities. The background
spectrum was collected and automatically deducted from the sample spectra. Then, a
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flow of toluene/20% O /He feed gas was introduced to the cell at a flow rate of 50
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mL/min. Afterwards, a series of DRIFTS spectra were recorded at an elevated
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temperature. As displayed in Fig. 7, the broad peaks at 3084, 3039 and 2961 cm are
assigned to C-H stretching vibrations of the aromatic ring, while at 2917 and 2870
cm−1 are attributed to asymmetric/symmetric C-H stretching vibrations of methyl
group,37 respectively. The weak band at 1256 cm−1 is characteristic of ring
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vibrations. These absorption peaks originated from the adsorbed toluene develop
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rapidly and reach steady level within a few minutes. Particularly, an intense peak at
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1180 cm with a small shoulder at 1084 cm correspond to C–O stretching vibration
of an alkoxide species.39 The peak at 954 cm−1 is associated with the bending
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vibration and torsional vibration of CH . The appearance of these absorption peaks
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suggest that the C–H bond of methyl group of aromatic ring is cleaved to generate
C–O bond where the O atoms come from dissociated pre-adsorbed O molecules on
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Pt NPs decorated rGO nanosheets, forming benzoyl species (C H –CH –O). The
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new doublet peaks at 2362 and 2341 cm are characteristic of asymmetric O=C=O
stretching vibrations,42,43 and the intensities of their peaks drastically increase with
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temperature on stream, owing to the contribution of more generated CO as final
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product during progressively deeper catalytic oxidation reaction of adsorbed toluene
species at a higher temperature.
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Fig. 8 On-line TPD-MS result for used Pt-rGO hybrid illuminated in a continuous in a flow of
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7254 mg/L toluene/21% O /N feed gas with IR light intensity of 88 mW/cm for 60 h, and the
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released species in outlet gas containing CO (m/z = 44), CO (m/z = 28), H O (m/z = 18), toluene
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(m/z=92, 91, 65, 63, 39), benzyl alcohol (m/z = 79), benzaldehyde (m/z=77, 106) and maleic
anhydride (m/z = 54, 26).
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hybrid that was illuminated in a continuous flow of 7254 mg/L toluene/21% O /N
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feed gas with IR light intensity of 88 mW/cm for 60 h was performed using
temperature-programmed desorption coupled with mass spectrometry detection
(TPD-MS). 600 mg of used Pt-rGO hybrid was loaded in a U shaped quartz tube,
installed in a furnace coupled to a temperature controller-programmer and exposed to
a He flow at 30 ml/min for 20 min to remove O from the system. The temperature
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was then raised from room temperature to 810 °C at a rate of 10 °C/min , and the
effluents were analyzed by thermal conductivity (TCD) and MS (Tilon, TC-D200 M).
The signals at mass to charge (m/z) ratios were recorded from 0 to 180, as shown in
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Figure S1. Signals for m/z = 44 (CO ), m/z = 28 (CO; relative contributions of
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residual N and CO fragmentation characteristic to the overall 28 signal were
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indistinctive), m/z = 18 (H O), m/z = 54 (maleic anhydride) and m/z = 79 (benzyl
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alcohol), m/z = 77 (benzaldehyde) and m/z = 91 (toluene) were monitored by on-line
MS during the TPD experiment, as displayed in Fig. 8. With the analysis of
composition in the effluent gas, benzyl alcohol, benzaldehyde and maleic anhydride
were identified as reaction intermediates resulted from partial oxidation of toluene.
Under light irradiation, photo-thermal responsive Pt-rGO composite achieves broad
spectrum utilization for thermalization where the generated heat increases the local
temperature of the system and thus trigger the toluene catalytic oxidation reaction. On
the Pt-rGO catalyst, pre-adsorbed O molecules are firstly dissociated on Pt NPs
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decorated rGO nanosheets to form activated O atoms, subsequently spill to the surface
of rGO nanosheets, which could actively react with the chemisorbed toluene
molecules, giving intermediates of benzoyl, benzoate and maleic anhydride species.
Finally, the adsorbed maleic anhydride is further oxidized by activated O atoms to
final products of CO and H O.
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Scheme 1 Proposed reaction pathway for photo-thermal catalytic oxidation of toluene on Pt-rGO
hybrid.
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4. Conclusion
We have constructed large-area contiguous rGO nanosheets adsorption layer
supported Pt NPs that has surprisingly giant infrared absorption, well dispersed active
Pt nanoparticles and strong reactant adsorption capacity with light irradiation. Such
unique properties lead to high light-mediated activity of Pt-rGO hybrid for typical
catalytic reaction through photo-thermal effect, giving a toluene conversion of 98%
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and CO yield of 96% under IR irradiation intensity of 146 mW/cm , together with
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remarkable stability. Comparative characterizations disclose that the synergistic
effects of ultrabroadband strong light absorption, efficient light-to-heat conversion,
considerably high adsorption ability and preferentially exposed more catalytically
active sites, enable largely improved performance. Research effort along this direction
could potentially provide the new strategy to effectively utilize solar energy in a wide
range of chemical reactions.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgments
This work was supported by the One Hundred Talent Project and the Key
Program for Frontier Sciences from Chinese Academy of Sciences
[QYZDB-SSWDQC022]; National Natural Science Foundation of China [No.
21703233]; “Cooperation of Industry-University-Institute and Scientific and
Technological Cooperation” of Xiamen [No. 3502Z20182006].
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Notes and references
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