Photocatalytic Activity of Reduced Graphene Oxide–Gold Nanoparticle

Article Cite This: Energy Fuels 2018, 32, 2673−2680

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Photocatalytic Activity of Reduced Graphene Oxide−Gold Nanoparticle Nanomaterials: Interaction with Asphaltene and Conversion of a Model Compound Maria Luiza de O. Pereira,*,† Daniel Grasseschi,†,‡ and Henrique E. Toma*,† †

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, 05508-000 São Paulo, Brazil Graphene and Nanomaterials Research Center (MackGraphe), Mackenzie Presbyterian University, 01302-907 São Paulo, Brazil

‡

S Supporting Information *

ABSTRACT: Asphaltenes are residual materials from the oil industry and are usually converted, after exhaustive distillation, into coke and asphalt. However, conversion of asphaltenes into more valuable raw materials, for instance, by photocatalytic cracking using suitable catalysts, would be a better, more economic option. To explore this idea, we combined the electronic and chemical properties of asphaltene and graphene derivatives with the plasmonic nature of gold nanoparticles. For this purpose, a hybrid material was generated in situ, containing reduced graphene oxide and gold nanoparticles (RGO@AuNP). Evaluation of the interaction between the hybrid material and asphaltenes by hyperspectral dark-field optical microscopy indicated the occurrence of charge transfer between the two species. Using 9-anthraldehyde (9-ATA) as a model compound for asphaltene, photocatalytic experiments performed with RGO@AuNP at room temperature, under visible light irradiation, revealed the formation of cyclic endoperoxides, which undergo further reactions, resulting in their cleavage, with 90% yield for the 9-ATA degradation.

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Ni.18,19 Because of the variable composition and matrix complexity, investigation of chemical processes involving asphaltenes can be rather complicated and problematic. For this reason, suitable model compounds bearing some similarity to asphaltene species are sometimes employed to obtain more conclusive results.20−23 In recent years, nanotechnology has been incorporated into oil research, opening new economic and environmental perspectives,9,24 including the improvement and recovery of heavy oils.8,25 The interaction of inorganic nanoparticles (NPs) and asphaltenes is a relevant subject in oil nanotechnology. For instance, Nassar et al.8−10 employed different types of metal oxide NPs, namely, Fe2O3, Co3O4, NiO, and Al2O3, for asphaltene adsorption and catalytic steam gasification/cracking. The unique optical properties of gold nanoparticles (AuNPs), associated with their surface plasmon resonance, have inspired their use in photodegradation reactions.26,27 The energy of excited plasmons can be harnessed in different ways, owing to radiative and non-radiative decay processes.28,29 Accordingly, there are three basic ways in which the energy absorbed through the excitation of surface plasmons is used: (i) enhanced generation of excited states, (ii) creation of electron− hole pairs, or (iii) localized surface heating.26,27,30 When supported on two-dimensional materials, such as graphene and its derivatives, the catalytic properties of AuNPs can be enhanced by the high surface area, chemical and thermal stability, and tunable bandgap.31,32 The bandgap of chemically

INTRODUCTION Petroleum, despite sustainability concerns, is still the main source of energy in the world. As one of the most complex molecular materials existing in nature, petroleum has great economic relevance. Current technologies highlight the importance of extracting and exploring oil with maximum efficiency and reduced environmental impact; asphaltenes, the heaviest fraction of petroleum, are in the counter-current flow of technological development. Asphaltenes are well-known in oil refineries for forming hard scales that can obstruct pipelines, compromising flow assurance.1,2 For this reason, most studies in this area are focused on aggregation and precipitation processes,2−4 because asphaltenes can cause obstructions and affect, in many different instances, the oil production chain. Many efforts have been concentrated on eliminating the problems caused by this fraction, such as the use of acid treatment,5,6 degradation at high temperatures,7 and adsorption on metal oxide nanoparticles.8−10 Aggregation studies11−14 can also improve the understanding and contribute to the development of more effective prevention methods. At the present time, however, asphaltenes are converted into asphalt and coke by destructive distillation,15 despite the high cost and polluting technology involved, which employs high temperatures and releases sulfur compounds into the atmosphere. Environmental and sustainability concerns are pushing scientists to find innovative ways toward a more rational use of asphaltenes, e.g., by converting them into more valuable raw materials for the chemical industry. Asphaltenes are a complex material composed of a mixture of polycyclic aromatic hydrocarbons, comprising seven or more condensed rings as cores,16,17 and side alkyl chains with various functional groups, including elements, such as N, S, V, and © 2017 American Chemical Society

Special Issue: 18th International Conference on Petroleum Phase Behavior and Fouling Received: September 12, 2017 Revised: December 15, 2017 Published: December 18, 2017 2673

DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680

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field emission electron microscope operating at 7 kV and 3 mm working distance. Dark-Field Hyperspectral Microscopy. A CytoViva ultraresolution imaging system, composed by a dark-field hyperspectral arrangement mounted on an Olympus BX51 microscope, was used for recording single AuNP Rayleigh scattering spectra. In the dark-field configuration, a hollow light cone is generated, which is focused on the specimen. Only the scattered and diffracted light inside the cone reaches the objective, which has a numerical aperture smaller than the dark-field condenser numerical aperture. The zero-order diffracted light (transmitted light) is not collected, and in this way, the particles appear as bright spots on a dark background. The resolution power is limited by light diffraction. The CytoViva system uses an annular cardioid condenser40 with a high annular aperture that enables the collection of higher order diffracted light by the objective, increasing the resolution power up to λ/5. Allied to the intensified light scattering from plasmonic AuNP nanoparticles, this ultraresolution optical system enables the study of a variety of nanoparticles, in wet media. With this configuration, it is possible to record a dark-field optical image composed by three-channel RGB spectra with a spatial resolution of 64 nm and a hyperspectral image, where each pixel has 64 nm and carries full visible spectra information. In general, for a better spectral signal-to-noise ratio, a binning process can be made, where four pixels are summed and a new pixel of 128 nm with an average spectrum is generated. However, as a result of the enhanced light scattering of plasmonic nanoparticles, a full-resolution hyperspectral image can be achieved. The samples were prepared by drop casting 1 μL of each suspension on a NEXTERION ultraclean glass (Schott). An ultraclean NEXTERION glass coverslip (Schott) was put over the drop and sealed with adhesive tape to avoid oil penetration into the sample. For asphaltene, a 20 ppm toluene solution was used; for RGO@AuNP, the concentration was the same as obtained from the synthesis and the AuNP suspension was diluted by half with deionized water. The darkfield optical images and Rayleigh scattering spectra were recorded with the sample wet, to keep the same refraction index. Photocatalysis. A 1.8 mL solution of 1 mM 9-ATA in acetonitrile was added in a quartz cuvette with 0.2 mL of RGO@AuNP suspension in water. The system was kept under synthetic air, with magnetic stirring, and irradiated with an 150 W halogen lamp (500−2500 nm) in a temperature-controlled chamber. After excitation, the products were extracted with an ethyl acetate/water mixture and characterized by gas chromatography−mass spectrometry (GC−MS). For nuclear magnetic resonance (NMR) analysis, a Bruker Avance III, 500 MHz, model DRX500 has been employed. An infrared spectrum was recorded using a Fourier transform infrared (FTIR) spectrometer from Bruker Alpha model, using the attenuated total reflectance (ATR) mode with a germanium crystal, providing a spectral window of 4000− 600 cm−1. Gas chromatography (GC) analyses were performed using a Shimadzu 17A with a flame ionization detector (FID), RTX-5, 30 m × 0.25 mm × 0.25 mm, with N2 as the gas carrier. GC−MS was carried out with a Shimadzu model QP 5050A, DB-5 ms, 30 × 0.25 × 0.25 mm, with He as the gas carrier, quadrupole detector, and electron ionization at 70 eV. Column temperature, 60°C (5 min) to 280 °C (10 min) (at 10 °C min−1); injector temperature, 200 °C (5 min) to 280 °C; and detector temperature, 280 °C.

derived graphene can be modulated by the degree of oxidation or interactions with metal or metal oxide NPs33 to obtain better semiconductors for photocatalysis in water treatment34 and energy storage devices.35 In graphene oxide−AuNP hybrid materials (GO@AuNP), the graphene oxide (GO) sheet can act as an electron or a hole receiver, increasing the number and lifetime of photogenerated electron−hole pairs.32,36 This type of hybrid system has been claimed to have a higher catalytic activity than AuNPs for the reduction of o-nitroaniline to 1,2benzenediamine and is also a better substrate for surfaceenhanced Raman spectroscopy.37 Wang et al. have reported that reduced graphene oxide−AuNP (RGO@AuNP) exhibits catalytic activity for the oxidation of p-aminothiophenol to p,p′dimercaptoazobenzene, which involved the generation of activated O2 species.38 Here, AuNPs were synthesized in situ in the presence of GO by thermal reduction with sodium citrate. The process led to spherical AuNPs directly attached to the reduced graphene oxide (RGO) surface, providing a synergetic material, in which the plasmonic waves interact with the RGO bandgap. This hybrid material was considered as a potential catalyst for asphaltene degradation at room temperature. Investigation of the electronic interaction between AuNPs and RGO using hyperspectral dark-field optical microscopy suggested the occurrence of charge transfer involving the AuNP plasmons and the RGO conduction band. After asphaltene addition to RGO@AuNP, aggregates were detected in the dark-field images. Broadening of RGO@AuNP scattering spectra was also observed, supporting the occurrence of an electronic interaction between asphaltene and the hybrid nanomaterial. As a proof of concept, the RGO@AuNP hybrid material was applied to the photodegradation of 9-anthraldehyde (9-ATA), a model compound for asphaltene, in the presence of dioxygen. Photodegradation was observed at room temperature under visible light, with effective yields of ∼90%.

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EXPERIMENTAL SECTION

Materials. GO suspension in water, HAuCl4·3H2O, and sodium citrate were obtained from Sigma-Aldrich and used without further purification. Ultrapure water from Direct-Q 5UV, Millipore, was used throughout the work. RGO@AuNP Synthesis. First, the glassware was previous washed with aqua regia (3HCl/1HNO3). The RGO@AuNP synthesis procedure was slightly modified from the literature.39 A total of 1 mL of GO (1 mg mL−1) aqueous dispersion was added to 20 mL of HAuCl4 (0.01 wt %) aqueous solution, and the mixture was sonicated for 30 min. After heating to 80 °C (water bath), 1 mL of sodium citrate (1 wt %) aqueous solution was added under stirring. After 1 h, the system was cooled to room temperature with continuous stirring. The sample was centrifuged for 10 min at 5000 rpm, and the solid residue was collected and resuspended in ultrapure water and then centrifuged at 10 000 rpm for 60 min. The final residue was resuspended in 15 mL of ultrapure water and stored in this form, in a dark, closed bottle. Atomic force microscopy (AFM) images was recorded on a Bruker ICON system in taping mode using a RTESPA-300 tip. For the asphaltene film, 1 μL of a 10 or 200 ppm toluene solution was dropcasted onto a highly ordered pyrolytic graphite (HOPG) substrate. For the GO film, a silicon wafer was kept under the corresponding 1 mg mL−1 water dispersion for 12 h, washed with Milli-Q water, and dried under a N2 stream. In the case of the RGO@ AuNP film, 1 μL of the suspension from the synthesis was dropcasted onto a NEXTERION ultraclean glass, from Schott. A WITec confocal raman microscope, equipped with a He−Ne laser, was used to obtain the Raman spectra at λexc of 633 nm. The scanning electron microscopy (SEM) images were obtained using a JEOL model 7200

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RESULTS AND DISCUSSION RGO@AuNP Characterization. AuNPs supported on RGO were obtained by mixing a HAuCl4 solution with a GO suspension, followed by the addition of sodium citrate as a reducing and stabilizing agent. In this process, the oxygen functional groups on the GO surface can also be involved in AuNP nucleation and growth. During the reduction process, both gold ions and the GO surface are reactive, leading to the formation of RGO incorporating AuNPs, denoted here as RGO@AuNP hybrids. Figure 1 shows an AFM topography 2674

DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680

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shift of the D band were observed, indicating that, during AuNP formation, citrate ions also reacted with the oxygen groups on the GO surface, leading to the formation of RGO.42,43 The scattering spectra of RGO@AuNP were measured using hyperspectral dark-field microscopy (Figure 2). An optical darkfield image of an RGO@AuNP film on a glass slide, where the AuNPs are seen as colored spots, is shown in Figure 2B. The green and red spots correspond to isolated particles, and the large yellow spots are composed of NP aggregates. The AuNPs are mainly deposited over the RGO surface, with only a few particles observed on RGO flake edges, suggesting that AuNP nucleation and growth occur preferentially on the flake surface. The SEM image in Figure 2A confirms the presence of both isolated and aggregated AuNPs on the surface of RGO with a size distribution of 20 ± 4 nm, in agreement with the AFM height profile in Figure S2 of the Supporting Information. The optical properties of AuNPs are associated with enhanced light absorption and scattering processes, owing to excitations of their conducting electrons, known as surface plasmons.44,45 Excited surface plasmons can decay by radiative and non-radiative pathways, giving rise to enhanced scattering and absorption of light and characteristic AuNP colors. These properties are revealed by the scattering spectrum, where the scattering intensity is dependent upon the scattering crosssection.44 Particle size, shape, composition, surface functionalization, and chemical environment have a direct effect on the optical properties of NPs, owing to changes in charge localization on the metallic surface.44,46−49 In this context, surface plasmon spectroscopy can play a key role, allowing for the optical properties of individual NPs to be studied. In this technique, an optical microscope in a dark-field configuration can be used to record the Rayleigh scattering spectra of single particles.50−52 The dark-field configuration is generally used, owing to the higher resolution power achieved by oblique illumination. The scattering spectra of RGO@ AuNP, measured using hyperspectral dark-field optical microscopy, revealed regions with contrasting behavior, owing to differences in particle sizes and aggregation. For isolated particles on a RGO sheet, the spectra showed one band in the region between 550 and 700 nm (green and red curves in Figure 2C). In contrast, AuNP aggregates showed a broad band with λmax > 700 nm, owing to plasmon coupling between nearby particles.53 Further, increased scattering intensity was observed at wavelengths below 500 nm, owing to light scattering at RGO edges and wrinkles (Figure S4 of the Supporting Information). To obtain a better understanding of the optical properties, the scattering spectrum of RGO@AuNP was compared to that of isolated AuNPs with a similar size distribution (25 ± 5 nm) (panels C and F of Figure 2). In this case, most particles showed a green color (Figure 2E), and the scattering spectra were characterized by a single plasmonic band at 500−700 nm (Figure 2F).54 The plasmonic band full width at half maximum (fwhm) is an important parameter for characterizing the optical and electronic properties of AuNPs, because it is related to the coherence of the plasmonic oscillations and, consequently, the lifetime of excited electrons. The coherence of plasmonic oscillations is determined by radiative and non-radiative damping effects of AuNP plasmons.51,55 Radiative damping is related to the decay of electrons to photons and is dominant in particles larger than 50 nm.51 In contrast, non-radiative

Figure 1. AFM topography images of (A) GO and (B) RGO@AuNP deposited on a Si/SiO2 wafer. (C) Raman spectra of GO (black curve) and RGO@AuNP (pink curve) acquired at λexc = 633 nm.

image of the GO sheets before and after NP formation. The GO sheets show typical heights between 1 and 4 nm (see the height profile in Figure S1 of the Supporting Information), confirming the presence of flakes with few layers. The reduction of gold ions by sodium citrate leads to an increase in the surface root-mean-square (RMS) roughness from 0.9 to 4.3 nm, owing the formation of AuNPs and some particle aggregates on the RGO surface (Figure 1B). In this case, the height profiles show the presence of particles with heights varying between 10 and 40 nm (see Figures S2 and S3 of the Supporting Information). The Raman spectrum of GO shows the characteristic D and G bands of graphitic materials41 (Figure 1C), where the D band at 1350 cm−1 is related to defects in the crystalline structure and the G band at 1590 cm−1 is assigned to CC stretching vibrations of graphitic materials with sp2 carbons. For RGO@ AuNP, a small increase in the D/G intensity ratio and a red 2675

DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680

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Figure 2. RGO@AuNP: (A) SEM image, (B) optical dark-field image, and (C) scattering spectra from selected points on image B. AuNP: (D) SEM image, (E) optical dark-field image, and (F) scattering spectra from selected points on image E.

Figure 3. (A) AFM topography image of the asphaltene deposit formed on HOPG, (B) optical dark-field image, and (C) scattering spectra from selected points on image B of the asphaltene deposit formed on a glass slide. RGO@AuNP: (D) optical dark-field image after the addition of 20 ppm asphaltene solution, (E) hyperspectral dark-field image, where the RGB channels are chosen as the scattering intensities of 635, 545, and 465 nm, and (F) scattering spectra from selected points on image E.

damping is related to electron−phonon, electron−electron, electron−defect, and electron−surface scattering processes and electron−hole pair creation, owing to inter- and intraband electronic transitions. This non-radiative process is more effective in small particles, and the efficiency of electron− surface scattering processes increases as the particle size

decreases, owing to limitations of the electron free path in the conduction band, resulting in a broadening of plasmon bands for smaller particles.55 This effect is evident in the AuNP scattering spectra shown in Figure 2F, where λmax and fwhm for the green particles are 564 ± 9 nm and 73 ± 10 nm, respectively, whereas those for the red particles are 634 ± 10 2676

DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680

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Energy & Fuels nm and 53 ± 5 nm, respectively. Therefore, the green particles are smaller than the red particles and have a broader plasmon band. Charge transfer transitions from the AuNP Fermi level to the conduction band or from the valence band to the AuNP Fermi level can occur in materials displaying electronic levels with appropriate energies. Smaller differences between donor and acceptor energy levels result in stronger coupling, increasing the probability of charge transfer transitions. These transitions cause a loss of coherence for the excited electrons, leading to an increase in the fwhm of the plasmon band.56 In the case of RGO, an excited electron from an AuNP can be transferred to the RGO conducting band, creating a hole in the NP and increasing the plasmon bandwidth. For the RGO@AuNP sample, the fwhm varies between 65 and 115 nm for particles with λmax around 550 nm (green particles). For red particles, with λmax around 650 nm, a fwhm of 91 ± 45 nm was measured for over 100 particles. The broadening of the RGO@AuNP bands in relation to those of isolated AuNPs indicates that the interaction between AuNP and RGO allows for charge transfer transitions from the AuNP Fermi level to the RGO conducting band. Because the excited electrons are located in the RGO conduction band and the holes in the AuNP Fermi level, longer lifetimes can be achieved, which is important for the catalysis38 of asphaltene photocracking reactions. Interaction between Asphaltenes and RGO@AuNP. The interactions between RGO@AuNP and asphaltene were carefully studied before evaluating the possibility of photocracking. Figure 3A depicts the AFM topography image of an asphaltene film made by drop casting a 200 ppm toluene solution on a HOPG substrate. Asphaltene formed a continuous film with island aggregates with dimension of less than 2 μm, heights of ∼3 nm, and a RMS roughness of ∼1.3 nm (Figure S5 of the Supporting Information). The film appeared thicker at the edges, with heights of around 10 nm. Owing to surface tension effects, bubbles and rings are probably formed, as shown in Figure 3A. In the dark-field optical image of the asphaltene film (Figure 3B), the islands are seen as round bright spots with diameters of ∼2 μm, and the thickest part of the film edge and the holes can be clearly identified in the optical image. The scattering spectra of this film exhibit a characteristic profile, where the intensity increases as the wavelength decreases, as shown in Figure 3C. The intensity increase observed at wavelengths above 800 nm is related to the second diffraction order of the light scattered between 400 and 500 nm. Because dark-field microscopy only measures scattered light, light absorption, owing to the electronic transition of asphaltenes, does not contribute to the observed spectra; however, fluorescence can influence the measured spectra, leading to a slight broadening of the scattering profile (Figure S4 of the Supporting Information). The AFM analysis of Raj et al. indicated that the size and shape of asphaltene aggregates are dependent upon the substrate surface.57 For hydrophilic substrates, such as mica, asphaltene forms particle aggregates, and for hydrophobic substrates, such aggregates show higher thicknesses, owing to poor surface adhesion. HOPG substrates are an exception, and thin film aggregates are formed,57 as shown in Figure 3A, owing to π−π interactions between the graphitic surface and the asphaltene aromatic core. The same behavior was also expected for RGO@AuNP, with π−π interactions between asphaltene and the graphene surface; however, because the nanomaterial

surface has some hydrophilic sites (from oxygen functional groups), particle-like aggregates were observed.57 To evaluate the electronic interaction between RGO@AuNP and asphaltenes, a film was made by drop casting a 20 ppm asphaltene solution over an RGO@AuNP film, and its optical properties were monitored by hyperspectral dark-field microscopy (panels D−F of Figure 3). As seen in the optical dark-field image in Figure 3D, the AuNPs are still visible as red and yellow spots. Additionally, owing to asphaltene aggregation, some bluish spots with an average size of 1 ± 0.2 μm are observed; however, we cannot distinguish the formation of particle- or film-like asphaltene aggregates, and our attempts to evaluate the heights of asphaltene aggregates by AFM were inconclusive because of the intrinsic roughness of RGO@AuNP. In the hyperspectral image in Figure 3E, bluish asphaltene aggregates can be clearly distinguished from isolated and aggregated AuNPs because they exhibit stronger scattering at 465 nm, as shown in Figure 3F and Figure S6 of the Supporting Information. Because RGO scatters light preferentially at the flake edges and folds, the recorded images are not consistent with such features,58 and the blue spots can be ascribed to asphaltene aggregates. After asphaltene addition to RGO@AuNP, a small red shift of the plasmon band was observed, indicating that asphaltene changes the environment surrounding the AuNPs. The fwhm, measured for 60 particles with λmax between 600 and 750 nm, increased to 128 ± 46 nm, indicating that charge transfer transitions can occur between RGO@AuNP and asphaltene. In this case, the formation of an asphaltene film facilitates charge transfer transitions, owing to the close contact between the asphaltene aromatic core and RGO through π−π interactions. As shown in the scattering spectra in Figure 3F and Figure S6 of the Supporting Information, in regions where the contribution of asphaltene is higher, the AuNP plasmon band is red-shifted and broader. Photocatalysis. Photoexcitation of RGO@AuNP was expected to promote electron transfer from AuNPs to the RGO conduction band and then to asphaltene orbitals. For this purpose, a halogen lamp was employed because of the broad emission between 600 and 770 nm (Figure S7 of the Supporting Information), which matches the spectra shown in Figure 2C and Figure 3F. However, because of the high complexity of the asphaltene matrix, initial studies with a model species are more convenient for exploring the photoreactivity of RGO@AuNP. For this reason, we carried out photocatalytic studies using 9-ATA as a model molecule, inspired by previous work by Scaiano et al. on AuNPs supported on a diamond.59 According to these authors, 9-ATA is converted to anthraquinone via a mechanism involving photoexcitation of AuNPs, with electron transfer to O2, generating reactive oxygen species (ROS) and a hole, which can be transferred to 9-ATA, initiating conversion of 9-ATA into the observed product. Analogously, we investigated the photocatalytic oxidation of 9-ATA by RGO@AuNP in the presence of air, using visible light from a halogen lamp. Recently, it has been shown that RGO alone is also a powerful photocatalyst for ROS generation60 because of accessible excitation from the visible bandgap. Surprisingly, Some et al.61 observed that thiophene species can react under such conditions, generating hydrocarbon radicals that remain anchored on the carbon plates of the formed RGO, thus increasing the number of carbon atoms in the substrate. 2677

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Figure 4. (A) Chromatogram obtained after 24 h of reaction. (B) Formation of 9-ATA photooxidation product on the RGO@AuNP surface, under broad band illumination.

tenes into more interesting chemical products, further efforts are required in this area, particularly involving the investigation of RGO@AuNP in many different solvents and conditions, including the evaluation of size effects and asphaltene aggregation conditions. This is a challenging task, but preliminary studies on asphaltene films and RGO@AuNP have indicated that a significant extent of photodegradation can be achieved, as indicated by the broadening and shift of the D and G bands in the Raman spectra as a function of laser power (Figure S13 of the Supporting Information).

Therefore, by encompassing two active species for generating ROS photochemically, RGO@AuNP is expected to be an even better photocatalyst, with a great possibility of performing synergistically through the concerted action of both species. Figure 4A depicts the GC analysis after 24 h of reaction, showing that a major product (89%) with a retention time of 24.8 min is obtained, in addition to the starting species (9-ATA, retention time of 19.9 min) and anthraquinone (retention time of 18.1 min). Analysis of the products by mass spectrometry (MS) (major (m/z) peaks at 149, 167, and 279), FTIR (Figure S11 of the Supporting Information), and 1H NMR (Figure S10 of the Supporting Information) was consistent with the formation of phthalate esters. We believe, from the previous work by Scaiano et al.,59 that the primary photooxidation species is a cyclic endoperoxide intermediate, preceding the anthraquinone product. The concerted action of RGO@AuNP could promote further photochemical cleavage of locally formed anthraquinone to generate products, such as those suggested in Figure 4B based on GC−MS (Figures S8 and S9 of the Supporting Information), 1H NMR (Figure S10 and Table S1 of the Supporting Information), and FTIR (Figure S11 and Table S2 of the Supporting Information) analyses. These reactions occur owing to the influence of light and heat (generated by electron−phonon coupling of AuNP excited plasmons)62 and reactive radical ion species (generated by photoinduced electron transfer on the catalyst surface). It should be noted that, without the RGO@AuNP catalyst, the reaction led to the formation of 16.5% anthraquinone only (Figure S12 of the Supporting Information). Further, without air bubbling, no product was observed, confirming that the reaction involves dioxygen. These results are quite relevant, because they demonstrate that RGO@AuNP is a potential photooxidation catalyst in the presence of dioxygen and that the presence of RGO can promote the formation of useful organic products, opening interesting perspectives for asphaltene photoconversion. This task is currently being pursued in our laboratory, but unfortunately, as asphaltenes are poorly soluble in acetonitrile and aqueous solvents, attempts to carry out comparative experiments, guided by 9-ATA as a model compound, have not yet been successful. In contrast, in toluene, which is a good solvent for asphaltenes, the chemistry, photochemistry, and photophysics of RGO@AuNP are not known, and the reactivity of the ROS species may also be quite different. Therefore, despite the possibility of photoconverting asphal-

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CONCLUSION AuNPs form an interesting hybrid material with RGO (RGO@ AuNP) that displays charge-transfer interactions, as revealed by hyperspectral dark-field microscopy. The RGO@AuNP species interact with asphaltene, leading to detectable changes in plasmonic extinction spectra, mainly in the plasmon bandwidth. Using 9-ATA as a model compound for asphaltene, RGO@ AuNP was revealed to exhibit enhanced photocatalytic activity in the presence of dioxygen, leading to the formation of anthraquinone and promoting further photochemical cascade reactions to yield phthalate products. According to the reported results, this photocatalyst is a good candidate for converting asphaltene into more valuable chemical products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02715. AFM topography images of GO (A) and height profiles of selected regions (B) (Figure S1), AFM topography images of RGO@AuNP (A) and height profiles of selected regions (B) (Figures S2 and S3), hyperspectral image and scattering spectra of a dropcast film with GO (1 mg mL−1) and asphaltene (20 ppm, toluene) (Figure S4), AFM topography images of a 200 ppm asphaltene film on HOPG (A) and height profiles of selected regions (B) (Figure S5), spectra of RGO@AuNP after the addition of a 20 ppm asphaltene solution and the corresponding hyperspectral dark-field image (inset) (Figure S6), emission spectrum of the halogen lamp used in the photocatalysis experiments (Figure S7), mass spectrum of the product with tR = 24.8 min (89%), showing a molecular peak at m/z 391 and corresponding fragments at m/z 279, 167, 149, 113, 71, 57, and 43, with 2678

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corresponding assignments (Figure S8), mass spectrum of anthraquinone, a product of the photooxidation of 9ATA (Figure S9), 1H NMR spectra of the 9-ATA photocatalysis products, Bruker 500 MHz (Figure S10), 1 H NMR chemical shifts (Table S1), infrared spectrum of the 9-ATA photocatalysis products in CCl4 (Figure S11), tentative assignment of infrared spectra (Table S2), chromatogram after irradiation for 24 h (with a halogen lamp) of 9-ATA in acetonitrile and water without catalyst, leading to the formation of 16.5% anthraquinone (Figure S12), and Raman spectra of an asphaltene film as a function of laser power (Figure S13) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Maria Luiza de O. Pereira: 0000-0002-9316-5386 Daniel Grasseschi: 0000-0001-6066-0869 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP 24725-4) and Conselho Nacional de ́ e Tecnológico (CNPq fellowships) Desenvolvimento Cientifico is gratefully acknowledged.

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NOMENCLATURE 9-ATA = 9-anthraldehyde RGO@AuNP = reduced graphene oxide@gold nanoparticle GC−MS = gas chromatography−mass spectrometry fwhm = full width at half maximum AuNP = gold nanoparticle GO = graphene oxide

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DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680

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DOI: 10.1021/acs.energyfuels.7b02715 Energy Fuels 2018, 32, 2673−2680