Photocatalytic activity of reduced graphene oxideâgold nanoparticle

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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 Oliveira Pereira, Daniel Grasseschi, and Henrique Eisi Toma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02715 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Submitted to Energy and Fuels – Petrophase 2017 Special Number

Photocatalytic activity of reduced graphene oxide– gold nanoparticle nanomaterials: Interaction with asphaltene and conversion of a model compound Maria Luiza de O. Pereira*1, Daniel Grasseschi1,2 and Henrique E. Toma*1 1

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo,

05508-000, São Paulo, Brazil 2

Mackgraphe-Graphene

and

Nanomaterials

Research

Center,

Mackenzie

Presbyterian

University, 01302-907, São Paulo, Brazil KEYWORDS: Asphaltenes, Gold nanoparticles, Reduced graphene oxide, Photocatalysis.

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

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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.

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 as 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 in spite of the high cost and polluting technology involved,


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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 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 NPs (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 nonradiative 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


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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 band gap of chemically derived graphene can be modulated by the degree of oxidation or by 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 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 1,2-benzenediamine and is also a better substrate for surface enhanced 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 synthetized in situ in the presence of graphene oxide (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


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proof of concept, the RGO@AuNP hybrid material was applied to the photodegradation of 9anthraldehyde (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%. EXPERIMENTAL SECTION Materials. Graphene oxide (GO) suspension in water, HAuCl4.3H2O, and sodium citrate were obtained from Sigma-Aldrich and used without further purification. Ultrapure water from DirectQ® 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 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 up to 80ºC (water bath), 1 mL of sodium citrate (1 wt%) aqueous solution was added under stirring. After 1 h, the system was cooled down 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 10000 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. AFM images was recorded on a Bruker Icon System in taping mode using a RTESPA300 tip. For the asphaltene film, 1 µL of a 10 or 200 ppm toluene solution was dropcasted onto a 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 RGO@AuNP film, 1 µL of the suspension from the synthesis was dropcasted onto a


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NEXTERION® ultra-clean glass, from Schott. A WITec Confocal Raman Microscope, equipped with He-Ne laser, was used for obtaining the Raman spectra at λexc 633 nm. The scanning electron microscopy images (SEM) were obtained using a JEOL model 7200 field emission electron microscope operating at 7 kV and 3 mm working distance. Dark-Field Hyperspectral Microscopy. A CytoViva ultra-resolution 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 a 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 ultra-resolution 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 a three channel RGB spectra with a spatial resolution of 64 nm, and a hyperspectral image where each pixel has 64 nm and carries a 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, due the enhanced light scattering of plasmonic nanoparticles, a full resolution hyperspectral image can be achieved.


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The samples were prepared by drop casting 1 µl of each suspension on a NEXTERION® ultra-clean glass (Schott). An ultra-clean NEXTERION® glass cover slip (Schott) was put over the drop, and sealed with an 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 obtained from the synthesis, and the AuNP suspension was diluted by half with deionized water. The dark-field optical images and Rayleigh scattering spectra were recorded with the sample wet, in order to keep the same refraction index. Photocatalysis. 1.8 mL solution of 9-anthraldehyde 1 mM 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 150W halogen lamp (500-2500 nm) in a temperature controlled chamber. After excitation, the products were extracted with ethyl acetate/water mixture and characterized by gas chromatography−mass spectrometry (GC-MS). For NMR analysis, a Bruker Avance III, 500 MHz, model DRX500 has been employed. Infrared spectrum was recorded using a Fourier Transformed Infrared spectrometer (FTIR) from Bruker Alpha model, using the 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 FID; RTX-5 30 m x 0.25 mm x 0.25 mm, with N2 as the gas carrier. Gas chromatography/mass spectrometry (CG-MS) was carried out with a Shimadzu Model QP 5050A; DB-5ms 30 mm x 0.25 mm x 0.25 mm with He as the gas carrier, quadrupole detector, and electron ionization at 70 eV. Column temperature: 60ºC (5 min) to 280 ºC (10 min) (10ºC min-1); injector temperature: 200ºC (5 min) to 280ºC; detector temperature: 280ºC.


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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, here denoted as RGO@AuNP hybrids. Figure 1 shows an AFM topography 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). 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 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


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Figure 1. AFM topography images of GO (A) and RGO@AuNP (B) deposited on Si/SiO2 wafer. Raman spectra of GO (black curve) and RGO@AuNP (pink curve) acquired at λexc = 633 nm (C). The scattering spectra of RGO@AuNP were measured using hyperspectral dark-field microscopy (Figure 2). An optical dark-field image of an RGO@AuNP film on a glass slide,


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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 scanning electron microscopy 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. 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 nonradiative 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 on the scattering cross-section.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 the optical properties of individuals NPs to be studied. In this technique, an optical microscope in a darkfield 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 an RGO sheet, the spectra showed one band in the region between 550 and 700 nm (Figure 2C, green and red curves). In contrast,


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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). To obtain a better understanding of the optical properties, the scattering spectrum of RGO@AuNP was compared with that of isolated AuNPs with a similar size distribution (25 ± 5 nm) (Figure 2C and F). 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, as it is related to the coherence of the plasmonic oscillations, and consequently to the lifetime of excited electrons. The coherence of plasmonic oscillations is determined by radiative and nonradiative 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, nonradiative damping is related to electron–phonon, electron–electron, electron–defect, and electron–surface scattering processes, and to electron– hole pair creation, owing to inter- and intraband electronic transitions. This nonradiative 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 nm and 53 ± 5 nm, respectively. Therefore, the green particles are smaller than the red ones and have a broader plasmon band.


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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. 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


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those of isolated AuNPs indicates that the interaction between AuNP and RGO allows charge transfer transitions from the AuNP Fermi level to the RGO conducting band. As 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 high ordered pyrolytic graphite (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). 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. As 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).


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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 addition of asphaltene 20 ppm solution, (E) hyperspectral dark-field image, where the RGB channels are chosen as the scattering intensity at 635, 545 and 465 nm respectively, and (F) scattering spectra from selected points on image E. 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 a 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


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RGO@AuNP, with π–π interactions between asphaltene and the graphene surface; however, as the nanomaterial surface has some hydrophilic sites (from oxygen functional groups), particlelike 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 (Figure 3D–F). As can be 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 aggregate 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. As 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


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in the scattering spectra in Figure 3F and Figure S6, 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), 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 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 9ATA 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 band gap. 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. 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


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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 (major (m/z) peaks at 149, 167, and 279), FTIR (Figure S11) and 1H NMR (Figure S10) 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 GCMS (Figures S8 and S9), 1H NMR (Figure S10 and Table S1), and FTIR (Figure S11 and Table S2) analyses. These reactions occur owing to the influence of light and heat (generated by electron–phonon coupling of AuNP excited plasmons)62 and of reactive radical ion species (generated by photoinduced electron transfer on the catalyst surface).


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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.

It should be noted that without the RGO@AuNP catalyst, the reaction led to the formation of 16.5% anthraquinone only (Figure S12). Further, without air bubbling, no product was observed, confirming that the reaction involves dioxygen. These results are quite relevant, as 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


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be quite different. Therefore, despite the possibility of photoconverting asphaltenes 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 function of laser power (Figure S13). CONCLUSIONS AuNPs form an interesting hybrid material with RGO (RGO@AuNP) that display 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. ASSOCIATED CONTENT Figure S1: AFM topography images of GO (A) and height profiles of selected regions (B); Figure S2 and S3: AFM topography images of RGO@AuNP (A) and height profiles of selected regions (B); Figure S4: Hyperspectral image and scattering spectra of a dropcast film with graphene oxide (1 mg·mL-1) and asphaltene (20 ppm, toluene); Figure S5: AFM topography images of a 200 ppm asphaltene film on HOPG (A) and height profiles of selected regions (B); Figure S6: Spectra of RGO@AuNP after addition


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of a 20 ppm asphaltene solution, and the corresponding hyperspectral dark-field image (inset); Figure S7: Emission spectrum of the halogen lamp used in the photocatalysis experiments; Figure S8: 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 corresponding assignments; Figure S9: Mass spectrum of anthraquinone, a product of the photooxidation of 9-ATA; Figure S10: 1H NMR spectra of the 9-ATA photocatalysis products, Bruker 500 MHz; Table S1: 1H NMR chemical shifts; Figure S11: Infrared spectrum of the 9-ATA photocatalysis products in CCl4; Table S2: Tentative assignment of infrared spectra; Figure S12: 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; and Figure S13: Raman spectra of an asphaltene film as a function of laser power.

AUTHOR INFORMATION Corresponding Author *To

whom

correspondence

should

be

addressed.

Email:

[email protected]

and

[email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 24725-4) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq fellowships) is gratefully acknowledged. ABBREVIATIONS


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Asphaltenes are residual materials from the oil industry, which are usually converted into asphalt after exhaustive distillation. Conversion of asphaltenes into more valuable raw materials, for instance, by photocatalytic cracking, would be a better and more economic option. Accordingly, in this work, we combined the electronic and chemical properties of asphaltene and graphene derivatives with gold nanoparticles. After evaluating the interactions of these species using hyperspectral dark-field optical microscopy, we performed photodegradation experiments, revealing the hybrid material as a promising photocatalyst for conversion of asphaltenes into more valuable chemical products.


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Photocatalytic activity of reduced graphene oxideâgold nanoparticle

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