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Nanostructured photo-catalysts based on different oxidized graphenes for VOCs removal martina roso, Carlo Boaretti, Maria Guglielmina Pelizzo, Annamaria Lauria, Michele Modesti, and Alessandra Lorenzetti Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02526 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017
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Industrial & Engineering Chemistry Research
Nanostructured photo-catalysts based on different oxidized graphenes for VOCs removal Martina Roso*1, Carlo Boaretti1, Maria Guglielmina Pelizzo2, Annamaria Lauria1, Michele Modesti1, Alessandra Lorenzetti1 1
University of Padova Department of Industrial Engineering, Via Marzolo, 9, 35131 Padova Italy; *Corresponding author E-mail:
[email protected] 2
National Reseach Council of Italy, Institute for Photonics and Nanotechnology, via Trasea 7,
35131 Padova Italy KEYWORDS Nanostructured mats, methanol photo-degradation, Graphene, Graphene oxide, reduced Graphene Oxide, Titanium dioxide
ABSTRACT The use of graphene based materials as co-catalysts are explored in the production of active filter media for the methanol gas-phase photo-oxidation. The present work is meant to show the relationship between the composition and the morphology of the photo-catalytic systems with the performance of nanostructured mats. Three different graphene based co-catalysts have been used (graphene oxide, reduced graphene oxide, and few layers graphene) and the effect of the different degree of oxidation has been explored. The comparison of the obtained results has revealed that the structure of the media united with the affinity of the graphene based co-catalysts with the polymer matrix, strongly affect the photo-catalytic efficiency. The reduced graphene
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oxide has given the best performance in terms of higher rate of gas-phase methanol degradation thanks to the enhanced electron mobility ensured by its deep contact with the photo-catalyst itself.
1. INTRODUCTION Volatile Organic Compounds (VOCs) represent relevant pollutants both in indoor and outdoor environment. Aromatics, alcohol, ketons and esters can be cited as typical VOCs emitted from a wide range of anthropogenic activities, especially those related to the use of solvents, such as, printing, spray painting, coil coating, wood treatment etc. Among the methods available for air purification, photocatalytic oxidation processes are the most feasible ones and there is an extensive literature about their application for this purpose1-3. While some VOCs may be present at concentrations that are not considered acutely harmful to human health with a short-term exposure, a long-term exposure may result in mutagenic or carcinogenic effects. Moreover the most important and common consequence of emitting VOCs to the atmosphere is that they are responsible for both ozone depletion in the stratosphere and ozone formation in the throposphere. Within this scenario, there is a growing demand of innovation in the detection and abatement of industrial VOCs in order to comply with tightening legislation. An extensive review on VOCs, their main sources and effects can be found on the web and in the scientific literature4,5. Graphene oxide (GO), reduced graphene oxide (rGO) and graphene (G) occupy the interface of macromolecules and nanoscale objects. As such, they can be readily modified using several reactions and subsequently employed as nanoscale building blocks in a wide range of applications such as the assembly of papers and thin films or as fillers in nanocomposites with polymeric and/or inorganic materials6. Recent scientific studies focus on the use of graphene based materials in photo-catalysis7-22.
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Most of the current research8,9 focuses on different strategies for enhancing the photo-catalytic activity of semiconductors testing the decomposition of dye in liquid phase. For example Guo et.al.10, studied the effect of wrapping rGO sheets around TiO2 inorganic nanofibers with a further addition of Ag nanoparticles. They reported a significant improvement of the photocatalytic activity, which was attributed to the synergic effect of rGO, and Ag nanoparticles, that is their ability to extend the absorption spectrum to the visible region as well as to delay the recombination of photogenerated carriers. Just few papers refers to gas-phase experiments19-23 and before focusing on them, it is very important to observe that in gas-phase photo-catalysis, the dominant photo-degradation mechanism is not always straightforward and more generally a myriad of radical-based reactions are potentially available. Moreover, it has to be emphasized that the local environment as well as the target species govern all these reactions. The gas phase degradation of 2-propanol using titania photo-catalysts has been explored in the work of Batista et al.19 under UV and Sunlight-type illumination conditions revealing that the maximum photo-catalytic efficiency is equally related both the electron-hole recombination and to the adequate solid optical properties. Among VOCs, the photo-degradation of n-decane and perchloroethylene (PCE) has been studied by Monteiro et al.20 who compared the performance of two commercial TiO2 photo-catalysts. The impregnation of the nanocrystalline TiO2 P25 with the colloidal graphene oxide was studied with respect the gas-phase oxidation of ethanol and benzene by air oxygen in Andryushina’s work22. A 4-fold increase of the degradation rate was observed and it was related both to the more efficient electron transfer, and to the affinity of the substrate to the GO and rGO sheets. In another study, Chun et al.23 reported the graphene and TiO2 incorporation into carbon nanofibers and their performance under visible-LED irradiation: the
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initial concentration resulted as a crucial parameter for the photocatalytic treatment of gas-phase target pollutants. According to our previous work7,11, photo-catalytic oxidation has been proven to be a promising technology for air purification and nanostructured mats based on electrospun nanofibers offer a proper support in the production of active filtering media in order to combine the physical activity of the media with the chemical degradation of harmful pollutants. The present work is meant to give a better understanding of the effect of graphene based materials as co-photo-catalysts in the gas-phase abatement of organic pollutants. A more detailed characterization of the catalytic system has been carried out in order to provide information about the absorption limit and the band gap of the implemented photo-catalysts. Moreover, a complete different approach for mat preparation has been adopted. 2. EXPERIMENTAL 2.1. Materials TiO2 nanoparticles (Aeroxide P25, Evonik Industries) were used as a benchmark photo-catalyst, while graphene oxide, GO, (Sigma-Aldrich, USA), reduced Graphene Oxide, rGO, produced by hydrothermal method15, and few layer graphene, G, (Avanzare Innovacion Technologica S.L. (La Rioja Spain) were employed as co-photo-catalysts. Polyacrylonitrile (PAN, Mw 150.000 Dalton purchased from Sigma-Aldrich, USA) has been chosen as a reference polymer for nanofibers production thanks to its high UV resistance and N,N-dimethylformamide (DMF, from Sigma-Aldrich, USA) was used as solvent after dehydration by storage over molecular sieves.
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2.2. Nonwowen mats production The electrospun mats have been produced by one-step approach, according to which the ink to be electrospun is characterized by the simultaneous presence of polymer and catalytic system. The procedure is the same for each type of mat and requires a first dispersion of the catalyst in DMF for an hour followed by the addition of the polymer. The solution is kept under stirring overnight before electrospinning to ensure complete polymer solubility. The composition of the suspensions to be electrospun have been the following: •
PAN/TiO2 (pristine TiO2 as photo-catalyst): the catalyst-to-polymer ratio has been fixed to 70 (photo-catalyst):30 (polymer), with a polymer (PAN) concentration of 5% w/w in DMF.
•
PAN/G+TiO2: even in this case the catalyst-to-polymer ratio has been fixed to 70:30, with a 5%w/w polymer solution in DMF and employing a fixed mass ratio of grapheneto-titania equal to 1:10.
•
PAN/rGO-TiO2: in this case the reagents (TiO2 and GO) were used in such quantity to get a final mass ratio of graphene-to-titania, after hydrothermal treatment, equal to 1:10. The photo-catalyst-to-polymer ratio has been fixed at 70:30 with a with a 5%w/w polymer solution in DMF.
•
PAN/GO+TiO2: in this case two different ratio of catalyst-to-polymer have been employed (70:30 and 50:50) with a fixed graphene oxide-to-titania mass ratio of 1:10. In both cases the polymer concentration is the same as the previous systems.
The polymer concentration of PAN has been fixed at 5%w/w according to previous screening experiments, in order to ensure the spinnability of the whole ink.
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The electrospinning conditions employed for the production of all the samples are reported in Table 1. As it can be observed, the conditions for the processing of the nonwovens have been optimized one by one in order to obtain a stable electrified jet and a uniform mat deposition.
Table 1. Electrospinning process parameters for the production of all the composite mats with the one-step approach.
PANTiO2
PANG+TiO2
PANrGO-TiO2
PANGO+TiO2
Flow-rate (ml/h)
0.5
0.9
0.9
1
Voltage (kV)
15
15
18
15
Tip-to-collector (cm)
15
17
15
15
Needle (i.d)
0,7
0.7
0.7
0.7
Relative humidity %
