Electrochemical and photocatalytic reactions of polycyclic aromatic hydrocarbons investigated by raman spectroscopy

Abstract

Este trabalho apresenta o estudo do comportamento fotoquímico de hidrocarbonetos policíclicos aromáticos (HPAs), potenciais poluentes secundários em reações em aerossóis, através de espectroscopia Raman em comparação com seu comportamento eletroquímico. Os HPAs estudados incluem pireno, antraceno, fenantreno e fluoreno. Esses foram adsorvidos sobre o catalisador TiO 2 e irradiados com radiação ultravioleta (254 nm). A oxidação eletroquímica foi estudada via espectroscopia Raman intensificada pela superfície (SERS) in situ e levou à formação de produtos carbonilados. Intermediários oxidados contendo o grupo C=O foram também observados nos processos de fotodegradação. A comparação entre os intermediários formados nos processos fotoquímicos e eletroquímicos revela produtos idênticos para o antraceno, mas não para o pireno. Esse resultado advém da diferença de mecanismo para esses dois processos. A degradação fotocatalítica sobre TiO 2 é iniciada por radicais hidroxila, enquanto a oxidação eletroquímica é iniciada por transferência eletrônica direta com o HPA adsorvido no eletrodo, gerando cátions radicais do HPA que desencadeiam as reações subseqüentes. This paper presents the study of photochemical behavior of polycyclic aromatic hydrocarbons (PAHs), potential pollutants in secondary reactions in aerosols, through Raman spectroscopy compared with its electrochemical behavior. The PAHs studied include pyrene, anthracene, phenanthrene and fluorene. These were adsorbed onto TiO 2 and irradiated with ultraviolet light (254 nm). Their electrochemical oxidation was studied by in situ Surface-enhanced Raman Scattering (SERS) and led to the formation of carbonyl-containing products. Oxidized intermediates bearing the C=O group were also formed during photodegradation. The joint analysis of the photodegradation data with those produced by electrochemical means -using spectroscopic techniques for the identification and characterization of the products -revealed the formation of identical products for anthracene, but not for pyrene. A reasonable explanation for this difference in results is that photochemical and electrochemical oxidation reactions proceed via different mechanisms. While photocatalytic degradation over TiO 2 is initiated by hydroxyl radicals, electrochemical oxidation is initiated by the direct electron transfer from adsorbed PAH to the electrode, generating PAH cation radicals that undergo subsequent reactions.Keywords: advanced oxidative processes, photocatalysis, TiO 2 , spectroelectrochemistry, SERS IntroductionExcessive concentrations of particulate matter in the atmosphere are cause for significant concern in many metropolitan areas around the world. This particulate matter exhibits dynamic behavior owing to potential chemical and photochemical reactions, changing its composition according to time and environment. Polycyclic aromatic hydrocarbons (PAHs), a class of chemicals present in particulate matter, are of special interest due to their mutagenic and carcinogenic properties. 1,2 PAHs are character...

Physicochemical analysis of individual atmospheric fine particles based on effective surface-enhanced Raman spectroscopy

“…reported in previous SERS studies (Craig et al, 2015;Fu et al, 2017). This indicates that our work has made great improvements on the sensitivity of the SERS technique for PM 2.5 study, with an EF factor of at least 56.…”

“…The highest intensities of two previously reported works for real particles were about 500 a.u. (Craig et al, 2015) at 1370 cm -1 and about 300 a.u. at 1039 cm -1 , respectively.…”

“…(Fig.S2A, Table S2). There were many small peaks between 1250 cm -1 and 1750 cm -1 in the Raman spectra of the PM 2.5 particles, possibly from organic components (Cordeiro and Corio, 2009;Kuo et al, 2011).…”

“…SERS provides a rapid and efficient platform for chemical analyses due to the explosive development of nanofabrication and improvements in instrumentation (Li et al, 5 2010;Halvorson and Vikesland, 2010). This technique has great potential in the field of atmospheric science and has been applied preliminarily (Craig et al, 2015;Ayora et al, 1997;Fu et al, 2017;Ofner et al, 2016). For instance, Craig et al (Craig et al, 2015) first applied it to atmospheric aerosol particles and showed rich vibrational spectra for individual submicron particles with Ag nanoparticles (NPs) as SERS substrates.…”

“…This technique has great potential in the field of atmospheric science and has been applied preliminarily (Craig et al, 2015;Ayora et al, 1997;Fu et al, 2017;Ofner et al, 2016). For instance, Craig et al (Craig et al, 2015) first applied it to atmospheric aerosol particles and showed rich vibrational spectra for individual submicron particles with Ag nanoparticles (NPs) as SERS substrates. Recently Fu et al used commercial Klarite for a chemical component study of individual atmospheric aerosols, and further increased substrate sensitivity by an 10 enhancement factor of 6.…”