Decomposition of benzene in the RF plasma environment. Part II. Formation of polycyclic aromatic hydrocarbons.

This study investigated the characteristics of polycyclic aromatic hydrocarbons (PAHs) formed during the decomposition of benzene (C(6)H(6)) in radio-frequency (RF) plasma environments. The identification and quantification were accomplished by using a GC/MS for PAHs and an on-line Fourier transform infrared (FT-IR) spectrometer for the reactants and gaseous products. The analytical results show that PAHs were formed in both C(6)H(6)/Ar and C(6)H(6)/H(2)/Ar systems. In terms of individual PAHs, naphthalene (C(10)H(8)) was the predominant species found among the 21 PAHs under all operational conditions, phenanthrene and chrysene are the next. High-ring PAHs did not form easily in the C(6)H(6)/Ar and C(6)H(6)/H(2)/Ar system, especially at high input power and high C(6)H(6) feed concentration (C(C(6)H(6))) for the former system. Yields of PAHs with different ring numbers decreased with increasing ring number. At low input power, increasing C(C(6)H(6)) would promote yields of PAHs, while adding hydrogen as the auxiliary gas suppressed PAHs formation. Higher input power or addition of oxygen not only effectively suppresses PAHs formation but also completely destroys C(6)H(6). Owing to the absence of the principal intermediate species, phenol (C(6)H(5)OH), from the gas products of C(6)H(6)/O(2)/Ar system, H-abstraction-C(2)H(2)-addition (HACA) pathway is proposed as the primary mechanism for PAHs formation in the present study. Gas phase distribution of total-PAHs accounts for 20-95.3% at 2% of C(C(6)H(6)) among C(6)H(6)/Ar, C(6)H(6)/H(2)/Ar and C(6)H(6)/O(2)/Ar systems. This study suggests that gas-phase PAHs should not be ignored, particularly in C(6)H(6)/Ar systems under high input power and high C(C(6)H(6)) , or in C(6)H(6)/O(2)/Ar systems.