Pt-black catalysts sintered at 473 and 633 K ("Pt-473" and "Pt-633"), respectively, have been characterized by X-ray diffraction and analyzed by XPS, UPS, and AES after carrying out n-hexane reactions. The analysis has been repeated after O2-H2 regeneration in the preparation chamber of the UHV apparatus. The surface of the blacks contains some carbon and oxygen impurities even after regeneration. Both Pt-473 and Pt-633 show high Pt 4f line intensity. Decomposition of the C 1s line reveals a higher amount of oxidized carbon polymers on Pt-633. This sample is free from lattice strain. The likely higher abundance of exposed hexagonal symmetry faces, namely the (111) plane and analogous stepped and kinked structures, may be one of the reasons why fragmentation and aromatization are favored on Pt-633, as opposed to the higher selectivity of isomerization and C5-cyclization on Pt-473. The higher amount of oxidized carbonaceous polymer overlayer (serving as "hydrogen catcher") on Pt-633 as well as the less-retained hydrogen by its crystallites without lattice strain may also contribute to the different selectivity and hydrogen pressure response of the n-hexane reaction over the two samples. Product ratios are suggested as an additional diagnostic tool for characterizing the hydrogen availability on the catalysts
In
this study, the
two-channel and differential dielectric spectroscopy
(TD-DES) technique has been applied to study different degrees of
oxidative degradation for two series of simulated oxidized lubricating
oils at low temperatures ranging from 20 °C to −55 °C.
The deep oxidation, general oxidation, nitration, and sulfation products
of the degraded lubricating oil were precisely identified by the Fourier
transform infrared spectroscopy (FT-IR) and increased with the level
of oxidation, which agreed well with the TD-DES data at room temperature
(20 °C). For the severely degraded lubricating oil, the TD-DES
real and imaginary data and relaxation characteristic changes from
20 °C to −55 °C were dramatically reduced; the interlacing
characteristics of Cole–Cole plots at low temperatures could
be ascribed to the formation of high-molecular-weight products during
the oxidative degradation process. It was found that the two-dimensional
(2D) synchronous and asynchronous dielectric spectroscopy were able
to qualitatively describe the degrees of simulated oxidative degradation
and the formation of highly oxidized products, as well as to explain
the polarization relaxation mechanism of degraded lubricating oil.
The partial least-squares (PLS) and multilinear-PLS (N-PLS) regression
results indicated that 2D synchronous and asynchronous dielectric
spectroscopy could better predict the FT-IR deep oxidation, general
oxidation, nitration, and sulfation peak areas than TD-DES real data
at 20 °C with regard to lower root-mean-square error of cross-validation
(RMSECV), better correlation coefficients (R), and
smaller predicted errors.
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