By using the in situ IR spectroscopy, the superoxide species (O2−), characterized by the O–O stretching peak at 1130 cm−1, was detected on the SrF2/La2O3 catalyst at temperatures up to 973 K. The introduction of 18O2 isotope caused the 1130 cm−1 peak to shift to lower wavenumbers (1095 and 1064 cm−1), consistent with the assignment of the spectra to the superoxide species. A good correlation between the rate of the disappearance
of the O2− species and that of the formation of C2H4 was observed, suggesting that O2− was the active oxygen species responsible for the oxidative coupling of methane (OCM) on the SrF2/La2O3 catalyst. This conclusion was reinforced by the EPR experiments (gxx = 2.0001, gyy = 2.0045, gzz = 2.0685), showing that O2− was the only paramagnetic oxygen species detectable on the O2-preadsorbed SrF2/La2O3 catalyst. These results suggest that superoxide O2− can be a stable active oxygen species, whose role in the OCM reaction cannot be overlooked. 相似文献
With the rapid development of information technology, social media has been widely used, and Internet information has been exploded, and consumers may experience information overload. Recommender systems using the social recommendation method that integrates social relationship information can provide users with target information that meets their needs. However, most of the existing methods only rely on the user’s ordinary friends to make recommendations, neglecting another influential group, the opinion leaders. In this study, we propose a new social recommendation method based on opinion leaders. The proposed method assumes that the influence of the opinion leader on the user is much greater than that of the user’s ordinary friends. The experimental results on two real datasets show that the proposed method not only has a better recommendation effect than the state-of-the-art recommendation algorithms, but also has a good performance in the cases of cold-start users.
Chlorpyrifos is a Malaysian Pesticide Board-approved organophosphate insecticide, which may become concentrated during fractionation. The objective of this project was to develop and validate a method to detect and quantify chlorpyrifos in food-grade fatty acids ingredients, e.g. caprylic-capric acid mixture and oleic acid (OLA) used to synthesize triacylglyceride based food additives and in the cosmetic industry. A selective ion monitoring gas chromatography–mass spectrometry method with a matrix-matched calibration curve calibration was selected. The method involved the direct injection of chlorpyrifos spiked into the fatty acids matrix. The percentage recoveries at spiking levels of 0.5, 0.75, 2.5, and 4.0 μg g−1 of chlorpyrifos in OLA and caprylic-capric acid ranged from 85.7%–101.1% to 97.2%–112%, respectively, with a relative standard deviation of within 11%. The intra-day and inter-day precision were deemed acceptable as indicated by an relative SD value of within 10%. A good linear relationship with a coefficient of correlation >0.99 for the matrix-matched calibration was achieved between 0.5 and 5 μg g−1. The limit of detection and limit of quantification corresponded to 0.5 μg g−1 for OLA and 0.55 μg g−1 for caprylic-capric acid. 相似文献
Castor oil (CO) is an environmentally friendly renewable green resource and ideal alternative to petroleum resources. The preparation of high strength and high toughness castor oil-based polyurethane prepolymer (COPU) composites has significant applications such as supporting material and engineering plastic sheet. In this study, unmodified clam shell powder (CSP) with a unique CaCO3-proteoglycan structure was used as a filler to prepare compatible reinforced COPU composite materials. Investigation of the mechanical properties revealed that the elastic modulus of the composite COPU reinforced with 50 wt% of CSP had increased to 5859.0 ± 8.4 MPa representing 187.77% to obtain stiffer and stronger material over pure COPU (2036.6 ± 196.9 MPa). Moreover, the scanning electron microscopy, thermogravimetric analysis and contact angle results demonstrated that the reinforced COPU composites have better compatibility, thermal stability, and water resistance than pure COPU. This work will promote the application prospects of CO-based polyurethane. 相似文献
The hydraulic erosion of concrete is often found in civil and marine engineering construction. The present study explores
the effects of several erosion parameters on the material loss of a concrete specimen subject to the hydraulic flow produced
by a submerged jet. Such an investigation has rarely been reported in the literature. The concrete specimen has a typical
compressive strength of 35 MPa, and the experimental parameters include the exposure time, incidence angle, standoff distance,
and the hydraulic jet pressure. The impinging velocity is estimated based on the distance considered in the round jet model.
The regression analysis shows the relationship between each parameter and the material loss. One finds that the material loss
is proportional to the exposure time and the hydraulic pressure. The maximum erosion lies at an incidence angle of around
30° to 45° and is affected by the fourth-order polynomial of impinging velocity. The result of the present study provides
a reference to engineering practice where concrete erosion is a concern. 相似文献