In the case of the composite with graphite oxide (ZnGO), the weight loss patterns revealed the peaks centered at 150 and 200 °C, assigned to the removal of physically adsorbed water and the dehydration of Zn(OH)2, respectively. The latter peak presents a smaller intensity comparing to that of ZnOH. We link it to hydrophobicity introduced by the addition of the graphene-based phase. At temperature higher than 400 °C the weight loss related to the dehydroxylation of the zinc hydroxide phase is revealed [25]. Finally, the weight loss occurring at over 800 °C is assigned to the reduction of zinc oxide by the carbonaceous phase.
After exposure to H2S, the low temperature peaks decrease significantly in intensity. A new intense peak at 280 °C appeared that might be linked to sulfur/oxysulfur species. It is suggested that water, OH, and O centers of Zn(OH)2 may take part in surface reactions towards sulfur/oxysulfur species [9]. The peak at 650 °C is assigned to the decomposition of zinc sulfates [37].
The AuZnGO sample presents an entirely different weight loss pattern than that for the ZnGO composite. For it the peak representing dehydration at about 150 °C almost disappeared. This suggests that the surface of the composite is the most hydrophobic among all samples tested. At about 250 °C the decomposition of species in which both zinc and oxygen from epoxy groups of GO are involved takes place [25].
After exposure to H2S, on the DTG curve for the AuZnGO-E composite an intense peak at <100 °C is revealed. It suggests formation of water in surface reactions. The new peaks which appeared at 325 °C and 400 °C can be linked to the removal of SO2 from of oxysulfates and/or sulfites from micropores that are high energy adsorption centers [36] and [38]. These sulfur species were also noticed for the AuZnOH-E sample. Moreover, the new peak at 480 °C, close to the boiling point of elemental sulfur, that is 445 °C [35], suggests that some S° can be present on the surface. The peak at 680 °C is linked to ZnSO4 decomposition [23]. Sulfides formed and deposited on the surface decompose at a temperature higher than 1000 °C. An offset of the high temperature peak is visible at the end of the experimental window. The results presented indicate the strong effects of AuNP on the reactivity of H2S on the surface of the AuZnGO composite and on the heterogeneity of the reactive adsorption products.
Potentiometric titration results support the differences in the chemistry of our materials discussed above. The experiments were carried out only in a very narrow pH window in order to avoid reactions of zinc hydroxide with titrants. In Fig. 5 the comparison of the proton binding curves is presented for the initial and exhausted samples. The negative values are linked the proton release related to surface acidic groups [13]. As seen, ZnGO is more acidic than ZnOH. Interestingly, while the addition of AuNPs to ZnOH caused a decrease in the acidity of this sample, its addition to the ZnGO increased the acidity, since AuNPs received electrons from the graphite oxide [39].
การแปล กรุณารอสักครู่..
