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Catalysts 2014, 4 301 In the third review by Wu et al., the progress in the synthesis, characterization and activity of Ni catalysts modified by gold for low-temperature steam reforming of methane is addressed [10]. Although there is a discrepancy between the results reported by different authors on the effect of gold on the catalytic activity of Ni-based catalysts, as a function of the support, preparation method and Au/Ni atomic ratio, it seems to be confirmed that the addition of gold improves the resistance of the Ni towards deactivation, due to coke deposition. These findings are fundamental in the design of new catalysts with a tailored gold effect on the activity and long-term stability. Among the research articles herein presented, three papers deal with the synthesis, characterization and catalytic activity of gold-supported catalysts [11–13]. In particular, Genty and coworkers have investigated the influence of gold on X6Al2HT (where X = Mg, Fe, Cu, Zn) hydrotalcite-like (HT) catalysts for CO and toluene total oxidation [11]. An interesting way to obtain mixed oxide catalysts is the use of hydrotalcite precursors. Indeed, HT compounds are a class of layered, double hydroxides that, after calcination treatment, form mixed oxides with high specific surface areas and porosity, as well as good thermal stability [14]. Many studies have established that nanosized Au over reducible oxide catalysts has a remarkable catalytic activity for oxidation reactions [15]. However, in the literature, only a few works have discussed the VOC oxidation activity of Au on HT-like catalysts [16]. In the quoted article, Genty et al. have demonstrated the beneficial effect of gold on the oxidation activity of CO and toluene over X6Al2HT compounds [11]. A correlation between catalytic properties and reducibility was observed. Catalytic results discussed in the present issue clearly point out that metal oxide-supported gold systems are receiving growing attention as catalysts for catalytic applications, both from academic and industrial points of view. However, if gold-based catalysts are ever to find commercial applications on a large scale, their long-term stability must be improved. The investigation of the procedures leading to the preservation of the catalytic performances of gold catalysts supported on zirconia and sulfated zirconia have been investigated in detail by Signoretto et al. [12]. Three potential causes of deactivation, namely particle growth by the sintering of gold nanoparticles, metal leaching and the formation of un-reactive species, which inhibit the reaction, have been recognized. The catalysts’ deactivation has been evaluated under different conditions: (1) storage conditions; (2) with time on stream for the water-gas shift reaction (WGSR); and (3) with time on stream for furfural oxidative esterification. The beneficial effect of sulfated zirconia in gold stabilization has been highlighted. In fact, even if sulfates are not present in the final catalyst, they act in the synthesis step increasing both, surface area and gold dispersion. Moreover, the calcinations pretreatment is an essential procedure for improving stability of Au/ZrO2 catalysts. Sanada et al. reported on the synthesis and catalytic activity of thermally-stable Au nanoparticles on ultrastable Y (USY) zeolites [13]. Au was successfully loaded on the NH4-form Y-type zeolite to yield 1.7 nm-sized Au2O3 along with the formation of NH4Cl. Interestingly, the formation of 1.8–3.7 nm Au particles was observed after thermal treatment at 773–973 K. Partial oxidation of benzyl alcohol in the liquid phase was employed as the test reaction. The catalytic activity of the Au/USY zeolites showed good correlation with the Au dispersion. The highest activity was attained with the Au/USY treated at 773–973 K. This study has demonstrated the potential use of the NH4-form of USY zeolites for supporting Au.PDF Image | New Trends in Gold Catalysts
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