Catalysts based on Na-mordenite (symbolized as 'M') exchanged with 5, 10 and 15 wt. % of Ag were characterized by XPS, EXAFS, XANES and UV-Vis DRS spectroscopic techniques in order to investigate the effect of different treatments on the chemical state and surface concentration of silver species. The Ag x M catalysts were analyzed in oxidizing (O 2 ) or reducing (H 2 /Ar) atmospheres and also after being used in the selective catalytic reduction of NOx or in successive cycles of toluene adsorption/desorption.In calcined samples, EXAFS profiles showed two types of Ag-O spheres of coordination, one due to a dispersed phase of silver oxide and the other, to Ag + ions in interaction with the oxygen of the zeolite framework. The UV-Vis DRS spectra showed the coexistence of isolated Ag + , Ag n δ+ (n < 10) cationic clusters and Ag x O particles. In addition, through the modified Auger parameter (α'), calculated from XPS measurements, it was possible to identify Ag + ions at exchange sites (α'~722 eV) and Ag x O (α'~725 eV) highly dispersed on the surface. Both species constitute stable active centers for the SCR of NOx under severe reaction conditions. However, during the adsorption-desorption of toluene, the reduction of silver oxides produces Ag(0) due to thermal hydrocarbon decomposition.
In this work, we studied the adsorption of butane, toluene and nitric oxide on NaMordenite exchanged with different amounts of silver. The reactions that occurred when the adsorbed hydrocarbons interacted with NO and the effect of water adsorption were also addressed. Different silver species were formed after ion exchange and they were detected by TPR analysis. Highly dispersed Ag 2 O particles were reduced at temperatures lower than 300°C whereas Ag + exchanged ions showed two TPR peaks, which can be ascribed to species exchanged at different mordenite sites. The TPD experiments after adsorption of NO at 25°C showed that the only desorbed species was NO 2 which was formed by the total reduction of Ag 2 O particles. When the adsorbed butane was exposed to NO (1000 ppm), isocyanate species were formed on Ag + ionic sites as well as Ag + -(NOx)-CO species. Toluene adsorption was stronger than butane since adsorbed toluene molecules were held even at 400°C. The characteristic bands of the aromatic ring C=C bond was observed as well as that of methyl groups interacting with Ag + and Na + ions. However, the appearance of carboxylic groups at temperatures above 300°C in inert flow indicated the partial oxidation of toluene due to Ag 2 O species present in the samples. After contacting adsorbed toluene with NO, different FTIR bands correspond to organic nitrocompounds, isocyanate, cyanide and isocyanide species adsorbed on Ag + ions, were detected. The presence of water inhibited the formation of NO 2 species and the hydrocarbon adsorption on Na + sites but did not affect the toluene-Ag + interaction.
Highlights 1. Crystalline cellulose is hydrolyzed into glucose with a yield of 63.8% 2. The Ag(I) ions exchanged allowed the hydrolysis of cellulose 3. The incorporation of silver produces a redistribution of acid sites 4. Al(III) species which are octahedrically coordinated were detected
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