CO Oxidation Catalyzed by Au−Ag Bimetallic Nanoparticles Supported in Mesoporous Silica

Abstract

We report a novel Au-Ag bimetallic nanocatalyst supported on an acidic mesoporous aluminosilicate Au-Ag@APTS-MCM prepared by a two-step synthesis procedure, which is very active for low-temperature CO oxidation. Its catalytic activity is still quite appreciable after 1 year of storage under room conditions. The silane APTS [H 2 N(CH 2 ) 3 -Si(OMe) 3 ] was used to surface functionalize mesoporous silica. The functionalized mesoporous silica was used to absorb the gold precursor AuCl 4and silver precursor AgNO 3 to form gold-silver bimetallic nanoparticles inside the nanochannels after chemical reduction. The catalysts were activated by calcinations, followed with hydrogen reduction at 873 K. Using various characterization techniques, such as X-ray diffraction, UV-vis, transmission electrom microscopy, and X-ray absorption fine structure spectroscopy (EXAFS), we elucidated the structure and surface compositions. As compared with the previously reported Au-Ag@MCM, prepared by one-pot procedure, the new method yields smaller sizes of AuAg bimetallic nanoparticles (4-6 vs 20 nm). They exhibited higher activity in catalysis for low-temperature CO oxidation with high stability. Moreover, the catalyst is resistant to moisture over a long storage time. A synergetic effect in relative composition was also found. The EXAFS study shows that Ag predominantly resides on the surface of the bimetallic nanoparticle. This distribution helps to yield a catalyst that is very active in both CO and O 2 neighboring sites.

Understanding the synergistic effects of gold bimetallic catalysts

“…2c) [28]. Evidently, for this one-pot method, the presence of Ag induced significant sintering of particles due to the formation of AgBr, which is quite different from the two-step method we developed later [31,32].…”

“…Nevertheless, if the two types of interactions can be manipulated well, e.g., by functionalizing the support surface to strengthen the metal-support interaction and choosing an appropriate reducing agent for rapid nucleation of the two metal species, uniform composition and size distribution might also be obtained in a well-controlled manner. In order to overcome the shortcomings of the first protocol (significant sintering by the activation process), we have developed a two-step method for the synthesis of gold bimetallic catalysts that proved effective in producing small, uniform, and sintering-resistant gold bimetallic nanoparticles on nonreducing supports such as silica and alumina [21,31,32]. A typical procedure for synthesis of silica-supported Au-Ag bimetallic catalyst using this two-step method is illustrated in Fig.…”

“…On the other hand, EX-AFS is a powerful technique for characterizing bimetallic nanocatalysts [71]. Table 1 shows the EXAFS data of Au-Ag bimetallic samples with different Au/Ag ratios after calcination and reduction treatments [32]. The contribution from Ag-O bonding is obviously seen in the calcined sample, while it disappears after reduction in H 2 , indicating the formation of silver oxide after calcination and realloying of Au and Ag after subsequent reduction treatment.…”

“…Based on the chemical properties of the second metal, we can classify gold-based bimetallic catalysts into two types. The first type is Au-BM catalysts, where BM refers to base metals such as Cu [21][22][23][24][25][26], Ag [27][28][29][30][31][32][33][34][35], Co [36][37][38], and Ni [39][40][41], and the second type is Au-PGM catalysts, where PGM refers to platinum group metals such as Pt [42,43], Pd [44][45][46][47], Rh [48], and Ir [49]. This classification may not cover all the gold bimetallic catalysts, but it will facilitate understanding the synergistic effect between gold and the second metal in catalysis.…”

Development of novel supported gold catalysts: A materials perspective

“…For instance, Crooks and co-workers synthesized PdAu/TiO 2 using dendrimer-encapsulated alloy nanoparticles as the precursor, and found that the activities of the catalysts in CO oxidation followed the sequence PdAu/TiO 2 > Pd/TiO 2 > Au/TiO 2 [235]. Mou and coworkers synthesized AuAg/Al-MCM-41 catalysts via both a one-step approach [236][237][238][239] and a two step method [55], and demonstrated enhanced catalytic activity in CO oxidation associated with the alloyed silver. Zhang and co-workers used a similar two-step approach to prepare AuAg/SiO 2 [53], AuAg/Al 2 O 3 [53], AuCu/SBA-15 [240], and AuCu/silica gel [63] catalysts, and demonstrated their good performance in CO oxidation.…”