Quick XAFS Studies on the Y-Type Zeolite Supported Au Catalysts for CO−O₂ Reaction

Abstract

Au/zeolite catalysts prepared with a deposition-precipitation method were characterized with quick XAFS (QXAFS) in combination with IR. The data were correlated with the catalytic performance in the CO-O(2) reaction conducted at 273 K. On the basis of the XANES analysis of Au loaded on H-Y, the deposited Au(2)O(3) was observed at the initial stage. The transformation of Au(2)O(3) to form metal Au clusters was observed at 473 K in a H(2) atmosphere. The fact was supported by the IR measurement of adsorbed CO and the subsequent reaction with O(2). Detailed clustering process of Au supported catalysts could be directly followed by EXAFS analysis. The growth of metal Au proceeded via the formation of a Au(55) cluster at 473 K. Then it agglomerated to give metal Au with diameter of 2 nm at 723 K. The addition of H(2) was effective to retard the sintering of Au clusters. A similar phenomenon was observed over Au loaded on USY zeolite. In marked contrast to the H-Y and USY supports, significantly agglomerated Au particles generated on Na-Y zeolite, indicating the importance of the presence of acid sites in keeping the Au clusters with highly dispersed form. The performance of 5 wt % Au loaded on H-Y and USY in the CO-O(2) reaction was remarkably sensitive to the pretreatment temperature and the gas atmosphere. The catalyst pretreated with hydrogen showed a two-spike pattern with respect to the pretreatment temperature. Namely, the optimum activity was observed after the pretreatment at 373 and 723 K, where the temperatures corresponded to the generation of Au(2)O(3) and metal Au clusters with 2 nm diameter as evidenced by QXAFS analysis, respectively. The reason for enhancement of the activity of Au/H-Y by the addition of H(2) in the pretreatment step could be attributed to the formation of metal Au with appropriate size. In contrast to the H-Y and USY support, Au loaded on Na-Y prepared under the same condition was almost inactive in the reaction due to the formation of aggregated metal Au.

Formation of nanometer-sized Au particles on USY zeolites under hydrogen atmosphere

“…Propene epoxidation by H 2 and O 2 was carried out over Au/TS-1 [11]. We have also utilized H-Y zeolites as a support for Au [12,13]. Zeolites are promising supports for metals because the presence of strongly acidic sites in these materials promotes high dispersion of metal particles.…”

Structure of a Gold(III) Hydroxide and Determination of Its Solubility

“…9 However, no peak appeared at 11.938 keV in the spectrum of the Au solid sample, which suggests that no AuCl interaction was present. Because the XANES spectrum for the Au solid sample resembled the [Au(OH) 4 ] ¹ solution spectrum, the Au oxidation state in the Au solid sample was demonstrated to be Au(III) that OH ¹ is coordinated with. This conclusion is supported by the 197 Au Mössbauer measurement result.…”

“…Solutions were prepared with deionized-distilled water. 4 ] and Na 2 CO 3 solutions were mixed, and the obtained solution was magnetically stirred for 8 h. The obtained precipitate was filtered using a 0.20¯m membrane filter, washed with water and then freeze-dried under vacuum. Here, we refer to the precipitate as the "Au solid sample".…”

“…In the curve fitting analyses, the backscattering amplitude and the phase shift were calculated by FEFF 6L, with the following used as fitting parameters: the number of neighboring atoms, interatomic distances between the absorbed atom and the neighboring atoms, the DebyeWaller factor and the absorption edge energy. The intrinsic loss factor was obtained from the curve fitting analysis of the EXAFS data of the [Au(OH) 4 ). The TG/DTA measurement for the Au solid sample was performed by using a TG/DTA7300 (HITACHI, Japan) between 298 and 773 K with a heating rate of 0.5 K/min under air flow (200 cm 3 /min).…”

“…1,2 The precipitate from alkaline solution of [AuCl 4 ] ¹ , which has been believed to be gold(III) hydroxide, is used as a precursor to prepare these catalysts. 3,4 However, it has remained uncertain whether the precipitate is gold(III) hydroxide (Au(OH) 3 ) or hydrous gold(III) oxide (Au 2 O 3 ¢nH 2 O). Although Johnston and Leland attempted to measure the solubility of the precipitate, 5 no investigation of the physical and chemical properties has been performed.…”