Size and Support Effects for the Water–Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al₂O₃ and TiO₂

Abstract

The water-gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO(2), 22% H(2)O, and 37% H(2) at 1 atm for Au/Al(2)O(3) catalysts at 180 °C and Au/TiO(2) catalysts at 120 °C varies with the number average Au particle size (d) as d(-2.2±0.2) and d(-2.7±0.1), respectively. The use of nonporous and crystalline, model Al(2)O(3) and TiO(2) supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au(0) (near 2100 cm(-1)) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al(2)O(3) and Au/TiO(2) catalysts, respectively, and 10 times more active for Au on TiO(2) versus Al(2)O(3). From operando Fourier transform infrared spectroscopy (FTIR) experiments, it is determined that the active Au sites are metallic in nature. In addition, Au/Al(2)O(3) catalysts have a higher apparent H(2)O order (0.63) and lower apparent activation energy (9 kJ mol(-1)) than Au/TiO(2) catalysts with apparent H(2)O order of -0.42 to -0.21 and activation energy of 45-60 kJ mol(-1) at near 120 °C. From these data, we conclude that the support directly participates by activating H(2)O molecules.

CO Oxidation Catalyzed by Oxide-Supported Au25(SR)18 Nanoclusters and Identification of Perimeter Sites as Active Centers

“…The issue of the perimeter Au sites versus low-coordinated corner Au sites being the active sites is particularly intriguing in gold catalysis. 43 Since the atomic packing structure of Au 25 (SR) 18 is known, future work should allow for gaining deep insight into the catalytic mechanism, including identification of the exact form of the active oxygen species and the mechanism for CO and O 2 activation and detailed surface reactions. We believe that the nanocluster catalysts, as a new type of well-defined nanocatalyst, hold promise in fundamental studies of the catalytic mechanisms by correlating their catalytic performance with atomic structures.…”

Hydrogen-Etched TiO2−x as Efficient Support of Gold Catalysts for Water–Gas Shift Reaction

“…Because the surface oxygen vacancies of TiO2−x-N are far richer than those of TiO2-N, during the deposition of gold species (i.e., Au(OH)3), the amount of metallic Au 0 species supported on the TiO2−x-N is far larger than on TiO2-N. The metallic Au 0 species have been regarded as a requisite for a high CO oxidation and WGS activity of Au/TiO2 catalysts [11,12,66,67]; thus, Au-TiO2−x-N present higher catalytic activities. Therefore, the difference in their catalytic activities mainly depends on the difference in Au valence, which is influenced by the amount of support surface oxygen vacancies.…”

“…To develop new and more efficient WGS catalysts, supported catalysts have been believed to be good candidate. For example, both precious metals (e.g., Pt, Au, Pd, Rh, Ir or Ru) [2,3] and cheap transition metals (e.g., Cu or Ni) [4,5] were deposited on various supports including TiO 2 [6][7][8][9][10][11][12][13][14][15][16][17], CeO 2 [18][19][20][21][22][23], Mo 2 C [24][25][26], FeO x [27][28][29], Co 3 O 4 [30], Al 2 O 3 [31,32], ZrO 2 [33,34], CeO 2 -TiO 2 [35], CeO 2 -ZrO 2 [36,37] and CeO 2 -La 2 O 3 [38]. Au-TiO 2 catalysts have been given considerable attentions for WGS activities, due to the high activity and low side reactions of dispersed Au [38], and some advantages of TiO 2 supports (e.g., low price, easy preparation, adjustable properties and strong interaction with active metal).…”

Fabrication of Organic Solar Cells with Branched Cauliflower-Like Nano Structures as a Back Electrode Replicated from a Natural Template of Cicada Wing Patterns

“…In this regard, branched metal NS materials are of specific interest that arises from their unique surfacesensitive properties, large number of multiple surface edges and large surface-to-volume ratio [14][15][16][17] . It would be also feasible to tune the fascinating properties of branched structures such as their optical and electrical properties by size and shape modifications [18,19].…”