DFT and In Situ EXAFS Investigation of Gold/Ceria−Zirconia Low-Temperature Water Gas Shift Catalysts:  Identification of the Nature of the Active Form of Gold

Abstract

A combined experimental and theoretical investigation of the nature of the active form of gold in oxide-supported gold catalysts for the water gas shift reaction has been performed. In situ extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) experiments have shown that in the fresh catalysts the gold is in the form of highly dispersed gold ions. However, under water gas shift reaction conditions, even at temperatures as low as 100 degrees C, the evidence from EXAFS and XANES is only consistent with rapid, and essentially complete, reduction of the gold to form metallic clusters containing about 50 atoms. The presence of Au-Ce distances in the EXAFS spectra, and the fact that about 15% of the gold atoms can be reoxidized after exposure to air at 150 degrees C, is indicative of a close interaction between a fraction (ca. 15%) of the gold atoms and the oxide support. Density functional theory (DFT) calculations are entirely consistent with this model and suggest that an important aspect of the active and stable form of gold under water gas shift reaction conditions is the location of a partially oxidized gold (Audelta+) species at a cerium cation vacancy in the surface of the oxide support. It is found that even with a low loading gold catalysts (0.2%) the fraction of ionic gold under water gas shift conditions is below the limit of detection by XANES (<5%). It is concluded that under water gas shift reaction conditions the active form of gold comprises small metallic gold clusters in intimate contact with the oxide support.

Non‐Thermal Plasma Activation of Gold‐Based Catalysts for Low‐Temperature Water–Gas Shift Catalysis

“…For each spectrum, the gas phase CO was subtracted from the original spectra recorded during the reaction. During thermal activation at 150 °C, the predominant feature was the band at 2095 cm −1 associated with the CO adsorbed on Au 0 , specifically at the step sites or the perimeter of Au nanoparticles, consistent with previous studies 23, 27, 34. A small contribution of Au δ+ −CO species at 2120 cm −1 was also observed.…”

“…The calculated specific energy input for an applied voltage of 7 kV was 9.2 W. No change in the selectivity was observed for the different voltages tested or under thermal conditions. In all cases, the reaction was selective to CO 2 , with mass balances of 97 %±3 % (Table S1) which is consistent with the previous studies under thermal control 22, 23. Furthermore, temperature‐programmed oxidation of the used samples confirmed that little carbon deposition took place and that the carbon‐containing species evolution could be attributed to carbonate decomposition after both thermal and plasma reactions24 (Figure S3).…”

“…Similar results were reported for a 2.0 wt % Au/CeZrO 4 by Pilasombat et al 22. and Tibiletti et al 23. Importantly, CO conversion was observed at low temperatures, for example, 15 % conversion at 100 °C.…”

“…Exposure of the fresh catalyst to WGS feed under plasma activation (first cycle) led to the appearance of similar adsorbed features, with Au 0 −CO species more dominant than the Au δ+ −CO species (Figure 3). Previous studies showed that under thermal conditions, metallic Au is the most stable and an active species for the water‐gas shift reaction 23, 26, 27, 41, 42. No CO bands were observed when the pure support was examined under similar conditions.…”

“…These features have been used extensively to characterize the state of the surface gold sites 23, 38, 39. For example, IR bands at 2110–2090 cm −1 have been assigned to CO on metallic Au particles (Au 0 −CO)25, 38, 39, 40 whereas bands between 2120 and 2180 cm −1 are thought to be due to CO adsorbed on Au δ+ .…”