Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles

Abstract

Well-defined Pt-nanoparticles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different monovalent (H + , Na + , K + , Rb + , and Cs + ) and divalent (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperatureprogrammed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CtO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CtO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter R. A decreasing R results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T 50% for CO oxidation decreases with decreasing R. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T 50% of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

Fabrication of supported Pd–Ir/Al2O3 bimetallic catalysts for 2‐ethylanthraquinone hydrogenation

“…Therefore, the single Pd sites, isolated by Ir atoms, may inhibit the formation of unwanted H 4 eAQ products. Moreover, it can be seen that the peak for bridging CO on Pd atoms becomes slightly blue‐shifted after addition of Ir, suggesting that there is some charge transfer from Pd to Ir . As reported elsewhere, eAQ is an electrophilic reagent, and the Pd catalyst may be considered as a nucleophilic reagent in the hydrogenation reaction .…”

CO Adsorption on Pt Nanoparticles in Low E-Fields Studied by ATR-IR Spectroscopy in a Microreactor

“…In Figures 2(a) and 2(b), the assignments of the CO absorption peaks at 2068-2050 cm -1 , 1840 cm -1 , 1708 cm -1 to linear (COL), bridging (COB) and other multiply-bonded (COMB) CO, respectively, is in accordance with literature 17,18 . The weak absorption at 1656 cm -1 (for which is corrected in the inset of Figure 2(a)) is most likely due to the presence of water 19,20 .…”

“…Pt/SiO2 33 and Pt/Zeolites 34 . In these studies, the integrated intensity ratio of the COL/CO bridging drops significantly, suggesting an increase in the electron density on the metal atom with increasing alkalinity of the support 5,18,24,33,34 . Our results are in agreement with J. T. Miller et al, who reported a shift to lower frequency and a decrease in the linear to bridging ratio as the alkalinity of the support increases 35 .…”

“…increases of the COB and COMB integrated areas) due to an increase in temperature are unlikely. For such temperature effects a sharp drop (or increase) in the COB or COMB integrated area in Figure 3 are expected at higher temperatures (~100 o C) 18 .…”

“…The weak absorption at 1656 cm -1 (for which is corrected in the inset of Figure 2(a)) is most likely due to the presence of water 19,20 . The exact structural assignment of the adsorption configuration for the 1708 cm -1 absorption band remains unclear, but likely candidates are a long 2-fold bridge (which could for instance occur between 2 adjacent ridges of the missing row reconstructed (110) surface) or CO molecules σ-bonded to one Pt atom and σ or π bonded to a second one 17,18 .…”