Influence of support ionicity on the hydrogen chemisorption of Pt particles dispersed in Y zeolite: consequences for Pt particle size determination using the H/M method

Zeolite Y-supported Pt particles were studied by high-resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine structure (EXAFS), and H2 chemisorption experiments. The ionicity (i.e., electron richness of the zeolite oxygen atoms) was altered by using different cations (NaY, MgY, and LaY) and by steaming and introduction of protons (H-USY). Highly dispersed Pt particles were introduced inside the zeolite pores using a very careful synthesis procedure. Computer analysis of HRTEM images exhibited similar Pt particle size distributions with calculated average Pt particle sizes ranging from 0.98 to 1.26 nm for the various zeolite Y-supported Pt particles. The EXAFS results revealed that the first-shell Pt-Pt coordination number was around 6.5, in good agreement with the observed HRTEM results. H2 chemisorption experiments found a much higher H/Pt value for Pt/NaY (1.63) than for Pt/H-USY(0.88), implying no relationship with the Pt dispersion as determined by HRTEM. Using the values for the dispersion (Pts/Pt) as calculated from the HRTEM results, one can obtain a value for the number of chemisorbed hydrogen atoms per surface platinum atom (H/Pts ratio). This value demonstrates a clearly decreasing trend with decreasing electron richness of the support oxygen atom: 1.96 (Pt/NaY) > 1.74 (Pt/MgY) > 1.51 (Pt/LaY) > 1.11 (Pt/H-USY). These results indicate that the hydrogen coverage on supported Pt particles depends strongly on the support ionicity. This has significant consequences for the catalytic behavior of hydrogenolysis and hydrogenation reactions catalyzed by Pt. These results also indicate that one should be cautious in interpreting H2 chemisorption results only in terms of Pt dispersion. This approach is valid only when comparing Pt particles dispersed on the same support. Serious deviations in the real Pt dispersions occur if Pt is supported on oxides with different ionicities.