NMR of platinum catalysts. II. Relaxation

Abstract

The authors report measurement of ' Pt spin-lattice relaxation times Tl and spin-spin relaxation times T2 of small particles of Pt supported on alumina. Tl and T2 were measured at various static fields H p, for frequencies vp of 45, 55, and 74 MHz, and at temperatures of 4.2, 77, and 300 K. Though strong functions of H p/vp at any given vp, the relaxation times Tl and T2 at fixed Hp/vp are independent of particle size. Tl is longest at the position (Hp/vp) corresponding to the "surface peak" described in paper I (the preceding paper), indicating that conduction-electron spins are largely tied up for surface Pt atoms. The peak in Tl shifts position with change in surface coating exactly as does the peak in NMR echo amplitude, showing that the change in Hp/vp of the surface peak as a function of surface coating is most likely a chemical shift.

NMR-based gap behavior related to the quantum size effect

“…2 (a), which also shows the previous results obtained by Rhodes et al 16 . The 1/T 1 results obtained in this study are in good agreement with the results of previous studies 16 . As discussed below, 1/T 1 showed the metallic behavior (T 1 T = constant) in the high-temperature range, termed the metallic range in this paper.…”

“…To discuss the difference in the electron correlations on the surface and in the interior, the Knight-shift dependence of (1/T 1 T ) 1/2 in the "metallic range" is shown in Fig. 2 (a), which also shows the previous results obtained by Rhodes et al 16 . The 1/T 1 results obtained in this study are in good agreement with the results of previous studies 16 .…”

“…To date, many NMR studies have been conducted on various-sized Pt nanoparticles [10][11][12]16 to clarify the surface effects but not the QSE. Rhodes et al 10 observed the 195 Pt-NMR spectra with a broad linewidth at the LN 2 temperature; their observations showed a peak in the small Knight shift side.…”

Novel Flower‐Like Ag‐SiO2‐MgO‐Al2O3 Material: Preparation, Characterization and Catalytic Application in Methanol Dehydrogenation

“…It is commonly accepted that the absorption band at 250 nm is ascribed to the isolated Ag ＋ , 270 nm to Ag n δ＋ , 315-320 nm to metallic silver particles or silver foil, 370-390 nm to microcrystalline silver, and 400-420 nm to silver clusters. [14][15][16][17] In Figure 2a, there is only one band centered at 260 nm for the catalysts with alumina content exceeding 16.5%, while another peak centered at 407 nm ascribed to silver clusters can be seen in the catalysts with alumina content lower than 16.5%. The result suggests that alumina in the catalysts play an important role to stabilize ionic Ag ＋ .…”

“…After that, the Al 2 O 3 content was set based on the results both from the catalytic performance and the determination of the acid centers. From Entry 7 to 11, it was found that no CO by-product produced when Al 2 O 3 content increased to 16.5%, which still maintains at 100% methanol conversion. Further addition of Al 2 O 3 exceeding 16.5% will decrease the selectivity to formaldehyde, which may be attributed to the stronger acidic property of the catalysts.…”