NMR of platinum catalysts. I. Line shapes

Abstract

The authors report spin-echo NMR studies of ' 'Pt in small particles of Pt supported on alumina. For the samples studied, the fraction of Pt atoms on the surface (called the dispersion) ranged from 4%%uo to 58%. The studies were at fields Ho of 80 to 85 kG, frequency vo-74 MHz, and at a temperature of 77 K. The lines are broad (3-5 kG), requiring special methods which permit substantial signal averaging (-50000 echoes). In the low-dispersion (large-particle) samples, there is a strong absorption at the position of the ' Pt NMR in bulk Pt metal (Ho/vp=1. 138 kG/MHz) which becomes progressively weaker as the particle size decreases. A peak which is near the ' Pt resonance in typical diamagnetic compounds (Ho/v0-1. 09. kG/MHz) is found in samples which are coated with adsorbed molecules. It disappears when the surface is cleaned. The authors show that this peak arises from the surface layer of Pt atoms, and that its position (Ho/vo) shows that coating the Pt atoms on the surface largely ties up the electron spins of the surface Pt atoms in bonds. The exact position of this peak depends on the chemical species which is adsorbed. The authors show that when Pt is cleaned, then exposed to air for long times, the surface peak reveals that the surface has reconstructed to form Pt(OH) 6.

Broadband excitation in solid-state NMR using interleaved DANTE pulse trains with N pulses per rotor period

“…The limited probe detection bandwidth can be circumvented by: (i) reducing the quality factor of the probe, at the expense of decreased sensitivity and/or (ii) piecewise spectral acquisition [4,16,17,26]. Wide spectra can be excited uniformly by short pulses, however at the expanse of a decrease in sensitivity [5].…”

“…In that case, MAS powder spectra feature numerous spinning sidebands. Such wide spectra are encountered when: (i) the nuclei are close either to unpaired electrons in paramagnetic samples [2][3][4][5][6][7][8][9][10][11][12] or to conduction electrons in electrical conductors, (ii) the nuclei are subject to large quadrupole interactions (I > 1/2) [13][14][15][16][17][18][19][20][21][22][23][24][25], or (iii) their chemical shift anisotropy (CSA) is large, particularly when the static magnetic field is high [26,27].…”

Novel microfibrous‐structured silver catalyst for high efficiency gas‐phase oxidation of alcohols

“…Figure 5 shows the UV-vis DRS spectra of 10 wt % Ag/Ni-fiber, 10 wt % Ag/SS-fiber and electrolytic silver catalysts. As noted in previous work, [27][28][29][30] the metallic silver particles provide the UV-vis absorption band in the region of 310-320 nm; the isolated Ag þ ions and charged Ag n dþ clusters deliver those in region of 230-250 nm and 270-290 nm, respectively. As shown in Figure 5, a strong sharp peak at about 320 nm, a weak peak at $230 nm and a faint peak at $275 nm, appeared on the electrolytic silver catalyst, indicating the existence of a large amount of metallic silver film or large particles with a relatively low concentration of isolated Ag þ ions and charged Ag n dþ clusters.…”

OPENCORE NMR: Open-source core modules for implementing an integrated FPGA-based NMR spectrometer

“…14 is a 195 Pt NMR experiment in a platinum powder sample [29][30][31]. In this sample, the T 1 value of the 195 Pt spin was found to be less than 1 ms, and sensitivity is quite low because its spectral width as large as several MHz due to Knight shift distribution only allows partial excitation of the spin packets at a time, necessitating rapid signal accumulations over many times.…”

“…(c) The 195 Pt echo intensities plotted as a function of the external field. acquisition phase was cycled as (x, Àx) and (x, Àx) while those of the p pulses were fixed to y [29], and the signals were accumulated over 63 times inside the FPGA before transferred to the PC through the USB module.…”