Platinum Knight shift

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

A simple test of this suggestion is the comparison of a five-layer slab calculation for the Knight shift in platinum (70) with the spectral fits of the layer model (Fig. 48). In both cases the surface resonance is shifted about 4% to low field wuth respect to the bulk signal, and the subsurface signal is found at approximately the halfway point. Another test is qualitatively to compare experimental results for hydrogen chemisorption on platinum (Fig. 55) with a calculation for hydrogen on palladium (175) in both cases an important diminution of the surface LDOS on the metal is found. 

Metal NMR is interesting in catalysis because of the relation of the (spin) Knight shift with the Pauli susceptibility, which in turn is related to the local density of states at the Fermi energy at the site of the nucleus. In practice, a detailed analysis of spin-shift-related effects requires that the orbital shift be relatively weak. This is not the general case for transition metals, but fortunately it applies to the catalytically important metal platinum. This is the subject of the next section. 

Figure 20 Correlation between the total iif-LDOS found on clean platinum catalyst surfaces and the Knight shift of chemisorbed afterward. 

Figure 6. Correlation between surface/subsurface frequency shifts and the Allred-Rochow electronegativity of the adsorbates. The dashed horizontal line indicates the Knight shift of bulk platinum atoms. The solid straight lines are linear fits to the surface and subsurface shifts as a function of the electronegativity. (Reprinted with permission from Copyright 2000 A n. Chem. Soc.)...

Other experimental method at a eomparable level of detail, although in some eases the conclusions reached support previous work by XANES using synchrotron radiation. Our results clearly demonstrate that due to a quantum mechanical electron density spillover from platinum, the interface is metallized, as evidenced by Korringa relaxation and Knight shift behavior. Thus, the adsorbate on a platinum electrode belongs to the metal part of the platinum-solution interface, and most likely other d-metal interfaces, and should be considered as such in any realistic models of the structure of the electrical double layer of interest to electrocatalysis. 

Correlation between the Knight Shift of a Platinum Surface and the Electronegativity of the Adsorbate... 

In complete contrast to the Friedel-Heine invariance of the electronic properties observed for the innermost platinum particle layers, the surface and subsurface NMR signals undergo major frequency shifts as different chemical species are adsorbed, as can be seen in Fig. 5. The NMR layer-model spectral deconvolution [31] technique was applied to these spectra in order to obtain the variations in the surface and subsurface Knight shifts. Because of the Friedel-Heine invariance, the NMR parameters (peak... 

The high field peak at 1.110 G kHz" is caused by platinum atoms in a metallic environment. Simple calculations yield a metallic particle whose volume is represented by a sphere with a radius of about 8.7 A. This agrees quite well with the radius of an inner cluster core consisting of 147 atoms. From the location of the metallic peak one can conclude that the density of states is remarkably reduced compared with bulk platinum, however, the Knight shift of the observed peak clearly indicates metal like character for the inner core of the cluster. 

P up to 8.8 GPa. The Knight shifts of copper and platinum, and the inplane shift of metalhc tin, were also measured. The Cu NQR frequency was considered to have excellent resolution in determining the pressure, while the Knight shifts of platinum and tin were suitable manometers when the addition of CU2O to the sample was to be avoided. 

The synthesis and full characterization of platinum nanoparticles prepared are reported. In these studies, ( CO) vras used as a prohe molecule to investigate the surface of the particles, using IR and solid-state NMR spectroscopies with magic angle spinning (MAS-NMR). Spectroscopic data suggest a modification of the electronic state of the nanoparticles between 1.2 and 2.0 nm which can he related to the presence of Knight shift. ... 

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