Platinum surfaces spectra

Fig. 54, Korringa relationship for the Pt spin lattice relaxation in the surface peak of the spectrum. The straight lines show that the platinum surface keeps its metallic character,...

An analysis of a Pto.sRhos with dispersion 0.40 by energy-dispersive X-ray spectroscopy showed that the individual particles had approximately the overall composition. The CO/ Pt double-resonance spectrum of CO close to Pt in the Pto.sRho.s surface is shown in Fig. 40b. The mere existence of this double-resonance signal shows that there is platinum in the surfaces of these particles. Its position, however, is different from that of CO on a pure platinum surface, showing that these particles are alloys. From the analysis of the Pt/ CO double-resonance spectrum of platinum in Fig. 62b, it is found that a fraction 0.49 + 0.07 of the Pt atoms are attached to CO, whereas the dispersion is estimated to be between 0.40 and 0.67 (Sec-... 

A vibrational spectrum of surface species is obtained by monitoring the sum frequency signal as a function of the incident infrared photon energy, but it should be noted that for a vitautional mode to be observable by SFG, it must be both infrared and Raman active. Only those modes which lack centrosymmetry can in the dipole approximation simultaneously obey both rules. Therefore, in the experiments described in this paper the (isotropic) gas phase and the fee lattice of the bulk platinum sample possess inversion symmetry and give nearly zero contribution to the signal. Henceforth, the dominant coniribution is generated by the modes of the adsorbed monolayer at the platinum surface, where inversion symmetry is always broken [14]. 

Figure A3.10.24 UPS data for CO adsorption on Pd(l 10). (a) Clean surface, (b) CO-dosed surface, (c) Difference spectrum (b-a). This spectrum is representative of molecular CO adsorption on platinum metals [M].

Measurements were performed on a potassium nitrite melt (KNO3) at 450°C. Fig. 73, curve 1 presents the spectrum obtained for a melt layer 0.05-0.1 mm thick, which was placed on a reflective surface (polished platinum). Fig. 73, curve 2 presents the inverted spectrum (relative to curve 1) of a relatively thin layer placed on an absorptive bottom surface (carbon-glass). 

Attempts have also been made to obtain the radicals (CF3)3C and CeFs as products of vacuum pyrolysis of (CF3)3CI and CeFsI (Butler and Snelson, 1980b). However, only perfluoroisobutene was observed in an IR spectrum of pyrolysis products of (CF3)3CI. Thermolysis of CeFsl led to formation of CF4, CF3 and CF2 as a result of decomposition of the aromatic ring. This behaviour was explained as due to catalytic effects which take place on the platinum reactor surface. 

To use the DCI probe, 1-2 xL of the sample (in solution) are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. DCI probes have the capability of very fast temperature ramping from 20 to 700 °C over several seconds, in order to volatilise the sample before it thermally decomposes. With slower temperature gradients, samples containing a mixture of components can be fractionally desorbed. The temperature ramp can be reproduced accurately. It is important to use as volatile a solvent as possible, so as to minimise the time required to wait for solvent evaporation, which leaves a thin layer of sample covering the coil. The observed spectrum is likely to be the superposition of various phenomena evaporation of the sample with rapid ionisation direct ionisation on the filament surface direct desorption of ions and, at higher temperature, pyrolysis followed by ionisation. 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

This system was subsequently investigated by Christensen et at. (1990) also using in situ FTIR, who observed identical product features (see Figure 3.48). In order first to compare directly the IR spectrum of oxalate generated in situ, the authors took advantage of the surface reactivity of Pt and the poor diffusion of species to and from the thin layer. Thus, a solution of oxalic acid in the electrolyte was placed in the spectroelectrochemical cell, the potential of the platinum working electrode stepped to successively lower values and spectra taken at each step. The spectra were all normalised to the reference spectrum collected at the base potential of 0 V vs. SCE. As a result of the deprotonation of adventitious water ... 

Pyridine was found to polymerize on a Pt electrode from a solution of 1 M pyridine in 1 M LiC104/CH3CN at potentials above 0.8 V vs Ag/AgCl. A colorless film was formed, but it could be oxidized and reduced when placed in plain electrolyte solution. The infrared spectrum of the electrochemically formed poly(pyridine) film is shown in Figure 5. It displays a very intense, narrow band at 1500 cm indicative of C=C stretches that are perpendicular to the surface. 3,5 Lutidine also was polymerized on a platinum electrode under the same conditions, and its infrared spectrum is similar to that for the surface catalyzed poly(lutidine). The C=C stretching band for the poly(lutidine)... 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

Khulbe and Mann [155] have obtained infrared spectra of allene adsorbed on silica-supported cobalt, nickel, palladium, platinum and rhodium. The spectra were similar for all the metals, although variations in band intensity from metal to metal were observed. Addition of hydrogen to the allene-precovered surface resulted in similar spectra to those found for chemisorbed and hydrogenated propene in which the surface species was thought to be an adsorbed prop-1-yl group. The authors concluded that the initial allene spectrum was consistent with the adsorbed species being a 1 2-di-o-bonded allene (structure K)... 

Figure 13.5 Potential modulated reflectance spectrum of p-aminonitrobenzene (PANB) on platinum (solution phase 0.5 mM Na2S04 + 0.05 mM PANB). Applied dc 0.44 V vs. SHE. Modulation amplitude 50 mV. Modulation frequency 33 Hz. Incidence angle 65°. 11 signifies incident polarization parallel to incident plane and perpendicular to electrode surface. J signifies incident polarization perpendicular to incident plane (hence parallel to electrode surface). [From Ref. 50.]...

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