NMR studies of acetylene adsorbed on supported platinum catalysts

Abstract

This thesis describes NMR measurements involving carbon-13, deuterons, and protons in acetylene ($\rm C\sb2H\sb2$) adsorbed at room temperature on supported platinum catalysts.

Measurements from 77$\sp\circ$K to 158$\sp\circ$K of the spin-lattice relaxation behavior of carbon-13 reveal multiple-exponential behavior. Calculations rule out an anisotropic T$\sb1$ as an explanation for the multiple-exponential behavior. Carbon-proton SEDOR measurements indicate that each time constant, T$\sb1$, in the spin-lattice relaxation of carbon corresponds to a mixture of adsorbate structures. The T$\sb1$ seems to exhibit a linear temperature dependence, indicating interaction with conduction electrons of the platinum clusters. However, the spectrum measurements indicate no Knight shift, a deviation from the Korringa relation. Such a deviation implies that electronic states at the Fermi energy involve hybrids of the platinum conduction band with adsorbate molecular orbitals with no spatial density at the carbon nucleus.

Measurements of the deuterium spectrum reveal no obvious features that would prove useful in identifying different adsorbate structures. Spectrum and spin-locking measurements indicate no motion of deuteron groups at 77$\sp\circ$K. The deuterium spin-lattice relaxation rate exhibits multiple-exponential behavior.

Measurements of the coupling between the magnetic dipole moments of deuterons determine bond angles and internuclear distances. A single-resonance "slow beat" measurement finds, in deuterated ethylidyne (CCD$\sb3$), a distance between deuterons of $\rm r\sb{DD} = 1.673\pm0.004$A. A double resonance method, demonstrated in a test sample of D$\sb2$O in gypsum, can provide bond angle and internuclear distance information. Coherence transfer between carbon and deuterium indicates that deuterium NMR measurements at lower temperatures may offer significant advantages in terms of signal to noise ratios.