Methane Activation on Model Catalysts and the Effects of Vibrational Excitation on Methane Reactivity.
Nowak, Jeremy A.
2013
- In the steam reforming process, methane and water react on the surface of a metal catalyst to form hydrogen (H2) and carbon monoxide (CO). The rate-limiting step of this reaction is the cleavage of a carbon-hydrogen bond in methane to form surface-bound hydrogen and methyl (CH3) fragments. This thesis describes eigenstate-resolved molecular beam-surface scattering experiments of vibrationally ... read moreenhanced methane dissociation on both the (111) and (110) faces of an iridium single crystal. These experiments revealed different reaction pathways available when a vibrationally excited methane molecule interacted with iridium. On a hot (1000 K) Ir(111) surface, two reaction pathways were observed. At high incident translational energies, the direct (single collision) dissociative pathway dominated, while at low incident translational energies, a precursor trapping-mediated pathway dominated. A shift in the reaction probability curve in the direct channel was observed when comparing reaction probabilities at v=0 (the vibrational ground state) and v3. This shift indicated that the 36 kJ/mol of vibrational energy in the 3 C-H stretch (Seets et al, 1997) was about 0.46 times as effective as translational energy in promoting methane's dissociation via the direct channel. In the precursor mechanism, v3 excitation led to an approximately six-fold enhancement in reactivity. Ir(110) has a lower average barrier to reaction, approximately 35 kJ/mol (Seets et al, 1997), and both direct and precursor-mediated channels at surface temperatures of 1000 K and 500 K were observed. Vibrational energy was about 0.86 times as effective as translational energy in promoting direct dissociation of methane on 1000 K Ir(110), suggesting that vibrational energy is more effective at promoting activation in the direct channel on Ir(110) than Ir(111), regardless of surface temperature. There was less of an immediate decrease in the contribution of the trapping-mediated regime to reactivity on 1000 K Ir(110) with increasing translational energy, indicating that the trapping-mediated pathway was more pronounced on Ir(110) than it was on Ir(111). While the trapping pathway had a minimal effect on reactivity on 500 K Ir(110) when the methane molecules existed in v=0, it contributed greatly to reactivity when the molecules were excited to v3. These experiments reveal mechanistic insights into the rate-limiting step of the steam reforming process. Future studies will include finalizing the Ir(110) reaction probability curves at higher translational energies in order to further study the effect of vibrational enhancement on the direct mechanism of dissociation, purchasing a new laser system to excite methane molecules into higher energy bending or stretching states, or using a palladium or platinum crystal to act as the transition metal catalyst in order to further study how different metals will affect the different channels of reactivity.read less
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