| AVS 63rd International Symposium & Exhibition | |
| In-Situ and Operando Spectroscopy and Microscopy for Catalysts, Surfaces, & Materials Focus Topic | Thursday Sessions |
| Session IS-ThP |
| Session: | In-Situ and Operando Spectroscopy and Microscopy for Catalysts, Surfaces, & Materials Poster Session |
| Presenter: | Eric Dombrowski, Tufts University |
| Authors: | E.K. Dombrowski, Tufts University E.H. High, Tufts University A.L. Utz, Tufts University |
| Correspondent: | Click to Email |
The steam reforming of methane to produce hydrogen occurs on an industrial scale at catalyst temperatures exceeding 1000 K, but most of the published sticking data on the methane / nickel system focuses on low surface coverages and surface temperatures, Ts, below 650K. At higher Ts, the carbon products of methane dissociation dissolve into the nickel bulk, which prevents post-dose measurements of reactivity.
To address this limitation, we couple King & Wells molecular beam reflectivity measurements with modulated infrared laser excitation to quantify methane’s dissociative chemisorption probability, S,over a wide range of incident fluxes and Ts. This new method simultaneously measures ground state and eigenstate resolved reactivities in real time. Each dose produces upwards of 20 independent reactivity measurements, increasing our precision greatly. Our ability to quantify S in real time reveals the coverage-dependent reaction probability, S(q,Ts). Measuring the full adsorption isotherm further constrains our measured value of the initial sticking probability, S0.
We measure S(Θ,Ts) for methane on Ni(111) over a wide range of Ts (500 - 1000 K) and reactive flux (from 0.004 to 0.40 ML/s). Under these conditions, methane initially dissociates into H and methyl fragments. The surface-bound methyls then dehydrogenate to C + 3H, and recombinative desorption of H at these surface temperatures is prompt. Initial measurements of S(Θ=0, Ts) show how elevated surface temperatures promote methane dissociation. As the dose proceeds, we observe coverage-dependent changes in S(Θ,Ts) that arise from the accumulation of C on and beneath the surface. These effects reveal the kinetics of carbon dissolution into bulk nickel starting with the initial CH bond cleavage event. We observe a sharp transition for the onset of observed site blocking. At all investigated reactive fluxes no site blocking occurs above Ts = 900 K. Below 900K, we observed a reactive flux dependent induction period as the carbon dissolution kinetics approach steady state. Observing how these kinetics change with reactive flux shows how the presence of surface and subsurface C can enhance or inhibit methane activation under the high temperature conditions present in a steam reforming reactor.