This project started because of our interest in performing the reaction CH4 → C + 2H2 which has the potential of producing hydrogen without carbon oxide side products. It turns out that the carbon formed by the reaction covers the catalyst, deactivating it. In addition, if the reaction is performed in gas phase, at high temperature, carbon is deposited on the walls of the reactor and the deposit grows until the gas is blocked. A possible solution is to perform the reaction by passing the gas through a bed of molten metal or molten salt, because the carbon segregates and floats to the surface and can be removed. The metals that melt at reasonably low temperature (e.g. Sn, Pb, Bi) are chemically inert and it seemed like a good idea to dissolve in them Pt, Pd or Ni, to confer to them some catalytic activity. We were hoping that Pt, Pd, and Ni will disperse, forming single, neutral-atom catalysts; this has been a desired and elusive goal so far. Ab initio, constant temperature molecular dynamics simulations show that the metals do disperse surprisingly rapidly but they form negative ions, not neutral atoms. The reactivity of these melts is lower than that of metallic Ni, Pt or Pd. In the case of molten salts (LiCl+ KCl), we had a similar problem: the carbon segregates but the salts are not catalytically active. To give them some chemical activity we dissolved in them alkali metals. Alkali metals ionize spontaneously, forming positive ions and solvated electrons. Ab initio molecular dynamics simulations indicated that the solvated electrons have lower energy if they pair up to form a singlet “bipolaron”. The solvated electrons are extremely strong Lewis bases. H2 or N2 in contact with the alkali-containing melt will form negative ions. If the melt is exposed to methane it forms CH3- and H-; if AgCl is dissolved in the melt the solvated electrons reduce Ag+ to Ag-.

Molecular dynamics and possible catalytic chemistry by molten metals or salts

253rd National Meeting (2017)

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