Horia Metiu

Molten salts have been recently used as catalysts for methane pyrolysis to generate hydrogen and carbon. It was found that molten alkali chlorides, which are poor catalysts, become rather active upon the addition of a small amount of FeCl3. Calculations have shown that this takes place through an unusual mechanism. Due to fluctuations in the position of the ions, the charge on the iron fluctuates between Fe3+ and Fe2+. The electron required for forming Fe2+ is donated by several chlorine ions, and this causes them to be active in breaking the carbon hydrogen bond in methane. It was suggested that this mechanism is general: it takes place whenever the dopant is such that a lower charge state is possible. We test this hypothesis here by examining KCl doped with MoCl5, MoCl4, or MoCl3. We find that MoCl5 and MoCl4 activate KCl by the mechanism described above. MoCl3 does not because Mo3+ is not converted by fluctuations to Mo2+. We also show that the gas-phase dissociation energy of molybdenum chloride can be used as a descriptor to assess its ability to activate methane. Because it is easy to remove one Cl from MoCl5 or MoCl4, they are effective dopants for methane activation. The energy required for the dissociation of MoCl3 is very high, and doping with MoCl3 does not improve catalytic activity.

We probe the adsorption of molecular H2O on a TiO2 (110)-(1 x 1) surface decorated with isolated VO clusters using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and temperature-programmed desorption (TPD). Our STM images show that preadsorbed VO clusters on the TiO2 (110)-(1 x 1) surface induce the adsorption of H2O molecules at room temperature (RT). The adsorbed H2O molecules form strings of beads of H2O dimers bound to the 5-fold coordinated Ti atom (5c-Ti) rows and are anchored by VO. This RT adsorption is completely reversible and is unique to the VO-decorated TiO2 surface. TPD spectra reveal two new desorption states for VO stabilized H2O at 395 and 445 K, which is in sharp contrast to the desorption of water due to recombination of hydroxyl groups at 490 K from clean TiO2(110)-(1 x 1) surfaces. Density functional theory (DFT) calculations show that the binding energy of molecular H2O to the VO clusters on the TiO2 (110)-(1 x 1) surface is higher than binding to the bare surface by 0.42 eV, and the resulting H2O-VO-TiO2 (110) complex provides the anchor point for adsorption of the string of beads of H2O dimers.

We have performed a combined quantum mechanical and microkinetic modeling study to understand the nascent decomposition pathways of methane pyrolysis, catalyzed by gas-phase ZnCl2, in a constant pressure batch reactor at 1273 K. We find that ZnCl2 catalyzes methane pyrolysis with an apparent activation energy of 227 kJ/mol. We have also performed sensitivity analysis on a reaction network comprising initiation, termination, and primary propagation reactions. The results suggest that the whole reaction network can be simplified to four reactions, which contributes to the initial rate of methane decomposition. Based on these insights, we have also explored the catalyzing effects of gas-phase AlCl3, CoCl2, CuCl2, FeCl2, and NiCl2 for methane decomposition. Our calculations suggest that gas-phase CuCl2 and NiCl2 are the most active catalysts among the metal halides studied in this work.