Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces

Abstract

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li-doped MgO that are much smaller (12±6 kJ mol(-1)) than the barriers inferred from different experimental studies (80-160 kJ mol(-1)). This raises further doubts that the Li(+)O(˙-) site is the active site as postulated by Lunsford. From temperature-programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH4 on both Li-doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol-gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH4 binds heterolytically on Mg(2+)O(2-) sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O2.

Models in Catalysis

“…clusters, 20, 23, and 6 kJ/mol for n = 2, 4, 7, respectively [23], are in a very similar range as the values of 11, 23, and 19 kJ/ mol for the neutral LiO(MgO) n-1 clusters with n = 4, 6, and 9, respectively. The calculated barrier for the surface model is also very close, 27 kJ/mol [21], showing that the gas phase clusters are indeed useful models for studying the reactivity of oxygen radical sites as they may occur on Li-doped MgO. Figure 3 reveals an important detail of the computational modeling.…”

“…The activity of Li-doped MgO catalysts was found to vary strongly over the catalytic runs and depend also on the preparation conditions [21,28]. The reported barriers for the stationary state of the catalysts were 90-160 kJ/mol [28] and 133 kJ/mol [21].…”

“…From microkinetic schemes fitted to kinetic OCM data, a barrier as high as 147 kJ/mol has been deduced for the generation of methyl radicals by hydrogen abstraction [27]. The activity of Li-doped MgO catalysts was found to vary strongly over the catalytic runs and depend also on the preparation conditions [21,28]. The reported barriers for the stationary state of the catalysts were 90-160 kJ/mol [28] and 133 kJ/mol [21].…”

“…To examine the active site at terraces, steps or corners we applying periodic boundary conditions to large super cells or embed geometrically constraint clusters in their crystal environment [21]. Computationally, we can also study neutral Li-doped clusters and compare the results with those for cationic pure MgO clusters.…”

“…Density functional theory has been used (B3LYP functional) to determine the energy profile for hydrogen transfer from the C-H bond to the oxygen radical site in the different model systems [21][22][23]. Figure 3 shows the transition structures localized on the potential energy surface and their energies with respect to methane separated from the catalyst, i.e., the apparent energy barriers.…”