(689b) The Marriage of Activity and Selectivity in the Oxidative and Non-Oxidative Activation of Methane on Gold-Palladium Alloys

The grandest challenge in the direct upgrade of
methane to useful chemicals is finding a catalyst that is active for the
difficult C-H bond scission, yet selective such that not all of the C-H bonds break
to form coke. These requirements inevitably lead to the downfall of single
metals, including palladium and gold. Palladium is highly active for the C-H
bond scission of methane, but the dehydrogenation is carried out to completion,
indicating poor selectivity. Meanwhile, gold is selective for methane
activation because the methyl adsorbate would rather desorb than dehydrogenate
further, but breaking the initial C-H bond of the inert methane molecule is a
great obstacle, which illustrates the catalyst’s poor activity.

To maximize the strengths and minimize the weaknesses
of the gold and palladium single metals, we combined the metals to form a
binary alloy; gold-palladium alloys have been used previously for the selective
oxidation of methane to methanol [1]. Further motivated by the unique surface
properties of single atom alloys [2], where one highly active promoter atom
sits within the surface of a less reactive host atom, we selected the Au₃Pd
alloy, where every surface Pd atom is fully surrounded by Au atoms. Using
density functional theory, we compared the pathways and mechanisms of
non-oxidative and oxidative methane activation on Au₃Pd(111)
with the monometallic Pd(111) and Au(111) systems. In the non-oxidative
mechanism, the inclusion of small quantities of Pd in Au drastically decreases
the activation energy for the activation of methane, yet the formed methyl
remains more likely to desorb than to further dehydrogenate, representing the
unison of catalytic activity and selectivity. In the oxidative mechanisms, we
elucidated the interaction of methane with pre-adsorbed O*, OH*, O₂*,
and OOH* and found that the reaction energies (DE) and the energies
required to add hydrogen to the pre-adsorbed motifs (E_H) formed a
linear relationship, as depicted in Figure 1. We determined that the true role
of Pd on the Au₃Pd(111) surface is to draw
oxygen species to the noble Au surface, which is highly active if oxygen-based
adsorbates are present. Since the oxygen adsorbates occupy the Pd atom, the
product methyl adsorbate must bind to the Au sites, where it is selective to
further chemistry. Thus, we find that the Au₃Pd alloy constitutes an
ideal marriage of activity and selectivity for both the non-oxidative and
oxidative activations of methane.

[1] Rahim, M. H. A.;
Murphy, D. M.; Kiely, C. J.; Hutchings, G. J.; Angew. Chem. Int. Ed. 52 (2013) 1280-1284.

[2] Kyriakou, G.;
Boucher, M. B.; Jewell, A.D.; Lewis, E.A.; Lawton, T.J.; et al.; Science. 335 (2012) 1209-1212.

Figure 1. On Au₃Pd(111), a linear relationship exists between
the energy required to add hydrogen to a pre-adsorbed motif (E_H) and
the reaction energy (DE). (top left) The inclusion of Pd in the
surface of Au(111) allows for the selective upgrade of
methane.