Effect of Steps on the Decomposition of CH<sub>3</sub>O at PdZn Alloy Surfaces
Zhao-Xu Chen
Kok Hwa Lim
Konstantin M. Neyman
Notker Rösch
10.1021/jp044843e.s001
https://acs.figshare.com/articles/journal_contribution/Effect_of_Steps_on_the_Decomposition_of_CH_sub_3_sub_O_at_PdZn_Alloy_Surfaces/3295870
The decomposition of methoxide (CH<sub>3</sub>O) on a PdZn alloy is considered to be the rate-limiting step of steam
re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (<i>Langmuir</i>
<b>2004</b>, <i>20</i>, 8068; <i>Phys. Chem. Chem. Phys</i>. <b>2004</b>, <i>6</i>, 4499) revealed only a very low propensity of defect-free
flat (111) and (100) PdZn surfaces to promote C−H or C−O bond breaking of CH<sub>3</sub>O. Thus, we applied the
same DF periodic slab-model approach to investigate these two routes of CH<sub>3</sub>O decomposition on PdZn(221)
surfaces that expose Pd, (221)<sup>Pd</sup>, and Zn, (221)<sup>Zn</sup>, steps. C−H bond cleavage of CH<sub>3</sub>O is greatly facilitated
on (221)<sup>Pd</sup>: the calculated activation energy is dramatically reduced, to ∼50 kJ mol<sup>-1</sup> from ∼90 kJ mol<sup>-1</sup> on
flat PdZn surfaces, increasing the rate constant by a factor of 10<sup>8</sup>. The lower barrier is mainly due to a
weaker interaction of the reactant CH<sub>3</sub>O and an enhanced interaction of the product CH<sub>2</sub>O with the substrate.
The activation energy for C−O bond scission did not decrease on the (221)<sup>Pd</sup> step. On the (221)<sup>Zn</sup> step, the
calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the
stronger adsorption of CH<sub>3</sub>O. Steps (and other defects) appear to be crucial for methanol steam re-forming
on Pd/ZnO catalyst; the stepped surface PdZn(221)<sup>Pd</sup> is a realistic model for studying the reactivity of this
catalyst.
2005-03-17 00:00:00
bond
PdZn Alloy Surfaces
product CH 2 O
CH 3 O
CH 3 O decomposition
reactant CH 3 O
catalyst
DF
surface
activation energy