Subject: QM study of heterogeneous reactions

From: Dr Mark J Biggs <[email-masked]>
Date: Wed, 26 Apr 2000 16:15:42 +0100

Hi All!

I have a question relating to QM study of reactions relevent to heterogeneous
catalysis.  I have never done such simulations before, but have a number of
papers which report such work.

One of the papers (Schneider et al., J. Phys. Chem. B, V101, p4353, 1997)
I have seeks to determine the mechanism for NO decomposition over Cu-ZSM5.

This paper indicates that the --Cu .. O-N configuration (i.e. oxygen down)
is the lowest energy configuration.  From this start point, a mechanism is
then determined.

However, they also say that the --Cu .. N-O configuration is only slightly
more energetic than the O-down configuration.

My question is:

Why should we pick any one particular starting configuration?

Let me outline my motivation for this question so that I do not get the
obvious answer (i.e. minimum energy configuration as revealed by QM sim
on model used)!

The models used in QM are normally small cluster models which cannot take
into account the steric effects imposed by cages or other fluid molecules.
It is possible that the solid structure and other fluid molecules may make
it difficult to acheive an O-down configuration as simulated above.

Even if the cluster is adequate, there is likely to be various angles of
approach and orientations of the molecule relative to the Cu site at
typical reaction temperatures, and the molecule may not ever have time to
"get itself" into the O-down position before a second NO arrives and a
reaction possibly occurs.

The reaction configuration may, therefore, never be able to start from the
initial configuration assumed on the basis of minimum energy as determined
by the cluster model, and may never follow the path of minimum PE.

Whilst I have used the above paper as an example, it appears as if this
approach is used in all papers I have (even so-called "dynamic QM" using
DFT MD).

Answers can be sent to [email-masked]

Many thanks in advance.

Mark Biggs