Transition state optimisation on the surface of periclase I want to model a reaction catalysed by periclase ($\ce{MgO}$) using DFT.
I have a good guess on the transition state (TS) of the reaction that goes in gas phase/solvent (produced using MOPAC).

The proposal for this reaction goes by the Eley–Rideal mechanism and I have a good guess on how the reactant adsorbs.

Now comes the question: I would like to optimise the TS on the surface using DFT (either ORCA or NWChem), but in tractable time.

I am tempted to use a simple, fixed model for the surface, e.g., a set of fixed atoms in bulk arrangement.
Can this work?
How am I supposed to find a structure with just one imaginary frequency using a partially fixed model (maybe using some clever vibrational projections)?

Even though NEB could be applied, the implementation of it in NWChem does not work with fixed atomic coordinates.
Besides, I am confident on the TS I found without the catalyst, so I would like to use that instead of starting all over again.

Edit: what about using a very small cluster model, say, 8-16 atoms, and let it relax? Do you think it could work (in the sense of giving a reliable TS in terms of geometry and energy)?
To study surface reactions I recommend the growing string method (GSM) for surfaces. Here is a nice quote from the paper which developed it (Ref. 1)


  GSM’s efficacy was confirmed by comparison with CI-NEB on
  an extensive set of reactions characteristic of modern surface chemistry studies. In these cases, GSM reduces the computational cost (in terms of gradient computations) by about 45% on average over CI-NEB. In addition to high efficiency, GSM has the advantage of operating in single-ended way to enable explorative study of chemical reactions. 


Key to the success of GSM is the use of the efficient hybrid delocalized coordinate system which greatly improves optimization and interpolation steps in comparison to NEB which uses Cartesian coordinates. 

The exact code to reproduce their work is not publicly available, although you may be able to request a copy. However you should be able to reproduce it and more, with this python version of the code that I have developed. 

Also, as others have pointed out, you should consider a higher level of theory like periodic wavefunction DFT (e.g. as implemented in VASP) . 

References: 


Jafari, M, Zimmerman, P. M.. "Reliable and efficient reaction path and transition state finding for surface reactions with the growing string method", J. Comput. Chem. 2017, 38, 645– 658. https://doi.org/10.1002/jcc.24720