pH is a meaure exclusively to acids diwolved in a solvent (mainly water). Maybe, you refer to find the quantity of acid sites in the surface of your catalyst. DRIFT or TPD using a molecule probe like pyridine, amonnia, can be useful for you.

Whether you want to determine pH or acid amount? For acid amount determination, you may refer to TPD or IR analysis with probe molucules such as ammonia, pyridine etc. But if you want to determine pH of a heterogeneous catalyst, it may not give you proper result. If the acidic species of catalyst is leached out in water then only pH determination will give proper result.

If you want to measure bronsted acid site, I suggest you to go with NH3-TPD. The amount of ammonia adsorbed on the catalyst surface is the acidity of the catalyst. I like to give you an article about this topic.

If you want to measure number of exchangeable proton in a solid acid catalysis, you may use  insitu ammonia adsorption over the activated solid and followed by FT-IR analysis.    We may get separate peaks (N-H bending vibration)  for  Lewis and Bronsted acid bound ammonia molecules.    By comparing their area ratio, their relative amount you can get.  Another way, you may use is as others told NH3 - TPD.    If you want strictly the pH, you may take known quantity of the solid and mix it  with known amount distilled water, shake it for a day, analyse the pH of the resulting solution before and after the shake up.  The difference will give pH of the solid. 

Surface acidity measurements

 Potentiometric titration. The total acidity of the solid samples

was measured by means of potentiometric titration .

The solid (0.05 g) was suspended in 20 ml acetonitrile (Merck), and

agitated for 3 h. The suspension of solid in acetonitrile was then

titrated using 0.025 N n-butylamine in acetonitrile at 0.005 ml/min.

The electrode potential variation was measured with an Orion 420

Digital A Model using a double junction electrode.