Here are some basic principles

1) The valence band of catalyst should be lower (more positive potential) than the oxidation potential of the pollutant

2) The Conduction band of catalyst should be higher (more negative potential) than reduction potential of mediating species.

3) The band gap must smaller than the energy of the incident radiation.

Other good features are:

- Catalyst must be soundly stable and should not be degraded under reaction conditions

- Sometimes you may need charge transfer catalysts between catalyst susrface and reactants

- More stable contaminants need catalyst with wider band gap

- Typically, the TiO2 photo-degrades ant organic contaminant in water if given enough light with wavelength shorter than 390 nm. 

- My advice is also: avoid using catalysts which may yield hazardous species during reactions (such as CdS which yields cadmium ions)

I am agree with  Prof. Dr Hilal.

Dear Eng. Cao

so whats your advices for CdS as a visible light driven photocatalyst?

does supporting the cds with a more stable support improves it's stability?

what about the use of sacrificial agents? I know it works for water splitting process but don't know works in this area?

Thanks all for great answers ...

Cao's answer is correct.

We have worked hard on stabilizing CdS but it decomposes into ions, worst of which are Cd2+ ions. Cd ions are very bad, and you do not wish to purify water from one contaminant and pollute it with Cd ions. We tried to stabilize CdS but it did not work.

Pleas read our paper on this:  http://www.sciencedirect.com/science/article/pii/S030438940901382X  

We are trying another method. We use TiO2 or ZnO (both have wide band gaps) and sensitize them with safe natural dyes (such as anthocyanine or curcumine). See our manuscript:  http://www.sciencedirect.com/science/article/pii/S1293255811001051   

Using a natural dye is good because if it degrades it should yield non-hazardous products. Moreover, you can add more natural dyes to re-sensitize TiO2 or ZnO. 

Best wishes

Hikmat S. Hilal

Hi,

All of the comments are correct. But, they are parts of a whole story. Heterogeneous photocatalytic activity depends on many parameters such as production of electron-hole pairs, speed of charge transfer in the photocatalyst, adsorption constant of the pollutant over the photocatalyst, separation of charge carriers, VB and CB energy positions, band gap of the photocatalyst, and so on. Hence, in my opinion, we cannot simply predict proper photocatalyst for a pollutant. We can only propose something that needs to confirm. For more information, please refer to the papers in my Researchgate homepage.