I don't fully agree. Alumina will change the pH of the solution due to protonation/deprotonation of surface hydroxyls (it is like introducing an acid or a base to a solution); actually the pH shift of a solution upon introducing an oxide is a way to measure the PZC of this oxide. Of course this effect will depend on the surface area of the alumina and of the oxide to solution ratio. Moreover, the surface of alumina can convert to bayerite in water and the dissolution may not be negligeable (it depends of the time scale)

you could for example have a look at this paper

http://www.sciencedirect.com/science/article/pii/S0021979703006532

Gamma alumina is very poorly soluble in water, so I don't expect that it changes the pH of water in which it is introduced. What can be said is that, because of its low ionic potential, Al3+ fails to repel H+ ions from the surface at neutral pH, resulting in a surface of positive charge. The point of zero charge is around pH 9.5, and pH higher than this lead to negatively charged surface.

Alain

Mr. Alain Celzard what is the relation between ionic potential and surface activity of solute in solution ....Can u explain me please

Metal oxides are crystals made of generally small metal cations located at the centre of cages made of much bigger oxide ions. The higher the charge of the cation, the lower the cation size, such as Al3+. Therefore, the surface of Al2O3 is mainly made of oxyde ions, which are negatively charged. These anions can adsorb H+ from the solution. If the pH is "low", there are many H+ ions which can completely cover the surface. The latter is consequently positively charged. At "high" pH, most oxides have not adsorbed H+, and the surface is consequently negatively charged. For each oxide, there is a point of zero charge, i.e. a pH at which the surface is neutral. Such a pH depends on the ability of the cation to repel H+ ions, what I called "the ionic potential". For instance, silica is made of Si4+ ions, very small and highly charged, hence having a high ionic potential. A high concentration of H+ is therefore required for making the surface of silica positively charged, and the point of zero charge is indeed around 1.5. In other words, the surface of silica is always negatively charged above pH 1.5. In contrast, MgO is made of bigger and less charged Mg2+ cations, so that repelling H+ ions is much less effective. As a consequence, the surface of MgO is positively charged at pH up to 12.5. As alumina is made of Al3+, intermediary between SI4+ and Mg2+ in terms of ionic potential, the point of zero charge is between those of silica and magnesia, i.e. 9.5.

Hope it helps,

Alain

Thank you very much.

Does addition of few dopants can change these properties significantly?

By definition, dopants are introduced at very low concentrations, so I don't expect any significant impact neither on solubility nor on surface properties.

Alain

I agree with Alain. The PZC of oxide is the key parameter to prepare precious metal supported catalyst

I don't fully agree. Alumina will change the pH of the solution due to protonation/deprotonation of surface hydroxyls (it is like introducing an acid or a base to a solution); actually the pH shift of a solution upon introducing an oxide is a way to measure the PZC of this oxide. Of course this effect will depend on the surface area of the alumina and of the oxide to solution ratio. Moreover, the surface of alumina can convert to bayerite in water and the dissolution may not be negligeable (it depends of the time scale)

you could for example have a look at this paper

http://www.sciencedirect.com/science/article/pii/S0021979703006532

I agree with most of what was said here. but would add a cautionary note.  The statement that dopants will not alter surface properties seems very much too general in this context.  Dopants are used in minute quantities in semiconductors and for that case, the statement is likely true.  However, the term is also used more generally for metal oxides and is sometimes applied to components that are present in very large amounts (e.g. in excess of 10%, for example in doped ceria).  My only point here is that whether or not the dopant has an effect is a function of what it is, how much of it is present, whether it segregates to the surface etc. So, in the case KN Rao is asking about it may be possible depending on the specifics of the situation.  After all, what is the purpose of adding a dopant if not to alter the properties of the material being doped?