What substance(salt) facilitates the self-ionization of water when it dissolves in water?

What about ammonium chloride? Does it accomplish the job above?

Which reaction for it has a greater dissociation constant?

the reaction of NH4+ with water or the reaction of it with hydroxide ion?

under a given temperature, I doubt if the salt added to water would have any effect on the self-dissociation of water. as we know that at a given temperature, the dissociation constant of water would be a constant. for instance, the dissociation constant of water at 298K is 1*10^14. so I think the added salt would not have an effect on this matter.

 

for a substance which consumes hydroxonium or hydroxide ions in the water, I think there should be a equilibrium shift rather than a fostering effect of self-dissociation of water. for example, if ammonium ions are added, there should be an equilibrium of:

NH4+ + OH- (from water) <--> NH3 + H2O

which in turns consuming parts of the hydroxide in water, hence the solution would probably be slightly acidic due to the excess of hydroxonium ions in water.

See water ion product to check how water dissociation constant changes with temperature.

 

In general no ion can facilitate water dissociation, however, if you dissolve an inert salt increasing ionic strength of the solution, you will observe changes in the water dissociation. But this effect is universal, it is not limitied just to water dissociation. See ionic strength and activity coefficients lecture for details.