POMs catalysis of benzyl alcohol oxidation, why Ni is the best cation with POM for that catalysis?

Why is nickel the optimal cation for benzyl alcohol oxidation with POMs and hydrogen peroxide?


In this article ([Chinese Journal of Chemistry, Volume 30, Issue 2, 2012](http://dx.doi.org/10.1002/cjoc.201100144)), they came to a conclusion that Ni gives better conversion then the other metals (Zn, Co, Mn, Cu, Fe) when they used it with Keggin type POM $\ce{Ni3(PW12O40)2}$. See the next note about the active structure:


[![its the active structure](https://i.stack.imgur.com/UqTZj.png)](https://i.stack.imgur.com/UqTZj.png)


How would you prove that Nickel is the best?


[![the table from the article describes the catalytic activity](https://i.stack.imgur.com/zTqrX.png)](https://i.stack.imgur.com/zTqrX.png) 


