Phosphomolybdic acid (H3PMo12O40) along with niobium, pyridine and niobium/pyridine exchanged phosphomolybdic acid compounds were prepared. These compounds were converted to selective oxidation catalysts by pre-treating to 693 K in an inert atmosphere. As shown previously, the active catalyst consists of partially decomposed, partially reduced Keggin units and MoOx fragments with some MoOx fragments collected around the Nb. The amount of surface Mo species reduced to the 5+ oxidation state varied among the catalysts. Ammonia adsorption microcalorimetry and methanol oxidation studies were carried out to investigate the acid sites strength and the acid/base/redox properties of each catalyst. The addition of niobium, pyridine or both increased the ammonia heat of adsorption by 30-40 kJ/mol and the total ammonia uptake. The catalyst with both niobium and pyridine demonstrated the largest number of strong sites. For the parent H3PMo12O40 catalyst, methanol oxidation favors the redox product (similar to 95% selectivity). However, catalyst deactivation occurs. The presence of niobium results in similar selectivity to redox products (similar to 93%) but also results in no catalyst deactivation. Incorporation of pyridine to the precursor compound, in contrast, changes the selectivity to initially favor the acid product (similar to 62%). Again, the catalyst deactivated and selectivity changed during deactivation to favor the redox product (similar to 55%). Finally, the inclusion of both niobium and pyridine results in strong selectivity to the acid product (similar to 95%) while also showing no catalyst deactivation and stable selectivity. Specific activity for the niobium and pyridine exchanged catalyst for the methanol oxidation reaction was twice any other catalyst. Selectivity to acid products was correlated with the amount of reduced surface Mo species. Thus, the presence of pyridine appears to enhance the acid property of the active site in the catalyst while niobium appears to stabilize the active site. (C) 2013 Elsevier B.V. All rights reserved.