Comparative Catalytic and Photocatalytic Reduction of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; by the in Situ Generated Solar H&lt;sub&gt;2&lt;/sub&gt; over Pt-Cu/TiO&lt;sub&gt;2&lt;/sub&gt; Catalysts