The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase

Abstract

The thermodynamic state of H₂ adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H₂ reacting with D₂ O to HDO, HD, and D₂ , and by DFT-based ab initio molecular dynamics simulations of H₂ adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H₂ on Pt is significantly weakened in the aqueous phase compared to adsorption at gas-solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19-22 kJ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>m</mml:mi> <mml:mi>o</mml:mi> <mml:msubsup><mml:mi>l</mml:mi> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msubsup> <mml:mspace></mml:mspace></mml:mrow> </mml:math> while inducing an additional entropy loss of 50-70 J <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>m</mml:mi> <mml:mi>o</mml:mi> <mml:msubsup><mml:mi>l</mml:mi> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> K^-1 . Upon dissociative adsorption of H₂ , the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H-Pt bond strength.