| Popis: |
Field emission microscopy (FEM) and thermal desorption techniques have been used to study the adsorption of oxygen, hydrogen, carbon monoxide and their mutual interaction on Rh surfaces. One form of adsorbed carbon monoxide is detected which displays the typical characteristics of non-dissociated adsorption. Three forms of oxygen can be distinguised: (i) oxygen that is desorbed around 150 K; (ii) strongly bound oxygen adatoms which are quite reactive with hydrogen and (iii) a less reactive oxide which is obtained after heating the Rh surface above 600 K. From the last two species oxygen is desorbed at ∼950 K. The catalytic reduction of oxygen has been studied by exposing adsorbed oxygen to hydrogen gas. The apparent activation energy of the O-H reactions is 2 kcal/mol. Visual observation of the FEM pattern shows that both reactions proceed with a sharp reaction boundary indicating a Langmuir-Hinshelwood mechanism. The O-H reaction occurs at about 280 K, the most active regions being the stepped (100) and (110) surfaces, while the (210) surface has a much lower activity. Quite remarkably, this (210) surface has a very high activity in the oxidation of adsorbed CO. The behavior of Rh in these reactions is compared with those of Ir and Pt surfaces. |
