Structure and reactivity of clean, potassium promoted and iron modified ruthenium
Various aspects of the surface chemistry of a ruthenium (1010) single crystal have been investigated under ultra-high vacuum conditions, employing the techniques of Auger Electron Spectroscopy, Low Energy Electron Diffraction, Multimass Thermal Desorption Spectroscopy, photoemission spectroscopies and work function measurements. The studies were undertaken with a view towards the applicability of ruthenium and iron-ruthenium alloys to the ammonia synthesis, though work relevant to the Fischer-Tropsch synthesis was also performed. The interactions of the gases nitrogen, hydrogen and ammonia with the clean surface were all explored. Molecular nitrogen was found to have an extremely low sticking probability of less than 10-9 at room temperature, but surface nitrogen atoms were deposited via two separate means, using either a mixture of nitrogen ions or nitrogen atoms themselves as the impinging species. Both chemisorbed and bulk implanted states were thereby observed. Hydrogen uptake at 310 K saturated at small doses but an estimate of 256 kJ mol-1 was made for the Ru-H bond strength from thermal desorption traces. Ammonia readily adsorbed at and above room temperature. Partial dissociation occurred at 300 K, the extent of fragmentation increasing as the crystal temperature was raised. Strong electron beam perturbations of the adlayer occurred, accelerating the rate of adsorption and resulting in the appearance of otherwise unobservable LEED patterns. The behaviour of the model promoter potassium was relatively typical of alkali metal/transition metal systems, though the anisotropic substrate potential was found to induce a series of interesting one dimensionally incoherent compressed overlayer structures. A further striking observation was the occurrence of substantial bulk dissolution of potassium following small doses at 430 K. The promoting effects of potassium on CO adsorption were investigated and interpreted interms of a recent modification of the Blyholder model, which combines indirect, through metal and direct, through space interactions. Finally, the deposition of iron on Ru(10bar 10) was studied. At 300 K the iron film grew in a metastable layer by layer mode, which rapidly rearranged on heating to either an alloy phase or a regime of 3D crystallites lying above one or two iron monolayers. Adsorption at elevated temperatures produced essentially the same results as heating layers deposited at room temperature.