Adsorption and Reaction Microcalorimetry on Nickel and Platinum Single Crystal Surfaces
The unique Cambridge single crystal adsorption microcalorimeter has been applied to a range of systems of catalytic interest to extract fundamental thermodynamic data. Coverage-dependent adsorption heats and sticking probabilities have been obtained with CO, NO, oxygen and ethylene adsorbates and Pt{110}, Ni{100}, Ni{110} and K/Ni{110} substrates. The initial adsorption heats for CO, NO, and O2 on Pt{110} are 183, 160 and 335 kJ mol$^{-1}$ respectively. Catalytic heat of reaction data for CO + O2 on Pt{110}, the first obtained on a single crystal surface, suggest that "hot" adatoms are involved in producing thermally excited CO2 product molecules. The first measurement of the heat of adsorption of a hydrocarbon on a single crystal surface has been made for ethylene on Pt{110}. The initial value of 200 kJ mol$^{-1}$, together with coverage dependent values, yield an average Pt–C binding energy of 231 kJ mol$^{-1}$. A detailed comparison of new data for oxygen adsorption on Ni{100} and Ni{110} with existing data for Ni{111} is presented, with initial adsorption heats of 550, 475 and 440 kJ (mol O2) -1 respectively, while the corresponding sticking data suggest that the oxide film created is four atomic layers thick in each case. Novel temperature-dependent data have been successfully collected using a pyroelectric detector for oxygen on Ni{100} at 100, 300 and 410 K with a Monte Carlo simulation of the highest-temperature data yielding a strong second-nearest neighbour interaction energy of +30 ± 5 kJ mol$^{-1}$. The adsorption of CO on K-predosed Ni{110} was found to differ markedly from previous data collected on Ni{100} for potassium coverages up to 0.35 ML, with the adsorption heat promoted by only ~ 30 kJ mol$^{-1}$ compared to ~ 180 kJ mol$^{-1}$ for the {100} surface, a difference ascribed to the missing row reconstruction of Ni{110}.
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