Desorption kinetics of small n-alkanes from magnesium oxide(100), platinum(111), and carbon(0001)/platinum(111) and studies of palladium nanoparticles: Growth and sintering on aluminum oxide(0001) and methane dissociation on magnesium oxide(100)

Small alkane interactions with surfaces are of interest for a wide range of applications including catalysis, atmospheric chemistry, geochemistry and chemical sensing. We have investigated in fundamental detail the chemical physics underlying relevant elementary steps for heterogeneous catalysis by studying model surfaces that are well defined yet present the same local structure as on real industrial catalysts.; We measured alkane desorption energy on MgO(100), Pt(111), and C(0001)/Pt(111) surfaces. The molecules, CNH2 N+2 (N = 1--4, 6, 8, 10), were each studied by temperature programmed desorption (TPD) over a range of initial coverages and heating rates. We developed a novel TPD analysis technique which allows for accurate determination of prefactors and coverage-dependent desorption energies. The prefactor for desorption was found to increase by several orders of magnitude with chain length. This increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gas-like transition state for desorption. The desorption energy from each surface increased linearly with chain length with a small y-intercept.; We also measured the dissociative sticking probability of methane on MgO-supported Pd nanoparticles and on Pd(111). On Pd(111), the sticking exhibits "normal energy scaling." We show that the Pd nanoparticle (∼3 nm wide) geometry must be decoupled from the measured sticking probabilities in order to compare the intrinsic reactivity of the Pd particles with Pd(111). We find that the sticking probability on ∼3 nm Pd particle surfaces is at most twice as large as on Pd(111).; We have also examined the growth and sintering kinetics of Pd nanoparticles on alpha-Al2O3(0001) by non-contact atomic force microscopy and low-energy ion scattering spectroscopy. This is the first study of metal nanoparticles on a well-defined oxide surface where both of these techniques are used for characterization. The Pd grows initially as 2D islands at 300 K, with the transition to 3D particle growth at 0.25 monolayers. Upon heating at 1 K/s, the Pd particles (∼2.5 nm diameter for 0.8 monolayers at 300 K) sinters at a nearly constant rate, doubling in particle diameter by ∼1000 K.