The thermodynamic properties of n-octane, n-nonane, and n-decane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature ranges of 225–295, 245–280, and 250–310 K, respectively. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. The temperature dependence of the two-dimensional compressibility was used to identify that two-phase transitions occur for the first and second layers, respectively. The average area per molecule of 137 ± 5,128 ± 5, and 202 ± 5 Å2 for octane, nonane, and decane, respectively, suggests that the carbon backbone is preferentially oriented parallel to the MgO(100) surface. A substantial set of molecular dynamics (MD) temperature and coverage runs for all three molecules using the COMPASS force field were used to calculate both the minimum energy configurations for individual molecules and near-completed layers on the MgO(100) surface. These calculations support the thermodynamic evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule along the ⟨11⟩ and ⟨10⟩ directions in the surface (100) plane. The MD simulations were used to evaluate the distribution of the molecules perpendicular to the MgO(100) surface as a function of temperature and nominal surface coverage. Evidence is also recorded that suggests that the interface broadens, the orientational order decreases, and liquid-like layers appear as the temperature is increased. As observed previously with the behavior of heptane on MgO(100), an entropic contribution to the free energy is apparent as a result of the odd number of carbon atoms in the nonane backbone. The near surface layer of all three molecules appears to remain orientationally ordered (i.e., noticeably more planar) and stabilized (i.e., more solid-like) at substantially higher temperatures when the second and third layers are observed to be liquid-like. This observation is consistent with earlier experimental observation of rare gases and alkanes on graphite that indicate that the near-surface layer melts at temperatures above the bulk melting temperature.

Au/ZnO catalysts have been used for liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol and compared with Au/Fe2O3 catalysts. To investigate the influence of the support on the hydrogenation activity and selectivity, three different Au/ZnO catalysts were synthesized, including Au/rod-tetrapod ZnO, Au/porous ZnO, and Au/ZnO-CP prepared using a coprecipitation method. The influence of calcination temperature was also systematically investigated in this study. The characterization of Au/ZnO catalysts was performed using ICP, N2 adsorption/desorption isotherms, X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy. Among all the supported Au catalysts prepared in this study, Au/ZnO-CP exhibits both the highest hydrogenation activity and selectivity. Using a 1.5% Au/ZnO-CP catalyst, 100% selectivity could be achieved with 94.9% conversion. We find that the Au particle (size and shape), the ZnO support (size and surface texture) and the interaction between Au and ZnO are three important parameters for achieving a highly efficient Au/ZnO catalyst.

Thermodynamic measurements using high-resolution volumetric adsorption isotherms were performed on n-pentane films physisorbed on MgO(100) surfaces between 181 K and 244 K. The isotherms show two distinct adsorption steps before the saturated vapor pressure is reached. The heat of adsorption is found to be 33.7 ± 0.3 kJ mol–1 for the first layer and 32.9 ± 0.3 kJ mol–1 for the second layer. Evolution of the two-dimensional compressibility, as a function of temperature, suggests that a phase transition occurs at 185.5 ± 1 K in the second layer. Neutron diffraction is used to establish that the melting of the pentane monolayer takes place between 101 K and 105 K. Computer modeling studies indicate that the pentane molecules adsorb with the molecular axis parallel to the substrate plane. These results suggest that the monolayer forms a solid with a rectangular unit cell, consistent with the neutron diffraction measurements.

Thermodynamic properties of n-heptane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature range of 205–275 K. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. Using the temperature dependence of the two-dimensional compressibility, two phase transitions were observed at 246.0 ± 2.0 K and 249.3 ± 1.2 K for the first and second layers, respectively. The average area per n-heptane molecule adsorbed on MgO was estimated to be 99 ± 10 Å2, suggesting that the carbon backbone is preferentially oriented parallel to the surface. The COMPASS force field was used to calculate the minimum energy configurations of an n-heptane molecule on the MgO(100) surface. The calculations support experimental evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule along the ⟨11⟩ and ⟨10⟩ directions in the surface (100) plane.

The wetting behavior of ethylene adsorbed on MgO(100) was investigated from 83−135 K using high resolution volumetric adsorption isotherms. The results are compared to ethylene adsorption on graphite, a prototype adsorption system, in an effort to gain further insight into the forces that drive the observed film growth. Layering transitions for ethylene on MgO(100) are observed below the bulk triple point of ethylene (T = 104.0 K). The formation of three discrete adlayers is observed on the MgO(100) surface; onset of the second and third layers occurs at 79.2 ± 1.3 K and 98.3 ± 0.9 K, respectively. Thermodynamic quantities such as differential enthalpy and entropy, heat of adsorption, and isosteric heat of adsorption are determined and compared to the previously published values for ethylene on graphite. The average area occupied by a ethylene molecule on MgO(100) is 22.6 ± 1.1 Å2 molecule−1. The locations of two phase transitions are identified (i.e., layer critical temperatures at Tc2(n=1) at 108.6 ± 1.7 K and Tc2(n=2) at 116.5 ± 1.2 K) and a phase diagram is proposed. Preliminary neutron diffraction measurements reveal evidence of a monolayer solid with a lattice constant of ∼4.2 Å. High resolution INS measurements show that the onset to dynamical motion and monolayer melting take place at ∼35 K and ∼65 K, respectively. The data reported here exhibit a striking similarity to ethylene on graphite which suggests that molecule−molecule interactions play an important role in determining the physical properties and growth of molecularly thin ethylene films.

Recent experimental investigations of the rotational motion of methane and molecular hydrogen using inelastic neutron scattering (INS) measurements in combination with thermodynamic techniques have provided a unique view of the evolution of the interaction of these two molecules with the MgO (100) surface and graphite basal plane. Despite significant differences in the chemical and physical properties and surface symmetry of these two adsorbents, the dynamical behavior of the adsorbed films is remarkably similar. The interaction of a CH4 monolayer solid with MgO and graphite, as monitored by the behavior of the J = 0 → J = 1 free rotor transition, is so strong that there is no evidence for unhindered rotation of the molecule below 20 K. Using this same transition as a probe, H2 monolayer solids exhibit nearly free or significantly hindered motion on graphite and MgO (100) surfaces, respectively. Investigations of CH4 and H2 multilayer films on MgO find that once the film thickness exceeds ∼3 layers, the molecule−molecule interactions predominantly determine the dynamical properties of the molecular film furthest from the surface. INS signals indicate that the dynamical motion in thicker films is closely related to that observed in the bulk system. The results of these studies serve as a valuable pathway for developing a qualitatively accurate description of the potential energy surfaces that govern the microscopic properties of these systems.