Water and Carbon Dioxide Adsorption on CaO(001) Studied via Single Crystal Adsorption Calorimetry

A new method to analyze microcalorimetry data was employed to study the adsorption energies and sticking probabilities of D2O and CO2 on CaO(001) at several temperatures. This method deconvolutes the line shapes of the heat detector response into an instrument response function and exponential decay functions, which correspond to the desorption of distinct surface species. This allows for a thorough analysis of the adsorption, dissociation, and desorption processes that occur during our microcalorimetry experiments. Our microcalorimetry results, show that D2O adsorbs initially with an adsorption energy of 85–90 kJ/mol at temperatures ranging from 120 to 300 K, consistent with prior spectroscopic studies that indicate dissociation. This adsorption energy decreases with increasing coverage until either D2O multilayers are formed at low temperatures (120 K) or the surface is saturated (> 150 K). Artificially producing defects on the surface by sputtering prior to dosing D2O sharply increases this adsorption energy, but these defects may be healed after annealing the surface to 1300 K. CO2 adsorbs on CaO(001) with an initial adsorption energy of ~ 125 kJ/mol, and decreases until the saturation coverage is reached, which is a function of surface temperature. The results showed that pre-adsorbed water blocks adsorption sites, lowers the saturation coverage, and lowers the measured adsorption energy of CO2. The calorimetry data further adds to our understanding of D2O and CO2 adsorption on oxide surfaces.

Energies of Adsorbed Catalytic Intermediates on Transition Metal Surfaces: Calorimetric Measurements and Benchmarks for Theory

Better catalysts and electrocatalysts are essential for the production and use of clean fuels with less pollution and improved energy efficiency, for making chemicals with less energy and environmental impact, for pollution abatement, and for many other future technologies needed to achieve environmentally friendlier energy supply and chemicals industry. Crucial for rational design of better catalyst and electrocatalyst materials is knowledge of the energies of elementary chemical reactions on late transition metal surfaces. This knowledge would also aid in designing more efficient and stable photocatalysts and batteries for harvesting and storing solar energy. These are all crucial for sustainable living with high quality. Herein, I review measurements of surface reaction energies involving many of the most common adsorbates formed as intermediates on late transition metal surfaces in catalytic and electrocatalytic reactions of interest for energy and environmental technologies. I focus on calorimetric measurements of the heat of molecular and dissociative adsorption of gases on single crystals (i.e., single crystal adsorption calorimetry, or SCAC) that allow the heats of formation of adsorbed intermediates in well-defined structures to be directly determined. Adsorption reactions are often irreversible, and in such cases SCAC is required to get these heats, since the other methods for measuring adsorption energies (equilibrium adsorption isotherms and temperature-programmed desorption) work only for reversible adsorption. Common examples of irreversible adsorption reactions are ones that produce adsorbed molecular fragments or adsorbed molecules such as olefins and aromatic molecules that bind very strongly to non-noble metals. When the heats of formation of different adsorbed molecular fragments are compared to each other, and to their values on different metal surfaces, they reveal which properties of the metal surface and the molecular fragments determine metal-adsorbate bond strengths, and clarify differences in catalytic reactivity between different metals. When combined with earlier adsorption energy measurements, these heats also provide a database of reliable energies of adsorbed catalytic intermediates that serve as crucial benchmarks to guide the development of improved computational methods for calculating the energetics of elementary steps on late transition metal surfaces (i.e., reaction energies and activation barriers), such as density functional theory. The energy accuracy of such computational estimates is crucial for the future of catalysis research and catalyst discovery.