Single Crystal Adsorption Calorimetry Research Articles

Accurate and Ultrafast Simulation of Molecular Recognition and Assembly on Metal Surfaces in Four Dimensions.

Understanding molecular interactions with metal surfaces in high reliability is critical for the development of catalysts, sensors, and therapeutics. Obtaining accurate experimental data for a wide range of surfaces remains a critical bottleneck and quantum-mechanical data remain speculative due to high uncertainties and limitations in scale. We report molecular dynamics simulations of adsorption energies and assembly of organic molecules on elemental metal surfaces using the INTERFACE force field (IFF). The force field-based simulations reach up to 8 times higher accuracy than density functional calculations at a million-fold faster speed, as well as more than 1 order of magnitude higher accuracy than other force fields relative to accurate measurements by single-crystal adsorption calorimetry. Uncertainties of prior computational methods are effectively reduced from on the order of 100% to less than 10% and validated by experimental data from multiple sources. Specifically, we describe the molecular interactions of benzene and naphthalene with even and defective platinum surfaces across a wide range of surface coverage in depth. We discuss molecular-scale influences on the heat of adsorption and clarify the definition of surface coverage. The methods can be applied to 18 metals to accurately predict binding and assembly of organic molecules, ligands, electrolytes, biological molecules, and gases without additional fit parameters.

Adsorption and adhesion energies of n-Decane on the Pt(111) surface by calorimetry

The heat of adsorption of n-decane molecularly adsorbed on Pt(111) at 150 K was measured by single-crystal adsorption calorimetry (SCAC). The heat of adsorption was initially ∼152 kJ/mol on terrace sites up to a coverage of Θ = 0.045 ML . Previous TPD measurements of the desorption energies of shorter n-alkanes (C1 to C6) on Pt(111) terraces showed a linear increase with chain length which predicts a desorption energy for n-decane in very good agreement with this new result. A heat of 169 kJ/mol was measured below 0.004 ML and attributed to defect sites, probably step edges. At coverages above Θ = 0.27 ML (∼3 molecular layers), the multilayer heat of adsorption was constant at ∼85 kJ/mol. Using the heat of adsorption versus coverage at 150 K up through this bulk-like multilayer, the adhesion energy per unit area of liquid n-decane on Pt(111) terraces at room temperature was estimated to be 0.148 J/m2. This is very similar to the value estimated for liquid n-hexane on Pt(111) from TPD energies, which shows that while adsorption energy increases nearly proportional to the number of CH2 plus CH3 groups, as the adhesion energy is a per unit area value, it remains nearly constant (0.15 J/m2) between hexane and decane.

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.

Energetics of Ag Adsorption on and Adhesion to Rutile TiO2(100) Studied by Microcalorimetry

The adsorption and adhesion energies of vapor-deposited Ag on rutile TiO2(100) films have been measured using single-crystal adsorption calorimetry and He+ low-energy ion scattering spectroscopy. A...

Energetics and Structure of Nickel Atoms and Nanoparticles on MgO(100)

The growth morphology and interfacial energetics of vapor-deposited Ni on the MgO(100) surface at 300 and 100 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 300 K, the Ni atoms grow as three-dimensional nanoparticles with a saturation number density of 5 × 1016 particles/m2. The differential heat of adsorption at 300 K increases rapidly with coverage, from 276 (initially) to 335 kJ/mol by 0.4 ML. Thereafter, it slowly increases asymptotically to the sublimation enthalpy of bulk Ni (430 kJ/ml) by 9 ML. The Ni 2p3/2 XPS peak binding energy at 300 K is initially (i.e., at 0.16 ML) 1.4 eV higher than that for bulk Ni(solid), but it decreases to that value at high coverage. The Ni atoms form a metastable hcp phase at 300 K when in nanoparticles with diameter <2.5 nm, and the adhesion energy of such Ni nanoparticles to MgO(100) was found to be 3.05 J/m2. At 100 K, the Ni atoms form single adatoms and then 0.17 nm thick 2D islands at low coverage with fewer Ni–Ni bonds compared to the Ni nanoparticles formed at 300 K. Thus, the initial heat (i.e., for the first ∼0.03 ML) is 148 kJ/mol at 100 K, 128 kJ/mol lower than that at 300 K, and remains lower for the 2D islands. With increasing coverage at 100 K, the tiny 2D Ni islands grow in size to cover nearly the entire surface before thickening. The XPS Ni 2p3/2 peak binding energy for 0.21 ML Ni on MgO(100) at 100 K is 2.2 eV higher than that for bulk Ni(solid), suggesting charge transfer from Ni to MgO(100) and formation of Ni2+ at very low coverage.

Ni Nanoparticles on CeO2(111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO2-x(111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO2-x(111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO1.95(111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape showed an increase in Ce3+/Ce4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO2 catalysts for which metal-oxide interactions play a very important role in the activation of O-H and C-H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

Leveraging Thermochemistry Data to Build Accurate Microkinetic Models

Ab initio microkinetic modeling, parameterized using density functional theory (DFT) energies, is a common tool to quantify reaction rates and analyze reaction mechanisms a priori in heterogeneous catalysis. Such models, however, often have large prediction errors even if they include plausible reaction steps and correctly model the active sites; this is partially due to the intrinsic inaccuracies of the chosen DFT functional. Borrowing concepts from Bayesian calibration theory, we show that transferable data-driven corrections to DFT energies in the form of Gaussian process models trained on single crystal adsorption calorimetry data can improve the accuracy of microkinetic models substantially. Specifically, we demonstrate that such corrections improve the predictive accuracy of the microkinetic model of the water gas shift reaction on single crystal Cu(111) surface by three (3) orders of magnitude. We finally show that Gaussian Process corrections serve as informed priors in a Bayesian experimental des...

C[sbnd]H versus O[sbnd]H bond scission in methanol decomposition on Pt(111): Role of the dispersion interaction

The dispersion interaction plays an important role in the organic-metal interaction, particularly in the case where the organic molecule is weakly bound to the metal substrate. Here, we report the effects of the dispersion interaction on the equilibrium structure of adsorbed methanol and the energetics of methanol decomposition via the CH and OH bond scissions on Pt(111) based on the results of density functional theory calculations by PBE and optB86b-vdW functionals. The binding mechanism of methanol with the surface is also clarified by the detailed analysis of the density of states, the electron density difference, and the effective Bader charge. The DFT calculated results show that the dispersion interaction significantly reduces the equilibrium distance and increases the adsorption energy of methanol on the top site of Pt(111) to 0.63 eV, thereby making it accord with the experimental result by single-crystal adsorption calorimetry. Moreover, the DFT calculations by PBE functional predict methanol tends to desorb from Pt(111) rather than undergoes decomposition whereas those by optB86b-vdW functional demonstrate that the dispersion interaction facilitates methanol decomposition via the CH bond scission. Our theoretical results provide new insights into the surface chemistry of methanol on Pt(111) under ultrahigh vacuum conditions.

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.

Energetics of Au Adsorption and Film Growth on Pt(111) by Single-Crystal Adsorption Calorimetry

Bimetallic catalysts are an important class of heterogeneous catalysts with catalytic properties distinct from either of their bulk metal constituents. The structural, electronic, chemisorptive, and catalytic properties of bimetallic surfaces have been widely studied. Surface reactivity often correlates with adsorption energy of one metal on a single-crystal surface of the other as measured using temperature-programmed desorption (TPD). However, TPD only works for systems where the metals are immiscible. For bimetallic systems that form an alloy or intermetallic compound, TPD generally fails because the adsorbed metal penetrates into the bulk upon heating. The metal-on-metal adsorption energy is unmeasured for all but one such system previously but often calculated because these adlayers often have interesting catalytic properties. We report here calorimetric measurements of the adsorption energy versus coverage of an adlayer of one metal on another for such a bimetallic system, where the metals prefer to alloy: Au on Pt(111). This bimetallic combination is important in catalysis and electrocatalysis. The first monolayer (ML) of Au grows pseudomorphically with the Pt(111) surface at 300 K, with an average heat of adsorption of 389, ∼21 kJ/mol greater than the bulk heat of sublimation of Au. The heat increases with coverage by ∼11 kJ/mol in the first 0.03 ML and then by another ∼2 kJ/mol up to a maximum of 395 kJ/mol at 0.7 ML, and it then decreases to near the bulk heat of sublimation (368 kJ/mol) at 1 ML. The increase in heat is attributed to the increase in size of the two-dimensional Au islands that nucleate at a very low coverage and their corresponding increase in the average number of Au–Au nearest neighbor bonds. The high-coverage decrease in heat is attributed to the buildup of strain associated with the 4% Au/Pt lattice mismatch. The second and possibly third layers of Au show similar but much smaller oscillations in heat around 370 kJ/mol, attributed to the same two effects.

Energetics of Adsorbed Methanol and Methoxy on Ni(111): Comparisons to Pt(111)

The heats of adsorption of methanol molecularly adsorbed on clean Ni(111) and dissociatively adsorbed on oxygen-precovered Ni(111) were measured by single-crystal adsorption calorimetry (SCAC). The...

Energetics of Adsorbed Phenol on Ni(111) and Pt(111) by Calorimetry

The heat of adsorption and sticking probability of phenol were measured on Ni(111) at 150 K and Pt(111) at 90 K using single crystal adsorption calorimetry (SCAC). Phenol adsorbs molecularly on bot...

Energetics of adsorbed benzene on Ni(111) and Pt(111) by calorimetry

The heat of adsorption and sticking probability of benzene were measured on Ni(111) and Pt(111) at 90 K using single crystal adsorption calorimetry (SCAC). Benzene adsorbs molecularly with a heat of 208 kJ/mol on terrace sites in the low-coverage limit on Ni(111) and has a standard enthalpy of formation (∆Hf0) of C6H6,ads of −251 kJ/mol at a coverage of 1/9 ML. These results are compared to calorimetric results on Pt(111), where the standard enthalpy of formation (∆Hf0) of C6H6,ads was found to be −244 kJ/mol at a coverage of 1/9 ML. The slightly stronger bonding to Ni is attributed to stronger CNi covalent bonding partially compensated by weaker van der Waals (vdW) attractions to Ni compared to Pt. The measured energetics for benzene are compared to Density Functional Theory (DFT) calculations from previous literature, showing that functionals that do not contain corrections for vdW interactions badly underestimate the bond energies of benzene to both Ni(111) and Pt(111), while many of those that correct for vdW interactions are much more accurate.

Adsorbed Hydroxyl and Water on Ni(111): Heats of Formation by Calorimetry

Adsorbed hydroxyl is a common intermediate in many catalytic reactions on Ni catalysts. The heat of the dissociative adsorption of D2O onto an O-precovered Ni(111) surface producing adsorbed hydroxyls is measured here by single crystal adsorption calorimetry, with an average value of 67.3 kJ/mol up to 0.23 ML coverage. From this, the enthalpy of formation for adsorbed hydroxyl on Ni(111) and the bond energy of hydroxyl to Ni(111) were estimated to be −278.3 and 314.1 kJ/mol, respectively. The molecular adsorption of D2O on clean Ni(111) gave an integral heat of 53.6 kJ/mol at 0.5 ML. In comparison to Pt(111), adsorbed hydroxyl binds to Ni(111) 67 kJ/mol more strongly and adsorbed D2O binds to Ni(111) ∼ 2 kJ/mol more strongly. These energetics help clarify the different catalytic properties of Ni versus Pt, and serve as benchmarks to evaluate the energy accuracy of quantum mechanical methods used in surface chemistry.

Formic Acid Dissociative Adsorption on NiO(111): Energetics and Structure of Adsorbed Formate

The dissociative adsorption of carboxylic acids on oxide surfaces is important for understanding adsorbed carboxylates, which are important as intermediates in catalytic reactions, for the organo-functionalization of oxide surfaces, and in many other aspects of oxide surface chemistry. We present here the first direct experimental measurement of the heat of dissociative adsorption of any carboxylic acid on any single-crystal oxide surface. The enthalpy of the dissociative adsorption of formic acid, the simplest carboxylic acid, to produce adsorbed formate and hydrogen (as a surface hydroxyl) on a (2 × 2)-NiO(111) surface is measured by single crystal adsorption calorimetry. The differential heat of adsorption decreases with formic acid coverage from 202 to 99 kJ/mol at saturation (0.25 ML). The structure of the adsorbed products is clarified by density functional theory (DFT) calculations, which provide energies in reasonable agreement with the calorimetry. These calculations show that formic acid readily...

Adsorption of Acetonitrile, Benzene, and Benzonitrile on Pt(111): Single Crystal Adsorption Calorimetry and Density Functional Theory

Single crystal adsorption calorimetry on Pt(111) surfaces allows us to study the influence of different functional groups on heats of adsorption. At zero coverage we find 103, 194, and 325 kJ/mol for acetonitrile, benzene, and benzonitrile, respectively. This leads to the idea of summing up the heats of adsorption of acetonitrile and benzene, assuming that the total heat of adsorption of benzonitrile is due to the combined effect of isolated functional groups. This idea is discussed in light of the recent literature regarding nitrile adsorption on metal surfaces and further investigated theoretically by density functional theory. In order to figure out the importance of dispersive effects on the heats of adsorption, van der Waals corrected calculations are performed considering different binding modes and surface reconstructions.

Energetics of adsorbed formate and formic acid on Ni(111) by calorimetry

Surface carboxylates are common intermediates in many catalytic reactions and are also used for organo-functionalization of surface. Herein, adsorbed formate, the simplest carboxylate, was produced by dissociative adsorption of formic acid on the Ni(111) surface and the heats of adsorption were measured by single-crystal adsorption calorimetry (SCAC). At 240K, the integral heat of dissociative adsorption to make adsorbed bidentate formate and adsorbed hydrogen is 117.1kJ/mol at 1/7ML. The estimated accuracy of measured heats is within 3kJ/mol. Therefore, the standard enthalpy of formation of bidentate formate on Ni(111) is derived as −449.3kJ/mol and the bond enthalpy to the Ni(111) surface is 319.6kJ/mol. Comparison to our earlier results for Pt(111) shows that bidentate formate is 54kJ/mol more stable on Ni(111). This is the first experimental measurement of the energy of any molecular fragment bonded to any Ni surface through metal-oxygen bonds, so it is an important benchmark for validating the accuracy of quantum mechanical calculations of a wide range of adsorbed oxygenates on Ni catalysts. Compared to previous density functional theory (DFT) calculations, we found that DFT calculations systematically underestimate the bond energy of formate to Ni(111), but correctly predict that it is stronger to Ni(111) than to Pt(111). The integral heat of HCOOH adsorption to make molecularly adsorbed formic acid was also measured and found to be 63.2kJ/mol at 0.45ML and 120–155K.

Energetics of 2D and 3D Gold Nanoparticles on MgO(100): Influence of Particle Size and Defects on Gold Adsorption and Adhesion Energies

The adsorption of gold vapor onto MgO(100) films grown on Mo(100) was studied at 300 and 100 K using single crystal adsorption calorimetry (SCAC). The Au particle morphology was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). The LEIS data combined with particle shape measurements from the literature (Benedetti, S.; Myrach, P.; di Bona, A.; Valeri, S.; Nilius, N.; Freund, H.-J. Phys. Rev. B 2011, 83 (12), 125423) reveal that, at both 300 and 100 K, Au grows as 2D islands with bilayer thickness (∼0.41 nm) up to a diameter of ∼7 nm. At higher coverage, the islands thicken with little increase in diameter. The island densities are 3.0 × 1011 and 5.4 × 1011 per cm2 at 300 and 100 K, respectively. The initial sticking probability of Au is 0.90 at 300 K and 0.95 at 100 K. The surface residence time of the Au atoms that do not stick is <10 ms, implying that gold monomers bind to MgO(100) weakly (<69 kJ/mol). The adsorption energies indicate that Au...

Energetics of Adsorbed Methyl and Methyl Iodide on Ni(111) by Calorimetry: Comparison to Pt(111) and Implications for Catalysis

The heat of adsorption and sticking probability of methyl iodide were measured on Ni(111) at 100 and 160 K using single-crystal adsorption calorimetry (SCAC). At 100 K, methyl iodide adsorbs molecularly with a heat of 102 kJ/mol on terrace sites in the low-coverage limit, giving a standard enthalpy of formation (ΔHf°) of CH3I(ad) of −87 kJ/mol. A heat of 122 kJ/mol is also measured on defect sites, probably step edges. Calorimetry of the dissociative adsorption of methyl iodide on Ni(111) at 160 K yielded an integral heat of adsorption of −270 kJ/mol at 0.04 ML, providing the energetics of adsorbed methyl, with ΔHf°[CH3(ad)] = −71 kJ/mol and a CH3–Ni(111) bond enthalpy of 218 kJ/mol. This is 22 kJ/mol stronger than the reported value for H3C–Pt(111) bonds, explaining the greater activity of Ni catalysts for hydrogenolysis in comparison to Pt. The measured energetics for methyl were compared to density functional theory (DFT) calculations from previous literature, showing that these methods systematically ...

ChemInform Abstract: Energies of Formation Reactions Measured for Adsorbates on Late Transition Metal Surfaces

AbstractReview: 117 refs.