Single Crystal Adsorption Calorimetry Research Articles

Calorimetric measurement of adsorption and adhesion energies of Cu on Pt(111)

The adsorption energies of submonolayer amounts of one metal on the surface of another metal have been measured for decades by temperature programmed desorption. However, that method fails for metals that alloy. We report here the first measurement of the adsorption energy for any such metal-on-metal combination that forms a bulk alloy. The adsorption and interfacial energetics of vapor deposited Cu onto Pt(111) at 300K has been studied using single crystal adsorption calorimetry (SCAC) and X-ray photoelectron spectroscopy (XPS). The Cu grows as 2D pseudomorphic islands in the first layer and its heat of adsorption decreased linearly from 358 to 339kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2kJ/mol more weakly in the 2nd layer than above 3ML, where it reaches the bulk heat of sublimation of Cu(solid), 337kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12kJ/mol, compared to a difference of ~83kJ/mol for underpotential deposition of Cu on a Pt(111) electrode, with the difference attributed to stronger bonding of Cu to the solvent and double layer compared to Pt.

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

The energies of adsorbates containing H, N, C, O, and halogens that are of interest as intermediates, poisons, and promoters in catalytic reactions have been measured on well-defined single-crystal surfaces by equilibrium adsorption isotherms, temperature-programmed desorption (TPD), and single-crystal adsorption calorimetry (SCAC). Here we tabulate a large collection of those experimental adsorption energies which we consider to be particularly reliable based on reproducibility by other groups, comparisons to results for closely related systems, and/or reliability of other results reported by the same group. Specifically, we list the enthalpies and energies of 81 molecular and dissociative adsorption reactions that were measured on 26 different metal single-crystal faces of 12 different late transition metals, and we extract from these the standard enthalpies of formation of the adsorbates thus produced. These can serve as benchmarks for validating computational methods for estimating surface reaction en...

Water Dissociative Adsorption on NiO(111): Energetics and Structure of the Hydroxylated Surface

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ∼92 kJ/mol. Density functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorime...

Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

The adsorption of vapor-deposited Au onto CeO2-x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to ...

A simultaneous volumetric adsorption–isothermal titration calorimetry study of small molecules on supported metallic nanoparticles

Adsorption microcalorimetry studies can determine the energetics involved in different adsorbate–surface interactions. These measurements can be taken using various techniques such as single-crystal adsorption calorimetry, isothermal titration calorimetry and differential scanning calorimetry. The incremental dosing of a small molecule adsorbate with simultaneous calorimetric measurement enables differential heats of adsorptions to be measured at a specific temperature. In this study, an automated volumetric adsorption apparatus (Micromeritics ASAP2020) was interfaced with a Differential Scanning Calorimeter (Setaram Sensys EVO) to perform simultaneous volumetric uptake and calorimetric measurements. Heats of adsorption of small gas molecules (CO, H2) on supported metal catalysts (Pt, Pd, Au) were measured using this setup. Similar calorimetric measurements for the adsorption of CO (0.5 % Pt/SiO2; 5 % Pd/Al2O3) and H2 (0.5 % Pt/SiO2) were also taken by interfacing a Micromeritics ASAP2020 with a custom-built calorimeter. From these studies, it was observed that the initial heats of CO adsorption on supported Pt (0.5 % Pt/Al2O3, 135 kJ mol−1; 1 % Pt/SiO2, 104 kJ mol−1) and Pd (5 % Pd/Al2O3, 148 kJ mol−1) catalysts were significantly higher than the corresponding Au (1 % Au/TiO2, 57.2 kJ mol−1; 1 % Au/Al2O3, 54.1 kJ mol−1) catalysts. It was also observed that the initial heat of adsorption of H2 on the supported 0.5 % Pt/Al2O3 (~80 to 89 kJ mol−1) catalyst is higher than that on 1 % Pt/SiO2 (~61 kJ mol−1). CO and H2 adsorption studies on the 0.5 % Pt/Al2O3 and the 5 % Pd/Al2O3 catalyst using both calorimetric setups yielded similar volumetric uptakes and initial heats.

Energetics of methanol and formic acid oxidation on Pt(111): Mechanistic insights from adsorption calorimetry

The catalytic and electrocatalytic oxidation and reforming of methanol and formic acid have received intense interest due to potential use in direct fuel cells and as prototype models for understanding electrocatalysis. Consequently, the reaction energy diagram (energies of all the adsorbed intermediates and activation energies of all the elementary steps) have been estimated for these reactions on Pt(111) by density functional theory (DFT) in several studies. However, no experimental measurement of these energy diagrams have been reported, nor is there a consensus on the mechanisms. Here, we use energies of key intermediates on Pt(111) from single crystal adsorption calorimetry (SCAC) and temperature programmed desorption (TPD) to build a combined energy diagram for these reactions. It suggests a new pathway involving monodentate formate as a key intermediate, with bidentate formate only being a spectator species that slows the rate. This helps reconcile conflicting proposed mechanisms.

Energy of Supported Metal Catalysts: From Single Atoms to Large Metal Nanoparticles

Many catalysts consist of late transition metal nanoparticles dispersed across oxide supports. The chemical potential of the metal atoms in these particles correlate with their catalytic activity and long-term thermal stability. This chemical potential versus particle size across the full size range between the single isolated atom and bulklike limits is reported here for the first time for any metal on any oxide. The chemical potential of Cu atoms on CeO2(111) surfaces, determined by single crystal adsorption calorimetry of gaseous Cu atoms onto slightly reduced CeO2(111) at 100 and 300 K is shown to decrease dramatically with increasing Cu cluster size. The Cu chemical potential is ∼110 kJ/mol higher for isolated Cu adatoms on stoichometric terrace sites than for Cu in nanoparticles exceeding 2.5 nm diameter, where it reaches the bulk Cu(solid) limit. In Cu dimers, Cu’s chemical potential is ∼57 kJ/mol lower at step edges than on stoichiometric terrace sites. Since Cu avoids oxygen vacancies, these mono...

Energetics of Cu Adsorption and Adhesion onto Reduced CeO2(111) Surfaces by Calorimetry

The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO2(111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼1013 particles/cm2 on CeO2–x(111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO1.95(111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle ...

A temperature dependent investigation of the adsorption of CO on Pt(111) using low-temperature single crystal adsorption calorimetry

In this work we present a detailed investigation of the adsorption of CO on a well defined Pt(111) surface with low-temperature Single-Crystal-Adsorption-Calorimetry (SCAC) by measuring the coverage dependent adsorption enthalpies at 150 K, 250 K, 300 K and 350 K. Considerable changes in the coverage dependent differential and integral adsorption enthalpies with temperature indicate an order–disorder transition into the fluid phase which becomes apparent in a significant increase in the corresponding heat capacity of the CO adlayer.

Chirally-modified metal surfaces: energetics of interaction with chiral molecules.

Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for the production of enantiopure pharmaceuticals. One of the major current challenges in heterogeneous chiral catalysis is the fundamental-level understanding of how such chirally-modified surfaces interact with chiral and prochiral molecules to induce their enantioselective transformations. Herein we report the first direct calorimetric measurement of the adsorption energy of chiral molecules onto well-defined chirally-modified surfaces. Two model modifiers 1-(1-naphthyl)ethylamine and 2-methylbutanoic acid were used to impart chirality to Pt(111) and their interaction with propylene oxide was investigated by means of single-crystal adsorption calorimetry. Differential adsorption energies and absolute surface uptakes were obtained for the R- and S-enantiomers of propylene oxide under clean ultrahigh vacuum conditions. Two types of adsorption behavior were observed for different chiral modifiers, pointing to different mechanisms of imparting chirality to metal surfaces. The results are analyzed and discussed in view of previously reported stereoselectivity of adsorption processes.

Method for direct deconvolution of heat signals in transient adsorption calorimetry

Abstract A method of heat signal analysis is presented for transient adsorption calorimetries including single crystal adsorption calorimetry (SCAC) which uses fast Fourier transforms (FFT) to determine the instrument response function and deconvolute the heat-versus-time signals. The method utilizes a heat signal generated by a laser pulse of known power-versus-time to extract the instrument response function for the calorimeter. The instrument response function is then used to extract the heat power signal from a molecular beam heat pulse of unknown intensity. This method allows for the extraction of the total heat deposited by the molecular beam pulse without any kinetic modeling even in the event of complex reaction dynamics. This method is compared to previous methods used to analyze SCAC data using example data from the two-step dissociative adsorption of methyl iodide on Pt(111). It is found to be equally accurate for extracting total heats and simpler to perform than the previous methods.

Single-crystal adsorption calorimetry on well-defined surfaces: from single crystals to supported nanoparticles.

Single-crystal adsorption calorimetry (SCAC) measures the energetics of gas-surface interactions in a direct way and can be applied to a broad range of well-defined model surfaces. In this Personal Account we review some of the recent advances in understanding the interaction of gaseous molecules with single-crystal surfaces and well-defined supported metallic nanoparticles by this powerful technique. SCAC was applied on single-crystal surfaces to determine formation enthalpies of adsorbed molecular fragments typically formed during heterogeneously catalyzed reactions involving hydrocarbons. On supported metal nanoparticles, the binding energies of gaseous species were determined by SCAC as a function of the particle size. The reported data provide valuable information for ongoing research in many fields of heterogeneous catalysis and materials science. In addition, direct calorimetric measurements serve as benchmarks for the improvement of computational approaches to understanding surface chemistry.

Energetics of Adsorbed CH2 and CH on Pt(111) by Calorimetry: The Dissociative Adsorption of Diiodomethane

The enthalpies of molecular and dissociative adsorption of CH2I2 on Pt(111) at 100–210 K were studied using single-crystal adsorption calorimetry and density functional theory (DFT). Gaseous CH2I2 was found to adsorb on the Pt(111) surface at 100 K to form CH2,ad + 2Iad, with a calorimetric heat of adsorption that decreases with coverage as 222–480θ kJ/mol for θ < 1/8, where θ is the coverage in monolayers (ML), defined as the number of dissociatively adsorbed CH2I2 molecules per Pt(111) surface atom. These coadsorbed iodine atoms greatly destabilize the methylene species even at the lowest coverage, which we attribute to their inability to diffuse away from the near-neighbor sites where they are initially produced on the short time scale of the heat measurement. A mixture of dissociative adsorption fragments of methylene and methylidyne were detected at elevated temperatures between 125 and 190 K. At 210 K, CH2I2 adsorption produced CHad, Had, and 2Iad, with the Iad now able to diffuse away to minimize r...

A compact low-temperature single crystal adsorption calorimetry setup for measuring coverage dependent heats of adsorption at cryogenic temperatures.

Here we present the modification of an already existing Single Crystal Adsorption Calorimetry (SCAC) apparatus which has been extended by a compact cooling system to measure the coverage dependent heats of adsorption of gaseous compounds on thin metal substrates in a temperature range from 80 K to 430 K. The setup is characterized and its performance is tested by studying the adsorption of CO on Pt(111) at 150 K and 300 K. Coverage dependent sticking probabilities and heat of adsorption measurements are compared to previous experimental and theoretical studies proving the reliability of our compact low-temperature-SCAC setup.

Energetics of Formic Acid Conversion to Adsorbed Formates on Pt(111) by Transient Calorimetry

Carboxylates adsorbed on solid surfaces are important in many technological applications, ranging from heterogeneous catalysis and surface organo-functionalization to medical implants. We report here the first experimentally determined enthalpy of formation of any surface bound carboxylate on any surface, formate on Pt(111). This was accomplished by studying the dissociative adsorption of formic acid on oxygen-presaturated (O-sat) Pt(111) to make adsorbed monodentate and bidentate formates using single-crystal adsorption calorimetry. The integral heat of molecular adsorption of formic acid on clean Pt(111) at 100 K is 62.5 kJ/mol at 0.25 monolayer (ML). On O-sat Pt(111), the integral heat of the dissociative adsorption of formic acid to make monodentate formate (HCOOmon,ad) plus the water-hydroxyl complex ((H2O-OH)ad) was found to be 76 kJ/mol at 3/8 ML and 100-150 K. Similarly, its integral heat of dissociative adsorption to make bidentate formate (HCOObi,ad) plus (H2O-OH)ad was 106 kJ/mol at 3/8 ML and 150 K. These heats give the standard enthalpies of formation of adsorbed monodentate and bidentate formate on Pt(111) to be -354 ± 5 and -384 ± 5 kJ/mol, respectively, and their net bond enthalpies to the Pt(111) surface to be 224 ± 13 and 254 ± 13 kJ/mol, respectively. Coverage-dependent enthalpies of formation were used to estimate the enthalpy of the elementary reaction HCOOHad → HCOObi,ad + Had to be -4 kJ/mol at zero coverage and +24 kJ/mol at 3/8 ML.

Adsorption Energy oftert-Butyl on Pt(111) by Dissociation oftert-Butyl Iodide: Calorimetry and DFT

Dissociative and molecular adsorption of tert-butyl iodide on Pt(111) has been studied by single-crystal adsorption calorimetry (SCAC), photoelectron spectroscopy (XPS), reflection/adsorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. Up to a t-BuI total coverage of 0.07 ML, t-BuI adsorbs dissociatively at 100 K to form t-Buad plus Iad, with an integral heat of reaction of 223 kJ/mol. At higher coverage, up to a total coverage of 0.15 ML, t-BuI adsorbs molecularly directly to the Pt surface atoms with an average heat of adsorption of 91 kJ/mol. At 0.15 ML, the first layer is saturated. Between 0.15 and 0.38 ML, t-BuI adsorbs molecularly on top of the first layer with a constant heat of adsorption of 44.5 ± 1.9 kJ/mol. The standard enthalpy of formation of adsorbed tert-butyl on Pt(111) at 1/25 ML coverage is estimated from the heat measurements to be −168 ± 20 kJ/mol, giving a (CH3)3C–Pt(111) bond enthalpy of 216 ± 20 kJ/mol. DFT calculations were performed using the GGA-PBE and optB86b vdW density functionals to determine the heats of reaction and binding energies of relevant adsorbates. Comparison to the measured energies shows that optB86b vdW-DF is more accurate than GGA-PBE for describing the adsorption and bonding of t-buI and t-Bu on Pt(111) because these energies contain large contributions from dispersion forces.

Surface kinetics and energetics from single crystal adsorption calorimetry lineshape analysis: Methyl from methyl iodide on Pt(1 1 1)

The first use of single crystal adsorption calorimetry (SCAC) to probe the kinetics of surface chemical reactions is presented. It is applied to study a common situation encountered in catalytic mechanisms, wherein a gas quickly populates a molecularly adsorbed state that then converts to more stable products on a slower timescale (10–1000ms). We show that for such a two-step process, detailed analysis of the heat-detector signal’s time-response lineshape to a single pulse of gas provides the rate constant for the second step and the heats of reaction for both of the elementary reactions. We apply this analysis to the dissociative adsorption of methyl iodide (CH3I) on Pt(111) at 270K, to measure the heats of reaction for the elementary steps involved and the rate constant for the slow step, all as a detailed function of coverage. At low coverage, the reaction is CH3Ig→CH3Iad→CH3,ad+Iad, followed at high coverage by CH3,ad→CHad+2Had and CH3,ad+Had→CH4,g (above 0.08ML total CH3,ad plus Iad). These results provide the rate constant for the dissociation of CH3Iad and the heats of formation of both CH3,ad and CHad. These two heats agree with values determined at 320K where the rates are so fast that lineshape analysis is not needed, proving the validity of the lineshape analysis method introduced here for analyzing SCAC data.

Energetics of Adsorbed CH3 on Pt(111) by Calorimetry

The enthalpy and sticking probability for the dissociative adsorption of methyl iodide were measured on Pt(111) at 320 K and at low coverages (up to 0.04 ML, where 1 ML is equal to one adsorbate molecule for every surface Pt atom) using single crystal adsorption calorimetry (SCAC). At this temperature and in this coverage range, methyl iodide produces adsorbed methyl (CH(3,ad)) plus an iodine adatom (I(ad)). Combining the heat of this reaction with reported energetics for Iad gives the standard heat of formation of adsorbed methyl, ΔH(f)(0)(CH3,ad), to be −53 kJ/mol and a Pt(111)–CH3 bond energy of 197 kJ/mol. (The error bar of ±20 kJ/mol for both values is limited by the reported heat of formation of I(ad).) This is the first direct measurement of these values for any alkyl fragment on any surface.

Energetics of Adsorbed CH3 and CH on Pt(111) by Calorimetry: Dissociative Adsorption of CH3I

The heat of adsorption and sticking probability of methyl iodide were measured on Pt(111) at 95, 215, 270, 300, and 320 K using single crystal adsorption calorimetry (SCAC). On clean Pt(111) at 95 ...

Single-crystal adsorption calorimetry and density functional theory of CO chemisorption on fcc Co{110}

Using single-crystal adsorption calorimetry (SCAC) and density functional theory (DFT), the interaction of carbon monoxide on fcc Co{110} is reported for the first time. The results indicate that adsorption is consistent with molecular chemisorption at all coverages. The initial heat of adsorption of 140 kJ mol(-1) is found in the range of heat values calorimetrically measured on other ferromagnetic metal surfaces, such as nickel and iron. DFT adsorption energies are in good agreement with the experimental results, and comparison between SCAC and DFT for CO on other ferromagnetic surfaces is made. The calculated dissociation barrier of 2.03 eV implies that dissociation at 300 K is unlikely even at the lowest coverage. At high coverages during the adsorption-desorption steady state regime, a pre-exponential factor for CO desorption of 1.2 × 10(17) s(-1) is found, implying a localised molecular adsorbed state prior to desorption in contrast to what we found with Ni surfaces. This result highlights the importance of the choice of the pre-exponential factor in evaluating the activation energy for desorption.