The adsorption energies of submonolayer amounts of one metal on the surface of another metal have... more 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 300 K 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 339 kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2 kJ/mol more weakly in the 2nd layer than above 3 ML, where it reaches the bulk heat of sublimation of Cu(solid), 337 kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76 J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12 kJ/mol, compared to a difference of ~83 kJ/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.
Thin films of metals and other materials are often grown by physical vapor deposition. To underst... more Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum.69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ∼18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.
Adsorbed molecules are involved in many reactions on solid surfaces that are of great technologic... more Adsorbed molecules are involved in many reactions on solid surfaces that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to theoretically predict rates for reactions involving adsorbed molecules. Theoretical calculations of rate constants require knowing both their activation energy and prefactor. Recent advances in ab initio computational methods (e.g., density functional theory with periodic boundary conditions and van der Waals corrections) promise to soon provide activation energies for surface reactions with sufficient accuracy to have real predictive ability. However, to predict reaction rates, we also need accurate predictions of prefactors. We recently discovered that the standard entropies of adsorbed molecules (Sad0) linearly track the entropy of the gas-phase molecule at the same temperature (T), such that Sad0(T) = 0.70 Sgas0(T) − 3.3 R (R = the gas constant), with a standard deviation of only 2 R over a range of 50 R. This correlation, which applies only to conditions where their surface residence times are shorter than ∼ 1000 s, provides a powerful new method for estimating the partition functions for adsorbates and the kinetic prefactors for their reactions. For desorption, we show that the prefactors obtained with DFT using transition state theory (TST) and the harmonic oscillator approximation to get the partition function predicts prefactors for desorption that are of order 103 times larger than experimental values while our approach gives much better estimates. We also explore the applications of this approach to estimate prefactors within TST for the main classes of adsorbate reactions: desorption, diffusion, dissociation and association, and discuss its limitations. We discuss general issues associated with applying TST to rate laws and multi-step mechanisms in surface chemistry, and argue that rates of adsorbate reactions which are often taken to be proportional to coverage (θ) might better be taken as proportional to θ/(1 − θ) (unless the adsorbate forms islands), to account for the configurational entropy or excluded volume effects on the adsorbate's chemical potential.
Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These a... more Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These are key elements in technologies for the clean production and use of fuels and chemicals. We show here that the chemical reactivity of the surface metal atoms on these nanoparticles is closely related to their chemical potential: the higher their chemical potential, the more strongly they bond to small adsorbates. Controlling their chemical potential by tuning the structural details of the material can thus be used to tune their reactivity. As their chemical potential increases, this also makes the metal surface less noble, effectively pushing its behavior upwards and to the left in the periodic table. Also, when the metal atoms are in a nanoparticle with higher chemical potential, they experience a larger thermodynamic driving force to sinter. Calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces in ultrahigh vacuum show that the chemical potential increases with decreasing particle size below 6 nm, and, for a given size, decreases with the adhesion energy between the metal and its support, Eadh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, Eadh increases with (ΔHsub,M − ΔHf,MOx)/ΩM2/3 for the metal, where ΔHsub,M is its heat of sublimation, ΔHf,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and ΩM is the atomic volume of the bulk solid metal. The value ΔHsub,M − ΔHf,MOx equals the heat of formation of that metal's oxide from a gaseous metal atom plus O2(g), so it reflects the strength of the chemical bonds which that metal atom can make to oxygen, and ΩM2/3 simply normalizes this energy to the area per metal atom, since Eadh is the adhesion energy per unit area. For a given metal, Eadh to different clean oxide surfaces increases as: MgO(100) ≈ TiO2(110) ≤ α-Al2O3(0001) < CeO2−x(111) ≤ Fe3O4(111). Oxygen vacancies also increase Eadh, but surface hydroxyl groups appear to decrease Eadh, even though they increase the initial heat of metal adsorption.
Adsorbed molecules are involved in many reactions on solid surface that are of great technologica... more Adsorbed molecules are involved in many reactions on solid surface that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to predict reaction rates and equilibrium constants for reactions involving adsorbed molecules. Theoretical calculation of both the rate and equilibrium constants for such reactions requires knowing the entropy and enthalpy of the adsorbed molecule. While much effort has been devoted to measuring and calculating the enthalpies of well-defined adsorbates, few measurements of the entropies of adsorbates have been reported. We present here a new way to determine the standard entropies of adsorbed molecules (Sad0) on single crystal surfaces from temperature programmed desorption data, prove its accuracy by comparison to entropies measured by equilibrium methods, and apply it to published data to extract new entropies. Most importantly, when combined with reported entropies, we find that at high coverage, they linearly track the entropy of the gas-phase molecule at the same temperature (T), such that Sad0(T) = 0.70 Sgas0(T) − 3.3R (R = the gas constant), with a standard deviation of only 2R over a range of 50R. These entropies, which are ∼2/3 of the gas, are huge compared to most theoretical predictions. This result can be extended to reliably predict prefactors in the Arrhenius rate constant for surface reactions involving such species, as proven here for desorption.
Chemical bonding at solid surfaces and interfaces is influential in a wide range of important tec... more Chemical bonding at solid surfaces and interfaces is influential in a wide range of important technological applications including catalysis, fuel cells, batteries, chemical sensors, and device fabrication for microelectronics, computers, solar cells, and all variety of coatings. Adsorption and adhesion energetics are key elements in understanding interfacial properties, and these properties can be used to develop functional industrial materials. First, the properties of single-crystalline oxide surfaces are reviewed in detail, particularly in regards to the adsorption energetics of these surfaces. This includes the largest collection of experimental adsorption data on single-crystalline oxide surfaces ever presented, from which trends in the thermodynamic properties of adsorbates are revealed which greatly expand our understanding of the physical processes occurring on these surfaces. Among these trends is the discovery that the entropy of adsorbed molecules tracks their gas-phase entropy, retaining ~2/3 of that entropy upon adsorption. This allows for a method of predicting not only entropies of adsorption, but also the kinetic prefactors associated with many classes of elementary surface reactions. These estimations of desorption prefactors are then used to improve calculations of adsorption energies from temperature programmed desorption (TPD) measurements for many systems. Metal adsorption on oxide surfaces and the strength of the binding at metal / oxide interfaces are then discussed. The motivation here is to understand oxide-supported transition metal nanoparticles such as those used in industrial heterogeneous catalysis. For metal atom adsorption, adsorption energetics and adhesion energies are directly related to the energy of the adsorbed atoms, which define their stability, sintering rates, and reactivity, and which are found to vary with both the size of the nanoparticle and the nature of the oxide support. The experimental techniques necessary for obtaining these values, as well as the data analysis involved, is explained, and in several cases improved upon. In particular, a new single crystal adsorption calorimeter capable of making the first direct measurements of adsorption energies for metals with high bulk cohesive energies has recently been completed. These studies greatly expand upon the understanding of and ability to measure the thermodynamic properties associated with adsorption on single-crystalline surfaces.
The adsorption energies of submonolayer amounts of one metal on the surface of another metal have... more 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 300 K 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 339 kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2 kJ/mol more weakly in the 2nd layer than above 3 ML, where it reaches the bulk heat of sublimation of Cu(solid), 337 kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76 J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12 kJ/mol, compared to a difference of ~83 kJ/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.
Thin films of metals and other materials are often grown by physical vapor deposition. To underst... more Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum.69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ∼18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.
Adsorbed molecules are involved in many reactions on solid surfaces that are of great technologic... more Adsorbed molecules are involved in many reactions on solid surfaces that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to theoretically predict rates for reactions involving adsorbed molecules. Theoretical calculations of rate constants require knowing both their activation energy and prefactor. Recent advances in ab initio computational methods (e.g., density functional theory with periodic boundary conditions and van der Waals corrections) promise to soon provide activation energies for surface reactions with sufficient accuracy to have real predictive ability. However, to predict reaction rates, we also need accurate predictions of prefactors. We recently discovered that the standard entropies of adsorbed molecules (Sad0) linearly track the entropy of the gas-phase molecule at the same temperature (T), such that Sad0(T) = 0.70 Sgas0(T) − 3.3 R (R = the gas constant), with a standard deviation of only 2 R over a range of 50 R. This correlation, which applies only to conditions where their surface residence times are shorter than ∼ 1000 s, provides a powerful new method for estimating the partition functions for adsorbates and the kinetic prefactors for their reactions. For desorption, we show that the prefactors obtained with DFT using transition state theory (TST) and the harmonic oscillator approximation to get the partition function predicts prefactors for desorption that are of order 103 times larger than experimental values while our approach gives much better estimates. We also explore the applications of this approach to estimate prefactors within TST for the main classes of adsorbate reactions: desorption, diffusion, dissociation and association, and discuss its limitations. We discuss general issues associated with applying TST to rate laws and multi-step mechanisms in surface chemistry, and argue that rates of adsorbate reactions which are often taken to be proportional to coverage (θ) might better be taken as proportional to θ/(1 − θ) (unless the adsorbate forms islands), to account for the configurational entropy or excluded volume effects on the adsorbate's chemical potential.
Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These a... more Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These are key elements in technologies for the clean production and use of fuels and chemicals. We show here that the chemical reactivity of the surface metal atoms on these nanoparticles is closely related to their chemical potential: the higher their chemical potential, the more strongly they bond to small adsorbates. Controlling their chemical potential by tuning the structural details of the material can thus be used to tune their reactivity. As their chemical potential increases, this also makes the metal surface less noble, effectively pushing its behavior upwards and to the left in the periodic table. Also, when the metal atoms are in a nanoparticle with higher chemical potential, they experience a larger thermodynamic driving force to sinter. Calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces in ultrahigh vacuum show that the chemical potential increases with decreasing particle size below 6 nm, and, for a given size, decreases with the adhesion energy between the metal and its support, Eadh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, Eadh increases with (ΔHsub,M − ΔHf,MOx)/ΩM2/3 for the metal, where ΔHsub,M is its heat of sublimation, ΔHf,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and ΩM is the atomic volume of the bulk solid metal. The value ΔHsub,M − ΔHf,MOx equals the heat of formation of that metal's oxide from a gaseous metal atom plus O2(g), so it reflects the strength of the chemical bonds which that metal atom can make to oxygen, and ΩM2/3 simply normalizes this energy to the area per metal atom, since Eadh is the adhesion energy per unit area. For a given metal, Eadh to different clean oxide surfaces increases as: MgO(100) ≈ TiO2(110) ≤ α-Al2O3(0001) < CeO2−x(111) ≤ Fe3O4(111). Oxygen vacancies also increase Eadh, but surface hydroxyl groups appear to decrease Eadh, even though they increase the initial heat of metal adsorption.
Adsorbed molecules are involved in many reactions on solid surface that are of great technologica... more Adsorbed molecules are involved in many reactions on solid surface that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to predict reaction rates and equilibrium constants for reactions involving adsorbed molecules. Theoretical calculation of both the rate and equilibrium constants for such reactions requires knowing the entropy and enthalpy of the adsorbed molecule. While much effort has been devoted to measuring and calculating the enthalpies of well-defined adsorbates, few measurements of the entropies of adsorbates have been reported. We present here a new way to determine the standard entropies of adsorbed molecules (Sad0) on single crystal surfaces from temperature programmed desorption data, prove its accuracy by comparison to entropies measured by equilibrium methods, and apply it to published data to extract new entropies. Most importantly, when combined with reported entropies, we find that at high coverage, they linearly track the entropy of the gas-phase molecule at the same temperature (T), such that Sad0(T) = 0.70 Sgas0(T) − 3.3R (R = the gas constant), with a standard deviation of only 2R over a range of 50R. These entropies, which are ∼2/3 of the gas, are huge compared to most theoretical predictions. This result can be extended to reliably predict prefactors in the Arrhenius rate constant for surface reactions involving such species, as proven here for desorption.
Chemical bonding at solid surfaces and interfaces is influential in a wide range of important tec... more Chemical bonding at solid surfaces and interfaces is influential in a wide range of important technological applications including catalysis, fuel cells, batteries, chemical sensors, and device fabrication for microelectronics, computers, solar cells, and all variety of coatings. Adsorption and adhesion energetics are key elements in understanding interfacial properties, and these properties can be used to develop functional industrial materials. First, the properties of single-crystalline oxide surfaces are reviewed in detail, particularly in regards to the adsorption energetics of these surfaces. This includes the largest collection of experimental adsorption data on single-crystalline oxide surfaces ever presented, from which trends in the thermodynamic properties of adsorbates are revealed which greatly expand our understanding of the physical processes occurring on these surfaces. Among these trends is the discovery that the entropy of adsorbed molecules tracks their gas-phase entropy, retaining ~2/3 of that entropy upon adsorption. This allows for a method of predicting not only entropies of adsorption, but also the kinetic prefactors associated with many classes of elementary surface reactions. These estimations of desorption prefactors are then used to improve calculations of adsorption energies from temperature programmed desorption (TPD) measurements for many systems. Metal adsorption on oxide surfaces and the strength of the binding at metal / oxide interfaces are then discussed. The motivation here is to understand oxide-supported transition metal nanoparticles such as those used in industrial heterogeneous catalysis. For metal atom adsorption, adsorption energetics and adhesion energies are directly related to the energy of the adsorbed atoms, which define their stability, sintering rates, and reactivity, and which are found to vary with both the size of the nanoparticle and the nature of the oxide support. The experimental techniques necessary for obtaining these values, as well as the data analysis involved, is explained, and in several cases improved upon. In particular, a new single crystal adsorption calorimeter capable of making the first direct measurements of adsorption energies for metals with high bulk cohesive energies has recently been completed. These studies greatly expand upon the understanding of and ability to measure the thermodynamic properties associated with adsorption on single-crystalline surfaces.
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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 300 K 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 339 kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2 kJ/mol more weakly in the 2nd layer than above 3 ML, where it reaches the bulk heat of sublimation of Cu(solid), 337 kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76 J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12 kJ/mol, compared to a difference of ~83 kJ/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.
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 300 K 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 339 kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2 kJ/mol more weakly in the 2nd layer than above 3 ML, where it reaches the bulk heat of sublimation of Cu(solid), 337 kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76 J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12 kJ/mol, compared to a difference of ~83 kJ/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.