We present calorimetric measurements of the effect of cluster size on the adsorption enthalpy of carbon monoxide on Pd nanoclusters sized from 120 to 4900 Pd atoms per particle, which were grown in situ on a well-ordered Fe 3 O 4 / Pt͑111͒ film. A substantial decrease in the initial heat of adsorption amounting to about 20-40 kJ mol −1 was observed on the smallest Pd nanoparticles as compared to the larger Pd clusters and the extended Pd͑111͒ single-crystal surface. We discuss this effect in terms of the size-dependent properties of the Pd nanoparticles. DOI: 10.1103/PhysRevB.81.241416 PACS number͑s͒: 68.43.Ϫh, 82.65.ϩr This Rapid Communication addresses the question: how does the heat of chemisorption of a molecule change when comparing a metal single-crystal surface with a supported metal nanoparticle, and how does it depend on particle size? This is an exceptionally important question that lies at the very heart of understanding particle size effects in catalysis. 1 The energetics of interaction of gaseous molecules, particularly carbon monoxide, with well-defined metal nanoparticles were previously addressed indirectly in nonisothermal temperature-programed desorption ͑TPD͒ experiments 2 and in isothermal modulated molecular-beam studies, 3 where the adsorption energies were obtained by modeling the desorption process and analyzing the lifetimes of the adsorbate on the surface. However, these indirect methods did not provide a clear trend in the changes in the adsorption energy with the particles size: whereas the TPD studies found a decrease in the adsorption energy by about 10 kJ mol −1 on the 2.5 nm sized Pd particles as compared to the extended single-crystal surfaces, the kinetic model used for analysis of the molecular-beam experiments predicted a pronounced increase in the adsorption energy by about 35 kJ mol −1 on the particles smaller than 1.5 nm.A strategy to overcome the shortcomings of such indirect methods based on the model assumptions is direct calorimetric measurement of adsorption enthalpies. Recently, two types of calorimeters were developed that allow direct adsorption energy measurements on single-crystal surfaces. 4,5 However, many phenomena inherent to dispersed supported catalysts and technically relevant nanoparticle-based materials cannot be addressed on such simplified model systems since they do not reproduce some properties of realistic surfaces such as different particles sizes or interactions between nanoparticles and their support material. Only a limited amount of calorimetric data is available today on dispersed supported metal powder catalysts, 6,7 suffering, however, from a high degree of inhomogeneity in metal particle size distribution and low support homogeneity. A strategy to surmount this shortcoming was the development of welldefined, single-crystal-based model surfaces, consisting of metal nanoparticles deposited on flat thin oxide films. 8 The structural properties of these model systems such as particle size and shape can be controlled and characterized in grea...

A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical∕thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques and in situ reflectivity measurements and (ii) the adsorption∕reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005-1.5 × 10(15) molecules cm(-2) s(-1) and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190-3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-2)). The absolute accuracy in energy is within ∼7%-9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K.

The heat of adsorption and sticking probability of CO on well-defined Pd nanoparticles were measured as a function of particle size using single crystal adsorption microcalorimetry. Pd particles of different average sizes ranging from 120 to 4900 atoms per particle (or from 1.8 to 8 nm) and Pd(111) were used that were supported on a model in situ grown Fe(3)O(4)/Pt(111) oxide film. To precisely quantify the adsorption energies, the reflectivities of the investigated model surfaces were measured as a function of the thickness of the Fe(3)O(4) oxide layer and the amount of deposited Pd. A substantial decrease of the binding energy of CO was found with decreasing particle size. Initial heat of adsorption obtained on the virtually adsorbate-free surface was observed to be reduced by about 20-40 kJ mol(-1) on the smallest 1.8 nm sized Pd particles as compared to the larger Pd clusters and the extended Pd(111) single crystal surface. This effect is discussed in terms of the size-dependent properties of the Pd nanoparticles. The CO adsorption kinetics indicates a strong enhancement of the adsorbate flux onto the metal particles due to a capture zone effect, which involves trapping of adsorbates on the support and diffusion to metal clusters. The CO adsorption rate was found to be enhanced by a factor of ∼8 for the smallest 1.8 nm sized particles and by ∼1.4 for the particles of 7-8 nm size.

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