Calorimeter for adsorption energies of larger molecules on single crystal surfaces

Abstract

A calorimeter for measuring heats of adsorption of large molecules on single crystal surfaces is described. It extends previous instrumentation for single crystal adsorption calorimetry by adding the capability for measuring larger (lower vapor pressure) molecules. This is achieved using a chopped and collimated (∼4 mm diameter) molecular beam capable of stable 100 ms pulses of low vapor pressure substances, and a line-of-sight modification of the King and Wells method for measuring their sticking probabilities at the single crystal’s surface. The heat input to the single crystal due to adsorption is detected using a pyroelectric polymer ribbon pressed against the back of the single crystal, following our previous calorimeter design. Measurements of benzene adsorption on Pt(111) prove the capability to produce a highly stable beam of flux ∼2×1014 molecules/(cm2 s) and measure adsorption energies with an absolute accuracy of ∼5% and a pulse-to-pulse standard deviation of 2 kJ/mol.

Heat of Adsorption of Naphthalene on Pt(111) Measured by Adsorption Calorimetry

“…3,4 Briefly, we employed an ultrahigh vacuum (UHV) chamber equipped with the SCAC, the MBS, and facilities for X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy ion scattering (LEIS), and low-energy electron diffraction (LEED).…”

“…3,4 Here, we have employed this technique to study the adsorption of naphthalene on a Pt(111) surface.…”

Experimental measurements of the energetics of surface reactions

“…Our group has recently measured the adsorption energies of several cyclic hydrocarbons on Pt(1 1 1) using a variable-temperature single crystal adsorption calorimeter [25,27,45,46]. In the case of benzene and naphthalene, only molecular adsorption was studied [45,46], but cyclohexene was studied at several temperatures so that the heats associated with forming molecularly adsorbed cyclohexene and several of its main dissociation products were independently determined [27].…”

“…Most recently, our group has developed [21][22][23][24] and further perfected [25][26][27] a heat detection method based on gently touching a soft pyroelectric polymer ribbon against the back surface of the single crystal sample during calorimetry measurements, and removing it during sample cleaning and annealing. This heat detection method has proven to have much, much higher sensitivity.…”

Chemisorption of benzene on Pt (1 1 1) surface: A DFT study with van der Waals interaction

“…For a brief summary of work done in the area so far, on the experimental side, the phenomena of benzene adsorption on Pt has been studied since late 1970s in a variety of ways, including high resolution electron energy loss spectroscopy (HREELS) [7,8], Diffuse LEED [9], scanning tunneling microscopy (STM) [10][11][12], reflection-absorption infrared spectroscopy (RAIRS) [13], temperature programmed desorption (TPD) [14,15] and single crystal adsorption calorimetry (SCAC) [16]. While most experiments agree about benzene being adsorbed parallel to the metal surface, there is still uncertainty about the preferred adsorption site and the adsorption energy.…”