Introduction
The Cambridge Single Crystal Adsorption Calorimeter (SCAC) is a unique instrument, the first in the world to directly measure the heat released when gases adsorb to well-defined metal single crystal surfaces. The heat of adsorption is a fundamental parameter in surface science and catalysis and hence its direct measurement leads to a more complete knowledge of the energetics of surface processes.
This parameter has been previously obtained indirectly, from thermal desorption or isosteric methods. These are limited to reversible systems, hence excluding the majority of surface processes. SCAC overcomes this, so direct studies of irreversible reactions such as dissociative and reactive adsorption, decomposition and bulk oxidation are possible.
Prior to the development of SCAC, calorimetry existed for poorly defined surfaces such as polycrystalline metal films evaporated into glass bulbs, so such experiments were of limited use - structural information could not be derived fairly and the data obtained was non-reproducible. Calorimetry is not possible on conventional single crystals (typically the size of a small coin) due to their large thermal mass - the heat released for an adsorption process would be negligible, and the long waiting time for thermal equilibrium after surface cleaning leads to subsequent contamination, so coverage-dependent studies are out of the question.
This has been overcome in SCAC by the use of ultrathin single crystal films typically 2000 Å thick, grown epitaxially on alkali halide substrates by Jacques Chevallier of the University of Aarhus, Denmark. Adsorption of a well-defined pulsed molecular beam on the unsupported central 2 mm diameter portion of such a film gives a heat release of the order of 1 K, which is measured remotely as infrared blackbody emission. This remote measurement retains the sensitivity of the low thermal mass film, and thermal equilibrium is achieved very rapidly in between pulses - with a molecular beam dosing approximately 0.01 ML of adsorbate per pulse, adsorption heats as a sensitive function of coverage are readily obtained. The other parameters needed in this highly quantitative experiment are: i) the beam flux, measured via a spinning rotor gauge; ii) the calibration of the crystal to a known amount of heat which is performed with a laser and an absolutely calibrated photodiode; iii) the sticking probability of every pulse, measured with the King and Wells technique [1]. Consequently, both adsorption heats and sticking probabilities as a function of coverage are obtained.
The SCAC Database
Many experiments have been performed to date, those as of 1997 have been summarised in a review [2] which interested parties are encouraged to read. Initial measurements were on various singular surfaces (i.e. {111}, {100} {110}) of the f.c.c. metals Ni, Pt, Pd and Rh. Interesting results have included the first measurement of the energy difference between two structural phases of a solid surface, namely between Pt{100}(1´1) and Pt{100}-hex [3]. Other information that has been extracted from calorimetric data includes lateral interaction energies between adsorbates [e.g. 4], and metal-carbon bond energies [e.g. 5].
In subsequent years, the database of adsorption heats has been extended to include stepped and step-kinked surfaces, such as for example Pt{211}, Pt{311}, Pt{411} and Ni{211}. These results are summarised in a second review [6].The motivation is the bridging of the 'materials gap' in surface science, looking at rougher (but still well-defined) single-crystal surfaces with low co-ordination surface sites similar to those on 'real catalysts' such as nanoscopic metal particles.
When additional structural information from other techniques (e.g. RAIRS or LEED) is available for a system under study, SCAC becomes an even more powerful technique, due to the assignment of heats or heat regimes to known surface structures. An example is the monitoring of the Pt{311}(1x2) reconstruction and its lifting upon CO adsorption, by SCAC and LEED in tandem [7].
The SCAC database provides a benchmark for DFT calculations - a comparison of the results of these experimental and theoretical calculations has been summarised in a recent review [7]. There is only one other known single crystal calorimeter worldwide, at the University of Washington, Seattle, which concentrates on the adsorption of metal atoms on metal and oxide surfaces, and uses an alternate heat detection method to ours.
References
1. D. A. King and M. G. Wells, Surf. Sci. 1972, 29, 454.
2. W. A. Brown, R. Kose, and D. A. King, Chem. Rev. 1998, 98, 797.
3. Y. Y. Yeo, C. E. Wartnaby, and D. A. King, Science 1995, 268, 1731.
4. Y. Y. Yeo, L. Vattuone, and D. A. King, J. Chem. Phys. 1996, 104, 3810.
5. A. Stuck, C. E. Wartnaby, Y. Y. Yeo, and D. A. King, Phys. Rev. Lett. 1995, 74, 578.
6. V. Fiorin, D. Borthwick and D. A. King, "Thermodynamics: Small Molecule Adsorption Calorimetry on Metal Single Crystals" in “Model System in catalysis: From Single-Crystals and Size-selected Clusters to Supported Enzyme Mimics”, Edited by Robert Rioux, Springer. In Press. 7. R. Kose and D. A. King, Chem. Phys. Lett. 1999, 313, 1.
8. Q. Ge, R. Kose, and D. A. King, Adv. Catal. 2000, 45, 207.