The lack of distribution network and storage problems limits the use of stored hydrogen in fuel cell applications. Thus, producing pure hydrogen from hydrocarbons, like LPG, on site, either for mobile or immobile uses, is of great scientific interest. The device called “fuel processor” converts hydrocarbon feeds to pure hydrogen. In a fuel processor, there are three catalytic reactions, namely reforming (either steam reforming, SR, or autothermal reforming, ATR), water-gas shift (WGS), and preferential oxidation (PROX), are conducted in series. From those reactions, PROX eliminates CO impurity from the resultant hydrogen rich stream. Since the Pt anode of a commercial fuel cell can operate stabily only at CO levels below 10 ppm, PROX should decrease CO concentration down below that level in the produced hydrogen stream.

The Pt-rich alloy form of Pt and Sn, Pt3Sn, has been studied for both its possible use in PROX and in the fuel cell anode. For those applications, the lure of Pt3Sn stems from its resistance to CO poisoning compared to monometallic Pt. Actually, the use of Pt3Sn in a fuel processor-fuel cell system has double-sided advantage; (i) when it is used as the active phase in PROX catalyst, it eliminates CO and led to stable operation of Pt-anode of the current fuel cells; (ii) when it is used as an alternative to Pt anode in fuel cells, it will lead to stable operation even when H2 feed is not free from CO, thus reduce the need of costly CO removal. Though its advantageous specs, experimental or theoretical investigations on the CO adsorption properties of different planes of Pt3Sn at atomic level are rare.

In this work, adsorption properties of CO on experimentally verified flat and stepped Pt3Sn surfaces, (111), (110), (001) and (102), were studied and the results are given in two parts; first for flat surfaces and then for the stepped surface (102). Prior to the calculations, all surfaces of Pt3Sn were generated with all possible bulk terminations, and on these terminations all types of active sites were determined. Then, adsorption energies and geometries of the CO molecule for all those sites were calculated.

CO-Pt3Sn system: flat faces of Pt3Sn [1]

The results obtained for flat Pt3Sn (111), Pt3Sn (110), Pt3Sn (001) surfaces were compared with the results obtained from the adsorption of CO on similar sites of Pt(111), Pt(110) and Pt(001) surfaces. The comparison reveals that adsorption of CO is stronger on Pt surfaces; this may be the reason why catalysts with Pt3Sn phase do not suffer from CO poisoning in experimental works. Aiming to understand the interactions between CO and the metal adsorption sites in detail, the local density of states (LDOS) profiles were produced for atop-Pt adsorption, both for the carbon end of CO for its adsorbed and free states, and for the Pt atom of the binding site. LDOS profiles of C of free and adsorbed CO and Pt for corresponding pure Pt surfaces, Pt(111), Pt(110) and Pt(001) were also obtained. The comparison of the LDOS profiles of Pt atoms of atop adsorption sites on the same faces of bare Pt3Sn and Pt surfaces showed the effect of alloying with Sn on the electronic properties of Pt atoms. Comparison of LDOS profiles of the C end of CO in its free and atop adsorbed states on Pt3Sn and LDOS of Pt on bare and CO adsorbed Pt3Sn surface were used to clear out the electronic changes occurred on CO and Pt upon adsorption. The study showed that (i) CO/Pt3Sn adsorption system, inclusion of a Sn atom at the adsorption site structure causes dramatic decrease in stability. All such sites on the three stable Pt3Sn surfaces studied have proved unstable. The results indicate that CO molecules diffuse from the unstable sites toward the neighboring stable sites, (ii) Sn present beneath the adsorption site strengthens the CO adsorption, whereas neighboring Sn on the surface destabilizes it for all Pt3Sn surfaces tested (iii) the presence of Sn causes angles different from 180° for M-C-O orientation, (iv) the presence of Sn in the neighborhood of Pt on which CO is adsorbed causes superposition of the 5R/1 derived-state peaks at the carbon end of CO and changes in adsorption energy of CO, (v) when Sn is not present at but is beneath the adsorption site, it may strengthen the adsorption for Pt3Sn surfaces, (vi) the most stable site for CO adsorption is the atop-Pt site of the mixed atom termination of Pt3Sn(110), (vii) Comparison of CO adsorption on the Pt and Pt3Sn surface sites having the same geometry showed that Sn presence in the alloy limits the number of stable CO adsorption sites on the alloy surface and weakens CO adsorption.

CO-Pt3Sn system: effect of stepped structure [2]

For the stepped Pt3Sn(102), the most favorable sites for adsorption were determined as the short bridge site on the terrace of pure-Pt row of the mixed-atom-ending termination, atop site at the step-edge of the pure row of pure-Pt-ending termination and atop site at the step-edge of the pure Pt row of the mixed-atom-ending termination. In general, Sn atom present beneath an adsorption site strengthens the adsorption. On the other hand, a neighboring Sn at the same plane or present above either weakens the adsorption or destabilizes the adsorbed CO. The results were compared with those for similar sites on the flat Pt3Sn(110) surface considering the fact that Pt3Sn(102) has terraces with (110) orientation. The LDOS analysis of bare sites clearly show that there are significant differences between the electronic properties of Pt atoms at stepped Pt3Sn (102) surface and the electronic properties of Pt atoms at flat (110) surface, which leads to changes in the CO bonding energies of these Pt atoms. Adsorption on Pt3Sn(102) surface is in general stronger compared to that on Pt3Sn(110) surface. The difference in adsorption strength of similar sites on these two surface terminations is a result of stepped structure of Pt3Sn(102). The local density of states (LDOS) of the adsorbent Pt and C of adsorbed CO was utilized. The LDOS of the surface metal atoms with CO adsorbed atop and of their bare state were compared to see the effect of CO chemisorption on the electron density distribution of the corresponding Pt atom. The downward shift in energy peak in the LDOS curves as well as changes in the electron densities of the corresponding energy levels indicate the orbital mixing between CO molecular orbitals and metal d states. The present study showed that the adsorption strength of the sites has a direct relation with their LDOS profiles.

The results of the current study show that by utilizing LDOS profiles, the electronic properties of bare surface atoms, the electronic property differences coming from surface geometry, the electronic changes of the surface metal atoms of the adsorption site and that of the adsorbate upon adsorption and the atoms which contributes to adsorption can be found; thus, the LDOS analysis coupled with a screening methodology can be used in screening of possible metal/alloy surfaces to be used in specific reactions without going into costly and time consuming adsorption/reaction studies.

ACKNOWLEDGEMENTS This work was supported by DPT (State Planning organization of Turkey) through projects DPT 01 K 120300 and DPT 03 K 120250. A. E. Aksoylu acknowledges TUBA-GEBIP program.

References

1. Gülmen, M. A., Sümer, A., Aksoylu, A. E., Adsorption Properties of CO on Low-Index Pt3Sn Surfaces, Surface Science, in revision (and the references therein)

2. Sümer, A., Gülmen, M. A., Aksoylu, A. E., “A Theoretical Investigation on Pt3Sn (102) Surface Alloy and CO-Pt3Sn(102) System”, Surface Science, in press (and the references therein)