Switch in Relative Stability between cis and trans 2-Butene on Pt(111) as a Function of Experimental Conditions: A Density Functional Theory Study

The adsorption of cis and trans 2-butenes on Pt(111) has been studied as a function of hydrogen coverage OH by means of calculations based on density functional theory (DFT) with the inclusion of dispersion forces. All hydrogen coverages have been considered, from 0 to 1.00 monolayer (ML). For each case, the di-sigma and pi adsorption geometries of the olefins have been compared at a surface coverage of theta(C4H8) = 0.11 ML. Calculations of the Gibbs free energies of these systems have identified the most stable 2-butene isomer (cis or trans) as a function of coverage, temperature, and pressure. In particular, focus was placed on two sets of conditions, namely, one with a pressure of 10(-7) Torr, a temperature of 80 K, and a gas ratio (P-H2/P-butene) of 25, similar to the conditions used in surface science studies, and a second with a pressure of 1 bar, a temperature range of 300-400 K, and a gas ratio (P-H2/P-butene) of 10, similar to catalytic hydrogenation conditions. With all selected functionals (PW91, PBE-TS, and optPBE), di-sigma bonding was found to be the most stable for both isomers of 2-butene and for all hydrogen coverages except for OH = 1.00 ML. At low pressures, 2-butene is physisorbed at low temperatures (<= 125 K with PBE-TS and <= 90 K with optPBE); however, when the temperature increases, coadsorption of the butene with 6 H atoms becomes the most stable configuration of the system (OH = 0.67 ML), and finally, 2-butene desorbs around 380 K, as estimated with PBE-TS (or around 325 K with optPBE). Interestingly, a switch in stability was observed with hydrogen coverage, from the adsorbed trans isomer being the more stable for theta(H) < 0.44 ML to the adsorbed cis isomer becoming the more stable at higher hydrogen coverages, in agreement with the cis-trans isomerization behavior previously reported for this system. At high pressures, the behavior is similar, but with transitions occurring at higher temperatures. 2-Butene is physisorbed until the temperature reaches 250 K, and desorbs above 500 K At hydrogenation reaction temperatures (between 300 and 500 K), a hydrogen coverage of roughly half a monolayer was calculated (0.66 and 0.44 for 300 and 500 K, respectively). Our results confirm that dispersion effects must be included to properly describe the 2-butene and hydrogen coadsorption on Pt(111), as PW91 predicts that 2-butene is never adsorbed on the platinum surface. On the other hand, DFT calculations including dispersion forces such as PBE-TS or optPBE afford a good understanding of catalytic systems under both ultra-high-vacuum conditions and catalytic hydrogenation conditions. For this system, the PBE-TS results are in good agreement with experiments: they correctly reproduce the coverage in hydrogen and the configuration of the 2-butene adsorbate (cis-trans isomer).