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

Abstract

The adsorption of cis and trans 2-butenes on Pt(111) has been studied as a function of hydrogen coverage θH 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-σ and π adsorption geometries of the olefins have been compared at a surface coverage of θC4H8 = 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 (PH2/Pbutene) 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 (PH2/Pbutene) of 10, similar to catalytic hydrogenation conditions. With all selected functionals (PW91, PBE-TS, and optPBE), di-σ bonding was found to be the most stable for both isomers of 2-butene and for all hydrogen coverages except for θH = 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 (θH = 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 θH < 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).