First-principles Studies For The Adsorption Of Sulfur And CO On Transition Metal Surfaces

Sulfur, is present as a common poisonious residual, its poisoning has a verynegative impact on the performance of catalysts currently used in oil refineries andchemical industry. Millions of dollars are lost every year as a consequence. Today, thepresence of sulfur constitutes a serious problem in our industrial society and for thedeterioration of environmental pollution.Transition metals(TMs) are extensively used as catalysts. It is well established thatsulfur adsorption results in modifications to the valence bands of transition metals,which lead to significant changes in catalytic properties of TMs. Industrially, even asmall amount of sulfur has been reported to inhibit the chemisorption of smallmolecules(CO, NO, H₂ and C₂H₂, etc) and CO methanation reactions. Moreover, thereexist different debates for the interaction between S and CO coadsorbed on metals, andfurther work are required to examine the poisoning effect of S on CO. Therefore, thebehavior of S on metals has received a lot of attention and understanding the interactionof S with CO has long been a hot subject, also much work are required for S and COcoadsorption studies.Secondly, for a few notorious cases, state-of-the-art density functional theory(DFT) calculations fail in predicting the correct site preference from the calculatedadsorption energies. This arouses much interest in predicting the site preference of COwith DFT calculations to clarify the available experimental results.More importantly, other experimental results have been reported for S adsorptionon Ir(100) surface, S and CO individual adsorption, as well as both S and COcoadsorption on Co(0001) surface. To our best knowledge, few theoretical studies arecarried out for these adsorption systems.In current work, a series of calculations based on first-principles density functionaltheory were carried out for the above adsorption systems, including adsorption energiesand site preference, optimized geometries and electronic structure analysis. Mainconclusions are summarized as follows:1. For S adsorption on Ir(100), the calculated results indicate that S favors in the hollow site at coverages of 0.25 ML and 0.5 ML, in agreement with others observedexperimental results. The Ir-S bond has an essentially covalent character. Unusually,calculated work function for S/Ir(100) decreases with respect to the cleanunreconstructed Ir(100) surface.2. Upon S adsorbed on Co(0001), hollow sites are identified as the most preferredsites for all considered coverages, and the adsorption energy decreases with thecoverage increasing to 1.0 ML, suggesting that the repulsive interaction betweenadsorbate S is important at higher coverages. At 1.0 ML, it tends to form molecule S₂adsorption on Co(0001). Compared to clean Co(0001), small changes in work functionare shown for coveragesθ≤0.25 ML and negligible changes for coveragesθ≥0.5ML. S induces magnetic moment reduction for nearest surface Co atoms and the highercoverage, the more the decreasing.3. When CO adsorption on Co(0001), only RPBE functional predicts that COprefer on-top site at the coverages of 0.33 ML and 0.25 ML, in agreement with availableexperiments. With energy correction, the site preference is corrected compared to thatwithout energy correction for PW91, PBE and PKZB functional results. The magneticmoment of nearest surface Co atom decreases the most for CO on-top site. 5(?) and 2(?)states of molecular CO contribute to the main interaction with the substrate d orbitals,mixing with 4(?).4. For S and CO coadsorption on Co(0001), calculated results show that the effectof S on the adsorption of CO depends on the coverage of S. At the S coverage of 0.25ML, the interaction between CO and S is complex, a combination of long-range alongwith some local bonding character. This agrees well with others presented experimentalconclusion, but is different from that on Rh(111). At a lower S coverage of 0.11 ML,however, it is shown that the interaction between adsorbates is mainly short-rangecharacter, in accordance with that on Rh(111).