Sulfur Oxidation on Pt(355): It Is the Steps!

Abstract

Platinum catalysts are frequently used in catalytic converters in cars [1][2][3] and also in oil refineries. [4][5][6] The catalysts' active sites are subject to deactivation through poisoning by sulfur (or sulfur oxides), which are common impurities in fuels. [1][2][3][4][5][6][7][8] These active sites are thought to be defects, such as step or kink sites, which are omnipresent on the surface of the highly dispersed catalyst nanoparticles. Adsorbed sulfur modifies the electronic properties of the catalyst surface, which leads to a decrease of chemical and catalytic activity. [9][10][11] The key step to regain catalyst activity is the removal of the sulfur atoms from the catalyst surface, for example by exposing it to molecular oxygen, thereby oxidizing the adsorbed sulfur, and then removing the resulting SO x species from the surface. The mechanism, the chemical nature of the intermediates formed, and the specific role of defects in this process are unknown for the most part. This lack of insight is exists, because the relevant information can be obtained directly only by in situ methods, which allow a quantitative determination of the surface species or intermediates on the timescale of seconds. However, up to now there have been only very few studies for the direct measurement of kinetic parameters such as activation energies. [12,13] In most cases kinetic parameters are determined by temperature-programmed desorption (TPD), where only the desorbing species are detected. Since important reaction intermediates can thereby easily be missed, the correct determination of kinetic parameters can be hampered.Herein we present the first in situ study of sulfur oxidation on a model catalyst surface, namely stepped Pt(355). We have clearly identified the steps as active sites and determined the activation energy directly. The Pt(355) surface has (111) terraces five atom rows wide, and monatomic steps with (111) orientation. The role of the steps is elucidated by comparison to data obtained on a flat Pt(111) surface. Using synchrotron radiation, we were able to measure high-resolution XP spectra in situ during adsorption and while heating the sample with short measuring times. Owing to the high resolution, different surface species could be identified and analyzed quantitatively and site selectively in a time-dependent fashion, also for very low adsorbate coverages. [14][15][16][17] This enabled us to investigate the oxidation of small amounts of sulfur with oxygen present on the surface in large excess, which simplifies the kinetic analysis and makes it possible to determine the activation energy of the rate-determining step.The information on sulfur oxidation is rather limited in the literature. Early TPD studies [18,19] on Pt(111) yielded no information on surface intermediates, and consequently only an apparent activation energy was derived. [19] Theoretical calculations [20] indicate that at the oxygen saturation limit S is oxidized to SO x (x = 1-4) and the total energy increases with x, but no information on the a...

Kinetics of the sulfur oxidation on palladium: A combined in situ x-ray photoelectron spectroscopy and density-functional study

“…Since no SO, but only SO 2 , SO 3 , and SO 4 are detected as reaction intermediates, the rate limiting step must be the reaction of atomic sulfur and oxygen to SO, as described in Figure 2, and similar to the situation found on Pt(111) and Pt(355). 28 In addition, isothermal experiments were also performed at 400, 425, 475, and 500 K with an oxygen pressure of 2.1 × 10 −6 mbar. Note that the initial precoverages of sulfur varied between 0.014 and 0.025 ML, which does, however, not affect the data analysis and the derived conclusions.…”

“…In the similar system Pt(111) the energy barrier is smaller: 74 ± 4 kJ mol −1 . 28 Interestingly, on Pt(111) significant amounts of SO 3 were observed during the oxidation, while on Pd(100) the SO 4 species is the dominating product and reaction intermediates such as SO 2 and SO 3 were found only in very minor amounts; this can be partly explained by the fact that SO 4 is more stable on Pd(100) than on Pt(111) by ∼53 kJ mol −1 , when comparing previous calculations of the adsorption energy of SO 4 on pristine Pd(100) and Pt(111) surfaces. 25,36 Nevertheless, one can not rule out different formation rates of SO 3 and SO 4 for the two surfaces.…”

“…19,26,27 Such blocking has been found to affect both hydrogen and oxygen adatoms 27 which are used to regenerate the catalyst through either a reductive mechanism (formation and desorption of H 2 S) 12 or an oxidative mechanism (formation and desorption of SO 2 or SO 3 ). 28 The only study on a thermal oxidation reaction is an early temperature programmed desorption study, 29 which included isothermal experiments. However, no experiments concerning the reaction mechanism, the surface species formed during the process, and the reaction barriers were presented.…”

Ultrafast x-ray photoelectron spectroscopy in the microsecond time domain

“…4 With this improvement it became possible to study time-dependent surface processes in situ, i.e., to follow adsorption as function of time or reactions as function of temperature. This gain in time resolution not only opens the possibility to access the kinetics of surface reactions, 5,6 but also to greatly improve the output in measurement data per time. On top of that further improvements were achieved by new technological efforts: The development of new detectors and the operation of electron analyzers in the so called snapshot mode enable a time resolution of down to ∼100 ms in the case of analyzers with a CCD camera as detector, while times down to 1 ms are projected for delay line detectors 7 and other advanced detector designs.…”

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…[29][30][31] In the past, this method has mostly been restricted to chemical reactions on solid surfaces [32][33][34][35][36][37][38][39][40][41][42] because conventional XPS requires ultrahigh vacuum (UHV) conditions, which are not compatible with the high vapor pressure of most liquids and solvents. An exception is the use of dedicated experimental setups with differential pumping stages, which allow for performing XPS at the so-called near ambient pressure (NAP) conditions.…”