R. Streber

Microscopic Insights into Methane Activation and Related Processes on Pt/Ceria Model Catalysts

Ceria-based supported noble-metal catalysts release oxygen, which may help to reduce the formation of carbonaceous residues, for example during hydrocarbon reforming. To gain insight into the microscopic origins of these effects, a model study is performed under ultrahigh-vacuum conditions using single-crystal-based supported model catalysts. The model systems are based on ordered CeO(2)(111) films on Cu(111), on which Pt nanoparticles are grown by physical vapor deposition. The growth and structure of the surfaces are characterized by means of scanning tunneling microscopy, and the electronic structure and reactivity are probed by X-ray photoelectron spectroscopy. Specifically, it is shown that the fully oxidized CeO(2) thin films undergo slight reduction upon Pt deposition (CeO(1.99)). This effect is enhanced upon annealing (CeO(1.96)), thus indicating facile oxygen release and reverse spillover. The model system is structurally stable up to temperatures exceeding 700 K. The activation of methane is investigated using high-kinetic-energy CH(4) (0.83 eV), generated by a supersonic molecular beam. It is shown that dehydrogenation occurs under rapid formation of CH or C species without detectable amounts of CH(3) being formed, even at low temperatures (100 K). The released hydrogen spills over to the CeO(2) support, which leads to the formation of OH groups. At 200 K and above, the OH groups start to decompose leaving additional Ce(3+) centers behind (CeO(1.97-1.94)). At up to 700 K, carbon deposits are quantitatively removed by reaction with oxygen, which is supplied by reverse spillover from the CeO(2) film, thus leading to substantial reduction of the support (approximately CeO(1.90-1.85)).

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

Platinum catalysts are frequently used in catalytic converters in cars and also in oil refineries. The catalysts’ active sites are subject to deactivation through poisoning by sulfur (or sulfur oxides), which are common impurities in fuels. 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. 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 SOx 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. 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. 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 on Pt(111) yielded no information on surface intermediates, and consequently only an apparent activation energy was derived. Theoretical calculations indicate that at the oxygen saturation limit S is oxidized to SOx (x = 1–4) and the total energy increases with x, but no information on the activation energy of the ratelimiting step is available. Furthermore, there are a number of studies on the adsorption of SO2 on Pt surfaces, which serve as reference for the identification of reaction intermediates and partial reaction steps in the present study. 11,21–23] The thermal evolution of a layer of coadsorbed sulfur and oxygen on Pt(355) provides a first overview of the relevant reaction steps. Figure 1a shows a series of S 2p spectra recorded before and after dosing of molecular oxygen at 250 K onto Pt(355) precovered with 0.020 monolayers (ML) of sulfur, and during subsequent heating of the coadsorbate layer. The spectrum in black (sulfur layer prior to exposure to oxygen) shows the S 2p3/2 and S 2p1/2 signals at 162.0 and 163.2 eV, respectively, with an intensity ratio of 2:1. As this ratio and the peak separation of these signals are identical for all sulfur species, only the stronger 2p3/2 signal will be discussed. The value of 162.0 eV is typical of S adsorbed at step sites on Pt(355). The spectrum in orange, which was recorded after saturation of the surface with oxygen, shows the S 2p3/2 peak at 162.2 eV, which is typical for S at terrace sites. The clearly discernable shift of 0.2 eV indicates that the S atoms were pushed away from the step to terrace sites by the O atoms, similar to a recent observation for the coadsorption of sulfur and carbon monoxide on Pt(355). When the sample is heated, the S 2p spectra in Figure 1a change dramatically. To visualize the thermal evolution more clearly, we have also plotted the data in a color-coded density plot in Figure 1b. For the quantitative analysis shown in Figure 1d, the spectra were fitted, with the energetic separation of the S 2p1/2 and 2p3/2 peaks fixed at 1.2 eVand their ratio set at 1:2 (see the Experimental Section). In Figure 1c a [*] R. Streber, Dr. C. Papp, M. P. A. Lorenz, Dr. A. Bayer, Prof. Dr. H.-P. Steinr ck Lehrstuhl f r Physikalische Chemie II Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) Fax: (+ 49)1931-852-8867 E-mail: steinrueck@chemie.uni-erlangen.de and Erlangen Catalysis Resource Center (ECRC) Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany)

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

We studied the reaction kinetics of sulfur oxidation on the Pd(100) surface by in situ high resolution x-ray photoelectron spectroscopy and ab initio density functional calculations. Isothermal oxidation experiments were performed between 400 and 500 K for small amounts (~0.02 ML) of preadsorbed sulfur, with oxygen in large excess. The main stable reaction intermediate found on the surface is SO(4), with SO(2) and SO(3) being only present in minor amounts. Density-functional calculations depict a reaction energy profile, which explains the sequential formation of SO(2), SO(3), and eventually SO(4), also highlighting that the in-plane formation of SO from S and O adatoms is the rate limiting step. From the experiments we determined the activation energy of the rate limiting step to be 85 ± 6 kJ mol(-1) by Arrhenius analysis, matching the calculated endothermicity of the SO formation.

Ethene adsorption and dehydrogenation on clean and oxygen precovered Ni(111) studied by high resolution x-ray photoelectron spectroscopy

The adsorption and thermal evolution of ethene (ethylene) on clean and oxygen precovered Ni(111) was investigated with high resolution x-ray photoelectron spectroscopy using synchrotron radiation at BESSY II. The high resolution spectra allow to unequivocally identify the local environment of individual carbon atoms. Upon adsorption at 110 K, ethene adsorbs in a geometry, where the two carbon atoms within the intact ethene molecule occupy nonequivalent sites, most likely hollow and on top; this new result unambiguously solves an old puzzle concerning the adsorption geometry of ethene on Ni(111). On the oxygen precovered surface a different adsorption geometry is found with both carbon atoms occupying equivalent hollow sites. Upon heating ethene on the clean surface, we can confirm the dehydrogenation to ethine (acetylene), which adsorbs in a geometry, where both carbon atoms occupy equivalent sites. On the oxygen precovered surface dehydrogenation of ethene is completely suppressed. For the identification of the adsorbed species and the quantitative analysis the vibrational fine structure of the x-ray photoelectron spectra was analyzed in detail.

Interaction between silver nanowires and CO on a stepped platinum surface.

We studied the interplay between Ag decoration of a stepped Pt(355) surface and CO adsorption by in situ high-resolution x-ray photoelectron spectroscopy. Varying amounts of Ag deposited at 300 K initially lead to a row-by-row growth starting from the lower Pt step edges. Such decoration of the step sites results in a change in the CO adsorption behavior. An apparent blocking of step sites for low CO coverages is attributed to a change in the electronic structure, resulting in a C 1s binding energy of CO at step sites being equal to that for CO at terrace on-top sites in the presence of Ag. Higher CO coverages induce the formation of embedded Ag clusters within the upper terraces, thus freeing up a part of the original Pt step sites for CO adsorption, as was derived by a comparison to density functional theory calculations in the corresponding surface models.

Adsorption and reaction of acetylene on clean and oxygen-precovered Pd(100) studied with high-resolution X-ray photoelectron spectroscopy.

We investigated the adsorption and thermal evolution of acetylene on clean Pd(100) and Pd(100) precovered with 0.25 ML oxygen. The measurements were performed in situ by fast XPS at the synchrotron radiation facility BESSY II. On Pd(100) acetylene molecularly adsorbs at 130 K. Upon heating transformation to a CCH species occurs around 390 K along with the formation of a completely dehydrogenated carbon species. On the oxygen-precovered surface partial CCH formation already occurs upon adsorption at 130 K, and the dehydrogenation temperature and the stability range of CCH are shifted to lower temperatures by ∼200 K.