Jens Hartmann

Catalytic activity and poisoning of specific sites on supported metal nanoparticles.

Typically, heterogeneous catalysts are based on nanometersized active particles, dispersed on an inert support material. In many cases it is assumed that the unique reactivities of such surfaces arise from the simultaneous presence of different active sites. On a molecular level, however, knowledge of the reaction kinetics of such systems is scarce (see e.g. refs. [1, 2] and references therein). Herein, we present first direct evidence for the different activity of coexisting sites on a well-defined supportednanoparticle system. As a model reaction we choose the decomposition of methanol on well-ordered Pd crystallites. For this reaction system two competing decomposition pathways exist (Figure 1): whereas dehydrogenation to CO

A molecular beam/surface spectroscopy apparatus for the study of reactions on complex model catalysts

We describe a newly developed ultrahigh vacuum (UHV) experiment which combines molecular beam techniques and in situ surface spectroscopy. It has been specifically designed to study the reaction kinetics and dynamics on complex model catalysts. The UHV system contains: (a) a preparation compartment providing the experimental techniques which are required to prepare and characterize single-crystal based model catalysts such as ordered oxide surfaces or oxide supported metal particles; and (b) the actual scattering chamber, where up to three molecular beams can be crossed on the sample surface. Two beams are produced by newly developed differentially pumped sources based on multichannel arrays. The latter are capable of providing high intensity and purity beams and can be modulated by means of a vacuum-motor driven and computer-controlled chopper. The third beam is generated in a continuous or pulsed supersonic expansion and is modulated via a variable duty-cycle chopper. Angular and time-resolved measureme...

The CO oxidation kinetics on supported Pd model catalysts: A molecular beam/in situ time-resolved infrared reflection absorption spectroscopy study

Combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum (UHV) conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ IR spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 product...

The interaction of oxygen with alumina-supported palladium particles

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (<500 K), only the surface oxygen, but not the bulk and subsurface oxygen is susceptible to the CO oxidation. The activation energies for the Langmuir–Hinshelwood step of the CO oxidation reaction were determined both in the regime of high CO coverage and high surface oxygen coverage. A comparison shows that the values are consistent with previous Pd(111) single crystal results. Thus, we conclude that, at least for the particle size under consideration in this study (5.5 nm), the LH activation energies are neither affected by the reduced size nor by the oxygen pretreatment.

IR absorption of SF6 excited up to the dissociation threshold

SF6 was pumped by a P20 TEA CO2 laser up to an average energy of 10 000 cm−1 in the absence of collisions and up to 20 000 cm−1 with a certain collisional relaxation. A second CO2 laser, very much attenuated, was used to determine absorption cross sections for frequencies from 915 to 985 cm−1. No coherent effects and no collisionless relaxations were found, in contrast to measurements with a cw laser as a probe. The ν3 band continuously shifts to longer wavelengths, but the shift of the ν2+ν6 band is 10 to 20 times smaller. Many more rotational states of the vibrational ground state are depopulated than expected. To explain it we suggest direct two‐photon excitation, for which independent evidence is also presented. Many peaks and holes were found, part of which we assign to high states. Evidence for inhomogeneity of these structures is found by comparison with single laser absorption spectra. Collisional relaxation of unidentified nature and with a time constant of 48±10 ns mbar generates new spectra, indicating large nonequilibrium rotational populations.

An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical∕thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques and in situ reflectivity measurements and (ii) the adsorption∕reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005-1.5 × 10(15) molecules cm(-2) s(-1) and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190-3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-2)). The absolute accuracy in energy is within ∼7%-9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K.

Complex model catalysts under UHV and high pressure conditions : CO adsorption and oxidation on alumina-supported Pd particles

Abstract The growth of metal particles on ordered oxide surfaces provides a strategy to prepare well-defined model systems for supported catalysts, which can by easily studied by most surface-science techniques. Here, we focus on Palladium particles grown on an ordered Al2O3 film on NiAl(110), a system which has previously been characterized in detail with respect to its structural, electronic and adsorption properties. In this contribution, we will provide several examples, showing how adsorption and reactivity phenomena on these systems can be addressed over a pressure range from ultrahigh vacuum (UHV) to near atmospheric pressure. In the low pressure region, we apply a combination of molecular beam methods and in-situ infrared reflection absorption spectroscopy (IRAS). For CO adsorption, angular resolved scattering and sticking coefficient measurements and structural information allow us to quantify different adsorption channels including reverse spillover effects. The coverage dependent kinetics of CO oxidation is derived and discussed in comparison with the single crystal kinetics. The adsorption of CO on alumina supported Pd aggregates at low and high pressure, i.e. from 10−7–200 mbar, is examined by IR–VIS sum frequency generation (SFG) vibrational spectroscopy. At low pressure, the CO adsorption site distribution (bridged vs. on-top) depends on the particle surface structure and temperature, but under reaction conditions, the site occupancy is mainly governed by the CO pressure. The impact of these results on the extrapolation of UHV data to high pressure catalysis is discussed.

Adsorption, decomposition and oxidation of methanol on alumina supported palladium particles

We have investigated the adsorption, decomposition and oxidation of methanol on a well-defined supported Pd model catalyst, utilizing a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The Pd model catalyst is prepared under ultrahigh-vacuum (UHV) conditions on a well-ordered Al2O3 film grown on NiAl(110). In previous studies, this model system has been characterized in detail with respect to its geometric and electronic structure. On the alumina support, two molecular adsorption states of methanol are distinguished by RAIRS and TPD. Moreover, we can differentiate between adsorption on the Pd particles and on the alumina support, enabling us to follow surface diffusion from the alumina film to the Pd particles during the adsorption process. Upon heating, methanol partially desorbs from the Pd particles and partially undergoes decomposition, with a reaction probability that is sensitively dependent on the initial methanol coverage. At 100 K, preadsorbed CO suppresses methanol adsorption on the Pd particles, whereas preadsorbed oxygen reduces the reaction probability. As a first intermediate, methoxy species are formed, which are stable up to temperatures of 200 K. Isotope exchange experiments indicate that a fast equilibrium is established between molecular methanol and methoxy species and that both species are rapidly exchanged with the gas phase. Further decomposition of methanol proceeds via two competing reaction pathways. The dominant pathway is dehydrogenation to CO, followed by CO2 formation in the presence of oxygen. Adsorbed oxygen has a pronounced inhibiting effect on the rate of decomposition. As a second pathway, we observe slow breakage of the carbon–oxygen bond, leading to formation of carbon and hydrocarbon species.

The Molecular Origins of Selectivity in Methanol Decomposition on Pd Nanoparticles

We have combined multi-molecular beam methods and in-situ time-resolved IR reflection absorption spectroscopy (IRAS) to explore the kinetics of methanol decomposition on a supported Pd model catalyst. The well-shaped Pd nanoparticles are prepared under ultra-high vacuum conditions on a well-ordered alumina film and have previously been characterized with respect to size, density, and morphology.Two competing decomposition pathways are observed: Whereas dehydrogenation to CO represents the dominating reaction channel, C-O bond scission proceeds at much lower rates and leads to the formation of carbon and hydrocarbon species. Using CO as a probe molecule, we show via IRAS spectroscopy that these carbon and hydrocarbon species preferentially block defect sites on the Pd particles such as steps or edges, whereas the (111) facet sites are affected to a lesser extent.Employing quantitative IR\Sigma AS and steady-state isotope exchange experiments, the reaction rates for both channels are measured as a function of carbon coverage. It is found that with increasing carbon coverage, the rate of carbon formation drops rapidly, whereas the kinetics of dehydrogenation is hardly affected. These results demonstrate that the rate of C-O bond scission is drastically enhanced at the particle steps and edges, whereas for the dehydrogenation pathway this is not the case.

Nanofacet-resolved CO oxidation kinetics on alumina-supported Pd particles

Employing a multi-molecular-beam approach, we have measured the angular distribution of CO2 molecules formed during CO oxidation under steady-state conditions on well oriented and shaped nm-sized Pd crystallites grown on an ordered alumina film. The experiment is combined with kinetic Monte Carlo simulations based on a realistic structural model. The results obtained allow us (i) to differentiate between local reaction rates on the particle nanofacets and (ii) to conclude that oxygen diffusion on and between the (1 1 1) facets is rapid compared to reaction. 2002 Elsevier Science B.V. All rights reserved.