Biomedical subjects

Anke S Wörz

Publications and source records attributed to Anke S Wörz.

4 recordsLinked to original sources

PubMed2005

Charging of Au atoms on TiO2 thin films from CO vibrational spectroscopy and DFT calculations.

Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (De < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (De = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Au(delta-)CO complexes formed on oxygen vacancies easily lose CO (De = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from Au to TiO2 with formation of strongly bound Audelta+CO complexes (De = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.

Journal Article↗

PubMed2005

Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO.

Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.

Journal Article↗

PubMed2004

Low-temperature cluster catalysis.

Free and supported metal clusters reveal unique chemical and physical properties, which vary as a function of size as each cluster possesses a characteristic electron confinement. Several previous experimental results showed that the outcome of a given chemical reaction can be controlled by tuning the cluster size. However, none of the examples indicate that clusters prepared in the gas phase and then deposited on a support material are indeed catalytically active over several reaction cycles nor that their catalytic properties remain constant during such a catalytic process. In this work we report turn-over frequencies (TOF) for Pd(n) (n = 4, 8, 30) clusters using pulsed molecular beam experiments. The obtained results illustrate that the catalytic reactivity for the NO reduction by CO (CO + NO --> 1/2N(2) + CO(2)) is indeed a function of cluster size and that the measured TOF remain constant at a given temperature. More interestingly, the temperature of maximal reactivity is at least 100 K lower than observed for palladium nanoparticles or single crystals. One reason for this surprising observation is the character of the binding sites of these small clusters: N(2) forms already at relatively low temperatures (400 and 450 K) and therefore poisoning by adsorbed nitrogen adatoms is prevented. Thus, small clusters not only open the possibility of tuning a catalytic process by changing cluster size, but also of catalyzing chemical reactions at low temperatures.

Journal Article↗

PubMed2003

Cluster size-dependent mechanisms of the CO + NO reaction on small Pdn (n < or = 30) clusters on oxide surfaces.

The CO + NO reaction (2CO + 2NO --> N(2) + 2CO(2)) on small size-selected palladium clusters supported on thin MgO(100) films reveals distinct size effects in the size range Pd(n) with n < or = 30. Clusters up to the tetramer are inert, while larger clusters form CO(2) at around 300 K, and this main reaction mechanism involves adsorbed CO and an adsorbed oxygen atom, a reaction product from the dissociation of NO. In addition, clusters consisting of 20-30 atoms reveal a low-temperature mechanism observed at temperatures below 150 K; the corresponding reaction mechanism can be described as a direct reaction of CO with molecularly adsorbed NO. Interestingly, for all reactive cluster sizes, the reaction temperature of the main mechanism is at least 150 K lower than those for palladium single crystals and larger particles. This indicates that the energetics of the reaction on clusters are distinctly different from those on bulklike systems. In the presented one-cycle experiments, the reaction is inhibited when strongly adsorbed NO blocks the CO adsorption sites. In addition, the obtained results reveal the interaction of NO with the clusters to show differences as a function of size; on larger clusters, both molecularly bonded and dissociated NO coexist, while on small clusters, NO is efficiently dissociated, and hardly any molecularly bonded NO is detected. The desorption of N(2) occurs on the reactive clusters between 300 and 500 K.

Journal Article↗