NOVEL PROSPECTS FOR A CHEMICAL CHARACTERIZATION OF SOLID STATE SURFACES AT HIGH PRESSURE AND HIGH TEMPERATURE: In situ X-RAY
ABSORPTION SPECTROSCOPY IN THE SOFT ENERGY RANGE Th. Schedel-Niedrig, M. Hävecker1, A. Knop-Gericke1, P. Reinke2, R. Schlögl1, and
M. Ch. Lux-Steiner
Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, 14109 Berlin, Germany
1 Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin, Germany
2Georg-August-Universität Göttingen, II. Physikalisches Institut, Bunsenstrasse 7-9, 37073 Göttigen,
Germany ABSTRACT
A contribution to the experimental overcoming of the "pressure gap" in material science is presented.
In situ X-ray absorption spectroscopy (XAS) investigations in the soft X-ray range (100 eV ≤ hν ≤ 1000 eV) at elevated pressures (mbar range) and sample temperatures (T ≤ 1000 K) can be performed by an instrument equipped with total electron yield detectors [1]. This allows, for the first time, XAS studies in a surface-sensitive mode of the light elements (Z = 3-15) and, additionally, the gas phase XAS can be collected simultaneously in order to correlate the gas/solid reaction rate with the surface electronic structure under working conditions in a flow-through mode. In this work examples are presented belonging to the field of heterogeneous catalysis [2-4] and to the reactivity of diamond surfaces [5].
X-ray absorption spectroscopy (XAS) investigations have long been performed due to their value in the study of the local structure of solid state materials, but it is only within the last decade that the development of in situ techniques has been progressed most significantly [6]. An in situ investigation means a study performed under real practical conditions which is necessary since the structure of solid state materials - e.g., a catalyst- can be very different from that found after the reaction or ex situ, i.e., under vacuum conditions.
Photons in the VUV range (100 – 1000 eV) are suitable probes for the electronic structure of reacting surfaces. Their interaction with solid matter leads to photoabsorption processes which can be detected via the Auger electrons created by the relaxation of the core holes. Photons of this energy
Igrid Isam
+ 4.5 V Polyimide
Window
Detector Gas Phase
Detector
Sample + Gas Phase Sample Collection Plate
Collection G rid Ceram ic Spacer
Signal W ire
Figure 1. Array of detectors used in the tank reactor. The detector electrodes are made of oxide-passivated nickel showing excellent stability in work function and absolute chemical inertness under reaction conditions employed. The electrodes are set to variable bias potentials via battery boxes and the electron current to ground is recorded by means of a precision current meters (Keithley).
interact also moderately strong with gas molecules giving rise to the same type of photoabsorption- electron emission processes. The gas phase absorption limits the useful pressure range of the experiments to about 1–10 mbar depending on the nature of the gas phase and the photon flux of the excitation source. The procedure we applied is described elsewhere [1,7] and uses a combination of several detector signals allowing to separate out either the solid or the gas phase signal as shown schematically in figure 1. In this way two sets of raw data are obtained which characterise at the very same moment the solid surface and the gas phase above it [7]. In this way correlation between spectral features and material properties like catalytic performance or wetness stability can be obtained.
The instrument consists of two chambers [1], one attaching the reactor to the front end of the synchrotron storage ring and the other serving as reactor. The reactor is separated from the UHV of the storage ring by a polyimide window array which is sufficiently transparent to low-energy X-rays [1] and allows to raise the pressure in the reactor up to 20 mbar. The reactor can be used as a batch or in a flow-through mode. The sample is heated resistively with the heating wires well shielded from the gas phase by a AlN/BN housing. The sample can be heated up to 1000 K. A photograph and a schematic drawing of the detector arrangement are shown in figure 1. The synchrotron light beam is monitored in the UHV chamber yielding the I0 signal. After passing through the window the beam is monitored again by a total electron yield detector in the form of a grid giving the absorption of the beam by the window and by the gas phase, Igrid. A consecutive detector electrode monitors a combined signal from the gas phase and from the solid sample absorption, Icol. Finally, the sample current is monitored which contains an independent but not surface-sensitive information on the sample XAS signal, Isam. The array of the detectors is mounted within 35 mm between the window and the sample surface. The sample is mounted on a precision manipulator allowing to position the sample between 5 and 25 mm in front of the detector plate. In this way a reasonable match between the detector dimensions and the mean free path of the Auger electrons at pressures in the millibar range is maintained. The gas phase is held constant in its inlet composition by means of differentially pumped (cold traps, membrane pumps) gas flow regulated with mass flow controllers. The conversion and calibration of the system can be controlled independently from the XAS data by a quadrupole mass spectrometer attached via a calibrated leak to the reactor tank. Further details of the instrumentation can be found in the literature [1,2].
EXAMPLE I: THE PARTIAL OXIDATION OF METHANOL OVER COPPER
Elemental copper is used as unsupported catalyst for the oxidehydrogenation with air of alcohols to aldehydes. The reaction of methanol to formaldehyde
2CH3OH + O2→ 2H2CO + 2H2O
is a good model reaction using a simple substrate. Its conversion vs. temperature profile is highly similar to that of the more relevant alcohols [3,7]. The catalytic performance of a polycrystalline copper foil within the in situ reactor set-up operated in the flow-through mode is very similar to that obtained for a conventional tubular reactor [3,7]. The oxygen K- and copper L2,3-edge XAS data of the catalyst (sub/near) surface region at various temperatures are shown in figures 2 and 3, respectively. At low temperatures up to about 560 K, XAS spectra typical of copper(I) oxide, Cu2O, are detected. It is seen that the spectral shapes of the XAS edges shown in the figures 2 and 3 change drastically at catalyst temperature of 560 K under steady-state reaction conditions indicative of a copper(I)-to-copper suboxide phase transition within few tens of minutes. Only a small contribution of Cu2O can be observed at 560 K in figure 3 due to the presence of a low-intensity peak at about 933 eV disappearing at 630 K. The copper-to-oxygen stoichiometric ratio changes also drastically from 2:1 for Cu2O to about 10:1 for the novel suboxide phase, Cu(x≥10)O. An estimation of the stoichiometric ratio was done by comparing the absorption edge heights of the copper(I) oxide solid state phase, Cu2O, and the catalytically active copper solid state phase both measured at the O K- edge [10] at a pressure in the millibar range.
∆∆∆∆t = 20 m in
∆
∆∆
∆t = 20 min.
Generally, it was found that the critical phase transition temperature, TCT, strongly depends on the oxygen proportions in the gas mixture and increases with increasing oxygen concentration [3,7]. The phase transformation is found to be reversible which can be seen in the copper(I) oxide-like spectrum of figure 3 detected after lowering the catalyst temperature from 750 K to 490 K. This strongly indicates that the copper(I) oxide is the thermo-dynamically stable phase at temperatures below TCT that is also found after quenching the reaction, i.e. ex situ (see refs.8,11,12). Thus, this observation strongly indicates that at low temperatures the solid state chemistry of copper and oxygen plays the dominant role and not the catalytic reaction. The phase transformation under reaction conditions leads contrary to expectation not to pure Cu and CO2 but to a phase with residual oxygen which is not stated in the known phase diagrams of the Cu-O system [13].
EXAMPLE II: IN SITU INVESTIGATION OF THE INTERACTION OF DIAMOND WITH WATER
The interaction of water and oxygen with a polycrystalline diamond film was investigated using the novel in situ XAS technique [5]. The diamond surface is inert with respect to the interaction with water in the investigated temperature below 500°C. A defect (sp2 carbon) rich surface layer, is created through ion irradiation (Ar+, 2 keV), and the ratio of the π*/σ* resonance was used as a measure of the extent of damage. A partial removal of the damage is observed in the first few minutes of water exposure, but a steady-state concentration of defects remains even after prolonged exposure periods.
The resultant steady-state damage is nearly independent of the water pressure. The reaction of the damaged surface with water leads also to the attachment of oxidic groups, mostly in the form of carbonyl groups, to the surface. The carbonyl groups seem to be located mostly at the surface and
Figure 2. The oxygen K-edge XAS data of the catalyst surface and sub/near-surface region are shown for different catalyst temperatures as indicated. The copper catalyst is exposed to a methanol-oxygen-gas flux (volume ratio 3:1;
total flux: 53 ml/min.) at different catalyst
Figure 3. The copper L2,3-edge XAS data of the catalyst surface and sub/near-surface region are shown for different catalyst temperatures as indicated. The spectra were recorded by using the second-order light of the monochromator and were subsequently shifted
result directly from the reaction with dangling bonds or sp2-carbon defects with water. The irradiation with oxygen ions leads likewise to an overall reduction in damage, the chemical etching dominates the creation of new defects through ion impact.
In this work two examples were presented demonstrating the strong mutual interaction between solid state chemistry, surface science, and material science of such "simple systems". Our new experimental tool presents a contribution to the experimental overcoming of the "pressure gap" in material research and, furthermore, demonstrates also that the "research gap" between materials science and surface science research can be closed. Thus, an expansion to relevant studies in the field of gas/solid interaction can be of high interest.
REFERENCES
1. A. Knop-Gericke, M. Hävecker, Th. Neisius, Th. Schedel-Niedrig, Nucl. Instr. Meth.
A 406 311 (1998); Deutsches Patent Nr. 198 10 539; United States Patent No. 6,212,253,B1 2. A. Knop-Gericke, M. Hävecker, Th. Schedel-Niedrig, Appl. Surf. Sci. 142, 438 (1999).
3. Th. Schedel-Niedrig, M. Hävecker, A. Knop-Gericke, R. Schlögl, Phys. Chem. Chem.
Phys. 2, 3473 (2000).
Table 1: Summary of processing conditions for the diamond films included in figure 4. The spectra were measured in vacuum unless otherwise indicated.
graph π*/σ* processing of sample
diamond polycrystalline diamond sample, as introduced
A 40’, 2.0 mbar H2O, and 20’, 0.2 mbar H2O, and 20’, 480°C, 0.2 mbar H2O (measured during gas exposure)
B A and 10’, 2 mbar H2O at 480°C, and 40’, 4 mbar H2O at 480°C (measured after cool down in vacuum)
C 0.5 ion irradiated diamond (Ar+, 2 keV)
D 0.37 C and 50’, 1·10-4 mbar H2O, and 25’, 1.0 mbar H2O E,H 0.79 ion irradiated diamond (Ar+, 2 keV)
F 0.52 E and 25’, 1.0 mbar H2O
G 0.74 same treatment as F, grazing incidence
Figure 4: Carbon K-edge spectra recorded at different stages of the modification of diamond films through the irradiation with ions (Ar+) and the exposure to water at different pressures. The processing conditions and assignment of the spectra is summarised in table 1. The spectra are normalised to unit height at the position of the σ* resonance.
π * σ *
305 300 295 290 285 280
photon energy (eV)
C K-Edge
diamond A B C D E F G
4. M. Hävecker, A. Knop-Gericke, Th. Schedel-Niedrig, R. Schlögl, Angew. Chem. Int. Ed. 37, 1939 (1998).
5. P. Reinke, A. Knop-Gericke, M. Hävecker, Th. Schedel-Niedrig, Surf. Sci. 447, 229 (2000).
6. R. Burch (Ed.), In situ Methods in Catalysis, Catalysis Today 9 (1991).
7. A. Knop-Gericke, M. Hävecker, Th. Schedel-Niedrig, R. Schlögl, Topics in Catalysis 10, 187 (2000).
8. Th. Schedel-Niedrig, X. Bao, M. Muhler, R. Schlögl, Ber. Bunsenges. Phys. Chem. 101, 994 (1997).
9. J.J. Yeh and I. Lindau, Atomic Data and Nuclear Data Tables 32, 1 (1985).
10. The copper-to-oxygen ratio was estimated by extracting the portion of the sample-related signal of the Cu2O-solid state phase from the total XAS collection plate signal, Icol, at the O K-edge detected at a pressure in the millibar range. Subsequently, this portion of the Cu2O-sample signal (copper-to-oxygen ratio = 2:1) was compared with the corresponding portion of the sample- related signal measured under catalytical conversion conditions at the same pressure. This sample-related portion was also extracted from the total XAS signal of the collection plate, Icol. Here, the sample-related signal of the solid state phase of the active copper catalyst was found to be 5 times smaller at the O K-edge if compared with the corresponding signal of Cu2O.
11. Th. Schedel-Niedrig, I. Böttger, Th. Neisius, E. Kitzelmann, G. Weinberg, D. Demuth, R. Schlögl, Phys. Chem. Chem. Phys. 2, 2407 (2000).
12. Th. Schedel-Niedrig: Application of Copper Suboxide for the Partial Oxidation: In situ and ex situ Characterization, Habilitationsschrift, (Technische Universität Berlin 1999).
13. Gmelins Handbuch der anorganischen Chemie: Kupfer (Verlag Chemie, Weinheim, 1958) Teil B, p 24; Gmelins Handbuch der anorganischen Chemie: Kupfer (Verlag Chemie, Weinheim, 1963) Teil D, p 26.
