X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1999-03-11
2001-04-03
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S057000
Reexamination Certificate
active
06212253
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for investigating the reactants involved in a reaction between a gas and a solid by means of X-ray absorption spectroscopy (XAS). The invention is also concerned with an XAS-method for investigating gas-solid reactions in situ using this apparatus.
For the investigation of gas-solid reactions and their reaction products it is frequently necessary to use spectroscopic techniques in which the intrusion of the probe and the disturbance of the reaction environment during the investigation is reduced to a minimum. This applies in particular to investigations in the field of heterogeneous catalysts. Here, in situ investigations are of particular significance. It has been established that ex situ or post mortem investigations which are not carried out under working conditions yield somewhat different results as compared with in situ investigations.
An apparatus and a method for the in situ investigation of catalytic agents by means of X-ray absorption spectroscopy is known for example from “Nuclear Instruments and Methods in Physics Research” B97 (1995) pages 28-32. The apparatus described there comprises a measuring cell with a sample holder for solid samples located in the cell and with a window for the entry of X-rays for the irradiation of the sample. The sample holder consists of a gold-coated plate of silica (silicon dioxide) which is arranged within a cylinder of the same material which is open at both ends. This cylinder, whose axis lies parallel to the direction of propagation of the X-rays, is biassed to a positive or negative potential with reference to the sample holder and serves as a collector arrangement for the collection of electrons which are emitted by the sample which is fixed to the sample holder as a result of the X-ray absorption. The electrical current which flows from the collector electrode to a ground connection is used as a measurement value for the plotting of the absorption spectrum. The principle belongs to the field of total electron yield detection (TEY detection), since only the total yield of electrons is determined, without analysis of their kinetic energy. Values >20V have been established as preferable for the potential difference between collector arrangement and sample holder. Investigations at pressures up to 1 atmosphere and at temperatures up to 500° C. in the range of middle and hard X-radiation (≧4.5 keV) have been carried out on samples such as methanol synthesis catalysts Cu/ZnO/Al
2
O
3
, a copper
ickel alloy as well as a nickel reforming catalyst.
Having regard to the aforementioned publication, further investigations are reported in “Faraday Discussions” 105 (1996) pages 317-336. There it is maintained that the TEY detection uses the proportionality between the absorption coefficient of the sample for X-ray radiation and the number of the Auger electrons emitted from the sample, with the TEY signal having a linear relationship to the absorption coefficient of the sample. In the book “Practical Surface Analysis” by D. Briggs and others (John Wiley, New York 1983) it had already been established that the average penetration depth of the most energetic Auger electrons from the sample determines the surface sensitivity, and that as a consequence the surface sensitivity of the TEY signal depends upon the energy of the absorption edge. It has been observed that the average penetration depth of a few nm for absorption edges with <1 keV changes to several hundred nm for X-ray energies >10 keV.
SUMMARY OF THE INVENTION
TEY X-ray absorption investigations under reaction conditions have until now been carried out only for elements with atomic numbers Z>15. It is the object of the invention to extend the apparatus for and a method of X-ray absorption spectroscopy with total electron yield detection (so-called TEY detection) so that, for in situ investigations of gas-solid reactions, evidential spectra can be obtained even for elements of lesser atomic numbers Z<15, and wherein, preferably, a high surface sensitivity at the solid sample should be achievable.
By the use in accordance with the invention of two sequentially arranged collector electrodes along the X-ray path in the gas atmosphere between the X-ray window and the solid sample, and by the possibility, created by means of suitable terminal devices, of separate current measurements at the two collector electrodes and at the sample holder, the respective total yields of the electrons originating from these three parts can be determined separately. By evaluation of the three measured currents, preferably in relation to the measured intensity of the X-rays directed at the window or passing through it, information about the X-ray absorption as well as at the surface of the solid and also in the gas atmosphere can be achieved.
This is advantageous in particular for the investigation of heterogenous catalyst processes. In such cases it is very desirable to monitor the catalytic activity of a sample simultaneously with the changes in the gas composition, since a close connection has been established between the spectra of gas and the chemical binding state at the surface of the catalyst. Consequently, the study of heterogenous catalyst processes is preferred, even if not exclusively the field of use of the invention.
In order specially to augment the strength of the evidence of the data detectable by means of the invention, in a preferred embodiment, the construction and position of the collector electrode which is closer to the sample holder is so chosen that it effectively screens in terms of measurement a solid sample held by the sample holder from electrons coming from the space between the two collector electrodes, while simultaneously the other collector electrode which is further from the sample is effectively screened in terms of measurement from electrons coming from the space next to the sample. By this means, a particularly clear differentiation is possible between the X-ray absorption in the gas phase and the X-ray absorption at the surface of the sample.
With embodiments of the present invention it is possible for the first time successfully to investigate gas-solid reactions in situ for atoms of low atomic number, such as carbon, nitrogen and oxygen, using soft X-rays in the energy range of 100 to 1000 eV under high gas pressures up to 25 mbar and sample temperatures up to 1000° K., and indeed with low expenditure in terms of apparatus. The achievable results can be optimised if, in a preferred embodiment of the invention, measures are adopted in order selectively to bias at least the collector electrode which is closer to the sample to a positive potential relative to the other collector electrode and to the sample holder, and/or to adjust the distance between the collector electrode which is closer to the sample and the sample holder.
REFERENCES:
patent: 4573008 (1986-02-01), Lischke
“X-ray Spectroscopy at a Buried Diode Interface”, IBM Technical Disclosure Bulletin, Dec. 1984, US, vol. 27, Issue No. 7B, pp. 4468-4469.*
“Analysis of Near-Surface Impurities Using Spectroscopy”, IBM Technical Disclosure Bulletin, Dec. 1984, US, vol. 27, No. 7B, pp. 4455-4456.*
Moggridge et al., “Enviromental Cells for In Situ X-ray Diffraction and X-ray . . . Catalysts”, Nucl. Instr. and Meth. In Phys. Res. B 97 1995, pp. 28-32.
Harris, “A Spinning Stage, Total Electron-Yield . . . Spectra”, Rev. Sci, Instrum. 68(1), Jan. 1997, pp. 23-29.
Havecker Michael
Knop-Gericke Axel
Schedel-Niedrig Thomas
Dennison, Scheiner Schultz & Wakeman
Max-Planck-Gesellschaft
Porta David P.
LandOfFree
Apparatus and method for X-ray absorption spectroscopy does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for X-ray absorption spectroscopy, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for X-ray absorption spectroscopy will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2510226