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Applications of inelastic incoherent neutron scattering in technical catalysis
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
3155
Applications of inelastic incoherent neutron scattering in technical catalysis P. Albers a, G. Preschera, K. Seibold a, S.F. Parkerb
aDegussa-Hiils AG, P.O.Box 1345, D-63403 Hanau, Germany blSIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. The inelastic neutron scattering technique was utilised in investigating cokes and technical catalysts whose properties caused serious problems when using conventional characterisation methods. The graphiticity and the intranetwork linking of carbon deposits from methane and from acetylene/ethylene/vinyl chloride were compared. [Fe(H20)C15]2was identified as one important intermediate in catalyst deactivation. The formation of macroscopic amounts of strongly bound Pd-CH3 species on hydrogenation catalysts caused unexpected deactivation by changing the adsorption properties of catalysts to sp2-type reactants. 1. INTRODUCTION In spite of the high potential benefits of neutron spectroscopic techniques [1,2] their application for investigations in the field of technical catalysis is still quite limited. For industrial users, neutron spectroscopic techniques may appear to be mostly confined to investigating predominantly basic research questions and not for directly studying properties and problems on materials of immediate technological relevance. The present contribution summarises some recent experimental results that illustrate the usefulness of neutron spectroscopy in characterising various technical catalysts and materials from large scale chemical reactors.
2. METHOD Inelastic Incoherent Neutron Scattering (INS) is a form of vibrational spectroscopy that is complementary to techniques such as Fourier transform infrared spectroscopy. The major advantages of INS are that there are no selection rules and it is possible to quantitatively model the spectra. In addition, the wide spectral range means that low energy vibrational modes down to 20 cm l are routinely measured (translational modes, deformation modes, optical and acoustic phonons in solids etc.) giving information on microstructural properties of catalysts and on materials of catalytic relevance. The high penetration power of neutrons and very different scattering cross sections of certain elements such as H and C or Pd allows the characterisation of the properties of macroscopic amounts of even highly dispersed materials such as activated carbons and carbon blacks (and related precious metal catalysts), cokes and metal-black catalysts even under in-situ conditions. INS thus provides helpful analytical information which is not accessible by and complementary to, conventional methods. A prominent feature of INS is the highselectivity for hydrogen which allows the utilisation of the proton dynamics of catalytic materials as a sensitive probe for microstructural features. This was already used to investigate the proton
3156 related properties of palladium catalysts and of carbonaceous catalyst support materials including the detection of strongly bound molecular-like species on highly porous activated carbon and the formation and decomposition of finely divided palladium hydride on Pd/C hydrogenation catalysts [3,4]. The INS spectra reported here were measured at 20 K using the spectrometer TFXA and its successor TOSCA at the ISIS neutron facility (Chilton, UK). Coke samples from different catalytic processes and active and deactivated palladium catalysts were analysed in thin walled aluminium cans. Additional experimental details were given in related papers [3-6]. 3. APPLICATIONS FROM TECHNICAL CATALYSIS 3.1 Characterisation of cokes from methane and from acetylene/ethylene/vinyl chloride Each INS spectrum in Fig. 1 gives an impression of the integral and hydrogen-related properties of up to 20 g coke per sample. Due to their conductivity or to the high absorbance of electromagnetic radiation of the cokes the application of techniques such as nuclear magnetic resonance or infrared spectroscopy was limited. Figs. 1 A and B show the INS spectra of material from thermally controlled carbon deposition occuring under very unfavourable conditions during the synthesis of HCN from CI-I4 and NH3 at high temperature (Tz 1200~ on Pt/A1203 catalysts. Spectra similar to carbon blacks or soots were expected for this kind of methane-derived coke. However, the spectra perfectly resemble the INS features of pure graphite and of graphitised carbon black as formed at about 3000~ [7,4]. The high purity and sp2 character of the material is indicated by the sharp graphite modes at 112 cm l and the other vibrational features are confined to the region below 1700 cm l. Traces of sp3 carbon [7] are largely missing. This indicates an accidentally very low partial pressure of hydrogen during the deposition and growth of this clean species of coke in spite of the, usually, high amount of hydrogen present in the reaction CH4 + NH3 --* HCN + 3 H2.
(1)
The presence of orientation effects which are known for well-defined graphites are indicated by varying intensifies of additional bands at about 143 and 323 cm 1 inside this coke as well. Completely different properties are exhibited by the low temperature cokes which were collected from technical reactors after premature catalyst deactivation (Fig. 1 C and D). These cokes were suddenly deposited during the selective hydrogenation of acetylene to ethylene on Pd/SiO2 catalysts in the HC1 recycle gas stream of the vinyl chloride process at T < 200~ aider 89 year (C) and 2 years (D) of smooth operation, respectively, leading to completely unusual premature loss of activity. Figs. 1 C and D indicate completely different coking mechanisms and coke properties. Principally, the spectra of polymer-like carbon originating from copolymerisation from acetylene and ethylene were expected. However, Fig. 1 C perfectly matches the spectrum of a well-defined, pure CVD-carbon [7]. More than 50 wt.% of this material are inorganic contaminants including Fe, C1 etc. which are closely linked to the finely divided, highly absorbing carbon component of the deposits. In the INS spectrum the inorganic component of the coke is largely suppressed and the hydrogen containing part of the coke is selectively revealed as a pure CVD material. As seen from Fig. 1 A and B the low energy band at 112 crn1 indicates the presence of sp2 entities originating from catalytically driven coke transformation of amorphous carbon into graphtitic entities of enhanced size under the influence of iron contaminants. The band at 270 crn1 is a measure for the degree of
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Fig. 1. INS spectra of cokes from large scale chemical reactors as collected from the catalysts surfaces. A, B: high temperature coke deposited at the surface of Pt/A1203 catalysts. 12, D: low temperature cokes fi~om the surfaces of Pd/SiO2 catalysts. D includes results of a computer simulation of the spectn~ of [Fe(tt20)C15] 2"(dashed line).
3158 cross-linking between sp2 and sp 3 type carbon (bands at 1300-1500 LTI1"1) erltities and, therefore, contains information about the interactions between different carbonaceous components of the bulk of the coke. Such information is of relevance with respect to the feasability and efficiency of a catalyst regeneration treatment under given conditions. In contrast to Fig. 1C the specman of a coke from the same process given in Fig. 1 D, surprisingly, strongly resembles the signals of the well-defined ~species [Fe(H20)C15]2 [8] whose simulated specmnn is also included. Alternative structures would show quite different vibrational spectra. The strongest band at 386 crn~ is assigned to the Fe-OH2 torsional mode indicating an other main reason for catalyst deactivation present. These examples illustrate that under certain conditions extensive growth of pure CVD-type carbon occurs which according to elemental analysis and surface analysis measurements contained significant amounts of well dispersed amounts of iron chlorides and other contaminants. On the other hand the presence of traces of humidity can lead to some corrosion in the HC1 recycle gas stream and to the formation of a [Fe(H20)C15]2 species dominating the whole INS spectnnn of this coke. Such findings on products or intermediates in catalyst deactivation processes may be helpful in investigating the changing mechanisms of coke deposition under varying conditions in more detail. INS as focused on the hydrogen related part of the coke allows a quite different view of partly finely divided, highly contaminated and bulk samples of technical cokes. In spite of the technical origin of the cokes Fig. 1 reveals the presence of well defined components dominating the INS-spectra of these tentatively ill-defined samples: pure oriented graphites, CVD-carbon and one certain iron complex of defined H20 content.
3.2 Catalyst poisoning by methyl groups In a screening study on catalyst deactivation INS was used for investigating various palladium catalysts which showed premature deactivation. The catalysts were used in the partial hydrogenation of-C=C- structures and of C=O-groups to C-OH groups in aromatic and polyaromatic systems of naphthalenes and related applications. In contrast to the common observations in investigating spent catalysts and in spite of using the broad spectrum of the well proven techniques for catalyst characterisation the loss of activity could not be traced back to catalyst degradation, inorganic catalyst poisons, particle growth, enhanced coke deposition, formation of carbides or to any other of the usual aging phenomena. INS was uniquely able to reveal the phenomenon leading to deactivation inthe present case. Fig. 2 compares the INS-spectra of deactivated palladium catalysts measured after cleaning by solvent extraction under strictly constant conditions. The spectrum in Fig. 2 A which was recorded under in-situ conditions under 1.5 bar of hydrogen equilibrium pressure inside the INS-cell is dominated by the strong asynunetric band of [3-paUadium hydride at 464 cm 1 and its first overtone. Atter dehydrogenation (degassing at room temperature) the hydride phase is completely decomposed however some sharp peaks remain indicating the presence of simple molecular like degradation products of geminal substitution at the surface of the catalyst (Fig. 2 B). Furthermore, two broad bands at about 800 and 1200 crnl which previously were hidden under the first overtone of the hydride band are lett which according to elemental analysis have to be assigned to non-extractable organic components. The bands at 800 and 1200 cm 1 were also observed on corresponding active catalysts and were clearly not of importance for the given deactivation phenomenon. At a later state of the catalyst deactivation process the strongly bound molecular-like as well as the strongly adherent presumably polycondensed carbonaceous species were transformed into simple molecular entities as indi-
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Fig. 2. INS spectra of palladium catalysts measured aiter extraction under strictly the same conditions. A: deactivated catalyst measured under 1.5 bar of H2 equilibrium pressure in-sire. B: as A, after dehydrogenation. C" deactivated catalyst [6], D: corresponding active catalyst.
3160 eated by the strong and sharp bands in Fig. 2 C. Detailed computer assisted analyses of the specmnn showed that the strong and sharp bands could uniquely and unambiguously be traced back to methyl groups chemically bound on top to the Pd surface in a C3v site. Reference spectra of the deactivated palladium catalyst as received from the technical application [6] revealed that the rotational modes of methyl groups of weakly adsorbed and completely extractable solvent molecules and reactants ranged at 237 em"l The signal at 301 em l dominated the spectrum of the catalyst before and after extraction. So the INS signal of the methyl groups is shifted from about 237 em ~ (extractable molecular species) up to about 301 em "l (surface-bound Pd-CH3). A perfect correlation between the normalised INS intensities of the Pd-CH3 peaks as measured on different catalyst samples and the degree of catalyst deactivation was observed. The unexpected formation of considerable amounts of the strongly bound Pd-CH3 species changed the adsorption properties of the catalysts surface for adsorption of sp2 type reactants and, therefore, changed the catalyst performance. It is remarkable that on the surfaces of deactivated technical catalysts well-defined, simple molecular structures are reproducibly detected which hitherto have only been revealed in surface science experiments. INS was exclusively able to supply this information.
4. CONCLUSION INS is a very helpful and versatile technique for studying problems in industrial heterogeneous catalysis. INS certainly deserves a much higher profile and more fi'equent use by investigators and practitioners in the chemical industry. ACKNOWLEDGEMENT The Rutherford Appleton Laboratory (Chilton, UK) is thanked for access to neutron beam facilities. REFERENCES
1. T. J. Udovic and R. D. Kelley in Hydrogen Effects in Catalysis, Eds. Z. Paal and P.G. Menon, M. Dekker, New York 1988 p. 167. And literature cited therein. 2. H. Jobic in Catalyst Characterisation, Eds. B. Imelik and J.C.Vedrine, Plenum Press, New York 1994 p. 347. And literature cited therein. 3. P. Albers, R. Burmeister, K. Seibold, G. Prescher, S.F. Parker, D.K. Ross, J. Catal., 181 (1999) 145. 4. P. Albers, G. Prescher, K. Seibold, D.K. Ross, and F. Fillaux, Carbon, 34 (1996) 903 and Carbon, 37 (1999) 437. 5. P. Albers, S. B6sing, G. Prescher, K. Seibold, D.K. Ross, S.F. Parker, Appl. Catal. A: General, 187 (1999)233. 6. P. Albers, H. Angert, G. Prescher, K. Seibold, S.F. Parker, Chem. Comm., No. 17 (1999) 1619. 7. J. Waiters, R.J. Newport, S.F. Parker, W.S. Howells, J. Phys.: Condens. Matter, 7 (1995) 10059. 8. S.F. Parker, K. Shankland, J.C. Sprunt, U.A. Jayasooriya, Spectrochim. Acta, A 43 (1997i 2333.
3155
Applications of inelastic incoherent neutron scattering in technical catalysis P. Albers a, G. Preschera, K. Seibold a, S.F. Parkerb
aDegussa-Hiils AG, P.O.Box 1345, D-63403 Hanau, Germany blSIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. The inelastic neutron scattering technique was utilised in investigating cokes and technical catalysts whose properties caused serious problems when using conventional characterisation methods. The graphiticity and the intranetwork linking of carbon deposits from methane and from acetylene/ethylene/vinyl chloride were compared. [Fe(H20)C15]2was identified as one important intermediate in catalyst deactivation. The formation of macroscopic amounts of strongly bound Pd-CH3 species on hydrogenation catalysts caused unexpected deactivation by changing the adsorption properties of catalysts to sp2-type reactants. 1. INTRODUCTION In spite of the high potential benefits of neutron spectroscopic techniques [1,2] their application for investigations in the field of technical catalysis is still quite limited. For industrial users, neutron spectroscopic techniques may appear to be mostly confined to investigating predominantly basic research questions and not for directly studying properties and problems on materials of immediate technological relevance. The present contribution summarises some recent experimental results that illustrate the usefulness of neutron spectroscopy in characterising various technical catalysts and materials from large scale chemical reactors.
2. METHOD Inelastic Incoherent Neutron Scattering (INS) is a form of vibrational spectroscopy that is complementary to techniques such as Fourier transform infrared spectroscopy. The major advantages of INS are that there are no selection rules and it is possible to quantitatively model the spectra. In addition, the wide spectral range means that low energy vibrational modes down to 20 cm l are routinely measured (translational modes, deformation modes, optical and acoustic phonons in solids etc.) giving information on microstructural properties of catalysts and on materials of catalytic relevance. The high penetration power of neutrons and very different scattering cross sections of certain elements such as H and C or Pd allows the characterisation of the properties of macroscopic amounts of even highly dispersed materials such as activated carbons and carbon blacks (and related precious metal catalysts), cokes and metal-black catalysts even under in-situ conditions. INS thus provides helpful analytical information which is not accessible by and complementary to, conventional methods. A prominent feature of INS is the highselectivity for hydrogen which allows the utilisation of the proton dynamics of catalytic materials as a sensitive probe for microstructural features. This was already used to investigate the proton
3156 related properties of palladium catalysts and of carbonaceous catalyst support materials including the detection of strongly bound molecular-like species on highly porous activated carbon and the formation and decomposition of finely divided palladium hydride on Pd/C hydrogenation catalysts [3,4]. The INS spectra reported here were measured at 20 K using the spectrometer TFXA and its successor TOSCA at the ISIS neutron facility (Chilton, UK). Coke samples from different catalytic processes and active and deactivated palladium catalysts were analysed in thin walled aluminium cans. Additional experimental details were given in related papers [3-6]. 3. APPLICATIONS FROM TECHNICAL CATALYSIS 3.1 Characterisation of cokes from methane and from acetylene/ethylene/vinyl chloride Each INS spectrum in Fig. 1 gives an impression of the integral and hydrogen-related properties of up to 20 g coke per sample. Due to their conductivity or to the high absorbance of electromagnetic radiation of the cokes the application of techniques such as nuclear magnetic resonance or infrared spectroscopy was limited. Figs. 1 A and B show the INS spectra of material from thermally controlled carbon deposition occuring under very unfavourable conditions during the synthesis of HCN from CI-I4 and NH3 at high temperature (Tz 1200~ on Pt/A1203 catalysts. Spectra similar to carbon blacks or soots were expected for this kind of methane-derived coke. However, the spectra perfectly resemble the INS features of pure graphite and of graphitised carbon black as formed at about 3000~ [7,4]. The high purity and sp2 character of the material is indicated by the sharp graphite modes at 112 cm l and the other vibrational features are confined to the region below 1700 cm l. Traces of sp3 carbon [7] are largely missing. This indicates an accidentally very low partial pressure of hydrogen during the deposition and growth of this clean species of coke in spite of the, usually, high amount of hydrogen present in the reaction CH4 + NH3 --* HCN + 3 H2.
(1)
The presence of orientation effects which are known for well-defined graphites are indicated by varying intensifies of additional bands at about 143 and 323 cm 1 inside this coke as well. Completely different properties are exhibited by the low temperature cokes which were collected from technical reactors after premature catalyst deactivation (Fig. 1 C and D). These cokes were suddenly deposited during the selective hydrogenation of acetylene to ethylene on Pd/SiO2 catalysts in the HC1 recycle gas stream of the vinyl chloride process at T < 200~ aider 89 year (C) and 2 years (D) of smooth operation, respectively, leading to completely unusual premature loss of activity. Figs. 1 C and D indicate completely different coking mechanisms and coke properties. Principally, the spectra of polymer-like carbon originating from copolymerisation from acetylene and ethylene were expected. However, Fig. 1 C perfectly matches the spectrum of a well-defined, pure CVD-carbon [7]. More than 50 wt.% of this material are inorganic contaminants including Fe, C1 etc. which are closely linked to the finely divided, highly absorbing carbon component of the deposits. In the INS spectrum the inorganic component of the coke is largely suppressed and the hydrogen containing part of the coke is selectively revealed as a pure CVD material. As seen from Fig. 1 A and B the low energy band at 112 crn1 indicates the presence of sp2 entities originating from catalytically driven coke transformation of amorphous carbon into graphtitic entities of enhanced size under the influence of iron contaminants. The band at 270 crn1 is a measure for the degree of
3157 ,
0 . 0 2 0
,
i
;
,
,
;
,
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0
,
0
|
2 O0
,
I
,
400 energy tror'sfer
I
600
,
I
800
1000
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Fig. 1. INS spectra of cokes from large scale chemical reactors as collected from the catalysts surfaces. A, B: high temperature coke deposited at the surface of Pt/A1203 catalysts. 12, D: low temperature cokes fi~om the surfaces of Pd/SiO2 catalysts. D includes results of a computer simulation of the spectn~ of [Fe(tt20)C15] 2"(dashed line).
3158 cross-linking between sp2 and sp 3 type carbon (bands at 1300-1500 LTI1"1) erltities and, therefore, contains information about the interactions between different carbonaceous components of the bulk of the coke. Such information is of relevance with respect to the feasability and efficiency of a catalyst regeneration treatment under given conditions. In contrast to Fig. 1C the specman of a coke from the same process given in Fig. 1 D, surprisingly, strongly resembles the signals of the well-defined ~species [Fe(H20)C15]2 [8] whose simulated specmnn is also included. Alternative structures would show quite different vibrational spectra. The strongest band at 386 crn~ is assigned to the Fe-OH2 torsional mode indicating an other main reason for catalyst deactivation present. These examples illustrate that under certain conditions extensive growth of pure CVD-type carbon occurs which according to elemental analysis and surface analysis measurements contained significant amounts of well dispersed amounts of iron chlorides and other contaminants. On the other hand the presence of traces of humidity can lead to some corrosion in the HC1 recycle gas stream and to the formation of a [Fe(H20)C15]2 species dominating the whole INS spectnnn of this coke. Such findings on products or intermediates in catalyst deactivation processes may be helpful in investigating the changing mechanisms of coke deposition under varying conditions in more detail. INS as focused on the hydrogen related part of the coke allows a quite different view of partly finely divided, highly contaminated and bulk samples of technical cokes. In spite of the technical origin of the cokes Fig. 1 reveals the presence of well defined components dominating the INS-spectra of these tentatively ill-defined samples: pure oriented graphites, CVD-carbon and one certain iron complex of defined H20 content.
3.2 Catalyst poisoning by methyl groups In a screening study on catalyst deactivation INS was used for investigating various palladium catalysts which showed premature deactivation. The catalysts were used in the partial hydrogenation of-C=C- structures and of C=O-groups to C-OH groups in aromatic and polyaromatic systems of naphthalenes and related applications. In contrast to the common observations in investigating spent catalysts and in spite of using the broad spectrum of the well proven techniques for catalyst characterisation the loss of activity could not be traced back to catalyst degradation, inorganic catalyst poisons, particle growth, enhanced coke deposition, formation of carbides or to any other of the usual aging phenomena. INS was uniquely able to reveal the phenomenon leading to deactivation inthe present case. Fig. 2 compares the INS-spectra of deactivated palladium catalysts measured after cleaning by solvent extraction under strictly constant conditions. The spectrum in Fig. 2 A which was recorded under in-situ conditions under 1.5 bar of hydrogen equilibrium pressure inside the INS-cell is dominated by the strong asynunetric band of [3-paUadium hydride at 464 cm 1 and its first overtone. Atter dehydrogenation (degassing at room temperature) the hydride phase is completely decomposed however some sharp peaks remain indicating the presence of simple molecular like degradation products of geminal substitution at the surface of the catalyst (Fig. 2 B). Furthermore, two broad bands at about 800 and 1200 crnl which previously were hidden under the first overtone of the hydride band are lett which according to elemental analysis have to be assigned to non-extractable organic components. The bands at 800 and 1200 cm 1 were also observed on corresponding active catalysts and were clearly not of importance for the given deactivation phenomenon. At a later state of the catalyst deactivation process the strongly bound molecular-like as well as the strongly adherent presumably polycondensed carbonaceous species were transformed into simple molecular entities as indi-
3159
n
o.o2o
e
r o n
o. OlS
s
/ m o. OlO t r a n
0.005
e r o
J
.
I
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e
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~
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I
,
_
m 0.3 t r a n
0.2
e r
o.i
C
0
500
~000
Energy
1500
Transfer
2000
2500
(cm-l)
Fig. 2. INS spectra of palladium catalysts measured aiter extraction under strictly the same conditions. A: deactivated catalyst measured under 1.5 bar of H2 equilibrium pressure in-sire. B: as A, after dehydrogenation. C" deactivated catalyst [6], D: corresponding active catalyst.
3160 eated by the strong and sharp bands in Fig. 2 C. Detailed computer assisted analyses of the specmnn showed that the strong and sharp bands could uniquely and unambiguously be traced back to methyl groups chemically bound on top to the Pd surface in a C3v site. Reference spectra of the deactivated palladium catalyst as received from the technical application [6] revealed that the rotational modes of methyl groups of weakly adsorbed and completely extractable solvent molecules and reactants ranged at 237 em"l The signal at 301 em l dominated the spectrum of the catalyst before and after extraction. So the INS signal of the methyl groups is shifted from about 237 em ~ (extractable molecular species) up to about 301 em "l (surface-bound Pd-CH3). A perfect correlation between the normalised INS intensities of the Pd-CH3 peaks as measured on different catalyst samples and the degree of catalyst deactivation was observed. The unexpected formation of considerable amounts of the strongly bound Pd-CH3 species changed the adsorption properties of the catalysts surface for adsorption of sp2 type reactants and, therefore, changed the catalyst performance. It is remarkable that on the surfaces of deactivated technical catalysts well-defined, simple molecular structures are reproducibly detected which hitherto have only been revealed in surface science experiments. INS was exclusively able to supply this information.
4. CONCLUSION INS is a very helpful and versatile technique for studying problems in industrial heterogeneous catalysis. INS certainly deserves a much higher profile and more fi'equent use by investigators and practitioners in the chemical industry. ACKNOWLEDGEMENT The Rutherford Appleton Laboratory (Chilton, UK) is thanked for access to neutron beam facilities. REFERENCES
1. T. J. Udovic and R. D. Kelley in Hydrogen Effects in Catalysis, Eds. Z. Paal and P.G. Menon, M. Dekker, New York 1988 p. 167. And literature cited therein. 2. H. Jobic in Catalyst Characterisation, Eds. B. Imelik and J.C.Vedrine, Plenum Press, New York 1994 p. 347. And literature cited therein. 3. P. Albers, R. Burmeister, K. Seibold, G. Prescher, S.F. Parker, D.K. Ross, J. Catal., 181 (1999) 145. 4. P. Albers, G. Prescher, K. Seibold, D.K. Ross, and F. Fillaux, Carbon, 34 (1996) 903 and Carbon, 37 (1999) 437. 5. P. Albers, S. B6sing, G. Prescher, K. Seibold, D.K. Ross, S.F. Parker, Appl. Catal. A: General, 187 (1999)233. 6. P. Albers, H. Angert, G. Prescher, K. Seibold, S.F. Parker, Chem. Comm., No. 17 (1999) 1619. 7. J. Waiters, R.J. Newport, S.F. Parker, W.S. Howells, J. Phys.: Condens. Matter, 7 (1995) 10059. 8. S.F. Parker, K. Shankland, J.C. Sprunt, U.A. Jayasooriya, Spectrochim. Acta, A 43 (1997i 2333.