achieved through various spectroscopic methods concerning thermal activation of
the Phillips catalyst, activation of the Phillips catalyst by CO or Al-alkyl
cocatalysts, activation of the Phillips catalyst by ethylene monomer, and modification of the Phillips catalyst by Ti.
2.1 Thermal Activation of the Phillips Catalyst
During the preparation of Phillips catalyst, thermal activation is a crucial stage in
which the chromium oxide is anchored into surface-stabilized chromate species. In
this calcination process, a highly dispersed state of surface-stabilized chromate
species, including mono-, di-, and polychromate, can be achieved through the
redispersion cycles of sublimation, volatilization, spreading, deposition, and stabilization of bulk CrO 3 on a silica support surface [2]. By measurement of molar
ratios of Δ[OH]/[Cr], McDaniel [5] suggested that the initial bonding was
monochromate at 200
C (Δ[OH]/[Cr] ¼ 2), but that the dichromate became dominant at 500
C (Δ[OH]/[Cr] ¼ 1), while polychromates might be formed above
800
C (Δ[OH]/[Cr] < 1). On the basis of DRIFTS and DRS results, Panchenko
et al. [78] confirmed that the reactions of CrO 3 with the silica calcined at 250
C,
400
C, and 800
C dominantly yield monochromates, dichromates, and
polychromates, respectively. However, two unfavorable problems might occur
during the thermal activation process: the calcination-induced reduction of
surface-stabilized Cr(VI) species into lower valence state (+5, +4, or +3) and the
creation of aggregated Cr 2 O 3 (usually in crystallized form) even in an oxidizing
atmosphere (O 2 or dry air). These affect to a great extent the physico-chemical state
of the surface Cr species and thus the properties and performance of the catalyst.
High resolution XPS, which has been demonstrated to be a very powerful method
for a better understanding of the physico-chemical nature of surface chromium
species through monitoring their transformation on Phillips catalysts calcined at
various conditions, has benefitted the investigation of the origins of these two
problems [8, 67, 68, 70, 79, 80].
For the Phillips catalyst calcined in dry air at 800
C for 20 h with 0.4 Cr nm
À2
loading, two oxidation states were found in the XPS measurement [8]. The
first, with a binding energy (BE) of 581.81 eV and a full width at half maximum
(FWHM) of 9.62 eV, was assigned as the surface-stabilized chromate Cr(VI)O x,surf
species with an oxidation state of +6 (atomic concentration 70.4%). The second,
with a BE of 577.21 eV and a FWHM of 4.43 eV, was assigned as an oxidation state
of +3 (atomic concentration 29.6%), which was quite different from the typical
values of the bulk Cr 2 O 3 , strongly suggesting a surface-stabilized and highly
dispersed characteristic of trivalent Cr species chemically bonded to silica surface
[Cr(III)O x,surf ]. Compared with bulk Cr 2 O 3 , the higher BE of Cr(III)O x,surf species
might result from the stabilizing effect of the silica as well as the environmental
effect of neighboring chromate species. The larger FWHM value could be ascribed
to its variety in molecule structure and the heterogeneity of the silica surface. The
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R. Cheng et al.
the Phillips catalyst, activation of the Phillips catalyst by CO or Al-alkyl
cocatalysts, activation of the Phillips catalyst by ethylene monomer, and modification of the Phillips catalyst by Ti.
2.1 Thermal Activation of the Phillips Catalyst
During the preparation of Phillips catalyst, thermal activation is a crucial stage in
which the chromium oxide is anchored into surface-stabilized chromate species. In
this calcination process, a highly dispersed state of surface-stabilized chromate
species, including mono-, di-, and polychromate, can be achieved through the
redispersion cycles of sublimation, volatilization, spreading, deposition, and stabilization of bulk CrO 3 on a silica support surface [2]. By measurement of molar
ratios of Δ[OH]/[Cr], McDaniel [5] suggested that the initial bonding was
monochromate at 200
C (Δ[OH]/[Cr] ¼ 2), but that the dichromate became dominant at 500
C (Δ[OH]/[Cr] ¼ 1), while polychromates might be formed above
800
C (Δ[OH]/[Cr] < 1). On the basis of DRIFTS and DRS results, Panchenko
et al. [78] confirmed that the reactions of CrO 3 with the silica calcined at 250
C,
400
C, and 800
C dominantly yield monochromates, dichromates, and
polychromates, respectively. However, two unfavorable problems might occur
during the thermal activation process: the calcination-induced reduction of
surface-stabilized Cr(VI) species into lower valence state (+5, +4, or +3) and the
creation of aggregated Cr 2 O 3 (usually in crystallized form) even in an oxidizing
atmosphere (O 2 or dry air). These affect to a great extent the physico-chemical state
of the surface Cr species and thus the properties and performance of the catalyst.
High resolution XPS, which has been demonstrated to be a very powerful method
for a better understanding of the physico-chemical nature of surface chromium
species through monitoring their transformation on Phillips catalysts calcined at
various conditions, has benefitted the investigation of the origins of these two
problems [8, 67, 68, 70, 79, 80].
For the Phillips catalyst calcined in dry air at 800
C for 20 h with 0.4 Cr nm
À2
loading, two oxidation states were found in the XPS measurement [8]. The
first, with a binding energy (BE) of 581.81 eV and a full width at half maximum
(FWHM) of 9.62 eV, was assigned as the surface-stabilized chromate Cr(VI)O x,surf
species with an oxidation state of +6 (atomic concentration 70.4%). The second,
with a BE of 577.21 eV and a FWHM of 4.43 eV, was assigned as an oxidation state
of +3 (atomic concentration 29.6%), which was quite different from the typical
values of the bulk Cr 2 O 3 , strongly suggesting a surface-stabilized and highly
dispersed characteristic of trivalent Cr species chemically bonded to silica surface
[Cr(III)O x,surf ]. Compared with bulk Cr 2 O 3 , the higher BE of Cr(III)O x,surf species
might result from the stabilizing effect of the silica as well as the environmental
effect of neighboring chromate species. The larger FWHM value could be ascribed
to its variety in molecule structure and the heterogeneity of the silica surface. The
142
R. Cheng et al.