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Titania: Photocatalysis and SMSI
N5
strength, good light olefin selectivity and high bottoms cracking ability. In a commercial run at 546°C, the product yields (wt.-%) were: dry gas 9.16, LPG 42.00 (C3 = 18.32, C4 14.02, i-C4 5.91), stabilized gasoline 26.60, cracked naphtha 13.49 and coke 0.51 at 86.51% conversion of heavy distillate. The MON and RON of the cracked naphtha were 82.3 and 97.6 respectively. The DCC-II technology for maximum isobutylene and isoamylene production was commercially applied in Jinan Refinery, using a CIP-I type solid acid catalyst with a feedstock of Linshang wax oil mixed with 20% of deasphalted oil [Chin. J. Petroleum Processing and Petrochem., 26 (5) (1995) 1]. It is claimed that the CIP-I catalyst was designed on the following principles: (i) having a low hydrogen transfer activity, increasing the olefin concentration in the products; (ii) using a matrix with high activity, enhancing the primary cracking of heavy oil; (iii) using multiple active components, comprising a large pore zeolite for primary cracking of heavy oil and a mesoporous zeolite for secondary cracking of the gasoline fraction; (iv) activation treatment of mesoporous zeolite, increasing its cracking activity and stability; and (v) adjustment of the ratio of large pore zeolite to mesoporous zeolite, giving consideration to both the olefins and gasoline yields. For DCC-II, two operation modes were used in a 150 kiloton/yr DCC unit: maximum isoolefins with naphtha; and maximum isoolefins with propylene. The product yields (%) were: propylene 12.52, isobutylene 4.57, isoamylene 5.78 and naphtha 40.98 from the former, and 14.43, 4.75, 5.93 and 38.45 from the latter. The DCC-II naphtha produced from both operation modes ex-
applied catalysis A: General
hibited high octane numbers and excellent stability. D.Z. Wang
Titania: Photocatalysis and SMSI
TiO2 and its solid solutions are of considerable interest in solid state chemistry, surface science and catalysis, and their wide range of uses and potential uses include the photocatalytic destruction of organic wastes [A. L. Linsebigler et al., Chem. Rev., 95 (1995) 735]. In this last connection, it has been found that small amounts of Nb205 added to TiO2 powder significantly increase surface acidity and photocatalytic activity [H. Cui et al., J. Solid State Chem., 115 (1995) 187]; possible reasons for this enhancement are provided by a recent study using single crystal rutile films with Nb contents up to 40 at%, deposited by molecular-beam epitaxy onto fully oxidized TIO2(110) substrates [Y. Gao et al, Surf. Sci., 348 (1996) 17]. Because morphology and the presence of surface defects may influence behaviour, the films were characterised in detail using three different diffraction techniques and Atomic Force Microscopy. All had the rutile(110) structure and those with up to 20 at% Nb were atomically smooth, highly ordered and contained Nb in an identical local environment to that of Ti. Thus, as expected on the basis of the bulk phase where NbO2 and TiO~ are mutually and completely soluble, the films were essentially solid solutions with Nb uniformly distributed throughout the near surface region. In terms of electronic structure, the most significant finding was the XPS observation that both types of metal ion were Volume 141 No. 1-2 - - 4 July 1996
N6
present in the 4+ oxidation state, with no evidence for Nb 5+ or T[ 3+. In bulk TiO2, each Ti contributes four valence electrons to six ionic bonds, or 0.67 electrons per bond. Each O contributes four electrons to three bonds, or 1.33 electrons per bond. Thus, each bond has a full complement of two electrons and, when N b 4+ (Nb 4dl) is substituted for Ti 4+, the additional Nb valence electrons cannot go into bonding states and instead are expected to form a Nb-derived nonbonding band. On the fully oxidized (110) surface, there are equal numbers of Ti ions and oxygens in 'bridging' positions, all with a dangling bond due to a reduction in their coordination of one relative to the bulk. A cation dangling state nominally contains 0.67 electrons but this charge is actually transferred to oxygen, leaving all cation dangling bonds empty and all bridging oxygen dangling bonds full. So, when N b4+ replaces Ti 4÷ at the surface, the extra electron must remain Iocalised in a (halffilled) Nb dangling bond state. Both in the bulk and at the surface, electrons in the Nb-derived non-bonding states should be easily excited to form electron-hole pairs or free photoelectrons. Therefore, their presence provides a plausible explanation for the enhanced photocatalytic activity which is observed with mixed Nb2Os/TiO2 powders and which peaks at ~2 at% Nb. Titania-supported Pt catalysts are known to display a number of unusual surface properties, including almost complete suppression of CO and hydrogen adsorption following high temperature reduction. Although these effects are generally ascribed to a strong metal-support interaction (SMSI), controversy remains as
applied catalysis A: General
to which of two types of mechanism may be involved, charge transfer or encapsulation. An investigation of Pt on single-crystalline TIO2(110), undertaken as part of a systematic study of the growth of transition metals on this substrate using XPS and Low Energy Ion Scattering (LEIS), throws some further light on the problem IF. Pesty et al., Surf. Sci., 339 (1995) 83]. The initial growth of Pt was in the form of 3D islands but, on annealing in vacuum above 450 K, the LEIS signal due to Pt disappeared while those for Ti and O were restored to their clean surface values. Platinum, however, had not been removed from the surface region since its signal reappeared rapidly on sputtering with Ne ÷ ions. These observations are consistent with Pt encapsulation by a titanium suboxide TiO× (x _< 1) and XPS confirmed the presence of reduced species, Ti n+ (0 n _< 3), the degree of reduction increasing with increasing annealing temperature. Migration or diffusion of Ti species occurred even when the initial Pt layer completely covered the substrate. Unlike most other known instances of encapsulation, with this system the process occurred in the absence of a reducing atmosphere and at a relatively low temperature. The original interpretation of SMSI emphasised the role of chemical bonding between the metal and the support and ruled out the possibility of encapsulation but the two are probably complementary since, even if blocking of chemisorption sites is a purely geometric effect, there must be some driving force for the migration of the suboxide. In the present case, it is suggested that this stems from an energetically favourable interaction between Pt and reduced species such as Ti 3+ .
Volume 141 No. 1 - 2 - - 4 July 1996
N7
In a wider context, encapsulation of Pt occurs also with substrates not as easily reduced as TiO2, such as AI203, Nb2Os, and to some extent V 2 0 3 and CeO2, but does not occur with SiO2. For various metals on TiO2, if the metal is reactive towards oxygen, it will wet the oxide easily. Thus, more reactive elements towards the left in the transition series, Cr, Hf or Ti itself, tend to grow in almost flat 2-D islands and are not encapsulated on heating. On the other hand, less reactive elements towards the right of the transition series, Fe, Pd, Rh, Pt,
do not wet TiO2 easily. This means that thicker, more three-dimensional metal islands develop. It means also that the suboxide should wet the metal and therefore encapsulation does occur. The one known example which does not fit into this scheme is Au/TiO2. Gold would be expected to behave similarly to Pt but there is a complete absence of encapsulation on annealing in UHV. C.S. McKee
News Brief is a forum for the free exchange of views and opinions. Views and opinions expressed in News Brief do not necessarily reflect those of the Publisher or the Editors. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.
applied catalysis A: General
Volume 141 No. 1-2 - - 4 July 1996
strength, good light olefin selectivity and high bottoms cracking ability. In a commercial run at 546°C, the product yields (wt.-%) were: dry gas 9.16, LPG 42.00 (C3 = 18.32, C4 14.02, i-C4 5.91), stabilized gasoline 26.60, cracked naphtha 13.49 and coke 0.51 at 86.51% conversion of heavy distillate. The MON and RON of the cracked naphtha were 82.3 and 97.6 respectively. The DCC-II technology for maximum isobutylene and isoamylene production was commercially applied in Jinan Refinery, using a CIP-I type solid acid catalyst with a feedstock of Linshang wax oil mixed with 20% of deasphalted oil [Chin. J. Petroleum Processing and Petrochem., 26 (5) (1995) 1]. It is claimed that the CIP-I catalyst was designed on the following principles: (i) having a low hydrogen transfer activity, increasing the olefin concentration in the products; (ii) using a matrix with high activity, enhancing the primary cracking of heavy oil; (iii) using multiple active components, comprising a large pore zeolite for primary cracking of heavy oil and a mesoporous zeolite for secondary cracking of the gasoline fraction; (iv) activation treatment of mesoporous zeolite, increasing its cracking activity and stability; and (v) adjustment of the ratio of large pore zeolite to mesoporous zeolite, giving consideration to both the olefins and gasoline yields. For DCC-II, two operation modes were used in a 150 kiloton/yr DCC unit: maximum isoolefins with naphtha; and maximum isoolefins with propylene. The product yields (%) were: propylene 12.52, isobutylene 4.57, isoamylene 5.78 and naphtha 40.98 from the former, and 14.43, 4.75, 5.93 and 38.45 from the latter. The DCC-II naphtha produced from both operation modes ex-
applied catalysis A: General
hibited high octane numbers and excellent stability. D.Z. Wang
Titania: Photocatalysis and SMSI
TiO2 and its solid solutions are of considerable interest in solid state chemistry, surface science and catalysis, and their wide range of uses and potential uses include the photocatalytic destruction of organic wastes [A. L. Linsebigler et al., Chem. Rev., 95 (1995) 735]. In this last connection, it has been found that small amounts of Nb205 added to TiO2 powder significantly increase surface acidity and photocatalytic activity [H. Cui et al., J. Solid State Chem., 115 (1995) 187]; possible reasons for this enhancement are provided by a recent study using single crystal rutile films with Nb contents up to 40 at%, deposited by molecular-beam epitaxy onto fully oxidized TIO2(110) substrates [Y. Gao et al, Surf. Sci., 348 (1996) 17]. Because morphology and the presence of surface defects may influence behaviour, the films were characterised in detail using three different diffraction techniques and Atomic Force Microscopy. All had the rutile(110) structure and those with up to 20 at% Nb were atomically smooth, highly ordered and contained Nb in an identical local environment to that of Ti. Thus, as expected on the basis of the bulk phase where NbO2 and TiO~ are mutually and completely soluble, the films were essentially solid solutions with Nb uniformly distributed throughout the near surface region. In terms of electronic structure, the most significant finding was the XPS observation that both types of metal ion were Volume 141 No. 1-2 - - 4 July 1996
N6
present in the 4+ oxidation state, with no evidence for Nb 5+ or T[ 3+. In bulk TiO2, each Ti contributes four valence electrons to six ionic bonds, or 0.67 electrons per bond. Each O contributes four electrons to three bonds, or 1.33 electrons per bond. Thus, each bond has a full complement of two electrons and, when N b 4+ (Nb 4dl) is substituted for Ti 4+, the additional Nb valence electrons cannot go into bonding states and instead are expected to form a Nb-derived nonbonding band. On the fully oxidized (110) surface, there are equal numbers of Ti ions and oxygens in 'bridging' positions, all with a dangling bond due to a reduction in their coordination of one relative to the bulk. A cation dangling state nominally contains 0.67 electrons but this charge is actually transferred to oxygen, leaving all cation dangling bonds empty and all bridging oxygen dangling bonds full. So, when N b4+ replaces Ti 4÷ at the surface, the extra electron must remain Iocalised in a (halffilled) Nb dangling bond state. Both in the bulk and at the surface, electrons in the Nb-derived non-bonding states should be easily excited to form electron-hole pairs or free photoelectrons. Therefore, their presence provides a plausible explanation for the enhanced photocatalytic activity which is observed with mixed Nb2Os/TiO2 powders and which peaks at ~2 at% Nb. Titania-supported Pt catalysts are known to display a number of unusual surface properties, including almost complete suppression of CO and hydrogen adsorption following high temperature reduction. Although these effects are generally ascribed to a strong metal-support interaction (SMSI), controversy remains as
applied catalysis A: General
to which of two types of mechanism may be involved, charge transfer or encapsulation. An investigation of Pt on single-crystalline TIO2(110), undertaken as part of a systematic study of the growth of transition metals on this substrate using XPS and Low Energy Ion Scattering (LEIS), throws some further light on the problem IF. Pesty et al., Surf. Sci., 339 (1995) 83]. The initial growth of Pt was in the form of 3D islands but, on annealing in vacuum above 450 K, the LEIS signal due to Pt disappeared while those for Ti and O were restored to their clean surface values. Platinum, however, had not been removed from the surface region since its signal reappeared rapidly on sputtering with Ne ÷ ions. These observations are consistent with Pt encapsulation by a titanium suboxide TiO× (x _< 1) and XPS confirmed the presence of reduced species, Ti n+ (0 n _< 3), the degree of reduction increasing with increasing annealing temperature. Migration or diffusion of Ti species occurred even when the initial Pt layer completely covered the substrate. Unlike most other known instances of encapsulation, with this system the process occurred in the absence of a reducing atmosphere and at a relatively low temperature. The original interpretation of SMSI emphasised the role of chemical bonding between the metal and the support and ruled out the possibility of encapsulation but the two are probably complementary since, even if blocking of chemisorption sites is a purely geometric effect, there must be some driving force for the migration of the suboxide. In the present case, it is suggested that this stems from an energetically favourable interaction between Pt and reduced species such as Ti 3+ .
Volume 141 No. 1 - 2 - - 4 July 1996
N7
In a wider context, encapsulation of Pt occurs also with substrates not as easily reduced as TiO2, such as AI203, Nb2Os, and to some extent V 2 0 3 and CeO2, but does not occur with SiO2. For various metals on TiO2, if the metal is reactive towards oxygen, it will wet the oxide easily. Thus, more reactive elements towards the left in the transition series, Cr, Hf or Ti itself, tend to grow in almost flat 2-D islands and are not encapsulated on heating. On the other hand, less reactive elements towards the right of the transition series, Fe, Pd, Rh, Pt,
do not wet TiO2 easily. This means that thicker, more three-dimensional metal islands develop. It means also that the suboxide should wet the metal and therefore encapsulation does occur. The one known example which does not fit into this scheme is Au/TiO2. Gold would be expected to behave similarly to Pt but there is a complete absence of encapsulation on annealing in UHV. C.S. McKee
News Brief is a forum for the free exchange of views and opinions. Views and opinions expressed in News Brief do not necessarily reflect those of the Publisher or the Editors. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.
applied catalysis A: General
Volume 141 No. 1-2 - - 4 July 1996