titania monolayer catalysts

r~

APPLIED CATALYSIS A: GENERAL

ELSEVIER

Applied Catalysis A: General 157 (1997) 91-103

Preparation and properties of vanadia/titania monolayer catalysts G.C. Bond Department of Chemistry, Brunel University, Uxbridge UB8 3PH, UK

Abstract Work performed on vanadia/titania catalysts at Brunel University over the past 20 years is summarised, and the thinking that led to the concept of reactive oxide monolayers is described. Both impregnation and grafting techniques lead to monolayers, and their structure, and that of the supramonolayer region is revealed through characterisation by a number of physical techniques. Factors responsible for the stability of monolayer systems are considered, and the basis of their activity in selective oxidations is discussed. Keywords: Vanadiamonolayers;Titania; o-Xylene,oxidationof; Isopropanoldecomposition

1. Historical introduction Our first studies of the vanadia/titania system [1] concerned in particular the solid-state reaction that occurred between the components at elevated temperatures (_>950 K). At the melting temperature of V205 (963 K) there began a reaction in which there was an irreversible weight loss due to evolution of 02, a change in the support phase from anatase to rutile, a change of colour to black, a loss of surface area, and the dissolution of a small amount of V 4+ ion into the support lattice. The relevance of impurities such as Na +, K + and P205 was noted and discussed. Materials subjected to the high temperature treatment were quite selective (~60%) for the oxidation of butadiene to maleic anhydride. The structural changes which we had observed are in fact one of the causes of deterioration of performance of industrial catalysts for o-xylene oxidation, as they can occur slowly in the hot spots that develop in the catalyst bed. After Dr. C. Cronan had with some difficulty assembled and tested apparatus for the study of o-xylene oxidation to phthalic anhydride (PA), Dr. Zoltan Schay began to investigate this reaction, with some emphasis on the use of basic promoters, as 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P l l S0926- 860X(97)00024-0

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G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

described in the patent literature, to improve selectivity. He was succeeded by Dr. Peter K6nig, also from the Institute of Isotopes of the Hungarian Academy of Sciences, and it was during the year-long visits to these talented scientists that we began to think deeply about the desirable structure of a highly selective catalyst. There was little difficulty in preparing samples that gave 60--65% selectivity to PA at high conversion, but industrial reactors operated with at least a 10% greater selectivity. One experiment was crucial in determining the direction of our thinking. In order to lessen the rate shown by a very active sample, we decided to lower the amount used, but to keep the bed volume the same by diluting it with a larger amount of the pure titania support; similar procedures are of course often used with other systems. The procedure is predicated on the assumption that the support itself has no activity, so we were considerably surprised to find that the mixture of catalyst and support showed lower selectivity to intermediate oxidation products, at equivalent conversion, than did the catalyst by itself. This implied that uncovered support could effect non-selective oxidation of intermediates such as o-tolualdehyde (TAL) and phthalide (PL), and hence that the ideal catalyst might comprise a thin coherent layer of vanadia totally covering the titania support, and allowing no Ti 4+ cations or Ti-OH groups to show. The question then became how such a catalyst might be constructed.

2. Preparation and reactivity of vanadia monolayers on titania formed by a vapour-phase process At this time we became aware of work performed in what was then East Germany on the reaction of silanol (Si-OH) groups with vanadium oxychloride (VOC13) [2]: what took place was in fact a hydrolysis of the V-C1 bond by Si-OH groups, leading to the chemical bonding of the vanadium atom to the surface. The stoichiometry of the initial step was not established, but the process would be represented as either WOC13 + 3SiOH ~ 3HC1 + VO(SiO)3

(1)

VOCI3 + 2SiOH ~ 2HC1 + VOCI(SiO)2

(2)

or

We therefore decided to try to apply this procedure to obtain a coating of vanadium species over the surface of titania. We were by no means certain that we should succeed, for several reasons. (i) Titania is a very different substance from silica; it is crystalline, anatase and rutile being the commonly encountered forms, but particularly the hydroxyl groups are much less stable, dehydration beginning at about 470 K. (ii) The HC1 might chlorinate Ti-OH groups which had not yet reacted with VOC13,and because of the stability of Ti-C1 bonds compared to Si-C1

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

93

100

625K

BOc

/

o 60e)

~- 4 0 k o

582K ~_

0

ao

.

/,,

60

s,,(mi x)

"~ ..~,

~ 4O 20

0

1

2 3 4 5 Number of VOCI3 treatments

6

Fig. 1. Performance of VOa/TiO2 catalysts prepared by vapour-phase grafting for selective oxidation of o-xylene. Dependence of number of VOCI3 treatments on (A) conversion at 582 and 625 K, and (B) product selectivities at 20% conversion, and maximum SpA [3].

Table 1 Comparison of a monolayer catalyst with unpromoted and doubly promoted impregnation catalysts for the selective oxidation of o-xylene at low and high conversions [3] SpL

SpA

SCOz

~Si d

11.7 9.7 13.6

5.6 5.5 8.3

18.5 20.0 14.3

34.7 28.2 27.2

70.5 63.6 63.4

0.4 0.4 0.6

0.2 <0.1 0.3

75.0 64.6 67.9

24.7 29,2 26.9

100.3 94.3 95.7

Catalyst

Conversion (%)

T (K)

STAL

Monolayera Unpromotedb Doubly promotedc

20 20 20

563 555 557

Monolayer~ Unpromotedb Doubly promotedc

99 99 99

611 606 585

a 1.7% V2Os/TiO2 prepared by grafting. b 10% V205/TiO 2 prepared by impregnation. c 10% V2Os+2% P205+2% Rb20 prepared by impregnation. d Sum of all detected products.

94

G.C Bond~Applied Catalysis A: General 157 (1997) 91-103

bonds this might impede the progress of the reaction. We therefore decided [3] to undertake the vapour-phase reaction of VOC13with a hydroxylated titania surface, using carefully controlled conditions of prior drying (4 h at 413 K in air) so as to remove all physically absorbed water, which we did not wish to have been present, without initiating the removal of Ti-OH groups as water. The advantage of such a method to which the name grafting came subsequently to be applied was that it should not be possible to obtain more than a single layer of vanadium species (VOx), the concentration of which would be controlled by the stoichiometry of the reaction forming them. Thus we came to speak of a vanadia/titania (VOfl'iO2) monolayer catalyst. In fact our first experiment [3] was to treat a dehydroxylated anatase surface with a stream of VOC13vapour; treatment in dry air at 720 K for 3 h should have been sufficient to remove all Ti-OH groups, but nevertheless we obtained after calcination 0.71 wt% V205 on a support of 9.8 m 2 g-1. We then re-hydrated the surface, and performed another VOC13treatment, and calcination; this cycle was carried out six times, the final V205 content being 1.74 wt%, and after each treatment the oxidation ofo-xylene was measured at a series of temperatures: some of the results are shown in Fig. 1, and in Table 1. They show the following features. (1) There is a progressive improvement in performance until at least the fifth treatment, this being more clearly demonstrated at high conversion. (2) The maximum selectivity to PA is higher than that shown by conventional catalysts prepared by impregnation, being 75% after the sixth treatment (Table 1). (3) The catalyst made by grafting is not superior to the others at low conversion (Table 1), but with all of them the totality of recovered products, including CO2, at 20% conversion is only about 63-70% of the o-xylene fed. This feature, which has been observed on numerous later occasions by ourselves [4-7] and others [8] is attributed to the formation of a polymer (probably a dimer [6]) of o-xylene which either remains on the surface, acting as a poison, or leaves the surface only to condense in the reactor outlet. It is thought that this residue can lead only to CO2, and that it is substantially responsible for non-selective oxidation. The finally obtained catalyst was employed for a detailed study of the reaction kinetics of o-xylene oxidation [3].

3. Preparing vanadia on titania monolayers by liquid-phase grafting Krnig's work established beyond doubt that a single atomic layer of a reactive oxide could act effectively in selective oxidation catalysis. This conclusion was not immediately accepted, and one distinguished scientist expressed grave reservations about the wisdom of working on such systems. Even now it is sometimes said that for selective oxidation one needs a bulk phase to provide a reservoir of oxide ions to replenish a reduced surface, Concern about the long-term stability of the monolayer under reaction conditions was also voiced. However it is now firmly

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

95

established that a VOx monolayer is capable of sustained and highly selective oxidation of o-xylene to PA; such a structure is not however the desideratum for cognate reactions such as oxidation of n-butane to maleic anhydride. We should now give further thought to exactly what we mean when we speak of a VO~gTiO2 monolayer. How is it defined? One may relate the surface concentration of VOx species formed in a given preparation to the number of VO2.5 units present in a single lamella of crystalline V205 of the same area as the support. Calculations of this type [3,9,10] show that the 'theoretical monolayer equivalent' corresponds to 0.145 wt% V205 per m 2 of anatase surface. Observed VOx concentrations can then be expressed as fractions of the equilvalent monolayer. The final VOC13 treatment made by K6nig (see above [3]), having 1.74% V205 per 9.8 m 2 anatase thus corresponds to a little more than a monolayer, a possible explanation being discussed later. Single grafting preparations typically give about 0.1 +0.01% V205 per m 2 anatase, i.e. about 0.70 of a monolayer equivalent, but lower fractions are found with the high area Degussa P-25, which may however contain traces of other oxides and may have some rutile at the surface. When a complete monolayer is present, all Ti-OH groups capable of reacting will have been removed. However, as was shown many years ago in another context [11], pairwise elimination of sites as in Eq. (2) from an infinite square array will leave about 8% of single sites; elimination of triplets (Eq. (1)) would leave a larger fraction of singletons. However, the quantitative determination of low concentrations of Ti--OH groups is not easy. It is also possible to express surface composition in terms of a V/Ti ratio; this is in many ways the most useful, as it serves to indicate the likely stoichiometry of the surface complex. Information on the concentration of VOx species on a number of supports besides titania is available [101. The vapour-phase (CVD) method used by KOnig and others [10] is neither speedy nor efficient, in that repeated treatments are needed to achieve maximum VO~ content. The likely explanation is that the calcination and re-hydroxylation steps do not effectively break all Ti-C1 bonds formed by chlorination of Ti-OH groups. Dr. Katherine Briickman [9] therefore tried to prepare grafted catalysts by shaking the support in organic solutions of VOC13. The use of this very simple method met with immediate success; benzene and toluene were suitable solvents, and V205 contents of 1.20-1.34 wt% V205 on anatase of 9.8 m 2 g - 1 were obtained (i.e. 85-95% of a monolayer equivalent). It is possible that the presence of the solvent controlled the chlorination of Ti-OH groups by HC1, but the high levels of VOx attained also owe something to the Law of Mass Action. It was noted that after washing to remove unreacted VOC13, treatment with water led to the detection of HCI; it was therefore concluded that Eq. (2) rather than Eq. (1) represented the reaction stoichiometry. Most satisfactorily, selectivities to PA in the high 80's at near full conversion were usually seen; optimisation of the variables of the method led to a selectivity of 88.5% at 99% conversion. A selection of the results obtained in this work is shown in Table 2.

G.C. Bond/Applied Catalysis A: General 157 (1997) 91-103

96

Table 2 Effect of varying conditions of liquid-phase grafting on performance of VOfl'iO2 catalysts for selective oxidation of o-xylene [9] Code

Time (h) a

T (K) a

[V205] (wt%)

SpA(max.) (%)

Tmax(K) b

Conv.(max.) (%)

KB25 KB4 KB28 KB15 KB16

0.5 5 14 5 5

313 313 313 283 348

0.90 1.20 0.80 0.33 0.92

59 80 70 65 85

623 593 603 608 583

97 99 93 92 99

a Time and temperature used for treating anatase support with benzene solution of VOCI3. b Reactor temperature and conversion at which SeA is maximum.

In our subsequent work, to be summarised below, more frequent use was made of refluxing with solutions of Vo(OiBu)3 as a method of grafting. This molecule may react with Ti-OH as Vo(OiBu)3 + 2TiOH ~ 2iBuOH + VO(OiBu)(OTi)2

(3)

It was intuitively thought that the alcohol was less likely to attack the surface or to adsorb on it than was HC1, so that reaction to completion would be facilitated. VO(acac)2 has also been used as a grafting agent [10,12].

4. Structure of the monolayer and of the supramonolayer region Having established beyond question the important catalytic properties of VOx monolayers on Tit2, and their apparent superiority to unsupported V205 and to VOx monolayers on other supports, we were naturally interested in establishing the nature of the species involved. We therefore began to apply the techniques of thermal analysis, especially temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), and X-ray photoelectron spectroscopy (XPS) [12]. It is worth stressing the value, indeed the necessity, of using as many techniques as possible, and certainly more than one, before attempting to arrive at conclusions about structures. The literature contains too many examples of where incorrect models have been proposed on the basis of only one (or sometimes even no) experimental technique. Each technique has its strengths and its limitations, and it is only when the results of all can be integrated into a single model that one can have any confidence in the result. Furthermore physiochemical methods are much more informative when comparing samples that have different vanadia contents, so that changesin spectra can be interpreted in terms of likely alterations in structure; looking at a single sample and trying to understand what it contains often leads to error. Some illustrations of this effect will be given below. Thus far we have emphasised the grafting procedure as the preferred route for obtaining VOx monolayers; however, perfectly respectable catalysts can be made simply by impregnating the titania support with an aqueous solution of ammonium

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

97

metavanadate or preferably the derived oxalato complex [10,12,13]. In this way one may prepare in one step either sub-monolayer or monolayer or multilayer materials and there is probably little difference between the first two and their analogues made by grafting, after the appropriate calcination. In an attempt to obtain perfectly complete monolayers, we have carried out second and subsequent graftings in the liquid phase mode, and have found that further deposition of VOx indeed occurs [9,12]. This is one of the reasons that led us to believe that the monolayer species might be represented as a hydroxo-vanadyl group, viz. =VO(OH), having approximately tetrahedral geometry. There has been some considerable debate as to the form of the vanadia which is in excess of that needed to form the monolayer. The question is important because industrial catalysts usually have 6-8% V2Os for an area of about 10 m 2 g-~, i.e. much more than is required for a monolayer. We can now illustrate how analytical techniques can differentiate between different forms of vanadia. The first informative method is TPR; now the TPR profile of V2Os is complex, and sensitive to experimental conditions such as heating rate [14], because of the number of intermediate phases encountered. By contrast, the VOx monolayer species is reduced in a single step from V 5+ to V 3+, the maximum temperature (Tmax) being about 740 K [4,10,12], which is well below the first peak in the reduction of crystalline V2Os [10,12]. As the vanadia content is increased, a second peak appears, initially as a shoulder at about 810-820 K, but later forming a separate and well-defined peak (Fig. 2); this is due to a form of V2Os that is more easily

. . . .

~

I

~

7"5

5"0

,

573

J

,

873 IlK

•

i 009

1173

Fig. 2. TPR profiles for precursorspreparedby aqueousimpregnationof low-areaTiO2 (anatase):the wt% V205 is indicated,and the profilefor unsupportedV205is shownfor comparison.

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

98

reducible than the bulk compound, the amount of which increases with vanadia content [12]. It is worth noting that many who use TPR fail to quantify peak areas, relying solely on Tmax values to identity surface species: they thereby miss much quantitatively useful information. LRS and especially its FT offspring are very useful in analysing surface species. As the concentration of vanadia on anatase is increased, the bonds characteristic of the support quickly decrease in intensity [12] as the material's efficiency as a Raman scatterer is diminished due to the growing depth of colour [15]. There appears a strong and sharp band at 995 cm-1 attributable to a V=O not present in monolayer VOx, but which starts to appear before the second peak in TPR. This led us to postulate two forms of vanadia in addition to the VOx monolayer species attached directly to the anatase surface, viz. a 'disordered' V205 and 'paracrystalline' V205, the latter taking the form of acicular crystals growing away from the surface, but having the basal plane parallel to it [12]. Both forms appear to contribute to the LRS band at 995 cm -1, but they are distinguished by their different reducibilities in TPR. The merit of examining a range of samples differing in their vanadia contents becomes apparent when quantitative measurements are made of the XPS V/Ti signal intensity radio [13]. Typical plots of this ratio vs. the vanadia content are shown in Fig. 3; the ratio ceases to increase shortly after the monolayer concentration is reached, and there is a quite prolonged plateau before a further increase begins. The interpretation of these results caused us considerable worry; in the end we opted [ 12,13] for a model that involved the growth of 'towers' of the paracrystalline V205 covering only about 10% of the support (or monolayer) I

I

I

0"75 v

0'50

..F -- 0.25

0 0

I 5

I 10 Wt °Io V~Os

I 15

20

Fig. 3. Dependence of V/Ti XPS peak intensity ratio on wt% V205 for catalysts prepared from Degussa P-25 by multiple grafting with either VOCI3 ( 0 ) or VO(iBuO)3 (O). The curve is derived theoretically from a model of 'towers' covering 12.5% of the surface.

G,C. Bond~Applied Catalysis A: General 157 (1997) 91-103

99

I Fig. 4. Illustration of possible structure of VOffl~iO2 catalyst containing more than sufficient V2Os to form a monolayer. Monolayer, black; disordered V205, shaded; paracrystalline V205, white.

surface (Fig. 4). While this model has received some support from other workers, it is not inconceivable that some other model would provide equally good fits with the experimental results. TEM results [16] appear to show the occurrence of 'amorphous' V205 on anatase in the 1-5 monolayer region, but it is not stated whether the materials had been calcined, and it has not been demonstrated quantitatively how this result can be harmonised with the others just described. What is however sure is that the XPS results firmly eliminate the progressive growth of complete coherent layers of V205 over the support surface, since this would mean that the V/Ti ratio would accelerate, and soon approach infinity. Models of this type were proposed by Vejux and Courtine [17] and by Murakami, Miyamoto and their associates [18]; the former, which is predicated on the close structural similarity between the basal plane of V2Os and the anatase (0 1 0) and (1 0 0) surfaces, is still often cited, but unfortunately is not supported by any physical evidence. Chemistry does not always follow the path which we would like it to.

5. Reasons for the stability of vanadia monolayers on titania There has been much discussion in the literature on the reasons for the apparently superior catalytic performance of VOx/anatase monolayers in o-xylene oxidation. It now appears that unsupported V205 may be not much less selective in forming PA than the monolayer catalyst, when prepared in a form of sufficiently high surface area that it can be used in the same kind of temperature range [ 19]. Thus it may be that the essential role of the anatase is to permit formation of a stable VOx monolayer, i.e. to provide essentially complete dispersion of the vanadia on a support that has an adequate but not excessive surface area. Factors determining the stability of supported oxides were considered at an early stage of work in this field [20-22]. Briefly what is required is that the two cations should have similar electronegativities, so that the Ti-O and V-O bonds have similar polarities; in this way the monolayer can act just as an extension of the support lattice. The factors at work can be illustrated by the fate of the supported oxide on

100

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

calcination; in the case of A1203, the V205 dissolves and a compound is formed, while with SiO2 the interaction energy is so low that V205 microcrystals are formed. Thus TiO2 (anatase) is a good compromise between these two extremes; rutile is not as good, but ZrO2 is also satisfactory [23]. It has recently been shown that several catalytically active oxides (i.e. those of V, Mo, Nb and W) dissolve into Bi203 and only form nuggets of active mixed phases after high-temperature calcination [24]. The problem can thus be stated in terms of solid-solid wetting: with SiO2, V205 does not wet the support; with A1203 the interfacial energy is too large, but with TiO2 (anatase) and ZrO2 it is just right. Much attention is now being given to mixed oxides (e.g. TiO2-SiO2) as supports for catalysts to be used for selective NOX removal; review of this area is however beyond the scope of this article.

6. Reactivity of VOx monolayer species The principal industrial application of VOx/titania catalysts is the selective oxidation of o-xylene to phthalic anhydride (PA), a reaction which has been the subject of a number of kinetic investigations [3-6] and reviews [10,26,28]. The value of V205 as a catalyst for this reaction has been known for at least 40 years [25]. It is now well established that the reaction proceeds through a 'rake' mechanism, summarised in Scheme l, in which a number of partially oxidised chemisorbed intermediates exist in equilibrium with gas-phase molecules, such as especially o-tolualdehyde (TAL) and phthalide (PL). In industrial practice, catalysts comprising 6-8% V2Os/titania afford selectivity to PA of at least 75% at conversions close to 100%; minimisation of the amount of PL formed is important because of the cost and difficulty of removing it from the PA. Detailed speculations have been made concerning possible unit steps in this complex reaction [29], in which 12 bonds have to be broken, and 12 formed. It is unrealistic to speak, as some do, of the 'direct' transformation of o-xylene to PA; this is not the way in which chemistry works. Mathematical modelling of experimental results [30] has confirmed that such multiple bond-changes do

o-xyl¢n¢

~.U~CH 3

Q,

o-totualdehyde o-toluic acid

j~ >Q

CHO CH 3

[~

COOH CH 3

phthalide

uLY cH

phthalic anhydride

~

CO,

CO"0

>®

Scheme 1. Simplified form of 'rake' mechanismbasedon five surface intermediates.

G.C. Bond/Applied Catalysis A: General 157 (1997) 91-103

101

H:IHO,iOH []

0% /OH

<-H I

o/V\o

H,o I

0

0z

"I ,

o/V\o

/OH

o/V\o

--

m

< +H

~x.

'

'°

o/V\o

Scheme 2. Proposed structures of catalytic species having single V atoms. The numbers are the oxidation states of the V atoms in the species shown, and the broken lines denote possible points of attachment of atoms connected to the substituent groups on the aromatic ring.

not occur. It is highly likely that the oxidation state of the active VOx groups alternates between 5, 4 and 3, and possible structures and the way in which they are transformed is shown in Scheme 2. Attention has also recently been focussed on the formation of a polymer of o-xylene [3-7], which occurs at low temperatures and low conversions; the presence of Lewis acid sites is implicated in the formation of this surface 'residue' (see also Section 2). The change in the chemical reactivity of the surface of the support when coated with VOx species is clearly indicated by the reaction ofisopropanol [31,32]. Titania (Degussa P-25) and V205 both catalyse dehydration, whereas monolayer VOx species, which are more reactive than V205, give substantial amounts of dehydrogenation. This behaviour has been attributed to the combined effects of adjacent V=O and V - O H groups, not present on the surface of V205. This reaction clearly shows that the surface of the VOx/titania monolayer is quite different from that of V205.

7. Conclusions It is not the purpose of this contribution to attempt a comprehensive review of vanadia/titania catalysts and their applications; it is simply a personal viewpoint, with emphasis on the contributions of our laboratory. A number of reviews are

102

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103

available for anyone wishing to pursue the matter further [12,26-28,33-35]. In particular the volume of Catalysis Today entitled 'EUROCAT Oxide' [33] is strongly recommended, since articles therein describe results obtained by less commonly available techniques such as secondary-ion mass spectrometry (SIMS), 51V nmr and electrical conductivity. Each adds to our knowledge of the structure of the catalysts, the last-named being especially informative. Two conclusions particularly deserve to be stressed. (1) In trying to obtain a structural description of any new catalyst, it is desirable to use as many different techniques of characterisation as possible, and to apply them to a number of different compositions, in order to be reasonably certain of the validity of the proposed model. Purely theoretical conjecture should be kept under control. (2) The structure of a catalyst deduced, as is usual, under ambient conditions, or the conditions of the instruments employed, is not necessarily that of the working catalyst: this is especially true in the field of selective oxidation, where partial reduction, and aggregation, of the active phase may occur during use, and loss of oxygen may take place under UHV conditions. Study of a catalyst while it is operating is never easy, but the next best thing is to examine it after use: there are however few studies of this kind [19], so that this should be a focus for future work.

Acknowledgements I wish to express my best thanks to my associates who have worked on various aspects of the vanadia/titania system over the years: Drs. A.J. Sfirkfiny, C. Cronan, Z. Schay, E K6nig, J. Perez Zurita, S. Flamerz Tahir, R. Shukri and L. van Wijk. I also acknowledge the value of the collaboration with Professor M. Farina Portela of the Instituto Superior T6cnico, Lisbon, and the outstanding contribution of Dr. Cristina Dias to the study of o-xylene oxidation.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

G.C. Bond, A.J. Sfirkfiny and G.D. Parfitt, J. Catal., 57 (1979) 476. W. Hanke, R. Bienert and H.-G. Jerschkewitz, Z. Anorg. Allg. Chem., 414 (1975) 109. G.C. Bond and E K6nig, J. Catal., 77 (1982) 309. C.R. Dias, M.E Portela and G.C. Bond, J. CataL, 157 (1995) 344. C.R. Dias, M.E Portela and G.C. Bond, J. Catal., 157 (1995) 353. C.R. Dias, M.E Portela and G.C. Bond, J. Catal., 162 (1996) 284. G.C. Bond, C.R. Dias and M.E Portela, J. CataL, 156 (1995) 295. R.Y. Saleh and I.E. Wachs, Appl. Catal., 31 (1987) 87. G.C. Bond and K. Briickman, Faraday Disc. Chem. Soc., 72 (1981) 235. G.C. Bond and S.E Tahir, Appl. Catal., 71 (1991) 1. J.K. Roberts, Proc. Cambridge Phil. Soc., 34 (1938) 399. G.C. Bond, J. Perez Zurita, S. Flamerz, EJ. Gellings, H. Bosch, J.G. van Ommen and B.J. Kip, Appl. Catal., 22 (1986) 361. [13] G.C. Bond, J. Perez Zurita and S. Flamerz, Appl. Catal., 27 (1986) 353.

G.C. Bond~Applied Catalysis A: General 157 (1997) 91-103 [14] [15] [16] [17] [18] [19] [20] [21] [22]

103

H. Bosch, B.J. Kip, J.G. van Ommen and P.J. Gellings, J. Chem. Soc., Faraday Trans., 80(1) (1984) 2479. D.N. Waters, Spectrocbim. Acta, 50A (1994) 1833. M. Sanati, L.R. Wallenberg, A. Anderson, S. Jansen and Y.-E Tu, J. Catal, 132 (1991) 128. A. Vejux and E Courtine, J. Solid State Chem., 23 (1978) 93. M. Inomata, K. Mori, A. Miyamoto, T. Vi and Y. Murakami, J. Phys. Chem., 87 (1993) 754. G. Centi, M. Lopez Granados, D. Pinelli and E Trifirb, Stud. Surf. Sci. Catal., 67 (1991) 1. F. Roozeboom, T. Fransen, R Mars and RJ. Gellings, Z. Anorg. Allg. Chem., 449 (1979) 25. G.C. Bond, S. Flamerz and R. Shukri, Faraday Disc. Chem. Soc., 87 (1989) 65. G.C. Bond, in: G. Ertl, H. Kn6zinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. A, in press. [23] J.R. Sohn, S.G. Cho, Y. I1 Pal and S. Hayashi, J. Catal., 159 (1996) 170. [24] N. Arora, G. Deo, I.E. Wachs and A.M. Hirt, J. Catal., 159 (1996) 1. [25] H. Clark and D.J. Berets, Adv. Catal., 9 (1957) 204. [26] V. Nikolov, D. Klissurski and A. Anastasov, Catal. Rev, Sci. Eng., 33 (1991) 319. [27] C.R. Dias, M.E Portela, G.C. Bond, Catal. Rev. Sci. Eng., in press. [28] G.C. Bond, J. Chem. Tech. Biotechnol., 68 (1997) 6. [29] G.C. Bond, J. CataL, 116 (1989) 531. [30] C.R. Dias, M.E Portela and G.C. Bond, J. Catal., 164 (1996) 276. [31] G.C. Bond and S. Flamerz, Appl. Catal., 33 (1987) 219. [32] B. Grzybowska-Swierkosz et al., Catal. Today, 20 (1994) 165. [33] J.C. Vedrine (Ed.), EUROCAT Oxide, Catal. Today, 20(1) (1994). [34] G. Deo, I.E. Wachs and J. Haber, Crit. Rev. Surf. Chem., 4 (1994) 141. 1135] G. Busca (Ed.), Vibrational Spectroscopy of Adsorbed Molecules and Surface Species on Metal Oxides, Catal. Today, 27 (3-4) (1996).