Surface Science 169 (1986) 289-298 289 North-Holland, Amsterdam

M E T A L - S U P P O R T I N T E R A C T I O N IN B E N Z E N E H Y D R O G E N A T I O N OVER Pt-TiO2: INFLUENCE OF 0 2 AND UV

Joseph C U N N I N G H A M and Ghas san H. A L - S A Y Y E D

Chemistry Department, University College Cork, Cork, Ireland

Accepted for publication 3 December 1985

A significant inhibiting effect of prior high temperature reduction (HTR) upon the activity of Pt-TiO 2 for the selective hydrogenation of benzene to cyclohexane at 293 and 373 K is reported. Negative results are reported from experiments designed to test whether the SMSI effect realised in this [C6H 6 + H 2 +Ar]/Pt-TiO 2 system in the absence of oxygen responded to UV-illumination in the manner expected if band-bending at the Pt-TiO2 microinterfaces contributed significantly to the inhibiting effect of HTR. Extent of reversal of the SMSI effect attainable by exposure to oxygen in the dark at room temperature was much enhanced under UV-illumination. A role is suggested for oxygen photoadsorption onto the surface of Pt-TiO 2 in the HTR condition.

1. Introduction

Phenomena indicat ive of s t rong m e t a l - s u p p o r t in terac t ions (SMSI) have been repor ted for m a n y systems compr i s ing G r o u p VI I I meta ls d ispersed upon reducib le meta l oxides [1-3]. Fo l lowing a so-cal led high t empera tu re reduct ion ( H T R ) - involving p re reduc t ion in H 2 or CO for some hours at T >/773 K - m a n y systems inc luding P t - T i O 2 and P t - A 1 2 0 3 evidence representa t ive SMSI phenomena . These include d iminu t ion of the capac i ty for chemisorp t ion of H e or CO in the H T R condi t ion [1-4], and also s t rong inh ib i t ion of the ca ta ly t ic ac t iv i ty of the suppo r t ed meta l for hydrogena t ion [5,6], dehyd rogena t i on [3e,4b], and hydrogenolys i s react ions [3b,4b,7]. "E lec t ron ic - type" factors in which some t ransfer of e lectrons is envisaged to occur f rom the p re reduced meta l oxide suppor t to the small par t ic le(s) of metal suppor t ed thereon - represent one of several mechanis t ic concepts b rought forward in efforts to expla in the observed deac t iva t ing effects of p r io r H T R [8] upon the surface p roper t i e s of the m e t a l - s e m i c o n d u c t o r systems. Ear ly jus t i f i ca t ions advanced for the p roposed t ransfer of e lectrons f rom the n- type semiconductor , TiO2, to metal l ic p l a t i num rested largely upon expec ta t ions of a higher Fe rmi level in the former, when reduced, and of a consequent ia l dr iv ing force for e lectron- t ransfer f rom TiO 2 to Pt. Schot tky bar r ie r voltages are known to arise across some macroscop ic interfaces f rom this t endency to equal ise the Fe rmi level

0039-6028 /86 /$03 .50 © Elsevier Science Publ ishers B.V. ( N o r t h - H o l l a n d Physics Publ ishing Divis ion)

290

Ecb

Evb

J. Cunningham, G.H. Al-Sayyed / Metal support interaction

. . . . . . . .

~ Ecs --~;-- Fc,

t s

E v s

Evs

Ef

- Esa

Fig. 1. Electronic energy levels adjacent to non-irradiated (solid lines) and irradiated (broken lines) gas/n-type semiconductor interface and their relationships to energy levels within the semiconductor. The magnitude of the Schottky barrier voltage and upward band bending resulting from excess negative charge on the surface is shown as V S for the non-irradiated and 1/* for the irradiated interface. The energy level corresponding to the bottom of the conduction band at the surface, Ecs, is located above that in the bulk, Ecb , and similar band-bending is shown for the valence band at the surface, Evs. Energies of the band gap, (Eg) and Fermi level, El, are also depicted.

wi th in the bulk of a semiconduc to r and a metal [9]. A l though impor t an t differences are evident be tween the P t - T i O 2 smal l -par t ic le systems and the macroscop ic m e t a l - s e m i c o n d u c t o r interfaces for which the concepts of band- bend ing and Schot tky barr iers were ini t ial ly developed, and a l though measure- ments of Schot tky bar r ie r heights in some real systems have fai led to reveal the expected dependence upon Fe rmi level differences [9b,10], it was nevertheless no t u n c o m m o n when our present s tudy commenced to f ind b a n d - b e n d i n g at the smal l -par t ic le T i O z - P t interfaces represen ted in the manner of fig. 1 [11]. N o r was it u n c o m m o n to f ind signif icant emphas is p laced upon charge- t rans- fer, in this or o ther manner , be tween the p l a t i num and TiO 2 par t ic les as an exper imenta l ly observab le a rb i te r of the SMSI condi t ion [11].

The s t rategy under ly ing the present s tudy at its onset der ived f rom pub- l ished evidence [12] that s i tuat ions of the type dep ic ted in fig. 1 can be apprec iab ly a l tered by inc idence of a flux of pho tons having wavelengths inside the band -edge of the semiconduc tor (in this work TiO 2 with edge at 390 nm). Pho togenera t ion of e l e c t r o n - h o l e pai rs within T iO z in the region of b o u n d a r y layer po ten t ia l can: (i) lead to local iza t ion and t r app ing of holes at a negat ively charged T iO 2 interface, and (ii) to a co r respond ing number of

J. Cunningham, G.H. Al-Sayyed / Metal-support interaction 291

electrons, photo-promoted into the conduction band, causing some neutraliza- tion of ionized donors within the space-charge region. Photo-diminution of pre-existing band-bending via such photo-electronic processes had indeed been identified at various UV-illuminated n-type semiconductor interfaces [13]. It thus appeared that, if a situation such as fig. 1 made the dominant contribution to SMSI, some photoreversal of SMSI should become possible in favourable conditions as a consequence of the indicated, predominantly pho- toelectronic, processes. Possibilities for attaining the requisite favourable con- ditions with TiO 2 as the semiconducting support for Pt particles appeared good, in view of the facile photogeneration of electron-hole pairs by near-UV illumination (band gap = 3 eV---390 nm), and of evidence that oxidisable species at the TiO 2 surface can capture a significant fraction of the photogen- erated holes [12a,14]. An experimental test of these predictions is made in this paper: firstly, by the experimental realization of a sizeable SMSI-type effect in the hydrogenation of benzene vapour [5] in a continuous-flow nficrocatalytic reactor over Pt-TiO2 and secondly, by use of this system, whilst in the SMSI condition, to look for reversal of SMSI by photoelectronic effects in the manner of fig. 1.

2. Experimental

Materials: The P t -T iO 2 material used as catalyst for the present series of experiments featured a loading of 0.5% Pt dispersed upon finely-divided TiO 2 (Degussa P25). Evidence has been given in previous work [15a] that such 0.5% P t -T iO 2 material, when treated in the manner indicated [17,18], feature a reproducible high dispersion of platinum, with an average of one Pt particle of size 2 nm per TiO 2 particle of size 27 + 13 nm. Low temperature reduced (LTR) surfaces were achieved by the following sequence of exposures to flowing gases at 1 atm in situ in the quartz microcatalytic reactor: (a) oxygen for 3 h at 773 K; (b) pure argon for 1 h during which the sample temperature was lowered to 473 K; and (c) H 2 for 3 h at 473 K. The LTR materials resulting from these treatments were first used to identify the level of activity of the 0.5% P t -T i O 2 material for benzene hydrogenation at 373 K in the absence of an SMSI effect. Full details have been given elsewhere [15a] of the differential flow microcatalytic reactor and associated gas chromatographic analyses by which the activity of the samples for conversion to product was measured. Benzene was AR grade and was redistilled before introduction into a saturator filled with molecular sieve pellets. This served to introduce C6H 6 vapour at a constant partial pressure of 40 Torr into the flow of argon carrier gas, which was also admixed with 380 Torr of H 2 prior to entry into the continuous-flow microcatalytic reactor. The P t -T iO 2 materials were then converted into the H T R form by repeating step (a) exactly, followed by steps

292 J. Cunningharn, G.H. AI-Sayyed / Metal-support interaction

(b) and (c) at 773 K rather than at 473 K. Conversion to product over the H T R surface were again measured using conditions identical to those for the sample in the LTR condition.

The foregoing comparisons were first routinely carried out without UV- illumination. Tests for possible effects of UV were then made by exposing the sample in situ within the quartz reactor to the output (~ > 310 nm) from a 125 W medium pressure mercury arc lamp, care being taken to maintain the sample temperature at normal reaction temperature during such exposures. Influence of illumination upon sample activity in the HTR condition was tested in two ways: (i) by exposure to UV during the flow of oxygen-free reactants and looking for changes in the activity profile relative to that observed without illumination; and (ii) by exposure to UV, and to gaseous oxygen in some cases, after the H T R treatment but prior to introducing the flow of oxygen-free reactants over the sample in the dark.

3. Results and discussion

High activities and selectivities >~ 99% for the hydrogenation of l~enzene to cyclohexane were observed over the LTR samples at 373 and 298 K. Results demonstrating the attainment of a sizeable SMSI effect upon this selective hydrogenation over (0.5% Pt-TiO2)~x R at 373 K are summarised as activity- profile plots in fig. 2. The rather low conversions and relatively flat nature of the activity-profile observed over the non-illuminated [C6H 6 + H 2 +

Ar]g/ (Pt -TiO2) HVR interface, are illustrated by plot (a) of fig. 2. These may be compared with an order-of-magnitude increase in the conversions attained in identical experimental conditions over the non-illuminated [C6H6+ H2+ Ar]g/(Pt-TiO2)Lx R interface (plot (b) of fig. 2). It may be noted that this order-of-magnitude difference in activity of the P t -T iO 2 material in the HTR and LTR conditions remained rather constant at times-on-stream >~ 100 min. This difference, which was reproduced with several samples, is attributed to an SMSI-type inhibition of hydrogen chemisorption upon the sample when in the H T R condition. Plot (b) of fig. 2 illustrates a reproducible feature of samples in the LTR condition viz. that prior to attaining the pseudo-steady-state activity level at times >/100 min, the samples exhibited activities which were higher by up to 50%. This can be attributed to a rapid deactivation of about one-third of the initially active sites, but an exploration of the origins of such deactivation lay outside the objectives of the present study.

In the case of benzene hydrogenation over the (0.5% Pt TiO 2) HXR material, good linearity was achieved in plots of conversion versus reciprocal space velocity (cf. fig. 3). This observation, when allied to the low conversions characteristic of this material, confirmed truly differential mode of operation of hydrogenation in this system and ruled out significant influence of

J. Cunningharn, G.H. Al-Sayyed / Metal-support interaction 293

\ \o

~ o e • ,~o

:\ ".,,.,, \ 1 5 o

" ! . . . . . ,.. ° 2 o .

~ . A A - O ~ o ~

U • • • • •

I I I I

100 300 Flow Time rain

Fig. 2. Activity profiles, for indicated times on-stream at 373 K, showing percentage conversions to cyclohexane observed with a reagent-flow (comprising: P(C6H6) = 40 Torr, P(H2) = 380 Torr and P(Ar) = 340 Torr) over samples of a [0.5% Pt-TiO 2 -B] type catalyst which had respectively been subjected to the following pretreatments: (a) Catalyst was activated by HTR (see text for details). (b) Catalyst was activated by LTR treatment (see text for details). (c) Same as (a) plus pre-illumination for 72 h in hydrogen flow at 373 K. (d) Same as (a) plus exposing the catalyst to a flow of argon at 373 K for 24 h. (e) Same as (a) plus pre-illumination in a flow of argon at 373 K for 2 h.

t . /

3

2 o / °

, / 0

i s p

1/F R

0

/o

x lO 2 cm-3min

Fig. 3. Plot of percentage conversion to cyclohexane achieved at 373 K at the (C6H 6 + H 2 + Ar)g/(0.5% Pt-TiO2) ~ interface as a function of reciprocal of the reactant flow rate.

294 J. Cunningham, G.H. AI-Sayyed / Metal-support interaction

mass-transport limitations. Conditions were thus ideally suited to test whether UV-illumination of the [C6H 6 + H 2 + Ar]g/(Pt-TiO2)HT R interface could en- hance the rate-determining process in the hydrogenation process and so bring about some reversal of the demonstrated inhibiting effect of SMSI on conver- sion to cyclohexane. However, no significant upward displacement of the activity profile for conversion to cyclohexane, relative to that shown as plot (a) of fig, 2, resulted whenever P t -T i O 2 was given the standard H T R pretreatment and the [C6H 6 + H 2 + Ar]g/ (Pt -TiO2) S interface was continually exposed to UV-illumination whilst again taking an activity profile in identical flow conditions. Neither was any product other than cyclohexane observed. The extent to which these negative observations provided definitive evidence against the band-bending depicted in fig. 1 as the dominant component of the SMSI condition, and against susceptibility of the latter to photo-electronic reversal was, in our view, qualified by a question as to whether efficient recombination of electron-hole pairs at P t / T i O 2 microinterfaces might not render ineffective the escape of sufficient electrons into the bulk (and holes to the surface) of the TiO 2 to affect any pre-existing band-bending. Two experi- ments were carried out to examine this question, the first of which was prompted by the contrast between the above absence of photocatalytic evi- dence for escape of a significant fraction of photoholes to UV-illuminated [C6H 6 + H 2 + Ar]g/ (Pt -TiO2) S interface and published evidence [18] support- ing such escape for [ ( C H 3 ) 2 C H O H + O z ] g / T i O 2 and [(CH3)2CHOH]g / (Pt-TiO2) ~ interfaces. Isopropanol vapour was therefore admixed with the benzene vapour flowing over a P t -T i O 2 interface in the HTR condition and this system was UV-illuminated to see if hole-capture by the alcohol vapour would result in the freeing of more electrons, some diminution of band-bend- ing and some photoenhancement of the yield of cyclohexane. No such effect was caused by UV-illumination. The second experiment was prompted by the expectation that, even if photoinitiated diminution of band-bending were rather inefficient due to strong competition from efficient electron hole recombination, prolonged illumination should gradually leak sufficient elec- trons back into the TiO 2 from the P t -T iO 2 microinterfaces to eventually diminish any preexisting band-bending by a significant amount. However, the triangular data points on plot (c) of fig. 2 show that even UV-illumination over a period of 72 h in a flow of prepurified H 2 did not result in any detectable enhancement of the low hydrogenation activity of (Pt-TiO2)H~ R. These experiments thus further strengthen the case against a dominant contri- bution by an equilibrium electronic situation of the type depicted in fig. 1 to the SMSI effect here observed upon hydrogenation at a [C6H 6 + H 2 + Ar]g/ (Pt TiO2)~v R interface at 373 K.

Other limitations and objections to fig. 1 as an adequate framework for the interpretation of the large body of experimental results involving SMSI phenomena have led different research groups to emphasise the role of various

J. Cunningham, G.H. Al-Sayyed / Metal-support interaction 295

structural features at surface regions of these systems [16,19-21]. It is not the intention of this paper to consider all these features. Rather, attention is focussed upon one which carries possibilities for modification by UV-illumina- tion in the presence of oxygen. The structural feature in question is the development by H T R pretreatments of small amounts of reduced titanium oxide upon the outer surface of the Group VII I metal particles [16a,16f,19]. Possibilities for its modification have their origins in the known ease of photoadsorption of molecular oxygen onto reduced TiO 2, and in the effect of oxygen in reversing the non-stoichiometry of reduced TiO 2. Recently, Sadeghi and Henrich concluded, from their careful study of a model system consisting of rhodium particles supported on well-characterised single-crystal rutile TiO 2 support, that H T R into the SMSI state was accompanied by the formation of a suboxide of titanium upon the Rh particles and that sites for CO chemisorp- tion were thereby blocked [19]. Likewise Takatani and Chung concluded, from their study of a model system consisting of a partially oxidised titanium foil covered with 120 A of nickel, that H T R brought about the segregation of monolayer amounts of reduced titanium oxide on the nickel surface [20]. Furthermore their observations on the efficiency with which the reduced titania inhibited CO chemisorption raised the possibility of inhibition at a distance from reduced titania, since an average of nine nickel atoms were deactivated per segregated titanium. Other evidence consistent with some role of an overlayer of reduced titania has been provided by work showing some reversal of the SMSI effect by various exposures to oxygen - either at high temperatures [1,2,7,16c] or at room temperature [5,21a]. Against this back- ground we carried out experiments to determine whether UV-ilhimination of the [C6H 6 + H 2 + Ar]g/ (Pt -TiOz)wv R interface in the presence of oxygen would cause reversal of SMSI to occur at greater rate or to greater extent, than for the same interface in the absence of illumination. Relevant results at room temperature are summarised in fig. 4, and at 373 K by plots (d) and (e) of fig. 2. Plot (ii) of fig. 4 demonstrates on an logarithmic scale the significant inhibiting effect induced by HTR, upon the activity for benzene hydrogena- tion at room temperature (relative to the greater activity of the same P t -T iO 2 material after LTR, cf. plot (i)). A measure of the extent to which large exposure to oxygen in the dark at room temperature could partially reverse the SMSI effect is provided by plot (iii) of fig. 4. This shows the activity-profile which resulted whenever the flow of reactant was interrupted after plot (ii) and the sample was first flushed at room temperature in a flow of argon for 10 min, then in a flow of 02 for 3 h, and again in a flow of argon only, before reestablishing the flow of reactants. This substantial reversal of SMSI in a flow of 02 at 1 atm pointed to likely difficulties in attempting to pick-up a small photoeffect in those conditions. A convenient method for maintaining the (Pt-TiO2)Hv R in a much lower dynamic pressure of oxygen during illumina- tion was indicated by recent work [22], demonstrating the formation of Cu20

296 J. Cunningham, G.H. Al-Sayyed / Metal-support interaction

100 200 300 400 f I

0.0

\o ~o <i)

0 0

0 ~ O~ O ~

° - ° - ° ~ o o--a--- o_ o_

U m~- an~ m .~.. m dii~

2 0 / , , i ,

100 200 100 2 0

F l o w T i m e m i n

Fig. 4. Activity profiles for indicated times on-stream at 295 K, showing percentage conversion of cyclohexane observed with the same reactant flow as for fig. 2 over 0.5% Pt -T iO 2 catalyst subject to the following pretreatments: (i) Catalyst was preactivated by LTR (see text for details). (ii) Catalyst was preactivated by HTR (see text for details). (iii) After (ii) followed by exposure to oxygen flow for 3 h at 293 K (see text for details).

upon Cu( l l0) when maintained in a flow of CP argon for 7 h at 900°C. Plot (e) of fig. 2, which shows the activity profile of a (P t -T iO 2) nXR sample after prior exposure to UV-illumination in a flow of CP argon for 2 h, makes it clear that hydrogenation activity was restored to the level of a LTR surface by this pre-illumination. Plot (d) of fig. 2 shows, however, that a much smaller restoration of activity was brought about by 36 h exposure to the flow of CP argon without UV-illumination. The greater reversal of SMSI achieved under UV-illumination during contact with ca. 5:4 1017 molecules of 02 allied to the absence of measurable photoreversal when illuminated under the reactant flow (see above) or a flow of hydrogen (cf. plot (c) of fig. 2) - pointed to crucial roles of photoassisted oxygen adsorption and reaction at the (P t -TiO2)uv R surfaces, rather than purely photoelectronic effects of the type envisaged in fig. 1. The literature provides much evidence for photoassisted adsorption and reaction of 02 on TiO? surfaces [23], and photoadsorption of O 2 onto reduced surfaces of 0.5% P t -T iO 2 has recently been reported [24]. Our findings would thus be consistent with photoassisted operation of a mecha- nism of oxygen-reversal of SMSI similar to one recently advanced by Anderson and Burch [21a], viz. an oxygen-induced formation of three-dimensional particles of stoichiometric TiO2 from the overlayer of reduced oxide on the metal surface.

J. Cunningham, G.H. Al-Sayyed / Metal-support interaction 297

Acknowledgements

T h e a u t h o r s a re g r a t e f u l to U n i v e r s i t y C o l l e g e C o r k fo r f i n a n c i a l s u p p o r t to

G . H . A1-Sayyed d u r i n g th i s w o r k a n d to O. N a s h a n d J. C a f f r e y fo r v a l u a b l e

t e c h n i c a l a s s i s t an ce ,

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