SURFACE AND INTERFACE ANALYSIS, VOL. 25, 292È294 (1997)

SHORT COMMUNICATION

Substrate E†ects and Chemical State Plots for theAnalysis of Supported CatalystsXPS TiO

2

V. M. Jime� nez, G. Lassaletta, A. Ferna� ndez, J. P. Espino� s and A. R. Gonza� lez-Elipe*Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla) and Departamento de Quimica Inorga� nica,PO Box 1115, 41080 Sevilla, Spain

The analysis by XPS of deposited on di†erent substrates MgO, Ag, SnO) shows the existence ofTiO2

(SiO2,

shifts in the Ti 2p binding energy and Auger parameter values. The magnitude of these shifts is a function of thesupport and of the coverage. A systematic representation of these shifts is possible with a chemical state plot. Theimplications of the existence of such shifts for the characterization of catalysts are discussed. 1997 by John(

Wiley & Sons, Ltd.

Surf. Interface Anal. 25, 292È294 (1997)No. of Figures : 2 No. of Tables : 0 No. of Refs : 18

KEYWORDS: XPS; Auger parameter ; catalyst ; chemical state plotTiO2 ;

INTRODUCTION

Binding energy (BE) and the modiÐed Auger parameter(a@) are the two most widely used parameters for thecharacterization by XPS of the chemical state of ele-ments. Both parameters have been utilized in a routineway to assess the oxidation state of the elements presentin a catalyst and other systems.1 However, generallyvery little attention is paid to a possible e†ect of thesubstrate on the BE and a@ values of cations in sup-ported metal oxides.

In an early paper by XPS on supported onTiO2we showed that BE and a@ of Ti4` di†er fromSiO2those of this cation in bulk oxide.2 These changes wereattributed to the inÑuence of the support, becauseSiO2unsupported colloidal particles did not presentTiO2any change with respect to the bulk material. Suchshifts in BE and a@ have been reported also for catalysisformed by ZnO supported on and other similarSiO2systems.3 Changes in the Auger parameter of silicon asa function of the surface coverage have already beenreported in catalysts (i.e.SiO2/Al2O3-supported SiO2supported on These changes were correlatedAl2O3).4h6with the catalytic activity of cracking n-hexane in H2and were explained as resulting from an increase of theelectron mobility of the system.5,6 In metals, similarchanges in these parameters can be found as a functionof the size of the metal particles or as a result of inter-action with the substrate on which the particles aredeposited.7h11

Recently, we have tried to contribute to the under-standing of such phenomena in supported metal oxides

* Correspondence to : A. R. Gonza� lez-Elipe.Contract grant sponsor : DGICYTContract grant : PB93-0183Contract grant sponsor : CICYTContract grant : MAT94-1039-C02-01

by studying a system formed by evaporated on aTiO2Ñat surface of From this study a result of partic-SiO2.11ular relevance for the present investigation is that theTi 2p BE and the a@ value both shift as a function ofcoverage. Thus, at low coverages, BE increases and a@decreases with respect to the values of the bulk material.We have also found a similar behaviour for SnO and

evaporated on and MgO.12h14SnO2 SiO2Here, we aim to extend this study further to check thee†ect of di†erent supports on the values of BE and a@ ofsupported at di†erent coverages by performingTiO2similar experiments on MgO (100), a polycrystallinesilver foil, highly oriented pyrolitic graphite (HOPG)and a polycrystalline SnO Ðlm. By comparison with

an insulating substrate where the MÈO bond isSiO2 ,rather covalent, these new substrates are representativeof an insulating metal oxide (MgO) where the MÈObond is ionic, a metal (Ag) poorly interacting with tita-nium oxides and where SMSI e†ects are not expected,15a covalent and poorly interacting substrate (HOPG)and an oxide (SnO) with electronic conductivity.

EXPERIMENTAL

Experimental conditions for the evaporation of TiO2and XPS recording were similar to those reported pre-viously for the system.11 Binding energyTiO2/SiO2reference was taken at the Mg 1s, Ag C 1s and3d5@2 ,

peaks of the substrates at, respectively, 1304.5,Sn 3d5@2368.3, 284.6 and 486.5 eV. These values yield the sameTi 2p BE for at high coverages (i.e. 459 eV). In allTiO2cases, formation of was conÐrmed at di†erentTiO2stages of the deposition by EELS (electron energy lossspectroscopy). Substrates MgO and Ag were cleaned bysuccessive Ar` bombardment and annealing cycles. TheHOPG was cleaned by peeling with Scotch tape andheating at 400 ¡C under UHV (P\ 10~8 Torr for 4 h).

CCC 0142È2421/97/040292È03 $17.50 Received 19 August 1996( 1997 by John Wiley & Sons, Ltd. Accepted 4 November 1996

CHEMICAL STATE PLOTS FOR SUPPORTED TiO2 293

A thick layer of SnO was obtained by evaporation ofand exposure to 0.1 Torr In all cases, theSnO2 O2 .12

coverage was measured as the Ti/M ratio determinedby XPS. A Degussa P-25 material was used as a refer-ence for bulk This sample consists of a poly-TiO2 .crystalline powder m2 g~1) formed mainly(SBET\ 49by anatase with some contribution of the rutilestructure.

RESULTS AND DISCUSSION

As an example of the type of results that are obtained,Fig. 1 shows the Ti 2p BE and a@ values for depos-TiO2ited on MgO(100) at di†erent coverages where oneequivalent monolayer corresponds approximately to aTi/Mg ratio of 0.15. Similar curves were obtained for

deposited on However, when silver isTiO2 SiO2 .11used as a support, the curves depicted an oppositebehaviour (i.e. BE decreases and a@ increases at lowcoverages with respect to bulk On HOPG noTiO2).signiÐcant changes were detected in the Ti 2p BE, buton SnO as a substrate small changes in the same direc-tion as on silver were found. Clearly, these results showthat the attribution of single values of BE and a@ toTi4`, irrespective of the support, can be misleadingwhen the oxide is highly dispersed on a substrate.

Recently, we have tried to account for the reasons forsuch shifts by considering polarization e†ects andquantum mechanical calculations with cluster models.16Here we propose an empirical approach that can beuseful in the practical work by XPS for the character-ization of supported oxide catalysts and related systems.

Figure 1. Evolution of Ti Ti and a¾ of sup-2p3@2 , L

3M

23V TiO

2ported on MgO(111) as a function of coverage, expressed as aTi/Mg ratio.

A general overview of the support e†ects can beobtained by means of the chemical state plots intro-duced by Wagner17 to deal systematically with the BEand a@ values of an element in di†erent compounds. AmodiÐed chemical state plot that includes the resultsobtained for the di†erent systems isTiO2/substrateshown in Fig. 2. Each point in this diagram correspondsto a given coverage on each support. The arrows indi-cate how the coverage increases in each experiment toreach the values of bulk Thus, for MgO andTiO2 . SiO2the sequence of points proceeds from low a@ and highBE values to reach the point of the bulk material, whilefor Ag and less clearly deÐned for SnO the sequence isthe opposite. In any case, the insulator or metal charac-ter of the substrates is the key property for the controlof the photoemission parameters of a supported metaloxide phase. On HOPG all the points appear concen-trated around the position of the bulk In the plotTiO2 .we have also included values for colloidal and aTiO2catalyst referred to previously.2 It is worthTiO2/SiO2mentioning that while the points of colloidal TiO2 ,bulk and a thick layer of (D1000 areTiO2 TiO2 Ó)18close in the diagram, the point of the cata-TiO2/SiO2lyst appears shifted in a@ and BE. Moreover, its positionÐts within the points obtained for the experiment con-sisting of evaporated on This conÐrms thatTiO2 SiO2 .in real catalysts there are also e†ects due to the supportthat lead to the observed changes in the values of thetwo parameters.

It is also interesting from Fig. 2 that, depending onthe substrate and the dispersion degree of the activephase, variations of up to 1.5 eV in BE and 5.5 eV in a@can be obtained for Ti4` in supported The con-TiO2 .

Figure 2. Chemical state plot for supported on various sub-TiO2

strates.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 292È294 (1997)

294 V. M. JIMEŠ NEZ ET AL .

sequences of this result for practical work in catalysisare obvious : BE and a@ might not be enough to estab-lish the chemical state of an element in a supportedcatalyst. In these systems, information about the natureof the support and about the dispersion degree of theoxide are necessary for an unambiguous character-ization of the system.

Finally, we have shown that a chemical state plot canbe used to describe systematically the e†ect of thesupport on the variation of the BE and a@ of a sup-

ported metal oxide. The elaboration of such plots forother oxide compounds would be a very useful tool inthe practical work of catalyst characterization.

Acknowledgements

We thank the DGICYT (project PB93-0183) and CICYT (projectMAT94-1039-C02-01) for Ðnancial support.

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( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 292È294 (1997)