Promotion of Ceria Catalysts by Precious Metals: Changes in Nature

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Promotion of Ceria Catalysts by Precious Metals: Changes in Nature of the Interaction under Reducing and Oxidising Conditions Nadia Acerbi, Stanley Golunski, Shik Chi Edman Tsang, Helen Daly, Chris Hardacre, Richard A P Smith, and Paul John Collier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp212233u • Publication Date (Web): 04 Jun 2012 Downloaded from http://pubs.acs.org on June 4, 2012

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The Journal of Physical Chemistry

Promotion of Ceria Catalysts by Precious Metals: Changes in Nature of the Interaction under Reducing and Oxidising Conditions Nadia Acerbi ,a,c* , Stan Golunski b, Shik Chi Tsang c, Helen Daly d, Chris Hardacre d, Richard Smitha and Paul Collier a a: Johnson Matthey Technology Centre, Blount’s Court Road, Sonning Common, Reading, RG4 9NH, UK

b: Cardiff Catalysis Institute, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT c: Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK d: CenTACat, School of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast, BT9 5AG

Abstract:

By depositing ceria over supported precious metal catalysts, and characterizing them with in situ diffuse reflectance UV (DR UV) and in situ Raman spectroscopy, we have been able to prove a direct correlation between a decrease in ceria band gap and the work function of the metal under reducing conditions. The PM-ceria interaction results in changes on the ceria side of the metal-ceria interface, such that the degree of oxygen vacancy formation on the ceria surface also correlates with the precious metal (PM) work function. Nevertheless, conclusive evidence for a purely electronic interaction could not be provided by XPS analysis. On the contrary, the results highlight the complexity of the PM-ceria interaction by supporting a spillover mechanism resulting from the electronic interaction under reducing conditions. Under oxidising conditions, another effect has

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been observed, namely a structural modification of ceria induced by the presence of PM cations. In particular, we have been able to demonstrate by in situ Raman spectroscopy that, depending on the PM ionic radius, it is possible to create PM-ceria solid solutions. We observed that this structural modification prevails under an oxidising atmosphere, whereas electronic and chemical interactions take place under reducing conditions.

Keywords: Ceria/precious metal catalysts, metal support interaction, DR UV, Raman, electronic effect, structural modification 1. Introduction Precious metal/ceria systems are very effective heterogeneous catalysts, in applications as diverse as three way catalytic converters for vehicle emission control,1 solid oxide fuel cells for energy generation,1 and CO removal by water gas shift2 and oxidation.3 The current theoretical understanding of this support-enhanced activity is usually based on the generic concept of strong metal support interaction (SMSI). The typical features of the SMSI phenomenon are: it occurs when reducible oxides are used as supports; it is induced by reduction temperatures higher than 550 °C; it provokes a disturbance of the chemical properties of the metal particles, resulting in a drop in CO and H2 chemisorption; and, finally, it is reversible.4 In PM/TiO2 systems, metal decoration (also known as the geometric effect) has been observed at reducing temperatures higher than 550 °C.4 The SMSI effect, is always accompanied by an improved reducibility of the reducible support. This has been attributed to chemical dissociation of H2 by the PM followed by abstraction of the neighboring support surface oxygen.5 The possible interactions between a PM and a reducible metal oxide may be electronic, chemical and/or structural (geometric).6 Electronic interactions can occur with charge transfer between the metal and metal oxide, while chemical interactions occur through a chemical reaction between PM and ceria, such as a redox reaction. If the structure of either the metal or the reducible metal oxide is modified in some way, then the interaction is assumed to be primarily of a structural type.


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The interaction between a precious metal and its reducible-oxide support often affects both the reducibility of the support and the catalytic performance of the precious metal in terms of activity and selectivity.7-12 Typically, the presence of the SMSI effect has been reported to decrease activities for hydrocarbon hydrogenolysis, isomerization and hydrogenation reactions, which are now often used as diagnostic reactions. Conversely, it does not affect the turnover frequencies of structure insensitive reactions such as hydrogenation of olefins and aromatics, even though the support-reducibility has increased12 There is also a growing body of evidence for it enhancing the activity of oxidation reactions,7 and the hydrogenation of CO and carbonyl bonds.12 Despite numerous studies of the PM/CeO2 system, there is still a lack of fundamental understanding of how the SMSI phenomenon modifies both support reducibility and surface activity in this system. Consequently, there is no scientific agreement on the mechanism occurring at molecular level to explain the different steps which lead to the SMSI effect. Different explanations abound, but can generally be grouped into three categories : i) alloy formation between Pt and Ce, ii) decoration or encapsulation of the Pt particles by reduced ceria, and finally iii) a purely electronic effect arising from a high degree of contact between the Pt particles and the ceria.13 Studies have demonstrated that in the case of PM/TiO2, SMSI induction is different compared to PM/CeO2. Bernal et al. have studied the nature of the metal/support interaction in PM/CeO2, compared to PM/TiO2 with increasing reduction temperatures, by using HRTEM and H2 chemisorption techniques.14,15 They found that for reduction temperatures below 550 °C, no significant nanostructural change was observed in PM/CeO2 catalysts. Nevertheless, they could observe a well defined epitaxial growth of PM particles on the ceria substrate, which was independent of the reduction temperature (200 – 550 °C). Interestingly, a correlation was found between changes in chemical properties (such as hydrogen adsorption) and the existence of this metal/support epitaxial growth relationship even when no metal decoration or alloying phenomena were observed. However, at temperatures of 900 °C, the formation of CePt5 alloys and Pd/CeO2 solid solutions were demonstrated. In contrast to PM/TiO2, the SMSI effect for PM/CeO2 is not fully reversible, and it has been concluded that partial deactivation effects observed in PM/CeO2 ACS Paragon Plus Environment

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catalysts could be due to electronic metal/support interactions, as decoration and alloying were not observed at temperatures below 550 °C under reducing conditions.14,15 Prior to the work of Bernal and co-workers, Dumesic and co-workers had proposed that an electronic effect was involved in the SMSI behaviour of Ni/TiO2 catalysts during CO and H2 chemisorption,16,17 while Burch et al. had suggested that new active sites are created at the interface between the metal and the support in these catalysts during n-hexane and ethane hydrogenolysis.18,19 Later, Somorjai et al. reported that a thin film of Pt on TiO2 can form a catalytic nanodiode. They measured a flow of current through the Pt-TiO2 interface during steady state CO oxidation, which they attributed to the formation of “hot” electrons during the chemisorption of CO on the metal nanoparticles.20,21 Wilson et al. have investigated the Pd/CeO2-x/Pt(111) system using density functional theory (DFT) and XPS, concluding that there is charge transfer from Pd to Ce(IV), which increases the density of Ce(III) cations.22 Recently we reported a practical demonstration of an electronic interaction between a metal and a metal oxide. By depositing ceria on top of supported PM it was shown that a direct correlation exists between ceria reducibility and PM work function .23 A purely electronic interaction, as postulated in the junction effect theory, has been used to explain the mechanism of charge transfer between a metal (Cu) and a semiconductor oxide (ZnO) during methanol-synthesis catalysis.24 The theory predicts that if a metal with high work function is in intimate contact with a semiconducting oxide with a high band gap, an equilibrium will be established in which electrons produced by ionizing the oxygen vacancies are distributed between the oxide conduction band states and the available Fermi levels of the metal.24 Potentially pioneering work has been recently published by Farmer et al. in which, by calorimetric techniques, they measured the heat of adsorption of Ag particles on partially reduced ceria surfaces. The very large adhesion energy (~2.3 joules per m2) of Ag nanoparticles to reduced CeO2 (111) resulted in strong bonding to both defects and CeO2 (111) terraces, apparently localized by lattice strain. This may well explain why ceria stabilizes precious metals and decreases sintering in precious metal catalysts. More importantly, the occurrence of chemical bonding reinforces the electronic interaction between precious metals and ceria.25 ACS Paragon Plus Environment

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Other mechanisms have been reported in the literature explaining enhanced activity and/or support reducibility of PM/CeO2 catalyst. Jacobs et al. proposed that the partial reduction of ceria by Pt generates active bridging hydroxyl groups on the ceria surface.26 The presence of hydroxyl groups was demonstrated by XANES at the Pt and Ce LIII edges. They suggested that two routes to hydroxyl group formation on ceria surfaces were possible. The first involves the reduction of oxidic Pt species to metallic Pt which dissociates H2. Molecular hydrogen spills over the ceria surface to generate bridging hydroxyl group active sites, directly accompanied by a reduction of Ce(IV) to Ce(III). The second route consists of H2 or CO abstracting ceria surface capping oxygen atoms to generate vacancies (Ce(III)), followed by H2O dissociation at the vacancies to generate the bridging hydroxyl group.26 De Leitenburg et al. found that the interaction of ceria with a series of PMs, namely Pd, Pt, Rh, Ir and Ru, is strongly dependent on the reduction temperature of the catalysts regardless of the metal employed.27 They suggested that, by increasing the reduction temperature, progressive reduction of bulk ceria takes place without promotion by the metal. Conversely, surface ceria reduction is facilitated by the precious metal by spillover, creating Ce(III) active sites.27 The transport of O2 from bulk to surface is believed to maintain the effectiveness of Ce(III)/Ce(IV) redox couple.27 A similar approach, but with Ce(IV) atom transport, is discussed by Zhang et al.28 They found that an oxidative/reducing atmosphere affected the structure and performance of Pt/CeO2 catalysts by the occurrence of SMSI. They proposed that Ce(IV) migration from the bulk to surface occurred during reducing treatment, whereas oxygen vacancies move during oxidizing treatment.28 Finally, there have been several reports of structural modification of ceria by PMs. Priolkar et al., observed the formation of a solid solution between CeO2 and Pd with PM insertion into the ceria fluorite structure.29 Similarly, CeO2-xPtxO2-δ30 and CeO2-xRhxO2-δ31 phases have been observed, while evidence for Pd-O-Ce bonding has also been reported in Pd/CeO2 catalysts.32,33 In this paper, in situ and ex situ spectroscopic techniques coupled with temperature programmed reduction (TPR) have been employed to investigate the type of interactions occurring between a series of supported PMs (Pt, Pd, Rh, Ru and Ag) and CeO2 under oxidising and reducing ACS Paragon Plus Environment

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conditions. We observe that a structural type of interaction prevails under oxidising atmosphere, whereas electronic and chemical interactions take place during reducing treatments. 2. Experimental

Preparation of ceria coated supported PM catalysts Alumina and silica (from Sasol, 140 m2/g and Grace, 540 m2/g respectively) supported catalysts with ceria deposited on PMs (Pt, Pd, Rh, Ru and Ag) were prepared by impregnation from platinum (IV) nitrate, palladium (II) nitrate, rhodium (III) nitrate, ruthenium (IV) nitrosyl nitrate and silver nitrate, all obtained from Johnson Matthey. Initially, the support pore volume was measured using water uptake measurements and determined to be 0.57 ml/g for alumina and 1.07 ml/g for silica; impregnation volumes were calculated for 20 g samples as 11.3 ml and 21.2 ml respectively. The mass of metal precursor solution required for 1 wt% metal loading was made up to the impregnation volume using demineralised water (from a Millipore water purifier). The diluted PM solution was mixed with the support powder to make a dry paste which was then dried at 105 °C for 4 h and then calcined in air at 500 °C for 2 h (10 °C min-1 ramp rate). Ceria was added to the supported PM catalyst by a deposition precipitation technique using cerium (IV) nitrate. The ceria precursor was prepared by reacting the Ce (III) hydroxide with nitric acid. 902 g of (Rhodia) Ce(OH)3 was added slowly to 1 L of nitric acid (Fisher Scientific, 69%), stirred at 70 °C for 6 h, filtered using Whatman 540, 541 and 542 filters and finally centrifuged and decanted. The desired amount of 1%PM catalyst was put in a beaker with 100 ml of distilled water, and the slurry stirred at 200 rpm for approximately 30 min at room temperature. A solution containing the required quantity of cerium (IV) nitrate to give a loading of 15 wt% of ceria, distilled water and a small amount of nitric acid was added dropwise along with 1 M sodium carbonate (Fisher Scientific, 95%) with stirring and the pH monitored in the precipitation vessel; precipitation occurred at pH 7.5±0.5. The precipitate was filtered and washed several times to remove adsorbed sodium until the conductivity of the washing water was less than 10µS. The material was then dried 6 ACS Paragon Plus Environment

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at 105 °C for 4 h and calcined at 500 °C for 2 h (ramp rate of 10 °C/min). CeO2/Al2O3 and CeO2/SiO2 were prepared by impregnation using Ce (IV) nitrate at a loading of 15wt%. The notation CeO2/PM/support (which will be used throughout the paper) refers to ceria deposited onto silica or alumina supported PM. Catalyst characterisation Temperature programmed reduction (TPR) profiles were obtained using a 10% H2/90% N2 gas mixture and an in-house TPR rig over the temperature range –25 °C to +900 °C. Firstly, the sample was cooled down to –25 °C using a flow of liquid nitrogen vapor. Once sub-ambient conditions were reached, the gas was passed to the thermal conductivity detector TCD cell, through either 0.2 or 0.4 g of the silica and alumina supported catalysts respectively in a quartz tube. The difference in mass is due to different densities of the samples; a different mass is used in order to obtain approximately the same volume of sample in the quartz tube. A molecular sieve/soda lime drier was used to remove the humidity in the gas flow prior to the TCD where any change in H2 content was monitored. Since the gas flow is constant, the change in H2 concentration is proportional to the extent of reduction in the catalyst sample. An instrumental error of ± 1.5% was estimated. H2 uptake measurement was carried out using in - house (JM) software that utilizes a calibration of the H2 response using a known injection volume of a 1 ml loop of H2. Samples for TEM analysis were prepared by embedding a small portion of sample powder in TAAB low viscosity resin and dispersed in an Agar Scientific lacy carbon film copper grid. The samples were examined using a Tecnai F20 Transmission Electron Microscope at a 200 kV accelerating voltage, with an aperture of 30 & 50 (m), a bright field (BF) and high angle annular dark field (HAADF) mode and energy dispersive X-ray analysis. In situ diffuse reflectance UV (DR UV) and ex situ DR UV were recorded using a Perkin Elmer Lambda 650S instrument with an integrating sphere detector and Praying Mantis diffuse reflectance attachment (Harrick). The in situ measurements were recorded using the commercial temperature controlled cell under a flow of 10%H2 in N2 (30 cm-1 min-1). Spectra were recorded using SiO2 and Al2O3 as reference materials. The absorption bands were determined by taking the apparent maxima 7 ACS Paragon Plus Environment


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in the spectrum. The band gap of the different ceria containing materials was measured by tangent analysis at the ceria absorption edge.34 Ex situ Raman spectra of ceria samples were recorded at room temperature using a Spectrum GX Raman spectrometer with the following instrument parameters: 4W Nd-YAG laser operating at near-infrared wavelengths (1064 nm), mode single beam, scan range 3600-100 cm-1, OPD 0.5 cm/s and resolution 4 cm-1. The measurement was repeated three times, performing 100 scans each time in order to minimize the error in the measurement of the Raman frequency shift. In situ Raman spectroscopy under reducing conditions (10% H2/90% N2, 10 cm-1/min-1) was carried out in a Linkham 1500 sample cell using a Horiba Yvon LabRam Infinity Spectrometer. The temperature was ramped at 10 °C min-1 and held at selected temperatures to allow spectral acquisition. The spectrometer operating conditions were: excitation wavelength of 633 nm with no filtration, confocal hole of 300 µ; exposure time of 60 s with 5 consecutive exposures; spectrograph position of 1500 cm-1, scan range of 200 to 1000 cm-1 and resolution of 8 cm-1. The electronic properties of the catalysts were assessed from plots of the Raman frequency shift of the ceria fluorite structure vs. reduction temperature. The Raman frequency shifts were obtained by considering the peak maximum. Additionally, the structural modification of ceria by the PM was monitored from the Raman peak area. XPS measurements were carried out using a Thermo VG instruments Escalab 250 instrument. Monochromatised aluminium Kα radiation was used for the studies, a 650 micron spot size at 200 W power. Charge compensation was activated, provided by the in-lens electron flood gun and argon ion source. Most samples were prepared by dusting onto carbon tape and thereby attaching to a standard sample stub; no significant signal from the tape was detected during measurements. Exceptions to this preparation procedure were for the in situ studies where the samples were compressed into pellets and mounted on a gold-plated stub and held down by gold foil. For the reduction studies, the Escalab's ex situ reaction cell was used; the sample was transferred (pressure of ~3x10-7 mbar or lower at all times) to the unit which was then pressurised to approximately 4 bar with 10% hydrogen in argon (99.999% purity gases) and a flow of ~20 ml min-1 was set at the outlet ACS Paragon Plus Environment

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of the unit. For the 80 °C treatment a ramp rate of 5 °C min-1 was set to reach that maximum temperature which was held for 30 minutes. The sample was then cooled under flow (~30 minutes), the cell then evacuated and the sample transferred to the system analysis chamber. The 300 °C treatment followed a similar pattern with a ramp rate of 10 °C /min-1, maximum temperature 300 °C held for 2 hours with the cooling under flow taking ~2 hours before evacuation and re-analysis. Regarding data analysis, Shirley-type backgrounds35 and sensitivity factors after Scofield36 were used where necessary; binding energy scale references used for the alumina-supported samples were the aluminium 2p signal maxima, set to 74.5 eV, and for the silica-supported samples the silicon 2p signal maxima, set to 103.6 eV. (The XRD experimental description can be found in the Supplementary Information.) 3.

Results

Ex-situ characterisation of as-prepared materials From TEM characterisation of the CeO2/PM/SiO2 and CeO2/PM/Al2O3 materials, they can be described as supported precious metals, where some of the added ceria is isolated, but where there are also distinct regions of contact between ceria and PM On the whole, no continuous coating of ceria was observed. Table 1 shows the measured PM particle size before and after ceria deposition. In the ceria-free supported PM samples the particle size varied depending on the PM. Dispersed nanoparticles, with sizes below 10 nm, were found for 1%PM/Al2O3 (PMs: Pt and Pd) and 1%PM/SiO2 (PMs: Pt and Ag) however, no nanoparticles of Rh or Ru were observed. Instead, very large (~200 nm) Rh and Ru rich areas were detected by EDX, suggesting either that the particles are very large, or that they are so finely dispersed at an atomic level that they are below the detection limit of the microscope used in this study. CO chemisorption analysis (data not reported) showed the Rh catalyst to have a very high dispersion which makes the second explanation more likely. In contrast, Ru showed a poor dispersion suggesting its deposition onto alumina in the form of large particles. The use of silica leads to formation of bigger PM particles compared to alumina, apart from the cases of Ag and Ru.


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In general deposition of ceria onto the supported PM leads to some aggregation of PM nanoparticles resulting in slightly larger clusters in comparison with the ceria free samples (Figure 1). CeO2/Ag/SiO2 and CeO2/Ag/Al2O3 were anomalous with no nanoparticles of Ag observable after ceria deposition.

Table 1

Figure 1

TEM characterisation also showed the presence of an interfacial interaction between CeO2 and Ru in CeO2/Ru/Al2O3 sample, which is shown in Figure 2.

Figure 2

XRD (see Supplementary Information) showed that the samples are generally not highly crystalline and in particular CeO2/PM/SiO2 (PMs: Pt, Rh and Ag) are amorphous. TEM results agreed with XRD showing crystalline CeO2 on alumina supported samples but not on silica , eg see Figure 3, where 0.314 nm is the typical d-spacing of CeO2 (111).1

Figure 3

No accurate binding energy values could be obtained by XPS for CeO2/PM/SiO2 (PMs: Pd, Rh, Rh and Ag). Detailed characterization, therefore, could only be performed on the as prepared alumina-supported PMs, before and after ceria deposition (PMs: Pt, Pd, Rh, Ru and Ag). From the data obtained (Table 2), the binding energies of the elements at the surface of the as-prepared supported catalysts are consistent with the topmost layers of the PM particles being in an oxidic ACS Paragon Plus Environment

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form rather than metallic. Namely, the surface platinum exists in form of PtO and PtO230, palladium as PdO29,30, rhodium as Rh2O331, and ruthenium as RuO.37 In the case of silver it was not possible to discriminate between Ag2O and AgO, due to the very close binding energy of the Ag 3d peak for the two types of oxide. In spite of the PM particle growth observed by TEM, on deposition of CeO2, the oxidation state of the PM was maintained when ceria was deposited on top, creating a metal oxide/metal oxide interface.

Table 2

In most cases, PM binding energy increased when ceria was deposited onto the supported PM, with the increase signifying increasingly ionic character of the PM due to Ce-PM-O phase formation.29,32 The one exception was CeO2/Ag/Al2O3, which showed a decrease in PM binding energy when ceria was deposited onto the supported PM. It was not possible to observe Pd and Ru when ceria was deposited on top for the silica supported catalysts. This suggests that ceria is completely covering the PM in the Pd and Ru samples. Low energy ion scattering (LEIS) analysis has also supported this observation (not reported here). In the Raman spectra (Table 3) obtained for as-prepared CeO2/PM/SiO2 and CeO2/PM/Al2O3 (PMs: Pt, Pd, Rh, Ru and Ag), the fluorite phase of ceria was identified by a band at 464 cm-1 for all samples, apart from CeO2/Pt/SiO2 and both the ruthenium containing samples. Figure 4 shows a typical Raman feature of ceria, in this case for CeO2/Ag/Al2O3. The ceria fluorite peak intensity is higher for the alumina supported samples compared to the silica supported samples. This is in agreement with XRD and TEM results which showed the silica supported samples to be largely amorphous. In addition, CeO2/Rh/SiO2 shows a very noisy peak that is difficult to distinguish from the baseline.

Figure 4


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Table 3

No significant shift in the ceria fluorite band to lower frequencies was obtained when ceria was dispersed on silica or alumina alone; values of 464 ± 2 cm-1 for ceria and 463 ± 2 nm for supported ceria (Table 3). Similarly, the band position obtained for the CeO2/PM/Al2O3 is comparable with the CeO2/Al2O3 reference (within the error of ± 2 cm-1). In comparison, the CeO2/PM/SiO2 samples exhibited a shift to lower frequencies relative to CeO2/SiO2. The Raman frequency shift depends on many factors, such as the extent of crystallinity, the presence of oxygen vacancies, and defects in the structure. The shift to lower Raman frequency may be partly explained by the lower ceria crystallinity of the silica supported samples compared to those supported on alumina.38-40 Figure 5 shows an example DR UV spectrum obtained for CeO2/Pt/SiO2. The PM/CeO2 catalysts typically show four different superimposed absorption bands with a maximum at 352 nm. The pure ceria spectrum (not reported here) shows two bands, the first one at 267 nm and the second at 326 nm. These can be ascribed to Ce3+ - O2- charge transfer and a Ce4+ - O2- interband transition.41,42 The introduction of the PM, induced a shift of the bands to higher wavelengths as well as a new feature in the spectra. In the case of Pt, the Ce3+ - O2- charge transfer band is shifted to 308 nm, and the Ce4+ - O2- interband transition to 352 nm. New bands at 386 nm, attributable to Ce4+ - O2charge transfer30, 23 and at ~500 nm, due to a d-d transition of platinum27 were also observed. In the literature, a redshift of the DR UV bands has been explained as defect induced introduction of energy levels into the interband gap, which implies doping with transition metal ions.43 Table 4 presents the three ceria absorption bands for CeO2/PM/SiO2, CeO2/PM/Al2O3 and metal free references. The presence of the PM leads to a large redshift for the Ce3+ - O2 charge transfer bands for all samples apart from for the CeO2/Pt/SiO2 samples. For the Ce4+ - O2- interband transition, the extent of the shift is very sensitive to both the PM and the support. The Ce4+ - O2- charge transfer transition is detectable when the PM is present and appears not to be dependent on the nature of the PM.


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Figure 5

Table 4

From the DR UV measurements, it was possible to calculate the band gap for all the samples apart from CeO2/Ag/SiO2 (Figure 6). The presence of PM in an oxidized state leads to a decrease in the ceria band gap, which can be interpreted as indicating the presence of partially reduced ceria on the surface of the catalysts..44 This phenomenon clearly depends on the nature of the PM oxide underlying the ceria. The decrease in band gap is greater for CeO2/PM/SiO2 samples compared to those in which alumina is the support. This may be due to the less crystalline nature of the ceria when deposited on the silica supported PM catalysts, as determined by XRD and TEM.45,46

Figure 6

From the results discussed so far it is possible to conclude that the presence of a PM oxide/ceria interface changes the redox properties of ceria. This is in agreement with the percentage of CeO2 reduction obtained from TPR results (see Supplementary Information).

In situ reduction study of ceria-coated PM materials Reducibility determined from TPR PMs supported on alumina and silica were used as the benchmarks in assessing the effect of the PM-ceria interaction on the reducibility of the ceria-coated materials. The TPR profile for Pt/Al2O3 (Figure 7e) shows three reduction peaks at 8, 100 and 370 °C. The highest temperature peak is associated with the reduction of PtOx stabilized by interaction with Al2O347, the intermediate temperature peak is attributed to less strongly interacting PtOx48 and finally, the low temperature peak corresponds to the reduction of surface Pt oxide to metallic Pt. Pd/Al2O3 (Figure 7d) also ACS Paragon Plus Environment

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shows three reduction peaks, though at lower temperatures (0, 50 and 305 °C). The two higher temperature peaks are due to PdO reduction49 while the very low temperature peak is generally attributed to Pd hydride formation.50 The TPR profile of Rh/Al2O3 (Figure 7c) looks different from the others, and it shows broad peaks rather than sharp reduction features. Burch et al., have related this broad nature of the peaks to the fact that RhOx species are strongly bound to the alumina support.51 The peaks at 140 and 260 °C are attributable to dispersed RhOx species.52 The peak at 75 °C, which was not found in the literature, suggests that there is also a Rh-containing species that is weakly bound to the alumina surface. The TPR profile of Ru/Al2O3 (Figure 7 b) shows a peak at 195 °C with a shoulder at 165 °C and a higher temperature peak at 380 °C, which have previously been assigned to well-dispersed and bulk RuO2.37,53 In the Ag/Al2O3 sample (Figure 7a), two reduction peaks at 100 and 200 °C are observed which are related with the reduction of surface oxygen on silver oxide clusters and bulk silver oxide reduction.54

Figure 7

The TPR profile of Pt/SiO2 (Figure 8e) shows two reduction peaks at 5 and 100 °C respectively. The lower temperature feature is related to the reduction of PtO2 to PtO, which has been previously observed at sub-ambient temperatures,55 and the higher temperature feature is due to reduction of PtO to the metal.56 As the reduction of PtO occurs at higher temperature than previously reported, it suggests that it is more strongly bound.57 The Pd sample (Figure 8d) shows a sharp peak at 70 °C which is related to the reduction of PdO.50 A peak is also observed at 750 °C which has previously been assigned to a palladium silicide that can be formed at high temperatures.58 Rh/SiO2 (Figure 8c) gives rise to a single reduction peak at 100 °C, during which Rh2O3 is reduced to metallic Rh. Ru/SiO2 (Figure 8b) shows a very sharp peak at 160°C which is the due to the reduction of RuO2.59 The reduction peak of oxidic species on Ag/SiO2 (Figure 8a) is barely observable at 55 °C, but a broad feature also attributable to Ag oxidic species is observed at ~ 375 °C. The peak at –20 °C is most likely an artifact of the TPR instrument. ACS Paragon Plus Environment

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Figure 8

Unsupported ceria and ceria supported on PM-free silica and alumina, show two reduction features, one at intermediate (~500 °C) and one at high temperature (775 °C) attributable to surface and bulk ceria reduction, respectively (figures 9 and 10).48,60-62 All TPR profiles of ceria-coated PM materials show similar features above 220 °C (Tables 9 and 10), which can be understood by reference to the published results for the reduction of unsupported ceria, supported ceria, and ceria in contact with PMs. The lowest temperature peak is due to combined reduction of PM and surface ceria, which is attributable to abstraction of O2- by hydrogen from surface ceria facilitated by the presence of the PM (* in Figures 9 and 10).48,60,62,63 An intermediate temperature peak is observed which is due to surface ceria distant from the PM, whereby O2- is abstracted from surface ceria which is not in direct contact with the PM (º in Figures 9 and 10).48,60,62,63 A high temperature peak is also observed due to bulk ceria reduction to Ce2O3 (▀ in figure9 and 10).48,60,62,63

Figure 9

Table 5 From the TPR profiles of ceria, ceria on alumina and ceria/PM on alumina (Figure 9), the three ceria reduction features, namely promoted surface ceria, surface ceria and bulk ceria can be distinguished (their reduction temperatures are summarized in Table 5). Additionally, in the TPR profiles for CeO2/Pt/Al2O3, CeO2/Pd/Al2O3 and CeO2/Ru/Al2O3, a low temperature peak is observed that is associated with PM oxide reduction. In CeO2/Pt/Al2O3 this appears at a subambient temperature (–10 °C), while the reduction of PdO to Pd0 occurs as two peaks (one at 20 °C and another at 90 °C). Ruthenium oxide shows a higher temperature reduction peak at 165 °C. In the case of CeO2/Rh/Al2O3 and CeO2/Ag/Al2O3, the PM oxide peak is convoluted and it is possible ACS Paragon Plus Environment

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to determine only its shoulder close to the ceria promoted PM peak. Interestingly, in CeO2/Ru/Al2O3 another reduction peak is observed just between the Ru oxide reduction and Ru promoted surface ceria reduction. This peak may be due to some so-far unknown ruthenium species dispersed on the alumina surface.37

Figure 10

Table 6

The TPR profiles of ceria on silica and ceria/PM on silica show three distinct ceria-reduction features (Figure 10); their reduction temperatures are summarized in Table 6. The reduction of PM and promoted surface ceria occurs as single step and is associated with a single reduction peak. A shift to lower temperatures of the three ceria reduction features for silica supported materials can be see along with the disappearance of the bulk ceria peak for CeO2/Pt/SiO2 and CeO2/Ag/SiO2 on silica catalysts compared with the alumina supported ones. This interpretation is based on the accepted literature explanation of the TPR features. We are currently using advanced characterization techniques to investigate the base of these assumptions.

Band gap as determined by DR UV In Figure 11 the temperature dependence of the ceria band gap under reducing conditions is shown for the CeO2/PM/Al2O3 materials. In CeO2/Pt/Al2O3 the band gap is constant up to 250 °C after which it decreases. A steeper drop was observed for CeO2/Pd/Al2O3, where the band gap decreases sharply up to 100 °C, remains almost stable up to 300 °C before decreasing further. In CeO2/Rh/Al2O3 the band gap decreases smoothly up to 300 °C before falling more steeply. CeO2/Ru/Al2O3 and CeO2/Ag/Al2O3 were unusual as the band gap increases for the first 100 °C before decreasing. This effect can be explained by a structural modification of the ceria by the PM.


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CeO2/Ag/Al2O3 showed a similar band gap trend as CeO2/Ru/Al2O3, however the effect was less severe indicating a possible lower extent of structural modification of CeO2 by Ag compared to Ru.

Figure 11

For the CeO2/PM/SiO2 catalysts (Figure 12), the decrease in the band gap was in general smoother, and more in the form of a decay curve. The difference observed in the band gap as function of temperature under reducing conditions, between the two supports, reflects the crystallinity of the ceria. When alumina was used as support, the ceria was crystalline (as shown by XRD and TEM); whereas when the ceria was coated over the silica-supported catalysts it was amorphous. For amorphous samples the change in the band gap with temperature may be due to electronic effects and/or reduction effects under our conditions. It is not possible to discount a structural effect in other cases. The fact that ceria is crystalline in CeO2/PM/Al2O3, adds complexity to the system, as the change of band gap is influenced by three factors, ie variations in the crystal lattice, electronic redistribution in response to increasing temperature, and partial reduction of CeO2 to Ce2O3.

Figure 12

The difference of band gap at room temperature, when the noble metal is in oxidic form, and the band gap at 300 °C, when the PM is already reduced to metallic form, is directly proportional to the work function for CeO2/PM/SiO2 (Figure 13). The higher the work function, the greater the decrease in band gap when the PM is reduced.

Figure 13


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Raman Peak Area The ceria fluorite peak area (band at 464 cm-1) decreases with increasing temperature for ceria and for the CeO2/PM/Al2O3 materials containing Pt or Rh, as a result of progressive reduction (Figure 14). By contrast, the materials containing Pd, Ru or Ag show a different trend. In CeO2/Pd/Al2O3 the ceria fluorite peak area dropped with increasing temperature up to 60 °C, then rose as the temperature was increased to 70 °C, before decreasing again with temperature. For the CeO2/Ru/Al2O3 sample, the increase in the ceria peak area was much more substantial than for the Pd catalyst (figure 14B); firstly the peak decreased until 120 °C, before dramatically increasing as the temperature was increased reaching a maximum peak area at 165 °C. As the temperature was ramped above 165 °C, a steep decrease was then recorded with the ceria band no longer observed above 300 °C. A similar effect was noted for CeO2/Ag/Al2O3 when the fluorite peak increased first up 125 °C and then decreased sharply at higher temperature. The peak areas from Raman spectra recorded under reducing conditions for CeO2/PM/SiO2 did not show any particular trend with temperature. This may be due to the fact that most of the ceria in these samples is very disordered, so little further modification of the fluorite structure is observed upon reduction.

Figure 14

More detailed interpretation of the changes in Raman spectra become possible by comparison with the TPR results. The loss of ceria fluorite structure peak area for CeO2/PM/Al2O3 during the first 100 °C of reduction can be attributed to the promoted reduction of surface ceria. The subsequent increase of the ceria band area (for PMs: Pd, Ru and Ag) may indicate that the ceria fluorite phase was re-forming after the reduction of the PM oxide, due to segregation of the metallic nanoparticles and collapse of the mixed oxide. This increase occurs above 60 °C in the palladiumcontaining material, and at 135 °C when Ru is present. These correspond to the reduction temperatures of the respective PM oxides.


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Raman Peak Shift In CeO2/Pt/Al2O3 the Raman frequency shift fell rapidly as the temperature rose from 25 °C to 140 °C and then remained constant until 350 °C followed by a slight reduction (Figure 15). In CeO2/Pd/Al2O3 there was a rapid fall from 25 °C to 90 °C, the band then remained constant for as long as the frequency could be measured accurately (up to 130 °C). CeO2/Rh/Al2O3 showed four reduction plateaus, each lower in Raman shift than the previous one with transition temperatures between the plateaus of 100 °C, 215 °C, 245 °C and 320 °C. The Ru sample showed unusual behavior with a decrease in Raman frequency shift occurring from 25 °C to 50 °C then remaining constant until 120 °C. A sharp decrease at 145 °C followed by a sharp increase at 165 °C with the frequency shift subsequently remained constant. The CeO2/Ag/Al2O3 sample showed similar behavior to CeO2/Rh/Al2O3 although the plateaus were punctuated with dips in Raman frequency shift. These variations in Raman frequency shift with temperature under reducing conditions can be correlated with the different ceria states identified by TPR. In CeO2/Pt/Al2O3, the PM promoted ceria resulted in a fast decrease of the ceria fluorite structure Raman frequency shift up to 110 °C. Subsequently, in the temperature range of the unpromoted surface ceria, Raman frequency shift decreased less dramatically and reached equilibrium up to 350 °C. In CeO2/Pd/Al2O3 the fluorite structure collapsed within the temperature range for Pd-promoted ceria reduction. At higher temperatures, only amorphous ceria is present which is not detectable by Raman spectroscopy. In CeO2/Rh/Al2O3 four different plateaus are observed. The two plateaus within the promoted surface ceria reduction temperature range (up to 200 °C) could be attributed to the ceria in direct contact with the Rh and ceria in close vicinity to Rh undergoing reduction at different rates. Between 200 and 260 °C the unpromoted surface ceria is reduced and this is observable by a drop in the Raman frequency shift in the in situ Raman and finally, at temperatures greater than 260 °C, part of the bulk becomes reducible. For CeO2/Ru/Al2O3 the increase in the Raman frequency shift at 165 °C, fits with previous evidence of a structural modification. The increase is observed in the temperature range in which the RuO2 is reduced. This indicates that the reduction of the crystalline RuO2, on ACS Paragon Plus Environment

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which ceria grows, modifying its fluorite structure, is leading to reconstruction of the ceria structure. For CeO2/Ag/Al2O3 it is possible to differentiate two forms of promoted surface ceria, one in direct contact, which is easily reducible and the other in the close vicinity of the Ag, by occurrence of two plateaus within the temperature of promoted surface ceria found by TPR. At temperatures higher than 200 °C, the unpromoted surface ceria starts to be reduced. For this material, the dips are at approximately the same temperature as the reduction of surface and bulk silver oxide (100 and 200 °C) which, similarly to CeO2/Ru/Al2O3, shows that reduction of the silver oxide results in a partial reconstruction of ceria fluorite structure, as attested by a slight increase in the Raman frequency shift.

Figure 15

Raman frequency shift (between room temperature and 300 °C) was found to be dependent on the work function of the noble metal (Figure 16). The greater the work function the greater is the relative variation of Raman frequency shift. In the case of CeO2/PM/SiO2 catalysts, a decrease in Raman frequency shift with temperature on reduction is not observed. The Raman frequency shift stays constant with temperature under reducing conditions. This may suggest that the ceria is already disordered to such an extent that Raman cannot detect any differences.38,39

Figure 16

Monitoring electron transfer by XPS Our XPS experiments were designed to clarify if the interaction between PM and ceria is predominantly electronic, as proposed in the junction effect theory.24 Platinum was chosen because it is the PM with the highest work function, therefore making it more likely for electron transfer to occur. The effects of ceria on platinum were studied by making a comparison between CeO2/Pt/SiO2 and the ceria-free Pt/SiO2, before and after exposure to H2 at 80 °C and at 300 °C. ACS Paragon Plus Environment

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The temperature of 80 °C corresponds to reduction of promoted surface ceria.The temperature of 300 °C was taken as reference all throughout the paper as temperature of complete reduction of the PMs (in this case Pt), to make sure that the PM is at the metallic state. The cerium 3d spectrum of CeO2 is known for its complexity. Besides the spin-orbit splitting of cerium 3d5/2 and cerium 3d3/2 there are several other splittings that are caused by a redistribution of the entire energy spectrum as a core hole is created.64 In addition, the Ce ion should not be considered as a pure Ce(IV) state, with configuration describable as 4f0Ln (Ln is the fully occupied valence band, formed by p orbitals of the ligand oxygen atoms), but as a mix of Ce(IV) – Ce(III) states. Due to cerium 3d photoelectron ejection, cerium ions may be left in three different electronic states:

v’’’-u’’’ spin orbit doublet [*]4f Ln v’’ - u’’ spin orbit doublet [*]4f 1Ln-1 v - u

spin orbit doublet [*]4f 2Ln-2

v, v’’’, u, and u’’’ are peaks only related to Ce4+ , and u’’, u’, v’’ and v’ are hybrid Ce4+/C3+ peaks.1

Figure 17

In Figure 17 cerium 3d XPS spectra of the fresh and the reduced catalysts at 80 °C and 300 °C are shown. The cerium 3d signal from the fresh material is as expected for cerium (IV) oxide. Reduction at 80 °C introduced some cerium (III), evinced by increased intensity at ~886 eV relative to ~883 eV. Reduction at 300 °C resulted in a line shape that clearly shows the presence of a large amount of cerium (III). The 80 °C and the 300 °C reduced samples contain ~11% of Ce(III) and ~49% Ce(III) respectively. The increasing extent of Ce(III) during reduction is to be expected.


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The platinum signal, before and after reduction treatment, is presented in Figure 18. Before reduction, the signals indicated two environments are present showing the presence of PtO and PtO2. For CeO2/0.85%Pt/SiO2 the spectra is shifted slightly to higher binding energies. This has been explained as due to the insertion of the Pt into the ceria matrix at the interface Pt-ceria, by formation of Pt-O-Ce(IV). After both reducing treatments the Pt(IV) is reduced to Pt(II) and this to metallic platinum. After reduction at 80 °C the spectra for CeO2/0.85%Pt/SiO2 and 1%Pt/SiO2 match exactly, with no shift to higher binding energies observed for the ceria containing sample. A net 0.26 eV shift is measured if compared with the ceria free catalyst to pass from Pt (II) signal to Pt(0). After reduction at 300 °C again the CeO2/0.85%Pt/SiO2 exhibits slight higher binding energies, in comparison with 1%Pt/SiO2. This may be explained by the fact that at this relative higher reduction treatment the platinum may re-dissolve into the ceria partially reduced structure. It is not, however, clear why this effect is not observed at 80 °C, where (according to the TPR results) the ceria is already partially reduced. The fact that there is no shift to higher binding energies for CeO2/0.85%Pt/SiO2 after reduction at 80 °C may be explained by segregation of the platinum on the surface of the catalyst at this mild reduction treatment. An indication that this occurs may be found in the estimated atom percentage on the surface. The atom percentage (Table 7) does not vary substantially for the 1%Pt/SiO2 before and after reduction. However, the estimated surface Pt atom percentage does change for the CeO2/0.85%Pt/SiO2 before and after reduction at 80 °C and 300 °C. A value of 0.59% is observed in the fresh sample against 0.28% when ceria is absent, which may be due to the well known effect of ceria in re-dispersing the platinum.60 At 80 °C the ceria containing sample showed an even higher atom percentage (0.65%), providing evidence of the platinum migrating to the surface. A decrease to 0.31% is recorded when the sample is reduced at 300 °C, indicating either that the platinum particles are becoming buried by ceria or that they are sintering.

Figure 18

Table 7 ACS Paragon Plus Environment

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3.

Discussion

XPS of the as-prepared supported catalysts indicated that the surface of the PM was present in an oxidized form. Where the PM remained uncovered after deposition of CeO2, the ionic character of the PM particles had increased in most cases as the result of Ce-PM-O phase formation. Additionally, XPS, Raman and DR UV all showed a modification of the ceria by the PM. The monitoring of the ceria fluorite structure peak area by Raman under reducing conditions helps to understand the nature of interaction between the PM oxide and the ceria. By comparison with the TPR results, the loss of ceria fluorite structure peak area for CeO2/PM/Al2O3 during the first 100 °C of reduction was due to the promoted reduction of surface ceria, as shown by TPR. The subsequent increase of the ceria band area (for PMs: Pd, Ru and Ag) may indicate that the ceria fluorite phase was re-forming after the reduction of the PM oxide, due to segregation of the metallic nanoparticles and collapse of the mixed oxide. This increase occurs above 60 °C in the palladium-containing ceria sample and at 135 °C in when ruthenium was present. The formation of ceria-PM (PMs: Pt, Pd and Rh) solid solutions is in agreement with other studies reported in the literature.29,31,32 However, in our case, if this hypothesis is valid, the insertion is not occurring throughout the catalyst but just at the interface created between the redox metal oxide (ceria) and the PM oxide. This limits the possible characterisation of these materials by EXAFS, XRD and/or TEM due to the localized formation of mixed phases. Shannon et al. reported that the ability to substitute one cation for another in a particular structure is largely dependent upon matching ionic radius.65 The cerium ionic radius in the ceria fluorite structure is 0.97 Å with a coordination number 866,67 and there is a substantial match with that of Pd(II) (with the assumption of CN=6) whose ionic radius is 0.86 Å. Unfortunately, in the case of Ag, it is not possible to distinguish between the presence of Ag(I) or Ag(II), however the ionic radii have similar values, 1.15 and 0.94 respectively. Ruthenium does not fit this hypothesis because there is a substantial mismatch in ionic radius (0.62 Å, CN: 6) with that of Ce(IV). Overlayer growth of ceria on top of a crystalline PM oxide is an alternative explanation for ceria regaining its ACS Paragon Plus Environment

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fluorite structure after the PM oxide reduction, which accords with the in situ Raman results. Evidence for crystalline RuO2 was found by XRD and TEM and in the more specific case of 1%Ru/Al2O3 (TEM image of figure 1(C)) confirms the crystallinity of RuO2 on Al2O3. We may suggest that overlayer growth phase rather than a substitution of the PM cation into the ceria fluorite structure occurs, in the Ru system. Additionally, the decrease in ceria band gap reflects a change in its redox properties, probably as the result of partial reduction of its surface. However, no decrease in the band gap was recorded between room temperature and the temperature at which TPR detected PM-promoted ceria reduction when Pt, Rh, Ru or Ag was present. In fact, the band gap increased in the CeO2/Ru/Al2O3 and CeO2/Ag/Al2O3 samples. This implies that the lattice parameter, modified by structural modification of ceria by cationic PM, dominates the band-gap change within the temperature range for ceria reduction.

After

collapse of the crystalline ceria structure, as the result of reduction, lattice constraints can be ruled out, and temperature and reduction must prevail as the dominant effects on the band-gap change. CeO2/Pd/Al2O3 represents an exception as a change is recorded within the temperature range for reduction of promoted surface ceria. This can be related to the enhanced rate of collapse of the ceria fluorite-structure in this sample, which is seen during in situ Raman spectroscopy (Figure 14 and 15) when the ceria peak area dramatically drops at 75 °C and becomes undetectable at 150 °C. The lattice constraint is not significant in the ceria when the PM is supported on silica, due to the amorphous nature of the ceria layer formed. Therefore, the change in the band gap points to an electronic redistribution resulting from the temperature increase and reduction of ceria. The difference between the band gap at room temperature (when the PM is in an oxidized state) and the band gap at 300 °C (when the PM has been reduced to its metallic form) is directly proportional to the work function of the PM (Figure 13). The overall trend is that, with increasing work function, there is a greater decrease in band gap when the PM is reduced. Therefore a PM with a greater work function leads to a narrower band gap and this could be due to electrons flowing from the conduction band of the ceria to the PM Fermi level, as the junction effect predicts24. Electron loss ACS Paragon Plus Environment

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from the ceria conduction band is possible and could lead to a redistribution of electrons with a narrowing of the ceria band gap. In addition, the change in band gap on reduction is related to the formation of partially reduced ceria, which also means higher oxygen vacancy formation. This is in agreement with our recent work on the demonstration of electronic promotion in the reduction of ceria PM catalysts.23 In addition, the apparent change in band gap under reducing conditions is associated with the formation of oxygen vacancies at the ceria surface.23 It has been reported that Raman shifts to lower frequencies are related to oxygen vacancy concentration.40 Thus, Raman frequency shift can be considered a qualitative measurement of the degree of ceria oxygen vacancy formation during reduction, which we found to be dependent on the work function of the noble metal (figure 16). The greater the work function the greater is the relative variation of Raman frequency shift. 38,39

With this weight of evidence for an electronic effect predominating in the CeO2/PM/SiO2 samples, we expected that in situ XPS could provide direct confirmation. However, the focussed set of experiments on Pt-containing materials did not indicate the presence of electron rich PM as previously reported by Khan et al. for a ceria-Pd system, where a shift of the Pd peak of 0.3 eV to lower binding energies compared to metallic Pd signal was attributed to charge transfer from the ceria to the PM.68 Electron rich metals have been also observed in PM/CeO2 catalysts for automotive applications.3 In this study, a net 0.26 eV shift in binding energy was indeed observed in the ceria-containing platinum sample in comparison to the ceria-free Pt/SiO2 when the Pt(II) was reduced to Pt(0) at 80 °C. Although the extent of the shift is consistent with the one reported in the literature, it does not give a clear indication of electron-rich Pt formation after reduction. Moreover, it has to be remembered that segregation of the platinum was observed after reduction at 80 °C in the CeO2/0.85%Pt/SiO2. This could provide support for the spillover model, since the segregation results in more platinum being exposed, providing a larger surface on which H2 molecules can be dissociated and a greater interface across which the dissociated hydrogen can be transferred. ACS Paragon Plus Environment

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The fact that XPS failed to detect charge transfer from the support to the metal may not be definitive. In effect, we have probed the core electrons (mainly K shell electrons), whereas charge transfer across the metal-support interface is likely to happen between high energy levels (near the Fermi level of the metal and the conduction band of the ceria). Therefore, the effect on core electrons could well be minimal. The study of band structure by UPS, or investigation of the Auger spectrum, may yet provide the key information.

4.

Conclusions

In this work we have investigated PM/ceria systems with reference to the interactions promoting ceria reducibility. We conclude that there are two different metal-ceria interactions, under reducing and non-reducing conditions. The majority of our experimental results indicate that an electronic interaction occurs when a metallic PM/ceria junction is created under reducing conditions, when the PM is in a metallic form. Improved reducibility of ceria, rearrangement of the ceria band structure and the degree of oxygen vacancy formation have been shown to be dependent on the work function of the PM. However, no compelling evidence for the junction effect theory was obtained by XPS analysis. On the contrary, results that would support a spillover mechanism have been found. This work underlines the complexity of the precious metal-ceria system, and offers a detailed set of experimental observations from in situ and ex situ techniques that cast light on the interactions that lead to the well-known improvement in the reducibility of ceria when in contact with a precious metal. Any model based solely on the electronic interactions between the two components, or a model which only takes into account the hydrogen spillover mechanism, is shown to be limited. It is apparent that the electronic and chemisorption mechanisms co-exist in these systems. In addition, a structural effect has been demonstrated by using ex situ spectroscopic techniques on the as-prepared samples. The redox properties of ceria are modified by the presence of PM oxide under oxidising conditions. In situ Raman helps explain the mechanism of this structural modification, with the support of XRD, TEM and ex situ Raman analysis. The structural modification can occur via PM-cation insertion, with ceria-PM solid solution formation confined to 26 ACS Paragon Plus Environment

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the interface, or via overlayer growth of ceria onto the crystalline PM oxide. The dominant promoting structural modification mechanism depends on the relative size of ionic radii between the PM ion and Ce4+ in the mixed oxide, on the nanoparticles distribution and the degree of crystallinity of the PM oxide.

Acknowledgements The authors are grateful to D. Boyd of Johnson Matthey for statistical analysis of the data; to M. Kett of Johnson Matthey for in situ Raman measurements; to Dr. D. Ozkaya and Dr. J. Dong of Johnson Matthey for TEM images, to H. Jobson of Johnson Matthey for XRD measurements; and to D. Leonarduzzi and G. Spikes of Johnson Matthey for useful discussion. The authors would also like to thank the EU Marie Curie Programme for funding (NA) under the contract NEWGROWTH (contract number MEST-CT-2005-020724).

Supporting Information Supporting information is enclosed. This information is available free of charge via the Internet at http://pubs.acs.org.


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Page 32 of 57

Table 1. PM particle size measured by TEM

Metal/ Support

Particle Size (nm) Before ceria deposition

After ceria deposition

1%Pt/Al2O3

0.2-5

0.7-3

1%Pd/Al2O3

4

2-10

1%Rh/Al2O3

-

-

1%Ru/Al2O3

-

-

1%Ag/Al2O3

5-30

-

1%Pt/SiO2

2-3

5-12

1%Pd/SiO2

20-30

50

1%Rh/SiO2

-

-

1%Ru/SiO2

>30 nm

20-50

1%Ag/SiO2

2.5

No nanoparticles observed


32

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Table 2. XPS results for the PM on alumina, ceria PM supported on alumina, Pt on silica, and ceria Pt supported on silica. The oxidation state of the metal is unchanged after ceria deposition.

Catalysts (as prepared) Pt/Al2O3 CeO2/Pt/Al2O3 Pd/Al2O3 CeO2/Pd/Al2O3 Rh/Al2O3 CeO2/Rh/Al2O3 Ru/Al2O3 CeO2/Ru/Al2O3 Ag/Al2O3 CeO2/Ag/Al2O3

Pt 4f (eV) Pt 4f 7/1 Pt 4f 5/1 72.56 75.86 72.68 76.01

Pt/SiO2

74.53

CeO2/Pt/SiO2

74.81

M 3d (eV)

PM oxidation state 2+ 4+

336.51 336.69 309.25 309.30 280.79 281.26 368.16 368.12

2+ 3+ 4+ Oxide 2+ 4+


33


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Page 34 of 57

Table 3. Raman spectroscopy shift for ceria supported PM samples

Sample CeO2/Pt/Al2O3 CeO2/Pd/Al2O3 CeO2/Rh/Al2O3 CeO2/Ru/Al2O3 CeO2/Ag/Al2O3 CeO2/Pt/SiO2 CeO2/Pd/SiO2 CeO2/Rh/SiO2 CeO2/Ru/SiO2 CeO2/Ag/SiO2 CeO2/Al2O3 CeO2/SiO2 CeO2

Raman spectroscopy shift (cm-1) 465 ± 2 462 ± 2 464 ± 2 N 463 ± 2 N 457 ± 2 459 ± 2 N 453 ± 2 463 ± 2 463 ± 2 464 ± 2


34

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Table 4. Ceria transition bands for CeO2/PM/SiO2 and Al2O3 and metal free reference materials

Catalyst

Ce3+ - O2- charge Ce4+ - O2- interband Ce4+ - O2- charge transfer (nm) transition (nm) transfer (nm)

CeO2/Pt/SiO2

308

352

386

CeO2/Pd/SiO2

315

341

385

CeO2/Rh/SiO2

320

346

385

CeO2/Ru/SiO2

320

347

383

CeO2/Ag/SiO2

317

334

384

CeO2/Pt/Al2O3

324

352

384

CeO2/Pd/Al2O3

322

343

385

CeO2/Rh/Al2O3 318

336

385

CeO2/Ru/Al2O3 323

353

385

CeO2/Ag/Al2O3 322

334

386

CeO2/SiO2

308

330

-

CeO2/Al2O3

268

301

340

CeO2

267

326

-


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Page 36 of 57

Table 5. Ceria reduction features for ceria coated PM on alumina

surface ceria Surface ceria reduction Bulk ceria reduction Metal Promoted reduction temperature (°C) temperature (°C) temperature (°C) Pt

110

390

780

Pd

140

325

765

Rh

200

270

770

Ru

220

260

755

Ag

200

530

730


36

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Table 6. Ceria reduction features for ceria coated PM on silica

surface ceria Surface ceria reduction Bulk ceria reduction Metal Promoted reduction temperature (°C) temperature (°C) temperature (°C) Pt

80

380

-

Pd

100

460

675

Rh

125

370

650

Ru

155

325

660

Ag

173

-

-


37


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Page 38 of 57

Table 7. XPS data for 1%Pt/SiO2 and 15%CeO2/0.85%Pt/SiO2 before and after reduction

Sample

Binding Energy (eV) Pt 4f

Atom %

Fresh

80 °C

300 °C

Fresh

80 °C

300 °C

74.53

71.08

71.14

0.28

0.27

0.23

15%CeO2/0.85%Pt/SiO2 74.81

71.10

71.36

0.59

0.65

0.31

1%Pt/SiO2


38

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 nm

5 nm

A

C

B

20 nm

D

Figure 1. TEM images of: (A) Pt/Al2O3; (B) CeO2/Pt/Al2O3; (C) Ru/Al2O3; (D) CeO2/Ru/Al2O3


39


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Page 40 of 57

Interface Ru-CeO2 Ru

CeO2 5 nm

Figure 2. Interface between Ru particles and ceria


40

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 nm

A

5 nm

B

Figure 3. TEM images of CeO2/Rh/SiO2 (A) and CeO2/Rh/Al2O3 (B)


41


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Page 42 of 57

Figure 4. Raman spectra for CeO2/Ag/Al2O3 and CeO2/Ag/SiO2


42

Page 43 of 57

0.12 0.1

Ce4+ - O2- interband transition Ce3+ - O2- charge transfer

Ce4+ - O2- charge transfer

0.08 K-M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ceria absorption edge

0.06

Platinum d-d transition

0.04 0.02 0 200

300

400

500

600

Wavelenght (nm)

Figure 5. Typical example of DR UV spectrum of CeO2/Pt/SiO2 catalysts


43


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Page 44 of 57

Figure 6. Comparison of band gap (eV) of unsupported ceria with CeO2/PM/SiO2 and CeO2/PM/Al2O3


44

Page 45 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. TPR profiles of supported PM on alumina catalysts: a. 1%Ag/Al2O3, b. 1%Ru/Al2O3, c. 1%Rh/Al2O3, d. 1%Pd/Al2O3, e. 1%Pt/Al2O3.


45


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Page 46 of 57

Figure 8. TPR profiles of supported PM on silica catalysts: a. 1%Ag/SiO2, b. 1%Ru/SiO2, c. 1%Rh/SiO2, d. 1%Pd/SiO2, e. 1%Pt/SiO2


46

Page 47 of 57

250

200

°

* *

· Kath Volts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


°

▀ ▀

b ▀

°

*

a

c

150

*

100

*

·

50

· ·

°

°

▀

°

*

▀

°

-25

175

375

▀ 575

e

f

▀

0

d

g

775

Temperature (°C)

Figure 9. TPR profiles of ceria, ceria on alumina and ceria PM on alumina catalysts: a. CeO2, b. CeO2/Al2O3, c. CeO2/Ag/Al2O3, d. CeO2/Ru/Al2O3, e. CeO2/Rh/Al2O3, f. CeO2/Pd/Al2O3, and g. CeO2/Pt/Al2O3 (·: PM oxide; *: promoted surface ceria; ° : surface ceria; ▀ : bulk ceria)


47


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

º

Page 48 of 57

▀

a

* *

b

º * * *

º º

▀

c

▀

d

▀

º

e f

Figure 10. TPR profiles of ceria on silica and ceria PM on silica catalysts: a. CeO2/SiO2, b. CeO2/Ag/SiO2, c. CeO2/Ru/SiO2, d. CeO2/Rh/SiO2, e. CeO2/Pd/SiO2, and f. CeO2/Pt/ SiO2 (*: promoted surface ceria; ° : surface ceria; ▀ : bulk ceria)


48

Page 49 of 57

in situ CeO2/Pd/Al 2O3 (DP) 2.90

2.90 2.85 2.80 2.75 2.70 2.65 2.60 2.55 2.50

2.80 Band Gap (eV)

Band gap (eV)

in situ CeO2/Pt/Al 2O3 (DP)

2.70 2.60 2.50 2.40 2.30 2.20

0

100

200

300

400

500

0

100

Temperature (°C)

200

300

400

500

400

500

Temperature (°C)

in situ CeO2/Rh/Al 2O3 (DP)

in situ CeO2/Ru/Al 2O3 (DP) 2.90

2.80

2.85

Band gap (eV)

2.90

2.70 2.60 2.50 2.40

2.80 2.75 2.70 2.65

2.30

2.60 0

100

200

300

400

500

0

100

Temperature (°C)

200

300

Temperature (°C) in situ CeO2/Ag/Al 2O3 (DP)

2.90 2.85 Band gap (eV)

Band Gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.80 2.75 2.70 2.65 2.60 0

100

200

300

400

500

Temperature (°C)

Figure 11. Band gap against temperature under reducing conditions for ceria PM on alumina (PMs: Pt, Pd, Rh, Ru and Ag). The random error on the band gap measurement was estimated to be of ± 5%.


49


in situ CeO 2/Pd/SiO

2.80

2.75

2.70

2.65

Band Gap (eV)

Band Gap (eV)

in situ CeO2 /Pt/SiO2 (DP)

2.60 2.50 2.40

2

(DP)

2.55 2.45 2.35 2.25

2.30 0

100

200 300 Temperature (°C)

400

0

500

100

200

300

400

500

400

500

Temperature (°C)

in situ CeO2/Rh/SiO2 (DP)

in situ CeO2/Ru/SiO2 (DP)

2.75

2.75

2.70

2.73

Band Gap (eV)

Band Gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 57

2.65 2.60 2.55 2.50

2.71 2.69 2.67 2.65

2.45 0

100

200

300

400

500

0

100

200

300

Temperature (°C)

Temperature (°C)

Figure 12. Band gap against temperature under reducing conditions for ceria PM on silica (PMs: Pt, Pd, Rh, and Ru). The random error on the band gap measurement was estimated to be of ± 5%.


50

Page 51 of 57

0.4

Pt

0.35 Band Gap Difference (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.25

Pd

0.2 0.15

Rh

0.1

Ru

0.05 0 4.5

4.7

4.9

5.1

5.3

5.5

5.7

5.9

Work Function (eV)

Figure 13. Plot of difference in band gap values after and before PM reduction vs. tabulated work function.69 The random error on the band gap measurement was estimated to be of

± 5%. The standard errors were assessed by linear

spline statistical analysis for the tangent analysis method for the CeO2/PM/SiO2 (PM: Pt, Pd, Rh and Ru) spectra at 30 °C and 300 °C . A maximum error of ± 2% was calculated.


51


A

1.E+06 1.E+06 9.E+05 8.E+05

Band Area

7.E+05 6.E+05 5.E+05 4.E+05 3.E+05 2.E+05 1.E+05 0.E+00 0

50

100

150

200

250

300

350

400

Temperature (°C)

Pt

Rh

Ag

B 2.E+05

Band Area Pt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 57

1.E+05

0.E+00 0

50

100

150

200

250

300

350

400

Temperature (°C)

Ru

Pd

Ce

Figure 14. Ceria fluorite structure peak area vs. temperature under reducing conditions for (A) and CeO2/PM/Al2O3 (PMs: Pt, Rh, Ag); (B) CeO2/Al2O3 and CeO2/PM/Al2O3 (PMs: Pd and Ru)


52

Page 53 of 57

In situ Raman CeO2/Pt/Al2O3

470 465 460 455 450 445 440 435 430 425

In situ Raman CeO2/Pd/Al2O3 470 465 460 455 450 445 440 435

0

50

100

150

200

250

300

430 425

350

0

50

100

Temperature (C)

In situ Raman CeO2/Rh/Al2O3

150 200 250 Temperature (C)

300

350

In situ CeO2/Ru/Al2O3

470 465

470

460 455

460

465 455

450 445

450

440 435

445

430 425

435

440 430

0

50

100

150

200

250

300

350

0

50

100

Temperature (°C)

150

200

250

300

350

Temperature (°C)

In situ CeO2/Ag/Al2O3 Raman Frequency Shift (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


470 465 460 455 450 445 440 435 430 425 0

50

100

150

200

250

300

350

Temperature (°C)

Figure 15. In situ Raman of CeO2/PM/Al2O3 (PMs: Pt, Pd, Rh, Ru and Ag). The error on the Raman frequency shift was estimated to be ± 4 cm-1.


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Variation of Raman shift after PGM reduction (cm-1)


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22 20 18

Pt

16 14

Pd

12 10 8

Ag

6

Rh

4

Ru

2 0 4

4.5

5 Work Function (eV)

5.5

6

Figure 16. Variation of the Raman frequency shift of the ceria fluorite structure before and after PM reduction vs. tabulated work function69


54

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Figure 17. 3d Cerium signal of fresh CeO2/Pt/SiO2, CeO2/Pt/SiO2 reduced in 5%H2/N2 at 300°C for 2 h (10°C/min) and CeO2/Pt/SiO2 reduced in 5%H2/N2 at 80°C for 30 min (5°C min-1)


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Figure 18. 4f platinum signal of CeO2/Pt/SiO2 and Pt/SiO2 fresh, after reduction at 80 °C and 300 °C


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Interface Ru-CeO2 Ru

CeO2 5 nm


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Promotion of Ceria Catalysts by Precious Metals: Changes in Nature

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