JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009, p. 196

Foundation item: MIUR for grant under PRIN projects

Corresponding author: Marta Boaro (E-mail: marta.boaro@uniud.it)

DOI: 10.1016/S1002-0721(08)60219-7

Redox behavior of gold supported on ceria and ceria-zirconia based catalysts Michela Vicario1, Jordi Llorca2, Marta Boaro1, Carla de Leitenburg1, Alessandro Trovarelli1 (1.Dipartimento di Scienze e Tecnologie Chimiche, Università di Udine, Via del Cotonificio 108, 33100 Udine, Italy; 2. Institut de Tècniques Energètiques, Universitat Politècnica de Catalunya, Diagonal 647, ed. ETSEIB, 08028 Barcelona, Spain)

Received 5 August 2008; revised 14 November 2008

Abstract: A series of gold-based catalysts were prepared by deposition precipitation or incipient wetness impregnation on CexZ1–xO2 solid solutions (0.28≤x≤1.00). The morphological and structural characterization of these catalysts were carried out with X-ray diffraction, trans-mission electron microscopy (TEM) analysis and physical adsorption technique, and their redox properties were studied by temperature pro-grammed reduction using both H2 and CO as probe molecules. Two cycles of oxidation/reduction were carried out in order to evaluate the effects of redox aging and gold sintering on the oxygen exchange capability. As observed with other noble metals, gold enhanced and pro-moted the ceria reduction at lower temperatures. Reduction by CO was shown to be dependent on the fine dispersion of gold and to be nega-tively affected by the ageing process more than reduction with hydrogen. This might have implications in reactions like water gas shift and CO-PROX which involve CO as a main reactant.

Keywords: gold; ceria; zirconia; catalysis; reduction behaviour; nanoparticles; temperature programmed reduction; X-ray diffraction; rare earths

Since the early studies by Haruta and coworkers[1,2] we

experienced an exponential growth of papers and reports about the unusual activity of nanosized gold-based materials in many field of catalysis[3,4]. The research efforts on these materials have been focused mainly on the development of novel catalysts for the key steps in H2 production by hydro-carbon reforming for fuel cell applications, such as CO oxi-dation[5], low temperature water gas shift (LT-WGS)[6,7] and selective oxidation of CO in the presence of a large excess of hydrogen (CO-PROX)[8]. Among all the factors which contribute to make gold an active component for these reac-tions, the nature of metal/support interface plays a very im-portant role. The design of the contact structure of the Au/metal-oxide interface depends on the method of synthe-sis, which brings about a great difference in the size of gold particles and in the interaction with the support[3,9]. Further-more, the catalytic properties of gold/metal-oxide interface strongly depend on the nature and textural structure of the support. The support must present a defective surface avail-able for strong interaction with gold precursors. Among the several compositions under study, ceria supported gold sys-tems have received a large attention. In this prospective, the easy reducibility of ceria, with the consequent formation of surface vacancies, makes the supports based on this oxide very peculiar[6,10,11]. Therefore, it is possible to design very

active gold catalysts improving the oxygen storage capacity of ceria by incorporation of other elements such as Zr4+ ions into its lattice[12]. Recently it has been reported a very active Au/CexZr1–xO2 composition for LT-WGS reaction[13] and CO oxidation[14]. In addition to enhancement of reducibility, doping the lattice of ceria with zirconia improves the ther-mal resistance of these catalysts[15]. This is a further advan-tage since the morphology and particle size distributions of ceria crystallites are also important parameters to determine their activity[16]. It has been shown that nanocrystalline ceria is much more effective in keeping gold dispersed and in ac-tive state in comparison to a micro structured support[17].

The present study investigates on the structural and redox properties of fresh and aged Au/CexZr1–xO2 compositions with 0.28≤x≤1.00 since these materials are promising cata-lysts for hydrogen portable reformer systems. Our main goal was to obtain insights about the nature of interaction be-tween gold and ceria based supports. On the one hand, we investigated the effect of introduction of Zr4+ into the lattice of ceria on the redox properties of supported gold species, on the other hand, the effect of gold on the reducibility of ceria zirconia has been studied through the choice of two different preparation methods: (1) deposition-precipitation that allows a fine dispersion of gold; (2) incipient wetness impregnation producing larger particles. For this purpose,

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Michela Vicario et al., Redox behavior of gold supported on ceria and ceria-zirconia based catalysts 197

temperature programmed studies have been carried out to investigate the nature of the metal/support interactions and the redox properties of materials under such conditions[18].

1 Experimental

1.1 Catalyst preparation

Gold catalysts (1wt.%) were prepared either by deposition precipitation (DP) or by incipient wetness impregnation (IW) using HAuCl4·3H2O (Aldrich, 99.9+%) as gold precursor ac-cording to the procedures described in Ref.[19]. In order to ensure the homogeneity of all samples we choose commercial ceria and ceria-zirconia mixed oxides as supports (Grace Davison). They will be indicated hereafter as CZXXDP or CZXXIW where the number XX refers to the ceria content and the suffix (DP or IW) indicates the synthesis method adopted (i.e., CZ75IW indicates Ce0.75Zr0.25O2 prepared by in-cipient wetness impregnation). Before gold deposition all the supports have been calcined at 923 K for 1 h, after deposition all the samples were dried at 383 K overnight and then cal-cined at 923 K for 1 h under air at a heating rate of 10 K/min.

The redox aging treatment of the catalysts consists of a tem-perature programmed reduction up to 1273 K at a rate of 10 K/min and a successive 1h oxidation at 773 K under air flow.

1.2 Catalyst characterization

The catalyst microstructure and morphology was deter-mined on fresh and aged samples by several different tech-niques. BET specific surface areas were measured with ni-trogen adsorption at 77 K on a Carlo Erba Sorptomatic 1900 instrument. Prior to the adsorption isotherm experiment, each sample was degassed at 423 K for 90 min. X-ray pow-der diffraction profiles were collected on a Philips PW3040/60 X’Pert PRO X-ray diffractometer, with a Ni- filtered, Cu Kα radiation source (0.1540598 nm) operating at voltage 40 kV and current 40 mA, equipped with an X’Celerator detector. The spectra were collected under ac-quisition conditions of 0.02° in step size and step time of 40 s over the 2θ range of 20°–145°. The assignment of the various crystalline phases was based on X’Pert High Score software provided with the instrument. The mean crystallite size of the supports was estimated using the Scherrer’s equation on FWHM of X-ray diffraction peaks, correcting the instrumental line broadening. The structural analysis was carried out by Rietveld refinement, performing the fitting with the Powder Cell Program, using Pseudo-Voigt profiles.

Conventional transmission electron microscopy (TEM) combined with energy-dispersive X-ray microanalysis was performed on a Hitachi MT-800 electron microscope work-ing at 200 kV and equipped with a Kevex analytical system

and a built-in CCD camera. Microstructural characterization by high resolution microscopy (HRTEM) was carried out with a Philips CM-30 instrument operating at 300 kV with 0.20 nm point-to-point resolution. Samples were placed on copper grids with a holey carbon film by depositing a drop of specimen suspended in methanol.

1.3 Reduction studies

Redox behavior was checked by running two consecutive TPRs, with an oxidation treatment under air at 773 K for 1 h in between. The second TPR was chosen as an indicator of the re-sponse of the catalyst to severe reduction/oxidation treatments.

TPRs were carried out in a flow system by placing the catalyst in a tubular quartz microreactor (6 mm i.d.), with a 5%H2/Ar or 2%CO/Ar mixture, flowing at 35 cc/min. For the TPR measurements, the temperature range tested was 295–1273 K and the heating rate 10 K/min. To minimize contribution from superficial adsorbed species and to pro-vide a uniform and reproducible redox state before analysis, prior to the TPR the samples were treated in air at 773 K for 1 h. Gas analysis was performed on a micro-GC (Agilent P200) and a quadrupole mass spectrometer (Balzers QMS200), measuring the consumption of reactants (hydro-gen or carbon monoxide) and the simultaneous release of products (water or carbon dioxide).

2 Results and discussion

2.1 Structural and morphological characterization

Table 1 summarizes the BET specific surface areas and XRD data of the bare supports after calcinations and of the fresh and aged gold-supported catalysts prepared by incipient wetness impregnation and by deposition precipitation. It is shown that introduction of gold by impregnation method causes a decrease of surface area, especially for ceria rich compositions. In fact, in comparison with the deposi-tion-precipitation method, the impregnation leads generally to larger gold particles[20,21] that could fill up the micro- and meso- pores of support contributing to a decrease of surface area.

The crystallographic parameters calculated by Rietveld refinement of the XRD profiles of the supports are always in good agreement with those obtained with Vegard’s law for the formation of ceria zirconia solid solutions[22]. Ceria and ceria rich solid solutions crystallize in a typical cubic fluo-rite structure (space group Fm3m). As expected, owing to the smaller ionic radius of Zr4+ (0.084 nm) with respect to Ce4+ (0.097 nm), the introduction of Zr4+ ions into the ceria matrix induces a contraction of cell dimensions. Conversely, Rietveld analysis of CZ28 diffratogram reveals the presence of a cubic and a tetragonal phase.

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198 JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009

Table 1 Surface areas and crystallographic parameters of supports in all investigated samples

Samples SBET/(m2/g) Phase (phase %) Cell parameter/nm Particle size/nma

CZ100 b 38.2 Cubic 0.5409 13.5

CZ75 b 58.5 Cubic 0.5367 7.1

CZ44 b 72.8 Cubic 0.5275 5.2

CZ28 b 78.9 Cubic 10% Tetragonal 90% c 6.4

Au/CZ100DP fresh 35.2 Cubic 0.5410 18.5

Au/CZ100DP aged Cubic 0.5409 68.1

Au/CZ100IW fresh 23.9 Cubic 0.5409 22.7

Au/CZ100IW aged Cubic 0.5410 39.4

Au/CZ75DP fresh 56.0 Cubic 0.5367 7.0

Au/CZ75DP aged Cubic 0.5353 13.7

Au/CZ75IW fresh 32.8 Cubic 0.5368 8.6

Au/CZ75IW aged Cubic 0.5355 12.1

Au/CZ44DP fresh 72.2 Cubic 0.5275 4.9

Au/CZ44DP aged Cubic 0.5267 8.4

Au/CZ44IW fresh 48.8 Cubic 0.5276 5.4

Au/CZ44IW aged Cubic 0.5266 8.0

Au/CZ28DP fresh 78.1 Cubic 16% Tetragonal 84% c 6.9

Au/CZ28DP aged Cubic 7% Tetragonal 93% c 7.9

Au/CZ28IW fresh 61.0 Cubic 19% Tetragonal 81% c 7.1

Au/CZ28IW aged Cubic 23% Tetragonal 77% c 10.1

a: Calculated from Scherrer’s equation after Rietveld refinement; b: Supports calcined at 923 K; c: Cubic s.s. a=5.344; Tetragonal s.s. a=b=0.3670; c=0.5259

No significant differences on XRD patterns were ob-

served before and after gold deposition, suggesting that the introduction of gold does not induce any modification of the support phase. The XRD profiles of all IW fresh samples show peak at 2θ=38.24, 44.61, 64.60 degrees corresponding respectively to the (111), (200), (220) reflections of metallic gold phase. Conversely, in the fresh DP samples the pres-ence of gold is barely visible. The fact that Au0 is not de-tected in fresh DP samples suggests that the metallic particle size obtained with this technique may be smaller than di-mension required for detection (ca. 5 nm).

The ageing treatment (H2 reduction at high temperature followed by oxidation at 773 K) causes an evident sintering of the support which is less remarkable for the zirconia con-taining materials, in agreement with their high thermal re-sistance[15]. The sintering process affects also the distribution of gold particles; in fact after ageing metallic gold is de-tected in all samples, independently on the preparation method. Moreover, after ageing, a reduction of the a cell parameter is observed in most samples. This contraction is attributed to the formation of CexZr1–xAuyO2–δ solid solutions; introduction of gold, due to its dimension (ionic radius of Au3+ is 0.085 nm) is expected to cause a reduction of the cell dimensions of ceria-zirconia. A reduction of the cell pa-rameter of pure CeO2 has been also observed with incorpo-ration of Au3+[23]; in our case deposition of gold over pure

ceria does not bring to a notable modification of cell pa-rameters in powder XRD. However, it is not excluded that the cell shrinkage observed could be related to other reasons. The reduction at high temperature and the further oxidation at middle temperature may decrease the Ce3+/Ce4+ ratio in the lattice or induce structural modifications responsible to a similar decrease.

A more detailed morphological characterization was car-ried out on Au deposited on CZ100 and CZ44 samples by transmission electron microscopy. Figs.1(a) and (b) show the micrographs of fresh Au/CZ100 catalysts prepared re-spectively by DP and IW methods. In both cases the mean particle size of ceria crystallites (6–10 nm for Au/CZ100- DP vs. 10–23 nm for Au/CZ100-IW) is slightly smaller than that determined with XRD. However, it is notable that the particle size measurement obtained from HRTEM and XRD are not always comparable. In particular, when the distribu-tion of particles is wide, Scherrer equation could overesti-mate the particle size because the smaller crystallites con-tributing to the tails of profiles are not properly taken into account[24].

The selected-area electron diffraction pattern of Au/ CZ100 DP sample and its corresponding averaged profile (Fig.1(a)) show only rings corresponding to CeO2. However, it should be highlighted that the lattice spacing obtained from the profile does not correspond exactly to pure CeO2.

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Michela Vicario et al., Redox behavior of gold supported on ceria and ceria-zirconia based catalysts 199

The most intense diffraction ring of CeO2 (111) is in fact centred at 0.310 nm, which is not 0.312 nm of pure CeO2. This may indicate that Au3+ was stabilized in the form of a solid solution of type Ce1–xAuxO2–δ, as suggested in[23]. There are no direct evidences for the existence of metallic Au or even Au2O3.

Conversely, the diffraction ring obtained in selected-area mode for fresh Au/CZ100 IW catalyst (Fig.1(b)) was located at 0.312 nm, nevertheless some crystallites exhibited 0.310 nm spacing (results not shown). This suggests the amount of possible solid solution in Au/CZ100 DP is much larger than that in Au/CZ100 IW. Moreover, further HRTEM investiga-tions on the latter (micrographs not shown) point out also the presence of Au and Au2O3 phases, in agreement with that reported in Ref.[25].

After the ageing treatment there is a remarkable sintering in both DP and IW samples but the structure of catalysts remains apparently unchanged (Figs.1(c) and (d)). Except that in the aged Au/CZ100 DP catalyst we observed spots related to Au2O3 or Au2O (JCPDS 43-1039 and 23-0278) phases which were not detected in the fresh sample. Notably, XRD technique does not evidence the presence of any gold-oxide phase for these samples, probably due to size or amount of gold-oxide phase well below the detection limit.

Fig.2 shows HRTEM results of Au/CZ44 samples as rep-

resentative of ceria zirconia solid solutions. Fresh Au/CZ44 DP (Fig.2(a)) and IW (Fig.2(b)) samples are really very homogeneous and consist of very small, well faceted crystallites, quite smaller for the DP samples (4–8 nm vs. 6–10 nm). Independently on the method of synthesis there is no evidence of segregated Au-containing particles by HRTEM and the electron diffraction pattern exhibits only diffraction cycles of a cubic phase. Although the low load-ing of gold makes the detection of its phases hardly possible, the absence of gold may find a good explanation either in a strong interaction between gold species and the support or in the incorporation of Au into it. Interestingly, in these cata-lysts the sintering is less pronounced compared to the zirco-nia-free samples prepared in a similar way. After ageing the crystallites are still well faceted and their size has increased only slightly (Fig.2(c) and (d)).

To summarize, despite of some discrepancies in the re-sults, due to the intrinsic limitations of each technique, XRD and TEM studies have been shown to be complementary for a structural characterization of the catalysts. In particular, TEM results suggest the possible formation of solid solu-tions between gold and the support by incorporation of Au3+ into the lattice of ceria-zirconia, especially when gold cata-lysts were synthesized by deposition-precipitation.

Fig.1 HRTEM micrographs of Au/CZ100 prepared by deposition precipitation (fresh (a) and aged (c)) and by incipient wetness

impregnation (fresh (b) and aged (d))

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200 JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009

Fig.2 HRTEM micrographs of Au/CZ44 prepared by deposition precipitation (fresh (a) and aged (c)) and by incipient wetness impregnation

(fresh (b) and aged (d))

2.2 Redox behavior

The comparison of redox behavior of the catalysts with that of the bare supports and the effect of the metal on ceria reducibility have been studied by temperature-programmed reduction using either CO or H2 as reductant. The choice of CO as reducing agent is related to the fact that, in most of the reactions catalyzed by these materials (PROX, WGS), carbon monoxide is the main component of the inlet gas mixture. Moreover for ceria based materials CO is a strong reductive agent[26], sensitive to the textural and morphologi-cal properties of the sample. On the contrary to what occurs with H2, CeO2 reduction by CO does not involve diffusion in the bulk particles via chemical pump effects[27]. Therefore, CO-TPRs would give information not only on redox proper-ties of the catalysts but also on their textural properties and on the nature of metal/support interface.

A typical experimental procedure consisted of two con-secutive TPRs, with a re-oxidation treatment in between. Following this scheme, the first experiment is representative of the behavior of fresh material, while the second one is taken to assess the effect of in situ redox ageing on the re-ducibility of the catalysts[15,28].

The TPR profiles of bare supports are reported in Fig.3. In contrast to the TPR profiles of ceria, those of ceria-zirconia solid solutions show a unique peak centered at an intermedi-ate temperature and a higher degree of reduction. This is as-

Fig.3 CO (a) and H2 (b) TPR profiles of fresh (n) and redox aged

(n') ceria-zirconia supports: CZ100 (1), (1)'; CZ75 (2), (2)'; CZ44 (3), (3)'; CZ28 (4), (4)'

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Michela Vicario et al., Redox behavior of gold supported on ceria and ceria-zirconia based catalysts 201

cribbed to higher oxygen mobility in the solid solutions compared with that in pure oxide. These results are in agreement with the results reported in the literature[12,29]. However, in the CO-TPR profiles the onset of reduction is located at lower temperature (513 K vs. 723 K) compared with that in the H2-TPR (Figs.3(a) and (b), samples a–d). Moreover, the profiles are characterized by broader peaks centered at around 700 K, with a shoulder at ca. 600 K. This behavior is explained with the presence of sites of oxidation with different strength and activity[30]. CO oxidation on ceria-zirconia based system involves the sites where the ex-change with the surface oxygen is rapid and causes a fast release of CO2, and the sites where CO is more strongly ad-sorbed forming carbonate species that decompose at higher temperature. Moreover, as the reduction proceeds, CO2 is re-adsorbed on surface vacancies formed, justifying the broadening of peaks.

It is interesting to note that, in the case of CO reduction, the profiles of second TPR are similar to those of fresh sam-ples (Fig.3(a), (1)'–(4)'), while those relative to H2 show an enlargement of the peaks (Fig.3(b), (1)'–(4)') and a shift of the onset of reduction toward lower temperature. This is a consequence of structural changes occurring under H2 at-mosphere[29].

Figs.4 and 5 show the TPR profiles and Table 2 summa-rizes the quantitative results of the two TPRs cycles carried out on gold supported samples. From a general point of view, the behaviour of metal supported samples is similar to that of bare supports (Fig.3): independently on the reactant used, the degree of reduction of Ce4+ to Ce3+ increases with the amount of zirconia, being almost complete for CZ28 and CZ44 compositions. Qualitatively the profiles of the two TPRs

Fig.4 CO-TPR profiles of Au-supported samples prepared by DP (a)

and IW (b) in a fresh (n) and redox aged state (n'): Au/CZ100 (1), (1)'; Au/CZ75 (2), (2)'; Au/CZ44 (3), (3)'; Au/CZ28 (4), (4)'

Fig.5 H2-TPR profiles of Au-supported samples prepared by DP (a)

and IW (b) in a fresh (n) and redox aged state (n'): Au/CZ100 (1), (1)'; Au/CZ75 (2), (2)'; Au/CZ44 (3), (3)'; Au/CZ28 (4), (4)'

Table 2 Reduction degree* measured after TPR

Samples Reduction of Ce4+ with CO /% Reduction of Ce4+ with H2 /%

First TPR Second TPR First TPR Second TPR

CZ100 46 42 61 51

CZ75 72 73 67 90

CZ44 78 98 91 88

CZ28 100 100 100 92

Au/CZ100DP 24 26 60 56

Au/CZ75DP 60 60 56 98

Au/CZ44DP 84 80 100 100

AuCZ28DP 100 100 100 100

Au/CZ100IW 42 40 34 34

Au/CZ75IW 80 72 60 60

Au/CZ44IW 92 92 90 92

Au/CZ28IW 100 100 100 94

* Calculated considering gold in metallic state

are different with the maxima of reduction slightly to higher temperature in the second TPR while total degree of reduc-tion remains similar.

The CO-TPR profiles of the catalysts investigated are shown in Fig.4. In the fresh DP samples (Fig.4(a), (1)–(4)) the onset of reduction is anticipated of more than 160 K compared with that for the unsupported catalysts. Now the main reduction peak is located in the range 400–600 K, and the broadness is reduced. Such a promoting effect is not ob-served in Au/CZ samples prepared by IW (Fig.4(b), (1)–(4)). In this case the TPR profiles are similar to those of bare supports, with the onset of reduction starting at a slightly higher temperature (580 K). It is interesting to note that the catalysts prepared by DP underwent a strong deactivation

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202 JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009

after ageing (Fig.4(a), (1)'–(4)') conversely to the IW sam-ples (Fig.4(b), (1)'–(4)'). In the latter the low temperature component disappeared but the average temperature of re-duction remains the same (770–800 K).

A possible interpretation of different behavior between DP and IW catalysts can be related to the higher dispersion of gold obtained with the former synthesis. TEM studies confirm the existence in the DP samples of a stronger inter-action between gold and ceria and the possible presence of cationic gold species. This would favor the adsorption of CO and would weaken the Ce-O bond[31,32]. Therefore, one possible mechanism is that CO is adsorbed at the perimeter of nanosized gold particles where it will react with the labile oxygen ions of support. The shape of peak and the low tem-perature of reduction suggest also that this kinetics should be fast[33]. The assumption that the sites of reduction are located mainly at the metal-support interface would justify the strong deactivation of the catalysts, especially of Au/CZ100, due to the aging treatment. TEM and XRD characterizations have shown that during the first TPR cycle occurs an irre-versible sintering of both gold particles and supports. This, in turn, could modify the nature of the metal support inter-face and consequently the mechanism of reduction. After aging the redox process appears to be controlled by the oxygen storage capacity of the support.

Fig.5, which shows the H2-TPR profiles of DP and IW catalysts, evidences that the effect of preparation method are less important when the reducing agent is H2. In this case gold promotes the reducibility of all supports examined in-dependently on the method used for its deposition. However, DP samples show a 100 K anticipation of the onset of re-duction in comparison with the IW samples.

Moreover, among the catalysts tested only the Au/CZ100 IW showed another peak at higher temperature (1100 K). This behavior agrees with the observed promoting effect generally exerted by noble metals on ceria: concurrently to the reduction of the supported metal precursor, it is induced a reduction of ceria surface via mechanism of spillover of the activated hydrogen[34].

For other noble metals the redox ageing induces generally a loss of reduction capabilities[35,36], it is worthy to note that gold is able to promote the reduction of Ce4+ to Ce3+ even after its sintering (Fig.5(a) and (b), samples (1)'–(4)'). In our case, especially for ceria, the absence of deactivation could be correlated with the possible formation of a solid solution between gold and ceria as suggested by HRTEM analysis[37]. However, the redox properties of DP samples are slightly affected by the high temperature reducing treatment. In fact, the second TPR profiles of these samples show a shift at high temperature and broadening of their main reduction peaks. This suggests that the nature of gold/ceria interface

plays a more important role in the kinetics of H2 spillover for the DP than that of the IW gold catalysts.

To conclude, gold promotes the reducibility of the ceria based materials to lower temperatures, however, the degree of reduction depends on the percentage of zirconia incorpo-rated. Moreover, on the basis of results discussed it is evi-dent that this promoting action relies on the size of gold par-ticles with differences between hydrogen and carbon mon-oxide. It has been shown that hydrogen is able to reduce the investigated compositions at temperatures inferior or as low as 500 K, even when gold particle size is larger than 10 nm; while using CO gold is an effective promoter of reduction only if it is dispersed very finely (<4–5 nm) or strongly in-teracts with the support. Therefore, especially when dealing with CO oxidation, the choice of an appropriate synthesis is very important. The deposition precipitation method leads to more easily reducible catalysts. In this case both hydrogen and carbon monoxide are able to reduce ceria in the same range of temperature (300–500 K). This suggests that the reduction of CexZr1–xO2 compositions via H2 or CO may follow different kinetics; however, in the catalysts prepared by deposition precipitation H2 and CO could be adsorbed on the same sites as it has recently been proposed in Ref.[38].

This could influence the kinetics of those reactions such as water gas shift or selective CO oxidation in which CO and H2 are both present in the reaction mixture.

3 Conclusion

The choice of an appropriate method of synthesis and a suitable support turned out to be fundamental in the develop-ment of gold-based catalysts. This is due to a strong syner-gism between reducibility of the support, dispersion of gold and activation of reagents. The technique of reduction under temperature-programmed conditions was demonstrated to be a valid instrument in order to characterize the nature of the metal-support interaction and to obtain information on the re-dox activity and potential use of these materials.

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