CeO2 catalysts in the preferential oxidation of CO in H2-rich gases

Applied Catalysis A: General 348 (2008) 42–53

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

On the role of redox properties of CuO/CeO2 catalysts in the preferential oxidation of CO in H2-rich gases Tiziana Caputo a, Luciana Lisi b, Raffaele Pirone b,*, Gennaro Russo a a b

Dipartimento di Ingegneria Chimica, Universita` degli Studi ‘‘Federico II’’ di Napoli, P.le Tecchio, 80 Napoli, Zip code: I-80125, Italy Istituto di Ricerche sulla Combustione (CNR), P.le Tecchio, 80 Napoli, Zip code: I-80125, Italy

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 November 2007 Received in revised form 19 May 2008 Accepted 11 June 2008 Available online 26 June 2008

CuO/CeO2 catalysts with CuO content ranging from 0.5 wt.% to 8 wt.%, prepared by wet impregnation of commercial ceria, have been tested for the preferential oxidation of CO (CO-PROX) under H2-rich conditions at 70–210 8C. Catalytic activity increases up to 4 wt.% CuO content, with less concentrated catalysts showing higher intrinsic activity. Catalysts have been characterized by means of XRD, BET analysis and UV spectroscopy. Formation of segregated CuO clusters has been detected for Cu richest CuO/CeO2 sample. Redox properties have been deeply investigated using TP analysis (H2 TPR, CO TPR, TPO) of fresh or pre-treated samples. Participation of surface ceria, induced by the strong interaction with copper, to reduction/oxidation reactions in the temperature range explored (up to 430 8C) has been demonstrated. Different copper species and their reactivity towards H2 and CO have been individuated by comparing TPR of fully oxidized catalysts with those of partially oxidized catalysts. Active species have been identified as copper-ceria sites able to oxidize CO even at room temperature and to be re-oxidized by O2 at the same temperature. Transient experiments have been carried out at different temperature using a diluted mixture starting from oxidized or reduced catalysts and followed by a H2 TPR of the used samples. The results of these tests have showed that active centres for CO oxidation contain copper in the +2 oxidation state. At T > 100 8C some reduced copper sites are stabilized which promote H2 oxidation thus lowering the selectivity of the CO-PROX process. ß 2008 Elsevier B.V. All rights reserved.

Keywords: PROX Copper Ceria CO oxidation Hydrogen Fuel cell CuO/CeO2 Catalysis CO removal TPR TPO

1. Introduction The high demand of clean energy increases the interest towards fuel cell powered systems for stationary and mobile source applications because of their high efficiency combined with practically zero emissions of pollutants. One of the most promising option for automotive applications is the polymer electrolyte fuel cell (PEMFC) operating at 80 8C with Pt catalysts for the electrodes, whose function is, however, greatly hindered by CO even in traces. To date, the tolerance limit is fixed in the range from 10 ppm to 100 ppm. Actually, the gas stream produced in a fuel processor by auto-thermal reforming, and subsequently further optimized by water-gas-shift stages, is typically composed by 50–75 vol.% H2, 10–20 vol.% CO2, 5–10 vol.% H2O and 0.5–1 vol.% CO; thus, the CO concentration must be significantly reduced. For non-stationary applications the catalytic abatement of CO is the only feasible way, because of the high costs and

* Corresponding author. Tel.: +39 0817682235; fax: +39 0815936936. E-mail address: [email protected] (R. Pirone). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.06.025

dimensions of the compressor in the pressure swing adsorption, and because of the presence of high CO2 concentrations which may complicate methanation [1]. A proper catalyst for CO-PROX should operate in a range of temperature fixed by the fuel processor at 80–200 8C and must be very active and selective towards the oxidation of CO more than of H2. Moreover, the catalyst should not catalyze the reverse water gas shift reaction in order to prevent limitations to the maximum CO conversion. Noble metals based catalysts were firstly explored for this application; despite of their high intrinsic activity in the oxidation of CO, they are not completely suitable for CO-PROX process for the low surface oxygen availability [2], overcome by supporting such active phases on easily reducible supports which provide oxygen for the reaction and significantly enhancing the performances of these systems [3–10]. Nevertheless, the strong CO adsorption on metal sites [11] limits the access to the other molecules, oxygen included, thus reducing the rate of surface re-oxidation by molecular oxygen and hence the overall CO oxidation rate [12– 14]. In addition, the selectivity of the entire CO-PROX process is lowered at the lowest CO concentration levels (to be reached to

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

fulfil the PEM requirements), due to the insufficient coverage of catalyst surface and the consequent occurrence of H2 oxidation. A completely different CO oxidation mechanism occurs on gold based systems [1–19] because of their high activity in CO oxidation even at room temperature [20]. Indeed, CO adsorption on gold is weaker than on Pt [14,21,22] but the former catalysts activate CO oxidation at lower temperature and with a faster rate than hydrogen oxidation. So, Au should be regarded as an ‘‘intrinsic’’ catalyst for the process, because it activates the desired oxygen species which enhance CO oxidation rate much more than H2 oxidation. On the other hand, oxygen has a very low sticking coefficient on gold. For this reason the use of ‘‘active’’ supports like Fe2O3, TiO2 and CeO2 was demonstrated by Schubert et al. [5,6] to increase the CO oxidation rate by providing oxygen adsorption sites and thus supplying O2 during the reaction. Nowadays, the most investigated alternative system to noble metals for CO-PROX is CuO supported on CeO2. Performances (95% conversion and 60% selectivity at 200 8C) even higher than those reported for noble metals were found by Avgouropoulos et al. [23] for CuO/CeO2 catalyst. These systems provide a very high value of selectivity (80%) up to 150 8C and do not promote the reverse water gas shift reaction in the temperature range of interest for COPROX reaction [24]. Liu and Stephanopoulos [25,26] explained the enhanced activity of these catalysts with respect to more conventional alumina-supported copper by the stabilization of Cu1+, originated from the interaction between copper clusters and cerium oxide, and addresses to ceria the role of oxygen source. The presence of Cu1+ in CuO/CeO2 samples was also detected by Avgouropoulos et al. [23], Hocevar et al. [27] and Zou et al. [28]. Martinez-Arias et al. [29–33] claimed that the facile redox interplay between copper and cerium is the key factor of high performances of CuO/CeO2 catalysts. Actually, ceria is able to enhance the reducibility and the chemisorption capability of both active phase and support itself [34]. The enhancement of the redox properties has been deeply investigated in recent years and firstly explained by Sanchez and Gazquez [35] by the occurrence of the so called strong-metalsupport-interactions between metal atoms and oxygen ion lattice vacancies in the oxide support. For oxide supports with fluoritetype structure, like ceria, it has been demonstrated that this interaction is limited to the catalyst surface while the structure of the cationic sub-lattice would represent a high barrier to penetration of the bulk by the metal atoms. The occurrence of this strong interaction between ceria and the metal should imply that the interface zones between the metal and the support are the most reactive ones due to the higher lability of the surface oxygen, for both noble and transition metals supported on ceria [36–38]. On the basis of structural and redox characterization studies, at low temperature a reaction path following a redox mechanism on CuO/CeO2 catalysts, involving concomitant changes of the oxidation state of both active phase (Cu2+ $ Cu1+) and support (Ce4+ $ Ce3+) has been assumed [30,31,33,39] for the CO oxidation reaction. Thus, the active sites for the CO oxidation reaction were proposed as the interface sites between copper oxide and the support. Proposals based on kinetic modelling of redox changes of Cu-Ce catalysts under CO and O2 partial pressures suggest redox exchanges between Cu2+ and Cu1+ (and concomitantly Ce4+ and Ce3+) as the key steps of the reaction mechanism of CO oxidation [40]. A somewhat different mechanism has been put forward by Wang et al. [41] for CuO supported on Sm doped CeO2. They proposed the active sites to be related to oxygen vacancies on the

43

top of the copper oxide component, while attributing to the interface vacancies a role limited to promoting oxygen activation during an induction step prior to light-off. The very good selectivity of CuO/CeO2 catalysts at low temperature has been associated to the higher reducibility of copper under CO than under H2 [33,41]. Wang et al. [41] attributed the rapid rise of CO conversion at 80 8C to the activation of interfacial centres which, at higher temperature, are able to interact with H2 as well, thus leading to a progressive drop of the selectivity. In a large number of papers redox properties of copper/ceria catalysts have been investigated by H2 TPR [28,39,42–49] and the peaks attributed to different copper species. Nevertheless, reduction of ceria has been often neglected both in the qualitative and quantitative analysis. In this paper TP analysis has been used to investigate the redox properties of the CuO/CeO2 catalysts and correlate the response to oxidative or reductive treatments of the catalytic performances. Catalysts with different copper content have been studied in order to explore a range of composition including both samples with highly dispersed copper and samples with significant presence of bulk-like CuO. Redox behaviour under either H2 and CO and re-oxidation capability have been analyzed to explain the catalyst selectivity. Finally, transient experiments under reaction conditions have further enlightened the role of the redox properties of CuO/CeO2 catalysts in the CO-PROX process.

2. Experimental 2.1. Materials A commercial CeO2 supplied by Grace was used as support. The CuO/CeO2 catalysts were prepared by wet impregnation method by using hydrate copper acetate ((CH3COO)2CuH2O) as copper precursor (Aldrich, assay 99.8%). Deposition of copper oxide was performed in a rotating evaporator at 50 8C, 90 mbar and with a velocity of 120 rpm. The materials were dried overnight at 120 8C and subsequently calcined at 450 8C under dry air flow (5 N l/h) for 3 h. The calcination temperature was chosen to assure a complete decomposition of copper acetate which occurs within 300 8C, as determined by thermogravimetric analysis. In the following, the notation ‘‘xCuCe’’ will be used to indicate catalysts supported on ceria, ‘‘x’’ being the CuO weight percentage. 2.2. Catalytic activity tests Reaction tests were performed at 100 8C on both CeO2 and on H2 pre-reduced CuCe samples. Before each test the samples were treated in air at 400 8C for 1/2 h and then purged in N2 at the reaction temperature. The standard composition of the reaction mixture (36 N l/h) was: 0.5 vol.% CO, 0.5 vol.% O2, 50 vol.% H2 in N2, obtained using high purity 5 vol.% CO/N2 mixture and pure CO2, O2 and H2. In order to avoid safety risks related to H2 storage, the H2 stream was provided by a hydrogen generator (CLAIND). The catalyst (300 mg) with a particle size of 200–300 mm was placed in a tubular quartz reactor. The temperature in the catalyst bed was measured by a thermocouple placed in a tube coaxial with the reactor. The analysis system consisted in a Fisher-Rosemount NGA2000 analyser equipped with four channels for the contemporary analysis of CO and CO2 with an IR detector, H2 with a thermoconductivity detector, and O2 with a paramagnetic detector. Each channel has four different measurement ranges.

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

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The CO conversion and the selectivity of oxygen to oxidize CO, are defined as follows: ! OUT DOðCOÞ 0:5  ðnIN CO  nCO Þ 2 SCO ¼ ¼ OUT nIN DOðCOÞ þ DOðH2 Þ O2  nO2 2

X CO

2

nIN  nOUT ¼ CO IN CO nCO

where SCO, XCO are, respectively, the O2 selectivity to CO2 and the ðH Þ ðCOÞ CO conversion, and DO2 and DO2 2 the oxygen moles consumed, respectively, for the CO oxidation and for the H2 oxidation. 2.3. Catalysts characterization The copper content in the catalysts was evaluated by spectrophotometric analysis using an UV Hach-Drel/2000 spectrophotometer. 0.1 g sample was dissolved in 4 ml water solution containing 50% HF at 150 8C for 5 min. After diluting to 1 l, pH was adjusted to 4–5 (at pH > 5 ceria precipitation takes place) with NH4OH. Then bicinconinic acid (Hach CuVer2 reactant), forming a violet complex with copper, was added to the solution and absorbance evaluated at 560 nm. XRD was performed with a Philips PW 1710 diffractometer with Rotating Anode Generators and monochromatic detector using Cu Ka radiation. The data were elaborated by applying the Scherrer equation for an estimation of the mean particle size. Specific surface area of the calcined catalyst was determined by the multi-points BET methods by means of a Carlo Erba Sorptomatic 1900. The reduction and oxidation measurements were performed in the same experimental apparatus described for the reaction tests placing the sample (300 mg) with a mesh size 200–300 mm in the quartz tube. A gas flow (14 N l/h) containing 1 vol.% CO or 2 vol.% H2 was used to carry out CO and H2 TPR analysis, respectively, diluting with N2 (99.9999% purity) further purified through an oxygen trap. Before each TPR analysis, the catalyst was pretreated at 400 8C in O2, in order to remove carbonates and hydrates, cooled down in O2, purged in N2 and then conditioned under the reducing gas mixture for 1/2 h at room temperature before rising the temperature up to 430 8C (if not otherwise specified) at 10 8C/min. This preliminary TPR was followed by a further re-oxidation in order to stabilize the catalyst. Indeed, reproducible TPR profiles were found only after a first TPR cycle and, as a consequence, TPR curves reported in the paper correspond to the second TPR cycle. The standard re-oxidation treatment of the reduced sample was carried out at 430 8C with 1 vol.% O2/N2 mixture (4 N l/h) for 1 h. It was verified that a complete re-oxidation occurs after 15 min. CO2 TPD was performed by equilibrating the catalyst with 1 vol.% CO2/N2 mixture (14 N l/h) for 1 h, then purging in N2 (14 N l/h) for 1/2 h before starting the temperature ramp (10 8C/min) under inert flux.

The same procedure was followed for CO TPD analysis. In this case the sample was equilibrated in a 1 vol.% CO/N2 mixture (14 N l/h). CO TPD of pre-reduced samples was performed on samples reduced in H2 at 430 8C, then purged in N2 for 10 min at the reduction temperature, and cooled down to room temperature before CO saturation. The TPO analysis was performed on samples pre-reduced at 430 8C, purged in N2 at 430 8C for 1/2 h and cooled down under N2 (14 N l/h). The sample was stabilized 1/2 h in O2 stream at room temperature, subsequently the temperature was raised up to 430 8C. Transient experiments were performed on 4 CuCe sample diluted 1:3 with quartz particles (200 mg of catalyst and 600 mg of quartz) to guarantee an isothermal profile along the catalyst bed. The particles size was the same for both catalyst and inert material for avoiding by-pass phenomena. A more diluted mixture (2 vol.% H2; 1000 ppm CO; 2000 ppm O2 in N2; Q = 36 N l/h) was used with respect to catalytic activity measurements in order to limit as much as possible the catalyst overheating in the transient phases, due to the occurrence of exothermal reactions. 3. Results 3.1. Physico-chemical properties and performances in the CO-PROX Table 1 reports the actual CuO content, the mean crystallite size and the BET surface area of all catalysts and CeO2 support. The commercial ceria used in the present investigation exhibits a surface area of 56 m2/g. This value remains unchanged up to 2 wt.% CuO load, then slightly decreases, as expected, when the theoretical monolayer, roughly corresponding to 4 wt.% CuO, is approached or exceeded. X-ray diffraction analysis (patterns not shown) revealed the characteristic diffraction peaks of CeO2 phase (cerianite, cubic: 2u = 28.68, 33.18, 47.58, 56.38, 59.18) while only in the sample containing 8.2 wt.% CuO, the characteristic diffraction peaks of bulk CuO (tenorite, syn, monoclin: 2u = 35.58, 38.78, 48.78, 61.58) were detected, suggesting the segregation of CuO clusters with a CuO bulk-like structure. CeO2 crystallites size (Table 1) is not significantly affected by copper deposition even in the presence of segregated CuO clusters, namely for the 8 CuCe sample. This can be related to the impregnation method used in this work which strongly limits the formation of bulk mixed phases [29]. The effect of copper concentration on the activity of ceria supported catalysts in CO-PROX has been widely investigated [23,28,33,41,48,49], but the results are dependent on the chemical and physical nature of the support and on the preparation method used. Using the same support and the preparation technique as in this work, we have found [24] that the best performances should be associated to 5 wt.% CuO content, in agreement with

Table 1 Results of chemical and physical analyses of CeO2 and CuCe catalysts Sample

CuO (wt.%)

Cu content (mmol/g)

Cryst. size CeO2(1 1 1) (nm)

BET (m2/g)

CO conversion (%)

k100 (cm3/g/s)

k100/Cu (mm3 s1/mol Cu)

CeO2 0.5 CuCe 2 CuCe 4 CuCe 8 CuCe

0 0.6 1.8 4.2 8.2

0 76 228 532 1038

11.2 11.8 12.1 11.6 12.5

56 56 55 50 47

0 7.2 17.2 33.5 33

– 3.1 7.9 17.6 16.7

– 17 14 14 7

CO conversion and kinetic constant expressed both per unit mass of catalyst and copper at 100 8C and feed composition: 50 vol.% H2, 0.5 vol.% CO, 0.5 vol.% O2 in N2.

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

Avgouropoulos et al. [23] and Ratnasamy et al. [50]. The results of the catalytic tests carried out at 100 8C, temperature low enough to assure 100% selectivity for all catalysts investigated [24], are reported in this paper as CO conversion and kinetic constant, referred to both catalyst weight and Cu content, evaluated under the hypothesis of first order rate dependence on CO. It is well-known that ceria itself has its own activity for the CO oxidation reaction [37,51]. Nevertheless, the results of catalytic tests (not reported here), carried out on CeO2 under the same experimental conditions used for the CuCe catalysts, showed that the temperature range of activity is significantly higher (250– 450 8C) than that required for the CO-PROX process (80–200 8C). On the other hand, it should be remarked that ceria shows an intrinsic selectivity towards CO oxidation also in the presence of large amounts of H2 (about 60% selectivity at 50% CO conversion at 360 8C). However, when even a small amount of copper is supported on ceria, a huge increase in the CO oxidation activity is obtained, as shown in Table 1. At temperature as low as 100 8C the sample with the lowest Cu content shows measurable activity, whereas under the same experimental conditions the oxidation of CO on pure ceria was not detectable below 250 8C. The activity increases up to a CuO concentration of 4 wt.%, further increase of the copper load (to about 8 wt.%) not changing significantly the CO conversion. The k100/Cu, namely the kinetic constant measured at 100 8C and referred to the amount of copper in the catalyst, decreases slightly when increasing the copper concentration up to 4 wt.% while it halves when doubling the copper concentration from 4 wt.% to 8 wt.% suggesting that the additional copper species formed when exceeding the theoretical monolayer are inactive for the CO oxidation. On the contrary, highly dispersed copper shows a greater intrinsic activity. Moreover, it should be noticed that for the less active catalysts, at the temperatures required to reach the same CO conversion obtained for 4CuCe sample at 100 8C, the selectivity will be lower than 100% since, under standard CO-PROX conditions, it basically depends on the reaction temperature [23]. This result confirms the best performances found for the catalysts with a composition close to that corresponding to the monolayer coverage due to the better compromise between the amount of copper and the activity value per unit mass of copper. 3.2. H2 temperature programmed reduction The results of H2 TPR tests carried out over pure ceria and copper based catalysts are presented in Fig. 1, in terms of hydrogen uptake plotted as a function of temperature. Pure ceria shows two reduction peaks at about 500 8C and 800 8C, respectively. According to the literature [37,38,44,47,50], ceria is reduced by H2 only at temperature higher than 350 8C and the two reduction peaks correspond to the reduction of superficial cerium (at about 540 8C) and bulk CeO2 (at about 840 8C). By the integration of the area of the low-temperature peak, a H2 uptake of 355 mmol/g was calculated, corresponding to the reduction of 12% cerium of the whole sample from the oxidation state +4 to +3. This amount is in good agreement with the fraction of reduced surface cerium reported by Zimmer et al. [44] and by Tang et al. [46] for CeO2 samples by taking into account the different surface areas. The amount of surface cerium ions was estimated about 6% of total cerium on the basis of the cell parameters of fluorite [37] for a CeO2 with a surface area of 56 m2/g, i.e. exactly half the amount evaluated by TPR. This result can suggest a deeper reduction of cerium to +2 oxidation state, as supposed by Zimmer et al. [44], or that more than one layer is involved in the reduction process, this latter hypothesis being more probable due to the high instability of

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Fig. 1. H2 TPR profiles of fully oxidized samples: ceria; 0.5 CuCe; 2 CuCe; 4 CuCe; 8 CuCe.

CeO. Moreover, the overall consumption of hydrogen in the TPR of pure ceria is 1165 mmol/g, corresponding to an average Ce reduction of 40%. As already mentioned, the reduction profile of CuCe samples reported in Fig. 1 refers to the second cycle of the redox treatment, because the TPR profiles were found reproducible only after the first one, as also reported by others and attributed to copper redistribution during the reduction process stabilizing copper in lower energy sites [44,46,47]. The hydrogen uptake evaluated during the first and the second TPR tests is very similar, within 2% difference, but the distribution and the shape of peaks change, suggesting a significant modification of the nature of reducible species in the sample as a consequence of the first reduction. When even a small amount of copper is supported on ceria, namely 0.5 wt.%, the TPR pattern is strongly modified showing two reduction peaks at temperatures much lower than for pure ceria. Peaks lie below 200 8C, while the ceria low-temperature peak (540 8C) disappears; on the contrary, the ceria high temperature (840 8C) reduction peak remains unchanged. In Table 2 the H2 uptake, evaluated from the integration of TPR curves, is reported for all samples. The amount of H2 globally consumed in each experiment increases with increasing the copper load; moreover, for all CuCe catalysts this amount far exceeds that necessary to the complete reduction of copper from Cu2+ to Cu0 suggesting that some ceria should be involved in the reduction process at low temperature. This phenomenon has been also observed by Tang at al. [46] for a 5 wt.% CuO/CeO2 sample and by Zimmer et al. [44] for mixed copper-ceria. The hypothesis about the nature of such a very reducible ceria seems quite trivial: the disappearance of the ceria low-temperature peak associated to the excess H2 uptake clearly indicates that reduction of surface ceria is promoted by copper addition and occurs at lower temperatures. The excess hydrogen uptake with respect to the complete reduction of Cu2+ to metallic copper exhibits a weak maximum in correspondence of 4 CuCe sample, but its value ranges from 200 mmol/g to 250 mmol/g whatever the copper content, suggesting that even very few copper ions are

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

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Table 2 Results of H2 TPR analysis of fully or partially re-oxidized samples Sample

CeO2** 0.5 CuCe 2 CuCe 4 CuCe 8 CuCe

Fully oxidized samples

Samples re-oxidized at room temperature

Total H2 uptake (mmol/g)

H2/Cu

Excess H2 uptake* (mmol/g)

H2 uptake (mmol/g)

H2/Cu

1135 278 442 786 1280

– 3.7 1.9 1.5 1.2

– 200 214 250 242

– 287 307 396 373

– 3.8 1.3 0.7 0.4

* Excess H2 has been evaluated with respect to that necessary to reduce Cu2+ to Cu0. ** TPR of ceria is conducted up to about 1000 8C.

sufficient to promote the reducibility of surface ceria at significantly lower temperature. The modification of the redox properties of ceria by the introduction of small copper amounts has been demonstrated for nanosized ceria supported Cu or Cu-doped CeO2 [33,52] likely related to the synergistic effect between Ce4+/Ce3+ redox couple and Cu2+ ions in CeO2 matrix [53]. Moreover, Tschoepe et al. [52] reported a limited solubility of Cun+ ions in CeO2 lattice and a stabilization of Cu2+ ions and Cu2+ pairs on ceria surface. All these results suggests that, notwithstanding the preparation method, some copper dissolution takes place together with the formation of highly dispersed copper ions promoting surface ceria reduction at lower temperatures. The minimum copper load explored in this work is sufficient to favour such a phenomenon. The slight decrease of the amount of ceria participating to the reduction in the 8 CuCe sample is probably related to the lower fraction of exposed surface ceria with respect to samples with lower copper content. The shape of the TPR curves is quite complex. Increasing the copper concentration the onset temperature of TPR patterns shifts to lower values, as also observed by others [42,48], having a minimum for 4 CuCe sample. In the TPR profile of 0.5 CuCe two weakly overlapped peaks, at 175 8C and 220 8C, are easily detectable, while at least three peaks can be observed in the TPR curves of Cu-richer samples. A large H2 uptake at high temperature appears in the TPR profile of 8 CuCe catalyst which can be reasonably associated to the reduction of CuO bulk-like species, detected by XRD for this sample, less reducible than species that are more interacting with the support. Indeed, it is widely accepted that the segregation of CuO micro-clusters in highly dispersed copper oxide materials produces Cu species much more reducible than bulk CuO, but that are generally the least reducible among those generated by the interaction with the support [42,54,55]. A qualitative attribution of the TPR peaks to different species has been proposed by many authors [42–49], although reduction of surface ceria in the same range of temperature of copper has not always been considered [44,46,47,49] and, as a consequence, peaks have been associated only to copper species excluding ceria contribution. Luo et al. [42] attributed the well defined low and high temperature peaks, detected for samples with a wide range of composition, to finely dispersed CuO interacting with the CeO2 and larger CuO particles, respectively. A similar interpretation was also reported by Avgouropoulos and Ioannides [48], whereas Zou et al. [28] proposed, for the more complex TPR curves they found, the contribution of cluster copper species, isolated Cu2+ ions and crystalline CuO. On the other hand, Zimmer et al. [44], by detection of both H2 uptake and water emission, concluded that the first peak of TPR of a mixed copper-ceria sample should have been related to

hydrogen adsorption on ceria surface rather than to copper reduction. Similarly, Pintar et al. [47] reported that some H2 storage in the catalyst structure occurs in parallel to the reduction of copper oxide phase. Opportune tests of hydrogen adsorption at room temperature (not reported here) have confirmed that H2 is adsorbed on CuCe catalysts only after a reducing pre-treatment. Anyway at the end of each TPR test, we have not found any adsorbed hydrogen on the catalyst, due the high temperature reached (430 8C). Deconvolution of the TPR profile of 0.5 CuCe catalyst into two peaks provided a value of the area of the high temperature peak (191 mmol/g) very close to the excess hydrogen estimated for this sample with respect to the complete reduction of the copper species, thus suggesting that the peak at 220 8C corresponds to the reduction of surface ceria occurring at lower temperature due to the interaction with copper. Moreover, the H2 amount associated to the first signal is that expected on the basis of the copper content of this sample. In conclusion, the quite simple surface composition of this catalyst allows an easy determination of the reducible species which can be associated to copper highly interacting with the support and to promoted surface ceria, respectively. Nevertheless, the presence of new peaks and, at the same time, their shift towards lower temperatures strongly complicates the correlation with different species for catalysts with higher copper loads, especially for the TPR profile of 8 CuCe where the large signal at the highest temperature, related to CuO micro-clusters hides the maximum of other peaks. For this reason, for more concentrated catalysts, we are not going to propose peak deconvolution, although the presence of a signal related to ‘‘copper-activated’’ surface ceria has been supposed in the TPR curves of all catalysts investigated. The TP experiments described in the following paragraphs aid the identification of the different peaks constituting the whole TPR profile through the elimination of one or more contributions upon suitable pre-treatments of the samples. 3.3. CO temperature programmed reduction The reducibility of copper/ceria catalysts was investigated by employing also CO as reducing agent in TPR tests. The interest towards CO TPR analysis is dual: on one side, it adds more detailed information on catalyst redox properties to the results of H2 TPR, due to the different reducing power and chemical properties of carbon monoxide with respect to hydrogen, on the other side CO and H2 are properly the two molecules in competition in the COPROX reaction. The results of CO TPR tests are shown in Fig. 2 and the corresponding quantitative analysis reported in Table 3. The major reduction of CuCe samples occurs below 300 8C, whereas pure ceria is reduced in a wider temperature range. Surprisingly, during the CO TPR, not only CO and CO2 but also H2 have been detected at the reactor outlet for temperature higher than 450 8C. This can be explained by the occurrence of a WGS reaction between CO and the OH groups of ceria, still present on the surface after the pre-treatment at 450 8C: CO þ OH ! CO2 þ 1=2H2

(1)

Indeed, it has been reported that only at 750 8C under O2 flux, hydroxyl groups can be completely removed [37]. As a consequence, a significantly higher reductant consumption is observed in the CO than in the H2 TPR for pure CeO2. However, by computing the amount of H2 produced at high temperature and taking into account the corresponding amount of CO consumed to the reduction of surface hydroxyls (according to the stoichiometry

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

Fig. 2. CO TPR of fully oxidized samples: 0.5 CuCe; 2 CuCe; 4 CuCe; 8 CuCe. CO uptake (—) and CO2 release ( ) as functions of time on stream (under isothermal conditions, before starting the programmed temperature ramp) and temperature (under heating rate of 10 8C/min). Table 3 Results of CO TPR analysis of fully oxidized samples Sample

CeO2 0.5 CuCe 2 CuCe 4 CuCe 8 CuCe

Fully oxidized samples

Sample re-oxidized at room temperature

Total CO uptake (mmol/g)

CO/Cu

Excess CO uptake* (mmol/g)

1177** 362 544 894 1362

– 4.8 2.7 1.7 1.3

– 286 316 363 324

CO uptake (mmol/g)

CO/Cu

585

1.1

* Excess CO has been evaluated with respect to that necessary to reduce Cu2+ to Cu0. ** TPR of ceria is conducted up to about 1000 8C.

of reaction (1)), the subtraction of such a quantity from the overall CO uptake, gives the same reductant uptake in both CO and H2 TPR tests for ceria, thus evidencing that the amount of sites involved in the reduction is the same whatever the probe molecule, but that the surface hydroxyls are reduced only by CO. In the presence of CO, ceria is reduced already at 100 8C. The reduction profile (not reported here) shows two partially overlapped reduction peaks at 295 8C and 410 8C, respectively, which corresponds to a CO consumption of 410 mmol/g and a further reduction peak at 900 8C, corresponding to a consumption of 755 mmol/g. The overall amount of ceria reduced by CO and by H2 is very similar (Tables 2 and 3); in particular the H2 consumed at lower temperature in the H2 TPR roughly corresponds to the CO related to the first two reduction peaks (410 mmol/g vs. 355 mmol/ g) which thus may be associated to the reduction of the surface ceria, while the high temperature reduction peak (900 8C) to the reduction of the bulk oxide. The CO2 production profile quite closely follows that of CO uptake. Nevertheless the amount of CO2 released in the lowtemperature region (<400 8C) is lower than the CO uptake, while the reverse occurs in the temperature range 400–700 8C. This is probably explainable by assuming that for temperature lower than 400 8C some carbonate species are formed on ceria which decompose or desorb at temperature higher than 400 8C. Such a thesis is supported by the results of Li et al. [56] and Martinez-Arias

47

et al. [29] who observed with IR analysis the formation of carbonates on ceria after a reducing treatment with CO. Very few data are available in the literature about reduction of ceria by CO; Wang et al. [41] found that a samaria-doped ceria was irreducible in CO up to a temperature of 500 8C. When copper is present in the sample, the reduction peaks at 300 8C and 400 8C detected in the CO TPR of ceria disappear while remaining unchanged the high temperature peak, corresponding to the reduction of bulk ceria (not shown). Furthermore, even for the sample with the lowest copper content (0.5 CuCe) a significant CO consumption, not observed for ceria, was detected at room temperature, as also reported by Martinez-Arias [29] for a sample with 1 wt.% CuO which was not associated to CO2 desorption up to a temperature of about 150 8C. This CO uptake increases with increasing the copper load up to 4 wt.%, remaining unchanged for the 8 CuCe sample. Also for CO TPR quite complex profiles were found. 0.5 CuCe catalyst shows three broad and partially superimposed peaks. An increase of the intermediate peak is observed for 2 CuCe sample which becomes more intense and shifts at lower temperature for 4 CuCe catalyst. Peaks are less defined for 8 CuCe catalyst, likely due to the contribution of the reduction of CuO clusters. The values of the overall CO uptake increase with increasing the copper load and the CO/Cu ratio suggests for all supported samples the participation of ceria to the reduction of the catalyst sample in the temperature range of the experiment, as also observed for H2 TPR tests. Also for CO TPR the amount of ceria involved in the reduction of CuCe catalysts was estimated by subtracting the CO consumed to reduce copper from Cu2+ to Cu0 from the total CO uptake. This amount is always lower than the whole amount of surface ceria estimated by the TPR carried out over pure CeO2 (410 mmol/g). As already mentioned, CO2 evolution was also monitored during CO TPR. For 0.5 CuCe, despite of a significant CO uptake at room temperature, CO2 is detected only at temperatures higher than 120 8C in accordance with the results of Martinez-Arias et al. [29]. At higher temperatures the CO2 profile follows the CO consumption. A similar behaviour is observed for 2 CuCe sample. Otherwise, when increasing the CuO concentration up to 4 wt.%, a slight CO2 emission is detected already at room temperature, while rising the temperature the intensity of the CO2 peak at 100 8C overcomes that of the CO consumption, suggesting the occurrence of desorption phenomena in correspondence of that temperature. The carbon balance is not closed very well in all CO TPR tests if the experiment is stopped at 430 8C with an error of about 15%. Nevertheless, in a check-test performed on the sample 4 CuCe carried out up to 1100 8C, a very good carbon balance was obtained (1963 mmol/g CO consumed versus 1936 mmol/g CO2 produced). Actually, similarly to what observed in the CO TPR of pure ceria, between 400 8C and 700 8C the CO2 production peak exceeds the CO consumed suggesting that a fraction of the carbon dioxide produced at temperature lower than 400 8C, due to the catalyst reduction with CO, forms carbonate species that remain adsorbed on ceria and decompose only at temperatures higher than 400 8C. In order to clarify the nature of the interaction of CO with the catalyst occurring even at very low temperatures, CO and CO2 TPD experiments were performed on the sample 4 CuCe (Fig. 3). The CO uptake measured after adsorption/saturation at 30 8C and subsequent N2 purge was estimated as 163 mmol/g which corresponds to a CO/Cu ratio of 0.3 (Table 4). This value is very close to the CO uptake evaluated at room temperature in the CO TPR before heating the sample. In the subsequent TPD carried out, after the sample washing under flowing N2 (room temperature peak in Fig. 3), the catalyst releases both CO (49 mmol/g) and CO2 (109 mmol/g) at about

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

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3.4. Temperature programmed oxidation

Fig. 3. CO TPD profile of 4 CuCe (CO: continuos line, CO2: dashed line); room temperature peak represents the evolution of CO in the sample washing with N2 (hold-up included).

Table 4 Results of CO TPD of 4CuCe sample

Total CO uptake (mmol/g) CO/Cu CO desorption (mmol/g) CO/Cu CO2 production (mmol/g) CO2/Cu C bal (%)

Fully oxidized sample

Sample re-oxidized at room temperature

163 0.3

98 0.18

49 0.09 109 0.2 2.9

Catalysts reduced with CO or H2 were re-oxidized under controlled conditions in order to study the regeneration step of catalyst sites in the reaction mechanism of CO and/or H2 oxidation. The re-oxidation profiles of the pre-reduced samples (as described in the experimental section) are shown in Fig. 4, while the corresponding quantitative analysis is reported in Table 5. The experiments show that the amount of oxygen uptake in each test is in good agreement with the corresponding H2 consumption estimated by TPR analysis. Indeed, a stoichiometric ratio 2:1 between the amount of H2 uptake in the TPR and the corresponding O2 consumption in the TPO was approximately found. The re-oxidation at programmed temperature exhibits different peaks. In particular, it is worth noticing that for all samples, support included, a significant re-oxidation is observed already at room temperature, namely before the heating starts. Pure support pre-reduced at 650 8C, i.e. at a temperature high enough to reduce surface ceria but not the bulk material, completely recovers the

10 0 84 0.18 4.1

100 8C, with a good carbon balance (uptake: 163 mmol/g; release: 158 mmol/g). The CO desorption peak is symmetric while carbon dioxide shows a tail so long that CO2 is completely desorbed only at about 400 8C. A similar trend has been observed by other authors [29,57] who explained the large CO2 peak tail with the desorption of adsorbed COx species. Likewise, Zou et al. [28] have supposed the occurrence of CO oxidation at T > 300 8C to account for the bad baseline recovery of CO TPD. In a separate CO2 TPD test, we found that the desorption temperature for carbon dioxide is 100 8C and the shape of the CO2 profile is symmetric not showing the long tail obtained for the CO2 profile in the CO TPD experiment, in contrast with Avgouropoulos and Ioannides [57] who observed the same extended tail shown in CO TPD. These results suggest that a fraction of CO fed to the catalyst during the CO TPD forms CO2 already at room temperature which probably remains adsorbed as carbonate as it occurs in the CO2 TPD, desorbing from the catalytic surface at about 100 8C. Another fraction of the CO fed at room temperature probably forms bidentate carbonate on the reactive sites which may evolve across intermediate species only rising up the temperature, and finally desorbs as CO2 at a temperature as high as 200–400 8C [58]. This latter species is probably not formed when CO2 is directly adsorbed on the catalyst. The comparison between the results of TPD and TPR of CO allows us to affirm that most of the CO uptake at room temperature should be associated to a partial site reduction, as evidenced by the formation of CO2 that remains adsorbed. Such catalytic species reduced by CO at room temperature are of great interest because, if easily re-oxidizable, can be involved in the preferential oxidation of CO at low temperature. Therefore, in the following study a particular attention will be devoted to characterize these species, their selectivity to oxidize CO more than H2 and their re-oxidation ability.

Fig. 4. TPO profiles of reduced samples: 0.5 CuCe; 2 CuCe; 4 CuCe; 8 CuCe (dotted lines correspond to the reactor hold-up).

T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53 Table 5 Results of TPO analysis of fully reduced samples

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3.5. TPR of partially re-oxidized samples

Sample

Total O2 uptake (mmol/g)

O2/Cu

O2 uptake at room T (mmol/g)

0.5 CuCe 2 CuCe 4 CuCe 8 CuCe

140 219 406 687

1.8 0.96 0.76 0.66

140 148 197 182

oxygen lost during the previous reduction at room temperature. Instead, the extent of the re-oxidation at room temperature is different among the different catalysts investigated. For instance, 0.5 CuCe sample is completely re-oxidized at room temperature whereas for the Cu-richer samples the amount of O2 recovered at room temperature only partially re-oxidize the sample although it increases with respect to 0.5 CuCe. The room temperature peak is larger with increasing the Cu content up to 4 CuCe, while remains almost unchanged with the further increase of copper load to 8 wt.%. The re-oxidation process at room temperature occurs very rapidly giving rise to a sharp peak that last no more than 10 min. Except for 0.5 CuCe, rising the temperature a shoulder at 100 8C and two peaks, respectively, at 200 8C and 430 8C have been observed. Nevertheless, it should be noted that the maximum of the latter signal is related to the end of the heating step. The intensity of the peaks at 100 8C and 200 8C increases with copper load. The peak at 430 8C is present only for the sample with 4 wt.% CuO and increases its intensity with increasing the copper load. This peak could be related to the re-oxidation of XRD-detectable CuO micro-clusters. In fact, the higher the copper load in the sample, the greater the amount of segregated bulk-like CuO particles which represent the less reducible species. Therefore, these species are also the most difficult to be re-oxidized, in good consistency with the idea that redox properties of the transition metal can be strongly enhanced by the interaction with ceria. Complex TPO profiles were also found by Pintar et al. [47] for copper-ceria mixed oxides, although they did not observe any O2 consumption at room temperature. They assigned the three main TPO peaks to the reoxidation of well dispersed copper species, to the reoxidation of segregate Cu phase and to the consumption of H2 stored in the material structure, respectively. On the other hand, in agreement with our results, Zimmer et al. [44] observed the partial reoxidation of copper-ceria catalysts at room temperature using N2O as oxidizing agent. They associated the oxygen uptake at higher temperature to the reoxidation of copper on ceria whereas the consumption at room temperature to the oxidation of hydrogen adsorbed on ceria surface. The presence of adsorbed or incorporated hydrogen has not been supposed by Martinez-Arias et al. [32] who ascribed the oxygen uptake at room temperature mainly to re-oxidation of ceria previously reduced. Our results suggest that surface ceria should be involved in the re-oxidation taking place at room temperature. Moreover, the trend observed for the amount of oxygen recovered at room temperature by the reduced CuCe catalysts (Table 5) is very similar to that evaluated as ceria involved in the reduction in the TPR experiments (Table 2) and attributed to copper-activated surface CeO2. Moreover, quantitative results indicate that re-oxidation of some copper at room temperature should also take place since the amount of oxygen uptake at room temperature exceeds the estimated quantity of ceria involved in the H2 reduction of the catalysts. In addition to these easy re-oxidizable species, highly dispersed and segregated copper are responsible for the peaks centred at 100 8C and 200 8C, respectively.

The catalytic species which can be very easily re-oxidized at room temperature deserves a particular attention because they can play a significant role in the catalysis of CO oxidation at relatively low temperature. More specifically, the importance of characterizing such species is related to their fast re-oxidation kinetics at temperatures even lower than in the reaction process. TPR analysis on samples with different reduction degree is a well-known technique used for the identification of latent peaks present in the TPR of completely oxidized samples [59]. In order to investigate the nature of the above mentioned species, we have performed TPR tests over partially re-oxidized catalysts at room temperature (for half an hour and 1% vol. O2). The H2 TPR profiles of these partially re-oxidized catalysts are shown in Fig. 5, where they are compared with the corresponding TPR curves of the fully oxidized samples, while the relevant quantitative analysis is reported in Table 2. The mass balance on hydrogen and oxygen atoms in these experiments is satisfactory. Indeed, the H2 uptake in the TPR is consistent with the O2 uptake preliminary measured in the treatment at room temperature (stoichiometric ratio H2/O2 = 2:1). Thus, it was confirmed that, by increasing the copper load, the absolute amount of H2 uptake due to the increase of the number of species re-oxidized at room temperature increases and, as expected, does not change enhancing the copper content from 4 wt.% to 8 wt.%. Generally speaking, TPR profiles after such a partial reoxidation subsequent to a deep catalyst reduction, is characterized by a main peak at temperature decreasing with enhancing the copper content and a very small signal at lower temperature. For the sample 0.5 CuCe, which was completely re-oxidized at room temperature, TPR profiles of completely and partially oxidized catalyst are overlapped as expected. Such an occurrence suggests that the copper species strongly interacting with ceria as well as the cerium atoms of the pertubated ceria structure are re-oxidized by room temperature O2 treatment, since 0.5 CuCe is fundamentally constituted by Cu-O-Ce species only, as seen by H2 TPR tests. The TPR patterns of catalysts with higher copper load show that the high temperature peak, attributed to the reduction of segregated CuO bulk-like microclusters, is not present. Such a

Fig. 5. Comparison between H2 TPR on fully oxidized (dashed line) and partially oxidized at room temperature (continuous line) samples: 0.5 CuCe; 2 CuCe; 4 CuCe; 8 CuCe.

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T. Caputo et al. / Applied Catalysis A: General 348 (2008) 42–53

Fig. 6. Comparison between CO and H2 TPR profiles of partially oxidized at room temperature 4 CuCe sample.

phenomenon is particularly evident for 4 CuCe and 8 CuCe, so showing that the weakly oxidizing treatment at room temperature (and low O2 partial pressure) cannot re-oxidize copper aggregates. This confirms that the easily re-oxidizable species are cerium and copper in the mixed Cu-O-Ce phase that, therefore, are the sites reduced at intermediate temperature in the H2 TPR of fully oxidized samples, as also supposed by Tang et al. [46]. Moreover, Tang et al. [46] also performed TPR tests over a 5 wt.% CuO/CeO2 samples pre-reduced and re-oxidized at different temperatures, and reported similar modification of the TPR profile, although they found at room temperature not only surface ceria and supportinteracting copper are re-oxidized, but also, in contrast with our results, some bulk-like CuO. The same approach used for H2 TPR was followed by using CO as the reducing agent. The test was carried out over the sample 4 CuCe (Fig. 6). Similarly to what observed on the fully oxidized sample, also in this case a CO uptake was detected at room temperature (as shown in the first isothermal part of the experiment). Nevertheless, this amount is half the quantity evaluated for the fully oxidized sample. The CO consumption increases by increasing the temperature giving a defined peak at 140 8C, superimposed to a very broad signal extending from 50 8C to 200 8C. The peak at 140 8C does not overlap with anyone of the TPR of the completely oxidized sample suggesting that it could be associable to the reduction of some copper(I) species stabilized upon the reoxidation treatment. This attribution is supported by the work of Wang et al. [60] who demonstrated that in the presence of CO, Cu2O is reduced at lower temperature with respect to CuO. In order to better characterize the species reduced by CO at room temperature, a CO TPD test was performed on the sample reoxidized at room temperature. An asymmetric CO2 desorption profile and a small symmetric CO desorption peak were observed. By making a deconvolution of the CO2 desorption peak it has been calculated that the peak centred at 100 8C corresponds to 84 mmol/ g which corresponds to the 77% of the species reduced at room temperature on the completely oxidized sample (Table 4). This experiment suggests that the re-oxidation quite completely restores the species which are reduced by CO at room temperature, while not replacing those species responsible for the CO adsorption, so explaining the half value of CO uptake in the RT treatment over the partially re-oxidized sample. 3.6. Transient experiments The redox properties of the CuO/CeO2 catalyst, studied with TPR/TPO techniques, allowed us to collect quite significant

information on such a system; however, the direct link between some of such properties and the catalytic performances in selectively convert CO to CO2 in the presence of hydrogen is not trivial. Thus, we have deeply analyzed the dynamics of the reaction test in order to characterize and identify the active species on the catalyst surface, the role of the oxidation state of surface sites in the catalysis and also the transformations occurring on the catalyst surface during the reaction. The experiments were carried out by introducing a reactive gaseous mixture to the catalytic reactor and waiting for the steady-state to be reached with continuously monitoring the reactor temperature and the gas composition at the reactor outlet. The transient response of the whole system to a proper pre-treatment carried out in each test was investigated after the step change of the feed gas from pure nitrogen to the diluted reacting mixture is applied, as described in the Experimental section. Moreover, once steady-state is established and the transient analysis of product composition and temperature is finished, a subsequent H2 TPR is performed to evaluate and measure the catalyst oxidation state under reaction conditions. As already mentioned, we chose to employ a more diluted reacting mixture compared to that generally involved in the present kinetic tests, due to the attempt to limit the thermal effects connected to the occurrence of the exothermic reactions of CO and/ or H2 oxidation and the willing of slowing down the transient phenomena to better observe them. Results of such investigation are reported for the 4 CuCe catalyst sample at different reaction temperatures (Table 6). In particular, the chosen values of temperature are those insuring a CO-PROX selectivity equal (70 8C) or lower (144–204 8C) than 100% under the corresponding reaction conditions of standard catalytic tests. In Fig. 7 one of the experiments here described, conducted at 70 8C on the pre-oxidized sample, is shown. It is evident that when the reaction mixture is switched to the reactor a sharp peak corresponding to reactor hold-up of reactants is observed. In this phase, a consumption of reactants due to adsorption or reaction is also possible, but the relative amount of the such phenomena is significantly different from the hold-up, which was measured in separate tests and subtracted in the quantitative analysis. Actually, the experiment is characterized by a negligible or inexistent reaction/adsorption phenomena or generally speaking no consumption of hydrogen (only the hold-up peak was measured). Such an occurrence is somewhat expected because the test is carried out in a range of conditions providing a complete CO-PROX selectivity and at a temperature at which the catalyst is not reduced by H2, neither hydrogen is adsorbed on oxidized copper in these conditions. On the contrary, during the 50 min time necessary to reach the steady-state, the gas phase concentrations of CO and O2 are systematically lower than the corresponding regime values. At steady-state the final conversion of CO is about 27% while the stable consumption of oxygen is in perfect stoichiometric agreement with it. Table 6 Transient experiments performed on pre-reduced (R) or pre-oxidized (O) 4 CuCe catalyst at different temperatures: steady state conversions and H2/Cu ratio measured in the subsequent H2 TPR Temperature (8C)

Pretreatment

CO conversion (%)

H2 conversion (%)

Selectivity (%)

TPR H2/Cu

70 70 104 144 144 204 204

O R O O R O R

27 17 69 90 88 98 98

0 0 0 0.2 0.3 3.4 3.5

100 100 100 96.5 94.6 56.9 54.6

1.49 0.85 1.47 1.52 1.10 1.18 1.00

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Fig. 7. Transient analysis over totally oxidized 4 CuCe: (a) evolution of gas phase composition at reactor outlet after a step change of feed composition from pure nitrogen to a mixture of CO (1000 ppm), O2 (2000 ppm), H2 (2 vol.%) N2 balance; (b) subsequent H2 TPR.

Fig. 8. Transient analysis at 70 8C over reduced 4 CuCe: (a) evolution of gas phase composition at reactor outlet after a step change of feed composition from pure nitrogen to a mixture of CO (1000 ppm), O2 (2000 ppm), H2 (2 vol.%) N2 balance; (b) subsequent H2 TPR.

The amount of CO and O2 consumed in the first 2 min is higher than that calculated for the reactor hold-up, indicating that a fraction of these compounds is reacting or adsorbing on the catalyst surface. Indeed, CO2 formation is contemporarily detected, although the production of carbon dioxide in the first 2 min is not large enough to satisfy the carbon balance. As evidenced by CO TPR and CO TPD tests, adsorption of carbon dioxide rather than carbon monoxide may explain this occurrence. The oxygen consumed corresponds to that necessary to oxidize CO in the whole experiment, first two minutes time included. The H2 TPR performed after the achievement of the steadystate, representing an indirect measurement of catalyst reduction degree under reaction conditions, gives rise to a profile similar to that observed on the fully pre-oxidized catalyst and to a H2 uptake of 792 mm/g. This value is almost the same of that obtained for the standard TPR test over fresh catalyst and hence would reveal that under reaction conditions and 70 8C, the catalyst is in the fully oxidized state, notwithstanding the exposition to a reducing atmosphere due to the very large excess of H2 (with respect to O2 and stoichiometry of CO and H2 oxidations). This probably happens because at 70 8C, hydrogen is not an actual reducing agent. In effect, it does not react with oxygen at steady state and should be considered as an inert, under such operating conditions. However, since not only the total H2 uptake but also the shape of TPR peaks are substantially the same, we may conclude that under reaction conditions at 70 8C, the catalyst does not significantly change its state from the initial one. In Fig. 8, it is shown the same transient experiment performed at 70 8C on the 4 CuCe catalyst but after having completely prereduced the sample in H2 (at 430 8C for 1 h). With such a different

pre-treatment, the concentration of the reacting species follows different trends with respect to those observed for the oxidized sample after the step change of the gas mixture entering the reactor. Oxygen concentration presents a huge transient peak, although the steady state seems to be reached faster (10 min). CO initially shows a sharp peak, whose area corresponds to that necessary to fill the reactor, and subsequently a broader CO consumption peak reaching a steady state condition after about 5 min from the beginning of the experiment. The CO2 production starts in correspondence of the CO consumption peak. The final conversion of CO is about 17% which is significantly lower than that obtained with the pre-oxidized catalyst. The TPR analysis performed after the test gives a H2 uptake of 452 mm/g, indicating that notwithstanding the complete reduction of catalyst attained in the pre-treatment, the catalyst has been partially re-oxidized under the reaction conditions. Indeed the amount of O2 consumed in the first two minutes of the reaction is much higher than that necessary to produce CO2. The oxygen consumed in excess with respect to the oxidation of CO, in the first two minutes has been estimated 250 mmol/g, a value not significantly different from 227 mmol/g that has been calculated for catalyst re-oxidation by the consumption of H2 in the TPR (Fig. 8b), within the error of such a complex experiment. These calculations support the idea that the extra oxygen consumed in the first minutes has partially re-oxidized the catalyst surface. However such a re-oxidation is not complete, as the H2 uptake in the TPR is lower than that measured over the fully pre-oxidized sample. Catalyst oxidation from a pre-reduced state under apparently strong reducing atmosphere may only improperly seem surprising.

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We have already affirmed that at relatively so low temperature, hydrogen is a sort of inert species or spectator; consequently the reacting mixture represents under such conditions an oxidizing atmosphere rather than a reducing one. The comparison with the TPR tests carried out over the partially re-oxidized catalyst at RT (Fig. 6) reveals that the oxidation degree reached under CO PROX condition at 708 is very similar to that of the catalyst re-oxidized at RT and that the type of the sites restored by oxygen is the same, as confirmed by the identical shape of the TPR profiles (compare Figs. 6 and 8b) with the same two peaks in both experiments. Moreover, by deeply analysing the response of the system to the step change of gas composition (Fig. 8a), it seems also clear that until the catalyst active sites are not at least partially re-oxidized the catalyst is not active at all. Actually, the CO concentration profile seems to follow the corresponding pattern of the reactor hold-up, with a sharp peak and a tendency to rapidly increase (to the value of no conversion of 0.1 vol.%) and a subsequent new decrease because of the restored activity of the catalyst towards CO oxidation. Similar trends are observed at the other temperatures investigated. At 144 8C the CO conversion is 90% and the selectivity is 96.5% for the pre-oxidized sample, while the subsequent H2 TPR reveals that also at that temperature the catalyst is in a completely oxidized state (H2 uptake = 782 mm/g). Actually, the difference of CO conversion with the pre-reduced sample appears to be significantly reduced (88%) suggesting that the higher temperatures promote the achievement of a quite common steady-state of the catalyst oxidation state regardless the initial condition or sample pre-treatment. The H2 consumption in the TPR performed immediately after the reaction tests at 144 8C shows that the prereduced sample is not yet completely re-oxidized after the reaction, even if such a phenomenon seems less pronounced than that observed at 70 8C. Moreover, the selectivity is lower on the pre-reduced sample (94.5% vs. 96.5%), so suggesting that at 144 8C a somewhat more reduced site can be more active towards the undesired reaction of H2 oxidation. Results are quite different at 204 8C, since a completely equal CO conversion (98%) is measured on the catalyst regardless the pre-treatment carried out. On the other hand, memory of the initial oxidation state remains in the O2 conversion that is a bit higher in the case of pre-reduced catalyst, as denoted by the slightly lower selectivity. No significance of pre-treatment was also observed for the stable final oxidation state, since a similar H2 uptake has been estimated by the TPR. However, the common value of H2 uptake in the TPR tests was lower than that corresponding to the fully preoxidized sample, thus suggesting that the steady-state foresees a partially reduced catalyst, even when the catalyst is initially in the completely oxidized state. The consequence of these findings implies that at such a relatively high temperature and under reaction conditions the catalyst is partially reduced, probably by hydrogen, that starts to be reactive (and converted) and is in large excess with respect to oxygen. 4. Discussion and conclusions It has been shown that ceria is reduced by H2 starting at 350 8C, while the reduction in the presence of CO starts at 100 8C. Nevertheless, the presence of CuO on ceria significantly changes the redox properties of the entire catalyst and its reactivity both in the presence of H2 and CO. In particular, TPR tests show different peaks (up to four) that can be roughly attributed to the reduction of: (i) isolated Cu species, (ii) Cu and (iii) Ce, both present in the (pertubated) structure of ceria (Cu-O-Ce species), and (iv) bulk-like CuO microclusters. The different reducibility of such species has been widely studied in the literature, although a not clear

consensus still exists. From our results, we may affirm that the order of reducibility in the presence of H2 is exactly that reported above. Actually, by analysing the behaviour of 0.5 CuCe sample, it clearly appears that the percentage of copper in strong interaction with support is maximum, if not exclusive; such a sample could be taken as the trace to understand the behaviour of Cu-O-Ce species. In the other catalysts, other species are also present, in particular some quite inactive and less reducible CuO bulk-like clusters, whose concentration increases with increasing the overall copper content. The comparison among the 0.5 CuCe sample and all the others shows that surface ceria is reduced at temperature lower than CuO microclusters (Fig. 5), and that both cerium and copper of the mixed phase are very easily reducible and oxidizable, even at room temperature under weak O2 partial pressure. Actually, it remains not completely clear the difference between the species (i) supposed isolated copper species and the species (ii) copper interacting with ceria. However, the order of reducibility changes if measured by CO: Cu-Ce interacting species are reduced in the easiest way, namely at room temperature in the CO TPR. Such species have been characterized to be the most easily re-oxidized by O2 too, as observed in the Cu-poorest sample. Moreover, such special redox properties of the catalyst sites are fundamental features to determine its catalytic behaviour and, in particular, its selectivity towards CO oxidation. Transient analysis of catalytic activity measurements in the CO PROX process carried out at different temperatures and catalyst pre-treatments shows that the active and selective state of the catalyst is the oxidized one. In addition, it has been demonstrated that, even under potentially strongly reducing atmosphere (CO PROX conditions), hydrogen does not reduce catalyst pre-oxidized sites up to about 150 8C, whereas O2 oxidises a fraction of catalyst active sites previously reduced with H2, restoring its active and selective state in the preferential oxidation of CO. Such sites have been clearly identified by carrying out TPR tests once the steady-state is established in the reaction tests and comparing them with those carried out after calcinations at high temperature or O2 treatment at room temperature (partial oxidation). In effect, the active and selective sites are those exhibiting in the operative range of temperature below 150 8C, a fast enough kinetics of CO oxidation (without oxidizing H2, as also confirmed by TPR of hydrogen) and a correspondingly high reoxidation rate under even small O2 partial pressures, and hence such Cu-Ce mixed species whose redox properties have been enlightened in TPR/TPO tests. By increasing the temperature above 150 8C a certain stabilization of reduced species occurs also on the pre-oxidized catalyst. At these relatively higher temperatures also hydrogen oxidation takes place. Thus it is plausible that in this range of temperature the rate of sites re-oxidation competes with hydrogen oxidation (H2 is also in large stoichiometric excess with respect to O2) so stabilizing reduced species. Moreover, it is well known in the literature and also demonstrated in this work that H2 does not adsorb on the fully oxidized catalyst while it adsorbs on a partially reduced one [47]. Then, the stabilization of reduced species Cu1+ and Ce3+ on the catalyst could favour hydrogen oxidation as suggested by the higher hydrogen conversions obtained on the initially reduced sample tested at high temperature. The steady-state measurements of the catalytic activity in the CO-PROX reaction also confirmed the assumption that mixed CuO-Ce are the active and selective system, as also proposed via different techniques by others [30,31,33,39]. They demonstrated that the dispersion of copper oxide on ceria drastically enhances the catalytic performances of the whole system, since even pure ceria is active towards CO oxidation and quite intrinsically

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