CeO2 catalysts prepared by impregnation and deposition–precipitation

Journal of Catalysis 320 (2014) 137–146

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Physicochemical characterization and catalytic performance of 10% Ag/CeO2 catalysts prepared by impregnation and deposition–precipitation Mira Skaf a,b,c, Samer Aouad c, Sara Hany a,b, Renaud Cousin a,b, Edmond Abi-Aad a,b, Antoine Aboukaïs a,b,⇑ a b c

Univ Lille Nord de France, 59000 Lille, France ULCO, Equipe de Catalyse-UCEIV, MREI, 59140 Dunkerque, France Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box. 100, Tripoli, Lebanon

a r t i c l e

i n f o

Article history: Received 16 July 2014 Revised 4 October 2014 Accepted 8 October 2014 Available online 1 November 2014 Keywords: Ag2+ Isotopes EPR Impregnation Deposition–precipitation Catalytic activity

a b s t r a c t Two silver–cerium oxide samples (Ag 10 wt.%) were prepared by two different methods: impregnation and deposition–precipitation. The XRD, EPR, XPS, and TPR techniques were used for physicochemical characterization. Catalysts were tested in C3H6, CO, and carbon black oxidation reactions. The impregnated catalyst showed better performance compared to the one prepared by deposition–precipitation in the different reactions. The EPR technique allowed the identification of three different Ag2+ sites in the impregnated solid along with the distinction of the Ag2+ isotopes (107Ag2+ and 109Ag2+) ions. The impregnated catalyst reduced at lower temperatures compared to the one prepared by deposition–precipitation. The catalytic activity of the impregnated solid was attributed to the presence of Ag2+ species that enhance the redox potential of the solid by creating three different redox couples: Ag2+/Ag+, Ag2+/ Ag0, and Ag+/Ag0. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Industrial chimneys, gasoline, and diesel engines emit considerable amounts of particulate matter (PM) as well as volatile organic compounds (VOC) [1]. One of the principal solutions to reduce their emissions is the catalytic oxidation technique [2–5]. Ceria (CeO2) has a great reputation in catalysis and was well investigated due to its high oxygen storage capacity and its use in catalytic oxidation reactions [6–10]. It is also well known that adding noble metals over ceria enhances the activity of the latter [2–5]; however, noble metals are unfortunately very expensive and are susceptible to poisoning. Transition metals are less expensive and are known for their good catalytic activity [11–14]. Silver catalysts were recently widely used [6,8,15–19] due to their catalytic performances in different reactions, especially in the epoxidation of ethylene [20,21] and carbon black oxidation [22,23]. However, few if any studies have been devoted to the total oxidation of volatile organic ⇑ Corresponding author at: Unité de Chimie Environnementale et Interaction sur le Vivant, EA 4492, France. Fax: +33 3 28 65 82 39. E-mail addresses: [email protected] (M. Skaf), samer.aouad@balamand. edu.lb (S. Aouad), [email protected] (S. Hany), [email protected] (R. Cousin), [email protected] (E. Abi-Aad), [email protected] (A. Aboukaïs). http://dx.doi.org/10.1016/j.jcat.2014.10.006 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

compounds in the presence of silver supported on ceria. It is well known that the catalytic activity of silver based catalysts depends on the particles size [19,24–27], the oxidation state [28,29], and the interaction between silver and the support [6,20,30,31]. In this study, deposition–precipitation and impregnation methods were used to prepare Ag/CeO2 solids. The first method is well known as it leads to the formation of nanoparticles that are efficient in catalytic reactions [32–37], while the second is known as one of the simplest preparation methods. The solids were tested in propylene, carbon monoxide, and carbon black oxidation reactions in order to consider their use as a potential solution for some types of air pollution emissions. 2. Methods 2.1. Catalyst preparation Cerium hydroxide Ce(OH)4 was precipitated from cerium(III) nitrate hexahydrated solution (Ce(NO3)36H2O, ACROS, 99.9%) with a sodium hydroxide solution (NaOH, PANREAC, 99%). The resulting hydroxides mixture (Ce(OH)3, Ce(OH)4) was filtered, washed, and dried overnight at 100 °C, before calcination under air flow (2 L h1) at 400 °C (1 °C min1) for 4 h to obtain ceria CeO2.

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Two silver containing solids (10% Ag/CeO2; 10 wt.% Ag) were prepared by impregnation (Imp) or deposition–precipitation (DP) methods. For the impregnation method, one gram of the support (CeO2) was added to 100 mL of an aqueous silver nitrate solution (AgNO3, PANREAC, 99%) containing the suitable amount of silver. The solution was then left under stirring for 2 h before its evaporation at low pressure. The obtained solid was dried overnight at 100 °C followed by calcination under air flow (2 L h1) at 400 °C (1 °C min1) for 4 h. For the deposition–precipitation method, the silver solution was heated to 80 °C and its pH was adjusted to 8 by addition of NaOH solution (0.1 M) drop by drop under stirring during 4 h. The suspension was filtered, and the solid washed several times with hot water in order to eliminate Na+ and NO 3 ions and it was then dried at 100 °C followed by a calination under air (2 L h1) at 400 °C (1 °C min1) during 4 h. 2.2. Catalyst characterization The specific surface areas were determined in a Thermo-Electron QSurf M1 apparatus using the BET method. Before analysis, the samples were treated under helium flow for 30 min at 120 °C. The simultaneous differential scanning calorimetry/thermogravimetry (DSC/TG) analyses were performed using a NETZSCH STA 409 apparatus. X-ray diffraction (XRD) diffractograms were recorded on a BRUKER Advance D8 powder X-ray diffractometer using Cu Ka radiation (k = 0.15406 nm) over a 2h range of 20–80° using a step size of 0.02°. X-ray photoelectron spectroscopy (XPS) experiments were performed using an Escalab 220XI vacuum generator spectrometer. A monochromatized aluminum source was used for excitation. Binding energy (BE) values were referenced to the binding energy of C1s (285 eV) and are given with an accuracy of ±0.1 eV. Electron paramagnetic resonance (EPR) measurements were performed at different temperatures from 196 °C up to room temperature on an EMX BRUKER spectrometer with a cavity operating at a frequency of 9.5 GHz (X band). The magnetic field was modulated at 100 kHz, and the power supply was sufficiently low to avoid saturation effects. The g values were determined from precise frequency and magnetic field values. Temperature programmed reduction analysis (TPR) was carried out with a flow type reactor. Hydrogen (5 vol.% in Ar) was passed through a reaction tube containing the catalyst under atmospheric pressure at 30 mL min1. The tube was heated with an electric furnace at 5 °C min1 and the amount of H2 consumed was monitored with a thermal conductivity detector (TCD).

to 1000 °C (heating rate: 5 °C min1) in air flow (75 mL min1). Three characteristics temperature values; Ti where the CB oxidation begins, Tm where the rate of oxidation is at its maximum, and Tf where the combustion ends; are determined from DSG–TG curves. Tm values were used to compare the catalytic performances and CB oxidation rates.

3. Results and discussion 3.1. Specific surface area Pure calcined ceria possesses a specific surface area of 122 m2 g1. Following silver impregnation, the specific surface area decreased slightly to 92 m2 g1 whereas, it was equal to 84 m2 g1 for the (DP) solid. In general and according to the literature [38], catalysts (M/support) prepared with the (DP) method possess high specific surface areas compared to catalysts prepared using the (Imp) method. This unexpected result is probably due to the chemical properties of the silver species formed on the surface of the different solids. In order to gain more insight into the nature of silver species, an X-ray diffraction study was done.

3.2. XRD Fig. 1 shows the XRD patterns obtained for the pure CeO2, 10% Ag/CeO2 (Imp), and (DP) catalysts. For all samples, the CeO2 cerianite phase (JCPDS-ICDD 34-0394) was obtained since peaks located at 2h: 28.55°; 33.07°; 47.48°; 56.34°; 59.4°; 69.86°; 77.08°; and 79.83° were observed. The silver Ag0 impregnated solid showed additional peaks located at 2h: 38.08°; 44.21°; 64.35°; and 77.23° ascribed to metallic silver (JCPDS-ICDD 01–1167). Furthermore, a peak (shouldering) at 2h = 37.86° can be attributed to Ag2O crystallites (File JCPDS-ICDD 41-1104). The remaining diffraction lines corresponding to the Ag2O structure have relatively low intensities at 2h values close to the cerianite diffraction lines which made its identification difficult. No other peaks related to any silver oxide species were observed. This might be due to the high dispersion of the silver particles over ceria making it non-detectable with the XRD technique [8,39,40]. The (DP) catalyst did not show any diffraction peaks related to any silver species. This could be due to the high dispersion of the silver particles over ceria [8,39,40], and these nanosized particles obtained after (DP) preparation [32–37] are not observed by the XRD technique. In order to identify the different silver states, the XPS and EPR studies were carried out on both silver containing solids.

2.3. Catalytic activity measurements

: CeO2- Cerianite : Ag 0 : Ag+ (c)

Intensity (a.u.)

Catalytic tests were carried out at atmospheric pressure in a conventional fixed bed micro-reactor using 100 mg of fine catalyst powder. In the case of propylene, the reactive flow (100 mL min1) is composed of air and 6000 ppm of C3H6 whereas, in the case of carbon monoxide, 1000 ppm of CO was mixed with 20% of molecular oxygen in helium. The reactants and the reaction products were analyzed with a VARIAN 4900 gas chromatograph equipped with TCD detectors. The catalysts were first activated at 400 °C for 4 h under air (2 L h1) and the conversion was calculated over a slow heating rate (1 °C min1) between 20 and 400 °C. The temperature rate of 1 °C min1 is considered to be slow enough to reach a pseudo-steady state at every point. Carbon black (CB) Degussa N330 was chosen as model for soot particles oxidation. The CB–catalyst mixture was performed and prepared in tight contact conditions. 2 wt.% of CB and 98 wt.% of catalyst were manually grinded for 15 min. About 30 mg of CB–catalyst mixture was loaded in an alumina crucible and heated from room temperature

(b)

(a)

20

30

40

50

60

70

80

2θ (°) Fig. 1. XRD patterns for (a) CeO2, (b) 10% Ag/CeO2 (Imp), and (c) 10% Ag/CeO2 (DP).

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3.3. XPS The XPS spectrum of Ce3d for pure calcined ceria (result not shown) showed a complex structure that was also observed in the case of the freshly calcined silver catalysts. The XPS spectra of Ce3d for 10% Ag/CeO2 (DP) and 10% Ag/CeO2 (Imp) are represented in Fig. 2A. For both silver containing solids, six peaks located at 882.0, 888.0, 897.8, 900.5, 907.0, and 916.3 eV (u1, u2, u3, u4, u5, and u6) were observed after deconvolution. These peaks were attributed to Ce4+ 3d final states and refer to three pairs of spin–orbit doublets [41,42]. Four additional peaks located at 881.0, 884.0, 899.3, and 902.3 eV (v1, v2, v3, and v4) referring to two pairs of doublets were ascribed to Ce3+ 3d final states [42,43]. These results indicate the coexistence of Ce3+ and Ce4+ species on the surface of both solids. It has already been demonstrated that with the presence of Ce3+ in the ceria, oxygen vacancies would

be formed [6]. The surface relative molar ratio of Ce3+/Ce4+ was calculated from the normalized peak areas of Ce3+ and Ce4+ core level spectra. The Ce3+/Ce4+ ratio on the surface of pure CeO2 (0.35) (result not shown) is much higher than that on 10% Ag/CeO2 (DP) (0.30) and 10% Ag/CeO2 (Imp) (0.23). These results show that the addition of silver on CeO2 leads to a decrease in the number of Ce3+ species, and consequently a decrease in oxygen vacancies that is more significant for the impregnated solid. Fig. 2B presents the O1s XPS spectra obtained for the freshly calcined 10% Ag/CeO2 (DP) (a) and (Imp) (b) solids, respectively. Each spectrum shows a sharp and intense component at 529.6 ± 0.2 eV (O(1)) and a broad weaker one (shoulder) at 531.6 ± 0.3 eV (O(2)). These peaks are due to the presence of oxide and hydroxide species in the solids, respectively [42,44,45]. Since for pure CeO2, the main peak of oxygen is often at around 529.5 eV [46], therefore the intense peak can be assigned to O2- of the ceria framework. In gen-

(A) Ce3d XPS spectra for the freshly calcined (a) 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp) u3

(a)

u6

u4

(b)

v4 u5

v3 v2 u2 v1

875

885

895

905

915

925

u3

u6 u4

u1

Intensity (a.u)

Intensity (a.u.)

u1

v3

u5

v4

v2 u2 v1

875

885

895

Binding energy (eV)

905

915

925

Binding energy (eV)

(B) O1s XPS spectra for (a) the freshly calcined 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp) 45000

34500

O(1)

(a)

40000

Intensity (a.u.)

Intensity (a.u.)

29500 24500 19500 14500

O(2)

9500

O(1)

(b)

35000 30000 25000 20000

O(2)

15000 10000

4500 522

527

532

5000 522

537

Binding energy (eV)

527

532

537

Binding energy (eV)

(C) Ag3d XPS spectra for (a) the freshly calcined 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp) (a)

(b)

Total Ag Intensity (a.u.)

Intensity (a.u.)

Total Ag

Ag+ Ag0

Ag+

Ag0 Ag 2+

363

368

373

Binding energy (eV)

378

363

368

373

378

Binding energy (eV)

Fig. 2. (A) Ce3d XPS spectra for the freshly calcined (a) 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp). (B) O1s XPS spectra for (a) the freshly calcined 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp). (C) Ag3d XPS spectra for (a) the freshly calcined 10% Ag/CeO2 (DP) and (b) 10% Ag/CeO2 (Imp).

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eral, the hydroxyl groups and adsorbed charged oxygen species are present on the ceria surface and characterized by XPS lines at BE  531.3 eV [46]. Therefore, the shoulder in our spectrum can be attributed to such species. The intensity of the O2 (O(1) + O(2)) peak for the (Imp) solid is higher than that obtained for the (DP) solid, which confirms the previous result about the relatively higher number of oxygen vacancies encountered in the (DP) solid. In addition, since the intensity of the shoulder peak in the (Imp) solid is higher than that obtained in the (DP) solid, this indicates that the adsorbed charged oxygen or/and the hydroxyl groups on the (Imp) solid are more abundant compared to those in the (DP) solid. Fig. 2C presents the Ag 3d spectra for the (a) 10% Ag/CeO2 (DP) and (b) the 10% Ag/CeO2 (Imp) solids. The peak de-convolution reveals two doublets in the 10% Ag/CeO2 (DP) spectrum. The first doublet with binding energies at 367.7 and 374.2 eV indicating the presence of Ag+ ions [47–49] while the second with binding energies at 369.2 and 375.2 eV corresponds to silver in the metallic state [50]. Moreover, a third peak was observed in the case of the impregnated solid with binding energies at 367.4 and 373.2 eV which corresponds, according to the literature [51–53] to Ag2+ ions. For both solids, total Ag on the surface was mainly composed by Ag+ ions, showing that, over ceria, these species are the most stable among the three silver states. The surface Ag/Ce atomic ratio reveals that a considerable concentration of silver is present at the surface of the (Imp) solid comparatively to the (DP) solid (Table 1). 3.4. EPR results Fig. 3 presents the EPR spectra recorded at 196 °C for CeO2 and 10% Ag/CeO2 (Imp) and (DP) solids treated at 400 °C for 30 min under vacuum. On all three spectra, a signal with an axial symmetry defined by g\ = 1.967 and g|| = 1.947 is observed. This signal was extensively studied by numerous authors [6,54,55] and was attributed to Ce3+ ions formed in the solid after its calcination under air. Adding silver on ceria by either method caused a decrease in the intensity of the Ce3+ signal. The decrease was more substantial in the (Imp) solid compared to the (DP) one. This is probably due to the chemical interaction between the silver and the Ce3+ ions which are present near oxygen vacancies at the ceria surface. Therefore, since in the (Imp) solid, the concentration of silTable 1 Experimental silver speciation at the surface of the (DP) and (Imp) solids.

10% Ag/CeO2 (DP) 10% Ag/CeO2 (Imp)

Ag/Ce ratio

% of Ag+

% of Ag0

% of Ag2+

0.6 4.5

87.2 86.7

12.8 8.8

0 4.5

ver on the surface is more substantial than in the (DP) solid, this explains the significant decrease in the Ce3+ concentration. In addition, the spectrum of (Imp) solid showed an additional Ce3+ signal (g\ = 1.965 and g|| = 1.941) related to a change in the chemical environment of some Ce3+ ions following silver addition. The EPR spectrum of the (DP) solid is similar to the one recorded for pure ceria. However, a slight difference in the g|| values (1.940) was noted due to the different chemical environment of these species. However, the EPR spectrum of the (Imp) solid is formed of several signals with different g values around g = 2. These signals are noted ‘‘a,’’ ‘‘b,’’ and ‘‘c’’. In order to assign these 3 signals, a solid with low silver content (2.14% Ag/CeO2 (Imp)) was prepared. In addition, different preparation methods (10% (wt.%)) and treatments were performed. 3.4.1. Effect of silver loading on the EPR spectra of the impregnated solids A solid with lower weight percentage of silver (2.14 wt.%) over ceria was prepared by the impregnation method. Fig. 4 shows the EPR signals of 2.14% Ag/CeO2 (Imp) solids recorded at 196 °C. For 2.14% Ag/CeO2 (Imp), two signals ‘‘a’’ and ‘‘b’’ are observed. Similar signals were obtained in different silver doped matrices particularly after irradiation at low temperature (196 °C) and were attributed to Ag2+ species (4d9; 42D5/2; S = 1/2; I = 1/2) [51,56–67]. Fig. 5 shows the EPR spectra of the impregnated 10% Ag/CeO2 solid, calcined at 400 °C, then treated under vacuum at different temperatures between room temperature and 400 °C. The intensity of the ‘‘c’’ signal decreased and then disappeared at 250 °C followed by signal ‘‘a’’ that disappeared at 300 °C. On the contrary, the ‘‘b’’ signal remained stable and its intensity increased with temperature up to 300 °C then slightly decreased at 400 °C. The ‘‘a’’ and ‘‘b’’ signals, presenting a hyperfine structures with two lines and having isotropic symmetries, are, respectively, centered at giso(a) = 2.018 and giso(b) = 2.013 with Aiso(a) Aiso(b) 30 G (29.8 G). As the ‘‘a’’ and ‘‘b’’ signals are isotropic with the absence of any axial symmetry, it can be attributed to Ag2+ ions in the form of clusters, and located in two different sites on ceria surface due to the different giso values. Consequently, the ‘‘c’’ signal was identified with the following EPR parameters: g\c = 2.010, g||c = 2.105, A\c = 30.5 G, and A||c = 47.0 G characterizing apparently, an axial symmetry. The ‘‘c’’ signal cannot be due to free silver atoms Ag0 (52S1/2; 5s1; S = 1/2; I = 1/2) as these latter species are only detected by EPR at low temperature (196 °C) and not at room temperature, and because its hyperfine coupling constants are much larger than those observed in the ‘‘c’’ signal [56].

g┴(Ce3+) = 1.967

(b) g||= 1.940

g||c= 2.105

g┴ = 1.965

(b) A||c= 42 G

g||= 1.941 giso a= 2.018 giso b= 2.013

a b

(a) giso(b) = 2.013 Aiso(b) = 29.8 G

g||= 1.947

3200

3250

3300

3350

3400

3450

b c

g┴ = 1.967 g┴c = 2.010

(a)

a

Intensity (a.u.)

Intensity (a.u.)

(c)

3500

3550

3125

3225

c

a b a b

3325

3425

giso(a) = 2.018 Aiso(a) = 29.8 G

3525

Magnetic field (G)

Magnetic field (G)

Fig. 3. EPR spectra for (a) CeO2, (b) 10% Ag/CeO2 (Imp), and (c) 10% Ag/CeO2 (DP) recorded at 196 °C after treatment under vacuum for 30 min.

Fig. 4. EPR spectra for (a) 2.14% Ag/CeO2 (Imp) and (b) 10% Ag/CeO2 (Imp) recorded at 196 °C after treatment under vacuum for 30 min.

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400°C b1

b1

b b2 2

350°C

g ||(1) =2.106

(E)

Intensity (a.u.)

A ||(1) =49.80G

c1

c1

A ┴(1) =32.88G g ┴(1) =2.010

300°C g ||(2) =2.106

(D)

250°C c2

c2

A ||(2) =43.30G

A ┴(2) =28.40G g ┴(2) =2.010

a

c b

3300

3350

3400

a

c b

3450

3500

Magnetic field (G) Fig. 5. EPR spectra for 10% Ag/CeO2 (Imp) recorded at 196 °C after treatment under vacuum at different temperatures for 30 min.

Intensity (a.u.)

200°C (C)

c2

3.4.2. Theoretical calculation of the EPR spectra Among the ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ signals, only the ‘‘c’’ signal showed a well-resolved hyperfine structure with clear perpendicular and parallel components. Moreover, the ‘‘c’’ signal showed that each perpendicular component is formed of a double line, whereas, the parallel component is relatively wide, therefore, no double lines are observed. The double lines are not observed at room temperature because the obtained ‘‘c’’ signal is broader (results not shown). The double lines can be ascribed either to a single Ag2+ species with orthorhombic symmetry, or to the superposition of two Ag2+ signals presenting an axial symmetry each. In order to identify the nature of the species responsible of the ‘‘c’’ signal, theoretical calculations of the EPR parameters were performed. The EPR parameter values are computed from the spectra that are simulated using the Bruker SIMFONIA software based upon perturbation theory [68]. The theoretical EPR signals are calculated using the effective spin Hamiltonian:

H ¼ bHj  Sj  g j þ Ij  Aj  Sj where, j is the component along one of the three axes x, y, and z, H is the applied magnetic field, S is the total electron spin, g is the spectroscopic factor, I is the nuclear spin, A is the hyperfine coupling constant, and b is the Bohr magneton. A poly-oriented sample EPR signal was simulated by generating 9000 random orientations of the magnetic field and by summing the corresponding 9000 absorption signals. The final signal was obtained by performing a convolution (Gaussian or Lorentzian line shape) of each transition line, adding all contributions, and calculating the first derivative of the signal; the line width of each component has been optimized in order to obtain the best fit with the observed experimental values. Fig. 6, spectrum B, illustrates the calculated ‘‘c’’ signal based on the hypothesis that the experimental one is due to a single Ag2+ site having an orthorhombic symmetry with the following EPR parameters: gxx = 2.009; gyy = 2.011; and gzz = 2.106 with Axx = 30.58 G;

c1

g zz=2.106

(B)

A zz =47.06G

The presence of the ‘‘c’’ signal just for high silver percentage in the solid, and its disappearance under vacuum treatment, confirms that the three ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ signals are independent and correspond to three different Ag2+ sites in the impregnated solid. However, the three signals showed that each line of the hyperfine structures is formed of a doublet. The doublets of the ‘‘c’’ signal are well resolved while it appears as a shouldering in the ‘‘a’’ and ‘‘b’’ signals. This confirms that the Ag2+ species responsible of the ‘‘c’’ signal are very well dispersed in the impregnated solid. The high resolution of the signal allows a good observation of the split. That was not the case of the ‘‘a’’ and ‘‘b’’ signals.

c2

c1

A yy =30.74G g yy =2.011

A xx=30.58G g xx =2.009

g ||(c) =2.106

(A)

A ||(2) =43.30G A||(1) =49.80G

g ┴(c) =2.010

c1 c2

c c12

A ┴(2) =28.40G

A ┴(1) =32.88G

3125

3175

3225

3275

3325

3375

3425

3475

3525

Magnetic field (G) Fig. 6. (A) Experimental EPR spectrum of 10% Ag/CeO2 (Imp) (196 °C); and simulated EPR signals for (B) Ag2+ in orthorhombic symmetry (D) Ag2+ with axial symmetry ‘‘c1,’’ (E) Ag2+ with axial symmetry ‘‘c2’’ and (C) the summation of ‘‘c1’’ and ‘‘c2’’.

Ayy = 30.74 G; and Azz = 47.06 G. The theoretical and experimental signals are very similar. Considering a second hypothesis, where the experimental ‘‘c’’ signal is due to the superposition of two signals resulting from two different Ag2+ species, two simulated signals ‘‘c1’’ and ‘‘c2’’ (Fig. 6, spectra D and E) with an axial symmetry each and the following EPR parameters: g\(1) = g\(2) = 2.010, g(1) = g(2) = 2.106 with A\(1)= 32.88 G; A\(2) = 28.40 G and A//(1) = 49.80 G; A//(2) = 43.30 G are computed. Fig. 6, spectrum C, is the algebraic summation of the theoretical ‘‘c1’’ and ‘‘c2’’ signals. It is also very similar to the experimental ‘‘c’’ signal (Spectrum B). Table 2 summarizes the theoretical and experimental g and A values. To validate one of the two hypotheses, the analysis of the EPR parameter values (g and A) was performed. Since the g\ and g|| values for the ‘‘c1’’ and ‘‘c2’’ signals are the same but their A\ and A|| values are different, the Ag2+ ions sites responsible of these signals must have the same chemical environment in the 10% Ag/CeO2 (Imp) solid. This is explained by different nuclear magnetic moments of the paramagnetic species responsible of ‘‘c1’’ and ‘‘c2’’ signals. In fact, it is well known that silver possesses two isotopes: 107Ag and 109Ag, with very close natural abundances (51.83% and 48.17%, respectively) [69]. Both isotopes possess the same nuclear spins (I = ½), but different nuclear magnetic moments: l(107Ag) = 0.1135700lN and l(109Ag) = 0.1306905 lN (with lN nuclear magneton = 5.050783  1027 J T1). In addition, the hyperfine coupling constant resulting from the electron spin ‘‘S’’ of the paramagnetic species and its nuclear spin ‘‘I’’ is proportional to its nuclear magnetic moment. A relation can be then established between them. In the case of two silver isotopes, the relation is:

lð109 AgÞ=lð107 AgÞ ¼ A?ð109Þ =A?ð107Þ ¼ Ajjð109Þ =Ajjð107Þ

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Table 2 Experimental and theoretical EPR parameters. g factor

A

Experimental 10% Ag/CeO2 (Imp) spectrum (A)

giso(a) = 2.018 giso(b) = 2.013 g\(c) = 2.010; g||(c) = 2.105

Aiso(a) = 29.8 G Aiso(b) = 29.8 G A\(c) = 30.5 G; A||(c) = 47.0 G

Orthorhombic symmetry spectrum (B)

gxx = 2.009 gyy = 2.011 gzz = 2.106

Axx = 30.58 G Ayy = 30.74 G Azz = 47.06 G

Superposition of the first (E) and the second (D) Ag2+ species spectrum (C)

g\(1) = g\(2) = 2.010 g(1) = g(2) = 2.106

A\(1)= 32.88 G A//(1) = 49.80 G A\(2) = 28.40 G A//(2) = 43.30 G

Ag2+ (c1) spectrum (E)

g\(1) = 2.010; g(1) = 2.106

A\(1) = 32.88 G A//(1) = 49.80 G

Ag2+ (c2) spectrum (D)

g\(2) = 2.010; g(2) = 2.106

A\(2) = 28.40 G A//(2) = 43.30 G

The ratio of nuclear magnetic moments of both silver isotopes is equal to:

lð109 AgÞ=lð107 AgÞ ¼ 0:1306905lN =  0:1135700lN ¼ 1:15075 and the ratios of the hyperfine coupling constants of the ‘‘c’’ signal that are identical to those obtained from the simulated signal (Fig. 6, spectrum C) are equal to:

A?ð1Þ =A?ð2Þ ¼ 32:88 G=28:40 G ¼ 1:15775 Ajjð1Þ =Ajjð2Þ ¼ 49:80 G=43:30 G ¼ 1:15011

Intensity (a.u.)

The computed ratios are practically the same; thus, the hypothesis that the doublets in the experimental ‘‘c’’ signal are the result of the presence of 109Ag2+ and 107Ag2+ isotopes is validated. Therefore, each ionic isotope, characterized by S = ½ (4d9; 42D5/2) and I = ½, gives an EPR signal with an axial symmetry and a two lines hyperfine structure. The difference between the perpendicular hyperfine coupling constants: A\(109)  A\(107) = A\(1)  A\(2) = 32.88  28.40 = 4.48 G and between the parallel constants: A//(109)  A//(107) = A//(1)  A//(2) = 49.80 – 43.30 = 6.50 G is due to an isotopic splitting effect (Fig. 7). The enlargement observed in the case of ‘‘a’’ and ‘‘b’’ signals is also attributed to the isotopic split, it was not well resolved due to broad signals. The distinction among the ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ signals was carried out in a different study (to publish), in which catalysts with different Ag contents were prepared using the impregnation method and were studied with the EPR technique. The adsorption of several probe molecules possessing different redox properties such as O2, CO, CO2, NO, NO2, and C3H8 was applied prior to EPR analysis. Each

3.4.3. Mechanism of the Ag2+ species formation In order to determine the origin of the Ag2+ species formation after calcination of the 10% Ag/CeO2 (Imp) solid under air flow at 400 °C for 4 h, an EPR study was carried out on a solid calcined at different temperatures, prepared from different precursors and on different supports. 3.4.3.1. Influence of the calcination temperature. Fig. 8 illustrates the EPR spectra obtained following the calcination of the 10% Ag/CeO2 (Imp) solid at different temperatures. Up to 200 °C, the obtained spectra are similar to that of pure CeO2. From a calcination temperature of 300 °C, the three ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ signals appear and their intensities increase to reach a maximum at 400 °C. Further heating leads to a decrease in their intensities before disappearing at 700 °C. At 500 °C, the doublet in the ‘‘b’’ signal is well resolved. The presence of the three signals in the temperature range of 300–400 °C corresponds to the same temperature range of the AgNO3/CeO2 decomposition with the temperature. Therefore, the formation of Ag2+ species in the solid can be due to the nitrate precursor decomposition following the reaction:

ðAgþ NO3 Þ=ðO2 Ce4þ O2 Þ ! ðAgþ NO2 Þ=ðO2 Ce4þ O2 Þ a

! ðAg2þ O2 Þ=ðO2 Ce4þ O2 Þ þ NO2

a b

c1

A┴ (107Ag2+) =28.40 G c2

3380

of the three signals behaves differently depending on the nature of the adsorbed gas. Moreover, at low silver loading, the ‘‘c’’ signal was not observed. The EPR spectra of the 10% Ag/CeO2 (Imp) solid showed three different signals: ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ ascribed to three different Ag2+ species. The Ag2+ species responsible of ‘‘a’’ and ‘‘b’’ signals are in their cluster form due to the isotropic shape of the signals. However, the well-resolved ‘‘c’’ signal shows that some Ag2+ species are well dispersed in the solid. The high resolution of the ‘‘c’’ signal allowed the accurate measurement of the A and g parameters that were used in theoretical signals simulation. The silver isotopic split of 107Ag2+ and 109Ag2+ was then confirmed. This split was not observed in the ‘‘a’’ and ‘‘b’’ signals because they were not well resolved and their EPR parameters were not well defined.

3390

3400

b

with the participation of O2 of ceria lattice. The oxygen vacancy will be regenerated either with the oxygen from nitrate or with gaseous oxygen.

c2

A┴(109Ag2+) =32.88 G c1

3410

3420

3430

Magnetic field (G) Fig. 7. The isotopic split observed in the experimental EPR spectrum recorded for the 10% Ag/CeO2 (Imp) solid at 196 °C.

3.4.3.2. Influence of the Ag+ based precursor and the support nature. Instead of silver nitrate, the 10% Ag/CeO2 (Imp) solid was prepared using silver sulfate (Ag2SO4) precursor. The solid was calcined under a flow of air at 400 °C for 4 h and then treated under vacuum for 30 min. Fig. 9 presents the EPR spectra for both

143

700 °C 600 °C 500 °C

Intensity (a.u.)

400 °C Ce 3+ Ag 2+ signals 300 °C 200 °C 100 °C

20

90°C 742°C

160°C

(c)

110°C

748°C

66°C

(b)

472°C

776°C (a)

120

220

320

420

520

620

720

TCD signal of 10% Ag/CeO2 (DP) (a.u.)

TCD signal of 10% Ag/CeO2 (Imp) (a.u.)

M. Skaf et al. / Journal of Catalysis 320 (2014) 137–146

820

Temperature (°C)

20°C

Fig. 10. TPR profiles for (a) CeO2, (b) 10% Ag/CeO2 (Imp), and (c) 10% Ag/CeO2 (DP).

3125

3175

3225

3275

3325

3375

3425

3475

3525

3575

Magnetic field (G) Fig. 8. EPR spectra for 10% Ag/CeO2 (Imp) calcined at different temperatures.

(c)

Intensity (a.u.)

(b) c

c Ce3+

(a) a

a b b c c

3300

3350

3400

3450

3500

Magnetic field (G) Fig. 9. EPR spectra for (a) 10% AgNO3/CeO2 (Imp) and (b) 10% Ag2SO4/CeO2 (Imp) and (c) 10% AgNO3/Al2O3 (Imp).

10% Ag/CeO2 (Imp) solids prepared from (a) the nitrate and (b) sulfate precursors. The spectrum obtained for AgSO4/CeO2 (Imp) solid is only formed by the Ce3+ and ‘‘c’’ signals with very weak intensities that slightly shifted to the left due to chemical environmental change. Consequently, the amount of Ag2+ species in the new solid is negligible compared to that obtained in the other catalyst. It is clear that the nitrate ions are essential in the formation of Ag2+ species from of Ag+. A 10% Ag/Al2O3 (Imp) solid was prepared from a silver nitrate precursor. The solid was calcined under a flow of air at 400 °C for 4 h and then treated under vacuum for 30 min (Fig. 9c). The absence of ‘‘a,’’ ‘‘b,’’ and ‘‘c’’ signals for the 10% Ag/Al2O3 can be due to the mobility of oxygen in the support which is hindered in the case of alumina. In conclusion, the simultaneous presence of nitrate in the silver precursor and the mobility of the oxygen in the support are necessary conditions to form Ag2+ species. The calcination temperature is another important factor where in our case (10% Ag/CeO2 (Imp)) it was found at 400 °C.

and 776 °C and were ascribed, respectively, to the reduction of ceria surface oxygen with the formation of Ce3+ species and an oxygen lacuna, and to the removal of bulk oxygen from the ceria structure along with the reduction of Ce4+ into Ce3+ ions [38]. After silver addition, new reduction peaks corresponding to the reduction of different silver oxides were observed for both. However, the peak at 472 °C that existed for pure ceria completely disappeared. Authors observed similar results [70–71] and assigned it to the formation of new reducible species due to the interaction between silver and the support. Two TPR peaks appeared at lower temperatures (66 and 110 °C) for the impregnated catalyst. Based on the standard redox potential of Ag2+, the first reduction peak at 66 °C is ascribed to the reduction of Ag2+ into Ag+ (2 AgO + H2 ? Ag2O + H2O) or Ag0 (Ag2O + H2 ? 2Ag0 + H2O). This peak is large and its width is due to the presence of three different Ag2+ species that reduce at different temperatures. While the second reduction peak (110 °C) corresponds to the reduction of Ag+ into Ag0. The latter peak may also overlap with the reduction of surface ceria. In literature [9,72], similar results were obtained for Ag/TiO2 and Ag/Al2O3, respectively. For the (DP) catalyst, the absence of Ag2+ with the EPR technique confirms that the reduction peak observed at 90 °C is due to the reduction of Ag+, in direct contact with oxygen, into Ag0. The interaction between Ag+ and the oxygen makes the latter more mobile. However, the peak observed at 160 °C is ascribed to the reduction of Ag+ species that are not very close to the oxygen, this distance reduces the mobility of oxygen and makes the reduction harder. The width of the peaks can be explained by the fact that the Ag+ species that are well dispersed over the CeO2 phase are more or less in interaction with the oxygen species which could be responsible of the Ag2O oxide formation. Indeed, it was demonstrated [6,73] that the presence of silver would weaken the Ce–O bond which is adjacent to silver species and facilitate the reducibility of surface capping oxygen of ceria. Table 3 shows the experimental H2 consumption of CeO2, 10% Ag/CeO2 (Imp), and 10% Ag/ CeO2 (DP) solids.

Table 3 Experimental H2 consumption of CeO2, 10% Ag/CeO2 (Imp), and 10% Ag/CeO2 (DP) solids. Experimental H2 consumption (lmol of H2/g of catalyst)

3.5. TPR results Fig. 10 shows the TPR profiles for the three studied solids. In the case of ceria, two broad reduction peaks were observed at 472 °C

CeO2 10% Ag/CeO2 (Imp) 10% Ag/CeO2 (DP)

130 (66 °C) 595 (90 °C)

559 (472 °C) 979 (110 °C) 81 (160 °C)

473 (776 °C) 293 (748 °C) 297 (742 °C)

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4. Catalytic performance 4.1. Catalytic activity

Intensity (a.u.)

(b)

Fig. 11 shows the catalytic conversion of (A) propylene and (B) carbon monoxide versus the temperature. Fig. 11 (C) shows the DSC curves versus temperature for the total oxidation of the carbon black. Adding silver over ceria enhanced the catalytic activity as it was previously observed in literature [15,74,75]. T at 50% of conversion (T50) was used to compare the catalytic activity of the solids in propylene and carbon black oxidation. T at maximum combustion (Tm) was used to compare the catalytic activity in carbon black oxidation. The performance of the (Imp) solid was better than the (DP) solid in the three different oxidation reactions, with a noticeable difference in temperatures (T50(Imp)  T50(DP)) of about 87 °C in the propylene oxidation, 41 °C in the CO oxidation, and a difference of (Tm(Imp)  Tm(DP)) 57 °C in the CB oxidation. It is reported in the literature [32–37], that the deposition– precipitation method leads to the formation of nanosized particles, which are highly efficient in catalysis compared to other preparation methods such as impregnation. In our case, an opposite trend was observed. The catalytic results were in accordance with TPR results that showed that impregnated catalyst reduced at lower temperatures compared to (DP) solid. EPR and XPS results showed the presence of Ag2+ ions in the impregnated catalysts. As mentioned before, the presence of silver would weaken the Ce–O bond adjacent to silver species and facilitate the reducibility of surface capping oxygen of ceria. This reducibility becomes more significant in the presence of Ag2+ than in the presence of Ag+ or Ag0. Consequently, in the presence of Ag2+, some oxygen species become more mobile making the solid more oxidant. The presence of such ions in the (Imp) solid creates three redox couples (Ag2+/Ag+, Ag2+/ Ag0, and Ag+/Ag0) with only one (Ag+/Ag0) in the (DP) one.

3300

60

173 °C

40

325 °C

30

3450

3500

4.2. Catalysts stability In order to evaluate the stability of the catalyst over time, two aging tests, one static and the second dynamic were carried over the 10% Ag/CeO2 (Imp) solid. For this purpose, 100 mg of freshly

20

(b)

(B)

(c)

80 70

90 °C

60

131 °C

50 40

300 °C

30 20

10 0 100

3400

EPR characterization after propylene test (Fig. 12), proved that Ag2+ ions intervene in this reaction. Signals ‘‘a’’ and ‘‘c’’ disappeared, showing the reduction of Ag2+ into Ag+ or Ag0. The intensity of signal (b) increased, showing that clusters of Ag2+ were affected after the propylene oxidation, becoming more dispersed species and more observable by EPR. Isotope split, which was previously ascribed to a shouldering on the ‘‘b’’ signal, appeared as a wellresolved line in the spectrum after test (b1 and b2 lines).

90

50

3350

c

Fig. 12. EPR spectrum recorded at 196 °C of 10% Ag/CeO2 (Imp) before (a) and after (b) propylene oxidation test (1 °C/min, 100 mg catalyst and 100 mL/min flow).

(c) 261 °C

a

b

Magnetic field (G)

80 70

b1

b

100

(b)

b2

c

CO conversion (%)

C 3H 6 conversion (%)

(A)

b2

(a) a

100 90

b1

10

(a) 150

200

250

(a)

0

300

350

400

20

70

Temperature (°C)

170

220

270

320

370

Temperature (°C) Tm= 355°C

(C)

Ti= 245°C Tm = 298°C

DSC Signal (a.u.)

120

(c)

Tf= 432°C

Tf= 350°C Ti= 208°C Tm= 350°C

(b) Ti= 261°C

Tf= 473°C

(a) 30

130

230

330

430

530

630

730

Temperature (°C) Fig. 11. Conversions of (A) propylene (B) carbon monoxide and (C) DSC curves for total carbon black oxidation over (a) CeO2, (b) 10% Ag/CeO2 (Imp), and (c) 10% Ag/CeO2 (DP).

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M. Skaf et al. / Journal of Catalysis 320 (2014) 137–146

100

100 90

T= 175°C

80

C3 H 6 conversion (%)

C3 H 6 conversion (%)

90

70 60 50

(a)

40 30

80

cycle 1

70 60

cycle 2

(b)

50

cycle 3

40

cycle 4

30 20

cycle 5

10

10

cycle 6

0

0

20

50

100

200

250

300

(c)

After Intensity (a.u.)

150

Temperature (°C)

Time (h)

b1 Before b2

3300

b1 b2

3400

3500

Magnetic field (G) Fig. 13. (a) Evolution of catalytic activity of 10% Ag/CeO2 (Imp) at 175 °C for 72 hours each (b) Evolution of catalytic activity of 10% Ag/CeO2 (Imp) for 7 consecutive catalytic cycles. (c) EPR spectra of 10% Ag/CeO2 (Imp) before and after aging test.

calcined catalyst was loaded in the reactor and used in the oxidation of propylene test. In the static test, (Fig. 13a) the catalyst was exposed to C3H6 at 175 °C for 72 h. The activity remained stable with time. In the dynamic aging test, different catalytic cycles were performed over the same catalyst (Fig. 13b), and catalytic activity remained practically unchanged after 7 cycles. The used solids were studied after the aging catalytic tests by the EPR technique. The EPR spectra for both used solids were the same showing that only the ‘‘b’’ signal is obtained with its isotope split marked as ‘‘b1’’ and ‘‘b2,’’ whereas, the ‘‘a’’ and ‘‘c’’ signals disappeared (Fig. 13c). In conclusion, the Ag2+ ions responsible of the ‘‘b’’ signal are the reason behind the better catalytic performance and stability of the 10% Ag/CeO2 (Imp) solid in the total oxidation of propylene. However, the species responsible of the ‘‘a’’ and ‘‘c’’ signals could participate in this reaction but to a lower extent. This does not exclude the role of Ag+ species in the catalytic reaction. But the presence of Ag2+ leads to the formation of the three different redox couples which makes the impregnated solid more efficient.

5. Conclusion The impregnated catalyst showed higher activity than the deposition–precipitation one in three different catalytic oxidation reactions. The EPR technique showed three different sites of Ag2+ ions in the impregnated solid, and allowed the identification of the 107 Ag2+ and 109Ag2+ isotope split that has not been reported in literature yet. These ions were absent in the (DP) solid. Ag2+ ions are proven responsible of the high performance of the (Imp) catalyst. Finally, Ag2+ sites giving the EPR signal (b) are responsible of the catalytic stability in the (Imp) solid.

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