Al2O3 Naphtha Reforming Catalysts: An

Aging of a Pt/Al 2 O 3 exhaust gas catalyst monitored by quasi in situ X-ray ... Mohamed Kacimi , François Bozon-Verduraz , Leonarda F. Liotta , Mahf...
8 downloads 0 Views 471KB Size
Download PDF
9068

J. Phys. Chem. 1996, 100, 9068-9076

Redispersion of Sintered Pt/Al2O3 Naphtha Reforming Catalysts: An in Situ Study Monitored by X-ray Absorption Spectroscopy Franc¸ ois Le Normand* LERCSI, UniVersite´ Louis Pasteur, Institut Le Bel, URA 423 du CNRS, 4 rue Blaise Pascal, F 67070 Strasbourg Ce´ dex, France

Armando Borgna, Teresita F. Garetto, and Carlos R. Apesteguia INCAPE, Facultad de Ingenierı´a Quı´mica (UNL), CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina

Bernard Moraweck Institut de Recherches sur la Catalyse, UPR 5401 du CNRS, 2 aVenue Albert Einstein, F 69626 Villeurbanne Ce´ dex, France ReceiVed: January 5, 1996; In Final Form: March 5, 1996X

Redispersion of sintered Pt (0.6-1.0%)/Al2O3 catalysts at 773 K under HCl/H2O/O2/N2 atmospheres was studied by hydrogen chemisorption, temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and in situ X-ray absorption spectroscopy (EXAFS) experiments. TPR and H2 chemisorption results showed that the concentration of hydroxychlorided Pt species on the alumina carrier and, as a consequence, the metallic dispersion constantly augmented during the 8 h redispersion treatment. In situ EXAFS experiments indicated that Pt(OH)4Cl22- species, which are formed by the attack of gaseous/ surface chlorided species to partially oxidized metallic Pt particles, are responsible for Pt redispersion. EXAFS studies also suggested that once the Pt-Cl/Pt-O coordination number ratio was stable, the Pt atoms were present on the alumina carrier either in the form of small rafts containing Cl and O atoms in a slighly distorted octahedral environment or in relatively large metallic particles covered by Pt(OH)4Cl22- species. This suggestion was supported by the bimodal distribution of metal platinum particles observed by TEM when the redispersed samples where examined after its reduction with H2.

Introduction Bifunctional Pt/Al2O3-Cl naphtha reforming catalysts are deactivated by side reactions such as coking, sintering, and poisoning in the course of industrial operation.1 The in situ regeneration of the catalyst involves two main steps: (i) burning of the coke in O2/N2 mixtures of low oxygen content and at temperatures lower than 773 K; because the reaction is highly exothermic and forms water, this step causes an additional sintering of the platinum particles;2,3 the growth of the platinum crystallites may be partially prevented by introducing chlorided compounds to the oxidizing mixture;4 (ii) rejuvenation of the catalyst by redispersing the metallic fraction and restoring the chlorine content on the alumina carrier using hydroxychlorinating atmospheres at 673-773 K.4,5 In both steps, the chemical nature of the surface intermediate species plays a key role in the sintering/redispersion process of the metallic fraction.5 Particularly, several papers employing TPR, UV-vis, XRD, and TEM techniques have reported that the formation of surface oxychlorided species of platinum is a requisite for redispersing it.5-7 However, knowledge on the exact chemical nature of these species and on its formation mechanism, particularly under industrial conditions, is still lacking. Extended X-ray absorption fine structure (EXAFS) spectroscopy seems to be a preferential technique for obtaining insight on this topic, since the local environment around * Address correspondence to this author. Present address: IPCMS-GSI, UMR 1046 du CNRS, Baˆtiment 69, 23, rue du Loess, F 67037 Strasbourg Cedex, France. X Abstract published in AdVance ACS Abstracts, May 1, 1996.

S0022-3654(96)00080-9 CCC: $12.00

platinum may be continuously monitored in situ under operative conditions approaching the real ones.8-10 In a previous work,9 we have used the EXAFS technique to investigate the chemical nature of the species involved during the sintering of unchlorided Pt/Al2O3 catalysts. In this paper, we employed the same technique for in situ investigating the redispersion kinetics of sintered Pt/Al2O3 catalysts under gaseous HCl/H2O/O2/N2 mixtures. The aim was twofold: (i) to establish the chemical nature of the surface species responsible for redispersion, and (ii) to study the effect the redispersing atmosphere composition has on the redispersion kinetics. Experimental Section Samples Preparation. Two alumina-supported platinum catalysts containing 0.95% Pt (sample S1) and 0.62% Pt (sample S2) were prepared as previously described.11 A high-purity γ-Al2O3 powder (175 m2/g; Cyanamid Ketjen CK-300) was impregnated with an aqueous solution of H2PtCl6 and HCl. After impregnation the samples were dried 12 h at 393 K and then calcined with flowing air at 773 K for 4 h. Sample S1 was sintered in a O2 (2%)/N2 atmosphere at 923 K for 8 h, whereas sample S2 was treated in the same gaseous mixture at 883 K for 12 h. It was checked that the sintering treatments did not modify the platinum content of the samples. The main characteristics of sintered samples S1 and S2 are given in Table 1. Redispersion Procedures. In all the cases, redispersion treatments were carried out in a flow system at 773 K and 1 atm (1 atm ) 101.325 kPa) using a gaseous mixture of HCl/ © 1996 American Chemical Society

Sintered Pt/Al2O3 Naphtha Reforming Catalysts

J. Phys. Chem., Vol. 100, No. 21, 1996 9069

TABLE 1: Main Characteristics of Sintered Samples S1 and S2, and Operating Conditions of in situ EXAFS Redispersion Treatments redispersion conditions Pt Pt Cl Cla O2b dispersion loading T td (wt %) (wt %) (ppm) (%) qc (K) (min) (%) sample S1 S2

4 21

0.95 0.62

0.58 0.61

2500 3300

20 2

56 773 56 773

245 180

a,b Chlorine and oxygen contents, respectively, in the hydroxychlorinated gaseous mixture of HCl/H2O/O2/N2. c q ) H2O/HCl, molar ratio in the gaseous mixture. d Duration of the redispersion treatments.

H2O/O2/N2 which contained 2-20% O2 and a H2O/HCl molar ratio of 56. Water and HCl were continuously added to the O2/N2 gas mixture by employing a motor-driven syringe. To avoid water condensation the gaseous mixture was kept at a temperature higher than 353 K before it entered to the reactor. The redispersion operative conditions used in EXAFS experiments are summarized in Table 1. In a parallel 8 h run, sintered sample S2 was also redispersed in a laboratory device and modifications in the metallic fraction followed by TPR and H2 chemisorption techniques. Excepting for the treatment length, the redispersion operative conditions employed were the same as described in Table 1 for sample S2. TPR and H2 Chemisorption Characterization. Accessible platinum fraction was determined by hydrogen chemisorption. The volumetric adsorption experiments were performed at room temperature in a conventional vacuum apparatus. Hydrogen uptake was determined using the double isotherm method as previously reported.9 The platinum dispersion was calculated by assuming a stoichiometry H/Pts ) 1, where Pts implies Pt atom on surface. The TPR experiments were performed in 5% H2/Ar gaseous at 20 cm3/min. Samples were heated at 5 K/min within the temperature range 298-800 K. Prior to TPR experiments, samples were treated in situ with air at 723 K for 1 h. Since water is formed during sample reduction, the gas exiting from the reactor was passed through a cold trap before entering the thermal conductivity detector. Quantitative TPR analysis was performed by integrating the H2 uptake curves and by calibrating the detector signals with injection of known volumes of H2 into the carrier gas. TEM Examination. Selected sintered and redispersed samples were examined by transmission electron microscopy (TEM) in a JEOL 100 CX microscope operating at 100 kV. An extractive replica technique was used to prepare the samples.12 EXAFS Spectroscopy. EXAFS experiments were carried out using the synchrotron radiation facility of the DCI storage ring at LURE (Orsay, France). The experiments were performed on the D42 (sample S1) and the D13 (sample S2) stations, which worked in the transmission mode by using a Si(111) channel cut and a Si(311) double monochromator, respectively. The two ion chambers were filled with argon at or below atmospheric pressure. Raw data were recorded in a in situ cell which allows thermal treatments up to 873 K at atmospheric pressure under various gaseous environments. The temperature was kept constant within (10 K throughout the experiment. The main characteristics of this cell were previously described.13 Samples (∼200-300 mg) were pressed between two graphite (from Papyex) sheets which ensure the gas tightness of the cell, with the pressure drop between the inside and outside of the cell never exceeding 5%. Sample thickness was about 4 mm, providing an absorption edge ∆µ of 0.2-0.3 at the Pt LIII edge. During redispersion treatments, EXAFS spectra were continuously recorded in the range 11 200-12 300 eV either in a step by step mode (D42) or in the fast acquisition mode (D13)

previously described.14 The counting time for each spectrum was 10 and 3 min, respectively. The EXAFS signal χ(k) was analyzed according to two independent procedures,15 both of which involve the conventional subsequent steps (Table 2): 1. The variation of the absorption coefficient was fitted with a Victoreen function before the edge and then extrapolated to higher energies. 2. The determination of the atomic-like absorption coefficient was fitted by a polynomial of convenient degree. Then the spectrum was normalized by the Heitler method16 and, after applying a Kaiser window to minimize the truncature effects, Fourier-transformed to get the radial distribution function. We then selected a range in R (0.88-3.23 Å) which corresponds to the three bonds possibly present around platinum, i.e., Pt-Pt, Pt-Cl, and Pt-O. 3. Then, a fitting procedure was performed on the k1- and k3-weighted inverse Fourier transform oscillations by using a simplex algorithm.17 The calculated EXAFS parameters did not differ by more than 5% whatever the weight functions k1 and k3. During the fitting process the k range was limited to 10 Å-1 because the signal to noise ratio becomes poor and also some effects of asymmetrical distance distribution may be important as the product k‚∆σ2 is then not far from 1.18 Due to the limited ranges in k (∆k ) 7 Å-1) and in R (∆R ) 2.35 Å) the maximum number of parameters N which may be optimized is given from the Shannon criterion which may be expressed by19

N ) 2∆kδR/π ) 2 × 7 × 2.35/π ) 10

(1)

Consequently, the coordination number nj, the radial distance Rj, and the Debye-Waller factor ∆σj of the jth shell around platinum (j ) Pt, O, Cl) may be optimized with only one unique phase parameter ∆E0. Nevertheless, this procedure does not allow to take into account asymmetrical disorder which has been already invoked by some authors for Pt-Pt bonds at high temperature.10,20 Moreover, as the possible reference compounds PtO2 and K2PtCl6 were unstable at the temperature of our experiments (773 K) we needed to use calculated backscattering amplitudes, Fj(k), and phase shifts, φj(k), for Pt, O, and Cl atoms. Pt central atom phase shift, δ(k), was extracted from previous theoretical works.21,22 Such a procedure leads to a phase correction factor varying between 5 and 10 eV. Results TPR and H2 Chemisorption. The TPR profiles corresponding to sintered sample S2 following consecutive redispersion treatments are presented in Figure 1. All the curves exhibited a main asymmetric peak at 493-523 K and a small H2 consumption band at higher temperatures. From previous work,4,5 the low-temperature peak may be attributed to the reduction of oxy- or more probably hydroxychlorided Pt species. The high-temperature consumption band, which presented a maximum at ca. 633 K, is probably caused by hydrogen uptake on the metallic Pt surface resulting from reduction of platinum in the low-temperature peak. Such a strong hydrogen chemisorption at high temperature on Pt-based catalysts is a very wellknown phenomenon which has been observed by several authors.23 It is important to note that no hydrogen consumption was observed around 368-378 K where reduction of PtO2 species takes place.9 The TPR traces of Figure 1 show that the H2 consumption was clearly increased by increasing the duration of the redispersion treatment, suggesting thereby that the concentration of

9070 J. Phys. Chem., Vol. 100, No. 21, 1996

Le Normand et al.

TABLE 2: Parameters of the EXAFS Analysis k power

k window (A-1)

k window shape

R window (Å)

R window shape

1 3

2.27-13.8 2.27-13.8

exponential Kaiser

0.88-3.23 0.88-3.23

Hanning Hanning

Figure 1. TPR profiles of sintered sample S2 following consecutive redispersion treatments at 773 K in HCl/H2O/O2/N2 gaseous mixture; (a) t ) 30 min; (b) t ) 60 min; (c) t ) 120 min; (d) t ) 180 min; (e) t ) 240 min; (f) t ) 360 min; (g) t ) 480 min.

Figure 2. Platinum dispersion from hydrogen chemisorption (square) and hydrogen consumption per surface Pt atom (H/Pts) from quantitative TPR analysis (circle) as a function of redispersion time (sample S2).

hydroxychlorided Pt species on the alumina carrier augmented during the redispersion of the sample. The accessible Pt fractions of the samples of Figure 1 were measured by H2 chemisorption. The results are given in Figure 2. It is shown that the metal dispersion of the sintered sample S2 was increased from 21 to 47% after 8 h of redispersion treatment. Finally, from the H2 consumption values of TPR profiles of Figure 1 and the metal dispersions of Figure 2, we calculated the H/Pts atomic ratio, without considering the H2 uptake of the hightemperature band of TPR traces. The H/Pts values as a function of time are given in Figure 2 and show that a nearly constant value of H/Pts ) 6 was reached after 2-3 h of redispersion treatment. This value was considerably higher than that measured for Pt oxides species, where H/Pts ) 4.9 Thus, quantitative TPR results strongly suggest that only oxy- or hydroxychlorinated Pt species are formed during the redispersion of sintered sample S2. Transmission Electron Microscopy. Electron micrographs showed that the sintered sample S2 exhibits a bimodal distribution of Pt crystallites, where large particles of 60-80 Å coexist with some unsintered or slightly sintered ones (Figure 3a). The Pt particle distribution of the sintered sample S1 (micrographs not shown here) was also bimodal, although the size of sintered

fit range in Å-1 (weight) 2.3-3.0 (0.1) 2.3-6.0 (0.3)

3.0-9.2 (1) 6.0-9.2 (1)

9.2-10 (0.3) 9.2-10 (0.1)

particles (150-200 Å) was considerably higher than that observed for sintered sample S2. This is in agreement with the metal dispersion values measured for these samples by H2 chemisorption (Table 1). Redispersed sample S2 exhibited a bimodal distribution of Pt crystallites with areas containing very small Pt crystallites and some domains where still subsists relatively larger Pt particles (Figure 3b). However, the mean Pt particle size was clearly lower than that observed in the same sample before redispersion (Figure 3a). Redispersed sample S1 also contained a bimodal Pt particle distribution. Such a bimodal distribution of Pt crystallites determined by TEM characterization in redispersed samples S2 and S1 suggests that platinum was not completely redispersed at the end of the respective redispersion treatments and the unimodal distribution of the fresh catalyst was never reached.4 X-ray Absorption Spectroscopy. The mean distances (Rj), the coordination numbers (nj), and the Debye-Waller factors (∆σj2) of the jth shell measured during redispersion of sintered samples S1 and S2 are reported in Tables 3 and 4, respectively. Due to the difficulty of performing EXAFS analysis at high temperatures, the coordination values reported in Tables 3 and 4 must be taken with care. The errors are estimated to 25%, specially for Pt-O and Pt-Cl coordinations. Before the redispersion runs were performed, samples were reduced in H2 at 573 K and analyzed by EXAFS at room temperature. Then, the sample was heated under pure helium up to 773 K in about 1 h and was again examined by EXAFS before the redispersion treatment was started under a HCl/H2O/O2/N2 mixture. This spectrum corresponds to zero time in Tables 3 and 4. Figure 4 illustrates the experimental radial distribution function obtained for sample S1 at increasing redispersion times from the direct Fourier transform of experimental data. Figure 5 compares the fit between the experimental and calculated k3χ(k) functions for some selected times of redispersion. We note an increase of the amplitude of the oscillations in the low k range. This can be qualitatively assigned to increasing contributions of the light elements. The amplitude of the oscillations drops at k ) 8 A-1, which corresponds to a maximum of the Pt-Pt backscattering amplitude.24 Similar quality of fit was obtained for all performed modeling. Furthermore, the agreement between the two EXAFS analyses was satisfactory on the absolute values and excellent on the ratio of the Pt-O and PtCl coordinations. The results given in Tables 3 and 4 show that the Pt-Pt coordination number decreased from an initial value of 10-11 to about 6, in 6 min in the case of sample S2 and in 100 min for sample S1. Such a difference in the variation kinetics of Pt-Pt coordination may be due to (i) the lower Pt dispersion exhibited by sintered sample S1 and (ii) the lower HCl and the larger O2 concentrations used in the gaseous mixture for redispersing sample S1 (Table 1). The Pt-Pt coordination decreased then slower in both samples, reaching a value of approximately 4.5-5 at the end of the runs. Moreover, a slight increase of the Pt-Pt distance was observed during redispersion of both samples. Regarding the Pt-Cl and Pt-O coordinations, Tables 3 and 4 show that Pt-O bonds were formed from the beginning of the runs whereas the presence of Pt-Cl bonds was only detected after an induction period larger than 15 min. In spite of some

Sintered Pt/Al2O3 Naphtha Reforming Catalysts

J. Phys. Chem., Vol. 100, No. 21, 1996 9071

Figure 3. TEM micrographs of sintered (a) and redispersed (b) sample S2. Magnification is 330 000.

irreproducibility in the respective coordination, probably due to local variations in the HCl and H2O relative partial pressures, the Pt-O and Pt-Cl coordination numbers seem to stabilize at about 3-4 and 1.5, respectively. The presence of both oxygen and chlorine atoms around platinum during the redispersion process suggests the existence of oxychlorinated complexes of the type (PtOx′Cly′)2- (x′ g 2 and y′ e 2) or [Pt(OH)xCly]2- (x + y ) 6, 4 e x e 6 and 0 e y e 2). In order to ascertain the chemical composition of these complexes, we calculated the r ) nPt-Cl/nPt-O ratio, which is plotted as a function of time in Figure 6. For both samples, it is observed that after the induction period needed to form Pt-Cl bonds, the r ratio continuously increased with increasing time up to reach a constant value close to 0.5. This constant level strongly suggests that the complex formed onto the surface after the induction time is [Pt(OH)4Cl2]2-. Additional information may be obtained from the mean

distance values. From Tables 3 and 4 it is inferred that the Pt-Pt distance at the end of redispersion treatments (2.802.85 Å) was slightly extended compared to the normal metal distance at room temperature (2.77 Å). This fact may be due to (i) a normal extension of the metallic distance due to the dilatation coefficient but this effect must still remain weak; (ii) an interface effect between the metallic core and the “cherry” of oxychlorided Pt species resulting in a slight distortion of the mean distance, and (iii) this mean distance actually corresponding to the addition of two different Pt-Pt shells, the first one corresponding to the metal distance at 2.77 Å and the second one to the Pt-Pt coordination in an oxide or an oxychloride species at around 3.18 Å for PtO2.25 Let us recall that the PtPt distance undergoes a contraction under reductive conditions, which is more pronounced the smaller the particle and the higher the temperature.20 Tables 3 and 4 also show that whereas the

9072 J. Phys. Chem., Vol. 100, No. 21, 1996

Le Normand et al.

TABLE 3: Structural Parameters Determined from EXAFS Analysis during Redispersion of Sintered Sample S1 Pt-Pt bond

Pt-Cl bond

Pt-O bond

T (K)

t (min)

n

R

∆σ2

n

R

∆σ2

n

R

∆σ2

Ra

300 773 773 773 773 773 773 773 773 773 773

0 5 25 45 100 130 140 160 215 245

11 10.8 10.3 9.2 7.4 6.4 6.0 6.4 5.0 5.9 4.75

2.78 2.79 2.80 2.80 2.85 2.85 2.85 2.84 2.83 2.82 2.84

0.0001 0.0090 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100

0.2 0.5 0.8 0.8 1.1 1.6 1.7 1.7

2.39 2.41 2.48 2.48 2.40 2.43 2.40 2.46

0.0140 0.0140 0.0140 0.0140 0.0140 0.0140 0.0140 0.0140

1.5 1.7 1.8 2.3 2.3 2.3 3.1 3.0 3.2

1.99 1.98 1.95 1.99 1.99 1.95 1.89 1.98 1.96

0.0110 0.0110 0.0110 0.0110 0.0110 0.0110 0.0110 0.0110 0.0110

0 0.1 0.28 0.35 0.35 0.48 0.52 0.57 0.53

a

R ) n(Pt-Cl)/n(Pt-O).

TABLE 4: Structural Parameters Determined from EXAFS Analysis during Redispersion of Sintered Sample S2 Pt-Pt bond

Pt-Cl bond

T (K)

t (min)

n

R

∆σ2

300 773 773 773 773 773 773 773 773 773 773 773 773 773 773 773

0 3 6 10 13 16 20 23 28 33 36 50 75 105 180

10.25 10.1 6.3 5.6 6.1 5.9 6.0 6.0 5.5 5.5 4.9 4.15 3.1 4.4 4.4 4.3

2.76 2.78 2.77 2.77 2.76 2.80 2.80 2.80 2.79 2.79 2.77 2.82 2.77 2.81 2.82 2.84

0.0009 0.0043 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0064 0.0120 0.0120 0.0120

a

n

0.95 0.9 1.0 1.1 1.05 1.8 1.8 1.4 1.4 1.2

R

2.64 2.62 2.60 2.50 2.35 2.49 2.49 2.30 2.28 2.30

Pt-O bond ∆σ2

n

R

∆σ2

ra

0.0140 0.0140 0.0140 0.0140 0.0140 0.0140 0.0140 0.0190 0.0190 0.0190

4.35 4.6 4.1 4.3 3.2 3.15 2.9 3.0 2.3 3.5 3.5 2.5 2.4 2.6

1.91 1.91 1.90 1.95 1.92 1.94 1.92 1.91 1.91 1.96 1.96 1.90 1.91 1.94

0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0220 0.0220 0.0220

0 0 0 0 0.30 0.29 0.35 0.37 0.46 0.51 0.56 0.56 0.58 0.46

r ) n(Pt-Cl)/n(Pt-O).

Discussion The EXAFS results summarized in Tables 3 and 4 and in Figure 6 show that redispersion of sintered samples S1 and S2 occurred in two steps. The first step, which can be considered as an induction period, was characterized by the absence of any Pt-Cl interaction (nPt-Cl ) 0). This suggests that initially the chlorine present in gas phase is preferentially adsorbed by the alumina support until a saturation is reached. According to Parera, the saturation yields about 20% of the total hydroxyl sites of the alumina at 773 K.26 This suggestion is supported by the fact that the chlorine equilibrium concentration at 773 K on the γ-alumina used in this work for a HCl/H2O/O2/N2 mixture with H2O/HCl ) 56 is 0.85%, which is a value considerably higher than those measured on sintered samples S1 and S2 (Table 1). In this first step, it was also observed that Pt-O bonds were almost instantaneously formed upon contacting the hydroxychlorinating mixture with the sample, thereby indicating that the oxygen in gas phase rapidly oxidized the metallic Pt particles according to Figure 4. Evolution of radial distribution obtained by Fourier analysis during redispersion process (sample S1): (a) t ) 0 min; (b) t ) 5 min; (c) t ) 45 min; (d) t ) 245 min.

Pt + O2 T PtO2

Pt-Cl distance in samples S1 and S2 during redispersion (2.382.45 Å) was higher than the Pt-Cl distance in the PtCl62complex (2.33 Å), the Pt-O distance (1.95-2.0 Å) was slightly lower than the normal distance in PtO2 (2.04 Å). These modifications can be explained in terms of a distortion of the platinum structural arrangement relative to the reference platinum structural arrangement in PtO2 (cubic) or more probably PtCl62- (octahedral).

Probably, the metallic crystallites were covered by some monolayers of PtO2 species, as we have determined in previous work performed over unchlorided Pt/Al2O3 catalysts.9 Scheme 1 illustrates the state of samples S1 and S2 in this first step. In the second step, the Pt-Cl and Pt-O coordinations continuously changed, thereby indicating an evolution in the chemical nature of the surface species involved in the redispersion process. Moreover, the decrease in Pt-Pt coordination

K

(2)

Sintered Pt/Al2O3 Naphtha Reforming Catalysts

J. Phys. Chem., Vol. 100, No. 21, 1996 9073

Figure 6. Evolution of r ) nPt-Cl/nPt-O coordination number ratio with redispersion time for sample S1 (square) and samples S2 (circle). In the full line is displayed the ratio expected for the complex Pt[(OH)4Cl2]2and in the dotted lines are displayed the expected ratios for the complexes Pt[(OH)5Cl]2- and Pt[(OH)3Cl3]2-.

SCHEME 1

) 6 determined by TPR analysis as the asymptotic value in the H/Pts vs redispersion time curve (Figure 2) can be explained by considering the following reduction reaction: k′

[Pt2(OH)4Cl2]2- + 6 H+ T Pt0 + 4H2O + 2HCl + 2e- (3) k

Figure 5. Experimental (triangles) and modeled (full line) EXAFS contributions around Pt atom for sample S2, during redispersion treatment: (a) t ) 0 min; (b) t ) 20 min; (c) t ) 105 min.

together with the simultaneous increase in Pt-Cl coordination suggested that the metallic nucleus of Pt particles was progressively attacked by both gaseous and surface chlorided species. The coexistence of both oxygen and chlorine atoms in the first shell around platinum indicates the formation of hydroxychlorided complexes such as [Pts(OH)xCl(6-x)]2- which are formed through ligand substitution. In the series of these complexes, only even values of x (0, 2, 4, and 6) which correspond to r ) nPt-Cl/nPt-O values of ∞, 2, 0.5, and 0, respectively, are known to be stable.27-29 The redispersion kinetics of Pt particles in samples S1 and S2 followed by EXAFS (Figure 6) showed that the r ratio progressively increased as a function of time until a value close to 0.5 was reached. This result strongly suggests the exclusive formation of [Pts(OH)4Cl2]2- complexes during the redispersion process. It is significant to note that the nearest r values corresponding to other [Pts(OH)xCl(6-x)]2- complexes are 0.2 and 1, which are far beyond of the experimental error and, besides, correspond to nonstable complexes. On the other hand, the existence of a Pt complex such as [Pts(OH)4Cl2]2- is also supported by the TPR results. In fact, the value of H/Pts

Reaction 3 occurs at 490-523 K. Lieske et al.5 reported a TPR peak at 533 K that they also attribute from UV-vis spectroscopy to a hydroxychloride species by contrast with the oxide or the oxychloride species which are reduced at 373 and 563 K, respectively. The relative experimental error in the determination of r values as well as the limited time range of the experiments precludes us, however, from drawing more precise conclusions. Particularly, the formation of others complexes such as Pt(OH)xCl(6-x)2- with x ) 2 and 0 cannot be excluded for much longer times of redispersion. Nevertheless, from our results it is apparent that (i) the redispersion process involves essentially the formation of only one platinum complex and (ii) the formation of complexes with an odd number of hydroxyl groups can be excluded. This is probably due to the destabilization of the complex during the introduction of an another type of ligand in the plane of the octahedra. Moreover, it can be suggested that the hydroxyl groups fill the basal plane of the octahedra, whereas the chloride ions fill the apexes. These complexes spread onto the support but are partially stabilized by strong or medium basic sites such as O2- or Cl-.30 This can explain the distortion of the octahedra, as the stronger electronegativity of the hydroxyl group relative to the chloride ion results both in a decrease of the planar Pt-OH distances and an increase of the Pt-Cl distances along the apexes of the octahedra. Figure 6 shows that the redispersion kinetics of sintered samples S1 and S2 are different, although in both cases an exponential variation of r with time was suggested. Such a difference in the redispersion rate may be related to the different partial pressure of chlorine and oxygen employed in both experiments (Table 1). If we consider that the [Pts(OH)4Cl2]2complexes are formed by a consecutive attack of chlorided species to the metallic Pt particles according to the inverse of reaction 3 with a kinetic constant k in competition with

9074 J. Phys. Chem., Vol. 100, No. 21, 1996

Le Normand et al.

SCHEME 2

oxydation according to reaction 2, and assuming that the reaction is first order, i.e., not surface or gas phase diffusion-controlled, then the rate of tetrahydroxydichloride platinate complex formation is proportional to PHCl/PO and the ratio r can be expressed as: 2 r ) rmax {1 - exp[-ApHCl (t - t0)/PO2]}

(4)

where t > t0 (induction time), rmax the r value for t f ∞ equal to 0.5, and A a kinetic constant only depending on the temperature. We used the experimental data of Figure 6 and eq 2 for calculating the values of the kinetic constant k ) (ApHCl/ PO) and we yield a value of 0.02 min-1 for redispersion of sample S1 and 0.16 min-1 for redispersion of sample S2. The ratio of these kinetic constants k (0.125) compares favorably 2 /PO2 (0.057) as well with the ratio of the partial pressure PHCl the kinetic constants are only indicative. These qualitative results strongly suggest that the formation kinetics of [Pts(OH)4Cl2]2- species and, as a consequence, the Pt redispersion rate, depend on the relative partial pressure of HCl over the oxygen partial pressure. The higher the HCl pressure, the faster the redispersion of the metallic phase. Finally, EXAFS results allowed us to speculate about the physical state of stable Pt chlorinated species which are present in the second step of the redispersion process. In previous work,9 we stated that if the platinum particles are covered by one or two layers of platinum oxide or hydroxychlorided species in a cubooctahedral structure, then the platinum-platinum coordination is between 6 and 7 for Pt particles of size 35-40 Å. This Pt-Pt coordination is considerably higher than that found here by EXAFS measurements at the end of the redispersion processes (nPt-Pt ) 4.5-5). We have investigated therefore the possible arrangement of platinum in other structures. Particularly, we estimated the coordination number of platinum for a two-dimensional (raft) platinum complex of octahedral structure. This complex can be the product of a dehydration-condensation process starting from the tetrahydroxy dichloride platinate ion according to

n[Pt(OH)4Cl2]2- T n[PtO(2-2/n) (OH)4/nCl2]2n- + (2n - 2)H2O (5) This assumption of a raft structure is strongly supported by TEM observations by Ruckenstein that platinum, unlike other transition metals such as Pd, Ni, or Fe, does not form a uniform film on the support, but more likely forms a nonuniform thin film around the original particle.31 It is highly probable that both the chloride and the water pressure control the size of these surface complexes. If we consider a square island containing m platinum atoms per edge, we obtain the following expression:

nPt-Pt ) [8 + 12(m - 2) + 4(m - 2)2]/m2

(6)

In the limit of an island of infinite size (m f ∞), this value tends to a maximum value of 4. Also, the overall coordination of the platinum with ligands (oxygen and chlorine) tends to 5-5.5, not far from the coordination of 6 in an octahedral arrangement. Thus, the above results may be interpreted by considering that at the end of the redispersion process the Pt atoms are present on the alumina carrier mainly in the form of small rafts containing Cl and O in a distorted octahedral arrangement but also in relatively larger metallic particles covered by one or two layers of Pt(OH)4Cl22- (Scheme 2). Thus, the mean Pt-

Pt coordination must be between 4 and 6-7, in good agreement with the mean platinum coordination number detected by EXAFS. This interpretation is supported by the TEM examinations which showed that, after reduction, redispersed sample S1 and S2 exhibited a bimodal distribution of the metallic Pt particles. The smallest particles would originate from reduction of rafts and the relatively larger Pt crystallites from reduction of the remaining particles exhibiting a “cherry” structure. It is well-known that the oxidation-reduction process of platinum particles is size-dependent.32 Besides, this model is also in agreement with the value of the mean Pt-Pt distance measured by EXAFS (2.80-2.85 Å) at the end of redispersion of samples S1 and S2 which was slightly extended as compared to the normal metal distance. As was noticed above, this result strongly suggests the existence of two different Pt-Pt shells. Comparisons with literature results on the real mechanism of the redispersion process may be useful. At the earliest, a crystallite6a or a molecular migration33 mechanism was postulated. Clearly, our results support the second type of mechanism, as the metallic phase is at least partially chemically transformed. The nature of the active complex has also been questioned. It was first postulated that the molecular migration occurs through the gas phase. In spite of the lack of data on the stability of the hydroxychloride platinate complexes, this pathway seems highly improbable. Furthermore, vaporization of PtO2 can be ruled out.32 Although the key role of chlorine was recognized early, it is only in recent reports that some insight is given.4,5 Lieske et al.5a claim the formation of hydroxychloride platinate, followed by further dehydration at 773 K by the formation of oxychloride PtIVOx′Cly′. Their results were obtained in the abscence of water. By contrast, in the presence of water the dehydration may be partial, according to reaction 5. In agreement with our results, the thermoprogrammed reduction of the hydroxychloride platinate complex occurs at 533 K, instead of 563 K for the oxychloride platinate complex. The occurrence of PtII complexes,34,35 although first postulated by some authors, can also be ruled out as a valence of IV for platinum has been found.4,5 This is in agreement with our hydrogen chemisorption and EXAFS results as no complexes can be postulated having the structure [PtII(OH)xCly]2with x + y ) 4. This means that the octahedral symmetry of the complex is preserved. On the other hand, to give some support to the occurrence of a nonuniform distribution of rafts n[PtO(2-2/n) (OH)4/nCl2]2n-, thermodynamical calculations could also be considered. Very few thermodynamical data are available for oxy- or hydroxychloride platinate species, while some data are available for PtO2.36-39 Especially Wynblatt considered the equilibrium between (i) metallic platinum particles, (ii) a 2-D PtO2 phase consisting of monomers dispersed on the substrate, and (iii) a 3-DPtO2 phase. It is believed that his result can be extrapolated to our case, as addition of both HCl and H2O increases the wettability of the hydroxychloride platinate phase, leading to easier spreading and fragmentation into two-dimensionnal islands. From the free energy of the reaction

Pt(s) + O2(g) f βPtO2(s)

(7)

it can be calculated that the oxide is the stable phase at temperatures lower than 803 K Above 773 K, the last two oxidized phases were in equilibrium between them, a result which has been experimentally confirmed.38 Besides, due to the higher interface energy of PtO2, the oxidized phase tends to spread onto the carrier surface. Hence the wetting angle, according to the Dupre´ formula, can be evaluated to 30° for PtO2 and 98° for metal platinum.39

Sintered Pt/Al2O3 Naphtha Reforming Catalysts

J. Phys. Chem., Vol. 100, No. 21, 1996 9075

A saturation coverage in monomers θ0 could then be calculated. Owing to the much larger interaction of the hydroxychlorideplatinate complex with the support than the platinum oxide, we can consider θ0 for PtO2 as a lower limit of the saturation coverage for the hydroxychlorideplatinate complex. Assuming a Langmuir type isotherm, the saturation coverage θ0 of 2-D PtO2 monomers can be expressed as:

θ0 ) A(T)p0PtO2(g)/[1 + A(T)p0PtO2(g)]

(8)

where

A(T) ) [σ/ν(2πmkbT) ] exp(Hdes/kbT) 1/2

(9)

Hdes is the desorption enthalpy estimated to 263 kJ/mol from the results of Yao,38 σ ) 1.05 × 10-19 m2 is the surface of a PtO2 molecule; ν ) 3.3 × 1012 s-1 is the vibration frequency; m is the mass of PtO2 molecule. p0PtO2(g) is the PtO2 saturation pressure given by the equilibrium

PtO2(s) T PtO2(g)

(10)

are present on the carrier in two different environments. A part of the Pt atoms would form small rafts containing Cl and O atoms in an octahedral environment. The other part is considered to form Pt particles consisting of a metallic core surrounded by a coating of [Pt(OH)4Cl2]2- species. Reduction with flowing hydrogen would lead to the formation of small Pt particles from rafts and relatively larger Pt crystallites from the partially oxidized Pt particles. This interpretation explains the bimodal Pt particle distribution observed by TEM when redispersed Pt/ Al2O3 samples were examined. Acknowledgment. The staff of the Laboratoire pour l’Utilisation du Rayonnement Electromagne´tique (LUREORSAY-France) is greatly acknowledged for dedicated runs under synchrotron radiation. F. Villain is specially acknowledged for her technical assistance during X-ray absorption experiments. Prof. J. M. Parera and Prof. J. Barbier are thanked for their constant interest during the progress of this work which is part of the Programme International de Coope´ration Scientifique (PICS # 74) between CONICET (Argentina) and CNRS (France). A.B. is specially indebted to Fundacio´n Antorchas for financial support. References and Notes

and then

p0PtO2(g) (atm) ) ∆G0{7}/kbT

(11)

where ∆G0{7}/(kbT) ) -41875/T + 26.3 is the free energy change of eq 7. From these values and from eqs 8-11 we calculate θ0 to be 0.17 at 773 K, which corresponds to 16.3 × 1017 atoms/m2, far beyond of the platinum surface concentration, Cs, of our catalysts (1.73 and 1.02 ×1017 atoms/m2 for samples S1 and S2, respectively). The Cs values were calculated by the formula

Cs ) CNA/SBETA

(12)

where C is the platinum weight loading given in Table 1, NA Avogadro’s number, SBET the specific area, and A the molecular mass. There is 1 order of magnitude between the saturation coverage by the dispersed two-dimensional phase and the actual platinum loading. Thus we conclude that there are no thermodynamical limitations to the occurrence of a two-dimensionnal phase of hydroxychlorideplatinate complexes. Conclusions Redispersion of sintered Pt/Al2O3 catalysts under HCl/H2O/ O2/N2 atmospheres was studied by TEM, TPR, and EXAFS. Conclusions can be summarized as follows: 1. Redispersion occurs in two steps. In the first step, the alumina support is covered by Cl- and OH- groups following the addition of the hydroxychlorinating mixture. The surface of the metallic fraction is oxidized forming a double coating of disordered PtO2. This first step is actually an induction period characterized by the absence of chlorine coordination around platinum. 2. In the second step the PtO2 coating of sintered metallic Pt particles is attacked by the gaseous and surface chlorided species forming n[PtO(2-2/n)(OH)4/nCl2]2n- rafts in equilibrium with [Pt(OH)4Cl2]2- species which cause the redispersion of the Pt particles by migration through the hydroxylated/chlorided alumina surface. The kinetics of this second step largely depends on the chlorine and the water content in the gaseous mixture. 3. The EXAFS analysis of the evolution of Pt coordination suggests that at the end of the redispersion process the Pt atoms

(1) Franck, J. P.; Martino, G. In Progress in Catalytic DeactiVation; Figueiredo, J. L., Ed.; NATO Adv. Study Inst. Series, E54, Kluwer Academic: Dordrecht, 1982; p 355. (2) Callender, W.; Miller J. J. In Proceedings of the VIIIth International Congress on Catalysis; Weitkamp, J., et al., Eds; Dechema: Frankfurt am Main, 1984; Vol. II, p 481. (3) Garetto, T. F.; Apesteguı´a, C. Appl. Catal. 1986, 20, 133. (4) Garetto, T. F.; Borgna, A.; Benvenutto, E. R.; Apesteguia, C. R. in Proc. 12th Iberoamerican Symp. Catal. 1990, 1, 585. (5) (a) Lieske, H.; Lietz, G.; Spindler, H.; Vo¨lter, J. J. Catal. 1983, 81, 8. (b) Lietz, G.; Lieske, H.; Spindler, W.; Hanke, H.; Vo¨lter, J. J. Catal. 1983, 81, 17. (6) (a) Ruckenstein, E.; Pulvermacher, B. J. Catal. 1973, 29, 224. (b) Ruckenstein, E.; Chu, Y. F. J. Catal. 1979, 59, 109. (7) Straguzzi, G. Y.; Aduriz, H. R.; Gigola, C. E. J. Catal.1980, 66, 171. (8) Le Normand, F.; Bazin, D.; Bournonville, J. P.; Dexpert, H.; Lagarde, P. In Proceedings of the IXth International Congress on Catalysis; Philips M. J., Ternan M., Eds.; Chemical Institute of Canada: Calgary, 1988; p 1401. (9) Borgna, A.; Le Normand, F.; Garetto, T. F.; Apesteguı´a, C. R.; Moraweck, B. Catal. Lett. 1992, 13, 175. (10) Guyot-Sionnest, N. S. Ph.D. Thesis, University of Paris XI, Sept 1991. (11) Borgna, A.; Garetto, T. F.; Monzo´n, A.; Apesteguia, C. J. Catal. 1994, 146, 69. (12) Dalmai-Imelik, G.; Leclercq, C.; Mutin, I. J. Microsc. Spectrosc. Electron. 1974, 20, 123. (13) Lytle, F. W.; Greegor, R. B.; Marques, E. B.; Sandstroem, D. R.; Via, G. H.; Sinfelt, J. H. J. Catal. 1985, 95, 546. (14) Prieto, C.; Briois, V.; Parent, P.; Villain, F.; Lagarde, P.; Dexpert, H.; Fourman, B.; Michalowicz, A.; Verdaguer, M. In Synchrotron Radiation and Dynamic Phenomena; Beswick, A., Ed.; American Institute of Physics: New York, 1992; p 621. (15) (a) Majerus, J. Ph.D. Thesis, University of Strasbourg, June 1992. (b) Moraweck, B.; Renouprez, A. Surf. Sci. 1981, 106, 35. (16) Lengeler, B.; Eisenberger, P. Phys. ReV. B 1980, 21, 4507. (17) Nelder, J. A.; Mead, R. Comput. J. 1965, 7, 308. (18) (a) Eisenberger, P.; Brown, G. S. Solid State Commun. 1979, 21, 481. (b) Crozier, E. D.; Seary, A. J. Can. J. Phys. 1980, 58, 1388. (c) Teo, B. K.; Chang, H. S.; Wang, R.; Antonio, M. R. J. Non-Cryst. Solids 1983, 58, 249. (19) Crozier, E. D.; Rehr, J. J.; Ingalls, R. In X-Ray Absorption, Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Koningsberger, D. C., Prins, R., Eds; John Wiley & Sons: New York, 1988; p 395. (20) Marques, E. C.; Sandstroem, D. R.; Lytle, F. W.; Greegor, R. B. J. Chem. Phys. 1992, 77, 1027. (21) McKale, A. G.; Veal, B. W.; Paulikas, A. P.; Chan, S. K.; Knapp, G. S. J. Am. Chem.Soc. 1988, 110, 3763. (22) Teo, B. K.; Lee, P. J. Am. Chem. Soc. 1979, 101, 2815. (23) (a) Menon, P. G.; Froment, G. F. J. Catal. 1979, 59, 138. (b) Menon, P. G.; Froment, G. F. Appl. Catal. 1981, 1, 31.

9076 J. Phys. Chem., Vol. 100, No. 21, 1996 (24) Lagarde, P.; Dexpert, H. AdV. Phys. 1984, 33, 567. (25) Lagarde, P.; Murata, T.; Vlaic, G.; Dexpert, H.; Freund, E.; Bournonville, J. P. J. Catal. 1983, 84, 333. (26) Castro, A.; Scelza, O.; Benvenuto, E.; Baronetti, G.; Parera, J. J. Catal. 1981, 69, 222. (27) Wells, A. F. In Structural Inorganic Chemistry; Wells, A. F., Ed.; Clarendon Press: Oxford, U.K., 1967; 556. (28) Dreyer, R.; Dreyer, J. Z. Chem. 1963, 3, 151. (29) Doronin, N. P.; Tsymbal, T. V.; Duplyakin, V. K. React. Kinet. Catal. Lett. 1986, 32, 399. (30) Adamiec, J.; Fiedorow, R. M. J.; Wanke, S. E. J. Catal. 1985, 95, 492. (31) Ruckenstein, E. In Metal Support Interactions in Catalysis, Sintering and Redispersion; Stevenson, S. A., Dumesic, J. A., Baker, R. T. K., Ruckenstein, E., Eds.; Academic Press: New York, 1991; Chapter 15, p. 278.

Le Normand et al. (32) Rickard, J. M.; Genovese, L.; Moata, A.; Nitsche, S. J. Catal 1990, 121, 141. (33) Fiedorow, R. M. J.; Wanke, S. E. J. Catal. 1976, 43, 34. (34) Wagstaff, N.; Prins, R.; J. Catal. 1981, 67, 255. (35) Escard, J.; Pontvianne, B.; Chennebaux, M. T.; Cosyns, J. Bull. Soc. Chim. Fr. 1976, 349. (36) Schaffer, H; Tebben, A. Anorg. Chem. 1960, 304, 317. (37) Berry, R. J. Surf. Sci. 1978, 76, 415. (38) Yao, H. C.; Sieg, M.; Plummer Jr, H. K. J. Catal. 1979, 59, 365. (39) Wynblatt, P. In Growth and Properties of Metal Clusters; Bourdon, J., et al., Eds.; Elsevier: Amsterdam, 1980; p 15.

JP960080M