Oxidation behavior of alumina-supported platinum metal catalysts

Oxidation behavior of alumina-supported platinum metal catalysts

Applied Catalysis A: General 209 (2001) 1–9 Oxidation behavior of alumina-supported platinum metal catalysts Chen-Bin Wang a,∗ , Chuin-Tih Yeh b a D...

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Applied Catalysis A: General 209 (2001) 1–9

Oxidation behavior of alumina-supported platinum metal catalysts Chen-Bin Wang a,∗ , Chuin-Tih Yeh b a

Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 33509, Taiwan, ROC b Department of Chemistry, National Tsinghua University, Hsinchu 30043, Taiwan, ROC Received 3 April 2000; received in revised form 26 June 2000; accepted 3 July 2000

Abstract Heats of adsorption of dioxygen and oxidation phenomena of supported platinum metals (Pd, Pt and Rh) have been investigated by a simultaneous TG-DSC technique over a wide temperature range between 280 and 900 K. The oxidation proceeds in three consecutive stages, i.e. surface adsorption (T < 300 K), progressive penetration of adsorbed oxide ions into bulk (300–700 K), and complete oxidation to form stable oxides (700–900 K) for Pd/Al2 O3 and Rh/Al2 O3 catalysts. For Pt/Al2 O3 , in the same temperature range, the surface oxide was decomposed when T > 800 K. The crystallite sizes of supported platinum metals have a profound effect on the heats of adsorption (1Had ), heats of stable oxide formation (1Hf ) and formation of oxide species. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Palladium; Platinum; Rhodium; Heat of adsorption; Calorimetry; Oxidation

1. Introduction Alumina-supported platinum metals (Pd, Pt and Rh) are widely used in industrial practice, i.e. hydrogenation [1,2], hydrocracking [3,4], oxidation reactions [5,6] and abatement of motor vehicle emissions [7–9]. In the oxidation reactions, the mechanism involves the adsorption of oxygen on the surface of platinum metals and reduction of the platinum metal oxides by hydrocarbon or CO, i.e. Ms ⇔ Ms Ox

(1)

Here, Ms denotes platinum metal atoms exposed to the surface of supported platinum metal crystallites. A detailed understanding of the interaction of platinum metals with dioxygen should be helpful in designing improvements of the oxidation reactions. ∗ Corresponding author. Fax: +886-33-891519. E-mail address: [email protected] (C.-B. Wang).

In previous papers [10–12], we have explained the interaction of Pd/Al2 O3 , Rh/Al2 O3 and Pt/Al2 O3 , respectively, with dioxygen using a simultaneous TGDSC technique. Both the 1Had and the 1Hf of these interactions were found to vary, to a different extent, with the particle size of the crystallites. In this review, we want to identify the oxidation phenomena and compare the distinctions among the platinum metals.

2. Experimental 2.1. Sample preparation Platinum metal (Pd, Pt and Rh) catalysts of various platinum metal loadings were prepared by impregnating ␥-Al2 O3 (Merck, surface area = 108 m2 /g) with different amounts of an aqueous H2 PdCl4 , H2 PtCl4 or RhCl3 ·3H2 O solution, respectively. Obtained slurries were dried up and pre-treated sequentially with an

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 7 4 6 - 8

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Table 1 Dispersion and particle sizes for alumina-supported platinum metals measured at 300 K Sample

10.1% 4.72% 2.04% 2.80% 1.07% 1.08% 0.55% 4.13% 4.13% 4.13% 4.13% 2.12% 0.82% 8.25% 8.25% 4.00% 3.26% 3.56% 2.45% 2.86% a b

Pd Pd Pd Pd Pd Pd Pd Pt Pt Pt Pt Pt Pt Rh Rh Rh Rh Rh Rh Rh

Hydrogen chemisorption

Oxygen chemisorption

Da

db (nm)

D

d (nm)

0.14 0.21 0.25 0.30 0.52 0.80 0.91 0.45 0.56 0.63 0.80 1.02 1.00

7.9 5.2 4.4 3.7 2.1 1.4 1.2 2.2 1.8 1.6 1.3 1.0 1.0 0.19 0.25 0.50 0.55 0.67 0.80 0.91

4.7 3.6 1.8 1.6 1.4 1.1 1.0

D: Dispersion of platinum metals. d: Average particle size.

overnight drying at 380 K and a calcination at 770 K for 4 h before storing as testing samples. 2.2. Dispersion measurement The dispersion (D, fraction of exposed atoms) of palladium and platinum on these samples was volumetrically determined by chemisorption of dihydrogen at 300 K. The average diameter (d) of palladium and platinum crystallites was calculated from 1.1/D [13,14]. The dispersion of rhodium crystallites was gravimetrically estimated from adsorption of dioxygen at 300 K, assuming a monolayer chemisorption of oxygen atoms on rhodium surface. The average diameter of rhodium crystallites was calculated from 0.90/D [15]. Table 1 lists the dispersions and particle sizes for the prepared samples. 2.3. Oxidation of the catalysts The amounts of dioxygen uptake and heats evolved were monitored over a temperature range between

280 and 900 K in a dual port calorimeter (Setaram TG-DSC 111) equipped with a sensitive balance (0.25 ␮g) and a calorimeter (10 ␮W). Before the dioxygen uptake, each testing sample was pre-reduced in flowing dihydrogen gas at the required temperature (Treq ) for 1 h and a subsequent evacuation at 720 K for 1 h. The reduced sample was cooled in the Ar flow to a predetermined oxidizing temperature (Tox ) and then oxidized by a flow of 10 ml/min dioxygen introduced from the auxiliary inlet into the Ar flow. A detailed description of this system had been given in the previous reports [10–12]. Pure ␥-Al2 O3 support was mounted in the reference port of the calorimeter as a blank to offset possible changes (1m and 1H) caused by the support.

3. Results and discussion 3.1. Phenomena of oxidation Phenomena of oxidation of nanometer platinum metal crystallites supported on alumina with dioxygen were studied with a commercial TG-DSC simultaneous system. Both gravimetric and calorimetric measurements give the same tendency over a wide temperature range between 280 and 900 K. Fig. 1 shows temperature profiles of dioxygen uptake (NO /NPd ) and heats of oxidation (−Qox ) on 4.72%Pd/ Al2 O3 sample. The NO /NPd ratio remained at a constant value at T < 300 K; it gradually increased from the lower value to a plateau value between 300 and 700 K; it remained at the plateau value between 700 and 900 K. In contrast, the −Qox remained at a constant plateau value at T < 300 K; it gradually decreased from the plateau value to a lower value between 300 and 700 K; it remained at the lower value between 700 and 900 K. According to the results, the oxidation proceeds in three consecutive stages, i.e. surface adsorption (T < 300 K), progressive penetration of adsorbed oxide ions into bulk (300–700 K), and complete oxidation to form stable oxide (700–900 K). Accompanying the variations of heats of oxidation with dioxygen uptake, a definition of heats of adsorption (1Had ) and heats of oxide formation (1Hf ) can be given as x (2) Ms + O2 → Ms Ox + 1Had 2

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Fig. 1. Temperature profiles of dioxygen uptake and heats of oxidation on 4.72%Pd/Al2 O3 .

y (3) O2 → MOy + 1Hf 2 Here, x denotes an adsorption stoichiometry of oxygen on metal atoms exposed on the surface (Ms ) of dispersed metal crystallites and y denotes the stoichiometry of bulk oxide formation. M+

3.2. Comparison of platinum metals (Pd, Pt and Rh) Fig. 2 compares the temperature profiles of the dioxygen uptake on the surfaces of various alumina-supported platinum metals obtained upon oxidation of 4.72%Pd/Al2 O3 (D = 21%), 3.26%Rh/Al2 O3 (D = 55%) and 4.13%Pt/Al2 O3 (D = 56%). Observed temperature profiles of the dioxygen uptake by these metals varied slightly with the nature of the metal. Oxygen uptakes of these three samples at room temperature are low and are confined on the metal surface after adsorption. The uptake increased gradually on raising the oxidation temperature and reached a plateau value for T > 700 K.

For Pd/Al2 O3 and Rh/Al2 O3 catalysts, the excessive oxidation into the bulk of palladium and rhodium became apparent at T > 300 K; this induced a formation of bulk oxides (PdO formed at T ∼ 770 K and RhO2 formed at T ∼ 670 K). However, in the case of Pt/Al2 O3 , the oxidation preserved only a superficial character by formation of a protective surface oxide layer till a temperature of around 750 K prevented the diffusion of oxygen into the bulk of the platinum (Table 2 lists dioxygen uptake on surface and in bulk of various alumina-supported platinum metals). The phenomena of surface oxidation are further supported by the previous reported results for samples of platinum single crystal [16–20]. The authors suggested that the oxide ions adsorbed into the subsurface regions of platinum crystallites at temperatures higher than 800 K. Also, Paryjczak et al. [21] studied the interaction of Pt/Al2 O3 with oxygen. They suggested that the oxidation of platinum preserved a superficial character between 200 and 920 K. The desorption of surface oxygen atoms and/or vaporiza-

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Fig. 2. Temperature profiles of dioxygen uptake on surface of various alumina-supported platinum metals: 4.72%Pd/Al2 O3 ( ); 3.26%Rh/Al2 O3 (䊉); 4.13%Pt/Al2 O3 (4).

tion of PtO2 [12,22,23] occurred when T > 800 K by the chemical equations of Pt s + O2 → Pts O2 (g) Pt s Ox → Pts +

(4)

x O2 2

(5)

According to the Cabrera–Mott theory [24], the initial formation of an oxide layer on a metal surface can affect the further incorporation of oxygen into the metal sublayers and/or bulk positions. The magnitude of the surface-incorporation barrier (EB ) of the oxygen will have a profound effect on the transition from

Table 2 Dioxygen uptake on surface and in bulk of various alumina-supported platinum metalsa T (K)

280 300 370 470 570 670 770 870 a

4.72%Pd/Al2 O3

4.13%Pt/Al2 O3

3.26%Rh/Al2 O3

s NO /NPd

b NO /NPd

s NO /NPt

b NO /NPt

s NO /NRh

b NO /NRh

0.67 0.67 1.00 1.50 2.52 4.00 4.70 4.80

0.14 0.14 0.21 0.32 0.53 0.84 0.99 1.00

0.55 0.55 0.58 0.65 0.78 0.85 0.95 0.55

0.31 0.31 0.32 0.36 0.44 0.48 0.53 –

1.00 1.00 1.18 1.70 3.10 3.55 3.55 3.55

0.55 0.55 0.65 0.94 1.70 1.95 1.95 1.95

s b NO /NM : Uptake of oxygen atoms on surface; NO /NM : uptake of oxygen atoms on bulk.

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Fig. 3. Temperature profiles of heats of oxidation (−Qox ) upon oxidation of various alumina-supported platinum metals: 4.72%Pd/Al2 O3 ( ); 3.26%Rh/Al2 O3 (䊉); 4.13%Pt/Al2 O3 (4).

chemisorption to oxidation [25,26]. The lower the EB , so that the adsorbate oxide ions become very mobile as the temperature is increased, the easier the incorporation of oxide ions into the sublayers of metal crystal to form bulk oxide. The higher the EB , so that the adsorbate oxide ions need to have a considerable mean free path along the surface into the sublayers of metal crystal to form bulk oxides, the easier the formation of surface oxide which is observed. If the temperature is too high, the adsorbed adsorbates desorbed or vaporized. Clearly, the results indicate that elements in the third transition metal series (Pt) probably have a higher EB than the metals in the second transition metals series (Pd and Rh). Fig. 3 presents the same tendency of the variation of heats of evolution upon oxidation of 4.72%Pd/Al2 O3 (D = 21%), 3.26%Rh/Al2 O3 (D = 55%) and 4.13%Pt/Al2 O3 (D = 56%). Large differences exist between the heats of adsorption of dioxygen and the heats of formation of oxides on platinum metals. The observed phenomena of −1H ad > −Qox become

somewhat logical when one considers that little or negligible energy is required for breaking metal–metal bonds in the chemisorption stage which possesses maximum binding energy. In contrast, the so-called ‘place-exchange’ mechanism [27] of surface-layers oxidation via chemisorption-induced-reconstruction to incorporate oxide ions into sublayers involves a series breakdown of the original metal lattices. The discrepancy between 1Had and Qox may therefore be attributed to the extents of metal–metal bond breakage. This indicates the complex nature of the adsorption/oxidation process and of the dependence of the heats on the extents of oxidation. The higher the extents of oxidation, the lower the heats of oxidation evolved. The abruptly increased of heats of oxidation over 800 K for 4.13%Pt/Al2 O3 may be linked to some reactions, i.e. desorbing of oxygen, vaporization of PtO2 , re-adsorbing of oxygen, and re-oxidizing of platinum crystallites. In general, crystallite size is an important factor that affects the activity and selectivity of catalysts

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Fig. 4. Variations of −1Had of dioxygen adsorption with the average diameter of platinum metal crystallites: Pd/Al2 O3 ( ); Rh/Al2 O3 (䊉); Pt/Al2 O3 (4).

[28]. Two major explanations for the crystallite size effect have been proposed [29,30] — geometric effect and electronic effect. Whether the geometric effect or the electronic effect is predominant is still ambiguous. Fig. 4 collects the variations of −1Had of dioxygen adsorption with the average diameter of platinum metals (also see Table 3). For Pd/Al2 O3 and Pt/Al2 O3 catalysts, the evolved heats of adsorption increased about 100 kJ (mol O2 )−1 as the size of palladium crystallites decreased from 8.0 to 1.5 nm and from 2.2 to 1.0 nm for platinum crystallites, respectively [10,12]. An obvious size-dependent relation is formed −1Had (kJ (mol O2 )−1 ) = 373 − 14d (nm), for Pd/Al2 O3

(6)

−1Had (kJ (mol O2 )−1 ) = 406 − 81d (nm), for Pt/Al2 O3

(7)

However, for Rh/Al2 O3 , the evolved heats of adsorption increased only about 20 kJ (mol O2 )−1 as the

size of rhodium crystallites decreased from 5.0 to 1.0 nm [11]. The crystallite size clearly affects the heats of dioxygen adsorption on platinum metals. Because both geometric and electronic properties vary simultaneously as crystallites shrink from 5 to <1 nm [31–33]. It is usually extremely difficult to separate the contributions of these two parameters. The observed −1Had for dioxygen adsorption on Pd/Al2 O3 catalysts increasing in Pd–O bond strength in ranges from 8.0 to 1.5 nm may be due primarily to an electronic effect. The small palladium crystallites have electronic properties different from those of bulk palladium. Since the different crystal planes of platinum had different reactivities toward oxygen [34,35], in the small size ranges (2.2–1.0 nm), the change in the 1Had for dioxygen adsorption on Pt/Al2 O3 catalysts is suggested to be due primarily to a geometric effect. For Rh/Al2 O3 catalysts, in the small variations of 1Had for dioxygen, crystallite size independence is assumed. Table 4 lists the formation of stable bulk/surface oxides and heats of formation for various alumina-

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Table 3 Heats of O2 adsorption on alumina-supported platinum metals measured at 300 K Sample

10.1% 4.72% 2.04% 2.80% 1.07% 1.08% 0.55% 4.13% 4.13% 4.13% 4.13% 2.12% 0.82% 8.25% 8.25% 4.00% 3.26% 3.56% 2.45% 2.86% a

d (nm)

Pd Pd Pd Pd Pd Pd Pd Pt Pt Pt Pt Pt Pt Rh Rh Rh Rh Rh Rh Rh

Oxygen adsorption

7.9 5.2 4.4 3.7 2.1 1.4 1.2 2.2 1.8 1.6 1.3 1.0 1.0 4.7 3.6 1.8 1.6 1.4 1.1 1.0

NOad /NM a

1Had (kJ (mol O2 )−1 )

0.10 0.14 0.19 0.21 0.33 0.48 0.35 0.10 0.31 0.38 0.60 1.02 1.01 0.19 0.25 0.50 0.55 0.67 0.80 0.91

253 293 326 330 334 346 359 230 264 270 303 328 329 282 289 293 293 298 299 305

NOad /NM : Uptake of oxygen atoms on alumina-supported platinum metals at 300 K.

Table 4 Formation of stable bulk/surface oxides and heats of formation for various alumina-supported platinum metals at 770 K Sample

Bulk oxide

−1Hf (kJ (mol O2 )−1 )

10.1% 4.72% 2.04% 2.80% 1.07% 1.08% 0.55% 4.13% 4.13% 4.13% 4.13% 2.12% 0.82% 8.25% 8.25% 4.00% 3.26% 3.56% 2.45% 2.86%

PdO PdO PdO PdO PdO PdO and PdO2 PdO and PdO2

188 184 184 188 188 185 173

Pd Pd Pd Pd Pd Pd Pd Pt Pt Pt Pt Pt Pt Rh Rh Rh Rh Rh Rh Rh

RhO2 RhO2 RhO2 RhO2 RhO2 and Rh2 O3 Rh2 O3 Rh2 O3

222 222 228 229 250 270 276

Surface oxide

−1Hf (kJ (mol O2 )−1 )

Pts O Pts O Pts O Pts O and Pts O2 Pts O2 Pts O2

171 169 166 186 189 193

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Fig. 5. Variations of −1Hf with the stoichiometry of platinum metal oxides: PdOx ( ); RhOx (䊉); Pts Ox (4).

supported platinum metals at 770 K. The crystallite size of supported platinum metals has a profound effect on the formation of stable bulk/surface oxides and −1Hf values. Fig. 5 collects the variations of −1Hf with the stoichiometry of platinum metal oxides. For Pd/Al2 O3 , a dioxide (PdO2 ) was formed as d < 1.8 nm, but this became PdO as d > 2.0 nm. The −1Hf was 186 ± 2 kJ (mol O2 )−1 and did not vary significantly with the oxide

species. For Pt/Al2 O3 , a surface dioxide (Pts O2 ) was formed as d < 1.3 nm, but this became Pts O as d > 2.0 nm. Also, the −1Hf varied with species from 190 kJ (mol O2 )−1 to 170 kJ (mol O2 )−1 . For Rh/Al2 O3 , a dioxide (RhO2 ) [−1H f = 225 ± 3 kJ (mol O2 )−1 ] or a sesquioxide (Rh2 O3 ) [−1H f = 273 ± 3 kJ (mol O2 )−1 ] was formed depending on whether the diameter was longer or shorter than 1.5 nm.

Table 5 Distinction of oxidation phenomena on alumina-supported platinum metals Sample/criteria

Pd/Al2 O3

Pt/Al2 O3

Rh/Al2 O3

Oxidation

T and size-dependent (bulk oxides formed at 770 K) Size-dependent (−1H ad = 373 − 14d (nm)) Oxides-independent (−1H f = 186 ± 2) Size-dependent (PdO2 (d < 1.8 nm); PdO (d > 2.0 nm)) Difficult T > 1000 K

T and size-dependent (surface oxides formed at 770 K) Size-dependent (−1H ad = 406 − 81d (nm)) Oxides-dependent (Pts O2 (190); Pts O (170)) Size-dependent (Pts O2 (d < 1.3 nm); Pts O (d > 2.0 nm)) Pts O2 vaporized at T > 800 K T > 800 K

T and size-dependent (bulk oxides formed at 770 K) Size-independent (−1H ad = 294 ± 6) Oxides-dependent (RhO2 (225 ± 3); Rh2 O3 (273 ± 3)) Size-dependent (Rh2 O3 (d < 1.5 nm); RhO2 (d > 1.5 nm)) Difficult T > 1100 K

−1Had (kJ (mol O2 )−1 ) −1Hf (kJ (mol O2 )−1 ) Oxide species Vaporization of oxide MOx decomposition

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4. Conclusions

References

The detailed distinctions of oxidation phenomena on palladium, rhodium and platinum are summarized in Table 5. The following results are found:

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1. Stable bulk oxides formed at T > 700 K for palladium and rhodium. However, stable surface oxides formed at temperatures around 750 K for platinum. 2. The crystallite sizes of supported platinum metals have a profound effect on the formation of stable bulk/surface oxides and −1Hf values. 2.1. A dioxide (PdO2 ) was formed as d < 1.8 nm, but became PdO as d > 2.0 nm. The −1Hf was 186 ± 2 kJ (mol O2 )−1 and did not vary significantly with the oxide species. 2.2. A surface dioxide (Pts O2 ) was formed as d < 1.3 nm, but became Pts O as d > 2.0 nm. Also, the −1Hf varied with species from 190 kJ (mol O2 )−1 to 170 kJ (mol O2 )−1 . 2.3. A dioxide (RhO2 ) [−1H f = 225 ± 3 kJ (mol O2 )−1 ] or a sesquioxide (Rh2 O3 ) [−1H f = 273 ± 3 kJ (mol O2 )−1 ] was formed depending on whether the diameter was longer or shorter than 1.5 nm. 3. The heats of adsorption decrease with an increase in the crystallite size of palladium and platinum and exhibit empirical relations of −1Had (kJ (mol O2 )−1 ) = 373 − 14d (nm), for Pd/Al2 O3 −1Had (kJ (mol O2 )−1 ) = 406 − 81d (nm), for Pt/Al2 O3 −1Had on rhodium surface varies only slightly with the crystallite size and has an average value of 294 ± 6 kJ (mol O2 )−1 .

Acknowledgements The authors acknowledge the financial support of this study by the National Science Council of the Republic of China.