Thermal chemistry of oxide-supported platinum catalysts: A comparative study

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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Thermal Chemistry of Oxide-supported Platinum Catalysts: A" Comparative Study J.F. Lambert, E. Marceau, B. Shelimov, J. Lehman, V. Le Bel de Penguilly, X. Carrier, S. Boujday, H. Pemot and M. Che Laboratoire de R6activit6 de Surface (UMR-CNRS 7609), Universit6 Pierre & Marie Curie 4, Place Jussieu, 75252 Paris Cedex 05, France This communication discusses some of the phenomena occurring during the thermal transformation of oxide-supported platinum catalysts. Based on previously established molecular understanding of the deposition step, the reactivity of several surface platinum species is followed through the later steps of catalyst preparation : drying, calcination (or thermal activation under inert atmosphere) and H 2 reduction. The phenomena observed include Pt complex self-reduction, H spillover to various support sites and formation of strongly held "residual chlorine". Their consequences on metal dispersion and catalytic activity are briefly presented. 1. INTRODUCTION Pt/A1203 and, to a lesser extent, Pt/SiO 2, are among the most widely used heterogeneous catalytic systems [1]. In spite of this, many questions pertaining to the chemistry of their preparation remain unanswered. The interfacial coordination chemistry of Pt complexes during their initial deposition on oxide surfaces from aqueous solution has recently been studied in our laboratory for several closely related systems [2-7], but there remains a gap in our understanding between the deposition step, which implies a relatively well-understood chemistry at the oxide/water interface, and the final catalyst that has been submitted to thermal treatments. The purpose of the present communication is to determine in what way the initial adsorption mechanism and speciation of supported Pt affect its subsequent transformations on thermal treatments of calcination and reduction, until the final catalyst is obtained. Both square planar P t II and octahedral Pt TM complexes have been studied by macroscopic methods (adsorption isotherms, elemental analysis, TGA, TPR followed by MS) as well as by spectroscopic techniques sensitive to local environment (UV-visible, ~95ptNMR, XAS). 2. SPECIATION DURING THE DRYING STEP. On alumina, Pt complexes are often electrostatically adsorbed initially but they can become grafted through a thermally activated ligand substitution. Figure l a illustrates this transition for the hexachloroplatinate/alumina system which has been most studied. Here, the initial adsorption mechanism involves electrostatic adsorption together with "specific" adsorption, probably by hydrogen-bonding. Drying causes the replacement of two chloride ligands by surface hydroxyls of the alumina ; the driving force appears to be the elimination into the gas phase of HC1. More details can be found in ref. [4]. On silica, in contrast, grafting seems to occur on drying (at 90~ but is quickly reversed on exposure to room humidity. The successive transformations implied are illustrated in figure lb, where it is seen that digrafted species [(SiOH)2PtC14] ~ are transformed into free [PtCln(H20)2] ~ upon rehydration. In the same way, monografted [(SiOH)PtC15] are transformed into free [PtC15(H20)], as revealed by 195ptNMR (see below). Thus, the oxidic surface groups of alumina have a higher affinity for Pt than those of silica. However, it was found that previous modification of an alumina surface by specific adsorption of isopolytungstate ions effectively prevented the grafting of [PtC16]2-, even though the fraction of the surface physically occupied by the tungstate was only 10 to 25% [5].

lO44

a

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+H+ Drying

v

A

A

b

B

B

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C

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+2H+ Fig. 1. (a) Change in the adsorption mode of Pt Iv in H2PtC16/AI203." A, after deposition, B, after drying ; (b) change in the adsorption mode of Pt ~v in HzPtC16/SiO2.: A, after deposition, B,. after drying, C, after rehydration (rewetting). It seems therefore possible to modulate the interaction between the support oxide surface and the transition metal precursor by an appropriate pretreatment. As we will see, this may provide a way to control the sequence of events occurring during subsequent thermal treatments. 3. SPECIATION DURING THE CALCINATION STEP; SELF-REDUCTION OF Pt =v Calcination under a neutral atmosphere, and even under 0 2 at moderate temperatures (under 500~ sometimes has the unexpected result of causing a partial reduction of Pt species. The reducing agent is necessarily constituted by the Pt ligands, and consequently the products of this redox reaction should be detected in the gas phase. In the case of silicasupported chloroplatinates, two successive steps may even be identified, as shown by the DTG trace of Figure 2. To ascertain the nature of thermal events 1 and 2 in DTG traces, the thermal treatment was interrupted at various points (labelled A, B and C in figure 2) and the nature of the existing phases was checked by XRD. At point A, only the diffractogram of the alumina support was apparent, meaning that Pt species were still molecularly dispersed. At point B, sharp diffraction lines were observed at positions corresponding to PtC12. At point C, these had disappeared but narrow peaks of metallic platinum were observed. This allows to interprete events 1 and 2 in DTG as corresponding to the following reactions, respectively: IV II HzPt C16 --->Pt C12 + 2 HC1 + C12 (1) ptncl2 --->pt ~ + C12 (2) The occurrence of these reactions is strongly detrimental to Pt dispersion as the volatile PtC12 intermediate forms very large crystallites, whose large size is maintained upon further transformation to Pt~

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Fig. 2. DTG (first derivative of thermogravimetric curve) of a H2PtC16/SiO2 sample heated under N 2 at 10~ Labels A, B and C indicate points where TG analysis was interrupted to check the X-Ray Diffractogram. For alumina-supported chloroplatinates, on the other hand, self-reduction is definitely inhibited. We have suggested [4] that this inhibition is due to the nature of chloroplatinate interaction with the support surface. As explained in w1, at moderate temperatures (RT to 90 ~ depending on preparation conditions), the chloroplatinate species are irreversibly grafted onto the alumina surface to give [(A1OH)2PtC14] ~ The grafted species is much more stable than its hypothetical analogue on the silica surface and this effectively prevents platinum self-reduction. This model can be checked by using modified systems in which grafting will be inhibited by pretreatment of the alumina surface prior to Pt deposition, such as WOx/A120 3. As we have said earlier, metatungstate and paratungstate ions specifically adsorb on those sites that cause Pt grafting, effectively blocking them 9 Pt then remains as [PtC16]2- and is self-reduced to Pt ~ at temperatures of the order of 500~ very much like on the silica support. Chlorination of the alumina support by treatment with dilute HC1 constitutes another way to inhibit Pt grafting, and it has the same result as polytungstate modification : self-reduction to Pt ~ NH 3 is a more efficient reducing agent than C1-. Samples containing NH4 + as a counterion ((NHn)2[PtC16] precursor) were reduced to Pt~ at least for treatments under neutral atmosphere at T_<400~ as observed on samples where ammonia was introduced as a ligand ([Pt(NH3)4] 2+ precursor). The mechanism of self-reduction by ammonia is still open to discussion. For instance, in the case of [Pt(NH3)4] 2§ one could write: 3 [Pt(NH3)4] 2+ ---->3 Pt ~ + 6H + + 10 NH 3 + N 2 (3) However, mass spectroscopy indicates that NH 3 and N 2 are not the only gases released : some NO is also observed, perhaps suggesting participation of adsorbed moisture in the redox reaction. Quite expectedly, treatments under 02 at higher temperatures cause a reoxidation of Pt to PtO and/or PtO 2. 4. EFFECTS OF SPECIATION ON H 2 REDUCTION AND CATALYTIC ACTIVITY

4.1.Selective reduction of platinic species in Pt/SiO 2 Even during this final preparation step, marked effects of the initial speciation during deposition and drying are still manifested. A clear example is provided by the [PtC16]Z-/SiOa system. The evolution of surface platinic species in this system is sketched in Fi~. lb, which shows how digrafted species exposed to humidity regenerate the [PtC14(H20)2] ~ complex. In the same way, monografted species regenerate [PtC15(H20)]-, Both these species, as well as untransformed [PtC16]2-, can be detected and quantified using 195pt NMR, as illustrated in figure 3a. Furthermore, we have demonstrated elsewhere that the distribution of Pt w between these different species (in other

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words, Pt TM speciation)can be controlled by a simple procedure (partial titration of the initial chloroplatinate solution with NaOH - see ref. [6] for details). If the samples obtained in this way are reduced under H 2, each surface species is reduced at a characteristic temperature: there is an obvious correlation between the ~95ptNMR spectra of Fi~. 3a and the TPR traces of Fig. 3b.

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Figure 3: 195Pt NMR spectra (A) and TPR traces (B) of dried and rehydrated [PtC16]2-/SiO2 systems, as a function of the amount of NaOH added to the precursor solution before deposition (expressed as the OH/Pt ratio). It is therefore possible to selectively reduce one ~!ven platinic species, say [PtCla(H20)2] ~ to the metallic state while leaving the others as Pt", as shown by NMR. This selective reduction, occurring at low temperatures, gives great flexibili~ for the control of Pt particle size. At any rate, direct reduction gives rise to particles with 10-25~ average diameters, as compared to over 100A when a calcination step is previously applied with occurrence of selfreduction (see w3). 4.2 Spillover hydrogen in Pt/AI20 3 The interpretation of TPR traces in Pt/A1203 is more difficult than in Pt/SiO 2. Several H 2 consumption peaks are usually apparent, even when it may be thought that only one Pt species is present. An example can be found in Figure 4a, corresponding to the TPR of a 1.5 % [PtC16] / A120~ sample dned at 90 C, and containing mostly or uniquely the dxgrafted [(A1OH)2PtC14]U species. No less than four peaks are apparent (labelled 1 to 4 in Figure 4). Figure 4b shows the TPR of the same sample calcined at 450~ under flowing 0 2 prior to reduction, a treatment which does not modify the nature of platinic species. The intensity of TPR peak 1 is unaffected, but all the others are much weaker than in the uncalcined sample. Since this kind of thermal treatment is expected to decrease the degree of surface hydroxylation, TPR peaks 2-4 are correlated with the presence of A1-OH groups. Most probably, they are in fact due to hydrogen spillover on the alumina surface and not to Pt x reduction, which is complete at 250~ 2-

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It turns out that modification of the alumina surface by polytungstate adsorption has a similar effect to calcination as it strongly suppresses peaks 2-4. Probably, the A1-OH groups with which tungstates are in specific interaction are those that were able to promote H spillover.

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Fig.5. TPR of a 3 % [Pt(NH3)412+/A1203, (a) prepared by incipient wetness impregnation ; (b) prepared by "wet impregnation" with excess solution. Both procedures included a 24h ageing time before drying. Similar TPR traces are observable when other Pt precursors are used, as for instance in [Pt(NH3)a]E+/AI203 (Fig. 5). However, peaks 3 and 4 are only observed if the preparation procedure involves prolonged contact between the support and a basic, bulk aqueous phase (compare traces a and b). Characterization of the support in both cases revealed that such an ageing in the presence of the solution resulted in extensive restructuring, including formation of AI(OH)3 particles. Consequently, it is tempting to assign TPR peaks 3 and 4 to inter-particular spillover (possibly to neoformed AI(OH) 3) while peak 2, which is always present, would correspond to a spillover to less remote sites (possibly at the perimeter of Pt ~ particles). 4.3. Residual chlorine in Pt/AI20 a The fate of chlorine during thermal treatments of ptX/oxide catalysts is another open question. We have mentioned that Pt reduction was complete at 250~ and a naive view would be to consider that all of the chlorine is eliminated as HC1 at this step, along reactions such as:

[(A1OH)EPtC14] 0 + 2 H E --) Pt 0 + 4HC1 + 2 (A1OH) (4) In fact, significant amounts of residual chlorine are still present in the system after reduction at 450~ This was found to have an important influence on the properties of the reduced catalysts: for instance, reduced Pt/A1203 catalysts prepared by impregnation of HEPtC16, calcination in He

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and reduction, exhibit lower activity for total oxidation of methane than catalysts prepared from Pt(NH3)4(OH)2, despite identical particle size (15 A) [6]. The limited H 2 and O 2 adsorption and low oxidative activity of the ex-H2PtC16 catalysts were clearly related to the presence of residual chlorine (about 0.4 wt%). Therefore, additional experiments were carried out on chlorinated catalysts; some of the results are summarized in Table 1. Table 1. Evolution of the conversion at 450~ and activation energy with the chlorine content and mode of introduction on Pt/A1203 catalysts for total oxidation of methane (Pt = 20 I.tmol ; He = 95 mol.L -t, C H 4 = 1 mol.L -~, O 2 = 4 mol.L ~, T=450~ Precursor salt Pt(NH3)4(OH) 2 Pt(NH3)4(OH)2 Pt(NH3)a(OH)2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH)E

Additional treatment reference ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1

Atomic CI/Pt 0 0.10 0.25 0.35 0.80 1.40 2.70

Conversion 0.77 0.60 0.45 0.36 0.23 0.18 0.14

Ea (kJ.mol-') 64 70 74 77 85 103 93

HEPtC16 HEPtC16 H2PtC1 ~

reference ref. + dechlorination ref. + NH4C1

1.60 0.50 2.80

0.21 0.19 0.22

110 103 113

.......

The inhibiting role of chlorine toward methane combustion was evidenced by the decreasing catalytic activity observed after impregnation of reduced ex-Pt(NH3)4(OH) 2 catalysts by NH4C1 prior to reaction [7]. However, impregnation by NH4C1 or dechlorination in boiling water of eX-HEPtC16 samples did not alter significantly their activity, though their chlorine content was modified. It has been supposed that the catalytic properties of reduced ex-HaPtC16 catalysts are ruled mainly by a strongly held fraction of residual chlorine species in the vicinity of the platinum particles, able to mask the possible influence of other chlorine atoms fixed on the alumina at a longer distance. 5. CONCLUSION This presentation only gives a brief overview of the complexity of Pt/oxide catalytic systems (some of which is more fully treated in the quoted references). However, we hope that the few examples treated here may convince the reader that 1) phenomena occuring in the "black box" between the Pt deposition and the obtention of the final catalyst may indeed be understood at a molecular level, and 2) they are strongly directed by the initial speciation of the platinum precursors and the mechanism of their adsorption on the support surface, which may therefore provide ways of controlling the final catalyst properties.

REFERENCES 1) J. P. Boitiaux, J. M. Dev~s, B. Didillon and C. R. Marcilly, in Catalytic Naphta Reforming: Science and Technology, G. J. Antos, A. M. Aitani, J. M. Parera (Eds.), Marcel Dekker, New-York, 1994, pp. 79-111. 2) B. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Am. Chem. Soc., 121 (1999) 545. 3) B. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Catal. (1999) in the press. 4) B. N. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Mol. Catal. (1999) in the press. 5) V. Le Bel de Penguilly, Ph.D. Thesis, Universit~ Pierre et Marie Curie, Paris, 1998. 6) B. Shelimov, J. Lehman, J. F. Lambert, M. Che and B. Didillon, Bull. Soc. Chim. Fr. 133 (1996), 617. 7) E. Marceau, J. M. Tatibou~t, M. Che and J. Saint-Just, Catal. Today 29 (1996) 415. 8) E. Marceau, Ph.D. Thesis, Universit6 Pierre et Marie Curie, Paris, 1997.