Mg(Al)O Catalysts for Light Alkane

14732

J. Phys. Chem. C 2007, 111, 14732-14742

Characterization of Pt,Sn/Mg(Al)O Catalysts for Light Alkane Dehydrogenation by FT-IR Spectroscopy and Catalytic Measurements Anastasia Virnovskaia,† Sara Morandi,‡ Erling Rytter,§ Giovanna Ghiotti,‡ and Unni Olsbye*,† Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway, Department of Inorganic, Physical and Material Chemistry, and NIS Center of Excellence, UniVersity of Turin, Via P. Giuria 7, 10125 Torino, Italy, and Statoil Research Centre, N-7005 Trondheim, Norway ReceiVed: June 17, 2007; In Final Form: July 12, 2007

The Pt function in Pt,Sn/Mg(Al)O and in Pt/Mg(Al)O catalysts has been studied by a combination of catalytic testing for ethane dehydrogenation at 450-650 °C under a C2H6/H2/CO2/N2/Ar ) 10:1.6:6.8:5.9:75.7 flow and Fourier transform infrared spectroscopy (FT-IR), using CO as a probe molecule. The acid-base properties of the support were also investigated by FT-IR, using CH3CN as a probe molecule. CO adsorption experiments revealed the presence of Pt terraces, as well as Pt sites with low coordination number (steps, edges, corners, or defects) on Pt/Mg(Al)O. The same experiments on Pt,Sn/Mg(Al)O revealed that Sn covers the steps, corners, edges, and defects of the Pt particles, thus developing simultaneously a geometric and a chemical effect on the surface properties of the exposed Pt atoms. Accordingly, the ethane dehydrogenation reaction proceeds with a lower activation energy over Pt,Sn/Mg(Al)O compared to Pt/Mg(Al)O. Further, Sn addition leads to more selective and more stable ethane dehydrogenation catalysts. The higher dehydrogenation selectivity of Pt,Sn catalysts was correlated to the masking of low-coordinated Pt sites. The Pt/Mg(Al)O and Pt,Sn/Mg(Al)O catalysts were subjected to an activation procedure consisting of several test-regeneration cycles. Good correlation was found between the number of accessible Pt sites and the catalytic activity after each cycle.

1. Introduction Detailed knowledge about the correlation between a catalyst’s surface properties and its activity and selectivity for a given reaction is a prerequisite for rational catalyst design. In this study, our aim was to elucidate the reason for the extraordinary high selectivity and stability of Pt,Sn/Mg(Al)O as a light alkane dehydrogenation catalyst. It was found that SnOx covers the low-coordinated Pt sites which are active for ethane reforming to synthesis gas, while the terrace sites which are active for ethane dehydrogenation are still accessible. It was further found that basic sites on the support further promote the dehydrogenation selectivity of the Pt,Sn/Mg(Al)O catalyst. Catalytic dehydrogenation of light alkanes is considered a potentially important route to selective production of high-purity alkenes, which are important base chemicals for the chemical industry. Two classes of catalysts are used in commercial dehydrogenation processes, supported platinum-tin catalysts and chromium oxide supported on alumina.1 Besides activity and product selectivity, heat addition is a main issue in catalytic dehydrogenation. The alkane dehydrogenation reaction is strongly endothermic and limited by thermodynamic equilibrium. The equilibrium conversion is shifted toward the product side at increasing temperatures.1 In Cr-based processes, heat is supplied by burning off coke deposits from the catalyst in either a fixed bed reactor with alternate feed (Catofin) or in a continuously regenerated fluid bed reactor * Corresponding author. Tel.: +4722855456. Fax: +4722855441. E-mail: [email protected]. † University of Oslo. ‡ University of Turin. § Statoil Research Centre.

(FBD-4). In Pt-based processes, heat is supplied by external heating of either a tubular fixed bed (STAR) or an adiabatic moving-bed reactor (Oleflex).1 Direct heating by combustion of parts of the alkene or hydrogen formed in the process inside the reactor would not only improve the heat balance of the process but even push the equilibrium conversion toward the product side. Cr-based catalysts are not suited for such direct heating methods, due to their intolerance toward water.2 Pt-based catalysts, on the other hand, have shown promise as combined dehydrogenationoxidation catalysts.3,4 Pt-based catalysts are also fairly stable even at temperatures g600 °C, which are necessary to overcome the unfavorable Gibbs free energy of the ethane dehydrogenation reaction.5 The addition of a promoter metal, such as Sn, Cu, or Au, to Pt-based catalysts has been shown to affect the selectivity and lifetime of the catalysts. However, while Sn addition has been reported to promote dehydrogenation reactions (vide infra), Cu or Au addition have been shown to have the opposite effect, i.e., to promote the selectivity toward C-C bond rupture relative to C-H bond rupture.6-9 Platinum, usually promoted with tin, is used as a dehydrogenation catalyst in several industrial processes.10,11 The promotion of Pt by Sn has been reported to have several advantages: it improves catalytic activity and dehydrogenation selectivity, prevents Pt sintering, and decreases deactivation due to coke formation (see, e.g., refs 5 and 12-19 and references therein). When the effect of Sn on Pt in the Pt-Sn bimetallic catalytic system is considered, a geometric and/or an electronic effect has been suggested. The geometric effect, i.e., a dilution of Pt by Sn, reduces the number of adjacent Pt atoms, thus decreasing the number of Pt atoms in the ensemble forming an active site

10.1021/jp074686u CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007

FT-IR and Catalytic Study of Pt,Sn/Mg(Al)O

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14733

TABLE 1: Mg/Al Ratio, Pt and Sn Contents, and BET Surface Areas for the Five Samples Studied sample label HT-A Pt-A Sn-A Pt,Sn-A Pt,Sn-B

sample

BET Mg/Al Pt Sn surface area [m2/g] [atomic ratio] [wt %] [wt %]

Mg(Al)O Pt/Mg(Al)O Sn/Mg(Al)O Pt,Sn/Mg(Al)O Pt,Sn/Mg(Al)O proprietary

4.8 4.8 4.8 4.8 4.8

0 1.2 0 2.3 0.5

0 0 5.2 5.4 1.2

178 172 198 186 130

for the formation of coke precursors or for the progress of undesired structure-sensitive reactions.20 The electronic effect, i.e., a change in the electronic properties of platinum by addition of Sn, decreases the strength of the bond between the adsorbed hydrocarbons and the surface metal atoms.21 Support materials can affect the properties of the metallic phase22,23 and modify the selectivity of the supported catalytic system. Nonacidic supports have been reported to result in less cracking and polymerization and enhanced stability in dehydrogenation reactions.15 Thus far, no general conclusion has been drawn concerning the reason for the effect of Sn on supported Pt catalysts. In this paper we study Pt,Sn/Mg(Al)O and Pt/Mg(Al)O catalysts. Their catalytic properties toward the ethane dehydrogenation reaction were investigated during an activation sequence. Pt,Sn/Mg(Al)O catalysts have previously been found to possess superior activity and stability as a propane dehydrogenation catalyst compared to conventional Pt,Sn/Al2O3 catalysts.24 It has further been reported that the activity pattern of a proprietary 0.25 wt % Pt, 0.5 wt % Sn/Mg(Al)O catalyst is quite peculiar, in that the initial activity increases during repeated dehydrogenation-regeneration cycles.25,26 This makes the catalyst system particularly suited for activity-characterization comparisons, with the aim of elucidating the active state of the catalyst. In this study the metallic phase and the acid-base properties of the support were investigated by Fourier transform infrared spectroscopy (FT-IR) of CO and CH3CN adsorption, respectively, at room temperature. Although a number of papers considering IR investigation of Pt and Pt,Sn supported on silica (SiO2) and alumina (Al2O3) exist in the literature,16,27,28 to our knowledge, no IR studies of the Pt-Sn system supported on the mixed oxides derived from hydrotalcites have been published. From previous in situ X-ray photoelectron spectroscopy (XPS) studies on high metal loading Pt,Sn/Mg(Al)O model systems (similar to those used in the present study), we observed that, during activation, part of Sn leaves the Pt,Sn particles and diffuses onto the carrier material.29 Unfortunately, overlap between Pt and Al peaks in Pt,Sn/Mg(Al)O catalysts prevented studies of the Pt metal by in situ XPS. The active state of the Pt metal, in the absence and presence of Sn, is therefore at the focus of the present study. 2. Experimental Methods 2.1. Sample Preparation and Preliminary Characterization. A Mg(Al)O support material, a high-loading Pt,Sn/Mg(Al)O catalyst, and for comparison, a high-loading Pt/Mg(Al)O and a high-loading Sn/Mg(Al)O catalyst were studied in this paper. The study has been extended to a semicommercial lowloading Pt,Sn/Mg(Al)O catalyst. The characterization details of the samples are shown in Table 1. The carrier material Mg(Al)O with a Mg/Al ratio of 4.8 was synthesized via the

corresponding hydrotalcite phase, using a standard coprecipitation method.26,30 Prior to characterization, the obtained Mg9.6Al2(OH)19.2CO3‚xH2O phase was calcined at 600 °C in air for approximately 15 h to yield the final Mg(Al)O phase. Hereafter, the sample will be named sample HT-A. The active metals in the high-loading Pt/Mg(Al)O, Sn/Mg(Al)O, and Pt,Sn/Mg(Al)O were deposited onto the uncalcined hydrotalcite by impregnation from aqueous solution as described in reference 26. Tin chloride (SnCl2‚2H2O, 0.3 g) was dissolved in concentrated HCl (1 mL). Hexachloroplatinic acid (H2PtCl6‚ 6H2O, 0.18 g) was dissolved in distilled water (50 mL). For coimpregnation of Pt and Sn, the two solutions were mixed, yielding a red solution due to Pt-Sn complex formation.26 It is known that SnCl2 and H2PtCl6 metal precursors form various colored anionic complexes in HCl acidic solution, such as the red-colored PtCl2(SnCl3)2-. The exact nature of Pt and Sn precursor reactions that take place in such solutions is, however, still not completely understood (see, e.g., ref 26 and references therein). The uncalcined Mg(Al)O carrier material (4.0 g), prepared as described above, was stirred with the appropriate salt solution for 1 h, then filtered and washed three times with water (approximately 750 mL). The solid material was dried (100 °C) and calcined at 800 °C in air for 5 h. Hereafter, the samples of this preparation will be named sample Pt-A, Sn-A and Pt,Sn-A. The Pt and Sn content of Pt-A, Sn-A, and Pt,Sn-A catalysts were determined by ICP analysis. Although the same preparation procedure was followed for samples Pt-A and Pt,Sn-A, only half the amount of Pt was deposited on the pure Pt catalyst, whereas the same amount of Sn was deposited on samples Pt,Sn-A and Sn-A (cf., Table 1). An increase in Pt deposition rate with Sn addition has been reported previously for the Al2O3 system.31 This might indicate that Sn (present in the Pt-Sn complexes formed in solution) can act as an anchor in Pt deposition on the support. The proprietary Pt,Sn/Mg(Al)O catalyst [Statoil C440-139] will hereafter be named Pt,Sn-B. The Brunauer, Emmett, and Teller (one-point BET) surface areas of the calcined samples were determined by N2 adsorption using a Monosorb instrument from Quantachrome. 2.2. Catalytic Testing. Ethane dehydrogenation tests were performed in a tubular fixed bed quartz reactor with an i.d. of 7 or 16 mm heated by a tubular furnace. The temperature in the catalyst bed was measured by a thermocouple contained in a quartz thermocouple well (o.d. 3 mm) which was centered axially inside the reactor. Prior to catalytic testing the catalyst was pressed (5 tons) to tablets, then crushed and sieved. Catalyst and quartz particles in the range of 0.125-0.25 mm were used in the tests. The in-house prepared Pt and Pt,Sn catalysts (PtA, Pt,Sn-A; 0.6 g, diluted with 2.4 g quartz particles) were pretreated at 450 or 600 °C and tested in the temperature range of 450-650 °C, all at atmospheric total pressure. The proprietary catalyst (Pt,Sn-B; 0.05 g, diluted with 0.2 g quartz particles) was pretreated at 600 °C and tested in the temperature range of 550-650 °C, all at atmospheric total pressure. The activation procedure consisted of cycles of 30 min of reduction (3 mL/ min H2), 2 h of ethane dehydrogenation (10.0% C2H6, 1.6% H2, 6.8% CO2, 5.9% N2, and 75.7% Ar, totally 126 mL/min) and 4 h of regeneration by O2 diluted with inert. The oxygen content was initially 4 mol % and was increased in four steps up to 24 mol %. During dehydrogenation, the reactor effluent was analyzed in an HP Quad Micro-GC with 10 m Molsieve-5 Å, PoraPlot-U and AluminaPlot columns, and thermal conductivity detectors. Ethane and CO2 conversions were calculated from the mass balance between feed and effluent concentrations, using N2 as internal standard. For all samples, the selectivity

14734 J. Phys. Chem. C, Vol. 111, No. 40, 2007

Virnovskaia et al.

Figure 1. Difference spectra of CH3CN adsorbed on (a) sample HT-A_R1, (b) sample Pt,Sn-A_R3, and (c) sample Pt,Sn-B_R3: (s) CH3CN adsorption at increasing pressure up to 8.4 mbar for samples HT-A and Pt,Sn-B, and up to 2 mbar for sample Pt,Sn-A, (- - -) sample evacuated at RT after 2 mbar, (‚ ‚ ‚) sample evacuated at RT after 8.4 mbar. Sample HT-A was stored in CH3CN overnight before evacuation for 20 min.

measurements and determination of activation energy (EA) were performed on fully activated catalysts. The selectivity was measured at 600 °C, whereas the activation energy was determined in the temperature range of 550-650 °C. Temperature variations were carried out in a nonlinear sequence, with a new measurement at the base temperature (600 °C) before each temperature change. 2.3. FT-IR Characterization. FT-IR spectra were collected by using a Bruker IFS 28 FT-IR spectrometer equipped with a Hg-Cd-Te cryodetector. A resolution of 2 cm-1 and the wavenumber range of 6000-400 cm-1 was used. All FT-IR measurements were performed at room temperature (RT) on self-supported pellets contained in a gold sample holder of an IR cell with KBr windows which allowed both heating (up to 700 °C) in situ under vacuum (ca. 1 × 10-4 mbar) or under controlled atmospheres. All gases used in the IR study were from Matheson C.P.; CH3CN (liquid, 99%) was obtained from Carlo Erba. For catalysts HT-A, Pt-A, Sn-A, and Pt,Sn-A pellets with weight of ca. 20 mg/cm2 were prepared by crushing and then pressing powders at 1.5 tons. For the Pt,Sn-B catalyst, a pressure of 5 tons was used. Because of the low metal loading of this sample, the density of the pellets used for CO adsorption experiments was doubled (ca. 40 mg/cm2) to obtain sufficient peak intensity. Prior to FT-IR measurements, the samples HT-A, Pt-A, SnA, and Pt,Sn-A were pretreated at 450 °C. The Pt,Sn-B sample was pretreated at 600 °C, as this is the standard pretreatment temperature used in catalytic tests of this catalyst.25,26 The pretreatment procedure was as follows. First step: The catalyst was heated in the cell to 450 or 600 °C in vacuum, outgassed for 30 min, and reduced in approximately 45 torr H2 for 30 min at the same temperature. After reduction, the sample was outgassed at 450 or 600 °C, cooled in vacuum to RT, moved to KBr windows, and spectra were collected at RT before and after admission of increasing doses of the appropriate gas. A sample submitted to this treatment will be named sample_R1. After the FT-IR analysis the samples were submitted to a second step: heating to 450 or 600 °C in vacuum, outgassing for 15-20 min; the reaction mixture (approximately 45 torr) consisting of H2/CO2/C2H6 ) 1:4:6 was then let in, and the sample left under reaction conditions for 1 h. The sample was subsequently outgassed for 15-20 min and oxidized in approximately 45 torr O2/N2 ) 1:3 for 1 h. Then the sample was resubmitted to reduction, outgassing, cooling to RT (as in the

first step), and FT-IR measurements were repeated. A sample submitted to this treatment will hereafter be named sample_R2. In some cases, the reaction-oxidation-reduction was repeated (sample_R3). All pretreatment and FT-IR measurements were performed in the same sample cell, without exposing the sample to air. Unless otherwise is indicated, the following conditions were used during IR measurements with CH3CN as a probe molecule: FT-IR spectra were recorded at increasing CH3CN pressure up to an equilibrium pressure of 8.4 mbar. At the maximum pressure the sample was let to react for 30 min, then spectra were collected before and after evacuation for 20 min at RT. When CO was used as a probe molecule, FT-IR spectra were recorded at increasing equilibrium pressure up to 20 mbar. The sample was subsequently evacuated to the base pressure at RT, and a spectrum was recorded. In some cases, the sample was further evacuated at higher temperatures (from RT up to 250 °C, 5 min at each temperature), and after each evacuation a spectrum was taken at RT. FT-IR data are reported as difference spectra obtained by subtracting the spectrum of the sample before the admission of the adsorbate from the spectra run in presence of the adsorbate. 3. Results and Discussion 3.1. Study of the Acid-Base Properties of the Support by Adsorption of Acetonitrile. 3.1.1. Sample HT-A. Figure 1a shows the spectra of adsorbed acetonitrile on the pure support at increasing pressure (sample HT-A_R1). The triplet of peaks on the high-frequency side of the spectra, at 2311, 2281, and 2254 cm-1, can be assigned as follows. The two first peaks are due to ν(CN) fundamental mode of CH3CN N-bonded to Al3+ ions, split by the coupling with ν(CC) + δsym(CH3) combination. As known, the blue shift of this doublet with respect to liquid acetonitrile (2292, 2254 cm-1) is due to the electron-withdrawal effect of the cationic site and can be taken as a measure of the Lewis acidity.32,33 The latter band at 2254 cm-1, emerging at higher coverage, is associated to ν(CN) fundamental mode of both H-bonded and liquid-like acetonitrile (the higher frequency component of the pair, expected at about 2292 cm-1, overlaps with the ν(CN) modes of acetonitrile coordinated to Al3+). Physisorbed acetonitrile (both H-bonded and liquid-like) desorbs during evacuation at RT. In the lower wavenumber region (2220-2050 cm-1), two weak bands at 2090 and 2114 cm-1 appear at very low pressure,

FT-IR and Catalytic Study of Pt,Sn/Mg(Al)O then disappear along with the increase of three strongly superimposed bands at 2135, 2154-2174, and 2205 cm-1. The three bands are almost unaffected by evacuation at RT. On the basis of this observation the weak bands at 2090 and 2114 cm-1 can be assigned to ν(CN) modes of anionic species [CH2CN]-, which are formed on strongly basic O2- sites, and adsorbed thereafter onto the adjacent Mg2+ cations, which may possess sensibly different Lewis acidity depending on the surroundings.34 These species disappear at higher coverage because they are solvated by CH3CN molecules leading to the formation of [CH3C(NH)CHCN]- dimeric anions or, for further CH3CN addition, of anions with increasing length (polymeric species), responsible for the three bands at higher wavenumbers.34 The precise assignment of the three peaks is not straightforward; they can be assigned to dimeric anions coordinated to Mg2+ ions with sensibly different Lewis acidity and/or to polymeric anions of different length.34-36 As already reported by some of us,37 we suggest to take as a criterion for the comparison of the acid-base properties of the samples the ratio between the integrated intensity of the ν(CN) modes of the species N-coordinated to the cationic acid sites (IA; the 2350-2220 cm-1 region) and the integrated intensity of the ν(CN) modes of the anionic species related to the presence of strong basic O2- sites (IB; the 2220-2050 cm-1 region). To overcome the interference between ν(CN) modes related to the species N-coordinated to the cations and those related to the physisorbed species, the integrated intensities were measured not at the maximum coverage but after evacuation at RT for a fixed time (20 min). The IB/IA value obtained for HT-A_R1 in repeated measurements is in the range of 2.0-2.6. 3.1.2. Samples Pt-A and Pt,Sn-A. IR measurements of adsorbed acetonitrile on samples prepared in the laboratory, Pt-A and Pt,Sn-A, revealed the presence of very low amounts of basic sites able to deprotonate acetonitrile. Only the triplet attributed to CH3CN adsorbed on Lewis acid sites and physisorbed CH3CN could be observed in the CtN stretching region. For both Pt-A and Pt,Sn-A, the ν(CN) modes of CH3CN chemisorbed on the acid sites (at 2314 and 2285 cm-1) were only slightly upward shifted in comparison to the modes observed for the support (2311 and 2281 cm-1), indicating a slight increase in the Lewis acidity. For the sake of brevity, only the spectra for Pt,Sn-A_R3 sample are reported in Figure 1b, as an example. The acetonitrile adsorption experiments were performed on samples Pt-A_R1 and Pt,Sn-A_R3 and showed similar results. This observation indicates that the activation treatment does not cause changes in the spectra of adsorbed CH3CN. The almost complete absence of the strong basic sites on Pt-A and Pt,Sn-A could be related to the presence of chlorine on the support surface, because the supported metals were obtained by impregnating the HT material with their chloride salts and the solution was acidified with HCl. It has been observed previously that even a small amount of Cl- present in a calcined Pd/Mg(Al)O catalyst (less than 1 wt %) is able to completely eliminate the detection of strong basic sites by CH3CN on the Mg(Al)O support.35,38 It was also reported that repeated washing and calcination at 450 °C did not remove Cl- completely from the Mg(Al)O structure.35 Another possible explanation is masking of the most basic or defect sites by the Pt clusters, as previously reported for Mg(Al)O support materials.39 3.1.3. Sample Pt,Sn-B. Contrary to the samples prepared in the laboratory, the spectra of acetonitrile adsorbed after evacuation at RT on the Pt,Sn-B catalyst reveal the presence of basic sites able to give anionic species. Only minor differences between the IR spectra of CH3CN adsorbed after each activation

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14735 step and, in particular, in the IB and IA integrated intensities (evaluated as described previously for HT-A) were observed. The IB/IA ratio, was 3.0 ( 0.3 after each step. For the sake of brevity, only the spectra for sample Pt,Sn-B_R3 are reported in Figure 1c, as an example. 3.1.4. Influence of Preparation and Pretreatment on the Acid-Base Properties of the Support. In a previous XPS study of Pt/Mg(Al)O and Pt,Sn/Mg(Al)O model compounds, with similar Pt and Sn loadings to samples Pt-A and Pt,Sn-A,29 indications were found that during activation, part of the Sn leaves the Pt-Sn metal particles and diffuses onto the support surface. Due to that observation, we expected Sn to gradually cover the basic sites of the support during activation of the Pt,Sn samples. However, owing to the inefficiency of the CH3CN molecule to probe basic sites on Pt-A and Pt,Sn-A samples, no such change was observed in the present study. It should be noted that samples Pt-A and Pt,Sn-A used in this study were reproductions of the samples used in the previous XPS study and that even the catalytic properties changed from the original to the present Pt,Sn-A sample, in accordance with the characterization results (vide infra). This observation points to the poor reproducibility of preparation of metal/Mg(Al)O catalysts, which has been observed also previously40 but is considered outside the scope of this study. Even for the Pt,Sn-B sample, no indication of coverage of basic sites by Sn is observed in this study, possibly due to the low metal loading in sample Pt,Sn-B (0.5 wt % Pt and 1.2 wt % Sn). Distributed in a monolayer at the surface of the catalyst, tin in either of the forms Sn0, SnO, or SnO2, would occupy below 6% of the total surface area. Therefore, the performed measurements are inconclusive concerning the effect of changes in dispersion of tin on the support, and we are still not in a position to conclude if spreading of tin on the surface takes place during activation. The presence of strongly basic sites on sample Pt,Sn-B, while such sites were not present on samples Pt-A and Pt,Sn-A, could be related to the lower metal loading or to different Pt and Sn precursors and to different procedures used in the preparation, avoiding the introduction of Cl- at the catalyst surface. Unfortunately, we are not in possession of the detailed preparation procedure for sample Pt,Sn-B. 3.2. Study of the Metallic Phase by CO Adsorption. 3.2.1. Sample Sn-A. The CO adsorption experiments performed at pressures up to 20 mbar on sample Sn-A (not reported) showed that CO does not substantially adsorb on the supported Sn at RT. This result is consistent with findings of Riguetto et al.41 for Sn supported on SiO2. 3.2.2. Sample Pt-A. Figure 2a displays the spectra of CO adsorbed at increasing coverage on sample Pt-A_R1. Peaks at 2205, 2184, 2114, 2052-2064, and 1844 cm-1 are present. The peaks at 2205 and 2184 cm-1 are assigned to the ν(CO) band, i.e., stretching modes of CO adsorbed on Al3+ of the support in the tetrahedral and octahedral position, respectively.42 Because Al3+ is observed both in the octahedral and tetrahedral position, we can conclude that some segregation of the mixed oxide into MgO and spinel (MgAl2O4) has occurred, probably during calcination at high temperature.10 The origin of the tetrahedrally coordinated Al3+ is the inverse spinel which is formed during decomposition along with the normal spinel, in which Al3+ is octahedrally coordinated.43 The main peak, showing an upward shift from 2052 to 2064 cm-1 at increasing coverage, and the small one at 1844 cm-1 are assigned to ν(CO) modes of terminal and bridged CO, respectively, adsorbed on reduced Pt. The ratio between their intensities is as expected for this metal, which shows a


Virnovskaia et al.

Figure 3. Determination of the singleton frequency for samples Pt-A (b), PtSn-A (9), and PtSn-B (2). A surface coverage equal to 1 is assumed after CO saturation at 20 mbar equilibrium pressure and subsequent evacuation at RT. The surface coverages were obtained by dividing the integrated area of the envelope related to the CO vibration registered after evacuation at each temperature by the integrated area of the envelope registered after saturation and, to avoid the difficulties due to the gas spectrum, after evacuation at RT.

Figure 2. Difference spectra of CO adsorbed on sample Pt-A_R1: (a) (s) CO adsorption at increasing pressure up to 20 mbar, (‚ ‚ ‚) sample evacuated at RT. (b) Evacuation at increasing temperature: (s ) CO equilibrium pressure of 20 mbar, (‚ ‚ ‚) evacuated at RT, (- -) evacuated at 100 °C, (- ‚ -) evacuated at 200 °C.

preference for bonding of CO in linear configuration. These observations are well in agreement with previous literature reports for supported15,27,44-48 or unsupported44,49 Pt. The peak at approximately 2114 cm-1 is assigned to CO adsorbed on Pt+;44,50 its very low intensity shows that the majority of surface Pt is in the reduced state, in line with our previous XPS measurements on a similar Pt/Mg(Al)O system.29 The spectra of the catalyst subjected to the second activation step (Pt-A_R2) (not reported for the sake of brevity) are practically identical to the spectra of sample Pt-A_R1, indicating that the surface properties as well as the amount of accessible Pt sites in Pt-A sample are unaltered upon activation. When examined in detail, the main band (at 2052-2064 cm-1) clearly shows a shoulder on its higher wavenumber side and a tail on its lower wavenumber side. The composite nature of this peak is better resolved during the desorption experiments at increasing temperature reported in Figure 2b: two strongly superimposed peaks, at approximately 2070 and 2059 cm-1, and a pronounced tail can be distinguished after outgassing at RT. After evacuation at 100 °C, both peaks shift to lower wavenumbers, and the tail transforms to a shoulder at approximately 2025 cm-1. At very low coverage, after evacuation at 200 °C, only a broad band at 2008 cm-1 is present. All linear CO was desorbed from the surface after evacuation at 250 °C. Several reasons are responsible for the spectral differences during adsorption and desorption and for the presence of the three components. As known, the bonding of terminal CO

chemisorbed on the surface of reduced Pt is described by a model where the 5σ electron pair of CO delocalizes into partially filled 5d-6s orbitals of Pt (σ-bond), while electrons from the filled d-orbitals of Pt are back-donated to the 2π* orbitals of CO (π-bond).50 Consequently, the lower the number of Pt-Pt bonds for each Pt surface atom, the higher electron density available for back-donation to the 2π* orbitals of CO and, as a consequence, the more important the decrease in the ν(CO) frequency. This is in agreement with the fact that the singleton frequency of CO adsorbed on a Pt0 single-crystal face depends on the face index.49 This is also in agreement with the fact that CO adsorbed at Pt sites with low coordination, like steps, kinks, corners, and edges, shows lower ν(CO) than CO adsorbed on terraces.49,51,52 A characteristic feature of CO adsorption on Pt faces of both single crystals and supported Pt particles is a shift toward higher wavenumbers of the ν(CO) mode upon increased coverage, caused by dipole-dipole coupling between nearly identical, adjacently adsorbed oscillators (the dipole shift) and to the reduced back-donation into the 2π* antibonding CO orbital as the surface density of the adsorbed molecules increases (the chemical shift).45,53-57 Furthermore, the inherent heterogeneity of the surface of supported particles may also contribute in another way to the shift observed with coverage. Actually, for CO adsorbed on supported Pt particles it is not possible to speak of strictly identical oscillators. CO adsorbed at high-energy sites like step, corner, and edge atoms shows ν(CO) with lower wavenumbers than CO adsorbed at the terrace atoms. During the adsorption experiments the high-energy sites should be occupied first, and during the desorption experiments they should be the last to be left by the CO molecules. This is in a certain way a chemical contribution to upward shift of the ν(CO) peak with increasing coverage. An additional mechanism may further complicate the peak shift behavior. In the case of parallel oscillators with different frequency (even for frequency separation of tens of wavenumbers), it has been demonstrated that the effect of the coupling interactions is a transfer of intensity from the lower frequency to the higher frequency band.49 Thus, for CO adsorption on metal particles with relatively small size in supported Pt catalysts, where a high proportion of CO molecules are adsorbed at the edges, corners, and defects, showing lower wavenumbers than those on the flat facets, at high coverage, when the terrace sites are also occupied,



Figure 4. Difference spectra of CO adsorbed on sample Pt,Sn-A (a) after one reduction (Pt,Sn-A_R1) and (b) after first pretreatment cycle (Pt,Sn-A_R2): (s) CO adsorption at increasing pressure up to 20 mbar, (‚ ‚ ‚) sample evacuated at RT.

the intensity transfer may prevent the detection of the molecules adsorbed at the defect sites. Eventually, it must be taken into account that the IR study of CO adsorbed at a metal phase during adsorption experiments does not always give information corresponding to the more stable situations from a thermodynamic point of view for each coverage. It is known that CO forms islands when it adsorbs on Pt at RT even for very low doses.58 Therefore, the coverage dependence of the ν(CO) is best determined by evacuating the sample at increasing temperature after saturation with CO at RT, to ensure the thermodynamic equilibrium at each coverage. In this way it is also possible to determine the singleton frequency of a family of nearly identical oscillators. Actually, the CO vibration frequency can then be plotted against coverage, and the singleton frequency can be obtained by extrapolation to zero coverage.45 On the basis of this argumentation we assign the two overlapping peaks at about 2064 and the shoulder at about 2070 cm-1 at full coverage to two different CO oscillator families adsorbed on two different terraces. No intensity transfer between the two overlapping peaks can be observed, indicating that the two families of oscillators cannot couple. Accordingly, the simplest explanation of the presence of two overlapping peaks can be found in an interparticle heterogeneity, i.e., being due to the presence of particles with two different sizes. As a matter of fact, Fanson et al.57 observed, for CO adsorbed on silicasupported Pt, ν(CO) peaks with positions (at full coverage) passing from 2058 to 2076 cm-1 with increasing metal loading and decreasing metal dispersion. They ascribed the difference in the peak position to the difference in the Pt particle sizes: the higher the metal dispersion, the smaller will be the particle size and the lower the frequency of the linearly adsorbed CO. The shoulder at 2025 cm-1 appearing and shifting downward during the desorption experiments can be assigned to CO adsorbed at edges, corners, and defects of the Pt particles. At high coverage, when the terrace sites are also occupied, the intensity transfer prevents the detection of this band, and only a tail remains on the lower wavenumber side of the main peak. Furthermore, the absence of this component in the spectra obtained during CO adsorption at RT, even for very low doses, shows that the surface situation is not the one corresponding to thermodynamic equilibrium and that CO islands are already formed at low coverage. Owing to the presence of low-coordination Pt sites, the singleton determination by desorption experiments will lead to the determination of ν(CO) singleton of this type of site.

Actually, if we consider the envelope of the three peaks as a unique band, and report the position of its maximum as a function of the surface coverage, as done in Figure 3, the singleton frequency obtained by extrapolation to zero coverage is very low, ca. 1999 cm-1. The contribution of the dipole and chemical shift is about 65 cm-1, i.e., nearly twice of that generally observed on terraces.57 This can be explained by the fact that at low coverage, mainly CO adsorbed on edge, corner, and defect sites (i.e., CO species showing stretching modes at lower wavenumbers than those of CO adsorbed on the flat facets) is present at thermodynamic equilibrium. Probably already the broad band at 2008 cm-1, still present after desorption at 200 °C, contains mainly contribution from CO adsorbed on edges, corners, and defects rather than on flat facets. 3.2.3. Sample Pt,Sn-A. Spectra of CO adsorbed on sample Pt,Sn-A_R1 are shown in Figure 4a. A main peak, with no visible broadening or shoulder, and slightly shifting with coverage from 2045 to 2050 cm-1 is present, assignable to linearly adsorbed CO on Pt0. Small amounts of bridged CO are revealed by a peak at 1887 cm-1. Two peaks at approximately 2205 and 2183 cm-1 due to CO adsorded on Al3+ ions of the support are also present. Spectra of CO adsorbed on Pt,Sn-A_R2 are very similar to those of Pt,Sn-A_R1 both for the positions and the intensities of the peaks, the only difference is the absence of bridged CO after activation (see Figure 4b). No changes are observed after further activation (Pt,Sn-A_R3, spectra recorded during adsorption are not reported for the sake of brevity). This result indicates that the number and properties of Pt sites is quite stable throughout several activation cycles on Pt,Sn-A, in agreement with catalytic test results (vide infra). The spectra recorded during desorption experiments on sample Pt,Sn-A_R3 are shown in Figure 5. As in the case of sample Pt-A, all linear carbonyls are desorbed from the surface of Pt,Sn-A after evacuation at 250 °C. However, the peak retains its symmetrical shape through the whole desorption sequence and, opposite to sample Pt-A, no modes assignable to defect sites are visible, even at very low coverages. When the peak position of linearly adsorbed CO on Pt,Sn-A is reported as a function of the surface coverage (see Figure 3), the singleton frequency, obtained by extrapolation to zero coverage, is approximately 2038 cm-1. 3.2.4. Influence of Sn on Pt0 Properties. A comparison between all adsorption/desorption experiments performed on Pt-A and Pt,Sn-A samples shows that the spectral features related to CO bonded at Pt0 sites are clearly different in the two cases. The main differences are observed for the ν(CO)


Figure 5. Difference spectra of CO adsorbed on sample Pt,Sn-A_R3, evacuating at increasing temperature: (s) CO equilibrium pressure of 20 mbar, (‚ ‚ ‚) evacuated at RT, (- - -) evacuated at 100 °C, (- ‚ -) evacuated at 200 °C.

mode related to linearly adsorbed CO, in particular: (i) after evacuation at RT, the integrated intensity of this band for Pt,Sn-A sample is significantly higher than for Pt-A sample (2.7 for Pt-A vs 4.0 for Pt,Sn-A); (ii) unlike Pt-A catalyst, for Pt,Sn-A this band is narrow, without shoulders or tails so that it can be recognized as a single peak; (iii) at full coverage the position of this band for Pt,Sn-A is shifted 14 or 20 cm-1 toward lower wavenumbers compared to that for the Pt-A catalyst (2050 vs 2064 or 2070 cm-1); (iv) during adsorption experiments the upward shift on increasing coverage is about 12 cm-1 for Pt-A and only 6 cm-1 for Pt,Sn-A; (v) during desorption experiments for Pt,Sn-A the peak retains its symmetrical shape through the whole desorption sequence and, opposite to sample Pt-A, no components assignable to defect sites have sufficient intensity to be visible, even at very low coverages; (vi) the overall shift observed during desorption is about 65 cm-1 for Pt-A catalyst and only 12 cm-1 for Pt,Sn-A. Observation i is in agreement with the lower amount of Pt present in the Pt-A than in the Pt,Sn-A catalyst. When the integrated intensity of the ν(CO) mode of CO linearly adsorbed on sample Pt,Sn-A is adjusted to reflect the same wt % Pt as on sample Pt-A, similar values are obtained (2.7 for Pt-A and 2.0 for Pt,Sn-A). This result indicates that the amount of exposed Pt per wt % Pt is similar on Pt-A and Pt,Sn-A. Observations ii-vi indicate that Sn markedly influences the surface properties of Pt. As mentioned in the Introduction, Sn may influence the properties of Pt in two ways. First, Sn addition to Pt may reduce the number of adjacent Pt atoms, thus increasing the distance between adsorbed CO. This would reduce the CO-CO dipole coupling with an effect of lowering of the CO stretching frequency compared to unpromoted Pt (geometric effect). Second, Sn, which has more outer-shell electrons than Pt, can assist the back-donation of d-electrons into the 2π* CO orbital, thus lowering the vibration frequency of CO adsorbed on Pt-Sn particles compared to pure Pt (electronic effect). Actually, at high coverage we observe that the position of the ν(CO) peak for sample Pt,Sn-A is 14 or 20 cm-1 lower than for Pt-A sample. But how much do the geometric effect and the electronic effect contribute to this shift? The presence of an electronic effect should be revealed by a lower singleton ν(CO) frequency for Pt,Sn-A than for Pt-A. It is not so. In fact, the singleton frequency of linearly adsorbed CO on Pt-A catalyst is much lower than for catalyst Pt,Sn-A. Furthermore, even though the temperature for full desorption is the same for the two samples (i.e., no peaks due to linearly adsorbed CO are visible after outgassing at 250 °C in either of the catalysts),

Virnovskaia et al. after outgassing at 200 °C the amount of CO, as measured by the integrated intensity of the residual peak present on the Pt,Sn-A surface (1.5% of that at full coverage), is an order of magnitude lower than in the case of Pt-A (17% of that at full coverage). Taking into account that, as discussed previously, at low coverage the main peak of Pt-A can be attributed to CO mainly adsorbed on edges, corners and defects, the features of the peak related to linearly bonded CO on Pt for Pt,Sn-A catalyst point toward the presence of more homogeneous Pt sites when Pt is diluted with Sn, indicating that it completely covers steps, edges, corners, and defects with the effect of smoothing the particles. In other words, the uncovered Pt sites should appertain mainly to terraces. Actually, linearly bonded CO on Pt atoms seems almost completely desorbed at 200 °C from Pt,Sn-A, whereas it is still present in perceptible amounts on Pt-A catalyst, on which higher energetic sites are present. The observations presented above prove that Sn has certainly a geometric effect, although they are not able to prove the presence an electronic effect, because it is hard to compare the two singletons. However, masking of Pt at edge, corner, and defect sites has certainly a chemical effect on the properties of Pt on the promoted catalyst because Sn covers the more energetic sites for CO adsorption. Minor differences observed between the FT-IR spectra of CO adsorbed on the promoted and unpromoted catalysts are as follows: (i) bridged CO present on Pt,Sn-A_R1 is absent when the sample is activated further, whereas for Pt-A, bridged CO is observed both before and after activation and (ii) in Pt-A, a small amount of Pt+ is observed, whereas no Pt+ is observed in Pt,Sn-A. These observations suggest that during activation, tin covers the small fraction of sites able to give bridged CO and that Pt is more easily reduced in the presence of tin. 3.2.5. Sample Pt,Sn-B (the Semicommercial Catalyst). Due to different pretreatment temperature, different preparation procedure, and different metal loading, the Pt,Sn-B sample is discussed separately. Spectra of CO adsorbed on Pt,Sn-B_R1, Pt,Sn-B_R2, and Pt,Sn-B_R3 pretreated at 600 °C are shown in Figure 6a-c, respectively. A number of interesting features can be observed on this sample after the different phases of the activation pretreatment: (i) on Pt,Sn-B_R1, the two ν(CO) modes of linearly adsorbed CO on Al3+ ions of the support (2200-2206 cm-1) and on Pt0 (2046 cm-1) show similar intensity at full coverage; (ii) at full coverage, passing from Pt,Sn-B_R1 to Pt,Sn-B_R2, the ν(CO) mode of CO linearly adsorbed on Pt0 markedly increases in intensity (by a factor of 7) and shifts about 10 cm-1 toward higher wavenumbers (2057 cm-1). Passing from Pt,Sn-B_R2 to Pt,Sn-B_R3 it increases in intensity by another 16% without a significant position shift. For one more cycle (not reported) no significant changes are observed; (iii) concerning the shift with CO coverage during adsorption, the frequency shift is low for Pt,Sn-B_R1 (4 cm-1), whereas it is more significant for Pt,Sn-B_R2 and after subsequent treatments (10-11 cm-1). The observations above show that during the activation treatments the surface properties of the metal phase change markedly for the Pt,Sn-B sample, differently from what is observed for the Pt,Sn-A sample. The marked increase in intensity of the ν(CO) mode accompanied by the frequency shift during the first activation cycle and the intensity increase during the second activation cycle can be explained by a marked increase in the amount of exposed Pt during activation. This observation is consistent with a model where part of the Sn diffuses away from the Pt,Sn particles during activation, as



Figure 6. Difference spectra of CO adsorbed on sample Pt,Sn-B (a) after one reduction (Pt,Sn-B_R1), (b) after the first pretreatment cycle (Pt,Sn-B_R2), (c) after the second pretreatment cycle (Pt,Sn-B_R3): (s) CO adsorption at increasing pressure up to 20 mbar, (‚ ‚ ‚) sample evacuated at RT.

Figure 7. Difference spectra of CO adsorbed on sample Pt,Sn-B_R3, evacuating at increasing temperature: (s) CO equilibrium pressure of 20 mbar, (‚ ‚ ‚) evacuated at RT, (- - -) evacuated at 115 °C, (- ‚ -) evacuated at 200 °C.

observed previously by XPS29 and transmission electron microscopy (TEM),5,12 although no such diffusion could be revealed by acetonitrile adsorption experiments in the present study. Another possible explanation could be that platinum and tin are covered by the support in the fresh catalyst and that during the pretreatment (predominantly during oxidation) Pt and Sn diffuse to the surface. An increase in the exposed Pt during repeated oxidation-reduction cycles has been observed previously for Pt/Mg(Al)O catalysts obtained by calcination of Ptcontaining Mg/Al LDHs (layered double hydroxide) prepared by coprecipitation of Mg, Al, and Pt precursors.37 Finally, the increase could be due to removal of remains of the impregnation salts, possibly carbonaceous compounds, on the surface of the fresh catalyst, during activation. Indications of such removal were found from IR measurements of CO adsorbed on Pt,Sn-B which was first submitted to an oxidation and then to a reduction step. After such pretreatment both the peak intensity, the frequencies, and the shift with increasing CO coverage were similar to the intensity, frequencies, and shifts obtained at Pt,Sn-B_R2 and Pt,Sn-B_R3. Results from desorption experiments performed on sample Pt,Sn-B_R3 are shown in Figure 7. All linear carbonyls are desorbed from the surface of Pt,Sn-B after evacuation at 250 °C. As for sample Pt,Sn-A, the peak retains its symmetrical shape through the whole desorption sequence and no modes

assignable to steps, edges, and corners are visible, even at very low coverages. The ν(CO) position of linearly adsorbed CO on Pt,Sn-B is reported as a function of surface coverage in Figure 3, and the singleton frequency, obtained by extrapolation to zero coverage, is 2033 cm-1. A comparison between the spectroscopic results obtained on Pt,Sn-A and on fully activated Pt,Sn-B sample shows that the CO adsorption at full coverage, as well as at increasing and decreasing coverage, give very similar results (compare ν(CO) positions at full coverage, the shifts during the adsorption and the singleton values). These results indicate that Pt surface properties observed for sample Pt,Sn-A are very similar to the fully activated Pt,Sn-B sample. However, a comparison between the integrated intensities of the ν(CO) modes normalized to the same Pt content reveals that the intensity value for the fully activated Pt,Sn-B sample is approximately 5 times higher than for the Pt,Sn-A sample. This result indicates that the metal dispersion is much higher on the semicommercial sample, as reasonably expected, due to the lower Pt loading. Furthermore, a minor difference can be observed between the spectra of Pt,Sn-A and activated Pt,Sn-B sample: a small shoulder at 1985 cm-1 appears in the CO spectra of Pt,Sn-B (best visible during desorption experiments in Figure 7) and not in the spectra of Pt,Sn-A. This absorption can be assigned to CO linearly bonded on Pt atoms that interact with the basic sites of the support.37 This observation is in agreement with the spectroscopic results obtained by acetonitrile adsorption: Pt,Sn-B shows the presence of species related to the presence of O2- basic sites (see Figure 1); on the contrary for Pt,Sn-A O2surface basic sites are almost absent. Finally, the stretching frequency of CO adsorbed on Al3+ ions of the support (22002206 cm-1) indicates that only tetrahedrally coordinated Al3+ is present in sample Pt,Sn-B, irrespective of pretreatment. This observation indicates that the Mg(Al)O support is not segregated in this sample. 3.3. Catalytic Tests. The ethane dehydrogenation activity of a reduced Sn/Mg(Al)O catalyst (Sn-A, 0.2 g, undiluted) was tested at 600 °C. The total feed flow rate was 31 mL/min, and the gas composition was as described in the Experimental Section. The ethane conversion was less than 0.4%, only slightly higher than in a blank test (using quartz particles under the same conditions), yielding 0.06% ethane conversion. Ethane dehydrogenation test results obtained over samples Pt-A and Pt,Sn-A during three activation cycles at 450 °C are shown in Figure 8. Ethene is the major product for both


Virnovskaia et al.

Figure 8. Catalytic activity for ethane dehydrogenation during activation of (a) sample Pt-A and (b) sample Pt,Sn-A. R1ssamples after first reduction, R2, R3, R4safter first, second, and third pretreatment cycle, respectively. Dehydrogenation conditions: 450 °C, C2H6/H2/CO2/N2/Ar ) 1:0.2:0.7:0.6:7.5, WHSV ) 1.6 h-1.

Figure 9. Catalytic activity for ethane dehydrogenation of sample Pt,Sn-B during activation sequence, R1safter first reduction, R2safter first pretreatment cycle, R3safter second pretreatment cycle. Dehydrogenation conditions: 600 °C, C2H6/H2/CO2/N2/Ar ) 1:0.2:0.7:0.6: 7.5, WHSV ) 18.5 h-1.

catalysts. Only minor amounts of carbon monoxide produced from ethane and traces of methane and propene (sample Pt-A) were observed. Further, only a minor increase in initial dehydrogenation activity is observed for both catalysts during the activation treatment. Two clear differences in the catalytic behavior of samples Pt-A and Pt,Sn-A can be observed: First, sample Pt-A is significantly more active than sample Pt,Sn-A. Taking into account that the amount of Pt is lower in sample Pt-A than in sample Pt,Sn-A (see Table 1), and that the amount of accessible Pt sites per similar Pt amount is not markedly different for the two samples (as indicated by the intensity of ν(CO) modes), the results in Figure 8 points to the turnover frequency being almost 4 times higher for sample Pt-A than for sample Pt,Sn-A at 450 °C. Second, sample Pt,Sn-A is more stable than sample Pt-A both before and after activation. A similar stability difference was observed at 600 °C (not reported). Ethane dehydrogenation test results for sample Pt,Sn-B at 600 °C are shown in Figure 9. The initial activity increases by a factor of 1.9 during the first activation cycle (passing from R1 to R2), while it increases to 2.4 times the initial R1 activity after the second activation cycle (R3). This result is very different from that observed during activation of the in-house prepared Pt-A and Pt,Sn-A samples in Figure 8. On the other hand, all results are in good agreement with what observed from FT-IR measurements (Figures 4 and 6) during activation,

indicating that the increase in initial activity observed for Pt,Sn-B sample after one or several reaction-regeneration cycles is mainly due to a dramatic increase in the number of accessible Pt sites. Activation energies at 550-650 °C and product selectivities at 600 °C and at 9% ethane conversion are shown in Table 2 for samples Pt-A, Pt,Sn-A, and Pt,Sn-B, all after three activation cycles. It should be noted that product selectivities, referring to the C1 amount in each product, are based exclusively on ethane conversion; the formation of CO from CO2 has been subtracted from the total CO amount, based on mass balances. Holding time variation experiments using sample Pt-A (not reported) indicated that ethene is a primary product, while methane and CO are secondary products from ethane. When product selectivities are compared, it is interesting to note that over Pt-A, CO is the main C-containing product (80% CO selectivity) from ethane at 600 °C, even at only 9% ethane conversion (Table 2). Over Pt,Sn-A ethene is the main C-containing product, and the CO selectivity is 18%, whereas over Pt,Sn-B only traces of CO were observed under the same conditions (Table 2). It is well-known from the literature that the ethane hydrogenolysis reaction, which bears strong resemblances to the ethane reforming reaction to syngas (CO and H2), is strongly surface sensitive, see, e.g., refs 59 and 60 and references therein. As such, the high selectivity to CO over sample Pt-A compared to samples Pt,Sn-A and Pt,Sn-B is well in line with the FT-IR results presented in Figure 3, which clearly shows that while sample Pt-A contains low-coordinated Pt sites, those sites are covered by Sn in samples Pt,Sn-A and Pt,Sn-B. Several groups have previously reported an increase in alkene selectivity and a corresponding decrease in cracking or reforming to syngas when Sn is added to Pt-based catalysts for alkane (ethane, propane, butane) dehydrogenation, see, e.g., refs 14 and 15 and references therein. However, to our knowledge, the clear correlation between product selectivity and masking of lowcoordinated Pt sites in supported Pt catalysts as observed in the present work has not been reported before. Finally, the FT-IR results obtained strongly suggest that the selectivity difference observed between Pt,Sn-A and Pt,Sn-B is related to an influence of strongly basic support sites on the metal function in the Pt,Sn-B catalyst (Figure 7). As for the apparent activation energies for ethane conversion, they increase in the order Pt,Sn-B ≈ Pt,Sn-A < Pt-A (Table 2). It would be expected that the activation energy of reaction is lower on low-coordinated Pt sites, and not higher, as observed in the present case. However, while C-H bond scission has



TABLE 2: Activation Energies and Product Selectivities for Ethane Conversion over Pt and Pt,Sn Samples selectivity [%] at 9% ethane conversiona sample label

sample

EA [kJ/mol] C2H4 CO CH4 C2H2 C3H6 C3H8

Pt-A Pt/Mg(Al)O 135 ( 5 Pt,Sn-A Pt,Sn/Mg(Al)O 117 ( 5 Pt,Sn-B Pt,Sn/Mg(Al)O 116 ( 10 proprietary

43 80 99

55 18 T

2 1 1

0 T T

T 0 0

0 0 0

a Selectivity determined after full activation for all catalysts. Conditions: 600 °C, C2H6/H2/CO2/N2/Ar ) 1:0.2:0.7:0.6:7.5, WHSV ) 1.6 h-1 for samples Pt-A and Pt,Sn-A and 18.5 h-1 for sample Pt,Sn-B. T ) Traces.

been reported as the rate-determining step in alkane dehydrogenation reactions,61 C-C bond scission is reported to be ratedetermining for ethane hydrogenolysis.62,63 A combined kinetic study of H/D exchange and hydrogenolysis of ethane over Pt(111) single-crystal surfaces revealed that the activation energy was 80 and 142 kJ/mol, respectively, for the two reactions.64 Provided that ethane reforming to synthesis gas, observed in the present work, is rate-limited by the same step as ethane hydrogenolysis, the trend in apparent activation energy for the three catalysts is well in line with the selectivities observed for the three catalysts. The faster deactivation of sample Pt-A compared to Pt,Sn-A and Pt,Sn-B further suggests that adsorption of reaction intermediates, subsequently leading to coke formation, is favored on the former catalyst. 4. Conclusions On the basis of the performed measurements the following conclusions can be drawn: (i) A large majority of surface Pt in Pt/Mg(Al)O, and all surface Pt in Pt,Sn/Mg(Al)O, is reduced after each reduction during the activation sequence. (ii) Sn is mainly placed at the steps, corners, edges, and defects of the Pt particles in Pt,Sn/Mg(Al)O, thus developing simultaneously a geometric and a chemical effect on the surface properties of the exposed Pt atoms. Actually, Sn covers the more energetic Pt sites. (iii) A higher selectivity toward ethene, as well as a higher stability, is observed for the Pt,Sn/Mg(Al)O catalysts compared to the Pt/Mg(Al)O catalyst. Since Sn covers the low-coordinated Pt sites, it could be suggested that terrace sites are active for dehydrogenation, whereas the low-coordinated sites are (also) active for C-C bond scission. (iv) During activation, a significant increase in ethane dehydrogenation activity for a semicommercial Pt,Sn/Mg(Al)O catalyst is accompanied by a dramatic increase in exposed Pt sites, observed by IR spectroscopy. (v) Together, the spectroscopic and catalytic results obtained for the three samples indicate that there is good correlation between their spectroscopic and catalytic properties. Acknowledgment. A.V.’s Ph.D. Grant is financed by Statoil through the VISTA programme, contract no. 6446. A.V.’s research stay at the University of Turin was financed through the Marie Curie programme. Aud I. Spjelkavik (SINTEF) is acknowledged for the preparation and BET measurements of catalyst samples used in this study. References and Notes (1) Moulijn, J. A.; Makkee, M.; van Diepen, A. Chemical Process Technology; John Wiley & Sons: Chichester, U.K., 2001.

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Mg(Al)O Catalysts for Light Alkane

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