17260
J. Phys. Chem. 1996, 100, 17260-17265
Microcalorimetric and Reaction Kinetic Studies of Alkali Metals on Pt Powder and Pt/SiO2 and Pt/Sn/SiO2 Catalysts B. E. Spiewak, P. Levin,† R. D. Cortright, and J. A. Dumesic* Department of Chemical Engineering, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706 ReceiVed: May 22, 1996; In Final Form: August 19, 1996X
Microcalorimetric measurements of CO adsorption at 403 K on reduced Pt powder gave an initial heat of 140 kJ/mol and a saturation CO coverage of 30 µmol/g Pt. The addition of metallic Rb and Cs to Pt increased the initial heats of CO adsorption by 20 and 40 kJ/mol and extended the CO adsorption capacity to 110 and 60 µmol/g Pt, respectively. The increase in CO adsorption capacity is caused by formation of an alkali-CO complex at 403 K, with an interaction heat near 120 kJ/mol. Microcalorimetric measurements of CO adsorption at 403 K on Pt/SiO2 (0.85 wt % Pt) and Pt/Sn/SiO2 (2.61 wt % Pt) catalysts showed initial heats of 140 and 130 kJ/mol and saturation CO coverages of 25 and 65 µmol/g, respectively. The addition of K, Rb, and Cs salts (1:5 atomic Pt/alkali) to Pt/SiO2 followed by treatment in H2 at 773 K did not alter the properties of these materials for CO adsorption at 403 K or for reactions with isobutane at 673 K. In contrast, the addition of Na and Cs salts (1:3 atomic Pt/alkali) to Pt/Sn/SiO2 significantly decreased the CO saturation coverage, did not affect the initial heat of CO adsorption, and increased the selectivity for isobutane dehydrogenation. These results indicate that while the alkali species are not present in the metallic state on Pt/Sn/SiO2 catalysts, these alkali species are probably associated with Sn on Pt/Sn particles, thereby inhibiting isomerization, hydrogenolysis, and coking reactions by decreasing the size of surface platinum ensembles.
Introduction Alkali promoters are commonly used in heterogeneous catalysts.1,2 For example, potassium has been used to promote the activity of iron catalysts for ammonia synthesis, to alter the selectivity of iron-based catalysts for Fisher-Tropsch synthesis, and to extend the lifetime of supported nickel catalysts for methanation.1 More recently, Cortright and Dumesic3 have shown that the presence of potassium on silica-supported Pt/ Sn catalysts promotes isobutane dehydrogenation, enhances isobutylene selectivity, and improves resistance of the catalyst to deactivation. Similarly, Imai and Hung4 have shown that the addition of potassium (atomic K:Pt ratio > 10) to aluminasupported Pt/Sn dehydrogenation catalysts increases olefin selectivity by inhibiting hydrogenolysis and isomerization and also improves catalyst stability by reducing coke formation. In a related study, Paa´l and co-workers5 have shown that the addition of KOH to platinum black enhanced the isomerization of n-hexane and suppressed hydrogenolysis activity. Various studies have addressed the role of metallic potassium and other alkali metals (Na, Rb, Cs) on metal surfaces under ultrahigh-vacuum conditions (for reviews see refs 2, 6-8 and references therein). In the particular case of CO adsorption on platinum single-crystal surfaces,9-28 the presence of metallic potassium (or cesium) (1) strengthens the Pt-CO bond, (2) weakens the C-O bond, increasing the probability for CO dissociation, and (3) promotes the adsorption of CO into bridgebonded platinum sites. Supported metal catalysts containing alkali promoters are typically prepared with alkali metal salts, followed by treatment in hydrogen at elevated temperatures (e.g., 723 K). These catalysts may thus contain ionic alkali species. For instance, Cortright and Dumesic3 suggest that potassium hydroxide † Present address: Department of Chemical Engineering and Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01493-1 CCC: $12.00
(KOH) may be the active form of the promoter on Pt/Sn/SiO2 isobutane dehydrogenation catalysts. Similarly, Aika and coworkers29 propose that alkali metal salts are converted to hydroxides at 673 K on Ru/Al2O3 catalysts for ammonia synthesis. In a previous study,30 we developed methods to prepare samples and conduct microcalorimetric studies of reduced nickel powders containing zero-valent alkali metal promoters. In the present study, we employ these methods to study platinum powders containing metallic rubidium and cesium, and also to study alkali-promoted Pt/SiO2 and Pt/Sn/SiO2 catalysts for isobutane dehydrogenation, prepared from alkali metal salts. The adsorption of carbon monoxide at 403 K is used on these samples to probe the promotional effects of the alkali on platinum powders and to address the chemical state of the alkali promoters on the supported platinum-based catalysts. Experimental Section Microcalorimetric measurements were performed at 403 K using a Setaram BT2.15 heat-flux calorimeter. The calorimeter was connected to a gas-handling and volumetric system employing Baratron capacitance manometers for precision pressure measurement ((0.5 × 10-4 Torr). The maximum apparent leak rate of the volumetric system (including the calorimetric cells) was 10-6 Torr/min in a volume of approximately 70 cm3 (i.e. 10-6 µmol/min). The ultimate dynamic vacuum of the system was 10-7 Torr. The calorimetric procedures used in this study have been described in detail elsewhere.30,31 Briefly, each sample was treated ex situ in ultrapure flowing gases, followed by sealing the sample in a Pyrex capsule. The Pyrex capsule containing the sample was then broken in a special set of calorimetric cells31 after the sample had attained thermal equilibrium with the calorimeter. In this manner, it was possible to expose the clean sample to the adsorbate gas without excessive surface contamination that can accumulate during the long times typically required for the calorimetric cells and sample to reach thermal © 1996 American Chemical Society
Reaction Kinetic Studies of Alkali Metals on Pt Powder equilibrium with the calorimeter (ca. 5-6 h). After the capsule had been broken, the microcalorimetric data were collected by sequentially introducing small doses of CO (1-10 µmol) onto the catalyst (0.5-1.0 g) until it became saturated. The resulting heat response for each dose was recorded as a function of time and integrated to determine the energy released (mJ). The amount of gas adsorbed (µmol) was determined volumetrically from the dose and equilibrium pressures and the system volumes and temperatures. The differential heat (kJ/mol), defined as the negative of the enthalpy change of adsorption per mole of gas adsorbed, was then calculated as a function of the adsorbate coverage. Sample treatments for the microcalorimetric experiments were performed in a Pyrex cell, equipped with a Pyrex 5 mm o.d. NMR tube (Wilmad Glass) sealed to the side of the cell (see ref 31). This tube served as both the fill port and a capsule in which to seal the samples. Ultrahigh-purity (99.999%) hydrogen and helium (Liquid Carbonic), used for the sample treatments, were purified by passage over heated (473 K) copper turnings to remove oxygen and activated molecular sieves (13×), cooled to 77 K, to remove water. These gases were further purified by passage over a bed of reduced Oxy-Trap (Alltech Association, Inc.) and a bed of reduced iron catalyst to remove oxygenated impurities. Carbon monoxide, used for the adsorption studies, was purified by passage over heated (473 K) quartz wool to decompose any metal carbonyls present and over activated molecular sieves (13×), held at room temperature, to remove water. Platinum powder (99.9%, Aldrich) for the microcalorimetric studies was calcined in flowing oxygen for 3 h at 623 K, reduced to the metallic state in flowing hydrogen for 1 h at 298 K and for 1 h at 373 K, and then purged with flowing helium for 0.5 h at 373 K and for 2 h at 473 K, to remove adsorbed hydrogen. The reduced platinum powder was then isolated in 350 Torr of helium at room temperature and sealed in the Pyrex NMR tube attached to the side of the treatment cell. Alkali-promoted platinum powders were prepared by first loading the platinum powder into the treatment cell and sealing the Pyrex NMR tube attached to the side of the treatment cell with a capped 1/4 in. Cajon Ultra-Torr. The platinum powder was then calcined in flowing oxygen for 3 h at 623 K and reduced in flowing hydrogen for 1 h at 298 K. The Pyrex treatment cell containing the platinum powder was next placed in a nitrogen-containing glovebox, and metallic rubidium (99.6%, Aldrich) or cesium (99.95%, Aldrich) was subsequently loaded into the Pyrex NMR tube attached to the side of the treatment cell. This side arm was sealed again with the capped 1/4 in. Ultra-Torr, followed by removing the cell from the glovebox and sealing the side arm with a torch. The platinum powder was then reduced in hydrogen for 1 h at 298 K and for 1 h at 373 K, purged with helium for 0.5 h at 373 K and for 2 h at 473 K, and finally sealed in 350 Torr of helium in the attached Pyrex NMR tube containing the metallic alkali. The Pyrex capsule formed in this manner was finally heated at 473 K for 12 h to evaporate the metallic alkali and deposit it onto the platinum powder. At 473 K, metallic rubidium and cesium have vapor pressures of 0.055 and 0.085 Torr, respectively.32 The average coverage of the alkali metal on the platinum powder was controlled by adjusting the relative amounts of these components in the sealed Pyrex capsule. Chemical analysis (Galbraith Laboratories, Inc.) of the rubidium-promoted platinum sample following microcalorimetric studies showed that this sample contained 148 µmol Rb/g Pt. Auger electron spectroscopy (AES) was used following microcalorimetric measurements to determine that the average surface concentrations of
J. Phys. Chem., Vol. 100, No. 43, 1996 17261 TABLE 1: Compositions of Silica-Supported Platinum Catalystsa catalyst
Pt loading (wt %)
alkali loading (wt %)
Pt/alkali (atomic ratio)
0.85 wt % Pt/SiO2 1:5 Pt/K/SiO2 1:5 Pt/Rb/SiO2 1:5 Pt/Cs/SiO2
0.85 0.85 0.85 0.85
0.79 1.71 2.94
1:0 1:4.64 1:4.59 1:5.07
a
Elemental analysis provided by Galbraith Laboratories, Inc.
TABLE 2: Compositions of Silica-Supported Platinum/Tin Catalystsa
catalyst
Pt loading (wt %)
Sn loading (wt %)
alkali loading (wt %)
Pt/Sn/alkali (atomic ratio)
2:1 Pt/Sn/SiO2 2:1:6 Pt/Sn/Na/SiO2 2:1:6 Pt/Sn/Cs/SiO2
2.61 2.61 2.61
0.66 0.66 0.66
0.84 0.84
1:0.42:0 1:0.42:2.73 1:0.42:2.73
a
Elemental analysis provided by Galbraith Laboratories, Inc.
Rb and Cs on these samples were equal to 0.13 and 0.58 monolayers, respectively. These AES measurements were performed on a Perkin Elmer Phi 660 Scanning Auger Multiprobe (SAM), using a 5.00 kV electron beam with a current of 0.0248 µA. After microcalorimetric measurements of carbon monoxide adsorption on a sample prepared in the aforementioned manner, the sample was exposed to air to allow the metallic alkali to oxidize. The sample was then treated in flowing oxygen for 3 h at 473 K, reduced in hydrogen for 1 h at 298 K and for 1 h at 373 K, purged with helium for 0.5 h at 373 K and for 2 h at 473 K, and then sealed in the attached Pyrex NMR tube with 350 Torr of helium. Microcalorimetric measurements of CO adsorption were then conducted for comparison with the results obtained on the sample prior to exposure to air. The average surface concentrations of Rb and Cs determined by AES on these samples were equal to 0.36 and 0.83 monolayers, respectively. The 0.85 and the 2.61 wt % Pt/SiO2 catalysts used in this study were prepared by ion exchange of Pt(NH3)42+ with H+ on the silica (Cab-O-Sil) surface, using the method of Benesi et al.33 The resulting material was filtered, washed with deionized water, dried overnight in air at 390 K, calcined in flowing oxygen for 2 h at 573 K, and then reduced in flowing hydrogen for 2 h at 773 K. These catalysts are similar to those studied previously by Cortright and Dumesic.3,34 Tin was added to the 2.61 wt % Pt/SiO2 catalyst by evaporative impregnation of a tributyltin acetate/pentane solution, following the procedure of Cortright and Dumesic,3,34 to produce a catalyst with an atomic Pt/Sn ratio of 2:1. After impregnation with tin, the catalyst was dried overnight in air at 390 K, treated in flowing oxygen for 2 h at 573 K, and then reduced in flowing hydrogen for 2 h at 773 K. Sodium, potassium, rubidium, and cesium were added to the Pt/SiO2 and the Pt/Sn/SiO2 catalysts (prior to calcination/ reduction) by incipient wetness impregnation with aqueous solutions of NaNO3 (99.995%, Aldrich), KOH (99.995%, Alpha), RbNO3 (99.99%, Aldrich), and CsNO3 (99.99%, Aldrich), respectively. The resulting Pt/K/SiO2, Pt/Rb/SiO2, Pt/ Cs/SiO2, Pt/Sn/Na/SiO2, and Pt/Sn/Cs/SiO2 catalysts were dried overnight in air at 390 K, calcined in flowing oxygen for 2 h at 573 K, and reduced in flowing hydrogen for 2 h at 773 K. The compositions of these catalysts, as determined by elemental analysis (Galbraith Laboratories, Inc.), are shown in Tables 1 and 2. Prior to microcalorimetric experiments, each silica-supported
17262 J. Phys. Chem., Vol. 100, No. 43, 1996
Figure 1. Differential heat of CO adsorption at 403 K on platinum powder (b), rubidium-promoted platinum (4), and cesium-promoted platinum (0), adapted from ref 31.
Figure 2. Differential heat of CO adsorption at 403 K on platinum powder (b), rubidium-promoted platinum (4), and cesium-promoted platinum (0), after exposure to air, calcination in O2 at 473 K, and subsequent reduction in H2 at 373 K.
catalyst was loaded into the Pyrex treatment cell, reduced in ultrapure flowing hydrogen for 2 h at 773 K, and then purged with ultrapure flowing helium for 2 h at 773 K to remove adsorbed hydrogen. The catalyst was then isolated in 350 Torr of helium and sealed in the attached Pyrex NMR tube. Microcalorimetric measurements of CO adsorption at 403 K were then performed following the procedures outlined above. Kinetic studies of isobutane conversion were performed in a stainless-steel flow system using a quartz down-flow reactor. Helium (Liquid Carbonic), used as a carrier gas, was purified by passage over a bed of reduced Oxy-Trap and over activated molecular sieves (13×) at 77 K. Isobutane (99.5%, Matheson) was purified by passage over a bed of reduced Oxy-Trap and over a bed of reduced nickel on alumina at 373 K, used to remove organic sulfur impurities. Hydrogen (Liquid Carbonic), used for catalyst treatments and in reaction kinetics studies, was purified by passage through a Deoxo unit (Engelhard) and activated molecular sieves (13×) at 77 K. The reactor inlet and outlet gases were analyzed with a HP-5890 gas chromatograph equipped with a FID and a 20 ft 15% Squalane Chromsorb PAW column held at 323 K. Turnover frequencies were calculated from the rates of product formation, based on the number of surface platinum atoms determined by the saturation uptakes of carbon monoxide at 403 K. Results Microcalorimetric results for CO adsorption at 403 K on platinum powder are shown in Figures 1 and 2. The initial heat of CO adsorption on platinum is 140 kJ/mol, in excellent agreement with the heats of ca. 140 kJ/mol reported for CO adsorption at 403 K on various silica- and zeolite-supported
Spiewak et al.
Figure 3. Differential heats of CO adsorption at 403 K on 0.85 wt % platinum catalysts: Pt/SiO2 (b), 1:5 Pt/K/SiO2 (0), 1:5 Pt/Rb/SiO2 ()), and 1:5 Pt/Cs/SiO2 (4).
platinum catalysts.34,35 The value of 140 kJ/mol is also consistent with the initial heats of 126-132 kJ/mol estimated from TPD measurements of CO desorption from Pt(111).10 The differential heat shown here remains nearly constant at 140 kJ/ mol to a CO coverage of ca. 26 µmol/g of platinum powder, and the heat decreases at higher coverages as the surface becomes saturated with CO at a coverage of ca. 30 µmol/g. The microcalorimetric results for CO adsorption at 403 K on platinum powders containing metallic rubidium and cesium are also shown in Figure 1. The addition of metallic rubidium to platinum powder increases the initial heat of CO adsorption from 140 to 160 kJ/mol. The differential heat remains nearly constant at this value for the first 20 µmol/g of CO coverage. At higher coverages the differential heat decreases to a value of 110 kJ/mol at a CO coverage of ca. 40 µmol/g, and the heat remains constant at this value until the sample becomes saturated with CO at a coverage of ca. 110 µmol/g. The addition of metallic cesium to platinum powder increases the initial heat of CO adsorption from 140 to 180 kJ/mol, and the heat remains nearly constant at this value for the first 18 µmol/g of CO coverage. At higher coverages, the differential heat decreases to a value of 115 kJ/mol at a CO coverage of ca. 30 µmol/g, and the heat remains nearly constant at this value until the sample becomes saturated with CO at a coverage of ca. 60 µmol/g. Figure 2 shows the microcalorimetric results of CO adsorption at 403 K on the rubidium- and cesium-promoted platinum powders, after exposure of these samples to air and subsequent calcination and reduction in hydrogen. The initial heat of CO adsorption on the rubidium-promoted platinum powder after exposure to air followed by reduction in H2 decreases from 160 to 150 kJ/mol, and the heat remains constant at this value to a CO coverage of ca. 19 µmol/g. Moreover, the second plateau of differential heat versus coverage, characteristic of this sample prior to exposure to air, is absent, resulting in a saturation CO coverage of ca. 23 µmol/g. Similarly, the initial heat of CO adsorption on the cesium-promoted platinum powder after exposure to air followed by reduction in H2 decreases from 180 to 150 kJ/mol, and the heat remains constant at this value to a CO coverage of ca. 22 µmol/g. In addition, the second plateau of differential heat versus coverage, characteristic of the cesiumpromoted sample prior to exposure to air, is absent, resulting in a saturation CO coverage of ca. 28 µmol/g. Figure 3 shows the microcalorimetric results for CO adsorption at 403 K on Pt/SiO2 catalysts containing 0.85 wt % platinum. The initial heat of CO adsorption on Pt/SiO2 is 135 kJ/mol, in good agreement with the value of 140 kJ/mol found in this study for platinum powder and reported for Pt/SiO2 catalysts containing 4 wt % platinum35 and 1.2 wt % platinum.34
Reaction Kinetic Studies of Alkali Metals on Pt Powder
Figure 4. Differential heats of CO adsorption at 403 K on 2.61 wt % platinum catalysts: 2:1 Pt/Sn/SiO2 (b), 2:1:6 Pt/Sn/Na/SiO2 (0), and 2:1:6 Pt/Sn/Cs/SiO2 (4).
TABLE 3: Isobutane Conversion and Selectivities over Silica-Supported Pt and Pt/Sn Catalysts at 673 K, 12.5 Torr Isobutane, 75 Torr Hydrogen, and 760 Torr Total Pressure catalyst Pt/SiO2
Pt/Sn/SiO2
Pt/Sn/Na/SiO2
WHSVa (h-1) 3.6 7.4 7.1 28.0 5.9 3.4 i-C4H10 conversion (%) 6.31 × 10-2 i-C4H10 conversion TOFb 2.32 × 10-1 3.95 × 10-2 CH4 TOFb 1.08 × 10-1 1.63 × 10-3 2.40 × 10-4 C2H6 TOFb 3.63 × 10-2 5.52 × 10-4 2.42 × 10-5 C3H8 TOFb 6.23 × 10-2 2.79 × 10-4
