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Adsorption Properties of Supported Platinum Catalysts Prepared using Dendrimers† Manuel A. Albiter and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521 Received February 19, 2010. Revised Manuscript Received April 6, 2010 The effect of different oxidation and reducing pretreatments on dendrimer-encapsulated platinum nanoparticles (PtDENs) dispersed on a high-surface-area sol-gel-made silica support was assessed by evaluating the capacity of the resulting catalysts to adsorb CO, NO, and acetylene using IR absorption spectroscopy. The untreated catalysts are themselves capable of CO uptake, but only slowly, in a diffusion-controlled process, and into a weak adsorption state. Either oxygen or hydrogen pretreatments are required for stronger adsorption. Under similar temperature and pressure conditions, O2 pretreatments result in higher uptakes, but also lead to partial oxidation and sintering of the Pt nanoparticles, and still do not fully eliminate the dendrimer matter. Hydrogen pretreatments alone at 525 K proved sufficient to expose the metal nanoparticles to the gas adsorbents and to activate the catalyst for hydrocarbon conversion reactions. NO adsorption is also seen in the H2-activated catalysts, much more extensive if adsorption is initiated at 125 K. Acetylene adsorption is via the π bonding favored on surfaces partially covered with carbonaceous deposits, suggesting that the dendrimer moieties that remain on the Pt surface of these catalysts may temper the dehydrogenation activity of the metal and favor hydrogenation and isomerization steps.
1. Introduction As reaction selectivity becomes an increasingly central criterion in the design of catalytic processes, more stringent requirements are being placed on the preparation of catalysts.1,2 In the case of heterogeneous catalysts, active nanostructures need to be prepared with high control on their size and shape. This task has been greatly supported in recent years by the advent of a new synthetic methodology based on colloidal and self-assembly chemistry.3-5 For instance, colloidal metal nanoparticles with well-defined structures have been used to prepare highly selective catalysts,6 in some cases for reactions previously thought to be structure insensitive.7,8 However, the colloidal synthesis of metal nanoparticles is not effective for the growth of small nanoparticles, with diameters below ∼5 nm, as is often desired in heterogeneous catalysis. An alternative for making such small nanoparticles, of diameters as small as 1-2 nm, is by using dendrimers as templates.9 The resulting dendrimer-encapsulated nanoparticles (DENs) have already been successfully tested in several catalytic † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. Email:
[email protected].
(1) Zaera, F. J. Phys. Chem. B 2002, 106, 4043. (2) Zaera, F. Acc. Chem. Res. 2009, 42, 1152. (3) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (4) Lee, I.; Morales, R.; Albiter, M. A.; Zaera, F. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 15241. (5) Zaera, F. J. Phys. Chem. Lett. 2010, 1, 621. (6) Somorjai, G.; Kliewer, C. React. Kinet. Catal. Lett. 2009, 96, 191. (7) Lee, I.; Delbecq, F.; Morales, R.; Albiter, M. A.; Zaera, F. Nat. Mater. 2009, 8, 132. (8) Lee, I.; Zaera, F. J. Catal. 2010, 269, 359. (9) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692. (10) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14. (11) Lang, H.; May, R. A.; Iversen, B. L.; Chandler, B. D. J. Am. Chem. Soc. 2003, 125, 14832. (12) Huang, W.; Kuhn, J. N.; Tsung, C. K.; Zhang, Y.; Habas, S. E.; Yang, P.; Somorjai, G. A. Nano Lett. 2008, 8, 2027. (13) Niu, Y.; Crooks, R. M. Chem. Mater. 2003, 15, 3463. (14) Mery, D.; Astruc, D. Coord. Chem. Rev. 2006, 250, 1965. (15) Albiter, M. A.; Crooks, R. M.; Zaera, F. J. Phys. Chem. Lett. 2010, 1, 38.
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applications,10-12 including some in liquid phase.13-15 There are, however, some unanswered questions concerning the best way to activate such catalysts in order to optimize their performance. After dispersion on a high-surface-area support, metal DENs often retain the organic matter used for their synthesis. It has been usually thought that the dendrimer structure needs to be removed in order to directly expose the metal particles to the outside environment and make them accessible to the reactants in catalysis.11,16 However, if severe treatments, typically oxidations at high temperatures, are used to achieve such a goal, there is the risk of inducing sintering of the metal nanoparticles and of losing their initial narrow size distributions.17 In this context, it has been shown recently that, at least in some instances, catalytic activity can be achieved with dendrimer-based catalysts even before the removal of the organic matter. In certain liquid solutions in particular, it appears that the dendrimers may open up and let the reactants diffuse to the surface of the metal nanoparticles,15,18 but significant catalytic activity has also been reported in the gas phase for Pt-DEN-based catalysts either without any pretreatment or after mild treatments with H2.12,19 What exactly determines the ability of metal-DEN-based catalysts to perform catalytically in the presence of the dendrimer material is still not known. In this Article, we report on infrared absorption characterization experiments on Pt-DEN supported catalysts aimed at providing a better understanding of the effect of pretreatment and the extent of removal of the organic matter on the adsorption ability of these catalysts. It is shown that oxygen treatments are in general more effective than hydrogen treatments for exposing the surface of the metal to the outside gases. However, severe oxygen treatments may lead to metal oxidation and to particle sintering and still do not fully (16) Singh, A.; Chandler, B. D. Langmuir 2005, 21, 10776. (17) Lafaye, G.; Siani, A.; Marecot, P.; Amiridis, M. D.; Williams, C. T. J. Phys. Chem. B 2006, 110, 7725. (18) Liu, D.; Gao, J.; Murphy, C. J.; Williams, C. T. J. Phys. Chem. B 2004, 108, 12911. (19) Albiter, M. A.; Zaera, F., Appl. Catal., A 2010, in press.
Published on Web 05/05/2010
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Figure 1. Transmission IR absorption spectra in the C-O stretching frequency region for carbon monoxide adsorbed at room temperature on an untreated 0.25 wt % Pt-DEN/SiO2 catalyst. The four lower traces correspond to adsorption in the presence of 5 Torr of CO, and the rest to the evolution of the adsorbed species over time once the gas is pumped away.
remove the initial dendrimer matter. Hydrogen treatments are shown to be milder and to still condition these catalysts for significant uptake of adsorbates on the platinum surface. Most of the adsorption tests performed on the Pt-DEN/SiO2 catalysts relied on carbon monoxide as the probe molecule, but the adsorption of nitric oxide and acetylene was briefly investigated as well. An increased ability of the Pt-DEN/SiO2 catalysts to adsorb NO at low temperatures was seen, presumably because condensation of the gas inside the pores of the catalyst facilitates the opening of the structure of the dendrimers, as in other liquidphase experiments. Acetylene adsorption was determined to be molecular and weak, suggesting that the dendrimers may passivate the surface of platinum in the same way as with the carbonaceous deposits that build up during hydrocarbon conversion reactions with regular Pt-based catalysts.20
2. Experimental Details The experiments were carried out with nanoparticles containing an average of 40 platinum atoms encapsulated in a fourthgeneration hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimer [G4-OH(Pt40)], prepared by mixing together appropriate aliquots of aqueous solutions of K2PtCl4 and G4-OH followed by reduction of the metal with a 10-fold excess of NaBH4.21 These Pt-DENs were then dispersed on a homemade sol-gel silica via wetness impregnation. Transmission electron microscopy (TEM) characterization of the resulting catalysts was carried out at the Central Facility for Advanced Microscopy and Microanalysis of the University of California, Riverside by using a FEI-Philips CM300 instrument equipped with an EDAX energy-dispersive X-ray spectrometer (EDS) with a Si/Li detector window. More details of the synthesis and characterization of these catalysts have been provided elsewhere.15,19 All liquid and solid reagents used in the synthesis of the Pt-DENs and sol-gel silica, including the PAMAM dendrimer, were purchased from Aldrich and used as supplied. Carbon monoxide (>99.998% purity), nitric oxide (>99.5% purity), and acetylene (>99.6% purity) were purchased from Matheson, and oxygen (>99.995% purity) and hydrogen (>99.995% purity) from Liquid Carbonic; they were all also used as received. Characterization of the adsorption of CO, NO, and acetylene on the Pt-DEN/SiO2 catalysts was carried out by transmission (20) Zaera, F. Catal. Lett. 2003, 91, 1. (21) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364.
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Figure 2. Summary of the time evolution of the peak areas (blue squares, left scale) and peak frequencies (red circles, right scale) during the CO adsorption experiment with the untreated Pt-DEN/ SiO2 catalyst reported in Figure 1. The time scale is plotted in logarithmic fashion and referenced to a value of t = 0 min at the start of the pumping of the IR cell. infrared (IR) absorption spectroscopy using a cell specifically designed for this type of in situ adsorption study.22 A sample of the catalyst is compressed in a self-supporting disk and then placed at the center of the IR cell capable of working at any temperature between 125 and 850 K and any pressure from 0.01 to 760 Torr. Pretreatments were carried out either in situ inside the IR cell before use or ex situ inside a quartz tube placed in an oven. The IR absorption data were taken at a resolution of 4 cm-1 by using a Bruker Vector 22 Fourier-transform infrared (FTIR) spectrometer in transmittance mode, and the raw traces ratioed against background traces obtained before adsorption to obtain the final transmission spectra.
3. Results The bulk of our work involves the adsorption of carbon monoxide on the Pt-DEN-based catalysts described above. Typical results from experiments on the catalysts as prepared, without any pretreatment, are reported in Figure 1. In addition to the two broad bands seen around approximately 2110 and 2170 cm-1 in the four bottom spectra due to gas-phase CO, a sharper peak is also observed around 2087 cm-1 due to the C-O stretching frequency (νC-O) of CO adsorbed on the surface of the platinum DENs. That signal grows with time of exposure and decreases slowly once the gas is pumped away. A summary of the total signal intensity of the νC-O absorption and the frequency of the peak maximum as a function of time extracted from those data is provided in Figure 2. The peak signal intensity, which roughly tracks the coverage of CO on the Pt surface,23 increases over a period of about 15 min during the exposure of the catalyst to gas-phase CO, at which point surface saturation is reached. The adsorbed CO then desorbs slowly once the gas is pumped away, with a half-life time of approximately 3 h. The changes in CO surface coverage are also reflected by the shifts in the frequency of the main peak seen between about 2060 and 2090 cm-1, which is typical of on-top adsorption of CO on single Pt atoms within flat terraces.24 The high frequency value in Figure 2 corresponds to a surface coverage of approximately θ = 0.2 ML (1 ML = 1 CO molecule/Pt atom), whereas the 2076 cm-1 peak seen after 210 (22) Zaera, F. Int. Rev. Phys. Chem. 2002, 21, 433. (23) Shigeishi, R. A.; King, D. A. Surf. Sci. 1976, 58, 379. (24) Sheppard, N.; Nguyen, T. T. The vibrational spectra of carbon monoxide chemisorbed on the surface of metal catalysts - A suggested scheme of interpretation. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, pp 67-148.
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Figure 3. IR absorption spectra for the adsorption of CO on 0.25 wt % Pt-DEN/SiO2 catalysts pretreated ex situ in an oven with 1 atm of either O2 (left panel) or H2 (right panel) at 575 K for 16 h. Both catalysts were then briefly reduced in situ in the IR cell with H2 for 15 min prior to the CO uptake. The bottom trace in each panel was obtained at room temperature in the presence of 1 Torr of CO, and the rest after heating in vacuum to the indicated temperatures.
min of pumping is estimated to correspond to θ = 0.1 ML;23 the subsequent changes cannot be reliably converted into surface coverages because of their high sensitivity to the nature of the platinum surface. The data in Figures 1 and 2 indicate that carbon monoxide adsorption on the original Pt-DEN/SiO2 catalysts is weak, and that under vacuum most of the adsorbed CO desorbs easily from the surface even at room temperature. The organic matter in the as-prepared catalysts hinders strong adsorption of CO on the platinum nanoparticles and needs to be removed to allow for stronger adsorbate bonding. Both oxidation and reduction pretreatments were tested for this purpose (Figure 3). The resulting catalysts show a higher capacity for CO adsorption than that reported in Figure 1, and also irreversible bonding to the surface at room temperature. On the O2 pretreated samples, the initial CO uptake at 300 K is indicated mainly by the C-O stretching peak seen at 2068 cm-1, but also by a second weaker feature at ∼1885 cm-1 typical of on-top adsorption on Pt atoms on defect (low coordination) sites.24 A gradual red shift of the main on-top C-O stretching peak is seen with increasing temperature until reaching a frequency value of 2007 cm-1 at 500 K, reflecting a decrease in surface coverage on the terrace sites. Simultaneously, the peak ascribed to adsorption on defects becomes more prominent, perhaps because some of the adsorbed CO migrates from the flat terraces to steps and kinks within the Pt nanoparticles. Most of the terrace on-top adsorption is suppressed after heating to 550 K, but a small amount of adsorbed CO remains on the surface until 575 K, about half on terrace sites and half on defects. Similar trends are seen on the H2-pretreated samples, with the main ontop CO peak shifting from 2058 cm-1 at 300 K to 2020 cm-1 after annealing to 500 K, and the low-frequency peak due to adsorption on defects following the same thermal behavior as on the samples pretreated with O2. Nevertheless, the use of oxygen appears to produce catalysts with a higher capacity for CO uptake. Figure 4 shows results from additional studies on the oxidation pretreatments as a function of temperature and the load of the metal in the catalyst. CO adsorption on all these catalysts is irreversible at room temperature, as their corresponding peaks in the IR spectra do not change upon evacuation of the cell, and 16206 DOI: 10.1021/la100753g
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Figure 4. IR absorption spectra for CO adsorbed on Pt-DEN/ SiO2 catalysts after different pretreatments, all based on ex situ exposures to 1 atm of O2 for 16 h. The two top traces in each panel correspond to catalysts with a 0.50 wt % Pt loading oxidized at 425 and 475 K, whereas the two bottom spectra were obtained with catalysts with 0.25 wt % Pt pretreated at 475 and 575 K. The left panel reports data obtained at room temperature in the presence of 1 Torr of CO, and the right panel the traces recorded after evacuation of the IR cell.
more extensive on the catalysts oxidized to higher temperatures: notice the 5-fold increase in the area of the peak due to CO adsorbed on on-top terrace sites between the catalysts treated at 425 and 575 K (even though the Pt load in the latter is half of that in the former). In addition, the maximum of that peak shifts to the blue, from 2032 cm-1 in the catalyst treated at 425 K to 2060 cm-1 after oxidation at 575 K, and an additional smaller feature is seen in some of the absorption IR traces in Figure 4 at 2100 cm-1 which we assign to on-top adsorption of CO on partially oxidized platinum. The latter CO is more weakly adsorbed on the surface and desorbs after heating the catalyst to 400 K; by contrast, some of the CO in the other adsorption states remains on the surface even at temperatures above 450 K (data not shown). Repeated adsorption-desorption cycles with CO were shown sufficient to reduce the PtOx layer. Finally, adsorption on defect sites is manifested by the peak at ∼1887 cm-1, which appears to be more intense on catalysts treated at lower temperatures and/or with lower loads of platinum. Oxidations at higher temperatures may be accompanied by some particle sintering and by a reduction in the number of defect sites, as also indicated by complementary TEM results (Figure 5): the small Pt nanoparticle sizes (
