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J. Phys. Chem. C 2009, 113, 2228–2234
Direct Spectroscopic Observation of the Role of Humidity in Surface Diffusion through an Ionic Adsorbent Powder. The Behavior of Adsorbed Pyridine on Nanocrystalline MgO Xiaoye Wang,† Sunhee Kim,† Corneliu Buda,‡ Matthew Neurock,‡ Olga B. Koper,§ and John T. Yates, Jr.*,† Department of Chemistry and Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904, and NanoScale Corporation, Manhattan, Kansas 66502 ReceiVed: September 19, 2008; ReVised Manuscript ReceiVed: NoVember 26, 2008
The influence of water vapor on the surface diffusion behavior of pyridine adsorbed on powdered MgO surfaces has been studied by Fourier transform IR (FTIR) absorption spectroscopy. It has been found that the introduction of water vapor significantly increases the pyridine surface diffusion coefficient. FTIR spectra showed that water vapor converted Lewis acid Mg2+ sites to Brønsted Mg-OH sites. These measurements also detected the change in surface bonding of pyridine to the two types of sites. The activation energy for escape of chemisorbed pyridine into a mobile precursor state is lower for pyridine bound to Mg-OH sites than for binding to Mg2+ sites, causing the hydroxylated MgO to exhibit a higher diffusivity than that found for dry MgO containing no surface hydroxyl groups. This effect was confirmed by DFT calculations of the binding energy of pyridine to MgO(100) sites and to defect sites on MgO(100), where hydroxylation decreases the binding energy by ∼30 kJ mol-1 on each type of site. I. Introduction The adsorption of trace contaminants from gas mixtures onto high area oxide surfaces is often compromised by the coadsorption of water in gas streams being purified.1-7 In the testing of sorbents for personnel protection, the breakthrough time of the toxic reagent is often used as a measure of the efficacy of the sorbent.2 The introduction of water vapor into the gas stream often reduces the breakthrough time considerably. This decrease in breakthrough time is thus used as a practical measure of the influence of water on the sorption efficiency of high surface area materials. These changes are likely the result of the molecular interactions that occur between water and the high surface area sorbent for which a number of questions exist. Does water preferentially occupy strong adsorption sites, thus resulting in a decreased sorption efficiency (decreased effective surface area), or does water occupy these sites and thus weaken the binding of the toxic reagents? Herein we present infrared spectroscopic evidence for an adsorbed molecule, pyridine, obtained during its surface diffusion/desorption through high surface area MgO powder, showing that the injection of water significantly increases the pyridine surface diffusion coefficient by displacement of the pyridine from strong adsorption sites (Lewis acid sites) to weaker hydrogen bonded sites. The method of using transmission IR spectroscopy as a kinetic probe for the study of surface diffusion is employed in our laboratory, using a thin deposit of powdered material in which molecular diffusion can be studied. The in situ absorbance changes in the vibrational modes of diffusing molecules can be fit to long-range Fickean diffusion kinetics very well.8-13 Our original method, used in the study of diffusion inward into high area γ-Al2O3, starting from a condensed film of the molecule * To whom correspondence should be addressed. † Department of Chemistry, University of Virginia. ‡ Department of Chemical Engineering, University of Virginia. § NanoScale Corporation.
of interest on the outer geometrical surface of the powder, was successful in measuring the coefficient of diffusion at a single temperature.11 Recent developments of this method involve measuring diffusion outward from a saturated adsorbed layer. Following adsorption to saturation, the surface is exposed to vacuum, which results in a slow depletion of the adsorbed layer which can be monitored conveniently by IR spectroscopy.12 This method has the advantage that the temperature dependence of surface diffusion can be measured over a short range of temperature, permitting the activation energy for the diffusion process to be measured. We observed two different pyridine diffusion regimes for the pyridine/MgO system.12 The first is the fast diffusion regime associated with diffusion of pyridine absorbed on terrace MgO(100) sites. The second regime is the slow diffusion regime which is associated with pyridine adsorbed onto defect Mg2+ sites, where the pyridine binding energy is larger than that on terrace MgO(100) sites. The thermal excitation of chemisorbed pyridine allows it to overcome the escape barrier and form a mobile precursor state which can either rapidly diffuse along the surface until it desorbs or readsorbs onto another chemisorption site.12,14,15 Using this method we have examined the role of injected water on the diffusion of pyridine through MgO powder. Dissociation of water molecules produces Mg-OH groups4-7 which convert the Lewis acid sites to Brønsted acid sites. Instead of bonding to the Lewis acid Mg2+ sites on the hydroxyl-free MgO surface through the lone pair of electrons on the nitrogen atom in pyridine, weaker hydrogen bonds form between the hydroxyl-covered MgO surface and the pyridine molecules, thus resulting in a more rapid diffusion/desorption process. II. Experimental Methods The experimental methods used in this work are described in detail elsewhere.12,16 An ultrahigh vacuum transmission IR cell and a FTIR spectrometer are used. The cell contains a flat
10.1021/jp808342t CCC: $40.75 2009 American Chemical Society Published on Web 01/15/2009
Role of Humidity in Surface Diffusion
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2229 The infrared spectra were recorded with a Bruker TENSOR 27 FTIR spectrometer, using a liquid nitrogen cooled MCT detector. Each spectrum was obtained in 12 s by averaging 32 interferograms at 2 cm-1 resolution. The background spectrum was taken through the empty grid region and was subtracted from the measured IR spectrum. Each diffusion/desorption experiment was observed for 2 h, with IR spectra being taken every 2-5 min.
Figure 1. Plots of the integrated absorbances of the pyridine ringbreathing vibrational mode ν19b (1443 cm-1) as a function of increasing diffusion time at 300 K for experiments I and II. Dark circles are data points from experiment I while triangles are data points from experiment II. Solid lines through experimental data points are the best fit lines to the double diffusion equation (1).
tungsten grid on which the powdered MgO sample is held. Gases can be dosed quantitatively into the cell. A type-K thermocouple is welded onto the top of the tungsten grid and allows programming of the sample temperature by resistive heating using an electronic temperature control program whose resolution is 0.1 K. The powdered MgO, which was provided by NanoScale Corporation (Manhattan, KS), consists of hexagonal platelet particles with individual crystallite sizes smaller than 8 nm.17 In order to prepare a coherent sample disk, the MgO powder was pressed into the grid using a special die under 60000 psi,18,19 achieving a density of 0.008 g cm-2. The compressed MgO sample was annealed to 1200 K in vacuum for 30 min to cause complete dehydroxylation prior to pyridine adsorption. The N2BET surface area of the compressed material is 23 m2 g-1 and the pore size distribution is reported elsewhere.12 We believe that the predominant surface planes on the MgO crystallites have the MgO(100) orientation.12 Pyridine was obtained from Sigma-Aldrich (anhydrous, 99.8% purity) and further purified with freeze-pump-thaw cycles using liquid nitrogen. In the pyridine diffusion/desorption experiment without water (experiment I), the MgO sample was exposed to pyridine vapor at about 1 Torr pressure and 295 K to form a saturated chemisorbed layer, as determined by reaching a maximum in pyridine absorbance. The chamber was pumped (
