18226
J. Phys. Chem. C 2007, 111, 18226-18235
Infrared Spectroscopic Study of ClCN Adsorption on Clean and Triethylenediamine-Precovered γ-Al2O3 Sunhee Kim,†,§ Dan C. Sorescu,‡ and John T. Yates, Jr.*,†,§ Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, United States Department of Energy, National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236, and Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904 ReceiVed: July 11, 2007; In Final Form: August 14, 2007
The effect of triethylenediamine (TEDA) (also named 1,4-diazabicyclo [2.2.2]octane, DABCO) on the adsorption of ClCN on a γ-Al2O3 absorbent has been investigated. Both Fourier transform infrared (FTIR) and theoretical studies indicate that no direct interaction between amine groups of TEDA and ClCN molecules takes place. Instead, we found that TEDA competes with ClCN for active surface sites on γ-Al2O3. In addition, the adsorption behavior of cyanogen chloride (ClCN) on a clean γ-Al2O3 surface has been studied. The sequence of the thermally activated processes of diffusion, adsorption, desorption, and decomposition of ClCN molecules on the clean γ-Al2O3 surface following icelike ClCN layer formation at lower temperature was observed. One of the decomposition products, Al-NCO, was assigned by using an Al-18OH labeled surface for reaction with ClCN. In addition, Al-CN and Al2-OCN species were also detected upon ClCN decomposition. Good correlation of the calculated vibrational frequencies for the adsorbed species with experimental data is found.
I. Introduction We have investigated the role of triethylenediamine (TEDA) (also named 1,4-diazabicyclo[2.2.2]octane, DABCO) functionalization of γ-Al2O3 on the adsorption of cyanogen chloride. In environmental and military applications, triethylenediamine (TEDA) has been used as an impregnant in filtration and aircleaning systems. TEDA is known to remove radio-active gases such as methyl iodide forming a quaternary ammonium salt.1-4 In the carbon filter, inorganic impregnants such as copper, chromium, silver, zinc, molybdenum, sulfur, manganese, and so forth are added to TEDA-functionalized adsorbents to provide additional chemical or catalytic interactions with low boiling toxic vapors and to promote decomposition of such gases into nontoxic deposits or innocuous gases. The amines are known to stabilize the inorganic impregnants shielding the active sites from water adsorption and increasing the shelf life of carbon filters. It was also found that organic amine compounds such as pyridine, picoline, and TEDA are able to reduce the aging effect of carbon filters, which is a characteristic problem under humid conditions.5-8 The retention of ClCN was tested in copper, silver, zinc, and molybdenum-incorporated carbon filters with varying water content by Deitz and Karwacki.7,8 As the relative humidity value increased from 0% to 80%, the ClCN retained in the filter decreased from 260 to 4 mg. In the presence of TEDA, the retained amount of ClCN was restored up to ∼230 mg, but it did not exceed the value measured under dry conditions. Thus, amine-impregnated materials show a considerably enhancement for the retention of cyanogen chloride. It has been reported that TEDA itself plays its role on the adsorption of cyanogen chloride directly, in analogy to the reaction with CH3I, as shown in Figure 1.9-11 There has been no direct experimental evidence to observe the (TEDA‚‚‚CN)+ species formation. †
University of Pittsburgh. United States Department of Energy. § University of Virginia. ‡
Figure 1. The proposed reaction between TEDA and cyanogen chloride.9-11 From ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; 2006; Pickett, J. L. et al. Sep. Sci. Technol. 2002, 37, 1079; Naderi, M. et al. J. Mater. Chem. 2002, 12, 1086.
Therefore, in the present work, the role of preadsorbed TEDA on the ClCN adsorption properties on a γ-Al2O3 surface has been investigated. The adsorption of TEDA on the γ-Al2O3 surfaces would be expected to occur in some cases by surface binding to one end of the molecule, leaving the other amine group exposed. Then, the adsorbed TEDA molecule might be able to provide the exposed amine moiety as a bonding site for subsequently adsorbed ClCN forming (TEDA‚‚‚CN)+ species as illustrated in Figure 1. Spectroscopic evaluation of the interaction of TEDA and ClCN has been made by comparing the IR spectra of ClCN adsorbed on a clean γ-Al2O3 surface and on a TEDA-precovered surface. In addition, the adsorption and decomposition behavior of ClCN on the clean γ-Al2O3 surface has been studied in this work. Even though the adsorption of other CN-containing molecules such as HCN, CH3CN, and CH3NC on oxide surface has a long history,12-18 there is a dearth of detailed studies of ClCN adsorption on such surfaces. We report the sequence of the processes of diffusion, adsorption, desorption, and decomposition of ClCN molecules, thermally activated on the γ-Al2O3 surface, following icelike ClCN layer formation on the outer geometric surface of a pressed disk of γ-Al2O3 at low temperature. The vibrational assignment of ClCN adsorbed on Lewis acid sites (Al3+), Brønsted acid sites (Al-OH), and Lewis base sites (Al-O-Al) is made and is correlated with theoretical results. Cyanate (Al2-OCN), isocyanate (Al-NCO), and cyanide (Al-CN) species are produced at elevated temperature.
10.1021/jp075409q CCC: $37.00 © 2007 American Chemical Society Published on Web 11/13/2007
Infrared Spectroscopic Study of ClCN Adsorption
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18227
Figure 2. FTIR spectra for ClCN adsorption on γ-Al2O3 at 110 K followed by sequential heating up to 160 K at vacuum (a) in the ν(O-H) region and (b) in the ν(CtN) region. The left insert in (b) shows the incremental increase in the FTIR spectrum from 130 to 160 K using difference spectra on the basis of the 110 K spectra.
TABLE 1: Comparison of the Calculated Bond Distances and Vibrational Frequencies of ClCN and NCO Systems in Gas Phase to the Corresponding Experimental Data or Other Theoretical Values system ClCN NCO CN
(calcd)a (exptl)b (calcd)a (MR-CI)c (exptl)d (calcd)a (exptl)b
d(Cl-C)
d(C-N)
1.623 1.629
1.166 1.160 1.230 1.230 1.200 1.173 1.172
d(C-O)
ν(C-Cl)
ν(C-O)
v(C-N)
1240
2200 2216 1961
729 744 1.191 1.190 1.206
1279
1951 2067 2068
a Values calculated in the current study. b Experimental values from ref 45. c Theoretical values indicated in ref 37. d Experimental values from ref 37.
II. Experimental and Computational Methods A. Experimental. The experimental methods used in this work were described in detail elsewhere.19 Briefly, a transmission IR cell contains a tungsten grid into which γ-Al2O3 powder is pressed by a hydraulic press.20,21 A type K thermocouple is welded on the upper empty region of the grid. The grid allows the sample to be heated electrically and to be cooled by a refrigerant in the range of 83-1500 K. The IR cell is connected to a stainless steel vacuum system with a base pressure
