XPS surface studies of activated carbon

Carbon Printed

Vol. 27. No. 4. PP. 611-613. in Great Britain.

1989 C 1989 Maxwell

o(K18.6223/X9 %3.(K)+ (Xl Pergamon Macmillan

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LETTERTOTHEEDITOR

XPS surface studies of activated carbon (Received 10, February 1989; accepted in revised form 18 May 1989)

Key Words - Activated carbon, ASC, ASC-TEDA, Whetlerite, XPS

ASC Whetlerite is used for removal of reactive gases from streams of air. Of importance to the military is its ability to remove HCN, (CN)2 and AsH3 through reaction/chemisorption with copper, chromium and silver impregnants. This letter reports a study of ASC Whetlerite (ASC) and ASC Whetlerite treated with triethylenediamine (ASC-TEDA) aimed at determining the oxidation state(s) and location of the metal impregnants. ASC Whetlerite and ASC-TEDA (lot numbers 1667 and 794, respectively) were obtained from Calgon Corporation, Pittsburgh, PA. These materials contain 8% copper, 3% chromium and 0.2% silver by weight in an activated carbon matrix. The ASC-TEDA sample contains 2% TEDA by weight. BPL carbon (lot number 7502) was also obtained from Calgon Corporation. BPL is similar to the ASC carbons but does not contain impregnants. All charcoal samples were received as 1230 mesh (U.S. standard sieve) granules. XPS spectra were recorded using a Perkin-Elmer Phi 570 ESCA/SAM system employing MgKa X-rays. Samples were analyzed for carbon, oxygen, copper, chromium, and silver. No silver was detected on any of the samples due to its low concentration, All binding energies were referenced to the carbon 1s photoelectron peak at 284.6 eV. Carbon samples were analyzed as both granules (as received from Calgon) and as fine particles which were prepared by crushing the granules to below 325 mesh. Quantitative XPS analyses were performed by dividing the peak area by its respective sensitivity factor, supplied by the manufacturer. Chromium (VI)/total chromium ratios were determined by using standard peak subtraction techniques. XPS spectra were recorded for ASC and ASCTEDA as received (in granular form) and crushed into a fine powder. Crushing the granules to a fine powder results in exposing the internal regions of the granule to the X-ray source. Spectra of the crushed and as-received granules are compared for two reasons: first, to obtain information regarding chemical states of species located within the granule and at the external surface, and second, to determine if the impregnation procedure resulted in depositing a large fraction of metals at the external surface of the manule. Figure 1 illustrates the XPS spectra of the copper 2p photoelectron region for ASC Whetletite as granules and crushed to c 325 mesh, crushed ASC-TEDA and copper(I1) oxide. The spectrum corresponding to ASCTEDA granules is similar to that of the crushed ASC-

TEDA. Note from this figure that all peaks are at approximately the same position. The binding energy of the 2p3/2 peak (933.5 + 0.1 eV), along with the characteristic shake-up peaks, indicate that copper is in the 2+ oxidation state. _All spectra are similar, which sueeests that treatment of ASC with TEDA does not alter the oxidation state of copper, at least over the age of the samples reported here. Small amounts of copper(I) and copper(O) have been reported to be present on impregnated carbons (1,2,3). No evidence for the Y”

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I A

B

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Figure 1. XPS spectrum of the copper 2p photoelectron region. (A) ASC (~325 mesh), (B) ASC granules, (C) ASC-TEDA (~325 mesh), (D) CuO. 611

Letter to the Editor

612

existence of lower oxidation state species is evident in the spectra shown in Figure 1. The XPS spectra of the chromium 2p photoelectron region of the impregnated carbon samples are illustrated for comparison in Figure 2. The spectrum of CrO? is reported as a representative chromium (VI) compound and is not intended to imply speciation. The position of the Cr 2~3~ photoelectron peak (578.7 f 0.2 eV) is indicative of chromium in the 6+ oxidation state (3,4,5) and is consistent with previous studies (3,4). The shoulder on the 2p3n is likely due to the presence of chromium in the 3+ oxidation state, as the presence of chromium(II1) on ASC carbons is well documented (l4,6). Following subtraction of the chromate spectrum from that of the impregnated carbons, a peak is observed at 576.0 f 0.2 eV. The position of this peak corresponds to chromium in the 3+ oxidation state (35). For Whetlerite samples, chromium(V1) species are required to react with cyanogen chloride (6,7). Note from Figure 2 that the magnitude of the Cr(II1) shoulder decreases upon crushing the impregnated carbon granule. Chromium(V1) to total chromium ratios, as determined by XPS, are reported in Table 1. Note from this table that the fraction of chromium(V1) increases upon crushing the carbon sample. This may be observed qualitatively by noting the decrease in the magnitude of the chromium(II1) shoulder in Fieure 2 uuon crushine the ASC sample.’ This indicate: that the fraction oyf chromium in the 6+ oxidation state is lower at the external surface of the granule relative to the bulk sample. Also Table 1 shows that the ASC-TEDA sample has a greater fraction of chromium in the 6+ oxidation state than does the ASC sample. These data suggest that treatment of ASC with TEDA either increases the fraction of chromium(V1) in the sample, or decreases the rate at which chromium(V1) species are reduced to species of lower oxidation state.

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585

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570

Figure 2. XPS spectrum of the copper 2p photoelectron region. (A) ASC (~325 mesh), (B) ASC granules, (C) ASC-TEDA (~325 mesh), (D) 003. The binding energy of the oxygen 1s photoelectron peak for BPL carbon (BE = 532.3 eV) is approximately 1.5 eV greater than that of the impregnated carbons (BE = 530.8 f 0.1 eV). The oxygen 1s binding energy of 530.8 eV for impregnated carbons is consistent with that reported for oxides of chromium and copper (5); while the binding energy of 532.2 eV for BPL agrees well with values reported by Marchon et al. (8) for oxygen intercalated graphite. Note from Table 2 that the oxygen to carbon ratios of the

Table 1 Chromium(V1) to Total Chromium Ratios for Whetlerite Samples as Determined by XPS Cr(VI)/Total Cr

Sample

0.69 0.80

ASC (granule) ASC (~325 mesh)

0.74 >0.90

ASC TEDA (granule) ASC TEDA (~325 mesh)

Table 2 Elemental Ratios as Determined by XPS Sample

Elemental Ratios Cu/C

BPL (granule) BPL (<325 mesh) ASC (granule) ASC (~325 mesh) Chemical analysis

Cr/C

O/C

C&r

0.075 0.059 0.061 0.030 0.251 2.11 0.028 0.012 0.125 2.25 0.020 0.009 0.125* 2.18

0.035 0.020 0.188 1.80 ASC TEDA (granule) ASC TEDA (~325 mesh) 0.022 0.010 0.097 2.21 0.019 0.009 0.097* 2.18 Chemical analysis *Elemental ratios determined from chemical analysis employ O/C ratio determined from XPS analysis of crushed sample.

impregnated carbons are significantly greater than those determined for BPL. These data suggest that a large fraction of oxygen may be associated with the impregnants and are consistent with the conclusions of Pytlewski (1). Quantitative XPS analyses, in the form of elemental ratios, are used to determine whether the impregnation process placed the metal into the internal pore network of the material, or deposited a large fraction of metal onto the external surface (9,10,11). Comparing the metal-to-carbon (M/C) ratio determined bv XPS to that determined bv chemical analvsis will Piovide the above information. Quantita&e XPS analyses, reported in Table 2, reveal that the Cu/C and Cr/C atomic ratios are approximately three times greater than the corresponding bulk analysis for ASC. This indicates that the outer regions of the granules, following preparation, are rich in impregnants, and thus the impregnant distribution is not radially uniform. ASC Whetlerlte is impregnated as 12-30 mesh granules from solutions of copper and chromium complexes. It is reasonable that, due to the rather large size of the granules, impregnants would accumulate in greater concentrations at the outer regions of the granules. For ASC-TEDA, the Cu/C and Cr/C ratios determined for the granules are approximately half those of the untreated ASC. This may suggest that treatment with TEDA reduces the metal accumulation at the external surface of the granule. Cu/C and Cr/C ratios calculated from analyses of the crushed granules (ASC and ASC-TEDA)

Letter to the Editor are closer to those of the bulk analysis, but slightly greater. It is interesting to note that despite the external surface enrichment of the granule with impregnants, the C&r ratios for both ASC and ASC-TEDA are in excellent agreement with those obtained from chemical analysis. This is somewhat surprising, as one may expect copper and chromium to concentrate at different radial locations due to the difference in their respective concentrations (8% copper versus 3% chromium). Acknowledgments

- I wish to thank D. Tevault and D. Friday for helpful discussion. I also extend thanks to the National Research Council and CRDEC for financial assistance.

U.S. Army CRDEC J.A. ROSSIN Atm: SMCCR-RSC-A/E3220 Aberdeen Proving Ground, MD 2101 O-5423

613 REFERENCES

1. L.L. Pytlewski, “Studies of ASC Whederite Reactivity”, contract DAAAIS-73-C-0263 (19731977). 2. J.L. Hammarstrom and A. Sacco, “Method for Determining Catalytic Activity of ASC Whetlerite Carbon Beds”, CRDEC Report CRDC-CR-84001 (1983). 3. J.L. Hammarstrom and A. Sacco, J. Catal. 100, 293 (1986). 4. J.L. Hammarstrom and A. Sacco, J. Catal. 112, 267 (1988). 5. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg (eds.), “Handbook of X-Ray Photoelectron Spectroscopy”, PerkinElmer Corp. (1979). 6. P.N. Krishnan, S.A. Katz, A. Brienzvige and H. Salem, Carbon 26,914 (1988). 7. J.E. Groose, C.K. Polinsky, J.R. Conlisk, C.M. Pinchback and P. Jones, “Prcc. 1985 Sci. Conf. Chm. Def. Res.“, 409 (1986).

8. B. Marchon, J. Carrrozza, H. Heinemann and G.A. Somojai, Carbon 26,507 (1988). 9. J.A. Rossin and M.E. Davis, Chem. Commun. 234 (1986).

10. J.A. Rossin and M.E. Davis, Zeolites 7,295 (1987).

11. R.D. Shannon, J.C. Vedrine, C. Naccache, and F. Lefebre, J. Cum/. 88,431 (1984).

EDITORIAL ANNOUNCEMENT

INTERNATIONAL SCHUNK AWARD Schunk Industrieverwaltung GmbH - formerly known as Schunk & Ebe GmbH - in Geikn, West Germany, has established a new international award for outstanding scientific studies of young international scientists in the fields of carbon, powder metal and structural ceramic technologies. The prize is to be awarded annually; alternately in one year for outstanding researchers in the field of carbon technology and the following year for outstanding researchers in the fields of powder metal or structural ceramic technologies. The award is endowed with DM 10.000, -The award is for outstanding scientific work which has been published following the Diploma degree of German Universities or any comparable international degree. The decision concerning the award will be made by the board of directors of Schunk Industrieverwaltung GmbH upon the recommendation of an independent board of scientists. The first award will be made on October 23, 1989 for research in carbon technology. Nominations, which should be accompanied by the following documents: copies of one or more publications which make clear the subject of the study a general explanation and evaluation of the paper(s) in question, should be sent before August 7,1989 to Schunk Industrieverwaltung GmbH, z. H. Herm Dr. Petef Schmidt, Postfach 64 20,630O Giepen, West Germany