Study of the Sur face Prop er ties of the Hopcalite Mod i fied with No ble Met als *

Transcription

1 Pol ish J. Chem., 82, (2008) Study of the Sur face Prop er ties of the Hopcalite Mod i fied with No ble Met als * by J. Rogowski, M.I. Szynkowska **, A. Wêgliñska, E. Wojciechowska and T. Paryjczak In sti tute of Gen eral and Ecological Chemistry, Technical University of Lodz, Zeromskiego 116, ódÿ, Po land, ** fax: (042) , miszynk@p.lodz.pl (Re ceived Sep tem ber 12th, 2007; re vised manu script Au gust 25th, 2008; ac cepted Au gust 30th, 2008) In this work hopcalite-based cat a lysts, mod i fied with no ble met als, have been stud ied in ox i da tion of thiophene. It was shown that mod i fied hopcalite dis plays higher ac tiv ity for thiophene ox i da tion rel a tive to com mer cial hopcalite. More over, dif fer ences in cat a lytic ac tiv ity of hopcalite mod i fied with dif fer ent met als (Pt, Pd, Au) were ob served. To ex - plain these dif fer ences in cat a lyst ac tiv ity both sur face and bulk prop er ties of cat a lyst were stud ied with Time of Flight Sec ond ary Ion Mass Spec trom e try (ToF-SIMS), Scan - ning Elec tron Mi cros copy with En ergy Dispersive Spec trom e ter (SEM-EDS) and X-ray dif frac tion method (XRD). Au mod i fied hopcalite shows the low est ac tiv ity for ox i da tion of thiophene. Ob tained im ages of cat a lyst sur faces in di cate that low est Au mod i fied cat a lysts ac tiv ity is ac com pa nied by pro nounced non-ho mo ge neous dis tri bu tion of Au at oms on the cat a lyst sur face. Key words: catalytic oxidation, hopcalite, odour gases Odour gases due to their low de tec tion thresh old are in tol er a ble even at low con - cen tra tion [1,2]. They can be clas si fied into sul phur com pounds, vol a tile fatty ac ids, phe nols and indoles, am mo nia and vol a tile amines [3 7]. Sev eral au thors have re - ported that sul phur-con tain ing com pounds such as: thiols, dimethyl sul phide, di - methyl disulphide or hy dro gen sul phide are im por tant con stit u ents of odour gases [6,8]. Sul phur com pounds can be pro duced from sul phate in the urine and pro teins or amino ac ids con tain ing sul phur in an i mal ma nure by var i ous bac te ria, un der an aer o - bic con di tions [8]. Cat a lytic ox i da tion is one of the pos si bil i ties for re moval of odour gases from the at mo sphere [9 13]. It has been shown that no ble metal cat a lysts show high ac tiv ity in ox i da tion of vol a tile or ganic com pounds [14 16]. Other prom is ing can di date are 3d-metal ox ide cat a lysts and mixed ox ides, which also show high ac tiv - ity for ox i da tion re ac tion and are less ex pen sive than no ble metal cat a lysts [17 19]. Among them hopcalite char ac ter ized as par tially in verse spinel with the gen eral for mula AB 2 O 4 de scribed as (Cu 2 Mn 1 ) 2 A(Cu 1 Mn 3 Mn 4+ ) B O 2 4 is widely used in CO ox i da tion at am bi ent tem per a ture but it has been also tested in ox i da tion of or - ganic com pounds [20 27]. * Presented at the 50th Jubilee Congress of the Polish Chemical Society and Polish Association of Chemical Engineers, September 9 12, 2007, Toruñ, Poland.

2 2360 J. Rogowski et al. Nu mer ous stud ies on the ox i da tive ac tiv ity of hopcalite in di cate co ex is tence of dif fer ently charged cop per and man ga nese ions in tet ra he dral A sites as well as oc ta he dral B sites. It has been sug gested that re sult ing re dox cou ples Cu 2+ + Mn 3+ Cu + + Mn 4+ can be re spon si ble for high ox i da tive ac tiv ity of the cat a lyst [5]. Fur ther stud ies showed that hopcalite is highly ac tive in the amor phous form but gen - er ally lose its ac tiv ity at higher tem per a tures. It has been sug gested that de ac ti va tion of hopcalite with spinel struc ture dur ing ther mal treat ment is mainly due to the shift - ing in pre dom i nantly ex ist ing re dox cou ples to the right in re dox equa tion dur ing crystallization of CuMn 2 O 4, and to sur face seg re ga tion of the im pu ri ties/pro mot ers dur ing ther mal treat ment due to or der ing of the spinel lat tice upon crystallization [6]. In this work we stud ied hopcalite ac tiv ity for ox i da tion re ac tion of sul phur con - tain ing com pounds. Fur ther more due to low ac tiv ity of the base cat a lyst we ex am ined pos si bil ity to ob tain more ac tive cat a lyst by no ble metal ad di tion. EX PER I MEN TAL Com mer cial hopcalite (70% MnO 2 and 30% CuO) sup plied by Chem i cal Com pany Inowroc³aw M¹twy (Po land) was tested in ox i da tion of thiophene with out ther mal treat ment. The no ble met als mod i - fied cat a lysts were pre pared by wet im preg na tion method of com mer cial hopcalite with aque ous so lu tions of H 2 PtCl 6 (Mennica, Po land), PdCl 2 (POCh Gliwice) and HAuCl 4 4H 2 O (POCh Gliwice). Met als load - ings in ob tained cat a lysts were 1 wt.%. Af ter im preg na tion the sam ples were dried at 130 C for 0.5 h and then cal cined in static air at 400 C for 4 h (Ta ble 1). Ta ble 1. The cat a lysts used in ox i da tion of thiophene. Cat a lysts Method of preparation Notation hopcalite Commercial (70%MnO 2, 30%CuO), non-cal cined CH 1%Pt-CuMnO x Wet im preg na tion, cal cined at 400 C for 4 h Pt/CH 1%Pd-CuMnO x Wet im preg na tion, cal cined at 400 C for 4 h Pd/CH 1%Au-CuMnO x Wet im preg na tion, cal cined at 400 C for 4 h Au/CH GA-S (0.1% Pt/Al 2 O 3 ) Commercial 0.1% Pt/Al 2 O 3 For com par i son com mer cial cat a lyst GA-S (0.1% Pt/Al 2 O 3 ) sup plied by Katalizator (Po land) was tested in this re ac tion. Cat a lytic ac tiv ity mea sure ments were car ried out in a quartz flow re ac tor us ing 200 mg of cat a lyst. The re ac tant mix ture 1.5% C 4 H 4 S and 20% O 2 in He was passed through the re ac tor with the flow rate of 40 cm 3 /min. The anal y ses of the re ac tor ef flu ent were per formed with on-line gas chro ma tog ra phy equipped with TDC de tec tor and HayeSep Q col umn. The thiophene con ver sion was cal cu lated us ing the equa tion: (C H S) (C H S) %C 4 H 4 S conversion (C H S) 4 4 in 4 4 out 4 4 in 100% where (C 4 H 4 S) in and (C 4 H 4 S) out are the in let and out let con cen tra tions, re spec tively. The sec ond ary ions mass spec tra were re corded with a ToF-SIMS IV mass spec trom e ter (ION-TOF GmbH, Ger many). The in stru ment was equipped with Bi liq uid metal ion gun and high mass res o lu tion

3 Study of the sur face prop er ties of the hopcalite mod i fied time of flight mass an a lyzer. Sec ond ary ion mass spec tra were re corded from an ap prox i mately 35 m 35 m area of the spot sur face. Dur ing mea sure ment analyzed area was ir ra di ated with the pulses of 25 kev Bi + ions at 10 khz rep e ti tion rate and an av er age ion cur rent 1 pa. The anal y sis time was 50 s for both pos i tive and neg a tive sec ond ary ions giv ing an ion dose be low static limit of ions/cm 2. Sec ond ary ions emit ted from the bom barded sur face were mass sep a rated and counted in time of flight (ToF) an a lyzer. Ion im ages were re corded us ing pulsed beam of 25 kev Bi 3 ions at the fre quency of 10 khz. Av er age pri mary ion cur rent was 0.14 pa. Be fore ToF-SIMS anal y sis pow der sam ples were tableted in or der to ob tain plain sur face, which al lows achiev ing higher lat eral and mass res o lu tion. The SEM-EDS mea sure ments were per formed us ing S-4700 scan ning elec tron mi cro scope (HITACHI, Ja pan), equipped with en ergy dispersive spec trom e ter EDS (Thermo Noran, USA). Im - ages were re corded at sev eral mag ni fi ca tions us ing back-scat tered elec tron de tec tor BSE YAG-type. El e men tal com po si tion of microareas of cat a lyst sur face were ob tained on the ba sis of the ob tained char - ac ter is tic X-ray spec tra re corded with EDS spec trom e ter. Maps of Cu and Mn and no ble met als dis tri bu - tion on the cho sen cat a lyst microarea were made. X-ray dif frac tion pat terns were re corded by a Simens D 5000 diffractometer us ing the Cu K ra di a tion. Data were col lected in the range of with the scan ning step RESULTS AND DISCUSSION All the cat a lysts were tested in thiophene ox i da tion ac cord ing to the ex per i men tal pro ce dure de scribed above. Thiophene is a suit able test mol e cule for study ing sul - phur com pounds ox i da tion be cause it is dif fi cult to ox i dize [27]. The re sults of cat a - lytic ac tiv ity for com mer cial hopcalite (CH), com mer cial hopcalite mod i fied with the no ble met als Pt, Pd, Au de noted as Pt/CH, Pd/CH, Au/CH and 0.1%Pt/Al 2 O 3 cat - a lyst are com pared in Fig. 1. Sam ples of CH show low ox i da tive per for mance in ox i - da tion of thiophene. It can be seen from light-off curve for CH that con ver sion grows to 30% at 340 C and de creases upon fur ther in crease of tem per a ture above 340 C in ac cor dance with re ported ther mal de ac ti va tion of hopcalite [23]. In the case of hopcalite mod i fied with no ble met als sig nif i cant in crease of ac tiv - ity for ox i da tion of thiophene can be seen in Fig. 1. The high est ac tiv ity was ob tained for Pd/CH with the to tal con ver sion at 310 C. The other cat a lysts Pt/CH and Au/CH Fig ure 1. Ac tiv ity of stud ied cat a lysts in ox i da tion of thiophene.

4 2362 J. Rogowski et al. were char ac ter ized by poorer per for mance and to tal con ver sion of thiophene was ob - tained at 340 C and 370 C, re spec tively. XRD mea sure ments of the sam ples be fore and af ter re ac tion were car ried out to eval u ate the in flu ence of cat a lyst struc ture on their per for mance in the stud ied re ac - tion. As it can be seen from Ta ble 2 com mer cial hopcalite is char ac ter ized by amor - phous form. Con trary to that, no ble metal mod i fied hopcalite dis plays pres ence of crys tal line CuMn 2 O 4 spinel form. This ob ser va tion is in agree ment with the for mer re sults. It has been shown that crys tal lized form of hopcalite can be ob served in sam - ples aged at 350 C and crys tal li za tion is com plete at 500 C. Af ter re ac tion for both com mer cial hopcalite and mod i fied cat a lysts, XRD anal y sis of the spent sam ples clearly ev i denced the de com po si tion of the spinel phase to non-stoichiometric Cu 1.5 Mn 1.5 O 4, man ga nese ox ide and coo per ox ide. It is in line with the for mer ob ser - va tion that ther mal treat ment can in duce the solid state re ac tion to wards the hopcalite de com po si tion/re duc tion [21,28]. Phase sep a ra tion from stoichiometric CuMn 2 O 4 to Cu x Mn 3 x O 4 and Mn 2 O 3 was re ported to oc cur above 600 C [28]: that tem per a ture is well above the Tamman tem per a ture of Mn 2 O 3 (403 C). Con tin ued heat ing to higher tem per a tures causes par tial de com po si tion of spinel to CuMnO 2 and Mn 3 O 4 [23]. Ta ble 2. Crys tal line phase of se lected catalysts before and after reaction. Cat a lyst Crystalline phase Com mer cial hopcalite amorphous Spent com mer cial hopcalite Cu 1.5 Mn 1.5 O 4, CuO, Mn 2 O 3, Mn 3 O 4 Fresh 1%Au-CuMnO x CuMn 2 O 4 Spent 1%Au-CuMnO x Cu 0.5 Mn 0.5 O 4, CuO, Mn 2 O 3, Mn 3 O 4, MnO x, Mn(OH) 2 Pres ence of crys tal line struc ture can sig nif i cantly in flu ence cat a lytic ac tiv ity as it has been re ported by Schwab and Kanungo [26], who have found that crys tal line CuMn 2 O 4 is an ac tive cat a lyst for CO ox i da tion. On the other hand Veprek [23] has re ported that hopcalite ac tiv ity de creases sig nif i cantly above amor phous to crys tal - line phase tran si tion. It may sug gest that also in thiophene ox i da tion amor phous form of hopcalite is ac tive. Ad di tion ally, dis cern ible dif frac tion lines of no ble met als were only ob served for me tal lic gold. These ob ser va tions sug gest that gold crys tal lites are char ac ter ized by larger di am e ter than those of Pd and Pt. Our re sults show that sig nif i cant de crease of the hopcalite cat a lytic ac tiv ity takes place at tem per a ture 350 C. More over, above this tem per a ture amor phous form is not ob served. This in di cates that amor phous form of hopcalite is re spon si ble for its cat a - lytic ac tiv ity. Amor phous form of hopcalite is also not ob served above 350 C for no - ble metal mod i fied hopcalite, but con trary to CH sam ples no ble metal mod i fied hopcalite dis plays high cat a lytic ac tiv ity. These ob ser va tions sug gest that dif fer ent ac tive cen ters are re spon si ble for cat a lytic ac tiv ity of hopcalite and M/CH cat a lysts. In the case of M/CH cat a lyst no ble metal at oms can be re spon si ble for the pres ence of me tal lic ac tive sites.

5 Study of the sur face prop er ties of the hopcalite mod i fied Fig ure 2. ToF-SIMS sur face im ages of fresh com mer cial hopcalite. Fig ure 3. ToF-SIMS sur face im ages of fresh 1%Au/CH. Fig ure 4. ToF-SIMS sur face im ages of fresh 1%Pd/CH. In or der to de ter mine the in flu ence of the chem i cal com po si tion of an a lyzed sur - face and sur face dis tri bu tion of at oms and mol e cules on cat a lyst per for mance, ToF-SIMS and SEM-EDS stud ies were car ried out. ToF-SIMS im ages of fresh cat a lyst sur face showed ho mo ge neous sur face dis tri - bu tion of hopcalite el e men tal com po nents (Figs. 2, 3 and 4). Anal y sis of the ob tained im ages showed that Pd at oms are evenly dis trib uted in mi crom e ter range on the cat a lyst sur face con trary to Au for which ar eas of en hanced sur face con cen tra tion can ob served in ToF-SIMS im ages. These re sults were fur ther more con firmed by SEM-EDS study (Fig. 5).

6 2364 J. Rogowski et al. Fig ure 5. SEM-EDS spec trum of 1%Au/CH. Fig ure 6. SEM-EDS map of el e ments dis tri bu tion of fresh 1%Pt/CH. Fig ure 7. ToF-SIMS sur face im ages of spent 1%Au/CH. Fig ure 8. SEM-EDS spec trum of spent 1%Pt/CH.

7 Study of the sur face prop er ties of the hopcalite mod i fied Un for tu nately Pt sec ond ary ions emis sion from the sam ple sur face is too low to pro vide dis cern ible sur face Pt im ages. Thus, the dis tri bu tion of Pt on hopcalite sur - face was de ter mined only by SEM-EDS study (Fig. 6). Fig. 7 shows ToF-SIMS im ages of cat a lysts sur face af ter re ac tion. It can be eas ily seen that re ac tion con di tions in duce changes of el e men tal sur face dis tri bu tion. It is dis played by the pres ence of large Cu and Mn sur face ag glom er ates in cor re spond ing ToF-SIMS im ages. Cu ag glom er ates sizes can be es ti mated as about m. Changes of Cu and Mn at oms sur face dis tri bu tion were not ob served for CH sam ples af ter cal ci na tion and re ac tion. These ob ser va tions sug gest that pres ence of no ble met als at re ac tion con di tion in duces changes in sur face dis tri bu tion of Cu and Mn at oms. SEM-EDS stud ies (Fig. 8) showed that, be side main com po nents as Mn and Cu no tice able con cen tra tion of po tas sium, cal cium and iron on the cat a lyst sur face can be ob served for all stud ied cat a lysts. Pres ence of these el e ments on the cat a lyst sur - face may cause ob served low ac tiv ity of hopcalite in the stud ied re ac tion. As it was shown this el e ments can block ac tive cen ter in ox i da tion re ac tion [23]. CONCLUSIONS The ob tained re sults show the ben e fi cial ef fect of no ble metal ad di tion to pro mote thiophene ox i da tion. The ac tiv ity of no ble met als mod i fied cat a lysts in ox i da tion of thiophene de creases in the or der Pd > Pt > Au. The low est Au mod i fied hopcalite ac tiv ity is ac com pa nied by pro nounced non-ho mog e neous dis tri bu tion of Au atoms on the catalyst surface. Acknowledgment The fi nan cial sup port of this work by the Pol ish Sci en tific Re search Coun cil (grant G 006/T02/2007) is grate fully ac knowl edged. REF ER ENCES 1. Zahn J.A., DiSpirito A.A., Do Y.S., Brooks B.E., Coo per E.E. and Hat field J.L., J. Environ. Qual., 30, 624 (2001). 2. Zahn J.A., Hat field J.L., Laird D.A., Hart T.T., Do Y.S. and DiSpirito A.A., J. En vi ron. Qual., 30, 635 (2001). 3. Schiffman S.S., Bennett J.L. and Raymer J.H., Agricul. For est Meteorol., 108, 213 (2001). 4. Cai L., Koziel J.A., Lo Y. and Hoff S.J., J. Chromatogr. A, 1102, 60 (2006). 5. McGinn S.M., Janzen H.H. and Coates T., J. Environ. Qual., 32, 1173 (2003). 6. Koœmider J., Mazur-Chrzanowska B. and Wyszyñski B., Odours PWN Warszawa, 2002 (in Pol ish). 7. Hobbs P.J., Misselbrook T.H. and Pain B.F., J. Agric. Eng. Res., 60, 137 (1995). 8. Le P.D, Baker P.D., Aarnink A.J.A., Jongbloed A.W and Van der Peet-Schwering C., Odour from Pig Pro duc tion Fa cil i ties: Its re la tion to diet, Re port 115, Agrotech. Food In no va tions (2004). 9. Wang C., Lin S., Liou S. and Weng H., Chemosphere, 49, 389 (2002).

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