Catalytic combustion of methane over palladium exchanged zeolites

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Applied Catalysts B Envuonmental3 (1994) 275-282

Catalytic combustion of methane over palladium exchanged zeolites YueJm LI, John N. Armor* Atr Products and Chemtcals, Inc. 7201 Ham&on Boulevard Allentown, PA 18195 USA (Received 5 October 1993, revised manuscnpt recewed 24 December 1993)

Abstract Palladtum cation exchanged zeohtes (ZSMJ, mordemte and femente) were studied as catalysts for methane combustion Pd-zeohtes showed much higher achvities than PdO/A1203 For comparable palladium loadmgs, PdO/A1203 requires a reachon temperature of ca 70-80°C higher than Pd-ZSM5 for converstons between SO-100% The catalytic acfivlty of Pd-ZSM-5 seems to be related to its reduablhty Temperature-programmed reduction expenments with carbon monoxide showed a lower reduction temperature (ca 157°C) for Pd-ZSMJ than for PdO/A1203 (225°C) Further, the ponhomng of the palladmm by ion exchange offers a highly Qspersed form of Pd” supported on the high surface area zeolite Key words catalyst preparahon(ton exchange), methane combustion, palladmm-zeohte, zeohtes

1. Introduction Because of its potential

economical

and envtronmental

advantages

over other hydrocar-

bons (high compresston ratio and low carbon content) and its relative abundance, natural gas has been widely used as a fuel for elech-tcal utthtles and has attracted considerable attention as an alternative fuel for motor vehicles [ 1] However, methane emlsslon itself 1s a potenttal envtronmental problem, 1 e tt contrrbutes to the greenhouse effect Most of attentton on global warmmg 1s focused on carbon dioxide m the atmosphere, but methane IS also a significant contrtbutor Although the volume of methane emlsston 1s much lower than that of carbon dioxide, it is 2 1 times more effective than carbon dioxide as a greenhouse gas [ 21 Currently, methane emtsston 1s not regulated but future regulations are anticipated Catalytic combustion of methane 1s an effecttve approach to solve thts problem It has been *Correspondmg Tel author ARMORJN@TTOWNAPCI COM

( + l-610)

4815792,

fax

0926-3373/94/.$07 00 Q 1994 Elscwer Science B V All nghts reserved SSDIO926-3373 (94)0006-Z

( + l-610)4812989.

e-mad

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known for a long hme that supported precious metals, especially palladium, are very good catalysts for tins reactlon [ 1,3-l 1] Among hydrocarbons, methane IS known to be the most difficult to oxidize, and high temperatures are usually required to ignite methane [ 12-141 For natural gas fueled vehicles, incomplete methane combustion IS a concern [ l] An effective catalyst that can destroy methane at low temperatures 1s needed to mmlmlze methane emlsslon to the atmosphere According to the literature, there are some uncertamtles regarding catalytic methane combustion For example, Hicks et al [ 151 reported that this reaction IS a structure-sensitive reaction on supported palladium and platinum, with turnover frequencies decreasing with increasing metal dispersion Baldwin and Burch [4 1, on the other hand, did not find any correlation between palladium particle size and the rate of reaction There are also different views about the role of the support on the methane combustion rate [ 4, lo] However, one consistent observation 1s that this reaction follows a first-order rate law wrth respect to methane partial pressure and zero order m oxygen [ 10,14,16-191 There 1s also a general consensus that oxygen strongly adsorbs on the metal surface and m that way prevents methane adsorption The strength of the metal-oxygen bonding IS recognized [ 201 as an important factor influencing the methane combustion activity A metal-exchanged zeohte, (e g Pd-ZSM-5)) which has an atomic dispersion of lomc Pdn, would provide a different form of metal species than supported metals and oxides m terms of llgand and Pd-0 strength There are only a few publications regarding the use of metal-exchanged zeohtes as catalysts for this reaction The use of Pd*+ exchanged zeohte 13X for methane combustion was first reported by Arth and Holland [ 201 then by Rudcham and Sanders [ 211 Pd*+ and other transltlon metal ion-exchanged mordemte catalysts were also studied for this reaction [ 22,231 All these studies focused on the kmetics Dunng our study of the selective reduction of NO, with methane [ 241, we noticed that Pd*+ exchanged zeohtes such as ZSM-5, mordemte and ferrlente were superior catalysts for the combustion of methane compared to supported palladium catalysts Our current N0.JCH4/02 process for NO, removal operates with excess methane relative to the NO, level Here, to remove the unburnt methane, we envision placing a secondary catalyst for complete methane oxldatlon after the NO, catalyst In this paper we report the activities of palladium ionexchanged zeohtes (ZSM-5, mordemte and femente) compared with that of PdO/Al,O, and attempt to explore key factors influencing the activity of these palladium catalysts

2. Experimental Pd-ZSM-5 was prepared by exchanging a Na-ZSM-5 with Pd*+ m an aqueous solution Five grams of the Na-ZSM-5 (G/Al = 14)) synthesized via a template-free method [ 251, were exchanged m a 500 ml palladmm nitrate solution (0 005 M) at 80°C for 24 h After the exchange, the zeohte suspension was filtered, and the exchanged zeohte cake was then washed with 1 1 deionized water After another filtration, the product was dried at 110°C overnight m air Elemental analyses (using mductlvely coupled plasma atonuc emission spectroscopy) showed that Pd-ZSMJ contams 3 39 wt -% Pd (W/Al atom ratio = 0 35) was stolchlometncally exchanged PdandO71%Na, (Na/Al=O33) suggestmgthePd*+ mordemte (WA1 = 5) was prepared using the same method with LZ-MS as a starting

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zeohte (LZ-MS IS a sodium form of mordemte obtamed from Union Carbide) For Pdfemente (Q/Al = 8)) NH,-femente was used to exchange Pd” ion, and NH,-femerrte was obtamed by exchanging a Na,K-femente (obtamed from TOSOH) with IQ&? ion [ 261 The palladium loadmgs for Pd-mordemte and Pd-femente are 5 61 wt -% (Pd/Al atom ratio=0 25) and 7 68 wt -% (Pd/Al atom ratio =0 51), respectively The alumma-supported palladium oxide catalyst was made via a standard mclplent wetness technique with palladium nitrate on a Kiuser/LaRouche Versal HG r_A1203 support (having a surface area of 135 m’/g) The product was calcmed at 450°C for 3 h m illf (ramp rate=2”C/mm) The palladium loadmg on the A1,03 supported catalyst was 4 19 wt -% based upon elemental analysis The particle size of PdO on alumma was measured by X-ray diffraction and calculated from the Scherrer equations From the ( 112) reflection near 55” (28)) the PdO crystallite size was calculated as 73 6 8, me expenmental reproduclbdlty 1s f 0 6 A m this size range The catalytic activities were measured using a micro-catalytic reactor operated m a steadystate plug-flow mode The reactor was a quartz glass tube with l/4 m 0 D at the inlet and 3/8 m 0 D at the outlet To reduce pressure drop, the powdered catalyst was pelletized, crushed and then sieved to 6&80 mesh before use A 0 074 g sample was used for activity measurement A K-type thermal couple was m contact with the catalyst bed The usual temperature ramping rate was S”C/mm, and the flow-rate of the feed was 74 cm3/m m (gas hourly space velocity GHSV = 30 000 h- ‘) controlled by a mass flow meter/controller (Brooks 5850) The feed was a CH,/alr nuxture ( 1% CH4) obtained from An Products and Chemicals, Inc , and the total pressure in the reactor was 1 atm A gas chromatograph (HP 5890) with a thermal conductlvlty detector was used to monitor catalytic activity A molecular sieve 5A column ( l/8 m X 10 ft ) was used to separate methane from other components Water vapor generated by the oxldatlon of methane was not observed m the chromatogram due to strong absorption of water on the column The molecular sieve column was penodlcally regenerated at 250°C overnight m a hehum flow to ensure its separation ability Methane conversion (x) was calculated based on Eq ( 1) , x( %) = ( 1 -A/Ao)

x 100

(1)

where A IS the mtenslty of the GC peak for methane after reactlon and & IS its intensity before reaction Intrmnc reaction rates (turnover frequencies) were measured with sample weights of 0 025 (for Pd-ZSM-5) and 0 05 g (for PdO/Al,O,) at a flow-rate of 140 cm31 mm and reaction temperatures ranging from 220 to 290°C Here, methane conversions were mamtamed below 13% to muumlze mass transport effects Temperature-programmed reduction (TPR) measurements with carbon monoxide were made with the reactor system using 0 1 g sample Carbon monoxide and carbon dioxide were contmuously monitored by a mass spectrometer (UTI 1OOC) her to the TPR measurement, a sample was oxidized m situ at 500°C for 1 h m a flowing 10% O,/He mixture and then cooled down to room temperature m the same mixture After flushing the sample with hehum at room temperature for ca 20 mm (0, level was comparable to the background of the mass spectrometer), a CO/He mixture (0 3% CO) was passed through the sample (150 cm3/mm), and the temperature was ramped to 500°C at 8”C/mm The mass spectrometer was carefully cahbrated for carbon monoxide and carbon dioxide to obtain quantitative results The procedures for temperature-programmed desorption (TPD)

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of carbon monoxide measurements were similar to those of TPR, except that the temperature was ramped m flowmg helium (instead of CO/He mixture) 3. Resultsand discussion Fig 1 compares methane conversions over Pd2+ exchanged ZSM-5 and PdO/A1203 as a function of temperature Obviously, the acfivlty for Pd-ZSM-5 1smuch higher compared to PdO/A1203 To obtam any given conversion between 50-lOO%, PdO/A1203 reqmres a reaction temperature of 70-8O”C higher than that of Pd-ZSM-5 Complete conversion of methane was obtamed at ca 280°C on the Pd-ZSM-5 catalyst while ca 370°C IS reqmred for PdO/A1203 Pd-ZSM-5 has some activity for the methane combustion even at 200°C A subsequent light off run was made on the Pd-ZSM-5 catalyst, and the curve m Fig 1 was reproduced Pd-mordemte and Pd-ferrlente also show remarkable activity for this reaction, and the overall methane combustion rates (normalized based on palladmm metal) for Pd2+ exchanged zeohtes and PdO/A1203 are compared m Fig 2 The palladium contammg zeohtes light-off at much lower temperatures and are much more efficient m utlhzmg palladium metal compared to the alumma supported palladium oxide catalyst The apparent activation energies for the methane combustion reaction were 27 3 and 26 6 kcal/mol (114 3 and 111 4 kJ/mol) for Pd-ZSMJ and PdO/A1203, respectively The turnover frequency on Pd-ZSM-5 was 0 013 s-l at 250°C (7 6 Torr CH,, 158 Torr 02, 1 Torr = 133 3 Pa) assuming that palladium IS atomically dispersed This number 1s comparable to the highest turn-over frequency (TOF) value, 0 016 s- ‘, reported by Hicks et al [ 151 (upon extrapolatmg to our condltlons), with a low palladium dispersion ( 12%) catalyst The rate normalized to total palladmm atoms for PdO/A1203 (Note, palladium dispersion was not determined for PdO/A1203 ) was 9 8 lop4 s- ’ at 250°C Large differences (two orders of magnitude) m TOF over supported palladium catalysts were reported m literature ( [ 151 and the references therem) Hicks et al [ 151 reported

225

275

325

375

Temperature (“C) Rg 1 Companson of methane conversions over Pd-ZSM-5 and PdO/A1203 The activltles were measured with a 0 074 g sample and a flow-rate of 74 cm3/mm (1% CH, m ar)

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-4

250

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Temperature

- -

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400

(“0

Rg 2 Comparisonof overall methane combustton rates over Pd-ZSMJ, Pd-mordemte.Pd-femente. and PdOl A1203 The rea&ons were run under the same condlhons as In Rg 1, and the rates we.m calculated based on a gram of palladmm

that methane combustion was a structure-sensItivereaction, and TOF decreases with mcreasmg palladmm dlsperslon They argued that the structure sensltlvlty of methane oxldatlon 1s related to the difference m the react~lty of the adsorbed oxygen, and the chenusorbed layer, not a surface oxrde, 1s a more reactive form of oxygen Baldwm and Burch [ 41, however, did not observe any “sensible correlation between actlvq and palladium aze” and mdlcated that only a small fraction of the palladium surface IS active for methane combustion One possible explanation for thy discrepancy IS that there are large vmations m particle size, oxidation state of palladmm and particle morphology on a smgle catalyst and the drstnbutlon vanes from preparation to preparation The preparation of metal exchanged zeohtes (unreduced), however, IS very reproducible In prmclple, the metal m a cation exchanged zeohte is atomlcally dispersed The TOF over Pd-ZSMJ observed m our experiment represents one of the highest reported m hterature This demonstrates that methane combustion can take place on a smgle atom and the palladmm cattons positioned m zeohte sites are highly active The overall actlvlty of Pd-ZSM-5 IS even higher due to its atonuc dlsperslon of palladmm Temperature-programmed reduction (TPR) expenments were camed out over Pd-ZSM5 and PdO/A1203 using carbon monoxtde to elucidate the relationship between reduction charactetlstlcs of a catalyst and ita activity for methane oxldatlon Fig 3 dlustrates the TPR profiles of these two catalysts, where the rate of carbon dloxlde formation 1splotted agamst temperature PdO/A1203 shows a major reduction peak at ca 225°C. and this peak corresponds to 0 41 10m3 mol of CO,/g or 105 0 atom/Pd atom Therefore, the supported palladmm 1s indeed a stolchlomemc bulk oxide Concomitantly, a carbon monoxide consumption peak (not shown m Fig 3) was observed havmg a mirror image to carbon dloxlde formation Below lOO”C,a trace amount of carbon dloxlde formation was detected, which may be attnbuted to the reactlon of carbon monoxide with the adsorbed oxygen A separate experunent was carned out to attempt to measure carbon monoxide chemlsorption on a fully oxldlzed PdO/A1203 After adsorpuon of carbon monoxide at room temperature, the

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0 0

100

200

Temperature

300

400

500

(“C)

Fig 3 Temperature-programmed reducbon (TPR) measurements on Pd-ZSM-5 aad PdO/A1203 The reductant

was a 0 3% CO/He mtxture Pnor the TPR measurements, a catalyst was oxldlzed III situ at 5OO“CIn flowmg a 10% OJHe nuxtunz for 1 h

TPD expenment did not indicate any appreciable amount of carbon monoxide desorphon, but a trace amount of carbon dioxide was formed below lOO”C,which corresponds to ca 12 pm01 of COJg or O/Pd atom ratio of 0 03 On Pd-ZSMJ a maJor carbon dloxlde peak at ca 157’C IS supenmposed on a broad peak extending from room temperature to 200°C Integration of the carbon dloxlde formation gives 0 35 10e3 mol/g or I 01 0 atom/Pd atom, which confirms that this IS a 2-electron reduction (Pd’+ + 2e- + Pd’) Like other transition metals m zeohtes, Pd2+ 1s an oxygen carrier, upon reduchon with carbon monoxide, an 0 atom 1s used to form a carbon dloxlde molecule A small shoulder observed at ca 210°C 1s hkely due to small amounts of palladium oxide clusters or par&les Most kinetic studies report a first-order rate law with respect to methane and zero order to oxygen [ 10,14,1619] On palladium and other precious metal surfaces the adsorpuon of oxygen IS strong [ 271 and the adsorption of methane 1sweak [ 281 Therefore the surface 1s predommantly covered by oxygen The stronger the O-metal bond, the higher IS the temperature required to activate the oxygen and the lower the activity for methane combustion Nlwa et al [ 191 found multiple peaks upon temperature-programmed reduction of supported platmum catalysts with hydrogen They pomted out the low-temperature reduction peak IS likely responsible for the methane combustion actlvlty and correlated this reduction peak temperature to the turnover frequency They found that the TOF dramatically decreased with mcreasmg peak temperature and concluded that methane combustion proceeded rapidly on the sites havmg weak Pt-0 bonds Thus, the dramatic difference m methane combustion actlvtty observed on Pd-ZSM-5 and PdO/A1203 can be explamed based on the different strengths of the Pd-0 bond or on differences m the reduclblhty of palladmm catalysts (Fig 3) It IS likely that the extra-lattice oxygen atoms m zeohte are very active forms of oxygen and are involved m the methane combustion reaction One disadvantage of the Pd-zeohte catalysts is their mstablhty to steammg at high temperatures Fig 4 shows the comparison between fresh and steamed (at 500°C for 18 h m a 4% H,O/He stream) Pd-zeohtes After steammg, the reaction rates of both Pd-ZSM-5

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w MOR @talncd)

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350

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Fig 4 Effect of steammg on methane combushon rate over Pd-ZSM-5 and Pd-mordemte The catalysts were steamed m a hehum steam contzunmg 4% Hz0 (100 cm3/rtun) at 500°C for 18 h

and Pd-mordemte decreased slgmficantly However, Pd/Al,O, was not slgmficantly affected by steaming Note, the rate for Pd-ZSM-5 after steaming 1s still more active than a fresh or steamed PdO/A1203 catalyst The deactivation of Pd-zeohtes may be caused by smtermg of palladium during steammg One challenge 1s to stabilize Pd2+ m the exchangeable sites of the zeollte agamst steaming and extreme condlhons likely encountered over emlsslon control catalysts Such a catalyst may be sultable for low-temperature methane emission control

4. Conclusron Palladium exchanged zeohtes, e g ZSM-5, mordemte and femente are very active catalysts for the combustion of methane Pd-zeohtes are much more active than supported palladium or palladium oxide Due to their very high metal dlsperslon (atomic dlsperslon) and very high mtrmslc reaction rates, the Pd-zeohtes described herem are suitable for lowtemperature methane enusslon control The higher catalytic acnvlty of Pd-zeohtes may be related to extra lattice oxygen, which 1s easily activated at low temperatures However, for commercial apphcatlons at high temperature, the stablhzatlon of Pd2+ against smtermg at hydrothermal condltlons remams a challenge

Acknowledgements Thanks are due to Paula Battavlo for the achvlty measurements and Tom Braymer for the catalyst preparations We thank Air Products and Chemicals, Inc for the pemusslon to pubhsh this work

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