Effect of water on the performance of Pd-ZSM-5 catalysts for the combustion of methane

Available online at www.sciencedirect.comJOURWLOF小ScienceDirectNATURAL GASCHEMISTRYEL SEVIERJoumal of Natural Gas Chemistry 17(2008)87-92www. elsevier.com/locate/jngcEffect of water on the performance of Pd-ZSM-5 catalysts forthe combustion of methaneBo Zhang，Xingyi Wang*，Ogtour M'Ramadj，DaoLi， Hua Zhang，Guanzhong LuLaboratory for Advanced Materials, Research Instiute of Industrial Catalysis, East China Universityof Science and Tehnology, Shanghai 200237, China[Manuscrip rceive July 10, 2007; revised September 27.2007 IAbstract: Palladium-based catalysts were prepared using impregnation (I) and ion exchange method (E) with ZSM-5 assupport. Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts presented the high activity for the combustion of methane. The orderof activity was consistent with Bronsted acidity of the catalysts: Pd-ZSM-5(I)>Pd-ZSM-5(E). It was shown by FT-IR thatmethane was adsorbed on the acidic bridging hydroxyl groups of ZSM-S-supported Pd catalysts. The effect of water on theactivity of Pd-ZSM-5 was investigated. The inhibition effect of water on the conversion of methane was observed. However,water promoted the stability of Pd-ZSM-5 obviously during extended time periods. XPS measurement showed that PdSiratio near the surface of Pd- ZSM-5(E) decreased more pronouncedly with time in dry stream than that of Pd- ZSM-5(D), this isatributed to the dispersion of Pd into the micropores. The addition of water, however, retarded Pd dispersion. And high partialpressure of methane reduced this effect of water vapor. The decrease in activity during the stability test can be explained on thebasis of the reduction of Pd/Si ratio.Key words: methane combustion; Bronsted acid; paladium; ZSM-S; water1. Introductionadditive which could be completely reversible or permanentin deactivation of Pd catalysts, depending on the time periodCatalytic combustion of methane is an important technol-of exposure [11]. Recently, Ribeiro tested the kinetic rate ofogy employed to decrease the emission of NOz in the ap-methane oxidation in the presence of water on Pd catalystsplication of natural gas. Supported palladium catalysts pos-and found the magnitude of inhibition effect of water on thesess an excellent activity for the oxidation of methane andoxidation of methane as a function of temperature. At 553 K,therefore have been extensively used for catalytic combustionthe reaction order of water was about - 1; above 723 K, thepurposes [1], but the thermal stability of Pd catalys remainswater reaction order was close to 0[12].to be a problem. The state and dispersion of Pd supported onCurrently, carbenium ion chemistry is considered to beAl2O3 [2], SiO2 [3] or zeolites [4,5] were less stable at lowerthe most probable route for the initiation of alkane via hydridetemperature than the decomposition temperature of PdO dur-transfer on a solid acid [13,14]. Truitt and co-workers [15]ing the catalytic combustion of methane. Even though the ini-discovered that trace amounts of Lewis acid sites facilitatedtial activity is high at this temperature, the catalysts are notthe abstraction of hydride from alkane and generate reac-able to maintain the high conversion level during extendedtive carbenium ions. Liu and Flytzani-Stephanopoulos [16]time periods, that is, the activity is not stable [6,7]. Previousproved that the strength and geometry of acidic sites on thestudies showed that the addition of other metal elements intocatalyst surface played an important role in the breakage ofPd catalysts can improve their stability to some extent [4,8,9].C-H bond and the formation of intermediate species. How-There were also studies on the effects of water and carbonever, several recent publications still proposed the direct pro-dioxide on the activity of Pd catalysts for the combustion oftonatiot'dby Sommer and co-methane [10]. The efect of water was generally considered asworke中国煤化工:otolyse the Cc- H bondan inhibition effect, which depended on the reaction tempera-and thYHC N M H G alkanes was achievedture. Burch observed that the inhibition effects of water were at low temperature[1小Accoraing to the recent result [15],●Corresponding author, Tel: +86-21-64253372; E-mail: wangxy@ecust.edu.cnThis work was supported by the National Nanral Science Foundation of China (No.20377012).88Bo Zhang e al/Joumal of Natural Gas Chemisty VoL. 17No. 12008alkane did not simply undergo physical adsorption on the ze-2.3. Characterization of catalystsolite but rather formed a specific adsorpion complex with theprolon on Bransted acid sites. The formation of adsorptionBET surface area of catalyst was measured by nitrogencomplex was the route of activating the C-H bonds. How-adsorption at 77 K on a Micromeritics ASAP 2000 surfaceever, the mechanismic steps catalyzed by solid acids and theanalyzer. XPS spectum of catalyst was recorded on a Perkin-role of acid sites for the combustion of methane are ambigu-Elmer PHl-5400spectrometer, using a monochromatic MgKaous. The proof related to the C- -H activation is inadequateradiation(1253.6eV). The binding energy of adventitiousCls[13,14]. Therefore, an explicit understanding is needed to(284.6eV with an accuracy of士0.2 eV) was used as a refer-identify the role of the surface acidity of catalyst in the activa-ence.tion of methane during the catalyic combustion of methane.The FT-IR spetra were recorded using a NICOLETIn this study, plladium-based catalysts sppred onNEXUS 470 spectrophotometer with a rsoutio of 4 cm-!.ZSM-5 were prepared. The efet of water vapor on the ac-A total of 15 mg sample was ground finely, pressed into ativity and sabiliy of Pd-ZSM-5 catalysts for the oxidation of sef-pporing wafer, and mounted into a quatz IR cell wihCH4 was studied.CaF2 windows. Before adsorbing pyridine at room tempera-ture, the wafer was heated at 300°C for 4 h under 10-3 torr2. Experimental(1 tor= 133 Pa). The FT-IR spectra of pyridine adsorbed onthe catalyst were recorded after subsequent evacuation with2.1. Preparation of catalystthe increase of temperature from room temperature to 300°C.The pre-adsorption of methane was also carried out before ad- .Plladium-based catalysts were prepared using impreg-sorption of pyridine. The background created by zeolite wasnation () and ion-exchange method (E). H-ZSM-5 zeolitesubtracted.(40- -60mesh, Si/AI= 80) was used as the spprt. In impreg-3. Results and discussionnation method, Pd-ZSM-5(I) catalyst was prepared by im-mersing support into an aqueous solution of palladium chlo-3.1. Catalyic ativityride at room temperature for 24 h. The impregnated solid wasdried at 110°C for 12 h and then calcined in air at 550 °C forThe conversion of methane on Pd-ZSM-5(1) and Pd-4 h. In ion-exchange method, the support was added into theZSM-5(E) as the function of temperaure is shown in Fig-aqueous solution of palladium chloride and the pH value ofure 1 and the data with regard to the activity and propertiesthe suspension was adjusted to 7.5 by the addition of NH4OHof catalysts are summarized in Table 1, in which T0o%, Tsor,solution. Sitred at 90°C for 24 h, Pd-ZSM-5(EB) catalyst wasand 1go% represent the temperatures for 10%, 50%, and 90%fitered, dried at 110°C for 12 h, and then calcined at S50°cconversion of CH4, respectively. It is obvious that both Pdfor4 h. The loading of Pd determined by inductively coupledZSM-5 catalysts were active, leading to almost total conver-plasma-atomic emission spectroscopy (ICP) was 0.55 wr%.sion ofCH4 at the temperatures of 200 to 460。C, of which2.2. Activity test of catalys for the combustion of methane10. Pd-ZSM-5(E)The catalytic activity for the combustion of methane was80”Pd-ZSM-5(1)evaluated at atmospheric pressure in a conventional fixed bedquarz reactor. Temperature was monitored by a thermocouple60at the bottomn of the catalyst bed and contolled by a tempera-ture contrller The catalyst of 1.0 g was placed on the reactorbed and a gas mixture (0.2%CH4, 2%O2 and N2 balance) wasfed at a rate of 420 mlmin-1 into the reactor (space veloc-ity 36000h~ l). For the measurement of thermal stability, thecatalyst bed was fed with reaction stream for 100h. For theactivity test on wet stream, the water vapour of the feed gaswas provided by a glass bubbler. Conversion measurementswere conducted in an ascending temperature mode. Afer the中国煤化工400 450 500reaction was kept for 30 min in the steady state, the compo-Figure.MHCNMH G。nthane over Pd-ZSM-5(Dsition in outlet of reactor was analyzed on stream by a gasand PA-ZSM-.5 (E),●and 1 in the ractant feed with 0.2%CH4.2%02 andchromatograph (G-120, Shanghai).N2 balance;。and 0 in the reactant feed with 0.2%CH, 0.4%O2 and Nzbalance; space velocity 36000h-1Joumal of Natural Gas Chemistry VoL 17 No, 1200889Table 1. The properties and activity of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalystsBETRelative amount of Bronsted acid sites 3Activity bCataly(m2/g)100°C200 °C300°CTio%/°CTo%/°CTos/.CPd-ZSM-5(I)3462.0811.8981.59027/380//30835//431//34134/456//380Pd-ZSM-5(E)3441.0450.880.65137/3//3423/3/22412435//456//471a The rlative amount of Brensted acid sites estimated from the inegrated areas of the band at 1540 cm-lat dffereot temperatures; b Reaction in drystream of O2/CH4 = 10/in dry stream of O2/CH4 = 2/in wet steam of O2/CH4= 10; Tio%, TSso%, and Too% are the temperatures required for 10%,50%, and 90% conversion of CH4. respectively.Pd-ZSM-5(I) was more active than Pd-ZSM-5(E). WhenO2/CH4= 10 in dry stream, the former presented Tio% ofa)/)277 °C, and Tgo% of 374。C, whereas for the latter, the con-5 90version of 10% and 90% was obtained at 337 and 435 °C, re-spectively. The reaction products were mainly CO2, and noother by-product was observed. When the partial pressure of8oxygen was changed from O2/CH4 = 10 to stoichiometric ra-tio (O2/CH4 = 2) and the concentration of methane was main-70 ttained at 0.2%, 10% conversion of methane over Pd-ZSM-5()+ 0.2%CH with wateroccurred at higher temperature than that over Pd-ZSM-5(E),- 0.2%CH, without waterand the temperature required for 90% conversion of methane一个 0.5%CH4 with wateron both Pd catalysts tended to be the same value.204060 80 100 120On the other hand, for the fresh Pd-ZSM-5 catalyst, its ac-Time ()tivity on dry stream was higher than that on Wet stream, whichindicated the inhibition effect of water. However, this effectb)decreased at high temperature over Pd-ZSM-5(I) and the con-version of methane reached more than 90% when temperaturewas raised to 380 °C. For Pd-ZSM -5(E) catalys, the inhibi-tion effect of water retarded the 90% conversion to occur at471。C, in line with the result reported by Ribeiro et al. [12].The thermal stability of Pd-ZSM-5 catalysts was testedin dry or wet reactant stream for 100 h (shown in Figure 2).◆0.2%CH4 with waterTo avoid the inhibition effect of water, the reaction tempera--0-0.2%CH without waterture was controlled at 430 and 480。C for Pd-ZSM-5() and5 0.5%CH with waterPd-ZSM-5(E), respectively. In Pd ZSM-5(I) first, the conver-68(00sion of methane in the dry stream (without water) was rapidlyTime (h)decreased from 100% to 82%, then increase up to 90%. OnFigure 2. The activity of methane oxidation over Pd-ZSM-5 1)(a) al430 °Cand Pd-ZSM-5 (E) (b) at 480。C in the reactant feed with different methanethe other hand, the conversion in wet stream over the same2%O2 and Nz balance with or without water vapor and spacecatalyst gradually decreased from 100% to about 80% fromconcentraloo,省velocity 36000 hthe start to 60 h, thereafter the catalytic activity kept constant.When the concentation of CH4 was increased from 0.2% to conversion gradually reached a consant,it can be seen0.5%, the decrease in the conversion occurred earlier. In thethat there were two types of active sites, of which one wascase of Pd-ZSM-5(E), the conversion of methane was changedstable and the other unstable. Obviously, water vapor re-from 90% to about 55% within 60 h in the dry stream contain-tarded this deactivation and enhanced the resistance to thermaling 0.2% methane. However, the catalyst was able to main-environment.tain the high conversion level during extended time period of3.2. Pridine FT-IR100 h, when water vapor was added into the stream. After theconcentration of methane in wet stream was raised to 0.5%，Py-FT-IR spectra over Pd-ZSM-5(I) and Pd-ZSM 5(E)the conversion of methane decreased gradually from 98% tocatalysts are shown in Fieure 3.For comparison, the rel-80% within the time range of 60-90 h, indicating that the in-ative中国煤化工stimated from the in-teraction between methane and active sites could result in thetegratdn~ I at dffenet tem-Hdeactivation of catalyst to some extent. From the way that theraturu山giul uU的 ”品canC NM.H.Gm be se from FIgF9(Bo Zhang et al./ Jounal of Narural Gas Chemistry Vol. 17 No.12008ure 3 that the intensity of bands at 1540 cm-1 is stronger onthat the Brpnsted acid sites accessible to pyridine decrease af-Pd-ZSM-5(I) catalyst than that on Pd-ZSM-5(E). The inten-ter the adsorption of methane on Pd-ZSM-5 catalysts, and thissity difference between 100 °C and 300 °C is similar in bothis attributed to the transfer of protons from strong Bronstedsamples, but the intensity of this band for Pd-ZSM-5(I) atacid sites to methane.300 °C is much higher than that for Pd-ZSM-5(E), indicatingthat larger amount of strong acid sites exist on Pd ZSM-5(I)Pd-ZSM-5(E)(shown in Table 1). By ion-exchange method, Pd2+ ions werefirstly substituted for H+ ions with strong acidity on ZSM-5,Pd-ZSM-0)whereas Pd species are anchored without choice on ZSM-5by using impregnation method. Therefore, the difference inacidity between Pd-ZSM-5(I) and Pd-ZSM-5(E) rflects the_Pd-ZSM-50), Ssmindifferent amount of strong Bronsted acid sites. The sequenceof catalytic activities is consistent with that of the Bronsted。 Pd-ZSM-5(D). 30minacidities (Pd-ZSM-5()>Pd-ZSM-5(E)), and the increase incatalytic activities is partly attributed to the existence of sig-Pd-ZSM-5(E), 30minnificant amount of Bronsted acid sites on the catalysts.Pd-ZSM-5(E). Smin4000390038037003600 3500 3400Wavenumber (cm ')Figure 4. FT-IR specra of hydroxyl group on Pd-ZSM-5 T) and Pd-ZSM-5 (E) before and after the adsorption of methane at room temperature fordfferent timeBronsted acidLewis acidAccording to the result of activity test of methanecombustion (Figure 1), the sequence of activity over PdPd-ZSM-.s mcatalysts supported on ZSM-5 with both Bronsted anLewis acid sites is consistent with Bronsted aciditiesPd-ZSM-5 (E)(Pd-ZSM-5(I)>Pd- ZSM-5(E)). On the basis of this consider-ation, it is plausible that the acidity of the support could affectthe catalytic performances of zeolite supported palladium cat-17001650 1600 1550 1500 1450 1400 135Warvenunber (cm )alysts toward the combustion of methane. In the presence ofwater, the acidic sites could be occupied to some extent, whichFigure 3. Py-FT-IR profiles of Pd-ZSM-5 () and Pd-ZSM-5 (E)at 150°Cwould result in the inhibition of the methane adsorption onThe acidic bridging hydroxyI group in the ZSM-5 struc-the Bransted acidic sites. As there were less acidic sites onture is a typical Bronsted acid site, whose vibration in FT-IRthe Pd-ZSM-5(E) than Pd-ZSM-5(I), the effect of water Va-spectrum appears at 3610 cm 1. Before and after the adsorp-por on catalytic activity of Pd-ZSM-5(E) was higher. Whention of methane, the acidic bridging hydroxyI group on Pd-the temperature was raised, the action of water molecules onZSM-5(E) and Pd-ZSM-5(I) was investigated by FT-IR (seethe acidic sites became weak. So, the magnitude of inhibi-Figure 4). And the results show that the band at 3610 cm~ 1tion effect on the oxidation of methane could be presented asappears on both Pd-ZSM-5(1) and Pd-ZSM-5(E), and the in-a function of temperature. On the other hand, Pd2+ hydroxyltensity of former is much stronger than that of the ltter, in linecomplexes formed because of the ionisation of the water mole-with the acidity shown in Table I. The appearance of negativecules are induced by the strong electronic field of Pd cations.band after the adsorption of methane indicates that the adsorp-This may inhibit methane oxidation to some extent.tion of methane occurs on the acidic hydroxyl groups of both3.3. XPSPd-ZSM-5(E) and Pd-ZSM-5(I). Therefore, it is reasonable toassume that adsorption of methane is more preferred on theFigure 5 shows the bonding energy of Pd on catalystsacidic sites of Pd-ZSM- 5(1). The equilibrium of methane ad-treated under different conditions measured by XPS, and thesorption takes a period of time as the inverse band becomesm the peak intensitylarge with the adsorption of methane until the intensity of in-of XPS中国煤化工:an be scen from Ta-verse bands becomes almost equal to the positive bands af-ble2tYHC N M H Gsts presened Pd3ds/2ter 30 min. In addition, the acidities after the adsorption ofpeaks at 337.0 eV, which indicated that Pd mainly existsmethane were tested by FT-IR. The experimental results showin the form of PdO (337.0 eV) [18]. When treated withJourmal of Natural Gas Chemisry VoL 17 No.1 20089the reactant stream with or without water vapor at 480 °C,24 h. After that, the ratio decreased at a slow rate. In wetPd3ds/2 bonding energy of Pd- ZSM 5(E) shifts with time to astream with 0.2%CH4, first, the Pd/Si ratio decreased at 24 hhigher value, 337.6土0.2 eV, corresponding to isolated Pd2+or 48 h from 0.015 to 0.012, but did not change significantly(337.8 eV) [18], which suggests that the structure of Pd was after that. Consequently, the peak intensity of Pd 3d of Pd-changed during the oxidation of methane. On the other hand,ZSM-5 catalyst treated for 100 h with wet stream was muchPd-ZSM- 5(I) treated with the reactant stream with or without stronger than that with dry stream (shown in Figure 5). How-water vapor at 430 °C maintained the value of 337.0eV.ever, the amount of Pd on Pd-ZSM-5(E) catalyst remainedconstant, 0.55 wt%, measured by ICP, before and after the re-action for 100 h with dry or wet stream, which indicates that337.60the decrease of Pd/Si ratio near surface did not result from theloss of Pd. Misono et al. [5] found a change in the Pd/Siratio of Pd-ZSM-5 in the NO-CH4-O2 stream because of the342.86gradual dispersion of Pd species into the micropores as iso-会lated Pd2+. Pecchi found a slight increase in the CIPd ratios inPd/SiO2 catalysts used [3]. Demoulin et al. considered the de-crease of Pd/Al ratios after the reaction as the results from sin-in wet streamtering of Pd species [18]. In this study, CISi ratios maintainedin dry streamabout 0.8 and did not increase obviously with reaction time in30 332 334 336 338 340 342 344dry stream. So, it is reasonable to assume that the decrease ofBinding energy (eV)Pd/Si ratio near surface is not resulted from the carbonaceousFigure 5. XPS spectra of Pd-ZSM-5 (E) catalyst treated in dry or wet stream.compounds covering on Pd species. To find the cause for the0.2% CH4, 2% O2 and N2 balance for 100 h a 480°C, space velocitydecrease of Pd/Si ratio, TEM analysis was carried out on Pd36000 h-Icatalysts before and after the aging test. The experimental re-For Pd-ZSM-5(E), Pd/Si ratio near surface of the freshsults showed that the particles of Pd on Pd-ZSM-5(E) werecatalyst was 0.015 much higher than the value expected bytoo small to see and almost not observed after the treatments.the uniform model which gives the Pd/Si ratio of 0.003 forAnd BET area was maintained at approximately 340 m2/g. ItPd-ZSM-5 with 0.55 wt% of Pd [6]. Therefore, Pd may beis suffice to say that most of the PdO species over Pd-ZSM-richer on the extemal surface of the catalyst than in the bulk.5(E) were transformed into isolated Pd2+, indicating that theDuring the reaction in dry stream (0.2% CH4), the ratio dra-decrease of Pd/Si ratio near surface can be partly ascribed tomatically decreased with time from 0.015 to 0.008 for firstthe dispersion of Pd2+ into the micropores of HZSM-5.Table 2. The data of XPS spectra of Pd-ZSM-5(I) and Pd-ZSM-5(E) catalysts used in oxidation of methanePd content (Wt%) :Pdsi ratio b/Pd3ds/2 (eVyeCatalystFreshresh24h，48 h72 h100hPd-ZSM-5(I)T0.540.014/377.00.012337.10.010/337.20.009/337.10.009/337.2Pd-ZSM-5(1)*0.550.013/337.10.012/337.20.012/337.1Pd-ZSM-5(E)*0.590.015/337.00.012/337.601337.90.012337.6Pd-ZSM-5(E)4008/337.70.008/337.60.006/337.50.007/337.6Pd-ZSM-5(E)f_:0.010/337.00.008/337.00.007/337.0a Measured by ICP: b the rlaive ratio of Pd/Si near the surface estimated by Ipl/s the peak intensity ratio of Pd 3d(3ds/2+3dj/2) and Si 2p; c the reactiontime at 430°C for Pd-ZSM-5(I) and at 480 °C for Pd-ZSM-5(E); d dry stream containing 0.2% CH4; * wet stream containing 0.2% CH;「wet strearncontaining 0.5% CH4Pd2+ ions in Pd-ZSM-5 have been proposed to anchor atas neutral PdO species attached to the zeolite where twooxygen anions of the zeolite structure in the crystallographic Bronsted acid groups proximate to each other serve to sta-position of the framework [18- -20] and can be dispersed grad-bilise it.ually into the micropores with reaction time when exposedIn this study, water on the surface of Pd-ZSM-5 was fur-to stream [5]， Pd2+ hydroxyl complexes formed becausether ionized hv the etrnna alertronic field of Pd cations, andof the ionisation of the water molecules are induced by thethe in中国煤化工10)-P8+ could occur,strong electronic field of the Pd cations. Aylor et al. [21] sug-whichYHCN M H GPd species. However,gested that palladium may be associated with the zeolite eitherwhen Tne concenuanon oI metnane was increased from 0.2%as charge compensating species, viz. Z-Pdn+(OH~ )(n-1) orto 0.5% in wet stream, it was observed that the drop of Pd/Si92Bo Zhang a al./ Joumal of Naural Gas Chemisty Vol. 17No. 12008ratio with the reaction time was enhanced. This implies that 5 markedly decreased with time in dry stream because of thethe decrease of Pd/Si ratio is related to the interaction between dispersion of Pd into micropores. The addition of water re-methane and [AlO]-Pd2+. This interaction reduced the inter-tarded the dispersion of Pd, whereas high partial pressure ofaction between Pd2+ and ZSM-5 framework and promoted themethane reduced the effect of water vapor. The decrease indispersion of Pd into micropores.activity during the stability test can be related to the reductionFor Pd-ZSM-5(), Pd/Si ratio near the surface of the freshof Pd/Si ratio.catalyst was 0.014, which is lower than the value of Pd/Si ratioAcknowledgementsover Pd-ZSM-5(E), and this can be atributed to the lower PdWe would like to acknowledge the National Basic Researchdispersion of Pd-ZSM 5(). Pd/Si ratio decreased with time inProgram of China (No.2004CB719500) and the National Naturaldry stream to greater extent than in wet stream. Pd speciesScience Foundation of China (NNSFC) (No. 20377012) for theiron Pd-ZSM-5(I) existed more in the form of PdO and less infinancial support.the form of Pd2+. Moreover, there were more Bronsted acidgroups to retard the dispersion of Pd2+ into the micropores,Referencesso that Pd/Si ratio decreased during the reaction to a smaller[1] Ciuparu D, Lyubovsky M R, Atman E. 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Appl Catal A, 199 188(1-2): 301Pd-ZSM-5() presented the high resistance to the thermal de-[6] Nanui K, Yata H, Furuta K, Nishida A, Kohtoku Y, Matsuzaki T.activation because of more stable Pd/Si ratio near surface ofApp Catal A: General, 199, 179(1-2): 165Pd-ZSM-5(I) during the reaction.[7] Ersson A, Kusar H, Carroni R, Grifn T, Jaras s. Catal Today,From the results in Figure 2(a), it can be seen that the2003, 83(1-4): 265activity for Pd ZSM-5([) dropped to a significant extent dur-8] Neyestanaki A K, Lindfors L E, Ollonqvist T, Vayrynen J. Appling the reaction in wet stream than in dry stream, althoughCatal A: General, 2000, 196(2): 233Pd/Si ratio dropped more significantly during the reaction in[9] Persson K, Ersson A, Jansson K, Iverlund N, Jaras S. J Catal,dry stream than in wet stream. 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