CO Selective Oxidation in Hydrogen-Rich Gas over Copper-Series Catalysts

Jourmal NtaralGas ChetisaryJournal of Natural Gas Chemistry 14(2005)29-34SCIENCE PRESSCO Selective Oxidation in Hydrogen-Rich Gas overCopper Series CatalystsHanbo Zou1*，Xinfa Dong'，Weiming Lin1,21. Schoo of Chemical and Energy Engineering, South China University of Technology, Guangzhou 510640, China;2. Department of Biological and Chemical Engineering, Guangzhou University, Guangzhou 510405, China[Manuscript received Decenber 13, 2004; revised February 16, 2005]Abstract: The performances of CO selective oxidation in hydrogen-rich gas over four catalytic systemsof CuO/ZrO2, CuO/MnO2, CuO/CoO and CuO/CeO2 were compared. The reducibility of these catalystsand the efect of CuO and CeO2 molar ratio of CuO/CeO2 catalysts on the activity of selective COoxidation are investigated by XRD and TPR methods. The results show that the catalysts with theexception of CuO/ZrO2 have the interactions between CuO and CoO, CeO2 or MnO2, which result in adecrease in the reduction temperature. Among the catalysts studied, CuO/ZrO2 catalyst shows the lowestcatalytic activity while CuO/CeO2 catalyst exhibits the best catalytic performance. The CuO(10%)/CeO2catalyst attains the highest CO conversion and seletivity at 140 and 160 °C. The addition of 9% H20 inthe reactant feed decreases the activity of CuO/CeO2 catalyst but increases its CO selectivity.Key words: hydrogen-rich gas, copper-series catalyst, CuO/CeO2 catalyst, selective oxidation, carbonmonoxide1. Introductionhave desirable activity and long-term stability, thehigh cost may limit their applicability for transporta-tion. Development of non-noble metal catalysts is at-Hydrogen fuelled polymer electrolyte membranetractive. It has been well documented that Cu and Aufuel cells (PEMFC) show considerable potential asshow superior low-temperature performance for selec-a cleaner, more efficient system than the currentlytive oxidation of CO than Pt[4]. It is ilustrated thatused compression engines in automobiles. In orderCuO can produce a strong interaction with severalto avoid problems associated with hydrogen distribu-3d transition metal oxide or rare earth oxide and im-tion and storage, H2 can be produced on-board byproves its catalytic activity[5-7]. In this study, foursteam reforming or autothermal reforming of hydro-types of copper-series catalysts, namely CuO/ZrO2,carbon fuels, such as gasoline and methanol. RemovalCuO/MnO2, CuO/Co0 and CuO/CeO2, are investi-of about 1%CO remaining in hydrogen-rich gas to .gated on the CO selective oxidation with simulated100 μL/L is a critical technology. Selective oxidationreformate gas. The properties of these catalysts wereof CO with oxygen appears to be the simplest andcharacterized by TPR and XRD techniques. Themost efective method for removing CO. The catalystseffects of CuO and CeO2 molar ratio in CuO-CeO2proposed in the literatures for this process are noblecatalys中国煤化工n the fed on themetal- based catalysts, such as Au/- Al2O3[1], Pt/Ce-catalyte also discussed.ZrO2[2], K Rh/SiO2[3]. Although precious catalystsYHCNMHG* Corresponding author. Tel: 020-87111884, E-mail: zoubb2000@sohu.com;This work was financially supported by Guangdong Province Natural Science Foundation of China(000435), the DoctoralProgram Foundation of the Ministry of Education (20010561003) and Guangzhou Municipal Science and Technology Project(2001J1-C0211)30Hanbo Zou et al./ Journal of Natural Gas Chemistry Vol. 14 No.1 20052. Experimentalof 50 mg. The sample was heated in nitrogen at 300°C for 0.5 h, and cooled to room temperature under2.1. Catalyst preparationnitrogen flow. Then H2-TPR was performed in a mix-ture of 10% hydrogen in nitrogen with the flow rate ofFour catalytic systems of CuO/ZrO2, CuO/30 ml/min. The temperature was raised at a constantMnO2, CuO/CoO and CuO/CeO2 were prepared byrate of 15 °C/min from room temperature to aroundthe alkaline coprecipitation method using Na2CO3 so-600 °C. The water produced by the reduction waslution as a precipitant. Under stirring, Na2CO3 so-trapped on 5A molecule sieve. The crystal structurelution and mixed nitrate solution containing the de-of the catalysts was determined on a Philips XPERP-sired ratio of cations were slowly dropped to a beakerPRO diffractrormneter using Cu Ka radiation, with thesimultaneously and pH of the solution was kept toaccelerated voltage of 40 kV, the filament current of10 at 70 °C. The precipitate were aged for 2 h at40 mA, and the scanning frequency of 5°/min in 20the same temperature, then filtered and washed withrange of 20° -80°.hot distilled water several times for removing residualsodium. They were dried in air at 110 °C overnight3. Results and discussionand calcined at 500 °C for 4 h.3.1. Comparison of catalytic activity over2.2. Catalytic reaction testdifferent copper catalystsCatalytic activity test were carried out in a con-CO conversion and selectivity of the four catalyststinuous fixed-bed reactor at atmospheric pressure.with the best performance in their types are given inThe micro-reactor was a quartz tube with 7 mm of in-Figure 1.ternal diameter. The bed was made of 0.3 g catalystsieved to 40- 60 mesh. Prior to all catalytic experi-100ments, the copper series catalysts were heated at 300°C under a flowing 20%O2/He mixture (30 ml/min)for 1 h. A thermocouple was inserted into the catalystbed to monitor the reaction temperature. The feed区stream contained 65%H2, 25%CO2, 1%CO (He as the60一balance gas) at a typical flow rate of 100 ml/min. Anexcess of oxygen was used for the selective CO oxi-dation experiments (\=2[O2]/[CO]= -3). The effect of+ Cu0(10%)VZrO3H2O was tested by the addition of 9%H2O in the feed.十CuO(20%VCoO- 0- Cu0(10%)CeO,Quantitative analysis of outlet gas was performed bygas chromatography (HP 4890) with TDX-01 column.When CO content is below the detection level, theinfrared CO/CO2 analyzer with the resolution of 1- C(10%)VZrO2μL/L was used. The catalytic activity was evaluated-◆. C0(209%/)Co0by CO conversion (Xco), CO selectivity (Sco), and30 Fto- Cu0(10%)CeQ2they are given as follows:8 60-Xco= nco,in二ncO.out x 100% .nco,in.40-Sco =0.5(nco.n = no.ou) x 100%nOs,in - nO2,out中国煤化工A2.3. Catalyst characterizationYHCNMHG80100 120 140 160180 200 220 240The temperature programmed reduction (TPR)Reaction temperature (C)of different catalysts was. carried out using TP5000Figure 1. Comparison of CO selective oxidationadsorption instrument (made in China) with a sampleover different catalystsJournal of Natural Gas Chemistry Vol. 14 No.1 200531It shows that CuO/ZrO2 has almost no activityIt indicates that the oxidative activity of eitherbelow 120 °C, the activity improves with the increaseCuO or CeO2 is relatively low, but when suitableof temperature. When the temperature reaches 240CuO and CeO2 are mixed the activity of the cata-°C, the CO conversion is even lower than 40%. Thelyst can increase to sorme extent. CuO/CeO2 catalystselectivity of CuO/ZrO2 decreases progressively fromwith 10%CuO molar ratio gives the highest activity100% at 120°C 30% at 240 °C. CuO/MnO2 performsand is nearly constant with the increase of CuO mo-the best catalytic activity to CO oxidation at lowerlar ratio up to 30%. However, the activity decreasestemperature (120 °C), but it also benefits to hydrogenwith atomic ratio of copper over 30%, due to the ap-oxidation, which is in agreement with the experimen-pearing and the increasing of crystalline CuO in thetal observation that its CO selectivity was lower thanCu0/CeO2 catalyst. XRD analysis shows that nothose of other catalysts at the same reaction temper-CuO phase is detected in CuO(10%)/CeO2 catalyst,ature. In the temperature region studied, the activitywhich suggested that the catalyst had high CuO dis-of CuO/CoO is lower than that of CuO/MnO2 un-persion, CuO has doped into the CeO2 matrix andtill 140 °C and higher. Furthermore when the tem-shows the strong interaction between CuO and CeO2.perature is higher than 160 °C, methanation can beIt is in agreement with the report that the catalystdetected by gas chromatography over the Cu0/CoOactivity was promoted greatly due to the synergisticcatalyst. The CuO(10%, molar ratio)/CeO2 catalysteffect between CuO and CeO2[8] .has the highest activity among all catalysts. At 140and 160 °C, it attains 99% CO conversion with 77%3.3. Effect of H2O on selective CO oxidationand 36%CO selectivity respectively. Hydrogen oxi-dation becomes predominant at higher temperatureThe results obtained for selective CO oxida-and the selectivity decreases dramatically. With ation over CuO(10%)/CeO2 catalyst, with or withoutwider temperature range for a 99% CO conversion9vol%H2O in the reactant feed, are given in Figure 3.and higher selectivity, Cu0/CeO2 is the most suit-100able catalyst to the selective CO oxidation comparedto other catalysts.3.2. Effect of CuO and CeO2 molar ratio onthe catalytic performance60(2For CuO/CeO2 catalysts, the effect of different840FCuO molar ratio on selective CO oxidation is shownin Figure 2.201090E/8(0年8 60oB 60(1)中国煤化工0h508CMHCNM+o2020Cu/(Cu+Ce) mol ratio (%)Reaction temperature (C)Figure 2. Relationship between Co conversion andFigure 3. Effect of H2O on CO conversion and sethe composition of CuO/CeO2 catalystslectivity over CuO(10%)/CeO2 catalyst(1) 140C, (2) 160 °C .(1) Without H2O, (2) With 9%H2O32Hanbo Zou et al./ Journal of Natural Gas Chemistry Vol. 14 No.1 2005Addition of H2O to the feed stream decreases thecatalysts with the exception of CuO/ZrO2 have somecatalytic activity for selective CO oxidation. Theinteraction between CuO and the other component oftemperature at which the 99% conversion of CO isCoO, CeO2 or MnO2. While for CuO/ZrO2 catalyst,obtained, shifts to a higher temperature by 20 °C.the higher temperature peak is in agreement with theWhen the temperature is lower than 120 °C, it is foundexperimental result that it is less active in the COthat the catalytic activity decreased markedly withselective oxidation.9vol%H2O. The selectivity is kept at a constant valueat lower temperatures (<120 °C ) as well as at higher(7)temperatures (> 180 °C) with or without the additionof H2O. But it increases obviously between 120 and180 °C in the presence of H2O. It is assumed that thedecrease in CuO(10%)/CeO2 catalytic activity maybe attributed to the blockage of catalytic active sites(6)by the absorbed water, as wll as to the formation ofCO-H2O surface comnplexes[9].5)3.4. TPR results4)_The reducibility of the catalysts was studied byH2-TPR technique. The H2-TPR profiles of different3)_catalysts are presented in Figure 4.2)_1)00Temperature(C)Figure 5. TPR profile of Cu0/CeO2 catalyst withdifferent CuO molar ratio(1) CuO(10%)/CeO2， (2) CuO(30%)/CeO2,3)Cu0(50%)/CeO2, (4) Cu0(70%)/CeO2, (5) CuO(90%)/CeO2,(4(6) Pure CuO, (7) Pure CeO2直t(3TPR profiles of CuO/CeO2 with different CuOloading (Cu0 molar ratio) are shown in Figure5. It isshown that the temperatures for the reduction peaksof pure CuO and CeO2 are 310.8 and 516.2 °C, respec-(1tively. For the sample of CuO(10%)/CeO2 catalyst,two reduction peaks with Tmax at 150.2 and 176.31002003004050060°C respectively can be detected clearly. In contrast,Temperature (C)for the sample of Cu0(30%)/CeO2, there are threeFigure 4. H2-TPR profiles of different copper-peaks at 155.0, 198.9 and 242.4 °C in its TPR profile.series catalystsAccording to Kundakovic et al.[10], when copper con-(1) CuO(10%)/CeO2,(2) Cu0(10%)/ZrO2， (3CuO(20%)/CoO, (4) Cu0(80%)/MnO2tent is sufficiently low (2.4wt% of CuO per gram ofcatalysts), copper is well dispersed and is only presentCu0/CoO and CuO/CeO2 show two peaks. Foras isolated copper Cu2+ ions or highly dispersed clus-the case of CuO/Co0 catalyst, the first peak at 184.4ters. In our case, CuO(10%)/CeO2 catalyst contains°C is not as distinct as for the case of CuO/CeO2 cata- 4.885wt% of Cn0 ner oram nf catalyst. Accordinglyst at 150.5C. While for CuO/MnO2 and CuO/ZrO2to中国煤化工:u0(10%)/CeO2 cat-catalysts only one distinct peak appears at higheralys:YHC N M H Gispersed copper andtemperature of 308.5 and 317.1 °C respectively. Thein cluster form, which is confirmed by XRD mea-reduction profile of pure CuO is characterized by asurements shown in Figure 6. Thus the first peak atsingle peak at 310.8 °C (shown in Figure 5). Because150.2 °C is due to the reduction of cluster species andof the lower temperature peak than that of CuO, the :the peak centered at 176.3 °C is assigned to isolatedJournal of Natural Gas Chemistry Vol. 14 No.1200533Cu2+ ions. For the CuO(30%)/CeO2 catalyst, thelyst. By collecting together the information obtainedexistence of crystalline CuO in addition to the highlyfrom TPR and activity test, one can conclude that thedispersed Cu2+ species causes the appearance of thelarger particle size is the plausible interpretation forthird reduction peak at 242.4 °C. With the continu-the deactivation of the catalyst after reaction. It is ap-ous increase in copper atomic ratio, copper is mainlyparent from Figure 7 that the fresh CuO(10%)/CeO2present as larger Cu0 particles, which causes a weakerpossesses the spheroidal structure in a uniform sizeinteraction with CeO2 and shows only one peak at thedistribution. While on the surface of the deactivatedhigher temperature. When combined with the activ-CuO(10%)/CeO2 sample, there are clearly many ir-ity data, it may be presumed that the well-dispersedregular and massive structures, which demonstratesCuO on the catalysts is benefital to the catalyst per- that the suface morphological structures of the freshformance.and deactivated CuO(10%)/CeO2 are quite differentand this may be a factor for the reduction of the cat-3.5. XRD characterizationalytic performance.Figure 6 shows the XRD patterns of the fresh anddeactivated CuO(10%)/CeO2 catalyst, and their SEMresults are shown in Figure 7.●CeOrb)203C400508C20/(0 )Figure 6. XRD patterns of fresh and deactivatedCuO(10%)/CeOz catalyst(1) Fresh, (2) Deactivated CuO(10%)/CeO2 catalystThe deactivated CuO(10%)/CeO2 catalyst canFigure 7. SEM photograph of (a) fresh and (b)not reach 90% for CO conversion at the optimal tem-deactivated CuO(10%)/CeO2 catalystsperature. According to the XRD patterns, only themagnified by 2000face-centred cubic cerianite phase is revealed for theCuO(10%)/CeO2 catalyst and the CuO peak can notbe observed. It is because that the Cu particles are4. Conclusionstoo small or finely dispersed on the catalyst sufaceand thus not shown on their XRD patterns. The(1) In the four catalytic systems of CuO/ZrO2,diffraction peaks of cerianite of the deactive sampleCu0/MnOo Cu0/Cn0 and Cm0/CeO2, CuO/ZrO2are weak and sharp, which means that the cerian-shows t中国煤化工hile Cu0/CeO2ite has formed relatively large crystals. The averageexhibitsiYHC N M H Gormance.crystallite size of deactivated catalyst in the directionmethanation reaction occurs between hydrogen andnormal to the (111) plane of cerianite was calculatedCO over the CuO/CoO catalyst. There are strong in-to be 76.3 nm by Sherrer equation, which is larger teractions between CuO and MnO2, CoO and CeO2than the crystallite size 24.2 nm of the fresh cata-which result in the decrease in the reduction temper-34Hanbo Zou et al./ Journal of Natural Gas Chermistry Vol. 14 No.1 2005ature of the catalysts.[2]RohHS,PordarHS,JunKWetal.CatalLett,(2) Comparing to the Cu0/CeO2 catalysts with2004(93): 203different CuO atomic ratio, CuO(10%)/CeO2 catalyst[3] Tanaka H, Ito S I, Kameoka S et al. Catal Commun,2003, 4(1): 1exhibits catalytic performance due to the synergisticeffect between CuO and CeO2, which is in agreement4] Kandoi S, Gokhale A A, Grabow LC et al. CatalLett, 2004(93): 93with the results of TPR profiles.5] Wang J B, Lin S C, Huang T J. Appl Catal A, 2002,(3) CO conversion of CuO(10%)/CeO2 catalyst32: 107decreases in the reactant feed with 9%H2O, while the[6] Avgouropoulos G, loannides T, Matralis H K et al.CO selectivity of the catalyst increases in the pres-Catal Lett, 2001, 73(1): 33ence of 9%H2O at the same temperature than the[7] Yu w G, Bai X, Liu Y et al.' Proceeding of 12th Na-case when no H2O is present. The temperature withtional Catalyst Science. Beijing: Petroleum & Chem-99%CO conversion is 20 °C higher than that of with-ical Industry Committee of China Chemical Industryout H2O in the feed.and Engineering Society, 2004: 3748}] Papavasiliou J, Avgouropoulos G, loannides T. CatalCommun, 2004, 5(5): 231References9] Snytnikov P V, Sobyanin V A, Belyaev V D et al.Appl Catal A, 2003, 239(1-2): 149[1] Bethke G K, Kung H H. Appl Catal A, 2000, (194- [10] Kundakovic L J, Flytzani Stephanopoulos M. Appl .195): 43 .Catal A, 1998, 171(1): 13中国煤化工MYHCNMHG