发表文章毕业论文2008-Xu-Sensors Actuators B-Thermoelectric carbon monoxide sensor using Co-Ce catalyst

Available online at Sensors and Actuators B 133 (2008) 7077 Thermoelectric carbon monoxide sensor using Co-Ce catalyst Tian Xu, Hu Huang, Weiling Luan, Yunshi Qi, Shan-tung Tu School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 12 October 2007; received in revised form 29 January 2008; accepted 29 January 2008 Available online 10 March 2008 Abstract A thermoelectric (TE) carbon monoxide sensor has been examined, which is composed of a Co-Ce oxide catalyst layer and a TE layer. The catalyst plays an important role in this sensor. The Co-Ce oxide catalysts were prepared by a co-precipitation and tablet compression method. Several factors including the atomic ratio of Co/Ce, pH value, tablet compression pressure, calcination temperature, and operating temperature were investigated. EDS analysis was utilized to determine the fi nal composition between Co and Ce. Methods of BET, XRD, SEM and H2-TPR wereusedtocharacterizethecatalysts.WhentheCo10.0Cecatalystshowedahightemperaturedifferenceoutput(44C)atanoperatingtemperature of 92C, the TE sensor possessed good sensing property to 3vol% CO with a high output voltage signal (42mV). The response and recovery time of the TE sensor was 72s and 68s, respectively. Furthermore, high selectivity of the sensor was also obtained in the operating temperature from 90C to 125C. 2008 Elsevier B.V. All rights reserved. Keywords: Co-Ce oxide; Carbon monoxide sensor; Thermoelectric fi lm 1. Introduction Carbon monoxide (CO) is one of the common poisonous gases; even 1vol% CO can be fatal. The colorless, odorless as well as non-irritating natures of CO make it diffi cult to be detected, and the hazard of CO is exacerbated by its wide sources. As a result, CO sensors are indispensable in many fi elds, such as measuring CO concentrations of vehicle emis- sions, industrial waste, and the indoor atmosphere. Currently, CO sensors based on electrochemical or semi-conductor meth- ods have been widely used, but they still have some shortage on selectivity, long-term stability, complex confi gurations and high cost. Recently, a novel TE CO sensor was reported by Matsumiya et al. 1,2. A TE SiGe thin fi lm was utilized with an Au-loaded Co3O4 thin-fi lmcatalystpreparedbysputteringmethodoranAu loaded TiO2 ceramic thick-fi lm catalyst serving as the sensing part. When this sensor is exposed to gas mixtures of air and CO, the exothermic reaction of CO oxidation heats up the catalyst- coatedsurfaceandthenaTEvoltagebuildsupalongthehotand Corresponding author. E-mail addresses: amber huang, lomn2005 (H. Huang), luan (W. Luan). cold region of the TE fi lm. The catalyst plays an important role intheoperationofthisTEsensor,asconfi rmedbyMatsumiyaet al. 3 and our previous work on Pt/ACC or Pt/?-Al2O3catalyst for a TE thin fi lm hydrogen sensor 4,5. Cobalt (Co) oxide catalysts are widely used in oxidation reactions with high catalytic activity in CO/O2mixtures at low temperature. Jansson et al. 6 found that Co3O4catalyst could facilitatetherapidoxidationofCOat200C,whileLinetal.7 and Cunningham et al. 8 reported different results on oxida- tion temperature as 120C and 54C, respectively. Recently, cerium (Ce) has been widely utilized as an additive because of its elevated oxygen transport capacity coupled with the abil- ity to shift easily between reduced and oxidized states of Ce (Ce3+Ce4+). Combination of CeO2with other metal oxides often accelerates the mobility of lattice oxygen on their sur- faces 9, disperses their supported metal 10, decreases the carbon formation on the catalyst surface 11 and promotes CO oxidation 12. In Co oxides, an appropriate amount of Ce ele- ment can stabilize the catalyst structure and lead to different coordinationoftheCoatoms13.Co-Ceoxidesshowbettercat- alytic activities than pure Co oxides, which is reported by Xu et al. 14. In this paper, the Co-Ce oxide was used to trigger the sensor possessinghighsensitivityandselectivitytoCOatlowoperating temperature. Following these considerations, the Co-Ce oxides 0925-4005/$ see front matter 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.01.064 T. Xu et al. / Sensors and Actuators B 133 (2008) 707771 overCOoxidationwerestudied.Alltheoxideswerepreparedby a co-precipitation and tablet compression method. Surface area, pore volume, crystal structure, oxidation-reduction behavior of the Co and Co-Ce oxides were characterized subsequently by BET, XRD, SEM and TPR methods. Finally, using the optimal Co-Ce oxide, CO sensing property of the TE gas sensor was discussed. 2. Experimental 2.1. Catalyst preparation The Co-Ce oxides were prepared by the co-precipitation methodinanaqueoussolution.Amixedsolutionof Co(NO3)26H2OandCe(NO3)36H2Owasgraduallyaddedinto a NH4HCO3solution dropwise with continuous stirring. The precipitation process was carried out at 4550C. The slurry wasagedfor9h,thenfi ltered,andwashedwithde-ionizedwater several times until the pH of the fi ltrate became 7. The precipi- tatewasdriedinanovenat100Cfor2h,afterthat,theas-dried powder was formed into tablets with the compression pressure varied from 1MPa to 6MPa. For all experiments, pellets with the same weight of 200mg were utilized. Finally, all samples were calcined at various temperatures ranged from 250C to 500C in air. The Co-Ce oxides were described as CoxCe (x is the atomic ratio of Co to Ce) in the below. The pure Co oxides with- out Ce additive were also prepared by similar procedures for comparison. 2.2. Catalyst characterization An energy dispersion spectroscopy (EDS, Falcon, USA) method was used to test the fi nal composition of Co/Ce of the formed Co-Ce oxides. A BET method was used to measure the surface areas of the samples by nitrogen adsorption at 196C. The pretreat- ment was degasifi cation at 190C for 6h, using a Micrometric ASAP2010 (USA) instrument in an automatic volumetric sys- tem. X-ray diffraction (XRD) spectra were obtained using a D/max2550 VB/PC X-ray diffractometer (Rigaku, Japan). The X-ray tube voltage and current during the data col- lection were 40kV and 100mA, respectively. Each sample was scanned from a 2 value of 1080at a scanning rate of 8/min. A scanning electron microscope (SEM, JSM-6360LV) methodwasusedtocharacterizethesamplestogetherwithX-ray diffractometry (JEOL, Japan). Temperature-programmedreduction(TPR)testswerecarried outinacontinuousfl owquartzreactorconnectedwithathermal conductivity detector (TCD) to monitor the hydrogen consump- tion, using a Micrometrics AutoChem II 2920 (USA). The test was performed using 10% H2 in air at a fl ow rate of 30ml/min. In each test, 100mg of the sample was used and the temperature was raised from room temperature (RT) to 900C at a constant rate of 10C/min. 2.3. Catalytic activity and TE sensing property BismuthtelluridewaschosenasaTEmaterialfortheremark- able Seebeck coeffi cient 5. A TE CO sensor with simple structure was fabricated by employing a TE thin fi lm (BiTe PN couples) and Co-Ce oxide. The structure of the sensor device is shown in Fig. 1. The TE layer was deposited onto a quartz glass substrate by magnetron sputtering with Co-Ce oxide cen- tered in the TE fi lm. The concrete size of the device was as follows: 20mm (length)20mm (width)1mm (thickness) forthesubstrate,8mm1mm350nmfortheTElayer,9mm (diameter)2mm (thickness) for the Co-Ce oxide. ThecatalyticactivityoftheCo-Ceoxideandtheperformance oftheTECOsensorhavebeeninvestigated.Theresponseofthe TE sensor was conducted by alternately fl owing 3vol% CO/air and N2 into the reaction chamber at a fl ow rate of 100ml/min. The measurement was conducted at a constant temperature, which was provided by a silica glass tube heated by a thermal stabilized electric furnace. The catalytic activity was evaluated by the temperature difference (?T) between one side of the TE covered with a catalyst and the other side without a catalyst. The TE fi lms, working as a feedback part, convert ?T into a voltage as an output signal. All data were monitored by a K- type thermocouple, automatically collected and analyzed by an Agilent 34970A data acquisition/switch unit and Labview (NI Crop., USA). 3. Results and discussions 3.1. Effect of parameters on sensing activity 3.1.1. Effect of atomic ratio Co/Ce By changing the value of x in CoxCe, different Co-Ce oxides were synthesized. The infl uence of nominal x value used in the preparationofprecursorsonthecatalyticactivityispresentedin Fig. 2. The co-precipitation was all carried out under the same pH, and actual x values of the formed precipitates were charac- terized by EDS. From Fig. 3, an approximate linear relationship was found between the nominal and fi nal compositions. After co-precipitation, the x values in the formed precipitates were smaller than the initial ratios, indicating that Ce was enriched in theCo-Ceoxides13.Therefore,theaddedCecanbedispersed in the Co-Ce oxides. From Fig. 2, when x was increased from 6.07 to 17.1, the ?T increased at fi rst and then decreased. At the turning point of x=10.0, the Co10.0Ce oxide exhibited the highest ?T. Therefore, the optimal atomic ratio of Co to Ce was 10.0. 3.1.2. Effect of pH value It is well established that the pH value during the co- precipitation procedure is an important factor. Under the same conditions, variable pH induced different precipitates, as con- fi rmed by several reports on the synthesis of nanoparticles by co-precipitation15,16.Theinfl uenceofpHonthe?Tismani- fested in its ability to control the grain size and dispersion of the formed precipitates. With an increase in pH value, the electro- staticforcesinducedbytheimprovedOHionconcentrationwill 72T. Xu et al. / Sensors and Actuators B 133 (2008) 7077 Fig. 1. Sketch (left) and picture (right) of the core structure of a TE CO sensor. Fig. 2. Effect of the Co/Ce atomic ratio on the catalytic activity at 92C. takeeffect,resultinginpoorsizedistributionandagglomeration of the particles. As can be seen from Fig. 4, the change in pH value of the solution from 6.6 to 7.2 resulted in the increased ?T, while the declined catalytic activity was observed for higher pH val- ues than 7.2. The highest catalytic activity was deduced at the pH value of 7.1. For pH values below 7.1, the precipitating process was continuously promoted with an increase in pH; Fig. 3. Comparison of the nominal Co/Ce molar ratios used in the preparation of precursors with those observed on the fi nal catalyst surfaces. however, a further increase in pH over 7.1 resulted in larger grain size and agglomeration of the particles, and the catalytic activity decreased. Therefore, the optimal pH value in the co- precipitation process was found to be 7.1 for Co10.0Ce oxide. 3.1.3. Effect of tablet compression pressure Different tablet compression pressures were applied for the precipitated Co10.0Ce oxides. The catalytic activities of the as- formed Co10.0Ce oxides are presented in Fig. 5 as a function of the compression pressure. The highest catalytic activity was obtained at the pressure of 1MPa, followed by a decreased ?T with an increase of the pressure. The lowest catalytic activity appeared at 4MPa; afterwards, ?T ascended a little with further increasing the pressure. The infl uence of com- pression pressure on the catalyst surface area and tiny pore structure was regarded as the main reason for the experimen- tal results. With an increase of the pressure, the surface area and total pore volume of the catalyst decreased fi rst, result- ing in a decreased catalytic activity. However, when a high pressure of 5MPa or 6MPa was applied, the cracking and realignment of the catalyst particles happened. Consequently, the uniform distribution of the total pores led to the improved catalytic activity, while ?T was much lower than their coun- terpart at the value of 1MPa. Therefore, the optimal tablet compression pressure of 1MPa was adopted in the following experiment. Fig. 4. Effect of the pH value on the catalytic activity of Co10.0Ce at 92C. T. Xu et al. / Sensors and Actuators B 133 (2008) 707773 Fig. 5. Effect of the tablet compression pressure on the catalytic activity of Co10.0Ce at 92C. Fig. 6. Effect of calcination temperature on the catalytic activity of Co10.0Ce at 92C. 3.1.4. Effect of calcination temperature The formed Co10.0Ce oxides were calcined at different tem- peratures. Fig. 6 shows the infl uence of calcination temperature on the catalytic activity of the Co10.0Ce. At 250C, Co10.0Ce showed the lowest activity. The ?T displayed no obvious dif- ference when calcination temperature ranged from 300C to 500C. It indicated that between 300C and 500C, the cal- cination temperature imposed little infl uence on the catalytic activity. This phenomenon was caused by the addition of Ce in the Co oxide, which improved the thermal stability of the oxide by avoiding their sintering process 13. The Co10.0Ce oxide exhibited the highest ?T of 47.5C at 400C. Therefore, the optimal calcination temperature was chosen at 400C. 3.1.5. Effect of catalyst temperature The catalytic activity of the Co10.0Ce oxide was studied at different operating temperatures. Fig. 7 shows the ?T variation overtheCo10.0Cesample.Noactivitywasobservedatroomtem- perature, but increasing the operating temperature from 50C to 90C resulted in a considerable enhancement of catalytic activity. Afterwards, the ?T kept constant when the operating temperature was raised over 90C, indicating that the catalytic activity of the Co10.0Ce sample exhibits no obvious change at the operating temperatures above 90C. The mechanism of CO oxidation over Co oxide has been proved 6,17, which indicated that CO fi rst adsorbs on the sur- face of cobalt oxide, afterwards the adsorbed CO reacts with a lattice oxygen atom forming CO2, and CO2can adsorb on the surface and form surface carbonate species. At ambient tem- perature, the carbonaceous species cannot desorb, resulting in the blocking of active sites and deactivation of the catalyst. The CO2concentration and the conversion of CO increase with an increase in temperature 17, implying that the desorption of CO2can be improved by increasing the temperature. In this way, the reduced sites created by the oxygen vacancies lead to decreased oxidation state of the Co oxide. Consequently, the catalytic activity keeps unchanged even though the operating temperature increases. 3.2. Characterization 3.2.1. BET surface Specifi c surface area and pore size distribution of Co10.0Ce and Co oxides were characterized by the BET method. Fig. 8 exhibits the pore size distribution of the two oxides. From the BET analysis, the average pore diameter of Co10.0Ce and Fig. 7. Response of the Co10.0Ce oxide at (a) different temperatures and (b) operating time. 74T. Xu et al. / Sensors and Actuators B 133 (2008) 7077 Fig. 8. Pore size distributions of Co10.0Ce and Co oxides. Co oxides was 17.1nm and 20.2nm, respectively. Meanwhile, it was observed that the pore size of the two oxides var- ied from each other in the range of 1050nm, and the pore volume of Co10.0Ce oxide was smaller than Co oxide. The BET surface area of Co10.0Ce and Co oxides was found as 85.7m2/g and 89.9m2/g, which indicated that the addition of Ce resulted in little change of BET surface in our experi- ment. 3.2.2. XRD spectra XRD analysis was employed to fi nd the crystal structure of Co10.0Ce oxide. Pure Co oxide was also investigated for com- parison. As can be seen in Fig. 9, only Co3O4spinel structure wasdetectedinpureCooxide,whilediffractionpeaksofCo3O4 and CeO2were observed in the Co10.0Ce oxide. Meanwhile, the Co10.0Ceoxideexhibitedlowercrystallinitythanthepureoxide. The crystallite sizes were calculated by Scherer equation; the Co10.0Ce oxide showed a low crystallite size of 7.2nm, while a high value of 21.4nm was observed for Co3O4. It indicated that the crystallites of Co3O4were greatly minimized by the addition of CeO2. In the Co10.0Ce oxide, the presence of the dispersed CeO2could improve the thermal stability of Co3O4, which is also confi rmed by Xue et al. 13. Thus, small crystal- lites and excellent catalytic activity are obtained with Co10.0Ce oxide. Fig. 9. XRD spectra of Co and Co10.0Ce oxides calcined at 400C. 3.2.3. SEM analysis Fig. 10 showed SEM images of Co3O4and Co10.0Ce oxide. TheapparentuniformsphericalparticlescanbeseenintheSEM of Co3O4. However, for the Co10.0Ce oxide, the average parti- cle size was much smaller than that of Co3O4. The particles of Co3O4were greatly minimized by the addition of CeO2. There- fore, the presence of highly dispersed CeO2in the Co10.0Ce oxidecouldimprovethethermalstabilityofCo3O4.Thus,small particles are obtained. 3.2.4. H2-TPR analysis In order to fi nd the reduction behavior, TPR analysis was performed on both Co3O4and Co10.0Ce oxides in the tem- perature range from RT to 900C. As shown in Fig. 11, two peaks appeared in the TPR profi le of Co3O4 . The fi rst peak (I) centered at 271C is resulted from the reduction of Co3O4 to CoO (Co3O4+H23CoO+H2O), and the second peak (II) at 320376C is due to the reduction of CoO to metallic Co (3CoO+3H23Co+3H2O) 13,18. The corresponding chemical reaction indicated that the H2consumption of II/I was 3:1 theoretically. The peak area ratio of II/I for Co3O4in Fig. 11(a) is consistent with the theoretical calculation. Fig. 10. SEM images of (a) Co3O4and (b) Co10.0Ce. T. Xu et al. / Sensors and Actuators B 133 (2008) 707775 Fig. 11. H2-TPR spectra of (a) Co3O4and (b) Co10.0Ce. As seen in Fig. 11(b), the TPR profi le of Co10.0Ce oxide showed four peaks at 177C, 286C, 333C and 567C. The fi rst and second peak appearing in Fig. 11(b) correspond to the reduction of Co3O4in Fig. 11(a), but with a shift to low temper- ature by 6090C. Thus, a proper amount of Ce could improve thereductionabilityofCo3O4 ,andthisresultisalsoaffi