DSpace at VNU: Kinetics of CO Oxidation over Pt-Modified CuO Nanocatalysts

Materials Transactions, Vol 56, No (2015) pp 1403 to 1407 Special Issue on Nanostructured Functional Materials and Their Applications © 2015 The Japan Institute of Metals and Materials Kinetics of CO Oxidation over Pt-Modified CuO Nanocatalysts Luu C Loc1,+, Nguyen Tri2, Hoang T Cuong1 and Ha C Anh2 Institute of Chemical Technology, Vietnam Academy of Science and Technology, 01 Mac Dinh Chi Str., Ho Chi Minh City, 70100 Vietnam Ho Chi Minh City University of Technology, Vietnam National University - Ho Chi Minh City, 268 Ly Thuong Kiet Str., Ho Chi Minh City, 70100 Vietnam Three Pt-CuO nanocatalysts PtCu/Al, PtCu/CeAl and PtCu/Ce have been successfully prepared The characterization of the catalysts was examined by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray energy dispersive analysis (EDS), temperatureprogrammed reduction (TPR), nitrogen physisorption measurements, and IR-CO adsorption The kinetics of CO oxidation using these catalysts was studied in a gradientless flow-circulating system at 398­498 K The obtained kinetic equation confirmed that the reaction proceeds in medium surface coverage with the participation of CO molecules and oxygen atoms [doi:10.2320/matertrans.MA201545] (Received January 30, 2015; Accepted May 29, 2015; Published August 25, 2015) Keywords: carbon monoxide oxidation, kinetics, platinum-modified copper oxide nanocatalysts Introduction The advantage of low-temperature oxidation is to reduce fuel consumption for conversion of large volume of polluted air Metal oxides and multioxide owning high activity and thermal stability are considered as alternative catalyst for the existing expensive noble metals In fact, promising results were obtained by adding a small amount of noble metals to metal oxide catalysts Particularly, the highest activity in oxidation of CuO/CeO2 catalysts modified with Pt is rationalized by the strong link between the Pt with CuO/ CeO2.1) The synergic effects between metal oxides and noble metals results in the increase of reducibility, which may enhance the oxygen transfer from the metallic oxides to the noble metals.2) Research on kinetics of oxidation of single CO on noble metal and oxide catalysts have intensively studied.3­11) Among various forms of suggested kinetic equations for the oxidation of CO, power-law kinetic expressions were repeatedly proposed Indeed, first-order of oxygen and zeroorder of CO concentrations for CO oxidation on bulk copper oxide were reported by Garner et al.3) In contract, over a silica-supported copper oxide catalyst, first-order of CO and zero-order of oxygen concentrations were observed and EleyRideal mechanism was proposed.4) In addition, a power-law rate equation was found to satisfactorily fit the experimental data of carbon monoxide oxidation with CO at partial pressure ranging from 0.0015 to 0.0125 atm over CuO supported on nanosized CeO2.5) Kinetics of CO oxidation in the CO-PROX process (H2-rich gases) has been investigated in a fixed-bed reactor by Caputo et al.6) and a power-law rate equation was found On the basis of Langmuir-Hinshelwood mechanism, the following expression for the reaction rate of CO oxidation on CuO/£-Al2O3 was proposed by Vannice et al.:7) p KCO PCO KO2 PO2 p rẳk 1ị ỵ KCO PCO ỵ KO2 PO2 ị2 where PCO, PO2 are the partial pressures of CO and O2; KCO + Corresponding author, E-mail: lcloc@ict.vast.vn and KO2 -adsorption coefficients for CO and O species, respectively However, order of O2 pressure was found to be near zero and the equation became power-law kinetic expressions:7) r ẳ kKCO PCO ị0:7 2ị 8,9) oxide catalysts 10 mass% In our previous publications, CuO/£-Al2O3 (Cu/Al), 10 mass% CuO/20 mass% CeO2 + £-Al2O3 (Cu/CeAl) have been reported to be the most active and stable catalysts in the complete oxidation of CO Furthermore, high active catalysts at low temperature reaction was obtained when Pt of 0.1 mass% was introduced to CuO catalysts.10) The kinetics of deep oxidation of CO8) and p-xylene and its mixtures with CO9) over Cu/Al and Cu/ CeAl have been investigated at the temperature range of 473­ 543 K The following rate equations were achieved for deep oxidation of sole CO:8) kCO PCO PO0:52 ỵ k1 PO0:52 ỵ k3 PCO2 ị 3ị kxyl Pxyl PO0:52 PO0:52 ỵ k3 PCO2 ỵ k4 Pxyl ỵ k5 PH2 O ị 4ị rCO ẳ and p-xylene:9) rxyl ẳ It has been revealed that a complicated mutual effect associated with the formation of new intermediates takes place in the simultaneous oxidation of CO and p-xylene to change the reaction kinetics The following rate equations were obtained for deep oxidation of CO and p-xylene in their mixture on the Cu/CeAl catalyst:9) ki Pi PO0:52 ri ẳ ỵ k1 PO0:52 ỵ k2 PCO ỵ k3 PCO2 ỵ k4 Pxyl ỵ k5 PH2 O Þ kÃÃ i PCO Pxyl À ð5Þ Ã ỵ k2 PCO ị1 ỵ k6 PCO Pxyl ị where ri* ­ reaction rate of CO or p-xylene oxidation in their mixture; ki*, ki** ­ constants; Pi ­ partial pressure of i components; i-CO or p-xylene The aim of this study is to establish the kinetics of CO oxidation using Pt-modified CuO catalysts 1404 L C Loc, N Tri, H T Cuong and H C Anh Experimental Procedure Two catalysts: 0.1 mass% Pt + 10 mass% CuO/£-Al2O3 (PtCu/Al) and 0.1 mass% Pt + 10 mass% CuO/(20 mass% CeO2 + 69.9 mass% £-Al2O3) (PtCu/CeAl) have been prepared by sequential impregnations as described in our previous works.10,12) Catalyst 0.1 mass% Pt + 7.5 mass% CuO/CeO2 (PtCu/Ce) was prepared by the urea nitrates combustion method described by H Matralis et al.11) with molar ratios of urea/nitrate = 4.17 Ce(NO3)3·6H2O, Cu(NO3)2·3H2O, H2PtCl6·6H2O complex, urea (CO(NH2)2) and £-Al2O3 were purchased from Merck All of the precursors were used without further purification The IR spectra were recorded from apparatus Nicolet ­ Spectrometer 460 in the range of 4000­400 cm¹1 with a resolution of cm¹1 The catalyst samples were pretreated in a pure oxygen flow of velocity L/h for h at 873 K for metal oxide catalyst and at 573 K for Pt containing sample The kinetics of CO oxidation was studied in a gradientless flowcirculating system at 398­498 K Ranges of initial partial pressures of CO, O2 and CO2 were 2.5­20, 35­140 and 0­ 25(hPa), respectively The follow gas have been used: O2 (99.999%); N2 (99.999%); Air (21 mol% O2 + 79 mol% N2); mixture CO (6 mol%) + N2 (94 mol%); and mixture CO2 (6 mol%) + N2 (94 mol%) 3.1 Results and Discussions Physico-chemical characteristics and activity of the obtained catalysts Physico-chemical characteristics of catalysts were previously reported.12) Briefly, from XRD pattern of catalysts (Fig 1), alumina exists in an amorphous state, cerium oxide exists in crystalline state of cubic fluorite structured CeO2 (2ª = 28.6°, 33°, 47.4°, 56.3°, 59°, 69.4°, 76.7° and 79°).13) The significantly weak intensity CeO2 peaks in PtCu/CeAl catalyst indicated that the interaction of CeO2 with Al2O3 resulted in cerium oxide to be crystallized in small agglomerate The XRD patterns of PtCu/CeAl and PtCu/ Ce showed very weak CuO reflections This can be explained by the existing of copper oxide phase in a highly divided or amorphous state on the surface of ceria or the formation of solid solution.11) At the same time on PtCu/Al catalyst, copper oxide exits in a state of good crystalline In all samples the characteristic peaks of Pt with low intensity were observed From Table and TEM image (Fig 2), platinum exists in fine dispersed state with particle size of 1­3 nm The EDS image of PtCu/Ce catalyst (Fig 3(a)) show that Cu and Cr are fairly evenly distributed on the surface of CeO2 On the surface of PtCu/CeAl catalyst (Fig 3(b)), the different regions of metal particles distribution can be observed; Pt and Cu are concentrated more on CeO2 than on £-Al2O3 TPR diagram on all CuO-based catalysts modified by Pt, showed only the peak of CuO reduction while the characteristic peak of Pt did not appear, probably due to its very low concentration.12) Thus, in comparison with non-Pt modified catalysts8) the addition of Pt did not change the character in XRD pattern of the sample Cu/Al Instead, it enhanced the reducibility of catalysts by decreasing reduction temperature and increasing reduction extent KRed (Table 1), further Fig XRD patterns of catalysts: (1) PtCu/Al; (2) PtCu/CeAl; (3) PtCu/ Ce (Pt-Pt; Cu-CuO; Al-Al2O3; CuAl-CuAl2O4; Ce-CeO2) Table The values of surface specific area (SBET), crystal size of CeO2 at 2ª = 28.6° (dCeO2 ) and CuO at 2ª = 35° (dCuO), particle size of Pt determined from TEM imagine (dPt), maximum reduction temperature (Tmax), reduction extent (KRed) and temperatures for 50% conversion of CO (T50) of the catalysts Catalysts SBET, m2/g dCeO2 , dCuO, nm nm dPt, nm Tmax, K Kred, % T50, K PtCu/Al 95.9 ® 18.8 1­3 547, 673 36.7 438 PtCu/Ce 14.8 11.8 n.d 1­3 457, 487, 818, 960 32.2 358 PtCu/CeAl 80.1 7.1 n.d ¯1 528 45.8 362 n.d: Not detected enhancing the activity of catalysts When 0.1 mass% of Pt was introduced to the Cu/Al catalyst, the temperature for 50%-conversion of CO reduced from 498 K to 438 K, and the temperature for 100%-conversion of CO reduced from 573 K to 548 K Similarly, the PtCu/CeAl catalyst was capable of converting 50% CO at 362 K and 100% CO even at 383 K (15 K lower than that for Cu/CeAl catalyst) It has been shown in Table 1, compare to PtCu/Al catalyst, the CeO2-contained catalysts (PtCu/CeAl and PtCu/Ce) offered much higher activity in CO oxidation, the temperature for 50%-conversion of CO was as low as 358­462 K (80 K lower than that of the catalysts without CeO2) The results might come from the fact that in catalysts containing CeO2 the copper oxide exists in a highly divided or amorphous state 3.2 Kinetics of CO oxidation over the obtained catalysts The Arrhenius plot of CO oxidation rate (rCO), log rCO versus 1/T is nonlinear, showing that the reaction rate obeyed a fractional rational equation rather than a power law one (Fig 4) The dependence of reaction rate upon CO partial pressure for all the catalysts was nearly linear (Fig 5) Kinetics of CO Oxidation over Pt-Modified CuO Nanocatalysts (a) (a) (b) (b) 1405 (c) Fig EDS images of the catalysts: (a) PtCu/Ce (Color: Ce-blue; Cugreen; Pt-red); (b) PtCu/CeAl (Color: Ce-blue; Al-green; Pt-red; Cupink) Fig TEM images of the catalysts: (a) PtCu/Al; (b) PtCu/CeAl; (c) PtCu/Ce Therefore, it is conclusive that CO pressure appears in the numerator of kinetic equation in first power The convex form of dependence of reaction rate versus O2 partial pressure in Fig indicates that oxygen concentration appeared in both the numerator and denominator of kinetic equation The concave shape of the conversion curves, rCO versus CO conversion (XCO), revealed that the reaction was inhibited by at least one of the products.14) The dependence of 1/rCO vs PCO2 is linear (Fig 7), meaning that PCO2 appeared in the denominator of kinetic equation in power of unit Thus, the reaction rate in general form should be described by the following equation: n1 n2 kCO PCO PO rCO ¼ 6ị m3 2Ă m1 m2 ỵ k1 PO2 ỵ k2 PCO ỵ k3 PCO ị Where: kCO, k1, k2, k3 - constants of kinetic equation; 2¡ surface coverage; PCO, PO2 , PCO2 - partial pressures of CO, O2 and CO2, respectively The optimal coincidence between Fig The Arrhenius plot of CO oxidation rate, log rCO versus 1/T, over the catalysts: (1) PtCu/Al; (2) PtCu/CeAl and (3) PtCu/Ce at XCO = 0.4; PCO = hPa; PO2 ¼ 104 hPa; PCO2 ¼ hPa experimental and calculated results has been observed when n1 = m2 = m3 = 1; n2 = m1 = 0.5; ¡ = 0.5, k2 = and reaction rate is described in form of eq (3) The values of the kinetic constants of eq (3) were given in Table The error of the calculation of the reaction rates via eq (3) was 19­22% Results in Table showed that the remarkable higher value of kCO was obtained on CeO2contained catalysts referring to high catalytic activity In comparison with non-Pt modified catalysts,8) the addition of Pt does not change the form of kinetic equation and expression (3) is the common equation for CO oxidation 1406 L C Loc, N Tri, H T Cuong and H C Anh Table Catalysts ¹1 ¹1 ¹1.5 kCO, mol·g ·h ·hPa k1, hPa¹0.5 k3, hPa¹1 Variance, % The values of the kinetic constants in the eq (3) PtCu/Al PtCu/Ce PtCu/CeAl 4.2 â exp(ạ1421/RT) 10ạ7 â exp(13846/RT) 1.1 â 10 â exp(ạ1496/RT) â 10ạ3 â exp(7028/RT) 9.7 â 102 â exp(ạ1139/RT) 6.9 â 10ạ8 â exp(16130/RT) â 10ạ16 â exp(30302/RT) 0.2 â 10ạ3 â exp(8756/RT) 1.7 â exp(1323/RT) 22 19 21 R = 1.987 cal.mol¹1.K¹1; ki = k0i â exp(ạEi/RT); Ei (cal.molạ1) (a) (b) Fig Rate of CO oxidation (rCO) versus the partial pressure of CO (PCO) over the catalysts: (1) PtCu/Al; (2) PtCu/CeAl and (3) PtCu/Ce at T = 448 K; XCO = 0.4; PO2 ¼ 104 hPa; PCO2 ¼ hPa Fig IR spectra of CO adsorption on the catalysts: (a) Cu/Al and (b) PtCu/Al Fig Rate of CO oxidation (rCO) versus the partial pressure of O2 (PO2 ) for catalysts: (1) PtCu/Al; (2) PtCu/Ce; and (3) PtCu/CeAl at T = 448 K; XCO = 0.4; PCO = Pa; PCO2 ¼ hPa on CuO-based catalysts under tested conditions However, it reduced activation energy of the reaction (reflected in the decrease of the value of ECO), subsequently increased the activity of Pt containing catalysts and lowered the temperature region of reaction Moreover, on the Pt-modified catalysts, value oxygen adsorption constants (k2) is much lower than that of non-modified catalyst,8) indicating the relatively strong adsorption of CO It is likely that the presence of Pt make oxidation degree of copper becomes lower Indeed, IR spectra of CO-adsorption showed characteristic bands Cu+­CO on PtCu/Al sample slightly shifted to the shorter wavenumber region as seen on the Cu/Al sample (2123 cm¹1 compared to 2125 cm¹1) (Fig 8) It has been demonstrated that shifts in CO band frequencies have often been related to the change in exposed Cu surface planes.15) Fig The dependence of reversed values of reaction rate (1/rCO) on partial pressure of CO2 (PCO2 ) for catalysts: (1) PtCu/Al; (2) PtCu/Ce; and (3) PtCu/CeAl at T = 448 K; XCO = 0.4; PCO = hPa; PO2 ¼ 104 hPa Conclusion Kinetic studies of CO oxidation over three Pt-modified CuO nanocatalysts were performed The experimental results provided that characteristics of the carriers and Pt additive affected the properties, activity, and adsorption capacity of catalysts However, form of the kinetic equation was kept intact On these catalysts the reaction proceeds in the average coverage with participation of CO molecules and oxygen Kinetics of CO Oxidation over Pt-Modified CuO Nanocatalysts atoms Furthermore, CeO2 depressed the formation of massive CuO leading to the increase in catalyst reduction and reaction rate Addition of 0.1 mass% Pt decreased the activation energy of the reaction and increased CO adsorption, leading increased the activity of CuO-based catalysts Acknowledgments This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grand No 104.03-2012.60 REFERENCES 1) C R Jung, A Kundu, S W Nam and H.-I Lee: Appl Catal A 331 (2007) 112­120 2) M Ferrandon: Ph.D Dissertation, Royal Institute of Technology, Stockholm, (2001) 3) W E Garner, F S Stone and P F Tiley: Proc R Soc A 221 (1952) 472­489 1407 4) R A Prokopowicz, P L Silveston, R R Hudgins and D E Irish: React Kinet Catal Lett 37 (1988) 63­70 5) J L Ayastuy, A Gurbani, M P González-Marcos and M A GutiérrezOrtiz: Ind Eng Chem Res 48 (2009) 5633­5641 6) T Caputo, L Lisi, R Pirone and G Russo: Ind Eng Chem Res 46 (2007) 6793­6800 7) K I Choi and M A Vannice: J Catal 131 (1991) 22­35 8) L C Loc, H T Cuong, N Tri and H S Thoang: J Exper Nanosci (2011) 631­640 9) L C Loc, N Tri, H T Cuong, H S Thoang, Y A Agafonov, N A Gaidai, N V Nekrasov and A L Lapidus: Kinet Catal 55 (2014) 611­ 619 10) L C Loc, D T T Mai, N Tri, H T Cuong, B T Huong and H S Thoang: J Chem 48 (2010) 84­89 (in Vietnamese) 11) G Avgouropoulos, T Ioannides and H Matralis: App Catal B: Env 56 (2005) 87­93 12) L C Loc, N Tri, H T Cuong, H M Nam and H C Anh: Adv Nat Sci.: Nanosci Nanotechnol (2015) doi:10.1088/2043-6262/6/1/ 015011 13) H I Chen and H Y Chang: Solid State Commun 133 (2005) 593­ 598 14) S G Bashkirova and S L Kiperman: Kinet i Katal 11 (1970) 631­ 637 (in Russian) 15) N Y Topsoe and H Topsoe: Topics Catal (1999) 267­270  ... the average coverage with participation of CO molecules and oxygen Kinetics of CO Oxidation over Pt-Modified CuO Nanocatalysts atoms Furthermore, CeO2 depressed the formation of massive CuO leading... equation: n1 n2 kCO PCO PO rCO ẳ 6ị m3 2Ă m1 m2 ỵ k1 PO2 ỵ k2 PCO ỵ k3 PCO Þ Where: kCO, k1, k2, k3 - constants of kinetic equation; 2¡ surface coverage; PCO, PO2 , PCO2 - partial pressures of CO, ... the numerator and denominator of kinetic equation The concave shape of the conversion curves, rCO versus CO conversion (XCO), revealed that the reaction was inhibited by at least one of the products.14)