Synthesis and catalytic properties of perovskite-related phases in the La-Sr-Co-Cu-Ru-0 system Yasutake Teraoka," Hiroshi Nii," Shuichi Kagawa," Kjell Janssonb and Mats Nygrenb "Departmentof Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan bDepartment of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden A series of oxides of the composition La,~,Sr,~,Co, -,,Cu,Ru,O, with the perovskite structure were synthesized and characterized by their X-ray diffraction (XRD) patterns and by element analysis in a transmission electron microscope (TEM) equipped with an energy-dispersive spectrometer (EDS). The XRD and TEM-EDS studies confirmed that almost monophasic samples with the perovskite structure having rhombohedral (hexagonal) symmetry were obtained for 0<y <0.20, although the simultaneous formation of a by product with the K,NiF,-type structure was observed for high y values.The lattice parameters and rhombohedral distortion increased monotonically with increasing y value due to the fact that the average ionic radius of the substituting Cu/Ru pair is larger than that of the host Co ion. Temperature-programmed desorption (TPD) studies showed that the substitution of the Cu/Ru pair for Co caused a decrease in the amount of oxygen desorbed from the bulk of the oxide, and the catalytic activity for CO oxidation and CO-NO reactions decreased. The Cu/Ru substitution did, however, selectivity increase the N, formation and decreased that of N20 in the CO-NO reactions.Synthesis of K,NiF,-type oxides with the overall compositions (Lao.8Sro~2),Coo~50Cuo~50~zRuz04with z =0.05 and 0.10 was also attempted. In both cases, tetragonal K2NiF4-type oxides having a composition close to (Lao,8Sr,~,)2(Co,Cu)o~97R~o~0304were obtained, implying that the solubility of Ru into the given K2NiF4-type framework is very low. Perovskite-type oxides (ABO,) are an important class of compounds in the realm of metal oxides, and they have been extensively studied with respect to their synthesis, structure and various physical and chemical properties.'.2 If the basic criteria of charge neutrality and geometric (ionic size) factors are satisfied, one can prepare many ternary or multicomponent oxides with the perovskite-type structure.Proper combinations of metal cations can stabilize cation or anion vacancies in the perovskite framework and anomalous formal oxidation states of metal cations can be obtained. These structurally versatile features offer a wide range of possibilities, not only in designing new perovskite compounds but also in tailoring their chemical and physical properties. In catalytic applications, ternary perovskite-type oxides of the composition ABO,, and quaternary ones such as A,-,A,'BO,, are most frequently although a few articles dealing with quinary A, -,Ax'Bl -,BY'03 perov-skite systems have been p~blished.~.~ Very recently, Skoglundh et aL8 reported the catalytic properties of a six-component perovskite catalyst of an entirely new composition, La, -,SrXAll -,,Cu,Ru,O,.This compound is reported to be active for the catalytic conversion of NO and CO to N, and CO,, respectively, and might be used as an active ingredient of a three-way catalyst (TWC) for automobiles. This result has triggered the present investigation concerning the syn- thesis and catalytic properties of the Co-based analogue, La,~,Sr,~,Co, -,,Cu,Ru,O,, partly because Co-based perov- skites are generally more catalytically active than Al-based ones. We have also attempted to synthesize K2NiF4- type (K-type) oxides of the nominal composition (La0.8Sr0.2)2C00.5Cu0.5 -zRUzo4* Experimenta1 Synthesis Perovskite-type and K-type compounds of compositions La0.,Sr0.,Co, -,,Cu,Ru,O,, with 0 dy d0.20 at intervals of -zR~z04,0.05, and (Lao.8Sro~2)2Coo~5Cuo~5 with z =0.05 and 0.10, were synthesized from p.u.grade La(NO,), 6H20, Sr(N03),, C0(N03), *6H20, CU(NO,)~ .3H20 and RuCl,. Appropiate proportions of the starting chemicals were dis-solved in water and this solution was evaporated to dryness at 120"C, and then heat-treated at 550°C for several hours until evolution of brown NO, gas was no longer observed. The resulting powder was ground, pelletized and then calcined at 900°C for 24 h, at 1000°C for 24 h and at 1100°C for 24 h. Perovskite-type oxides of the formula Lao,8Sro.2-Co, ~,,Cu,Ru,O, are hereafter designated as CCRlOOy, i.e. Lao~8Sro~2Coo~9Cuo~05Ruo~0503is abbreviated as CCR5. Considering the charge neutrality and variable valencies of the B-site cations, it is easily recognized that both the perov- skite-type and K-type oxides may have non-stoichiometric compositions. In this article, however, nominal formulae with the stoichiometric composition of oxygen are used, because the exact oxygen content has not yet been determined. Characterization X-Ray powder diffraction (XRD) patterns were recorded in a Guinier-Hagg focusing camera, using Cu-Kal radiation and Si as an internal standard.The obtained films were evaluated with a computerized scan system.' A portion of each sample was ground in butanol and then dispersed on a holey carbon film supported on an Ni grid, and analysed in a transmission electron microscope (TEM; JEOL 2000FX) equipped with an energy-dispersive spectrometer (EDS; LINK AN10000).The metal content of the samples was determined by the spot meas- uring technique and at least ten individual grains were ana- lysed. The obtained mean values of x and y in La, -,Sr,Co, -,,Cu,Ru,0, were within f0.01, in agreement with the weighed-in perovskite compositions. Oxygen desorption and catalytic activity La,.8Sr,.2Col -2,Cu,Ru,03 samples with y =0, 0.05 and 0.10 synthesized at 900°C for 48 h were used for these measurements. Temperature-programmed desorption (TPD) studies of oxygen desorption were performed as follows. (i) After being J. Muter. Chem., 1996, 6(l),97-102 granulated to 42-80 mesh sized particles and mounted in a quartz-glass reactor, each sample (0.5 g) was exposed to vacuum for 30 min and then to 13 kPa of oxygen for 30 min at 800"C, and allowed to cool to room temperature in the same atmosphere.(ii) Then, after changing the atmosphere to helium with a stream rate of 30 cm3 min-', the samples were heated at a constant rate of 10°C min-', and the oxygen release was monitored with a thermal conductivity detector (TCD). The CO oxidation and CO-NO reaction studies were carried out in a fixed-bed flow reactor, over 0.2 g of catalyst granules (20-60 mesh) fed at a rate of 30 cm3 min-' with the following gases: (i) CO (0.5 ~01%) and 0, (5%) balanced with He; (ii) CO (0.5%) and NO (0.5%) balanced with He. After reaching steady-state conditions at each temperature, the gas composition before and after passing through the catalyst bed was analysed by a TCD-type gas chromatograph (Shimadzu GC-8A) with a Porapak Q column for separating CO, and N,O and a 5A molecular sieve column for separating O,, N,, NO and CO.Results and Discussion Preparation and crystal structure of La,~,Sr,~,Co,-~,,Cu,,Ru,,03 La,~,Sr,~,Co, -,,Cu,Ru,O, samples with 0 <y d0.20 were syn- thesized at 900, 1000 and 1100°C. When calcined at 900°C, monophasic samples were obtained, but the diffraction peaks were too diffuse to allow accurate determinations of the lattice parameters. As the calcination temperature increased, the peaks became sharper and better resolved, most probably due to an increase in crystallinity and/or crystallite size.At 1100 "C a small additional peak assignable to a phase of the composition with the K,NiF,-type structure was observed. Typical XRD patterns of La,,8Sr,.,Col -,,CuyRu,03 prepared at 900, 1000 and 1100°C are shown in Fig. 1. The EDS spot analysis of individual grains of samples prepared at 1100 "C revealed the occurrence of both perovskite and K-type grains, but the observed composition of the great majority of the grains agreed well with the weighed-in perovskite composition. This indi- cates that almost monophasic samples of the composition Lao$r,~,Col -z,Cu,Ru,03 (0dy <0.20) were formed. The amount of K-type phase tended to increase with increasing y value. In these perovskite- and K-type oxides, the oxidation states of the B cations are basically trivalent and divalent, respectively. As the normal valence state of Cu in these types of oxides is 2+, an increasing amount of IS-type phase with increasing overall content of Cu is expected. The structural considerations outlined below are based on samples calcined at 1100 "C, which yielded sharp and well resolved diffraction peaks; this in turn allowed accurate determination of the lattice parameters.The samples calcined at 900°C were those used for oxygen desorption and catalytic measurements, because calcination at higher temperatures reduces the specific surface area. The XRD patterns of La,.8Sro.,Col -zyCu,Ru,O, (0<y <0.20) were satisfactorily indexed on the basis of rhombohedral (hexagonal) unit cells similar to the ones reported for LaCo03" and Lao$ro.lCo031' (see also Table 1).Attempts were made to synthesize perovskite-based oxides with this type of hexagonal unit cell for y=0.3, 0.4 and 0.5, but their XRD patterns could not be indexed on this basis. The main phases found Ft y=O.3 and 0.4 could be indextd with cubic [a: 3.9144( 6) A] and vrthorhombic [a=5.588(1)A, b =5.5995(9) A and c =7.862( 1) A] unit cells, respectively, and the product with y=O.5 yielded a complex diffractrogram. It is thus concluded that it is possible to replace Co with Cu/Ru up to a y value of 0.2 in the rhombohedral unit cell of Lao.,Sro.,Co,- ,,CuyRu,O,. The lattice parameters of Lao.,Sr,,,Col -z,Cu,Ru,03 98 J. Muter. Chem., 1996, 6(l), 97-102 I I I I I I 20 25 30 35 40 45 28(Cu-KaJdegrees Fig.1 XRD patterns of Lao~,Sro.zCoo,,Cuo,lRuo,103calcined at (a) 900°C for 48 h, (b) 1000°C for 24 h, and (c) 1100°C for 24 h. The indices are based on the hexagonal unit cell; 0,Si (internal standard) and V,K,NiF,-type oxide. Table 1 XRD data for Lao.8Sro.2Coo,8Cyo.1Ruo.103synthesized at 1100"C [aH =5.4755(4)A, CH =13.217( 1) A] 1 0 2 3.8501 3.8527 12 1 1 0 2.7374 2.7377 100 1 0 4 2.7112 2.71 10 95 1 1 3 2.3251 2.23 17 1 2 0 2 2.2309 2.2028 20 0 0 6 2.2025 2.2028 6 2 0 4 1.9265 1.9263 66 2 1 2 1.7289 1.7298 3 1 1 6 1.7159 1.7163 3 3 0 0 1.5805 1.5806 17 2 1 4 1.5757 1.5754 42 1 0 8 1.5602 1.5602 15 2 2 0 1.3691 1.3689 15 2 0 8 1.3558 1.3555 11 3 1 4 1.2222 1.2219 10 2 1 8 1.2151 1.2149 11 (0<y G0.20) are given in Table 2. As seen in Fig.2, the hexagonal unit cell axes and volume increase monotonically with increasing y value. The formal oxidation states of the Co, Cu and Ru ions in the present system are not known (see also below) but the normal oxidation states of these ions in similar perovskite- based structures are three, two and four or three, two and five, Table 2 Lattice parameters of La,,,,Sr,,,,Co, -,,Cu,Ru,03 sythesized at 1100"C lattice parameters' sample y value aH/A cH /A vH/A3 a,/degrees CCRO 0 5.4457(2) 13.161 5( 8) 338.54 5.403 60.53 CCR5 0.05 5.4595(4) 13.192( 2) 340.54 5.410 60.60 CCRlO 0.10 5.47554 4) 13.217( 1) 343.19 5.423 60.65 CCR15 0.15 5.4911(6) 13.247( 2) 345.94 5.436 60.67 CCR20 0.20 5.5 106 (6) 13.281( 1) 349.31 5.452 60.72 aH,cH and VH are the lattice parameters and cell volume of the hexagonal unit cell and a, and Q, are the rhombohedral cell edge and angle, respectively. 13.161.~ I I I I~ 5.52 5.50 5.48 5.46 5.44 0.00 0.05 0.10 0.15 0.20 Y in La0.8Sr0.2c01-2$UfiUp3 Fig.2 Hexagonal lattice parameters as functions of y in ~a,~,~r,,,~o,~,,~u,~u,~,.(a) a axis, (b) c axis, (c) unit-cell volume. respectively. The most stable oxidation numbers of the Co, Cu and Ru ions are, however, three, two and four, respectively and octahedrally coordinate? Co3+, Cu2+ and Ru4+ ions have radii of 0.61, 0.73 and 0.62 A, respectively.'2 This implies that the increase in the lattice parameters can be ascribed to the fact that the average ionic radius of the substituent pair, Cu/Ru, is larger than that of the host Co ion as long as both Cu and Ru ions are incorporated in the perovskite framework (see also below).The rhombohedral distortion, i.e. the deviation from 60" of the rhombohedral angle, aR,increases with increasing y value (see Fig. 3).The rhombohedral distortion parameter, Y (in %), is defined as: y= loo{ 1-(~H/UH,/~)} (1) where uH and cHare the lattice parameters of the hexagonal unit cell. The Y value is zero for perovskites with cubic symmetry but is >0 for distorted perovskite framework struc- tures.As shown in Fig. 3, the Y values display the same tendency to increase with increasing y value as the aR values do. The tolerance factor, t, is commonly used as a measure of the distortion or deformation of the perovskite structure and is defined as: t= {rav(A)+r(O)}/,/2 x {rav(B)+r(O)I (2) where r(0) is the ionic radius of oxygen and rav(A) and rav(B) 60.8 1 60.7 g1 2 A 0Q,zv b F 1 60.6 1.o 60.5 0.00 0.05 0.10 0.15 0.20 Y in La0.8Sr0.2Col-2fiufiuf13 Fig. 3 The rhombohedral angle, a,, and distortion parameter, Y, plotted as functions of y in Lao.8Sro.zCo, -,,Cu,Ru,O, (Y and aR are defined in the text) are the average ionic radii of the A-site and B-site cations, respectively. For the ideal cubic structure t=1, decreasing as the crystal lattice becomes more di~torted,'~ and as a rule the tolerance factor is 0.8dtd1.0. In the present oxide system, r(0) and rav(A) are constant, and rav(B) increases with increas- ing y value, implying that the tvalue decreases simultaneously. In this case it is difficult to obtain realistic tvalues, however, as the true oxidation states of the B ions are not known.The variation of the cell parameters and degree of lattice distortion with composition can be satisfactorily explained by the replace- ment of smaller host Co ions by larger Cu/Ru pairs. Oxygen desorption behaviour Oxygen desorption from Co-based perovskites samples has been investigated e~tensively,'~-~~ and it has been revealed that the oxygen desorption process yields information about the valence states of metal cations incorporated in the perov- skite structure and about the catalytic properties of the com- pounds.The TPD chromatograms for oxygen release from the samples Lao,8Sro,,Co, -zyCuyRuy03( y =0, 0.05 and 0.1) are given in Fig. 4, where the recorder response, which is pro- portional to the rate of oxygen desorption, is plotted us. the temperature. The oxygen desorption from CCRO started at around 150"C and after passing a maximum at around 300 "C the desorption rate attained a constant value up to 800°C. Above 800°C the rate of oxygen desorption increased again. This oxygen desorption behaviour of CCRO is very similar to that reported for Lao.8Sro.2Co03 in ref. 18. The oxygen released below 800 "C emanates from weakly bonded lattice oxygens, and this release is accompanied with a reduction by Co4+ to Co3+ (see below) and is closely related to the catalytic proper- ties of these The onset temperature for the oxygen desorption increased and the amount of oxygen released decreased with increasing y value, as seen in Fig.4. By the partial substitution of Cu/Ru for Co, the amount of weakly bonded oxygens is decreased, as is the redox capacity of the oxide lattice. J. Muter. Chern., 1996, 6(l), 97-102 99 'i p, 0.8 0 200 400 600 800 T 1°C Fig.4 TPD chromatograms for oxygen release from La,,8Sr,~,Col~2,Cu,Ru,03.(a) y=O (CCRO), (b) y=O.O5 (CCR5) and (c) y=O.lO (CCR10). When Sr2+ ions substitute for La3+ ions in LaCoO,, the charge compensation is achieved either by the forma- tion of oxide ion vacancies, according to the formula L~,-,S~,COO,-~,,~,or by the formation of Co4+ ions as described by the formula La,-,Sr,(Co, -,)"(CO,)~+~, (or more likely electron holes), or a combination of these two possibilities according to the formula La, -xSrx- ~~~1-,~3+~~~,~4+~3-~~x,2~+~2,2~11.It has been shown that when these types of oxides are prepared in air, oxygen vacancies and nominal Co4+ ions are simultaneously formed, and that oxygen released below and above 800°C corresponds to the reduction of Co4+ to Co3+ and of Co3+ to Co2+, respectively.18 The decrease in the amount of oxygen released below 800 "C with increasing Cu/Ru content (see Fig.4) can thus be interpreted in terms of the Cu/Ru substitution restraining the formation of Co4+ ions. Cu occurs as divalent or, in a formal sense, trivalent ions in perovskite-related structures, and it has been shown that the oxygen desorption which starts around 200 "C for both Lao~6Sr,~4Coo.8Cuo.20321the high-T, superconductor and YB~,CU,O~-,~~and is centred at 300 and 550 "C, respectively, is due to the reduction of Cu3+ to Cu2+. Thus the reduction of oxygen desorption capacity with increasing Cu/Ru substi- tution in our Lao.8Sro.2C01 2,Cu,Ruy0, samples, especially -that which occurs below 400"C, suggests that the Cu ions in our samples are predominatively divalent. It has been reported that the Ru ions are tetravalent in SrRuO,, C~RUO,,~and La2MRU06 (M=Mg, Co, Ni, Zn),25 while Ru in BaLaMRuO, (M=Mg, Co, Ni, Zn),25 S~L~CURUO,,~~~~~Ba2( LaRuo.5Sbo.5)06 and Ba2( TaRuo.,- Na0.5)0628 is pentavalent.Asmentioned above, the Cu ions seem to be divalent in La,.,Sr,.,Co, -,,Cu,Ru,03 and thus the substitution of Cu/Ru for Co gives rise to a reduction of the concentration of Co4+ ions in the present system. This can be explained by the occurrence of either tetravalent or pentavalent Ru ions, if the variation of the concentration of oxide ion vacancies is taken into account. In order to confirm the exact valence state of each metal cation as well as to give a more complete explanation of the oxygen desorption behaviour, further studies are necessary. Catalytic activity In the oxidation of CO over Lao~8Sro~2Col-2,CuyRu,03with y=O, 0.05 and 0.1, C02 was the sole reaction product, and the temperature dependence of the conversion of CO to C02 is given in Fig.5. Among three oxides tested, CCRO catalysed the reaction at the lowest temperatures and is accordingly regarded as the most active one, followed by CCRS and CCR10. The degree of conversion of CO over CCRO and CCRS 100 h 80 g0 .-E 60 8 40 0-g 20 8 0 0 200 400 600 T 1°C Fig. 5Catalytic oxidation of CO over La,,,Sr,,,Co, ~,,Cu,Ru,O,; 0, CCRO (y=O); 0,CCRS (y=0.05); A, CCRlO (y=O.lO) increased sharply with increasing temperature and reached 100% conversion within very narrow temperature ranges. Over CCR 10 the increase of CO conversion with increasing tempera- ture was lower and tended to level off at higher conversions.These results show that the CO oxidation activity of La,.,Sr,~,Co, -2,Cu,Ru,0, decreases progressively with the substitution of Cu/Ru for Co. In the CO-NO reaction, formation of N20, N2 and CO, was observed and is assumed to proceed according to eqn. (3) and (4). 2N0 +CO+N20 +CO, (3) 2N0 +2CO-+N2+2C02 (4) As shown in Fig. 6, N20 is predominantely formed below 350 "C and N2 above this temperature. The formation of N2 is preferable to that of N20, as the latter is a greenhouse gas, i.e. the selectivity to N2 formation should be as high as possible. In this regard, the substitution of Cu/Ru for Co played a positive role because the conversion to N,O decreased with increasing y value.The reason why no formation of N20 was observed over CCRlO seems to be related to the fact that this compound did not show any activity for NO conversion below 400 "C, where N20is formed in larger amounts than N2.From a comparision of the conversion curves of CCRO and CCR5, however, it is obvious that the conversion to N,O decreases with increasing y value. AsCO, is a common product of both 100 h85z" 80 -0 (d 60 0 c .-c e 40 .c0 c .-0 t! 20 a>c 8 0 0 200 400 600 800 T1°C Fig. 6 Temperature dependence of the degree of conversion of NO to N, and N20 in CO-NO reaction over La,,,Sr,,,Co, ~2,Cu,Ru,03; 0 and 0,CCRO (y=O); and H,CCRS (y=0.05); A and A,CCRlO (y =0.10).Open and filled symbols represent conversions to N, and N,O, respectively. 100 J. Muter. Chem., 1996, 6(l), 97-102 Table 3 Lattice parameters and composition of the prepared K,NiF,-type oxides of overall composition (Lao.8Sro.2)z(Coo.5Cuo,5-ZRuz)04 ~ ~ ~~ z value alA c/A 0.05 0.10 3.81 16(3) 3.8142( 1) 12.864( 2) 12.838( 1) reactions, the degree of conversion of CO in the CO-NO reaction is plotted us. temperature in Fig. 7. As seen in this figure, CCRlO was far less active than CCRO and CCR5, which in turn showed comparable activities although CCRO was more active than CCR5 in the lower temperature region, i.e. where the degree of conversion was less than 50%. These results indicates that the activity of CO oxidation in the CO-NO reactions decreased with the substitution of Cu/Ru for Co, though the substitution had a positive effect on the selectivity for N, formation.Over Co-based perovskite-type oxides, oxidation reactions of CO and hydrocarbons have been classified as being intra- facial and involve redox-type reactions in which lattice oxygens, especially weakly bonded species, takes part.16,19920 The CO-NO reaction has also been reported to proceed with an intrafacial mechanism., Accordingly, it is reasonably concluded that the decrease in the amount of weakly bonded lattice oxygens with the increasing extent of substitution of Cu/Ru for Co is primarily responsible for the decrease in the catalytic activity for both CO oxidation and CO-NO reactions.Synthesis of K,NiF,-type oxides As described above, a K2NiF,-type phase was formed in minor amounts in connection with the preparation of the perovskite- type oxides in the La-Sr-Co-Cu-Ru-0 system. The TEM-EDS studies of crystal fragments present in the sample with the overall composition Lao.8Sro.2Coo.6CUo.2RU0.203 revealed the presence of grains with an electron diffraction pattern different from that of the perovskite phase. These grains had a La :Sr :Co :Cu :Ru ratio of 52 :15 :17: 15: 1 yield- ing a (La +Sr):(Co +Cu +Ru) ratio close to 2. Samples of the composition (Lao.8Sro,,)2(Coo.sCuo,s-ZRu,)04 with z =0.05 and 0.10 were accordingly prepared. The XRD patterns of these two compositions revealed the presence of a predomi- nantly tetragonal K-type phase, minor amounts of a perovskite phase and some unreacted oxides.The K-type phase in the samples with z=O.O5 and 0.10 had quite similar average metal composition and lattice parameters, as seen in Table 3. If the formula (Lao.8Sro.2),(CoCu)l -ZRuz)04 is used to express the solubility limit of Ru in this phase one obtains a z value of 0.03. This low solubility value can be understood in terms 200 400 600 800 T1°C Fig. 7 Temperature dependence of the degree of conversion of CO to C02 in CO-NO reaction over Lao,8Sro,2Co, -,,Cu,Ru,03; 0, CCRO (y=O); 0,CCR5 (y=0.05); A, CCRlO (y=O.lO). VIA composition suggested by EDS 186.91 (La0.82Sr0. 18)(c046cu0.5 IRU0.03 )O4 186.79 (La0.83Sr0.17 )(C0~6CU0.~1RU0.0~)04 of the B-site in La-based K-type oxides being normally occu- pied by divalent ions, implying that the amount of tetravalent Ru ions that can enter this position should be small. The TEM-EDS studies of the grains with the perovskite structure showed that they had an average metal composition of La :Sr :Co :Cu :Ru =37.9 :12.6 : 19.0 :7.8 :22.7, which yields a composition of La~~~~Sr~~~~CO~~~~CU~~~~~U~~~~~~.This is in accordance with the observation that the La-based perovskite framework is more capable of incorporating tetravalent cations than the corresponding K-type framework. Conclusions Monophasic samples of the composition Lao~,Sro,,Col -2y-Cu,Ru,03 with 0 <y <0.2 possessing the rhombohedral (hexagonal) perovskite structure have been prepared.The lattice parameters and rhombohedral distortion increased monotonically with increasing y value, due to the replace- ment of smaller Co ions by larger Cu/Ru pairs. This re-placement brought about a decrease in the amount of weakly bonded lattice oxygens, which are known to be the most active oxygen species in connection with intrafacial CO oxidation and CO-NO reactions. Accordingly the catalytic activity for CO oxidation and CO-NO reactions was decreased by the replacement of Co ions by Cu/Ru pairs. The Cu/Ru substi- tution did, however, selectively increase the N, formation and decreased that of N20 in the CO-NO reactions. 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