ISSN 00231584, Kinetics and Catalysis, 2015, Vol. 56, No. 6, pp. 774–780. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Luu Cam Loc, Nguyen Tri, Hoang Tien Cuong, Ha Cam Anh, Ho Si Thoang, N.A. Gaidai, Yu.A. Agafonov, A.L. Lapidus, 2015, published in Kinetika i Kataliz, 2015, Vol. 56, No. 6, pp. 763–769.
Mechanism of Carbon Monoxide Oxidation on Supported Copper Catalysts Modified with Cerium and Platinum Luu Cam Loca, Nguyen Tria, Hoang Tien Cuonga, Ha Cam Anha, Ho Si Thoanga, N. A. Gaidaib, *, Yu. A. Agafonovb, and A. L. Lapidusb a
Institute of Chemical Technology, 01 Mac Dinh Chi Street, District 1, Ho Chi Minh City, Vietnam Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia *email:
[email protected]
b
Received December 30, 2014
Abstract—The mechanism of CO oxidation on copper oxide catalysts without additives and with cerium and platinum additives supported on γAl2O3 was studied. It was found that the presence of Ce and Pt facilitated the reduction of copper and increased the activity of the catalysts. It was established that the parent substances participated in the reaction from an adsorbed state; in this case, the surface coverage of all of the catalysts with oxygen was greater than the coverage with CO, and the reaction system components exhibited different adsorption capacity. The introduction of Ce increases the bond strength of CO with the surface and weakens the bond of oxygen with the surface. The presence of Pt increases the bond strengths of CO, O2, and CO2 with the surface. Keywords: CO, oxidation, mechanism, copper oxide catalysts, platinum, cerium DOI: 10.1134/S0023158415060087
INTRODUCTION The study of the reaction kinetics and mechanism of CO oxidation on platinum catalysts remains of interest until now, although it began since Langmuir’s publication [1] in 1922. This is related to a problem of the removal of CO impurities from air and hydrogen and the development of effective catalysts that do not contain expensive noble metals. Copper oxides should be noted among the catalysts that are closest to plati num systems in terms of activity [2–9]. The activity of copper catalysts can be increased by introducing the oxides of other metals, for example, cerium [10–13], and small quantities of noble metals. Mixed catalysts can be superior to those containing only one noble metal in terms of activity [14–17]. The introduction of Ce prevents the agglomeration of copper and increases its dispersity. The oxidation of CO on copper catalysts can occur by the following three different mechanisms: the Langmuir–Hinshelwood, Eley–Rideal, and Mars– van Krevelen mechanisms. It is assumed that adsorbed CO and О2 (in a molecular or atomic form) interact at the slow stage on the surface by the Langmuir–Hin shelwood mechanism [9, 18–20]. According to the Eley–Rideal mechanism, the CO or О2 molecule adsorbed on the surface interacts with the molecules of О2 or CO, respectively, from a gas phase [21–23]. The Mars–van Krevelen (MK) mechanism was proposed [9, 24–32]. The modified MK equation [9, 26] implies the interaction of a CO molecule adsorbed on the sur
face of copper with lattice oxygen. It was also shown [9] that the mechanism of CO oxidation depends on the nature of the support. The Langmuir–Hinshel wood mechanism occurs on the CuO/SiO2 and CuO/TiO2 catalysts, whereas the redox MK mecha nism, which can include a change in the degree of reduction of either only cerium [9, 24–27] or both cerium and copper [28–32], occurs on CuO/CeO2. It is believed [9, 26–28, 32] that the MK mechanism is applicable when the order of exponential equations for CO oxidation with respect to oxygen is close to zero and the catalyst activity is determined by the degree of its reduction. However, this is not always sufficient for the acknowledgement of the MK mechanism. The aim of this work was to determine which of the published mechanisms occurs in the oxidation of CO on copper catalysts with cerium and platinum addi tives and to study the effects of these additives on the adsorption characteristics of the catalysts. EXPERIMENTAL The copper catalysts supported onto γAl2O3 con tained 10 wt % CuO or 10 wt % CuO + 20 wt % CeO2. The concentration of platinum was varied from 0 to 0.3 wt %. The catalysts were prepared by the impreg nation of γAl2O3 with a specific surface area of 252 m2/g with the aqueous solutions of the nitrates Cu(NO3)2 · 3H2O and Ce(NO3)3 · 6H2O. The samples were initially dried at room temperature for 24 h and
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Table 1. Conversion of CO on the Cu–Al, Cu–Ce–Al, Pt–Cu–Al, and Pt–Cu–Ce–Al catalysts at different temperatures Catalyst Cu–Al Pt–Cu–Al Cu–Ce–Al Pt–Cu–Ce–Al
Conversion of CO, % 75°C
100°C
–* – 14.0 9.25
– – 53.3 76.2
125°C – – 97.4 100
150°C
175°C
200°C
225°C
250°C
– 46.4 – –
– 52.1 – –
13.6 60.3 – –
49.7 68.2 – –
73.7 93.2 – –
275°C 91.8 100 – –
300°C
310°C
99.6 – – –
100 – – –
Initial mixture, 0.5% CO and 10.5% O2; catalyst sample weight, 0.2 g. * Dashes indicate that experiments were not performed at the specified temperatures.
then at 80°C (2 h), 100°C (2 h), and 120°C (2 h); thereafter, they were treated with air (the space veloc ity V0 = 12 000 h–1) for 4 h at 600°C. Previously [20], it was found that the catalysts containing 10% CuO/γ Al2O3 (henceforth, they are referred to as Cu–Al) and (10% CuO + 20% CeO2)/γAl2O3 (henceforth, Cu– Ce–Al) should be considered optimum for the oxida tion of CO. The platinumcontaining samples were obtained by the impregnation of oxide catalysts with a solution of H2PtCl6. They were dried and calcined at 300°C for 2 h. The Pt content was varied from 0.05 to 0.3% (it is specified by the number before the symbol Pt in the catalyst designation). The specific surface area of the catalysts was determined from the adsorption of nitrogen. The activity of the catalysts was studied in a flow circulation system. The composition of the reaction mixture was as follows: 0.5 vol % CO and 10.5 vol % О2. The catalyst sample weight was 0.2 g. The temperatureprogrammed reduction of the samples with hydrogen (TPR H2) was carried out on a CHEMBET3000 instrument (Quantachrome, the United States). The rate of heating was 10 K/min; the initial mixture contained 5.0% Н2 and 95.0% N2. For determining the phase composition of the cat alysts, the Xray diffraction spectra were recorded using an XD5A instrument (Shimadzu, Japan) with CuKα radiation. The diffuse reflectance IR spectra were measured on a NicoletSpectrometer 460 instrument (Nicolet, the United States) in a range of 4000–1000 cm–1 with a resolution of 4 cm–1. The samples were loaded in a quartz cell with an optical window of CaF2. Before beginning measurements, the samples without Pt were treated with oxygen at 600°C for 1 h, and the plati numcontaining catalysts were treated at with 300°C; thereafter, they were evacuated to 10–3 Torr. The adsorption of CO was performed at a partial pressure of 8 or 13 Torr for 20 min at room temperature. Then, the cell was pumped out to 10–3 Torr at room temper ature or at 100°C for 20 min, and the spectrum was recorded once again. The 20% Се/γAl2O3 and 0.5% Pt/γAl2O3 catalysts were prepared for the IRspectro KINETICS AND CATALYSIS
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scopic studies. The supported copper catalysts also contained 0.5% Pt. The mechanism of the test process was studied by a response method. The experiments were carried out in two different smallvolume flow systems on the Cu– Al, Cu–Ce–Al, and 0.1Pt–Cu–Ce–Al catalysts using reaction mixtures of different composition. The relax ation curves were obtained by sharply replacing one composition by another. In one of the systems, the microreactor was con nected to a 6890N/MSD 5973 GC/MS instrument (Agilent Technologies, the United States). The reac tion mixture contained 0.5 vol % CO and 10.5 vol % О2 (the balance He). The experiments were carried out at 200°C and the reaction mixture feed rate V = 3 L/h with a catalyst sample of 0.1 g. The residence time, that is, a ratio of the reaction system volume to the flow rate, was no longer than 4 s, which was considered during the construction of relaxation curves. The other system was connected to an MSKh6 timeofflight mass spectrometer (Russia). The reac tion mixture contained 4.4% CO and 11.7% O2. The experiments were carried out in a temperature range of 200–270°C at a total reaction mixture feed rate of 9 L/h on a catalyst sample of 1.0 g. Furthermore, the adsorption and desorption studies and experiments on the displacement of carbon monoxide by oxygen and of oxygen by carbon monoxide were also performed at 30°C with the use of a mixture containing 10.0% CO and 21% О2 (the balance He). The residence time was 5 s. The process conditions were responsible for the occurrence of the process in the differential reactor mode. In each particular experiment, the concentra tion of only one substance was measured at regular intervals of 1 s. RESULTS AND DISCUSSION The experiments with a change in the platinum concentration from 0.05 to 0.3% in the CuAl and CuCeAl catalysts showed that the maximum rate of CO oxidation was reached on the samples with 0.1% Pt. All of the data given below refer to these catalysts, unless otherwise specified. Tables 1 and 2 summarize data on CO conversion at different temperatures and on the temperatures of 50% (T50) and 100% (T100) con
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Intensity
Ce
Ce Cu Pt
CuCe Al
Ce Cu
Al Pt
2
Al CuAl Pt
Cu Cu Al Pt Cu
Al Pt
Cu Al
Kubelka–Munk units
1.0
2125
0.8 1 0.6 2 2122
0.4 0.2
2194
1 0
25
30
40
50 2θ, deg
60
70
Fig. 1. Xray diffraction patterns of the (1) Pt–Cu–Al and (2) Pt–Cu–Ce–Al catalysts.
versions of CO on the test catalysts. As is evident, the introduction of Pt and, especially, Ce increased the activity of copper oxide catalysts. The complete con version of CO into CO2 on the most active Pt–Cu– Ce–Al sample was achieved even at 110°C. In terms of activity, the catalysts are arranged in the order Pt– Cu–Ce–Al > Cu–Ce–Al > Pt–Cu–Al > Cu–Al. Table 3 summarizes data on the specific surface areas of the catalysts (Ssp), the temperature maximums (Tmax) in the TPR curves, and the degrees of reduction of Cu2+ ions to Cu0 (xred). The presence of two TPR peaks of the Cu–Al catalyst was caused by the reduc tion of small CuO particles strongly bound to Al2O3, which are characterized by relatively high dispersity, and large blocks weakly bound to Al2O3 [27, 33]. The first peak in the curve of Cu–Ce–Al (Tmax = 276°C) relates to the reduction of copper particles bound to СеО2, and the second peak (Tmax = 321°C), to the reduction of copper on the surface of the sup port. Upon the introduction of CeO2, both of the peaks displaced to the region of lower temperatures. A Table 2. Temperatures of 50 and 100% conversion of CO on the Cu–Al, Cu–Ce–Al, Pt–Cu–Al, and Pt–Cu–Ce–Al catalysts Catalyst
T50, °C
T100, °C
Cu–Al
225
300
Pt–Cu–Al
175
275
Cu–Ce–Al
100
125
85
110
Pt–Cu–Ce–Al
Initial mixture, 0.5% CO and 10.5% O2; catalyst sample weight, 0.2 g.
2200
2150 2100 Wave number, cm–1
2050
Fig. 2. IR spectra of CO adsorbed on the 10% CuO/Al2O3 catalyst (1) at a pressure of 8 Torr and (2) after evacuation to 10–3 Torr at 20°С.
decrease in the temperature of copper reduction upon the introduction of Ce into the Cu–Al catalyst was observed earlier [34, 35]; it was also shown that the addition of Ce decreased the concentration of the bulk crystalline phase of CuO. The presence of Ce also facilitates the reduction of highly dispersed copper [36]. The TPR curves of the Pt–Cu–Al and Pt–Cu– Ce–Al catalysts exhibit only one peak, which relates to the reduction of small CuO particles; it is character ized by a lower Tmax in comparison with analogous cat alysts without platinum. Thus, the introduction of cerium and platinum facilitates the reduction of copper. The Xray diffraction patterns (Fig. 1) indicate that the Cu–Al catalyst exhibited peaks characteristic of a crystalline phase of CuO (2θ = 35, 38, 49, and 62 deg), γAl2O3 (2θ = 68, 45, and 37 deg), and a small amount of the spinel CuAl2O4 (2θ = 61 deg). The addition of 0.05–0.3% Pt did not change the shape of the spec trum because the characteristic peaks of Pt possess low intensity. The Cu–Ce–Al catalyst is characterized by the intense peaks of CeO2 (2θ = 28, 47, and 57 deg), and the Pt–Cu–Ce–Al catalyst, also by the peaks of Pt. A comparison of the IR spectra of CO adsorbed on the 20% Се/γAl2O3, 0.5% Pt/γAl2O3, and (0.5% Pt + 10.0% CuО)/γAl2O3 catalysts shows that the intensity of peaks corresponding to the adsorbed molecules of CO on the (0.5% Pt + 10.0% CuО)/γAl2O3 catalyst is considerably greater than that in the spectra of sam ples without copper. Nevertheless, the ions of Ce and Pt exert a considerable effect on the state of a copper oxide phase. Figures 2 and 3 show the IR spectra of CO adsorbed on Cu–Al and Cu–Ce–Al, respectively. As is evident, the bond vibration frequencies in the adsorbed molecules of CO are similar. The presence of Ce causes a shift of absorption bands to the region of KINETICS AND CATALYSIS
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1.0 Kubelka–Munk units
low wave numbers, which is indicative of a decrease in the oxidation number of copper. The IR spectrum obtained on Cu–Al at PCO = 8 Torr (Fig. 2, curve 1) exhibits a fundamental absorption band at 2125 cm–1 and a small band at 2194 cm–1, which corresponds to the adsorption of CO on Al2O3. According to pub lished data [37], absorption bands in a frequency range of 2130–2115 cm–1 correspond to the Cu+–CO com plex, whereas frequencies of 2180 and 2100 cm–1 cor respond to the Cu2+–CO and Cu0–CO complexes, respectively. Marban et al. [29] demonstrated that about 70% Cu2+ ions in the completely oxidized CuO– CeO2 catalyst was reduced to Cu+ at PCO = 1 Torr even at 27°C, and this state of copper was retained at PO 2 = 1 Torr and a temperature of 100°C. Tang et al. [31] also found that Cu+ is the main copper species in the CuO–CeO2 catalyst even after its reduction with hydrogen at 500°C. These conclusions were also con firmed in other works [32, 38]. Thus, an absorption band at 2125 cm–1 corresponds to the Cu+–CO com plex. After evacuation at 20°C, an absorption band at 2194 cm–1 almost disappeared, and a band corre sponding to Cu+–CO retained a considerable inten sity; this indicates the high strength of adsorption at these centers. The spectrum of CO adsorbed on Cu– Ce–Al (Fig. 3) also exhibits an absorption band at 2119 cm–1, which belongs to the Cu+–CO complex, and a small band at 2188 cm–1, which characterizes the adsorption of CO at the acidic centers of alumi num and disappears after evacuation at 20°C, as in the Cu–Al catalyst. A distinctive feature of the adsorption of CO on Cu–Ce–Al is a shift of the absorption band corresponding to the adsorption of CO on copper ions toward lower wave numbers, which is indicative of a decrease in the oxidation number of copper. This also leads to the stronger binding of CO with the surface. Indeed, on the Cu–Ce–Al catalyst, evacuation at 100°C was not accompanied by significant CO desorp tion; in this case, a band due to CO bond vibrations shifted to 2110 cm–1. Thus, the introduction of Ce increases the bond strength of CO with the catalyst. The IR spectra of the Pt–Cu–Al and Pt–Cu–Ce–Al catalysts also exhibit a fundamental absorption band in a range of 2110–2125 cm–1, which corresponds to the Cu+–CO complex. Table 4 summarizes data on the time of adsorption (t), the amount of adsorbed CO and О2 (Q), the sur face coverage with them (Σ), the time of desorption (t'), and the amount of irreversibly adsorbed CO and O2 (q) on the Cu–Al and Cu–Ce–Al catalysts at 200°C. The data were obtained by the analysis of He/(He + CO), (He + CO)/He, He/(He + O2), and (He + O2)/He responses (a slash indicates a sharp change in the composition). As can be seen, the total surface coverage with both of the substances does not exceed 50.4% in the case of Cu–Ce–Al or 20.4% in the case of Cu–Al. The degree of surface coverage with
777 2119
0.8
1
2115
0.6
2110 2
0.4
3
2188
0.2 0
2200
2150 2100 Wave number, cm–1
2050
Fig. 3. IR spectra of CO adsorbed on the (10% CuO + 20% СеО2)/Al2O3 catalyst (1) at a pressure of 8 Torr and after evacuation to 10–3 Torr at (2) 20 and (3) 100°С.
oxygen was greater than that with carbon monoxide on both of the catalysts. The ratio O2 : CO was 23 on Cu–Al, whereas it was 1.3 on Cu–Ce–Al. A comparison between the adsorption–desorption responses of CO and O2 on Cu–Ce–Al and Pt–Cu–Ce–Al showed that the presence of Pt increased the strength of CO and O2 binding with the surface, and the degree of sur face coverage with oxygen was greater than the degree of surface coverage with CO on both of the catalysts. The formation of CO was observed on all of the test catalysts in responses with the preliminary adsorption of CO and the subsequent supply of O2 and in the reverse responses, that is, (CO + He)/(O2 + He) and (O2 + He)/(CO + He), respectively. This fact suggests that CO and O2 participate in the reaction in an adsorbed state. The intermediate blowing of prelimi narily adsorbed CO and O2 with helium makes it pos sible to estimate the strength of their binding with the surface. On the Cu–Al and Cu–Ce–Al catalysts, the amount of СО2 formed in the (О2 + He)/He/(CO + He) responses at the same time of blowing decreased more slowly than that in the (CO + He)/He/(О2 + Table 3. Specific surface areas, reduction temperature maximums in the TPR curves, and the degrees of the reduc tion of Cu2+ ions to Cu0 (xred) Catalyst Cu–Al Cu–Ce–Al Pt–Cu–Al Pt–Cu–Ce–Al
Ssp, m2/g 177 87.0 151 80.1
Tmax, °C
xred, %
300; 375
13.0
276; 321
17.6
274
36.7
255
45.8
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LUU CAM LOC et al. (а) [CO2], arb. units
[CO2], arb. units
6 4 2
0
5
10 Time, s
(а)
4
8
15
3 2 1
0
20
2
4
6
8 10 Time, s
12
14
16
(b)
5 (b) 4 [CO2], arb. units
[CO2], arb. units
8 6 4
2 1
2
0
3
5
10 15 Time, s
20
25
0
5
10 Time, s
15
20
Fig. 4. Dependence of the concentration of СО2 on time in the (a) He/(CO+ O2) and (b) (He + О2)/(CO + O2) responses on the Cu–Ce–Al catalyst at 250°C.
Fig. 5. Dependence of the concentration of СО2 on time in the (a) He/(CO + O2) and (b) (He + О2)/(CO + O2) responses on the Pt–Cu–Ce–Al catalyst at 250°C.
He) responses. This means that the coverage of the catalyst with oxygen is greater than the coverage with CO. With the intermediate blowing of the Cu–Ce–Al catalyst with preliminarily adsorbed oxygen by helium for 6 s, the amount of CO2 formed decreased more rapidly than that with the analogous blowing of the Cu–Al catalyst. Therefore, oxygen is less strongly bound to the Cu–Ce–Al catalyst than to Cu–Al; that is, the presence of Ce weakens the bond of oxygen. In contrast to this, if the Cu–Ce–Al catalyst with prelim inarily adsorbed CO was blown with helium for 6 s, the amount of CO2 formed decreased more slowly than that on the Cu–Al catalyst. Consequently, the pres ence of Ce increased the bond strength of CO with the catalyst surface; this is consistent with the IRspectro scopic results. Table 5 gives data on the effect of the time of blowing with helium (tHe) of Cu–Ce–Al and Pt–Cu–Ce–Al catalysts with preliminarily adsorbed CO and О2 on the emergence time of СО2 (tCO2). As is evident, О2 was removed from the surface of these cat alysts much more rapidly than CO. The (O2 + He)/He/(CO + He) responses on Pt–Cu–Ce–Al were more prolonged than those on Cu–Ce–Al; this
was due to a higher degree of coverage with О2 and the stronger adsorption of oxygen; that is, the introduc tion of Pt increased the bond strengths of both CO and О2 and surface coverage with them. The longer dura tion of responses with the preliminary adsorption of О2 on these catalysts impelled us to test the possibility of the participation of lattice oxygen in the oxidation of CO. The formation of CO2 was not observed upon the treatment of the Cu–Ce–Al catalyst with helium at 300°C for 1 h and the subsequent supply of CO, so that lat tice oxygen did not participate in the oxidation of CO. Figures 4 and 5 show the relaxation curves of CO2 formation in responses with the introduction of a reac tion mixture into the reactor (a) after its blowing with helium and (b) after the preliminary adsorption of O2 on the Cu–Ce–Al and Pt–Cu–Ce–Al catalysts, respectively. СО2 appeared in the gas phase after a time necessary for the adsorption of parent substances and the occurrence of the reaction and СО2 desorption, whereas an additional time is required for the dis placement of adsorbed О2 in the responses with the preliminary adsorption of oxygen. On the Pt–Cu– Ce–Al catalyst, these times are longer than those on Cu–Ce–Al. The reverse responses, when either He or KINETICS AND CATALYSIS
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Table 4. Adsorption and desorption of CO and O2 on the Cu–Al and Cu–Ce–Al catalysts at 200°C* Adsorption Adsorbate
Catalyst
Q t, s
Cu–Al
Cu–Al O2 (10.5%)
Cu–Ce–Al
Σ, %
t', s
q, μmol/g
48
4.40
μmol/g
molecule/g
57
13.91
0.08 × 1020
195
317.03
1.91 × 1020
20.3
42
101.14
77
306.0
1.84 × 1020
19.6
66
125.93
151
485.2
2.92 × 1020
30.1
54
263.48
CO (5%) Cu–Ce–Al
Desorption
0.85
t is the adsorption time, Q is the quantity of adsorbed CO and O2, Σ is the surface coverage with CO and O2, t' is the desorption time, and q is the quantity of irreversibly adsorbed CO and O2. The calculated number of copper atoms per gram of catalyst was 9.41 × 1020. * The data were obtained by a response method.
Table 5. Effect of the time of blowing with helium on the emergence time of CO2 in the (CO + He)/He/(O2 + He) and (O2 + He)/He/(CO + He) responses on the Cu–Ce–Al and Pt–Cu–Ce–Al catalysts at 250°C tHe, min
Response
(10% CO + He)/He/(O2 + He)
(21% O2 + He)/He/(CO + He)
t CO2 , s Cu–Ce–Al
Pt–Cu–Ce–Al
0
58
51
0.5
52
57
1.0
44
50
2.0
40
45
3.0
38
40
0
8.0
1.0
6.9
95 8.0
2.0
6.4
7.5
3.0
5.8
7.0
4.0
5.0
6.2
5.0
3.5
5.3
6.0
3.4
5.0
tHe is the time of blowing the catalysts with helium, and tCO2 is the emergence time of CO2.
He + О2 was introduced into the reactor after the reac tion mixture, indicate that the time taken for a com plete decrease in the amount of СО2 to zero on the Pt– Cu–Ce–Al catalyst is longer, and this is indicative of the stronger adsorption of СО2. A small increase in the emergence time of СО2 in the (He + О2)/(CO + O2) responses in comparison with the He/(CO + O2) responses on these catalysts can serve as confirmation that the adsorption of oxygen is weaker than that of CO on both of the catalysts. An insignificant increase in the emergence time of СО2 into the gas phase (Fig. 4b) in the (He + O2/(CO + O2) response in com KINETICS AND CATALYSIS
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parison with the He/(CO + O2) response (Fig. 4a) on Pt–Cu–Ce–Al indicates that the adsorption coeffi cients of О2 and CO on this catalyst differ to a lesser degree than those on Cu–Ce–Al. Indeed, the experi ments with the displacement of CO by oxygen at 30°C showed that oxygen has greater difficulty in displacing CO from the surface of Pt–Cu–Ce–Al than from the surface of Cu–Ce–Al. Thus, the mechanism of CO oxidation on the cop per oxide catalysts both without additives and with the additives of cerium and platinum supported onto γAl2O3 is identical: the parent substances participate
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in the reaction from an adsorbed state; that is, the Langmuir–Hinshelwood mechanism occurs. In this case, the degree of surface coverage of all of the cata lysts with oxygen was greater than the degree of with CO, but the reaction system components are different in adsorption capacity. The introduction of Ce increases the bond strength of CO and weakens the bond of oxygen with the catalyst surface. The presence of Pt increases the bond strengths of CO, O2, and CO2. The introduction of Ce and Pt facilitates the reduction of copper. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research and the Vietnam Academy of Science and Technology (VAST) (joint grant no. 13 0393001 Viet_a) and NAPOSTED (no. 104.03 2012.60).
17. 18. 19. 20. 21. 22. 23. 24.
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS
Vol. 56
No. 6
2015
