Al2O3 Catalyst for

Jan 12, 2007 - Japan, and Catalysis Research Center, Hokkaido UniVersity, N21-W10 Sapporo 001-0021, Japan. ReceiVed: August 24, 2006; In Final Form: ...
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J. Phys. Chem. C 2007, 111, 2205-2211

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Mechanistic Investigation of Deactivation of Ru/Al2O3 Catalyst for Preferential CO Oxidation in the Presence of NH3 Hidenobu Wakita,*,† Kunihiro Ukai,† Tatsuya Takeguchi,‡ and Wataru Ueda‡ LiVing EnVironment DeVelopment Center, Matsushita Electric Industrial Co., Ltd., Moriguchi, Osaka 570-8501, Japan, and Catalysis Research Center, Hokkaido UniVersity, N21-W10 Sapporo 001-0021, Japan ReceiVed: August 24, 2006; In Final Form: NoVember 1, 2006

The preferential CO oxidation (PROX) on a Ru/Al2O3 catalyst was investigated at 115-185 °C in the presence of 21-75 ppm NH3. With an increase in [O2]/[CO] ratios, the CO conversion decreased, though it increased in the absence of NH3. In the presence of NH3, the catalytic activity was high at high temperature, which indicates that O2 was consumed over the upstream of the catalyst bed and that the reaction atmosphere in most portions of catalyst bed was reductive. Thus, a reduction atmosphere kept the catalyst active. For the H2 oxidation without CO, the catalyst was immediately deactivated in the presence of NH3, which indicates that the activation of oxygen was suppressed. In the in-situ IR spectra, a band assigned to nitrosyls appeared during the poisoning, and the intensity increased with time on stream. In addition, the band intensity of monocarbonyls decreased with an increase in that of multicarbonyls and nitrosyls. The presence of multicarbonyls indicates that the nitrosyls stabilized the oxidized Ru. In the presence of NH3, the oxidized Ru catalyst showed a low activity under an oxidation atmosphere, while the reduced Ru catalyst exhibited a high activity under a reduction atmosphere.

1. Introduction Polymer electrolyte fuel cells (PEFC) have recently attracted much attention for vehicle and residential use. Residential PEFCcombined co-generation systems using natural gas, liquefied petroleum gas, or kerosene as the fuel have been actively studied, especially in Japan. The fuel processor of the each system normally comprises not only a steam reformer but also a shift converter and a preferential CO oxidation (PROX) unit, since the CO concentration in the reformed fuel gas must be less than 10 ppm. Though the stoichiometric [O2]/[CO] ratio is 0.5 for the CO oxidation, it is necessary to supply the excess amount of O2 to achieve high CO conversion for the PROX in H2-rich gas. Thus, the PROX is usually conducted at the [O2]/ [CO] ratio of 1.5-2.5, though excess O2 consumes H2. When the autothermal reforming is carried out for the fuel processing, NH3 is formed from N2 and H2 to deteriorate the polymer electrolyte membrane; therefore, NH3 absorbers have been proposed.1,2 On the same score, NH3 is formed by the steam reforming of natural gas containing N2 over Ru catalysts at high temperature (>600 °C).3 In steam reforming of the natural gas containing 1 vol % of N2, the thermodynamic equilibrium concentration of NH3 at 650 °C, which depends on the steam-to-carbon ratio, is ∼20 ppm. Despite the practical importance, however, the influence of NH3 on the catalytic activity for the PROX has not been reported except for our recent letter,4 to the best of our knowledge. For the PROX, Ru/Al2O3 and Pt/Al2O3 catalysts have been widely applied.5-7 Though Pt catalysts are resistant to oxidation, they tend to show high activity for the reverse water-gas shift reaction at high temperature, and strongly adsorbed CO sup* Corresponding author. Phone: +81-6-6906-2430. Fax: +81-6-69062875. E-mail: [email protected]. † Matsushita Electric Industrial Co., Ltd. ‡ Hokkaido University.

presses the reaction at low temperature. Thus they show a high activity over a narrow temperature range.5 For Ru catalysts, however, the concomitant methanation also reduces CO at high temperature. Therefore they show a high activity over a wide temperature range, unless they are not oxidized at high temperature. Thus, Ru catalysts are suitable for the residential PEFC co-generation systems. A few studies on NH3 oxidation8-10 and CO-NH3 reaction11,12 over Ru/Al2O3 catalysts have been reported. In addition, a lot of studies have been reported on the NO-CO,13-23 NOH2,17,24-30 and NO-CO-H2 reactions17,25,31-34 over Ru/Al2O3 catalysts. However, the conditions for these reactions were quite different from that for the PROX, since the PROX is conducted in the presence of O2 and H2O under a H2-rich condition. Therefore, it is interesting to study the reaction behavior in the presence of a nitrogen compound (NH3, NO, or NO2) for the PROX. The objectives in this work are to investigate the influence of nitrogen compounds, especially NH3, on the activity of Ru/Al2O3 catalysts for the PROX and to elucidate the deactivation mechanisms on the basis of in-situ IR spectra. The PROX over Pt/Al2O3 catalysts was also conducted in the presence of a nitrogen compound, and the results were compared with those for the Ru/Al2O3 catalysts. 2. Experimental Section Catalyst Preparation. The catalysts were prepared by the impregnation method described in detail elsewhere.35 For the monolith Ru/Al2O3 catalyst, a θ-Al2O3 support was impregnated with an aqueous solution of RuNO(NO3)x, and the obtained 2.0 wt % Ru/Al2O3 catalyst powder was supported on cordierite honeycombs (20-mm diameter; 10-mm thickness; 400 cell/inch2) with an Al2O3-sol binder. The prepared monolith Ru/Al2O3 catalyst with Ru loading of 1.6 g/L was employed on transient response tests in a fixed-bed reactor. For the in-situ IR

10.1021/jp065497z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007

2206 J. Phys. Chem. C, Vol. 111, No. 5, 2007 measurements and O2 pulse adsorption, 3.8 wt % Ru/Al2O3 catalyst powder was prepared in a similar manner as above. For the monolith Pt/Al2O3 catalyst with Pt loading of 3.1 g/L, the above-mentioned honeycombs were dipped in the slurry containing a nitric acid solution of Pt(NH3)2(NO2)2 and the Al2O3 support, followed by calcination at 500 °C for 2 h and reduction at 350 °C for 2 h. Reaction Tests. Catalytic activities were measured in a fixedbed reactor. The reaction temperatures were measured with a thermocouple in contact with the top of the monolith catalyst bed and kept at 115-185 °C. For the Ru/Al2O3 catalyst, the PROX was conducted at 200 °C for 20 min under the condition mentioned below to obtain stable activity, and then the catalyst was cooled to the given temperature. The transient response tests were conducted in the following manner: the catalytic activity was measured in the absence of a nitrogen compound (NH3, NO, or NO2), followed by the addition of 21-75 ppm of a nitrogen compound for 1-41 h and the stop of nitrogen compound supply. When the nitrogen compound was not supplied, He was fed instead of it. The gas composition was 51.5 vol % H2, 0.3 vol % CO, 13.0 vol % CO2, 0.4-0.6 vol % O2, and 28.8 vol % H2O balanced with He ([O2]/ [CO] ratios of 1.1-1.9) for the NH3 poisoning, or 45.5 vol % H2, 0.3 vol % CO, 11.5 vol % CO2, 0.4 vol % O2, and 25.4 vol % H2O balanced with He ([O2]/[CO] ratio of 1.4) for the NH3, NO, or NO2 poisoning. The catalytic performances were examined at GHSV ) 9300 h-1. The outlet CH4 concentration was negligible under any conditions in this study. In a similar manner as above, the NH3 poisoning (75 ppm) was conducted for the H2 oxidation without CO, CO2, and H2O under a H2-rich condition and for the PROX ([O2]/[CO] ratio of 1.4) without CO2 and H2O. The gas composition was 93.2 or 93.6 vol % H2, 0 or 0.3 vol % CO, and 0.5 vol % O2 balanced with He. NH3 in the outlet gas was trapped by a 5% boric acid solution and was analyzed by liquid chromatography. The method of the other gas composition analysis was described in detail elsewhere.35 The O2 pulse adsorption on the NH3-poisoned Ru/Al2O3 catalyst powder (0.033 g) was conducted on a temperatureprogrammed desorption apparatus equipped with a quadruple mass detector (TPD-1-AT, BEL Japan Inc.). The poisoning was conducted at 150 °C in a flow of 90.7 vol % H2 and 0.4 vol % O2 balanced with He without CO in the presence of 500 ppm NH3 at 0.54 mgcat min mL-1 until O2 did not react at all, and then the catalyst was cooled to 50 °C in a He flow and the O2 pulse adsorption was conducted in a He flow. IR. IR spectra were obtained with an FTIR spectrometer (FTIR 8200, Shimadzu Corp.) equipped with a DRIFT apparatus (model 0030-103, Spectra-Tech Inc.) having CaF2 windows. First, the reaction was conducted over the Ru/Al2O3 catalyst powder at 200 °C in a flow of the reaction gas (gas composition (I): 75.5 vol % H2, 0.5 vol % CO, 19.0 vol % CO2, 0.7 vol % O2, and 1.6 vol % H2O balanced with N2; [O2]/[CO] ratio of 1.5). Then the catalyst was reduced at 200 °C in a flow of 5 vol % H2 diluted with He for 30 min and cooled to 150 °C. After a background spectrum was taken in a He flow, a spectrum was taken in the flow of the reaction gas (gas composition (I)). Next, the nitrogen compound diluted with He was added to the reaction gas. For the 14NH3 poisoning, the catalyst was poisoned with 31 ppm 14NH3 in the gas composition (I) for 330 min. Next, the reaction was continuously carried out in the absence of 14NH3 for 90 min. The outlet gas was analyzed in a manner similar to the reaction in the fixed-bed reactor. To confirm the assignment of the bands, in-situ IR spectra in the presence of 138 ppm 15NH3 were also taken in a similar manner as above.

Wakita et al.

Figure 1. Dependence of the activity of the Ru/Al2O3 catalyst on [O2]/ [CO] ratios (a) in the absence of NH3, and (b) at 200 min after the start of 21 ppm NH3 supply. Black bar, CO conv.; hatched bar, O2 conv.; feed, 51.5 vol % H2, 0.3 vol % CO, 13.0 vol % CO2, 0.4-0.6 vol % O2, and 28.8 vol % H2O balanced with He; temperature, 150 °C; GHSV ) 9300 h-1.

The H2 oxidation without CO, CO2, and H2O was also conducted at 150 °C in the presence of 31 ppm 14NH3. In this case, a gas mixture of 90.9 vol % H2, 0.7 vol % O2, and 2.6 vol % N2 balanced with He (gas composition (II)) was supplied. CO Adsorption. The CO adsorption was conducted at 50 °C by the pulse method on an automatic gas adsorption apparatus (R6015, Okura Riken Co., Ltd.). For the pretreatment, the catalysts were oxidized in an O2 flow at 300 °C for 15 min, followed by the reduction in a H2 flow at 400 °C for 30 min. 3. Results and Discussion 3.1. Effects of the [O2]/[CO] Ratios. Figure 1 shows the effect of the [O2]/[CO] ratios on the conversions of CO and O2 for the RROX over the Ru/Al2O3 catalyst in the presence/ absence of 21 ppm NH3. In the absence of NH3, the catalyst exhibited a stable high activity over 180 min, and the CO conversion at the low [O2]/[CO] ratio of 1.1 was slightly lower than those at the higher [O2]/[CO] ratios. Due to low consumption of H2 and high conversion of CO, the [O2]/[CO] ratio of 1.4 is desirable for PEFC in the absence of NH3. On the contrary, in the presence of NH3, both CO and O2 conversions at 200 min after the start of NH3 supply decreased with an increase in the [O2]/[CO] ratios. Figure 2 shows the changes in the catalytic activity with time in the presence of 21 ppm NH3. The CO and O2 conversions at the high [O2]/[CO] ratio of 1.9 decreased rapidly and simultaneously (Figure 2a). After NH3 was stopped, both conversions were slowly restored. At the [O2]/[CO] ratios of 1.9 and 1.4, the CO conversion began to decrease at 40 and 140 min, respectively. On the other hand, at the low [O2]/[CO] ratio of 1.1, the CO and O2 conversions were held almost constant at ∼90 and ∼100% for 710 min, respectively (Figure 2b). After NH3 was stopped, the CO conversion was restored rapidly and completely. Thus, the O2 concentration clearly affected the deactivation of the Ru/Al2O3 catalyst. Whenever the CO conversion decreased with time on stream, the decrease in the CO conversion accompanied the decrease in the O2 conversion. This indicates that the catalyst was deactivated under an oxidation atmosphere. At the low [O2]/[CO] ratio of 1.1, the deactivation took place only over the upstream of the catalyst bed, because O2 was consumed over that part. Thus the deactivation proceeded downstream very slowly. 3.2. Temperature Dependence. The activity of the Ru/Al2O3 catalyst in the presence/absence of NH3 (75 ppm) was investigated at 115-185 °C and the [O2]/ [CO] ratio of 1.4. As shown in Figure 3a, in the absence of NH3, the O2 conversions were ∼100%. Though the CO conversions were more than 99% in

Deactivation of Ru/Al2O3 Catalyst by NH3

Figure 2. Changes in the activity of the Ru/Al2O3 catalyst during the poisoning with 21 ppm NH3: (a) [O2]/[CO] ) 1.9, (b) [O2]/[CO] ) 1.1. (b) CO conv.; (4) O2 conv.; (0) NH3 conv. Reaction conditions are the same as those shown in Figure 1.

Figure 3. Temperature dependence of the activity of the Ru/Al2O3 catalyst (a) in the absence of NH3, and (b) at 60 min after the start of 75 ppm NH3 supply. Black bar, CO conv.; hatched bar, O2 conv. Reaction conditions are the same as those shown in Figure 1. [O2]/ [CO] ) 1.4.

the wide temperature range of 115-185 °C, they slightly decreased with an increase in the temperature. On the contrary, in the presence of NH3, the catalytic behavior strongly depended on the reaction temperature, as shown in Figure 3b. Both conversions at 60 min decreased with a decrease in the temperature. Figure 4 shows the changes in the catalytic activity with time at 115 and 185 °C. At the low temperature of 115 °C, both conversions rapidly decreased to 15% at 40 min (Figure 4a). After NH3 was stopped, the catalytic activity was gradually regenerated; however, the CO conversion was as low as 55% at 180 min. Thus, the high-concentration NH3 severely damaged the catalyst at the low temperature of 115 °C. However, at the high temperature of 185 °C, the O2 conversion was kept high, and the CO conversion immediately decreased to 83% and did not change for 240 min (Figure 4b). After NH3 was stopped, the CO conversion was rapidly and completely restored. It is suggested that O2 was consumed over the upstream of the catalyst bed, and most of catalyst bed was kept reductive.

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Figure 4. Changes in the activity of the Ru/Al2O3 catalyst during the poisoning with 75 ppm NH3 at (a) 115 °C and (b) 185 °C. (b) CO conv.; (4) O2 conv.; (0) NH3 conv. Reaction conditions are the same as those shown in Figure 1. [O2]/[CO] ) 1.4.

Moreover, after the stop of NH3, the catalyst was more easily regenerated at high temperature than that at low temperature. For the residential PEFC co-generation systems, the daily start-and-stop operation will be conducted for efficiency. Our results showed that the deactivation was reversible. Therefore, the reaction at the low [O2]/[CO] ratios at high temperature over an adequate amount of catalyst in combination with reactivation could significantly decrease the influence of NH3. 3.3. H2 Oxidation in the Presence of NH3. Figure 5a shows the behavior for the H2 oxidation without CO, CO2, and H2O at 150 °C in the presence of 75 ppm NH3. For comparison, the PROX without CO2 and H2O was conducted (Figure 5b). For the H2 oxidation, the O2 conversion has been over 95% for 30 min without the detection of NH3, and then the O2 conversion suddenly decreased with a decrease in the NH3 conversion. Therefore, most of the supplied NH3 contributed to the poisoning in the initial stage until most of the active sites were covered with the poisoning species, which suppressed the activation of oxygen. On the other hand, the CO and O2 conversions for the PROX without CO2 and H2O were held at 96 and 98% for 100 min, respectively (Figure 5b). As shown in Figure 5b, the NH3 conversion was relatively high in the presence of CO, which indicated that the poisoning species formed by the reaction of NH3 rapidly reacted with CO to be removed, though some of the active sites were covered with adsorbed CO. As a result, the coexistent CO kept the activity high. The reaction of NH3 will be discussed in section 3.5. In the absence of CO2 and H2O, the CO and O2 conversions hardly changed for 100 min (Figure 5b), though they decreased to 60-70% at 60 min in the presence of CO2 and H2O (Figure 3b, 150 °C). Since the PROX over Ru/Al2O3 catalysts was suppressed by the coexistent H2O,36 O2 remained downstream over the catalyst in the presence of CO2 and H2O compared with that in the absence of CO2 and H2O. Therefore the coexistent H2O accelerated the deactivation. To elucidate the deactivation mechanism, the H2 oxidation was conducted over the Ru/Al2O3 catalyst powder in the

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Figure 5. Changes in the activity of the Ru/Al2O3 catalyst during the poisoning with 75 ppm NH3 at 150 °C for (a) the H2 oxidation, and (b) the PROX in the absence of CO2 and H2O. (4) O2 conv.; (0) NH3 conv.; (b) CO conv. Feed: (a) 93.6 vol % H2 and 0.5 vol % O2 balanced with He; (b) 93.2 vol % H2, 0.3 vol % CO, and 0.5 vol % O2 balanced with He. [O2]/[CO] ) 1.4; GHSV ) 9300 h-1.

TABLE 1: Changes in the Activity of the Ru/Al2O3 Catalyst during the Poisoning with 75 ppm of Nitrogen Compoundsa poisoning for 30 min

poisoning for 60 min

nitrogen compound

CO conv. (%)

O2 conv. (%)

CO conv. (%)

O2 conv. (%)

NH3 NO NO2

91.7 24.2 13.4

98.3 26.9 13.2

80.8 14.8 8.8

86.7 16.4 8.7

a Reaction condition: feed, 45.5 vol % H2, 0.3 vol % CO, 11.5 vol % CO2, 0.4 vol % O2, 25.4 vol % H2O, and 75 ppm nitrogen compound balanced with He; [O2]/[CO] ) 1.4; GHSV ) 9300 h-1; temperature, 150 °C.

presence of 500 ppm NH3 until O2 did not react at all, and then O2 was pulse-injected into the deactivated catalyst at 50 °C. Though the fresh catalyst adsorbed 0.21 mmol/gcat of O2, the deactivated one did not adsorb O2 (detection limit: 0.02 mmol/ gcat). It also suggests that the activation of oxygen was suppressed. 3.4. NO and NO2 Poisoning. The Ru/Al2O3 catalysts were poisoned with NO and NO2 in the PROX at 150 °C and the [O2]/[CO] ratio of 1.4. Table 1 shows the results of the PROX in the presence of the nitrogen compounds (75 ppm). Both conversions in the presence of NO were lower than those in the presence of NH3, and the catalytic activity was restored after the stop of NO. During the NO poisoning, 5-15 ppm NH3 was detected in the outlet. Both conversions in the presence of NO2 were lower than those in the presence of NO, which indicates that NO2 oxidized Ru more strongly than NO. During the NO2 poisoning, NH3 was also detected in the effluent gas, which suggests that the same intermediate was formed during the NO and NO2 poisoning. Isocynates were probably the intermediate, since it has been reported that the isocyanates easily spilled over to the support for Ru/Al2O3 catalysts12 and that the isocyanates on Al2O3 were easily hydrolyzed to NH3.37,38 However, the

Figure 6. In-situ IR spectra over the region of the C-O and N-O stretching vibrations for the PROX on the Ru catalysts: (a) before the poisoning; (b) at 60 min after the start of 31 ppm 14NH3 supply; (c) at 180 min; (d) at 330 min; (e) at next 90 min after the stop of 14NH3 supply; (f) at 180 min after the start of 138 ppm 15NH3 supply. Feed: 75.5 vol % H2, 0.5 vol % CO, 19.0 vol % CO2, 0.7 vol % O2, and 1.6 vol % H2O balanced with N2; temperature, 150 °C.

deactivation was not mainly caused by the isocynates, because the catalyst was deactivated more slowly for the PROX than that for the H2 oxidation without CO, as shown in Figure 5. 3.5. In-Situ IR Measurements. To investigate the poisoning species, the reaction in the presence/absence of 31 ppm NH3 was conducted in the in-situ IR apparatus. The presence of NH3 decreased the CO conversion with time on stream, and the stop of NH3 slowly restored the catalytic activity. Figure 6a shows the IR spectrum over the region of C-O stretching vibrations before the poisoning. A broad peak comprised of a few bands was observed at 1850-2100 cm-1. It has been reported that the monocarbonyls linearly adsorbed on Ru0 were characterized by the band at 2040 cm-1,39,40 and the bands at 1850-2050 cm-1 were composed of the carbonyls on Ru sites with different oxidation states.16,41-46 The band at ∼1950 cm-1 might also arise from bridge-adsorbed carbonyls.46 Figure 6b-d shows the IR spectra at 60, 180, and 330 min after the start of 31 ppm 14NH3 supply, respectively. The intensity of the band at ∼2020 cm-1 assigned to the monocarbonyls linearly adsorbed on Ru0 decreased with time on stream. Yokomizo et al. reported that the monocarbonyls linearly adsorbed on a Ru/SiO2 catalyst constituted the main active species for the CO oxidation.47 With the decrease in the intensity of the monocarbonyl band, a shoulder band at 2060 cm-1 assigned to Run+(CO)2 dicarbonyls became clear.42,43,48-58 In addition, a new band assigned to Rum+(CO)3 tricarbonyls appeared at 2176 cm-1 and the intensity increased with time on stream, which suggests that Ru sites were partly oxidized to disrupt the Ru-Ru bonding.39,47 As shown in Figure 6c, a band attributed to nitrosyls linearly adsorbed on Ru appeared at 1830 cm-1 at 180 min,56 and the intensity increased with time on stream. The increase in the intensity of the nitrosyl band at 1830 cm-1 always accompanied the increase in the intensity of the multicarbonyl band. It indicates that the nitrosyls oxidized parts of Ru sites, which might be bonded to the nitrosyls or next to the Ru sites bonded to the nitrosyls.16 Therefore, the activation of both oxygen and CO was suppressed. After 14NH3 was stopped at 330 min, the CO conversion was slowly restored

Deactivation of Ru/Al2O3 Catalyst by NH3

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Figure 7. In-situ IR spectra over the region of the N-O stretching vibrations on the Ru catalysts for the H2 oxidation: (a) at 180 min after the start of 31 ppm 14NH3 supply; (b) at 10 min after the stop of the air supply over the catalyst, which had been poisoned with 14NH3 for 180 min and employed on the reaction in the absence of NH3 for 60 min. Feed: 90.9 vol % H2, 0 or 0.7 vol % O2, and 2.6 vol % N2 balanced with He; temperature, 150 °C.

coincidently with an increase in the intensity of the monocarbonyl band at ∼2020 cm-1 and a decrease in the intensity of the tricarbonyl band at 2176 cm-1 (Figure 6e). In addition, the intensity of the nitrosyl band at 1830 cm-1 decreased and became unclear. After NH3 was stopped, the nitrosyls were slowly decomposed and the oxidized Ru sites were reduced to restore the catalytic activity. To confirm the assignment of the bands, the catalyst was poisoned with 138 ppm 15NH3. Figure 6f shows the spectrum at 180 min. The band at 1830 cm-1 for the 14NH3 poisoning shifted to 1817 cm-1 for the 15NH3 poisoning. Thus it was confirmed that the band was assigned to the nitrosyls. The in-situ IR spectra during the H2 oxidation without CO, CO2, and H2O were also taken in the presence of 31 ppm 14NH3. The nitrosyl band appeared at 1834 cm-1 at 30 min, and the intensity increased with time on stream. The intensity of the nitrosyl band at 180 min for the H2 oxidation (Figure 7a) was larger than that at 330 min for the PROX (Figure 6d), which is consistent with the fact that the catalyst was more slowly deactivated in the presence of CO than that for the H2 oxidation without CO (Figure 5). For the H2 oxidation in the in-situ IR apparatus, the band intensity at 1834 cm-1 has hardly changed for the next 60 min after the stop of 14NH3, because the nitrosyls largely covered the Ru sites. However, the stop of air supply for 10 min significantly diminished the intensity of the nitrosyl band (Figure 7b). It is indicated that the nitrosyls were easily reduced under a reduction atmosphere, which is consistent with the fact that the catalyst was deactivated more slowly at a higher temperature or at lower [O2]/[CO] ratios. The catalysts were also poisoned with NO and NO2 (31 ppm) in the PROX in the in-situ IR apparatus. The nitrosyl band appeared at 1825 cm-1 on the Ru catalyst poisoned with NO as early as 40 min and shifted to 1844 cm-1 with time on stream. A broadband assigned to nitrates on Al2O3 at ∼1510 cm-1 also appeared at 40 min and increased with time on stream.59,60 The stop of NO decreased the nitrosyl band; however, it did not affect the nitrate band. It suggests that the nitrates did not contribute to the deactivation. The rapid formation of nitrosyls was also observed for the NO2 poisoning. Both conversions in the presence of NO were lower than that in the presence of NH3 (Table 1), which indicates that the deactivation depended on the rate of NH3 oxidation and that high O2 concentration promoted the NH3 oxidation. It has been reported that Ru adsorbed NO more strongly than CO, while the other precious metals adsorbed CO more strongly than NO.13,15-18,31 Therefore, the CO oxidation was suppressed over the Ru catalyst by the nitrosyls. To investigate the influence of the nitrosyls on the other species, the catalyst was poisoned with 31 ppm 14NH3 for 2 h

Figure 8. Changes in the IR spectra of the catalyst poisoned with 14 NH3 for the H2 oxidation at 150 °C by the following CO supply at 60 °C: (a) before the introduction of CO over the poisoned catalyst; (b) at 10 min after the start of CO supply; (c) at 30 min. The poisoning was conducted for 2 h under the same condition as that in Figure 7.

in the H2 oxidation without CO, CO2, and H2O in the IR apparatus, cooled to 60 °C in a He flow, and then CO diluted with N2 was supplied at 60 °C. Before the CO supply, a large band of nitrosyls was observed as shown in Figure 8a; however, the intensity of the nitrosyl band rapidly decreased after the start of the CO supply. At 10 min after the start of the CO supply, a small band attributed to the isocyanates was observed at ∼2230 cm-1,12,13,20,23 besides the carbonyl bands at 2175, 2046, and 1980 cm-1(Figure 8b). Since the isocyanate band disappeared at 30 min (Figure 8c), the isocyanates might spill over to the Al2O3 support and be hydrolyzed to NH3 by H2O adsorbed on the support. Therefore, the isocynates are probably the intermediate for the formation of NH3 for the NO and NO2 poisoning. In addition, the decrease in the intensity of the nitrosyl band accompanied an increase in the intensity of the monocarbonyl band at 2046 cm-1 and a slight decrease in the intensity of the tricarbonyl band at 2176 cm-1. It indicates that the monocarbonyls easily replaced the nitrosyls. The intensity of the monocarbonyl band was large even at 10 min, so that only small parts of the surface Ru sites might be oxidized by the poisoning. The proposed deactivation model is shown in Figure 9. NH3 was oxidized to nitrosyls, which hindered the adsorption of CO and stabilized the oxidized Ru under an oxidation atmosphere (Figure 9(1)). For the NO and NO2 poisoning, NH3 was generated, which indicated that isocyanates were formed by the reaction of the adsorbed N and CO and were hydrolyzed to NH3 and CO2 (Figure 9(2)). However, the isocynates did not mainly cause the deactivation, since the H2 oxidation was deactivated more rapidly than the PROX. The in-situ IR spectra for the NO poisoning indicated the oxidation to NO2 (Figure 9(3)); however, the stop of NO did not change the nitrate bands, which suggested that the nitrates were mainly adsorbed on the support. The nitrosyls were decomposed under a reduction atmosphere. Voorhoeve et al. studied the reduction of NO over various Ru catalysts in a gas composition of 0.13 vol % CO, 0.4 vol % H2, 1.3 vol % CO, 3 vol % H2O, and 3 vol % CO2 balanced with He at 150-500 °C.33 They reported that N2O, N2, and NH3 were exhausted at 150-200 °C. Thus N2O and N2 was probably formed under a reduction atmosphere (Figure 9(6, 7), in addition to the NH3 formation by the hydrolysis of isocyanates (Figure 9(2)). It is unlikely that NO reduction to NH3 by H2 (Figure 9(8)) occurred over this low-temperature range.25

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Figure 10. Activities of (a) the Pt/Al2O3 and (b) Ru/Al2O3 catalysts before the poisoning and at 240 min after the start of 75 ppm NH3 supply. Black bar, CO conv.; hatched bar, O2 conv.; gray bar, NH3 conv. Reaction conditions are the same as those shown in Figure 1. [O2]/[CO] ) 1.4; temperature, 150 °C.

Figure 9. Deactivation and reactivation models over the Ru/Al2O3 catalyst.

TABLE 2: Results of CO Adsorptiona sample

Ru dispersion (%)

fresh catalyst catalyst employed on the reaction in the absence of NH3 for 1 h catalyst poisoned with 21 ppm NH3 for 4 h catalyst poisoned with 21 ppm NH3 for 41 h

27.8 26.9 22.9 23.6

a Reaction conditions are the same as those shown in Figure 1. [O2]/ [CO] ) 1.9.

As shown in Figure 5b, for the PROX in the absence of CO2 and H2O, both CO and O2 conversions were kept high for 100 min, though the NH3 conversion was relatively high. On the other hand, the H2 oxidation without CO, CO2, and H2O was rapidly suppressed (Figure 5a). Since it has been reported that NO reduction by CO was faster at low temperature (