Infrared Studies of Adsorbed Dinitrogen on Supported Ruthenium

Publication Date: November 1994. ACS Legacy Archive. Cite this:J. Phys. Chem. 98, 44, 11293-11300. Note: In lieu of an abstract, this is the article's...
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J. Phys. Chem. 1994, 98, 11293-11300

11293

Infrared Studies of Adsorbed Dinitrogen on Supported Ruthenium Catalysts for Ammonia Synthesis: Effects of the Alumina and Magnesia Supports and the Cesium Compound Promoter Jun Kubota and Ken-ichi Aikaa Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama, 227 Japan Received: February 22, 1994; In Final Form: August 22, 1994@

Infrared absorption due to adsorbed dinitrogen on Ru metals supported on A1203 and MgO is observed at 2214 and 2168 cm-', respectively, under N2 atmosphere at 160-300 K. The different wavenumbers were obtained depending on the reduction temperature of the samples. On cesium nitrate doping, the bands shifted to lower frequencies such as 2095 cm-' for Ru-Cs+/A1203 and 1910 cm-' for Ru-Cs+/MgO. These frequencies are related to the electronic states of Ru particles, which are affected by the supports and the promoters. These surface natures are discussed in connection with the ammonia synthesis activities. The heat of adsorption of dinitrogen on 10 wt 9% RtdMgO is estimated to be 14 kJ mol-'. Through the infrared absorption of adsorbed dinitrogen, the change of the surface Ru state by the oxidation-reduction cycles was studied. Ru particles became more dispersed on MgO surfaces by the oxidation-reduction cycles. Active ammonia catalysts were characterized by an FTIR through the adsorbed dinitrogen.

Introduction Ruthenium catalysts are known to be most active for dinitrogen activation or ammonia synthesis; the activities are remarkably increased with the addition of alkali metal promoters.lS2 A type of promoted ruthenium catalyst was beginning to be used in a commercial ammonia plant in 1992.3 In the ammonia synthesis reaction, the rate-determining step is the dissociation of dinitrogen on the surface through the precursor of adsorbed dinitrogen. Thus, it is important to study the adsorbed dinitrogen species. We recently observed the infrared absorption of adsorbed dinitrogen on Ru catalysts by FTIR! Ru catalysts supported on basic oxides such as MgO are more active for ammonia synthesis than those on acidic oxides such as &03.5 The basic supports donate electrons to Ru surface atoms, which promote the dissociation of dinitrogen. Furthermore, Ru catalysts promoted with alkali metals or alkali metal oxides, which are considered to be more electronegative compounds, give the highest activities under atmospheric pres~ure.~Such an electronic change of surface Ru atoms, which affects the ammonia synthesis activities, must be reflected in the N-N frequencies adsorbed on various Ru catalysts. Eishens and Jacknow fiist obtained infrared spectra of adsorbed dinitrogen on Ni/Si02 (2202 cm-1).6 Numerous infrared studies have been done on adsorbed dinitrogens molecules on various supported metals such as Ir (2185 cm-'),' PdA1203 (2260 cm-1),8 WSi02 (2230 ~ m - ' ) ,Fe/MgO ~ (2200 cm-l),l0 Rh/A1203 (2256 cm11),11,12and CoIA1203 (2214 cm-l).13 There is only one report on the infrared spectra of dinitrogen on supported Ru surface. Lyutov and Borodko have reported the absorption at 2240 cm-' for a Nz-RdSiOz c0mp1ex.l~ However, most of the catalysts mentioned above are not effective ammonia catalysts. Even RdSiOz has no activity. Fe/MgO is the only active catalyst; however, the dinitrogen species are obtained not by gaseous N2 but by NH3 decomposition, and the spectra are not clear. We have reported infrared spectra of adsorbed nitrogen on the active ammonia

* To whom correspondence should be addressed. @

Abstract published in Advance ACS Absrracrs, October 1, 1994.

0022-365419412098-11293$04.5010

catalyst, Ru-WA1203; however, the species was quite stable even under vacuum at high temperatures.15J6 The peaks were assigned to species of dissociated nitrogen which interacted strongly with the supports15and the promoters.16 Thus, adsorbed dinitrogen, which should be the precursor of ammonia synthesis, was not investigated on the active ammonia catalysts including Ru catalysts. There are three kinds of adsorbed nitrogen on metal surfaces: end-on (y-state) dinitrogen, side-on (a-state) dinitrogen, and dissociated @-state) nitrogen atoms." Generally, dinitrogen is considered to be dissociated through the y -state to the a-state and finally the p-state. The end-on species are the weakest adsorbates that are desorbable at a temperature lower than 150 K under vacuum. The N-N stretching mode of this species is infrared active and observed around 2200 cm-'. The side-on species are important for dinitrogen activation because the form must be similar to the activated complex of the dissociation. However, the side-on species are generally hard to detect by infrared spectroscopy because of the symmetry, despite the importance. The side-on dinitrogen has been detected at 1590-1490 cm-' on Fe single crystals by highresolution electron energy loss spectroscopy (HREELS),18*19 where the surface selection rule is not so strict because of the existence of the impact-scatteringmechanism. The dissociated nitrogen atoms are the product after the rate-determining step of ammonia synthesis.' It is easily hydrogenated to ammonia with the equilibrium concentration. The dissociated species have long been investigated. The heat of dissociative adsorption has been well studied on metal surfaces, and the adsorption forms have been studied on single-crystal surfaces by lowenergy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). X P S spectra of N(1s) can indicate the kind of species because of the energy deference. Thus, electron spectroscopy is a very strong technique,*O but it should be measured in a vacuum. On the other hand, infrared spectra can be taken with the presence of gaseous molecules; thus, we can study the precursor y-state under the near in situ condition of dinitrogen activation or ammonia synthesis. We recently found the infrared spectra of adsorbed dinitrogen

(D 1994 American Chemical Society

Kubota and Aika

11294 J. Phys. Chem., Vol. 98, No. 44, 1994 Closed circulation system

1

50 mm

Thermoc0,uple

3729 3678

O-ring Epoxy resin

SUS tube for liquid Nz cooling

Figure 1. Schematic view of the infrared cell for transmission infrared spectroscopy. on the active Ru catalysts for the first time.4 In this paper, we report the results of a more detailed study of these spectra, which should reflect the surface states of the active Ru catalysts: R d A1203, RulMgO, R~-Cs+/A1203, and Ru-Cs+/MgO.

Experimental Section Sample Preparation. The Ru catalysts were prepared by the following methods: y-Al203 (184 m2/g) and MgO (90 m2/ g) were calcinated in air at 773 K for 5 h and then impregnated with Ru3(C0)12 in the stirred tetrahydrofuran solution at room temperature for 4 h. The solvent was evaporated at the room temperature or lower. Then, the samples were heated at 673 K Torr; 1 Torr = 133 Pa) for 2 h. under vacuum (ca. Cesium-promoted samples were obtained by impregnating the prepared supported Ru catalysts with CsN03 aqueous solution at ambient temperature for 4 h and drying at 373 K in air. The contents of Cs promoter are represented as the atomic ratio of Cs and Ru, and the Ru contents are indicated by the weight percentage: (Ru)/(Ru support). The dried samples were pressed to self-supported infrared disks (20 mm diameter, 20-50 mg) by an oil press (6000 kg/ 20 mm diameter disk) and then mounted in the center of the infrared cell. The samples were heated under vacuum at the pretreatment temperatures and then treated with circulated H2 (200 Torr) with a liquid nitrogen trap for 12 h. The nitrate of Cs-promoted samples was decomposed under H2 atmosphere. The samples were then evacuated at the same temperature for 1-2 h to remove the hydrogen. After the pretreatments, the sample temperature was controlled quickly for the sake of the measurement. Commercial grade N2, H2, and He were purified through a reduced active copper catalyst (473 K) connected to a liquid nitrogen trap. Oxygen gas was purified through a dry-ice methanol trap. A glass-bottled nitrogen isotope (15N2)was used without purification. Apparatus. The infrared cell made from quartz is illustrated in Figure 1. The temperature of the cell can be controlled between 160 and 1000 K by cooling with liquid nitrogen and heating electrically. The temperature was monitored by a thermocouple attached near the sample in a quartz tube. The sample disk was mounted on a sample holder at the center of the cell. NaCl windows were sealed by Viton O-rings attached to the aluminum flanges, which adhered to the cell by epoxy resin. The cell was connected to a closed-circulation glass system. The spectra were taken by an FTIR spectrometer (Japan Spectroscopic Co. Ltd. Model FTIR-5300) equipped with a triglycine sulfate (TGS) detector. The spectrometer sitting on a movable table was placed to give the infrared beam at the

+

1

1

4000

1

1

3000

"

"

l

2000

wavenumber

/

1000

cm-'

Figure 2. Background spectra of various Ru catalysts at 300 K after pretreatment at 773 K: (a) 2 wt % RulMgO; (b) 2 wt % Ru-Cs+/ MgO (CSRU= 1); (c) 2 Wt % RdA1203; (d) 2 wt % Ru-Cs'lA1203 (CsRu = 8). center of the sample. Usually 32 or 64 scans with 2 cm-' resolution, for 2 or 4 min, were repeated to obtain the spectra.

Results Background Spectra. FTIR spectra of 773 K-pretreated samples, 2 wt % RdAlzO3, 2 wt % RulMgO, 2 wt % RuCs+/A1203 (CS/RU= g), and 2 wt % Ru-Cs+/MgO (CS/RU= l), are shown in Figure 2. After pretreatment at 773 K, no adsorbed species were observed on RulMgO. Peaks at 987 and 846 cm-' were assigned to MgO phonon modes. Negative peaks around 2340 cm-' were due to instability of atmospheric C02 in the optical path. On Ru/A1203, there were four peaks around the 3600 cm-' region, which were assigned to surface hydroxyl groups of Al2O3. It has been reported that the hydroxyl groups are desorbed at a higher temperature.21 The cesium-promoted samples had characteristic peaks, which appeared at 3772 and 3757 cm-' on Ru-Cs+NgO. These absorptions were considered to be due to hydroxyl groups strongly interacting with Cs promoter because these peaks were absent on Ru/MgO. We have suggested that CsN03 on R d MgO is tumed to CsOH during the reduction with hydrogen. The appearance of the OH group supports this. Infrared peaks of nitrate and nitrous ions are usually observed at the 15001000 cm-' region. The absence of the absorption at this region (Figure 2b) also suggests the complete decomposition of CsN03.'s5 For Ru-Cs+/Al203, absorptions due to hydroxyl groups were not detected (Figure 2d), and new bands appeared at 1348 and 1222 cm-I instead. The absence of hydroxyl groups on A1203 may be due to the interaction of the Cs promoter with the hydroxyl groups. This sample contains more Cs (Cs/Ru = 8) than Ru-Cs+/MgO (Cs/Ru = 1). Each sample has a proper Cs content to give the maximum activity for the ammonia ~ynthesis.~The new bands at 1348 and 1222 cmcl were assigned to nitrate or nitrous species. The nitrato complexes generally have the vibrational modes in this frequency region, as summarized in Table 1F2 It was not possible to clarify the detail of the form of this species; however, obviously the precursor of the Cs promoter, CsN03, was not decomposed completely to hydroxylates or oxides in the case of RdA1203. The O N 0 3 promoter is completely tumed to CsOH on R d MgO surface, whereas it remains as CsNO, (x = 2 or 3) species

Adsorbed Dinitrogen on Supported Ru Catalysts

J. Phys. Chem., Vol. 98, No. 44, 1994 11295

TABLE 1: Vibrational Frequencies of Stretching Modes in Nitro and Nitrato CompoundsZ2

stretching frequencykm-' NOzN03a

K3Ba[Ni(N0~)61 [Ni(en)z(NO3)~1" [Ni(en)zNOs]Clog

unidentate bidentate

1343 1420 1290

1476

1306 1305 1025

1008

en: ethylenediamine. 2214

2198

I

I 973 K 873 K

I13 K 673 K 1

~

1

1

1

1

1

1

1

'

'

'

I

I

2400

2200

2000

wavenumber i cm

1800

-'

2400

I

I

I

I

2200

I

"

l

"

'

I

2000

wavenumber i cm

1800

-'

Figure 3. Isotope shifts of FTIR spectra of adsorbed dinitrogen at 300 K under 50 Torr I4N2(a, c) and l5N2 (b, d), on 2 wt % RdA1203 pretreated at 873 K (a, b) and on 2 wt % Ru/MgO pretreated at 673 K

Figure 4. FTIR spectra of adsorbed dinitrogen at 300 K under 50 Torr on 2 wt % RdAlz03 samples pretreated at various temperatures.

(c, d).

observed peaks were assigned to the N-N stretching mode of end-on species of dinitrogen adsorbed on Ru metals on the basis of the frequencies and isotope shifts. Change of Spectra Due to Treatment Temperature of Catalysts. The 2 wt % Ru/Al203 samples were treated at various temperatures under 200 Torr of circulated hydrogen and then under vacuum. FTIR spectra of dinitrogen adsorbed on those samples at 300 K are displayed in Figure 4. On the sample reduced at temperatures lower than 673 K, infrared absorption of adsorbed dinitrogen did not emerge. A peak due to adsorbed dinitrogen appeared at 2268 cm-' on the sample pretreated at 673 K. This was tentatively assigned to dinitrogen adsorbed on the partially oxidized Ru as described in the discussion. When the pretreatment temperature was increased, a new band appeared at 2214 cm-' (Figure 3a and 4) above 773 K and shifted to 2198 cm-'. These were assigned to dinitrogen adsorbed on the metallic Ru sites (see Discussion). Infrared spectra of adsorbed dinitrogen on 2 wt % Ru/MgO pretreated at various temperatures are shown in Figure 5 . The bands were observed at 2170 and 2100 cm-' on the sample reduced at 673 K. The band at 2170 cm-' increased in intensity and was shifted slightly to lower frequency (2165 cm-'), with increasing pretreatment temperature. The band at 2170 cm-' was tentatively assigned here to dinitrogen species adsorbed on Ru metal particles without interaction with the support MgO and another one at 2100 cm-' was due to dinitrogen adsorbed on Ru sites affected by the support. Heat of Adsorption of Dinitrogen. As will be shown in the Discussion, the spectra reflect the condition of Ru atoms. The adsorbed dinitrogens were roughly classified into two forms, the species on the Ru site free from the support effect (higher frequency) and that on Ru site interacting with MgO strongly (lower frequency), for 2 wt % Ru/MgO. However, the lowfrequency band was little observed on 10 wt % Ru/MgO pretreated at 673 K. Probably because of the larger Ru particle size, the Ru part interacting with MgO may be small. Further,

on Ru/A1203 surface. Of course it is possible to form partly cesium oxide, which is not observable by infrared spectroscopy. Cs promoters are thus described as Cs+ in this paper. On these samples, absorptions at around 2000 cm-' due to carbonyl groups were absent, and there were no peaks in the C-H stretching and C-H deformation region even after the reduction with hydrogen. It was concluded that carbonyl species of the R u ~ ( C O ) ~precursor Z were completely decomposed or desorbed, and there seemed to be no carbon contamination on the samples. The spectra shown in the remainder of this report are the differential absorbances between the spectra of the sample after the interaction with the adsorbates and that before the interaction, which can erase the background spectra. Isotope Shifts of Adsorbed Dinitrogen. FTIR spectra of dinitrogen (14N2) and heavy dinitrogen (15N2)adsorbed at 300 K on 2 wt % Ru/Al2O3 and 2 wt % Ru/MgO under 50 Torr are shown in Figure 3. A peak due to the N-N stretching mode was observed at 2214 cm-' with a shoulder at 2268 cm-' for 2 wt % Ru/Al203 pretreated at 873 K. The shoulder peak is discussed in the following section. The frequency ratio of the N-N stretching mode in the nitrogen isotopes, 2214 cm-'/2143 cm-' = 1.033, agreed closely with the theoretical value, (15/14)1'2= 1.035. The species were observed only under N2 atmosphere, which meant the adsorption was reversible. The frequency of this peak corresponds to the data on Ru(0001) single-crystal ~ u r f a c e ,although ~ ~ . ~ ~ it is a little lower than that on R ~ d S i 0 2 . ' ~ The infrared peaks of adsorbed dinitrogen on 2 wt % Ru/ MgO were observed at 2168 cm-' with a broad peak around 2 100 cm-' . The nature of the adsorbed species comparing these plural peaks is discussed in the following section also. These peaks were reasonably shifted to lower frequencies in the case of 15N2;(2168 cm-'/2090 cm-' = 1.037). Consequently, the

Kubota and Aika

11296 J. Phys. Chem., Vol. 98, No. 44, 1994 2165

n 0 N2

50 pressure I Torr

100

Figure 7. Adsorption isotherms of dinitrogen on 10 wt % RulMgO pretreated at 673 K estimated from integrated absorbance of the dinitrogen band. 1

1

1

1

1

2400

1

1

1

2200

1

1

1

1

wavenumber I cm

1

1800

2000

-'

A

2.0

Figure 5. FTIR spectra of adsorbed dinitrogen at 300 K under 50 Torr on 2 wt % RulMgO samples pretreated at various temperatures.

1.5

2204 I

0.5

0

B

3.0

6

'3 I

'

l

I

I

'

l

I

I

2200 2000 wavenumber / cm-'

Figure 6. Pressure dependence of l T I R spectra of adsorbed dinitrogen on 10 wt % R m g O pretreated at 773 K.

the 10 wt % Ru/MgO sample was free from the contamination in the vacuum system because of the large Ru surface area. These are the reasons the heat of adsorption of dinitrogen was measured on the 10 wt % Ru/MgO sample. Infrared spectra of dinitrogen on 10 wt % Ru/MgO were measured from 0 to 100 Torr (equilibrium pressure) NZat 303343 K. Some of these spectra are shown in Figure 6. The adsorption isotherms, which are obtained from the integrated intensities of the adsorbed dinitrogen bands, are displayed in Figure 7. The heat of adsorption at each amount of adsorption was obtained from the Clausius-Clapeyron equation:

Q = -R(a In P/a( l/Z",)),, where Q,R, T, P , and v are heat of adsorption, the gas constant, temperature, equilibrium pressure, and the amount of adsorption, respectively. The results of this analysis are shown in Figure 8a as log P versus UT, and Figure 8b as Q versus v . The initial heat of adsorption of dinitrogen on 10 wt % Ru/MgO was thus estimated to be 14 kJ mol-' by extrapolating v to 0 in Figure 8b. The heat of adsorption decreases with the increase of

3.3

1 5 r - - - - - 7

' 13

2400

3.1 3.2 1OOOm (K)

g

P

12

%

3

11

L:

10

-

0

0.5 1.0 integrated peak intensity / a. u.

Figure 8. Analysis of heat of adsorption of dinitrogen (see text). (A) Clausius-Clapeyron plots. (B) Obtained heat of adsorption as a function of the coverage.

amount of adsorption. The value obtained here is near the reported values: 15 kJ mol-' on Pt/SiOZ, 9.4 kJ mol-' on Rh/ A1203,and 43 kJ mol-' on M i 0 2 through measurements by infrared spectros~opy.~,~However, precise discussions seem to be difficult because each calculation method is different and the heat of adsorption depends on the amount of adsorption. Oxidation Treatment. FTIR spectra of adsorbed dinitrogen on 2 wt % Ru/A1203, which were treated under 10 Torr of 0 2 at various temperatures for 10 min followed by the reduction with 200 Torr of H2 at 773 K for 30 min, are shown in Figure 9. For the samples oxidized above 473 K, the peak was split to two (2225 and 2202 cm-I). The peak at 2225 cm-' disappeared, and the band at 2202 cm-' decreased as the oxidation temperature was increased. Although it was difficult to assign each peak split by the oxidation treatment, both may be assigned to dinitrogen adsorbed on the metallic Ru site on the A1203 support. The decrease of the peak intensity must be due to the irreversible oxidation of

J. Phys. Chem., Vol. 98, No. 44, 1994 11297

Adsorbed Dinitrogen on Supported Ru Catalysts

T

2220

I

2214

nonoxidized

n

473 K 523 K 573 K 623 K 673 K I

~

2400

f

i

~

I

*

~

~

Yd;\

J

2200 2000 wavenumber / cm.'

2095

Figure 9. FTIR spectra of adsorbed dinitrogen on 2 wt % RdAl203 oxidized under 10 Torr of 0 2 at various temperatures followed by Hz reduction at 773 K for 0.5 h. The spectra were measured at 300 K under 50 Torr of Nz.

I

2160

2400

1

T

-1

~

~

'

'

'

'

2200 2000 wavenumber / cm"

'

l

"

'

l

1800

Figure 11. FlTR spectra of adsorbed dinitrogen on various 2 wt % Ru-Cs+/Al203 samples pretreated at 773 K. The contents of Cs promoter are shown in the figure. The spectra were taken at 300 K under 50 Torr of Nz.

nonoxidized

473 K 523 K 573 K 623 K

2048

3 1

2400

1

1

1

1

2200

1

\=_ 1

1

1

2000

1

1

1

673K 723 K 1

1800

wavenumber I cm

Figure 10. FTIR spectra of adsorbed dinitrogen on 2 wt % RulMgO oxidized under 10 Torr of 0 2 at various temperatures followed by Hz reduction at 673 K for 0.5 h. The spectra were measured at 300 K under 50 Torr of N2.

Ru metal particles or evaporation of it as an Ru oxide, which reduces the number of surface Ru metal atoms. An interesting situation was found on 2 wt % RulMgO. FTIR spectra of adsorbed nitrogen on 2 wt % Ru/MgO, which were treated under 10 Torr of 0 2 at various temperatures for 10 min followed by the reduction with 200 Torr of H2 at 673 K for 30 min, are shown in Figure 10. The bands increased in intensity more than twice for samples treated at temperatures higher than 473 K. Furthermore, the band was shifted to lower frequency even by 32 cm-' and was split to three peaks. The details of the effects of the oxidation treatment are presented in the discussion. Effect of Cs+ Promoter. FTIR spectra of adsorbed dinitrogen on 2 wt % Ru/A1203 samples doped with various amounts of Cs+ promoter, which were treated at 773 K, are

shown in Figure 11. On the unpromoted sample, the peak was observed at 2214 cm-', which was shifted to lower frequency with increased Cs content. The peak was shifted to 2095 cm-' with shoulders at a lower frequency (1900-2000 cm-') for the sample with more Cs promoter (Cs/Ru = 8). The peak intensities were generally decreased with the Cs+ doping, which can be explained by Ru metal surfaces being covered with the Cs+ promoter. As discussed, the shifts to low frequency were due to an electron-donation effect from the Cs+ promoter to Ru metal surfaces. For R W g O , similar situations were observed as shown in Figure 12. These spectra were measured at 160 K because the absorption bands of adsorbed dinitrogen were weak and not detected clearly at 300 K. The metallic surface area becomes small or the adsorption becomes weak with doping of the Cs promoter. The peak at 2168 cm-' for unpromoted 2 wt % Ru/ MgO was shifted to lower frequency and decreased in intensity. At Cs/Ru = 0.1-0.2 a new band appeared at 1910 cm-'. The peak at 2070 cm-', which was assigned to species on a Ru site interacted with the MgO support, was affected a little by the promoter doping. The promoter effect to the bands appeared at lower Cs+ contents for Ru/MgO than for RdA1203. With higher Cs+ contents (Cs+/Ru =- 0.2), the strong promoter effect disappeared for Ru/MgO.

Discussion Nature of the Stretching Frequency of Dinitrogen Adsorbed on the Metal. When the dinitrogen is adsorbed on metal surfaces, the molecule donates the electrons of the highest occupied molecular orbital (HOMO, 3a,) and some of 2s to the surface metal atom and at the same time the metal atom donates its d electrons to the lowest unoccupied molecular orbital (LUMO, lng*) of N2.l' Because HOMO is the bonding and LUMO is the antibonding orbital, the N-N bond of adsorbed dinitrogen is weakened while the M-N bond is formed. Thus, the N-N stretching frequency reflects the electronic states of the adsorption metal site by the above rule. We have demon-

Kubota and Aika

11298 J. Phys. Chem., Vol. 98, No. 44, 1994

TABLE 2: Observed Frequencies (cm-') of Dinitrogen Adsorbed on 2 wt % Ru Catalysts, Nature of Adsorption Sites, and Ammonia Synthesis Activities pretreatment temperature Cs content condition low" highb Ru 10 wt % oxidation treatmentC lowd high' adsorption site Rus+ interacting with A1203 2 wt % R~IA1203 2268 2214 2220 metallic Ruo site 2198 2095 Cs interacting site (Ru6-) 214 pmol h-' g catal-I NH3 activity' 13 2204 metallic Ruo site 2 wt % Ru/MgO metallic Ruo site 2168 2165 (affected by MgO weakly) 2100 2128 metallic Ruo site 2092 2070 (interacting MgO strongly) 2048 2040 Cs interacting site (Rub-) 1910 strongly Cs interacting site (Ru6-) 420 1445 pmol h-' g catal-I NH3 activity' 60 873 K for RdA120, 673 K for Ru/MgO. 973 K for Ru/A1203,873 K for Ru/MgO. Oxidized at 673 K followed by reduction by hydrogen at 773 and 673 K for Ru/AlZ0and RulMgO, respectively. CslRu = 0.05-0.1 for Ru/MgO. e CslRu = 8 for Ru/A1203 and CsRu = 1 for Ru/ MgO. f Rate of NH3 synthesis from 760 Torr of N2 + 3H2 at 588 K @mol h-' g catal-I) reported in refs 2 and 26.

I

i

strated that the rates of ammonia synthesis on the Ru catalysts depend on the electronic state of Ru, which is affected chemically by the supports and the promoters.'~~The rates differ more than hundreds or thousands between the Ru catalysts. Thus, the infrared study of adsorbed dinitrogen is important to understand the state of an active site for ammonia synthesis. On the other hand, the adsorption states of dinitrogen itself can be the precursors of the rate-determining step of ammonia synthesis (dissociative adsorption of dinitrogen), although the lifetime might be quite short at the high temperature of the ammonia synthesis. The infrared study of NZ adsorbed on the active Ru catalysts has not been reported, although many investigations have centered on adsorbed dinitrogen and nitrogen atoms on single-crystal metal surfaces. Anton and colleaguesz3 and de Paola and associatesz4have reported spectra of adsorbed dinitrogen species on clean, potassium-precovered, and oxygenprecovered Ru(0001) surfaces using HREELS and infrared reflection absorption spectroscopy (IRAS). On clean Ru(0001) surfaces, adsorbed dinitrogen are observed at 2209-2195 cm-' at full coverage. The absorption frequency shifts to 30-40 cm-I lower position on the potassium preadsorbed Ru(OOO1) surface, whereas the frequency becomes 43-55 cm-' higher on an oxygen-modified one. It is evident that electron-rich Ru surfaces give low frequency and the poor Ru gives high frequency. Identification of Adsorption Site on RdA1203. Infrared absorption frequencies of adsorbed dinitrogen are summarized in Table 2. The bands at 2214-2198 cm-' were assigned to the stretching mode of the adsorbed dinitrogen on a metallic Ru site because the frequency corresponds to that on the Ru(0001) single-crystal ~ u r f a c e It . ~was ~ ~suggested ~~ that the shift from 2214 to 2198 cm-' and increased intensity with an increasing reduction temperature were due to the Ru particles growth (sintering) making more surfaces apart from the support A1203. The observed peak at 2268 cm-' was assigned to the adsorbed species on partially oxidized Ru sites because of the reduction characteristic and the higher frequency (see Figure 4). It was also speculated that this Ru site was directly interacted with the A1203 support because the peak at 2268 cm-' was only observed on the A1203 supported sample. The oxidation-reduction cycle just decreased the metallic Ru site (2220 cm-I); this may be due to the particle growth or loss of Ru through RuO4. It has been speculated that the Cs promoter does not interact well with Ru but does with Alz03.26 As shown in Figure 2,

tau atomic ratio 0

0.01 0.05

0.2 0.5

I

2400

#

1

1

1

2200

1

1

1

1

1

1

1

1

2000 1800 wavenumber I cm"

1

1

1

1

1600

Figure 12. FTIR spectra of adsorbed dinitrogen on various 2 wt % Ru-Cs+/MgO samples pretreated at 873 K. The contents of Cs promoter are shown. The spectra were taken at 160 K under 50 TOH of Nz.

the doped Cs promoter seems to react with hydroxyl groups of AIzO3, so that the promoter is suggested not to interact with Ru particles effectively. On the other hand, the promoter was decomposed to CsOH or Cs20 near the Ru particles and mainly located on the Ru particles in R ~ h l g O . ~

Identification of Adsorption Site on Ru/MgO. The absorption frequency of adsorbed dinitrogen on Ru/MgO (2165-2170 cm-') was lower than that on Ru/A1203. However, the authors assigned this to well-reduced Ru particle atoms on the support MgO, because the peak intensities at 2165-2170 cm-' did not increase with the increased reduction temperatures over 723 K. On the Ru/AI203 sample, the peak due to the dinitrogen species on the metallic site increased in intensity with reduction temperatures above 873 K. Thus, R a g 0 seems to be reduced easily at a lower temperture than RdAlzO3. In other words,

J. Phys. Chem., Vol. 98, No. 44, 1994 11299

Adsorbed Dinitrogen on Supported Ru Catalysts A1203 seems to resist the complete reduction of Ru particles. The particle size of Ru may also affect the reduction characteristics. The dinitrogen on well-reduced Ru was observed at 2204 cm-' on 10 wt % Ru/MgO sample, as shown in Figure 6, probably because this sample has large Ru particles. This suggests that the small Ru particles are also affected electronically by the support even if the site is not directly connected with the support. The MgO support, which is a basic oxide, affects the Ru particles being electronically negative; thus, the support effect is small for the large particle (10 wt % RulMgO). On the other hand, acidic oxide A1203 makes the frequency higher. We consider that the remarkable increase in the peak intensity by oxidation-reduction treatment of Ru/MgO is due to a reconstruction of Ru particles by oxidation, that is, the Ru particles on MgO are disrupted through oxidation and dispersed on the support, and the surface area of Ru metal on MgO is increased after the reduction. For Ru/Al2O3, Ru particles are already well dispersed on the support, so that an increase of the Ru surface area was not observed. The frequency shifts of adsorbed dinitrogen by alkali metal doping have been investigated on IURu(O001) surface by IRAS, and the frequencies are shifted to 45 cm-I lower through coadsorption of K metal (6' = 0.05).24 In the present work, the absorption band was shifted to a lower frequency by about 120 cm-' with addition of the Cs promoter. It is noteworthy that the Cs salts (weaker electron donor) on the Ru/MgO sample cause a stronger effect than metallic K atoms (stronger electron donor) on the Ru single-crystal surface. There, the measurements of HREELS are done for irreversible adsorbates at low temperature under ultrahigh-vaccum (UHV)conditions, whereas reversible adsorbates under gaseous atmosphere are measured in the present case. This is one of the reasons that the adsorbed dinitrogen on the highly promoted Ru site could be detected. The new broad shoulders at 1900-2000 cm-' were assigned to a dinitrogen species on Ru sites, which were interacted with the Cs promoter directly (short-range interaction), because an adsorption state was created with the Cs addition. On the other hand, the frequency shift from 2214 to 2095 cm-' was due to weak interaction of Ru particles with the Cs promoter as a longrange interaction. Relevance to Ammonia Synthesis Activities. The ammonia synthesis activities of Ru catalysts are drastically affected by the supports and the promoter^.^^^^^ It has been well-known that electronegative compounds promote the synthesis rate, Le., dinitrogen activation. The N-N stretching frequency must reflect the electronic conditions of the adsorption sites, which control the synthesis rate. The ammonia synthesis rates at 588 K over 2 wt % Ru/A1203, 2 wt % Ru/MgO, 2 wt % Ru-Cs+/ A1203 (Cs/Ru = 8), and 2 wt % Ru-Cs+/MgO (Cs/Ru = 1) have been reported in the previous paper,5126as summarized in Table 2. The ammonia synthesis activities over most of the Ru catalysts used in this work were checked again and proved to have values similar to the previous data. The 2 wt % Ru/ A1203 catalyst was the least active, which corresponds to the highest frequency (2214 cm-'). A more active catalyst, 2 wt % R a g 0 shows the dinitrogen absorption at 50 cm-' lower than that for Ru/Al2O3. The Cs-promoted samples, which are extremely active, show even lower frequencies (2050-2100 cm-l). Moreover, for active catalysts, the specific shoulder peaks were assigned to the dinitrogen species adsorbed on Ru atoms interacted to the supports and the promoters directly, and a detailed discussion is necessary to clarify the contribution of

these sites to the dinitrogen activation. These problems will be published elsewhere. Relevance to the Mechanism of Ammonia Synthesis. In the present work, the a-state (side-on) adsorbed dinitrogen was not observed because the species might be unstable or inactive for infrared spectroscopy. However, the side-on species can be an intermediate from the y-state (end-on) to the p-state (atoms). The importance of the side-on species for the dinitrogen activation has been reported on Fe surfaces under UHV conditions.lg On Ru catalysts, the side-on type of dinitrogen has been reported by TPD techniques, and it is clarified that the amount of adsorption of this type is It is considered that no detection of side-on species on Ru catalysts is due to not only the infrared selection rule but also the small coverages. The end-on species was found to be adsorbed with exothermal heat of 14 kJ mol-'. This species may not be observed at the temperature of ammonia synthesis like 573 K, because of the short lifetime or weak adsorption. However, the end-on type must still be the "short-lifetime" intermediate when N2 first interacts with the surface Ru atoms. This may proceed into the side-on intermediate.

Conclusions The dinitrogen adsorption on supported Ru catalysts for ammonia synthesis was investigated by infrared spectroscopy, and the characteristics of catalysts were clarified. The infrared absorptions of adsorbed dinitrogen were observed at 2214 cm-' on 2 wt % RdAl203, 2168 cm-' on 2 wt % Ru/MgO, 2095 cm-' on 2 wt % Ru-Cs+/A1203, and 1910 cm-' on 2 wt % Ru-Cs+/MgO. The wavenumber, which reflects the N-N bond strength, was related to the activity of ammonia synthesis over these samples. The catalyst with a low wavenumber is the active catalyst. The study of reduction temperature effect indicated that the metal particles in Ru/MgO samples were reduced by hydrogen more easily than those in Ru/A1203. The peaks of adsorbed dinitrogen have the characteristic shoulder (2268 cm-' for Ru/A1203,2100 cm-I for Ru/MgO, 1950 cm-' for Ru-Cs+/A1203, and 2020 cm-' for Ru-Cs+/MgO); suggesting the surface heterogeneity of Ru metal surfaces. The estimated initial heat of adsorption of dinitrogen on 10 wt % Ru/MgO was 14 kJ mol-', which is similar to values reported for other metal catalysts. This adsorption was weak and reversible. Oxidation-reduction cycles change the states of adsorption site of Ru particles (particle size and surface morphology) especially when supported on MgO.

Acknowledgment. This investigation was supported by a Grant-in-Aid for Scientific Research (No. 05225206) from the Ministry of Education, Science, and Culture. References and Notes (1) Ozaki, A.; Aka, K. Catalysis-Science and Technology; Anderson, J. R. Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 3, p 139. (2) Aika, K.; Ohya, A.; Ozaki, A.; Inoue, Y.;Yasumori, I. J . Catal. 1985, 92, 305. (3) Report article: Appl. Catal. 1991, 67, N18. (4) Kubota, J.; Aika, K. Chem. Commun. 1991, 1544. ( 5 ) Aika, K.; Takano, T.; Murata, S. J . Catal. 1992, 136, 126. (6) Eishens, R. P.; Jacknow, J. Proc. 3rd Znr. Congr. Catal.; 1965, 627. (7) Ravi, A.; King, D. A.; Sheppard, N. Trans. Faraday SOC. 1968, 64, 3359. ( 8 ) Hardeveld, R. V.; Montfoort, A. V. Su$. Sci. 1966, 4, 396. (9) Egerton, T.A.; Sheppard, N. J. Chem. SOC.,Faraday Trans. 1 1974, 70, 1357. (10) Okawa, T.;Onishi, T.; Tamaru, K. 2.Phys. Chem. (Munich) 1977, 107, 239.

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