CO-induced structural changes of supported Rh promoted by NO

~ ELSEVIER

APPLIED CATALYSS I A:GENERAL

Applied Catalysis A: General 149 (1997) 89-101

CO-induced structural changes of supported Rh promoted by NO E. NovS.k *, D. Sprinceana, F. Solymosi Institute of Solid State and Radiochemistry, A. Jdzsef University and Reaction Kinetics Research Group ~f the Hungarian Academy of Sciences I, P.O. Box 168, H-6701, Szeged, Hungary Received 14 March 1996; accepted 30 May 1996

Abstract The effects of NO on the CO-induced structural changes of Rh deposited on SiO 2, MgO and TiO 2 are investigated in the temperature range 110-503 K by means of infrared spectroscopy. It was observed that the addition of NO to CO dramatically enhanced the development of gem-dicarbonyl, RhI(CO)E, from R h x - C O species - - indicative of the occurrence of the oxidative disruption of the Rh x cluster - - for all the three samples at 2 3 3 - 3 0 0 K. This is explained by the formation of a strong bond between NO and Rh x crystallites which weakens the R h - R h bond. It is assumed that the NO and its dissociation product, the adsorbed O, participate in the oxidation of isolated Rh ° atoms to Rh 1 ions. On the other hand, the presence of NO slowed down the conversion of gem-dicarbonyl into R h x - C O at and above 448 K to a great extent, indicating that NO retards the reductive agglomerization of Rh ~ to Rh x crystallites on these supports. Keywords: Gem-dicarbonyl; NO; Rhodium

I. Introduction

It was recognized a long time ago that the catalyst is not always inert toward the reacting system, and depending on the nature of the catalyst and on the reaction, the chemical composition of the catalyst can undergo significant changes during the high temperature catalytic reactions [ 1]. Recent spectroscopic studies (extended X-ray absorption fine structure (EXAFS) and infrared spec* Corresponding author. 1 This laboratory is a part of the Center for Catalysis, Surface and Material Science at the J6zsef Attila University of Szeged. 0926-860X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 2 5 I-7

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troscopy (IR)) revealed that significant changes occur in the structure, morphology and state of the supported Rh crystallites even during the low temperature (150-400 K) adsorption of CO [2-11]. As a result of the strong interaction, the Rh crystallites are disrupted to smaller units and finally to isolated Rh atoms, which are probably oxidized by the OH groups of the supports to Rh t. The role of OH groups has been experimentally demonstrated by Basu et al. [8], who found a direct correlation between the depletion of isolated OH groups of the support and the development of gem-dicarbonyl indicating the direct participation of OH groups in the CO-induced disruption process. Another important observation is the detection of H 2 evolution following CO adsorption on NaY zeolite-supported Rh [10]. In many respects similar features were observed for alumina-supported Ru [12-14], Ir [15] and Re catalysts [16-19]. As was expected, the addition of other gases to CO affected the rate of the CO-induced structural changes [5,7,8,20,21]. A particular great effect was observed in the presence of NO for the CO-Rhx/A1203 system, which promoted the development of absorption bands of gem-dicarbonyl, Rhl(CO)2, at the expense of the band due to the Rhx-CO species [20]. The promoting effect of NO was confirmed by means of Fourier transform infrared spectroscopy (FTIR) of Chuang and co-workers [22,23] and by transmission electron microscopy (TEM) of Schmidt and co-workers [24,25]. It is well known that the nature of the support exerts a profound influence on the rates of several catalytic reactions involving CO on supported Rh catalyst [1]. The turnover numbers of the hydrogenation of CO and CO 2 on R h / T i O 2 are almost two orders of magnitude higher that those determined for R h / M g O and R h / S i O 2 [1,26,27]. Significant differences exist for the rates of the NO + CO reaction on supported Rh samples [1]. In the light of these findings it seemed important to examine the CO-induced structural changes of Rh thoroughly using different supports, SiO 2, MgO and TiO 2, with particular emphasis on the effect of NO. This study may contribute to the evaluation of the state of the Rh catalyst in the NO + CO reaction occurring in automobile exhaust catalysis. The main technique is IR spectroscopy combined with mass spectrometry. The state of the Rh in the reduced samples is characterized by X-ray photoelectron spectroscopy (XPS).

2. Experimental The oxide supports were impregnated at room temperature in an aqueous solution of R h C 1 3 • 3H:O (Johnson-Matthey), so as to nominally load 2 wt.-%. The impregnated material was dried at 330 K and stored over silica gel. The following supports were used: A1203 (Degussa P 110 C1), TiO 2 (Degussa P25), SiO 2 (Cab-O-Sil) and MgO (DAB 6). The gases used in the experiments were of research grade purity (Linde).

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Table 1 Some characteristic data for supported Rh samples

2% R h / S i O 2 2% R h / M g O 2% R h / T i O 2

Reduction temp. (K)

Dispersion %

Binding energy of Rh(3ds/2) for reduced sample (eV)

573 1073 573 873 573 673

36.0 14.9 26.5 7.1 30.4

306.35 306.8 306.95

The Rh content was 2 wt.-% in all samples. The binding energy of Rh(3d5/2) for Rh foil is 307.1 eV.

Self-supporting wafers (15-20 mg cm -2) of the catalyst material were subjected to heat treatment in situ in the infrared cell at 573 K under continuous evacuation, oxidation in 100 Torr of 0 2 for 30 min at 573 K, evacuation for 30 rain and reduction at R T = 573-1273 K (R T is the reduction temperature) in 100 Torr of H 2 (Linde, 99.999% purity) for 60 min. The gases were circulated during oxidation and reduction processes, and the water formed in the latter case was frozen in a cold trap. This procedure was followed by degassing at 573 K independent of the reduction temperature. Thereafter the samples were cooled to the adsorption temperatures. The rhodium samples reduced at different temperatures have been characterized by H 2 chemisorption and XPS measurements. Results are listed in Table 1. IR spectra were recorded with a Specord 75 IR double beam spectrophotometer (Zeiss, Jena). Difference spectra were produced and magnified with the help of a DATA system (Tracor-Northern, TN 1710). The wavenumber accuracy was better than 5 cm -~. An in situ IR cell was used, which permitted IR spectra to be recorded in the temperature range 100-573 K. The XPS measurements were performed in a Kratos XSAM 800 instrument at a base pressure of 10 -8 Torr using Mg K primary radiation (14 kV, 15 mA). To compensate for possible charging effect, binding energies were normalized with respect to the position of the metal peaks in the appropriate oxides; Si(2p) in SiO 2 = 103.4 eV, Ti(2P3/2) in TiO 2 = 458.8 eV, Mg(2p) in MgO = 49.75 eV. The pass energy was set at 40 eV, an energy step width of 50 meV and dwell time of 300 ms were used. Typically 10 scans were accumulated for each spectrum.

3. Results

3.1. Adsorption of CO It was observed in our previous works [3,11] that the oxidative disruption of 0.1-1.0% R h x / A 1 2 0 3 (R v = 573-673 K) is very fast at 300 K, as a result the

E. Noufk et al./ Applied Catalysis A: General 149 (1997) 89-101

92

B

2i00

2000

~4avenumber / cm -1

2i00 ~avenumber/cm -1

C

~00

2i00

2000

Vo.ve number/cm -~

Fig. 1. Changes in the IR spectra of (A) 2% R h / S i O 2 (R T = 1073 K), (B) 2% R h / M g O (R T = 873 K) and (C) 2% R h / T i O 2 (R T = 673 K) in the presence of CO (10 Torr) at 300 K as a function of adsorption time. (1) 5 min; (2) 15 min; (3) 30 rain; (4) 60 rain; (5) 120 min. The first spectra (a) in A - C were taken following CO adsorption of Rh samples with R T = 573 K.

adsorption of CO at 300 K gives immediately the dicarbonyl bands at 2100 and 2030 cm-1. Fig. 1 shows that the situation is practically the same for other Rh samples. Following the reduction of supported Rh at 573 K, the adsorption of CO gives at once intense absorption bands at 2090-2100 and 2016-2034 c m due to the asymmetric and symmetric stretches of dicarbonyl indicating the occurrence of the disruption process of Rh x crystallites. The presence of a band at 2060-2075 cm-1 due to Rhx-CO, however, suggests that a fraction of Rh x crystallites is still present on the samples. The intensity of this band is the highest for R h / S i O 2 followed by R h / M g O and R h / T i O 2. When the R h / S i O 2 was reduced at a higher temperature (R T = 1073 K), the dicarbonyl bands were missing: the adsorption of CO gave an intense band at 2064 cm -~. For R h / M g O (R T = 873 K) we obtained a strong band at 2072 cm-~ and very weak ones at 2096 and 2030 cm-~. The easy reducibility of titania, which leads to the loss of the transparency, prevented the application of a high reduction temperature for R h / T i O 2. As a compromise, 673 K was chosen for R v. In this case, as Fig. 1 shows, weaker absorption bands at 2096 and 2030 c m - l of gem-dicarbonyl can be also detected. Also, a weak band at 1890-1905 cm-1 due to bridge bonded CO (not shown in the spectra) was produced for all Rh samples. Evacuation of the sample at 300 K diminished only very slightly the intensity of these bands. Following the spectral changes in the presence of CO in time (2 h), a slight enhancement of dicarbonyl bands was

E. Noufk et al. / Applied Catalysis A: General 149 (1997) 89-101

93

observed for R h / M g O , and a more significant one for R h / T i O 2. There was, however, no spectral change for R h / S i O 2.

3.2. The co-adsorption of NO + CO gas mixture A completely different picture was obtained following the coadsorption of NO + CO gas mixture. Even the addition of 0.1 Torr of NO to 10 Torr of CO promoted the development of gem-dicarbonyl on R h / S i O 2 to a great extent. In the presence of 1-5 Torr of NO the dominant absorption bands were at 2096 and 2034 cm -1. In the case of R h / M g O the appearance of intense dicarbonyl bands required a somewhat lower partial pressure of NO compared to the R h / S i O 2. The presence of NO in the CO also influenced the spectra of Rh/TiO2: the gem-dicarbonyl became the dominant spectral feature even at 0.1 Torr of NO. Spectral changes in the presence of CO (10 Tort) containing different amounts of NO are presented in Fig. 2. As NO exerted a dramatic influence on the C O - R h x system at 300 K, it was interesting to extend the measurements to the low temperature region. In this case we applied a R h / S i O 2 sample which was reduced at 573 K. Adsorption of CO on this catalyst at 100-110 K produced a band at 2062 cm -l and weak shoulders at 2090-2096 cm-1 and 2030-2020 cm-~ indicating the presence of

/// T/~/°

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I

",

£

/

1

10TortC O / #

V

~J

2100 2000 Wavenumber/ rm~

IO*Z

2100 2000 Vavenumber/ crn-1

2100 2000 Vavenumber/cm-1

Fig. 2. Changes in the IR spectra of (A) 2% R h / S i O 2 (R T = 1073 K), (B) 2% R h / M g O (R r = 873 K) and (C) 2% R h / T i O 2 (R T = 673 K) in the presence of 10 Torr C O + 1 Tort NO and 10 Tort C O + 5 Torr NO gas mixture at 300 K in time. (1) and (3) 5 min; (2) and (4) 30 min. Dotted lines refer to the spectra taken in the absence of NO after 30 min.

94

E. Noudk et al. / A p p l i e d Catalysis A: General 149 (1997) 89-101 A

B

bose 233 K

--'..--

~x_/ ~ ~-k~ _...._ ~v.

253 K

U ~ . , ~ / . ~

273 K 2B3 K 293 K

10%

2i00

2600

Wovenumber / cm-I

2100

2000

Wovenumber/ cm-I

Fig. 3. Changes of the IR spectra of 2% Rh/SiO 2 (R T = 573 K) in the presence of (A) CO (10 Torr) and (B) NO (1 Torr)+ CO (10 Torr) gas mixture at 233-293 K.

RhI(CO)2 species. This may be due to the unreduced Rh I. In addition, other relatively intense absorption bands at 2155 and 2180 cm -1 due to weakly adsorbed CO appeared in the spectrum. These latter bands disappeared either upon degassing at 100 K, or on increasing the adsorption temperature to ca. 180 K. A slight enhancement of the dicarbonyl bands occurred above 253 K. Adding 1 Torr of NO to 10 Torr of CO, intense dicarbonyl bands appeared even above 223-233 K and they became the dominant spectral features above this temperature. Similar behaviors were observed for the other Rh samples. Some characteristic spectra are presented in Fig. 3.

3.3. Effects of the preadsorption of NO The effect of NO was also exhibited when the NO-covered Rh was exposed to CO. In Fig. 4 we show first the spectra obtained after CO adsorption at 300 K. The dominant spectral feature was the band at 2060-2067 cm -1 due to Rhx-CO. Admission of NO (5 Torr) onto the degassed R h / S i O 2 sample exerted a dramatic influence on the spectra: the band due to R h x - C O was removed completely and the NO bands appeared at 1910, 1710 and 1670 cm-1. No spectral changes occurred in 1 h. After evacuation at 300 K, the NO bands somewhat attenuated. When the NO-covered R h / S i O 2 was exposed to 10 Torr of CO again, absorption bands of equal intensity appeared at once at 2096 and 2038 cm-1 (Fig. 4A). There was no sign of the CO band around 2062 c m -

E. Nov6k et al. / Applied Catalysis A: General 149 (1997) 89-101

95

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A

COevoc

/~.o.s ro~ NO~+0.5Tort" NO/ * 1"fortCO ~ ~aC.

+ITortCO/, ,``,,./

10% 2100

2000

~/avenumber/cm-1

2100

2000

Wovenumber / cm-I

21bo

.

.

.

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~/clvenurnber/ cm"1

Fig. 4. Changes in the IR spectra following the interaction of adsorbed CO(NO) with NO(CO) on (A) 2% R h / S i O 2 (R T = 1073 K), (B) 2% R h / M g O (R T = 873 K) and (C) 2% R h / T i O 2 ( R T = 673 K) at 300 K.

observed before the NO introduction. The above bands intensified only slightly after extended adsorption time, while the NO bands slightly attenuated. Similar features were observed for R h / M g O sample (Fig. 4B). In this case while the readsorption of CO produced intense absorption band at 2090 and 2022 cm -~, a band at 2060 cm-1 also appeared on the spectrum indicating the presence of Rhx-CO species. Note that no effect of NO was observed when the R h / M g O samples were reduced at a higher temperature, R T = 1073-1273 K. In the case of R h / T i O 2 the readsorption of CO on the NO-covered surface yielded gem-dicarbonyl bands at 2090 and 2028 cm-1 and the Rhx-CO band at 2060 c m - 1 (Fig. 4C). An intensification of these bands occurred after extended adsorption time, but no changes were found in the region of NO vibration.

3.4. High temperature measurements As was mentioned before, in the presence of CO t h e R h I ( C o ) 2 species on alumina support is transformed back into Rhx-CO species at a higher temperature, which indicated the occurrence of the reductive agglomeration of highly d i s p e r s e d R h I [4]. Fig. 5A shows these spectral changes for R h / M g O . In Fig. 5B the intensity changes of 2096 c m - 1 band are plotted as a function of time at 448 and 473 K for supported samples. A general feature is that the attenuation of the dicarbonyl bands and the development of linearly bonded CO at 2050-2060 cm-1 start at

96

E. Nov6k et al. /Applied Catalysis A: General 149 (1997) 89-101 A

tg Io

B

o.~1 %

T/~/o

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2600 %/ovenurnber/cm-1

os " , , , .

01

0.1 I%

0.1-

C

~'~ 50 t/rain 100

s6 t/rain 106

Fig. 5. (A) Changes in the IR spectra of 2% R h / M g O (R r = 573 K) in the presence of 10 Ton" of CO at different temperatures. (1) 300 K, 10 rain; (2) 448 K, 1 min; (3) 448 K, 150 rain; (4) 473 K, 1 min; (5) 473 K, 10 min; (6) 473 K, 45 min (4,5,6 results of a new experiment). (B) Intensity changes of the CO band at 2096 cm -~ in the presence of CO (10 Torr) at 448 K (x) at 473 K ((9) and (C) in the presence of 10 Torr C O + 1 Torr NO mixture at 473 K for supported Rh.

B

A rnin

t,.48K ///.~150

~

a

r.in

,,

C

rain

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/10

150

~0%

2ioo

28oo

Wovenumber/ crn-1

2~oo Wovenurnber/cm -1

2800

21oo

2600

Wovenumber/crn"1

Fig. 6. Changes in the IR spectra of (A) 2% Rh/SiO 2, (B) 2% R h / M g O and (C) 2% Rh/TiO 2 at different temperatures in the presence of 10 Tort of CO and 1 Ton" of NO. R T was 573 K for all samples.

E. Novdk et al, /Applied Catalysis A: General 149 (1997) 89-I01

97

448 K, and the transformation of RhI(CO)2 into Rhx-CO is complete in 10-45 min at 473 K. This transformation takes place at the highest rate on Rh/SiO 2 and at the lowest rate on Rh/MgO. Adding NO to CO, however, the stability of the Rh~(CO)2 species is greatly increased. In the presence of 1-5 Torr of NO, only very little reduction of the intensities of dicarbonyl bands occurred at 448-473 K. The development of the band due to Rhx-CO proceeded at and above 503 K at the expense of dicarbonyl bands. The intensity-changes observed for the 2096 cm-~ band at 473 K are plotted in Fig. 5C. Fig. 6 shows some characteristic spectra obtained at different temperatures.

3.5. Adsorption-induced desorption process IR spectra presented in Fig. 4, suggest that the NO, which is bonded more strongly to the Rh than CO, forces the CO to desorb at 300 K. In order to confirm this assumption the gas phase was analyzed by mass spectrometry. As data presented in Fig. 7 demonstrate, the admission of NO (5 Torr) on the Rh sample containing only linearly bonded CO to Rh x crystallites causes an immediate release of CO into the gas phase. This process is fast, as no change in the composition of gas phase is observed after 5-10 rain. The situation was completely different when the sample containing only Rhx(CO)2 species was exposed to NO. In this case no release of CO to the gas phase was registered.

B

A 80

g

==

80"#

~ *

#0"

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1'o o

2~0 .

80"

:~ t/~n x ~ C

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20

80"

3b t/~in

O

4--

"5

/+0"

to IE

~b

2b

35 t~i0

~

2~

3'0 ~n

Fig. 7. Mass spectrometric analysis of the gas-phase following NO (5 Tort) adsorption of CO-covered (A) 2% Rh/SiOz, R-r = 1073 K, (B) 2% MgO, R T = 873 K, (C) 2% R h / T i O e, R T = 673 K and (D) 2% Rh/AI203, R T = 1073 K.

98

E. Novdk et al. /Applied Catalysis A: General 149 (1997) 89-101

4. Discussion 4.1. Effects of the supports on the CO-Rh interaction IR spectroscopic results clearly showed that the CO-induced oxidative disruption of Rh clusters, observed first for the Rh/A1203, also occur when Rh is deposited on SiO 2, MgO and TiO 2 supports. This process starts even below room temperature. We can count with the occurrence of following reactions Rh~ + CO ~ Rhx-CO R h x - C O -~- CO ~ R h x _ n - C O

(1) -~- nRh°-CO

(2)

Rh°-CO + O H - + CO ~ Rh'(CO)2 + O 2- + 1 / 2 H 2

(3)

Rh°(CO)2 + O H - ~ RhI(CO)a + 0 2- + 1 / 2 H 2

(4)

or

When all the Rh samples were reduced at 573 K, the adsorption of CO at 300 K gave immediately intense absorption bands at 2090-2100 and 2016-2030 cm -1 due to the Rh~(CO)2 species. In the case of R h / A l z O 3 there were no other absorptions bands for adsorbed CO [4]. This suggests that the oxidative disruption process was fast and complete. A weak absorption band (or a shoulder) at 2060-2070 cm-1 was seen for R h / T i O 2 and for R h / M g O , and a stronger one for R h / S i O 2 (Fig. 1). This means that the disruption of Rh x clusters was not complete on these supports. Accordingly, the reactivity of Rh x cluster on various supports decreases in the order A1203 > TiO 2 > MgO > SiO z. It is worthwhile to note that this order is divergent from that of the catalytic efficiency of supported Rh [1,26,27], and in this case the R h / T i O 2 sample does not exhibit a striking behavior. As there are slight differences in the crystallite sizes of Rh samples, we may assume that this is at least partly responsible for the above order. In addition, the reactivity of OH groups of the support may also contribute to the support effect. It was established before [4,8] that the OH groups of the supports are responsible for the oxidation of the isolated Rh atoms, formed in the disruption of Rh x clusters, to Rh I species. If this oxidation process is not fast enough, the isolated Rh atoms can not be stabilized in the form of RhI(CO)2, and thus they may agglomerize into small crystallites. This is no doubt slows down the transformation of Rhx cluster into xRh ~ species. At higher temperature we observed the reductive agglomerization of Rh ~ species as indicated by the transformation of Rh~(CO)z into Rhx-CO. This morphological change can be characterized by the following equations 2RhI(CO)2 --~ 0 2- ~ 2Rh°-CO + CO + CO 2

(5)

x R h ° - C O ~ Rhx-CO + ( x - 1)CO

(6)

E. Novdk et al. / Applied Catalysis A: General 149 (1997) 89-101

99

This process occurred fastest on R h / S i O 2 followed by R h / T i O 2, R h / M g O and R h / A 1 2 0 3. This sequence is opposite of the order established for the disruption process. 4.2. Effects of NO on the supported Rh x cluster

The addition of NO to CO caused a dramatic influence on the spectral features of adsorbed CO: it promoted the spectral changes attributed to the oxidative disruption of Rh x clusters (Figs. 2 and 4). The spectra taken at low temperature (Fig. 3) indicate that the promoting effect of NO is exhibited even at 223-233 K. In order to explain this influence we have to explore the effects of NO itself on the behavior of Rh x clusters. In the case of the effect of CO it was assumed that the formation of a strong adsorbate-Rh bond causes the cleavage of the weaker R h - R h bond [3]. As NO binds stronger to Rhx than the CO [28-30], we may expect that the adsorption of NO itself can lead to the disintegration of the Rh~ cluster. This assumption seems to be supported by the results of a separate study concerning the N O - R h / s u p p o r t interaction [31,32]. IR spectroscopic data reveal significant spectral changes following the adsorption of NO on Rhx/support at 200-300 K [32]. This consists of the development of absorption bands at 1740 and 1830 cm-~, which were assigned to the symmetric and asymmetric stretches of the RhI(NO)2 species [33-35]. XPS studies showed that the adsorption of NO on the Rh,./SiO 2, R h x / M g O and R h x / T i O 2 (R T = 673 K) caused a well observable increase, 0.4-0.8 eV, in the binding energy of the Rh [31]. In addition, STM studies clearly demonstrated that the size of Rh crystallites on TiO2(ll0) (1 × 2 ) surface appreciably decreases when it is exposed to NO or NO + CO gas mixture [32]. Accordingly we assume the occurrence of the following reactions Rh x + NO(g) ~ Rhx-NO

(7)

Rhx-NO + NO(g) ~ Rhx_y-NO + R h ° - N O ( y < x)

(8)

2Rh.v-NO ~ Rh2y_ I - N O + Rh°-NO

(9)

Rhx-NO ~ Rhx_y-N + R h y - O

(10)

(Rh ° is atomically dispersed Rh). The atomically dispersed Rh is easily oxidized by adsorbed oxygen, hydroxyl and nitric oxide to yield Rh ~ sites. As it was expected the addition of NO to CO markedly hinders the CO-induced agglomerization of the isolated xRh ~ species to the Rhx cluster. Whereas in the absence of NO the transformation of Rh~(CO)2 to Rhx-CO proceeds at measurable rates for all the three samples at 448 K, and is completed in a couple of minutes at 473 K, in the presence of even 0.1 Torr of NO in the CO the reductive agglomerization at 448 K is stopped, and is significantly delayed at higher temperatures. No doubt this influence can be attributed to the oxidizing properties of NO.

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E. Novdk et al. / A p p l i e d Catalysis A: General 149 (1997) 89-101

5. Conclusions (1) The ease of the disruption of the supported Rh x cluster, occurring above 233-253 K, is increased in the order SiO 2 > MgO > TiO 2. (2) The striking effect of titania, observed for the catalytic activity of Rh crystallites, is not exhibited in the CO-induced structural changes of rhodium. (3) The addition of a small amount of NO to the CO dramatically increased the rate of the oxidative disruption of R h clusters, and greatly hindered the reductive agglomeration of Rh t into Rh x crystallites.

Acknowledgements

A loan of rhodium chloride from Johnson-Matthey PLC and the financial support of OTKA (contract number T 014461) are greatly acknowledged.

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101