Pt-Rh/<FONT FACE=Symbol>g</FONT> Al2O3 Benzene Hydrogenation Reaction as a Characterization Technique

Pt-Rh/g Al₂O₃ Benzene Hydrogenation Reaction as a Characterization Technique

N.M. da Fonseca¹, T.G.S. Neto², M.N.S.C. Roma⁴, J. Felcman³, D.S. Cunha2,^*

and G.M. da Cruz⁴

¹White Matins Gases Industriais S.A., VPSA Plant Dev. and Degn Mgr, Rua Mayrink Veiga, 9

- Centro - Rio de Janeiro 20.090-050 - Brazil

²Laboratório Associado de Combustão e Propulsão (LCP), Instituto Nacional de Pesquisas Espaciais (INPE), Caixa Postal 01, Cachoeira Paulista, SP, 12630-000 - Brazil -

Fax: (+ 55-12) 561-1992, Phone: [phone] - e-mail: [e-mail]

³Pontifícia Universidade Católica do Rio de Janeiro, Rua Marques de São Vicente, 225 -

Rio de Janeiro 22453-900 Brazil

⁴Faculdade de Engenharia Química de Lorena - FAENQUIL, Rodovia Itajubá - Lorena, Km 74,5

- Lorena 12600-000 Brazil

(Received: November 5, 1997; Accepted: March 27, 1998)

Abstract - Pt-Rh/Al₂O₃ catalysts prepared by successive incipient impregnations and coimpregnation were characterized by H₂ chemisorption, temperature programmed reduction and benzene hydrogenation reaction in the vapor phase. The results showed that Rh plays the role of Pt reducting agent, which is very different from the effects of metal-metal interaction which appear mainly in solids with the highest metal contents. The most important parameter that results in bimetallic particles in the catalyst prepared by successive impregnation is the sequence of metal addition.

Keywords: Benzene hydrogenation reaction, catalyst, interaction.

INTRODUCTION

Pt-Rh bimetallic catalysts have their broadest application in the control of pollution produced by combustion engines

⁽¹⁾. Courty and Chauvel

⁽²⁾ showed that in 1993 the principal use of rhodium (90.7%) and platinum (35%) was in the production of such catalysts.

For monometallic catalysts (Pt/Al₂O₃ and Rh/Al₂O₃), the literature shows that Pt reduction is strongly related to reduction conditions^{(3 to 5 )}. These paper show that to obtain highly dispersed and well reduced Pt/Al₂O₃, it must be calcined at 673 K in O₂ before reduction in H₂ at 725 K and that reduction temperatures higher than 773 K must be avoided, due to the possibility of contaminating the surface platinum by species originating in the support. In the case of the Rh/Al₂O₃^(6,7) catalysts, it was observed that rhodium is easily reducible and sinterizes easily when treated in H₂ at temperatures of around 773 K. One can conclude that for the Pt-Rh/Al₂O₃ catalysts, the use of temperatures above 673 K favors the reduction of the metal but on the other hand, it causes a remarkable loss of sites due to the sintering process.

The main objective of this work is to show, by use of the benzene hydrogenation reaction, the role of rhodium in promoting the reduction of platinum and the existence of interaction between the metals, during the preparation of the bimetallic catalysts supported on alumina, by the coimpregnation and successive impregnation methods.

EXPERIMENTAL PART

Catalyst Preparation

Pt/Al

₂O

₃ and Rh/Al

₂O

₃ catalysts, containing 1, 2, 3, 4, 6 or 9 metal weight percent, were prepared by the incipient impregnation method, using gamma-alumina as the support (total specific surface area = 260 m

²/g and pore volume = 0.90 ml/g), which had been previously activated by calcination in air at 873 K during 4 hours. H

₂PtCl

₆ and RhCl

₃ solutions, with 0.5 N free acidity, were used. After impregnation, the solids were dried in air at 383 K during 30 minutes and later submitted to reduction in an H

₂ flux, by increasing the temperature slowly up to 423 K during two hours and then more quickly up to 673 K, and maintaining this temperature for four more hours.

Bimetallic catalysts were prepared by two different methods: a) successive impregnations - monometallic catalysts containing 1, 2, 3, 4 or 6 wt% of Pt or Rh, which had already been reduced, were impregnated a second time with a solution of the second metal, Rh or Pt, respectively. Then they were subjected to another thermal treatment similar to the first. Weight ratios of the first metal (M₁) to the second (M₂), M₁ /M₂, were equal to 1/2 and 2/1. Total metal weight percentages in these catalysts were 3, 6 and 9 wt%; b) coimpregnation - bimetallic catalysts, with total metal content and compositions identical to those of the bimetallic solids prepared by successive impregnations were prepared by a single impregnation of a mixture of Pt and Rh solutions, as described before. After impregnation, the catalysts were subjected to thermal treatments identical to those used with the monometallic materials.

Catalyst Characterization

Catalysts were characterized by H

₂ chemisorption at 298 K after pre-activation treatment

⁽⁸⁾.

The solids were also submitted to temperature programmed reduction (TPR), which was performed in a Micromeritics device (TPD/TPR 2900) at the Catalysis Division (DICAT) of CENPES/PETROBRAS. The catalysts, previously calcined at 673 K for four hours, were reduced in situ, by a 6% H₂/N₂ flux. A heating rate of 10 K/min was used up to 773 K.

Catalytic Test

The catalysts were tested in the benzene hydrogenation reaction at 333 K in the vapor phase. For this purpose, a differential dynamic microreactor was used at atmospheric pressure. Helium was used for diluting the reduction gas. Total gas flux was 52.0 cm³/min and the partial pressures of the reagents were 5.18 kPa for benzene and 16.3 kPa for hydrogen. Reaction products were analyzed by gas chromatography. Activation energy in the temperature range from 303 to 333 K was determined.

RESULTS AND DISCUSSION

Table 1 presents the values for the number of active sites per gram of catalyst (Y), metallic phase dispersion (D

_(%)) and average diameter of metallic particles(

), obtained by H

₂ chemisorption for the Pt/Al

₂O

₃ and Rh/Al

₂O

₃ catalysts.

One can observe that for the Pt/Al₂O₃ catalysts, dispersion values are around 30%. For the Rh/Al₂O₃ catalysts, the values are around 4 %. Such behavior shows the tendency of rhodium to sinter during reduction.

Tables 2 and 3 show the theoretical composition and the results of characterization obtained by H₂ chemisorption for the bimetallic catalysts prepared by coimpregnation and successive impregnations.

The results presented in these tables show that the bimetallic catalysts prepared by coimpregnation and those prepared by successive impregnations have dispersions in a range that varies from 4 to 30 %, similar to those shown by the monometallic catalysts.

Table 1:
Results of characterization of monometallic catalysts

Catalyst	%Pt	%Rh	Y . 10-19	D(%)	0p (nm)
3Pt	3.0	-	3.25	35.1	2.6
4Pt	4.0	-	3.64	29.3	3.1
6Pt	6.0	-	5.08	27.6	3.3
3Rh	-	3.0	0.99	5.6	16.4
4Rh	-	4.0	0.87	3.7	24.8
6Rh	-	6.0	0.77	2.2	41.9

0Catalyst% Pt% RhY. 10-19D(%)0p (nm)2Pt+1Rh(co)2.01.02.3219.34.84Pt+2Rh(co)4.02.04.0016.65.56Pt+3Rh(co)6.03.03.579.99.31Pt+2Rh(co)1.0201.107412.42Pt+4Rh(co)2.04.04.4014.76.33Pt+6Rh(co)3.06.05.3012.07.7

Catalyst	% Pt	% Rh	Y. 10-19	D(%)	0p (nm)
2Pt+1Rh(co)	2.0	1.0	2.32	19.3	4.8
4Pt+2Rh(co)	4.0	2.0	4.00	16.6	5.5
6Pt+3Rh(co)	6.0	3.0	3.57	9.9	9.3
1Pt+2Rh(co)	1.0	20	1.10	74	12.4
2Pt+4Rh(co)	2.0	4.0	4.40	14.7	6.3
3Pt+6Rh(co)	3.0	6.0	5.30	12.0	7.7

Table 2: Results of the characterization of the Pt+Rh/Al₂O₃ (co) catalysts prepared by coimpregnation

Table 4 shows the values of specific velocity (V) and turnover frequency (TOF) for the benzene hydrogenation reaction at 333 K for the Pt/Al₂O₃ and Rh/Al₂O₃ catalysts.

The absence of the external diffusion effect was verified by doubling the values for mass and for flow. No modification of the data presented in Table 4 was observed for this procedure.

The apparent activation energy for the benzene hydrogenation reaction had a value of 39 kJ/mol for Pt/Al₂O₃ and a value of 46 kJ/mol for Rh/Al₂O₃. Such values are in agreement with those in the existing literature (40 - 50 kJ/mol)⁽⁹⁾. This agreement shows the absence of the internal diffusion effect.

It is known from the literature that for the experimental conditions under which this research was performed, the average values for TOF are around 0.063 s^-1, which in this paper was considered standard for both the Pt/Al₂O₃⁽¹⁰⁾ and the Rh/Al₂O₃⁽¹¹⁾ catalysts. In Table 4, one can observe that the average value for TOF (0.066 s^-1) obtained for the Rh/Al₂O₃ catalysts is very close to that presented in the literature. Therefore, we can say that these catalysts are well reduced.

On the other hand, for the Pt/Al₂O₃ catalysts, the average value for TOF (0.053 s^-1) obtained for three catalysts reduced at 673 K is lower than that used as a standard for this research. This a result is the same as that found by Cruz et al.⁽⁸⁾, who explained this value based on the low reduction temperature. Because the thermal treatments used by them (reduction at 673 K) were the same as those used in this work (shown in Table 4), it was decided to determine the TOF values for the Pt/Al₂O₃ catalysts after they had been reduced at 773 K (Table 5). One observe now that the average value of TOF (0.060 s^-1) is practically identical to the standard value considered, which shows that this is the most adequate temperature condition for the best reduction of the platinum.

The results for specific velocity (V) and for turnover frequency (TOF) of the bimetallic catalysts, prepared by successive impregnations and by coimpregnation, are presented in Table 6.

Table 3:
Results of the characterization of the Pt-Rh/Al

Catalyst	% Pt	% Rh	Y . 10-19	D(%)	0p (nm)
2Pt-1Rh(a)	2.0	1.0	0.61	5.1	18.0
4Pt-2Rh(a)	4.0	2.0	5.00	20.7	4.4
6Pt-3Rh(a)	6.0	3,0	6.64	18.3	5.0
1Pt-2Rh(a)	1.0	2.0	1.60	10.7	8.6
2Pt-4Rh(a)	2.0	4.0	1.80	6.1	15.1
3Pt-6Rh(a)	3.0	6.0	3.59	8.0	11.5
2Rh-1Pt(b)	1.0	2.0	2.00	13.5	6.8
4Rh-2Pt(b)	2.0	4.0	4.30	14.4	6.4
6Rh-3Pt(b)	3.0	6.0	1.87	4.2	21.9
1Rh-2Pt(b)	2.0	1.0	1.40	11.9	7.7
2Rh-4Pt(b)	4.0	2.0	2.35	10.0	9.2
3Rh-6Pt(b)	6.0	3.0	9.70	26.6	3.4

₂O₃ catalysts prepared by successive impregnations

(a) Pt was added before Rh; (b) Rh was added before Pt.

Catalyst % Metal V . 10-18 (a) TOF(s-1) 3Pt 3.0 1.40 0043 4Pt 4.0 1.36 0.037 6Pt 6.0 4.03 0.079 3Rh 3.0 0.68 0.069 4Rh 4.0 0.60 0.069 6Rh 6.0 0.47 0.061

Catalyst	% Metal	V . 10-18 (a)	TOF(s-1)
3Pt	3.0	1.40	0043
4Pt	4.0	1.36	0.037
6Pt	6.0	4.03	0.079
3Rh	3.0	0.68	0.069
4Rh	4.0	0.60	0.069
6Rh	6.0	0.47	0.061

Table 4: Kinetic results of the benzene hydrogenation reaction (333 K) in the presence of monometallic catalysts reduced at 673 K

(a) molecules of C

₆

H

₁₂

obtained/s .g

_cat.

Catalyst % Metal Y . 10-19 V . 10-18 (a) TOF (s-1) 3Pt 3.0 4.29 2.09 0.049 4Pt 4.0 4.41 2.35 0.053 6Pt 6.0 5.12 4.01 0.078

Catalyst	% Metal	Y . 10-19	V . 10-18 (a)	TOF (s-1)
3Pt	3.0	4.29	2.09	0.049
4Pt	4.0	4.41	2.35	0.053
6Pt	6.0	5.12	4.01	0.078

Table 5 - Kinetic results of the benzene hydrogenation reaction (333 K) in the presence of Pt/Al₂O₃ reduced at 773 K

Table 6:
Kinetic Results of the Benzene Hydrogenation Reaction (333 K) on Bimetallic Catalysts

Successive Impregnations Coimpregnation
Catalyst	V . 10-18 (a)	TOF (s-1)	Catalyst	V . 10-18 (a)	TOF (s-1)	Catalyst	V . 10-18 (a)	TOF (s-1)
2Pt-1Rh	0.43	0.070	2Rh-1Pt	1.06	0.053	2Pt+1Rh(co)	1.67	0.072
4Pt-2Rh	6.78	0.136	4Rh-2Pt	3.07	0.072	4Pt+2Rh(co)	2.94	0.074
6Pt-3Rh	11.70	0.176	6Rh-3Pt	2.81	0.150	6Pt+3Rh(co)	5.07	0.142
1Pt-2Rh	1.17	0.073	1Rh-2Pt	0.95	0.066	1Pt+2Rh(co)	0.84	0.076
2Pt-4Rh	2.02	0.112	2Rh-4Pt	160	0.068	2Pt+4Rh(co)	3.57	0.086
3Pt-6Rh	5.96	0.166	3Rh-6Pt	15.00	0.155	3Pt+6Rh(co)	7.87	0.148

₆H

₁₂ obtained/s .g

_cat.

It can be observed that :

• the catalysts prepared by successive impregnations and those prepared by coimpregnation present two levels of TOF values, one close to 0.07 s

  ^-1 and the other around 0.15 s

  ^-1;

• for the bimetallic catalysts prepared by successive impregnations, where Pt was the first metal to be deposited on the alumina (Pt-Rh/Al

  ₂O

  ₃), Rh favors a better reduction of platinum, since the TOF values for such materials are at least approximately equal to 0.07 s

  ^-1 and have the tendency to approach 0.15 s

  ^-1, even when the total metal content is only 6 wt%;

• the possibility of Rh favoring the reduction of Pt is more difficult in the case of bimetallic catalysts prepared by successive impregnations when Rh is the first metal to be deposited on alumina (Rh-Pt/Al

  ₂O

  ₃), since the TOF values approach 0.07 s

  ^-1 only when the total metal content reaches 6 wt%; and

• all bimetallic catalysts containing a total metal content of 9 wt% present TOF values of around 0.15 s

  ^-1.

From the literature⁽¹²⁾ it is known that structure-insensitive reactions on bimetallic catalysts may have significantly higher TOF values (in relation to the standard values corresponding to monometallic catalysts). Increases in TOF values are attributed to an electronic effect (ligant effect), originating in the formation of bimetallic particles. On the contrary, the inexistence of such increases in TOF values indicates that the bimetallic catalysts are composed of monometallic particles⁽¹³⁾.

In this way, from the TOF results presented in Table 6, one can observe that:

• no electronic effect was observed for the 2Rh-1Pt catalyst, which indicates segregation of the two metals;

• all bimetallic catalysts containing a total metal content of 9 wt% and those with 6 wt%, prepared by successive impregnations (Pt-Rh/Al

  ₂O

  ₃) with the Pt solution being the first to be impregnated (before the rhodium solution), present the electronic effect, with the formation of bimetallic particles. Such a conclusion is corroborated by the apparent activation energy presented by these materials (~ 25 kJ/mol), which is about half that existing in the presence of monometallic catalysts; and

• all the other materials present TOF values equal to or close to 0.07 s

  ^-1, which indicates complete reduction of the metals without the existence of the electronic effect. One can guess, in this case, that Rh has the role of promoting Pt reduction, and that the particles of one metal are only decorated by the other.

The role of Rh in promoting the reduction of Pt is clearly recognized by observing the data in Figure 1. For all bimetallic catalysts, independent of preparation method, of total metal content and of sequence of addition, the presence of Rh causes the decrease in reduction temperature for values lower than 420 K.

CONCLUSIONS

In this work we can conclude that:

• Rh is easily reduced, while Pt is not completely reduced at 673 K;

• Rh has the role of promoting the reduction of Pt;

• the formation of metallic particles containing significant contents of the two metals occurs when one uses a total percentage of 9 wt%, by any preparation method, or below this percentage, when Rh is the last metal to be deposited. The existence of such particles is accompanied by a significant increase in TOF for the benzene hydrogenation reaction at 333 K.

Figure 1:
1: Temperature programmed reduction of the catalysts: (a) 9Pt; (b) 9Rh; (c) 4Pt-2Rh; (d) 3Pt-6Rh; (e) 3Pt+6Rh(co); and (f) 6Pt+3Rh(co).

ACKNOWLEDGEMENTS

We would like to thank Doctors Yiu Lau Lam and Maria Alice Duarte, as well as researchers Denise Costa, Eliane Mattos, Danielle Rosas and M. Vinicius Medeiros at CENPES/PETROBRAS, for their outstanding collaboration in the catalyst characterization by TPR.

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*

To whom correspondence should be addressed.

Publication Dates

• Publication in this collection
  09 Oct 1998
• Date of issue
  June 1998

History

• Received
  05 Nov 1997
• Accepted
  27 Mar 1998