In situ XAS studies of ammonia synthesis catalysts : XANES and EXAFS of Rb promoters

160

Physica B 158 (1989) 160-161 North-Holland, Amsterdam

IN

SITU XAS

STUDIES

OF

AMMONIA

SYNTHESIS

CATALYSTS

:

XANES AND EXAFS OF Rb PROMOTERS

Bjeme Haldor

S. Clausen,

Topsee

Wilhelm

Niemann and Henrik

Research Laboratories

, DK- 2800 Lyngby / Denmark

In a versatile in situ cell , Fe based ammonia synthesis and by an alkaline

Topsee

catalysts

metal (K, Cs, or Rb 1 have been studied.

promoted

by Al2 03

The reduction

of the

catalyst precursor in a flow of 3 H, : N, as well as the catalyst at synthesis conditions ( T = 400 ‘tl I have been investigated by XAS spectroscopy in the transmission mode . From Fe K-edge XANES and EXAFS it has been shown in a previous paper /l/ that the temperature T” at which small metallic Fe particles start to appear depends on the type of alkaline promoter. For the alkaline metals studied it was found that 250 e < T” < 300 k . In order to facilitate the XAS measurements at the edge of the promoter atoms the most often used alkaline metal , K , has been replaced by Cs or Rb . In this paper , results for catalysts containing about 2 wt. X Rb are reported . We have gained similiar results for the Cs promoted catalysts. In fig.1 some XAS spectra for various reduction

temperatures

are displayed

. EXAFS at the Rb K-edge

shows

that the calclned precursor contains a disordered Rubidium oxide phase . In these preliminary studies , we only got true EXAFS signals until k = 4-S A’( see fig. 2 1. Due to changes of the position of zero crossings we conclude that the mean NN distance is decreasing as a function of reduction temperature . Assuming a linear Rb-0 phase (0 =a-2b*k the changes can be written as AR= -(R-bl/k*Ak . Using theoretical data for Cp /2/ the mean Rb-0 distance is estimated to be R= 2.7 A at 30 b and R= 2.5 A at 400 e. Within the error bars , there is no evidence for bonding of alkaline atoms to other metals (i.e. Al, Fe, or Rb 1 within the first coordination shell. A gradual decrease in the amplitude of EXAFS may not only be due to increase of thermal disorder but also a decrease in the NN coordination number as also corroborated by the XANES results . An investigation of the properties of the White Line at the Rb K-edge ( see fig.3 1 confirms the trend observed in the EXAFS . Peak height , FWHM and peak area are changing monotonously up to T = 300 e . Then an abrupt change in height and width is observed. To fit the White Line, a gaussian peak and an arctan step function have been used . Our combined EXAFS and XANES results show that the Rb- 0 phase changes during reduction of the catalysts . From the decrease of NN distance and amplitude the reduction seems to form an oxide phase with Rb in a low-coordination environment with short Rb- 0 distances . 0921-4526/89/$03.50 (North-Holland

Physics

@ El sevier Publishing

Science Publishers Division)

B.V.

161

B.S. Clausenet al./Ammonia synthesiscatalysts

Fig.1 : Rb K-edge

XAS spectra

LOOY 360 300 250 200 105 30

Fig.2 : Fourier back transforms

Fig.3 : Properties

of the fine structures

the Rb K-edge

of the White Line at

Evidence for the alkaline in being present in oxidic environments in reduced ammonia synthesis catalysts has been found in studies of model systems /3,4/ . The observed changes in the XAS spectra may be explained by a spreading or redistribution of the alkaline phase during reduction . High mobility of the potassium oxide on Fe has been observed in model systems /3/ . The reduction process may also result in the formation of a completely new structure between the alkaline ions and the alumina structural promotor . Both of these mechanisms may give rise to the observed changes in the XAS spectra . Further studies are needed in order to more clearly reveal the nature of the alkaline species in the ammonia catalysts .

Support from FI’U grant 5.17.0.0.04 is gratefully HASYLAB

for offering

/l/ W. Niemann, B.S. Clausen, H. Topsee, 121 B.-K. Teo and P.A. Lee , JACS &l, 131 G. Connell and 141 Z.Paal,

G.Ertl,

acknowledged . We also acknowledge

beam time at the ROEMO-2

J.A. Dumesic

spectrometer

.

Ber. Bunsenges. Phys. Chem. X.1292 2815 (1979)

, Journ. Catal. 92.17 (1985 1

S.B.Lee , Appl. Surf. Sci. 4, 231 ( 1981)

(1987)