Gasification of deposit formed in steam reforming or cracking of n-butane on the promoted nickel catalysts

Catalyst Deactivation 1999 B. Delmon and G.F. Froment(Editors) 9 1999 Elsevier Science B.V. All rights reserved.

431

Gasification of deposit formed in steam reforming or cracking of n-butane on the promoted nickel catalysts B. Stasif~kaa, A. Got~biowski b and T.Borowiecki a

aFaculty of Chemistry, Maria Curie-Skbdowska University, 20-031 Lublin, Poland blnstitute of Fertilizers, 24-100 Pu|awy, Poland

Abstract

Small additions of molybdenum compounds (< 0.5 wt. %) cause a significant increase in resistance to coking of nickel catalysts in the steam reforming of hydrocarbons. The influence of promoter on the gasification rate of carbon deposits in different gaseous mixtures was studied. It was found out that the rate of carbon gasification on Ni-Mo catalysts is smaller than on Ni catalysts. I. INTRODUCTION The steam reforming of hydrocarbons is one of the most important methods of commercial production of hydrogen or synthesis gas. One of the significant properties of a good catalyst of steam reforming is its resistance to carbon formation [I ]. Small additions of molybdenum compounds (< 0.5 wt. %) cause a significant increase in resistance to eoking of nickel catalysts [2]. In contrast to the presence of potassium as an inhibitor of coking, molybdenum compounds do not decrease the specific nickel activity in the steam reforming of methane. A simplified scheme of the steam reforming can be presented as follows: { 2 ~ CO, CO2, H2, CH4 CnHm (g) --~ CnHrn (a)--~ C ( a ) ~

{4}

{3}- carbon deposit Decrease of the deposit amount formed on the catalyst can be caused, e.g.: by acceleration of the deposit gasification process {4}. The aims of the present research were: (1) to examine the influence of molybdenum additives on the gasification of carbon deposits in different gaseous mixtures, and (2) to find out the influence of the promoter on the decrease in the coking rate for the steam reforming reaction of hydrocarbons.

432 2. EXPERIMENTAL

2.1. Catalysts Studies were carried out on a series of catalysts containing various amounts of MOO3. Samples were prepared by the impregnation of the Ni/a-AI20 3 catalyst with the ammonium heptamolybdate aqueous solution. The samples were dried at 378 K and calcined at 723 K. Investigations were carried out after reduction at 1073 K for 2 h in deoxidized and dried hydrogen. 2.2. Methods The methods used for determination of catalysts properties are described in detail elsewhere [2,3]. The coking-gasification experiments were performed by the gravimetric method in a flow reactor [2,4] according to the scheme presented in Figure 1. Carbonaceous deposit was formed in the steam reforming or cracking, at a constant temperature (773K) and at a constant partial pressure of n-butane (6.1 kPa). The carbon gasification was carried out in four mixtures of different compositions, at a constant volumetric flow and at the same partial pressure of gasifying agent (H20, He or H20+H2), respectively. Carbon gasification in the mixture Coking of catalyst in steam reforming of n-butane at reagent ratios H20:C = 0.7 or 1.5

H2+N2

Catalyst with carbon deposit (amounts of carbon 15-20 wt. %)

Coking of catalyst in n-butane cracking

H20'+H2+N2

(H20:H2=l:l) ' H20+H2+N2 (H20:H2= 10:1)

Figure 1. Stages of experimental procedure 3. RESULTS The catalysts after the reduction showed no significant changes in total or active surface areas (3.4_+0.1 and 0.8_+0.1 m2gq), respectively. The mean sizes of nickel crystallites, determined by the method of XRD, do not either indicate any directe change in nickel dispersion [4,5]. The gasification of deposits formed on Ni and Ni-Mo catalysts in the steam reforming reaction depends on the composition of the reaction mixtures [4]. As shown in Figure 2 the rate of the deposit removal from the catalysts decreases in the series: H20+N2 > H20+H2+N: (H20:H2=10:I)> H2+N2 > H20+H2+N2 (H20:H2=I:l)

433 In the H2+N2 m i x t u r e , deposit gasification does not depend either on the presence or on the amount of additives [4]. 250

In the steam reforming of n-butane, deposits were formed with different rates and at different time, necessary to form the same initial amount of carbon. This fact could affect both the deposit properties and the way of its "distribution" on the catalyst samples. However, TPO studies have shown that the conditions of the deposit formation and the presence of Mo have no effect on the kind of the deposit [4].

9 [ H 2 0 + N 2]

~

200

m [H2+N2]

.c: ~

~ [ H 2 0 +H 2+N211:1

~ 150

~ 100-

[] mm

50 [ ,

0

100

,

,

,,

200 300 400 Time [min]

Figure 2. Gasification of carbon deposit in different mixtures (Ni-Mo(0.1) catalyst) In the cracking of butane the amounts of the promoter had no effect on the coking rate [5]. It enables to prepare Ni and Ni-Mo coked catalysts containing the same initial amount of deposits formed in identical conditions. Figure 3 presents changes in the weight of catalysts connected with the formations of carbon deposit in cracking and gasification of deposits in different gas mixtures. 2 50

250

o [ H 2 0 + H 2 + N2]10:l

9 [H20 + N2]

r~

~,

200

00

[] [H 2 + N2]

A[H20 + H 2 + N2]I: I

.

~

15o

~ ~

100

50

IG

AA A AA

~t~l:3,n 0 ~ rJ

AAAAAAAAAA

00

)onA~ t3 m O0~AA " D D~,., 50

|

50 0

O

100

200

300

400

500

m

Ni-Mo(0.5) ~

0 100 Time [min]

f

200

'

i

300

. . . . . . . . . . I

400

500

Figure 3. Carbon weight changes on the Ni and Ni-Mo(0.5) catalysts during coking in cracking and gasification in different mixtures The initial rates of carbon gasification in different mixtures have been set up in Table 1. The rates of the deposit gasification on Ni catalyst are practically the same in the gasifying mixtures containing hydrogen. In the three reaction mixtures containing steam, gasification of carbonaceous deposits on the Ni-Mo(0.5) catalyst was performed with a lower rate than on the

434 Ni catalyst. In the H2+N2 mixture only the rate of carbon gasification on both catalysts was the same. Table 1 .Rate of carbon gasification in the different mixtures Catalysts

, . Rates 9 f carbon gasification [~tg.,C/gc,~t xmin], H20 + N2 .

Ni ..Ni-Mo(0,5)

.

.

.

.

.

.

.

H20+H2+N2 (HzO:Hz=l:l) .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

,

H20+H2+N2 (H20:H2= 10:l)

.

.

.

.

.

.

.

.

.

.

.

2459

826

778

1023 . . . . . . . . .

761 . . . . . . . . . . . .

241

.

.

.

.

.

.

.

.

.

,

H2 + N2

.

738 . . . . .

,,,

776 ,

J

4. DISCUSSION An appearance of a deposit and the rate of its formation depend on the equilibria of reactions presented in the scheme (see Introduction) [ 1,6,7]. Gasification of deposits from the catalysts depends on the presence of promoter and the kind of atmosphere. In the presence of hydrogen, the rate of the deposit removal depends neither on the presence nor amount of the promoter (Table 1). It was shown [2,5], that at high partial pressures of hydrogen in the reaction mixtures the Ni-Mo catalysts become similar to the nickel sample. Such behaviour suggests a high dynamics of the surface states in Ni-Mo catalysts samples and dependence of various MoOx states distribution on reduction or oxidation properties of the reaction mixtures. The presence of molybdenum in the nickel catalysts decreases the coking rate in the steam reforming reaction. It is not the result of the promoter influence on the deposit gasification rate (reaction {4}). It seems that the re~tction {3 } course is restrained in the Ni-Mo catalysts which results in the decrease of the deposit amount (Table 1 or paper [4]). Obtained results suggest the influence of the promoter (MOO3) on the amount of filaments, i.e. number of filaments per unit surface area of catalyst. The recent pictures of carbon deposits obtained by means of the electron microscope HR [8] confirm this conclusion. Coking rates of the Ni-Mo catalysts are lower because the deposit forms only on a fraction of nickel crystallites. Therefore, rates of deposit gasification, which depend on the number of carbonaceous filaments per unit surface area of a catalyst, are also lower. References 1 J.R. Rostrup-Nielsen, in Catalysis - Science and Technology, (J.R. Anderson and M. Boudart, Eds.), Springer Verlag, Berlin, 1984, Vol.5, 1 2 T. Borowiecki, A. Got~biowski and B. Stasifiska, Appl.Catal., A: General, 153 (1997) 141 3 T. Borowiecki, Appl.Catal., 10 (1984) 273 4 B. Stasifiska, J.Gryglicki and T.Borowiecki, in Heterogeneous Catalysis, Proc. 8th Int.Symp.Heterogeneous Catalysis, A.Andreev et al. (eds), Varna, 5-9 October 1996, 879 5 B. Stasifiska, T.Borowiecki and A.Got~biowski, in preparation 6 T. Borowiecki, Polish J.Chem., 67 (1993) 1755 7 S.D.Jackson, S.J.Thomson and G.Webb, J.Catal., 70 (1991) 249 8 L.K~pifiski, B.Stasifiska and T.Borowiecki, submitted to Carbon