The kinetics of formation and transformation of silver atoms on solid surfaces subjected to ionizing irradiation

Radiar Phys. Chem. Vol. 32, No. 4, pp. 639-643, 1988 Int. J. Radiat. Appl. lnstrum. Part C

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THE KINETICS OF FORMATION AND TRANSFORMATION OF SILVER ATOMS ON SOLID SURFACES SUBJECTED TO IONIZING IRRADIATION G. M. POPOVICH Chemical Department of Moscow State University, Moscow 117234, U.S.S.R. (Received 1 June 1987)

Abstract--The paper discusses the results obtained in ESR-assisted studies of the kinetics of formation and transformation of silver atoms generated by y-irradiation of silver-containing carriers. Three types of dependences have been established: (1) extreme; (2) saturation curves and (3) step-like. All the kinetic curves display, after a definite period of time, stable concentrations of adsorbed silver atoms per unit of the surface at a given temperature. Depending on the temperature of the experiment, the composition and nature of the carrier, the number of adsorbed silver ions, the irradiation dose and conditions of the experiment, a stable concentration of silver atoms at a given temperature may be equal to, higher or lower than the number of silver atoms measured immediately after ),-irradiation at a temperature of liquid nitrogen. A kinetic scheme is proposed to explain the obtained curves. The model suggests that the silver atoms adsorbed on the surface, as well as those formed after ?-irradiation, are bonded to the surface by various energies, which are related to heterogeneity of the carrier surface.

Studying the processes of formation, kinetic transformations and stabilization of atomic silver particles in solids is of great interest for understanding the mechanism of catalytical action of silver-containing catalysts. The present paper discusses the results of investigations conducted by the ESR-method with the aim of elucidating the specifics of formation and transformation kinetics of silver atoms induced by 6°Co v-irradiation at 77 K on the surface of solids containing adsorbed silver ions. Silica gels were used as carriers with various geometric parametric, alumogels, alumosilicates of varying composition, synthetic zeolites of various types, silica gels and alumosilicates modified by metal oxides of the I, II, IV and VIII groups, as well as industrial catalysts (Zeocar-2 and others). The studies were carried out within a temperature range between 77 and 353 K. Studies of thermal transformation kinetics of silver atoms on v-irradiated solid surfaces of a various nature have shown that the behaviour of atoms in these matrices obeys the same laws: the obtained kinetic dependences are of an extreme nature; all these dependences are characterized by the presence of a stable number of silver atoms at a given temperature, tH0) Characteristic specifics of kinetic dependences of the number of adsorbed silver atoms at varied temperatures may be manifest in different ways, depending on the nature and composition of the carrier, content of silver atoms adsorbed in the sample and conditions of the experiment (both the mode of sample preparation including the conditions of irradiation, and the temperature of the experiment). The experimental data (7) have suggested the existence of three types of dependences of the number of

adsorbed silver temperatures:

atoms

on

time,

at

varied

(1) extreme Figs la, d; (2) step-like Fig. lb; (3) saturation curves Fig. lc. Strictly speaking, the general type of kinetic laws of silver atom transformations are extreme dependences. Their nature was explained by two various processes occurring on the surface of y-irradiated samples: transformations of silver atoms and emergence of new ones as a result of the reaction between stabilized electrons released from the traps and the excess of adsorbed silver ions.o) Following a certain period of time, the kinetic curves of all types display a quasi-stationary concentration of adsorbed silver atoms per unit of the surface at a given temperature (Ag~t) that ceases to depend on time within the range of the experimental error. This effect is a general qualitative regularity for all the types of kinetic transformations of adsorbed silver atoms studied. The magnitude of quasistationary concentration of silver atoms is determined not only by the temperature but also by the composition and geometric parameters of the carrier, the number of adsorbed metal atoms and the degree of their transformation to atomic condition. Depending on these parameters, it can be equal to, smaller or larger than the number of adsorbed silver atoms measured immediately after irradiation at liquid nitrogen temperature (Figs 1, 4 and 5). As was noted in Ref. (6), at this temperature the quasi-stationary concentration of silver atoms (Agst) determined through experimental curves, passes through its maximum after a change in the number

639

640

G . M . POPOVICH o

tural parameters of the carrier. {3-7) As the number of adsorbed silver atoms per unit of the carrier grows, the maximum in the kinetic curves shifts towards 7 shorter times, o,6) U n d e r certain experimental conditions (Figs lc and 6) the initial sections of the 0 saturation curves exhibit an increase in the number of 8 silver atoms per unit of the surface, which, upon reaching a certain value, remain constant for long,? L.-I term isothermal exposure of the samples. Step-like kinetic dependences of the number of adsorbed silver atoms were generally noted for samXA'A--"-,--b t I I 1 I ples with high local concentrations of adsorbed ions 40 80 120 160 2oo per unit of the surface, or at elevated temperatures t (min) (Figs lb and 4). In this case a maximum in kinetic Fig. 1. Kinetic dependences of the number of silver atoms on silica gel surface at varied temperatures and irradiation curves is located at short investigation times. O f doses; points represent the experiment, and the solid line is particular interest are kinetic dependences of the calculation by formula (7) for: (a) T = 133 K; 2.5 Mrad; number of silver atoms at rather elevated tem[ A g + ]total = 0.2 X 1019g- 1SiOz; [AgO]o= 4.6 x 1016g- 1 SiO2: peratures (room temperature--Fig. 4 - - a n d above), A =4.3 x 10t6; B = 32.1 x 1016;K; = 0.035 rain-1;/(2 +/(3 = since it has been established over the years that silver 0.06min -1. (b) T = 153 K; 1.5 Mrad; [Ag+]total= 55 x atoms adsorbed on the surface of solids can preserve 10198 - I SiO2; [Ag°]0= 4.65 x 1016; A = --4 x 1016; B=7xl0t6; K~=0.17min-I; K 2 + K 3 = l m i n -1. (c) a degree of freedom, and can be fixed in the ESRT = lI3K; 1 Mrad; [Ag+]tot~J= 1.0 x I019g -I SiO2; spectra only at low temperatures (77-150 K). lAg°]0= 7.6 x 1016g-I SiO2; A = 8 . 8 x 1016; B = 0 ; However, step-like kinetics is not typical of silver K2 + K3 = 0.1 rain -1. (d) T = 143 K; 1.5 Mrad; [Ag+]total= atoms stabilized on the surface. The authors of Ref. 0.2x 1019g -I SiO2; [Ag°]0=5.4x 1016g-~ SiO2; A = 1.8x 1016; B = 3 x 1016 K~ = 0.09 min-t; K2+K3= 1 min -I. (12) also observed "step-like" kinetics of the recombination of radicals formed in y-irradiated polymers. To explain these kinetics several models have been suggested, {13)one being that radicals are bonded o f silver atoms adsorbed on the surface, irrespective of the dose adsorbed in the sample (Fig. 6). The 'with the surface by different bonding energies, so that extreme dependence observed is satisfactorily ex- at each temperature only a part is transformed, while plained by the theory of active ensembles put forward another model suggests non-homogeneous spatial by N. I. Kobozev, whose mathematical basis is distribution of radicals. As has been shown, <2) during the heating of prePoisson's distribution. Once a maximum value has irradiated silver-containing samples an additional been reached, in all the kinetic curves the initial rapid decrease in the number of adsorbed silver atoms number of silver atoms form. Notably, during heatchanges to a slow one, and then remains virtually ing of y-irradiated samples a greater part of adsorbed constant for quite a long time. However, it is not silver ions is transformed to atoms as compared to always possible to detect a maximum in kinetic curves, since it may be located either at very long or very short investigation times, and be expressed in minutes, hours, days or months, depending on the conditions of the experiment: temperature, initial T number of adsorbed ions, the degree of their trans:6 b formation to atoms, as well as the nature and struc16

0

0

0

0

C

=< 4 o

.4

%, ,1.2

-0

1.0

o,---.-o-

I

I

I

I

10

20

30

40

t (rain)

Fig. 2. Dependence of the number of silver atoms adsorbed on the surface of y-irradiated catalyst Zcocar-2 on time, at 143 K.

I

I

40

80

I

I

420 160 t (min)

[

I

I

200

240

280

Fig. 3. Dependence of the number of silver atoms on time at 133 K, for samples containing the same initial number of silver atoms but a different number of silver ions per 1 g of silica gel; irradiation dose 0.5 Mrad; points stand represent the experiment, and the solid line calculated by formula (7) for: (a) [Ag+]to~l =0.29 x 10t9g -m SiO2; lAg°]o=2.6 x 1016g-I SiO2' A = 2 . 4 x 1016; B = 6 . 6 x 1016; K~= 0.013 rain-i; K2 + K3 = 0.088 min-i. (b) [Ag+]total = 55X 1019g-I SiO2; [Ag°]0=2.06x 1016g-1 SiO2; . 4 = 0.54x 1016; B = 4 . 0 4 x 1016; K~=0.018min-I; K2+K3= 0.12 min -I .

641

Silver atoms on solid surfaces 1.0

== o g :E

o.~

\

o •

o-

[

r

2o

10

30

t (min)

Fig. 4. A change in the number of silver atoms in y-irradiated aluminium silicate modified by yttrium oxide(III) (2% Y203, 10% A1203,88% SiO2) with duration of time at room temperature. the number of silver atoms formed during radiolysis of the systems under study at 77 K. One of the signs of step-like radical recombination is a linear dependence of the stable concentration of radicals on their initial concentration at a given temperature. "z'3) In our case, we cannot confirm an unambiguous dependence of the number of silver atoms stable at a given temperature on their initial number, since the initial concentration of silver atoms is a function of the number of adsorbed atoms and irradiation dose. The initial number of adsorbed silver atoms at a given temperature is the number of silver atoms formed during ~,-irradiation which are manifest in the ESR-spectra at 77 K. It is known(TJ' that the dependence of the yield of silver atoms stabilized on the surface at 77 K on the number of silver ions introduced into the system is of an extreme nature. Hence, it is possible to obtain samples with the same initial number of silver atoms but a different content of adsorbed ions. Experiments concerning the defrosting to a certain temperature of two samples, which were irradiated with the same dose, and contain the same initial number of silver

atoms but a different number of adsorbed silver ions, yield varying values of the number of silver atoms stable at a given temperature (Fig. 3).(3) Another sign of step-like recombination is independence of the stable radical concentration at a given temperature, to the mode of sample heating. (~2.~3) As in Ref. (12), studies of the effect of the mode of sample heating on the number of silver atoms stable at a given temperature, have shown that, indeed, NA~t is a function of the temperature and not the mode of heating,(2,6) irrespective of the nature and composition of the carrier, on whose surface the silver atoms are adsorbed (Fig. 7). However, all the known literature references(12,~3) report, in such experiments at intermediate temperatures, distinct step-like kinetic dependences, although they are of an extreme nature for silver atoms (Fig. 7). A third sign of step-like recombination of radicals is, according to the authors of Refs (12, 13) a linear dependence of the stable concentration of radicals on the temperature in a broad temperature range. Kinetic studies of the number of stable silver atoms in a broad temperature range (from 77 to 353 K) have shown that the temperature dependence bears various characteristics, being determined by the nature and composition of the carrier, the number of adsorbed silver ions, the irradiation dose, and the mode of ion introduction. Thus, the dependence of the stable number of silver atoms on the surface of silica gel on temperature is of an extreme nature. (6) In other cases such dependences may be different; linear, in particular, if the temperature range is not too broad. Previously, to explain the processes occurring on the surface of irradiated silver-containing matrices during their heating, a scheme(2) was proposed which included the following reactions: 1. Transformation of silver atoms by a first order reaction Ki Ag o~ product 2. Formation of silver atoms via a reaction of the

16

o

9

Q

24

T

V

9 le x

x w

m

~L.I 8

m

I

m

~

I

3

2

3

Dose (kGy)

Fig. 5. Dependence of the number of silver atoms stable at 123 K on the irradiation dose for samples containing (1) 0.2 x 1019; (2) 3.0 x 1019; (3) 55 x l019 and (4) 180 x l019 Ag+/l g of SiO2. R.P.C.

32/4---D

4

4 I

I

8

7q ~

I

I

I

I

I

4O

80

t20

160

200

t (rain)

Fig. 6. Dependence of the number of silver atoms adsorbed on silica gel, on time at I 13 K, for samples irradiated with same dose and containing: (a) 0.2 × 1019; (b) 3.5 × 1019 Ag+/l g of SiO2.

642

G.M. PoPOV[Crt

1123K I °..°2

I1~

I

I 133K

f

'143K I

I

i

6o

q20

-180

t (rain)

Fig. 7. Studies of the effect of the mode of sample heating on the number of silver atoms stable at a given temperature.

silver ion with the electron released from the trap Ag + + e-

K2

, Ag o

3. A side first order reaction of electron disappearance (loss): e

/(s

, product

The solution of the corresponding system of differential equations has the form: [Ag°] = lAg°]0[(1 - B) exp ( - K l t) + B exp[--(K~ + Ks)t] B=

(I)

K~[e -]0 [K. - (K~ + K3)]"[Ag°]o

where [Ag°] and [e-] are the initial concentrations of silver atoms and electrons stabilized at a given temperature; [Ag°] is the current concentration of silver atoms; K~=K2[Ag+]. At B < 1 expression (I) is a sum of exponential curves; if K~>>K~ + K3 one of the exponential curves will decline slowly and the other rapidly, their sum yielding a distinct step. At B > 1 expression (I) is the difference of both exponential curves, which describes the curve passing through a maximum, on condition that ( B - 1)K1 > 1

KI>>K'2+K 3

surface of solids. Supposing the adsorption of silver ions on the surface of silica gels and alumosilicates results in bonding of silver atoms with the surface by various energies, which are due to the non-uniformity of the surface of the carriers under study. Following v-irradiation silver ions are partially transformed to silver atoms. Silver atoms may be bonded in a different way with the carrier surface. This assumption is confirmed by our results: upon v-irradiation silver atoms located at various stabilization sites are formed on the surface of silica gels, alumosilicates and other carriers. (1~-17) To describe the obtained kinetic dependences it was assumed that the silver ions and atoms present on the carrier surface after v-irradiation can be divided into two groups: ions and atoms quite firmly bonded with the carrier at a given temperature, which do not react on the surface, and ions and atoms poorly bonded with the carrier that participate in reactions on the surface at higher or lower rates and at any temperature (let us designate the latter A,g+ and ~,g0). The number of silver ions and atoms belonging to the first and second groups may depend on the temperature of the experiment, irradiation dose, the initial number of adsorbed silver ions, as well as the nature and composition of the carrier. As a result of v-irradiation of silver-containing carriers the surface of a solid also possesses stabilized electrons or other poorly bonded electron forms, which participate in reactions on the surface. Reactions occurring on the surface of solids at a given temperature can be divided in the following manner: 1. Formation of an additional number of silver atoms following v-irradiation (in addition to the number of atoms obtained directly via v-irradiation at 77 K) through the reaction of non-transformed silver atoms and mobile electrons present on the surface. .4,g+ + e~

Ki

, .~g0

2. Transformation (loss) of silver atoms Ag o

, product

(II)

B(K~ + Ks) The above model given in Ref. (2) to explain the observed kinetic dependences of the number of silver atoms in adsorbed condition has allowed, in some cases, calculation of the rate constants and energy activation of kinetic transformations of silver atoms on the surface of solids; yet it fails to explain kinetic dependences with a distinct maximum followed by a sharp decline and the subsequent virtually complete stabilization of a certain number of adsorbed silver atoms (Fig. In). Analysing the results obtained as well as the literary data, ° H.~s-ts) the present paper suggests an amended scheme of reactions of silver atoms on the

(Possibly, the transformation (loss) occurs through the reaction AgO+ Ag ÷ ~ Ag~.) "~,16) Since after 7-irradiation of the surface of silvercontaining samples, silver ions are present in much larger quantities than silver atoms, reaction (II) can be regarded as a pseudo-first order reaction. "'") 3. Stabilization of silver atoms: Ag o

K3

, Ags°

(III)

(Silver atoms stable at a given temperature are formed not only through reaction (III), but also as a result of y-irradiation at 77 K).

Silver atoms on solid surfaces Thus, we have suggested a system of differential equations

where .4=

d['~g+] = -K~[.~,g +] dt

(I) B=

d[A'g°] = K~ [A,g+ ] - (K2 + K3) [Ag °] dt

(2)

dtAg~d = K3 [Ag°] dt

(3)

where

K3:

e+

- rd:

e°]o

K2+K3 (K~ -- K3) lag+]0

K2+K3-K~

It can be seen that with corresponding values of constants A, B, K~, K2 and K 3 all the observed cases of kinetic dependences (Figs I and 3) can be described. At T -* oo we have a step: [Ag°lst = [Ag°]o + A

K~ = K I[era]. The solution of equations (1) and (2), assuming that [em]>>[A~g+] and [era] remains constant, leads to an expression determining the number of surface silver atoms participating in reactions on the carrier surface at a given temperature:

[AGO]= K~[Ag+]o rs+r3-r~

643

exp(_Klt)

Depending on whether A is larger, smaller or equal to zero the step may be higher, lower or equal to the initial number of [Ag°]0 atoms (Figs 1 and 3). Mathematical treatment of the experimental results has shown that the physically simple model suggested in the present paper allows a quantitative description of the observed kinetic dependences of the number of silver atoms adsorbed on solid surfaces of varying nature and composition.

+ [,~g°]0 exp[-- (K2 + r3)t] K~ [Ag + ]o /(2 + K3 _ Ki exp[--(K2 + K3)t ]

(4)

By substituting equation (4) into (3), and solving the differential equation thus obtained we arrive at the expression for the number of silver atoms stable at a given temperature that have been formed through reaction (III): ^ &

[AGO]=

_

K3[~ g+]. [ 1 - e x p ( - K l t ) ]

K2+K3--~I 1

+ ~

(/(3 ['~g°]°

(5)

The number of silver atoms observed in the ESRspectrum at a given m o m e n t and imposed temperature can be expressed as the following algebraic sum: [Ag°] = [Ag°]o - [A,g°]o+ [Ag°] + [Ag°t].

(6)

lAg°]0 is the initial number of all the silver atoms observed in the ESR-spectrum upon irradiation, i.e. at 77 K. [A,g°]0 is the initial number of silver atoms poorly bonded with the carrier surface, i.e. the number of atoms participating in reactions on the surface. Thus, [Ag°]o - [Ag°]o is the number of silver atoms stable at liquid nitrogen temperature. Let us substitute equations (4) and (5) into (6) and, using the initial conditions at t = 0, [Ag°] = [Ag°]o, we obtain a final equation expressing the dependence of the number of surface silver atoms observed in the ESR-spectra, with time at a given temperature. [Ag°] = [Ag°]o + A + B e x p ( - K ~ ) -

(A + B)exp[-(K2 + K3)t]

1. G. M. Popovich, V. B. Golubev and L. T. Bugaenko, Zh. Fiz. Khim. 1972, 46, 1284 (in Russian). 2. G. M. Popovich, V. B. Golubev and L. T. Bugaenko, Khim. Vys. Energ. 1973, 7, 159 (in Russian). 3. G. A. Kuranova, G. M. Popovich and G. V. Sazykina, Zh. Fiz. Khim. 1978, 52, 725 (in Russian). 4. G. M. Popovich, G. V. Sazykina and Yu. V. Filippov, In Abstracts o f Reports at the All-Union conference on Applied Metallic Catalysts o f Hydrocarbon Transformatwn. Novosibirsk, 1978, p. 139 (in Russian).

K~ K 3[Ag + 1o /(2 + K3 -- K~

x {1 -- e x p [ - ( K 2 + K3)t]}.

REFERENCES

(7)

5. G. V. Sazykina, G. M. Popovich and Yu. V. Filippov, Zh. Fiz. Khim. 1979, 53, 941 (in Russian). 6. G. V. Sazykina, G. M. Popovich and Yu. V. Filippov, Zh. Fiz. Khim. 1979, 53, 2956 (in Russian). 7. G. M. Popovich, In Modern Problems o f Physical Chemistry, p. 152. Moscow University Press, 1982. 8. G. M. Popovich, L. T. Bugaenko and O. S. Povolotskaya, 5th " T I H A N Y " Symposium on Radiation Chemistry, Abstracts, p. 137. Hungary, Siofok, 1982. 9. G. M. Popovich, G. V. Pavilova and Y. V. Filippov, 5th " T 1 H A N Y " Symposium on Radiation Chemistry, Ab-

stracts, p. 138. Hungary, Siofok, 1982. 10. G.M. Popovich, 12th International Hot Atom Chemistry Symposium, Abstracts, p. 83. Hungary, Balatonfured, 1984. 11. G. M. Popovich and O. G. Medvcdeva, Zh. Fiz. Khim. 1976, 50, 3065 (in Russian). 12. A. M. Mikhailov, Ya. S. Lebedev and N. Ya. Bubcn, Kinet. Katal. 1964, 5, 1020 (in Russian). 13. A. M. Mikhailov, Ya. S. Lebedev and N. Ya Buben, Kinet. KataL 1965, 6, 48 (in Russian). 14. N. I. Kobozev, Zh. Fiz. Khim. 1939, 13, 1 (in Russian). 15. G. V. Pavilova, G. M. Popovich and Yu. V. Filippov, Zh. Fiz. Khim. 1984, 58, 1490 (in Russian). 16. G. V. Pavilova, G. M. Popovich and Yu. V. Filippov, Zh. Fiz. Khim. 1984, 58, 1759 (in Russian). 17. G. V. Pavilova, G. M. Popovich and Yu. V. Fifippov, Zh. Fiz. Khim. 1981, 55, 1000 (in Russian). 18. G. M. Popovich, G. V. Pavilova and Yu. V. Filippov, Vestnik Mosk. universiteta, Set. 2, "Khimiya", 1985, No. 5, 469 (in Russian).