zirconium phosphate - American Chemical Society

500

J. Phys. Chem. 1982, 8 6 , 500-506

Abbreviations daco 1,5-diazacyclooctane as-Me2- asymmetric-N,”-dimethylethylenediamine Et4dien 1,1,7,7-tetraethyldiethylenetriamine en ethylenediamine 2,3,2-tet 1,4,8,11-tetraazaundecane

dpp PPh3 PMea s-Et2en as-

1,3-bis(diphenylphosphino)propane

triphenylphosphine trimethylphosphine symmetric-NB‘cdiethylethylenediamine asymmetric-N,N’-dimethylethylenediamine

Mezen

Metal Dispersions on Zirconium Phosphates. 1. Hydrogen Reduction of Copper-Exchanged a-Zirconium Phosphate7 Abraham Clearfleld,’ Deepak S. lhakur, and Hosea Cheung

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 29, 2015 | http://pubs.acs.org Publication Date: February 1, 1982 | doi: 10.1021/j100393a017

Department of Chemlstty, Texas A & M Unlverslty. College Station, Texas 77843 (Recetved:April 20, 1981; I n Final Form: September 16, 198 1)

The reduction of Cu(I1) by hydrogen in copper-exchangeda-zirconium phosphate, Z ~ C U ( P O was ~ ) ~ found , to proceed in two stages. Below about 150 torr the product was Z~CUH(PO~)~, which in turn reacted further with hydrogen above Hzpressures of 150 torr to yield copper metal and A-zirconium phosphate. The rates of both reactions were found to conform to an Elovich-type equation and to be strongly pressure dependent. These results were interpreted in terms of the sorption and diffusion of H2 to the metal as the rate-controlling step. Activation energies were low (7.1 and 5.7 kcal/mol) for both stages of the reduction, as expected for a diffusion-controlled reaction. The concept of active-sitegeneration is introduced to account for the observed induction period.

Introduction Metals dispersed on supports represent an important class of catalysts. They are generally prepared by impregnating the support with a salt solution or ion exchanging the required cation onto the surface followed by reduction with hydrogen at elevated temperatures.lP2 The nature of the resultant metal dispersions strongly depends upon the experimental conditions. In many instances, the mechanism of the reduction reaction is in doubt and the nature of the dispersed metal poorly characterized. This is especially true of cation reductions in zeolite^.^ Therefore, the study of a simpler ion-exchanger system, in which facile reduction to metals occurs, might prove to be useful in shedding light on these questions. In this connection, we have chosen to examine hydrogen reduction of certain cations in the ion exchanger, a-zirconium phosphate, Zr(HP04)2.H20,often referred to as a-ZrP. a-Zirconium phosphate has a layered structure with an interlayer distance of 7.55 Cations can be exchanged for monohydrogen phosphate protons, as illustrated for Cu2+in eq 1.6 Copper(I1)-exchangedzirconium phosphate A.475

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Zr(HP04)2.H20+ Cu2+(aq) Z ~ C U ( P O ~ ) ~+ . ~2H+(aq) H ~ O (1) was shown to exhibit high activity for the air oxidation of C07 and the oxidative dehydrogenation of cyclohexene.8 However, in the absence of oxygen, cyclohexene reduced Cu(I1) to Cu(0) and the copper metal formed a reddish brown coating on the surface of the zirconium phosphate. This reduction process was accompanied by a decrease in catalytic activity for oxidative dehydrogenation. No copper remained inside the exchanger, as X-ray powder patterns revealed the presence of only Zr(HP04)z,along with the ‘Presented in part at the 176th National Meeting of the American Chemical Society, Sept 10-15, 1979, Miami Beach, FL.

~ o p p e r . ~The zirconium phosphate phase was A-ZrP, which can also be produced by reaction 2.1° Zr(NaP04)2+ HCl(g) Z I ~ H P O , ) ~ 2NaCl(s) (2) When the freshly reduced solids were allowed to stand in air, reoxidation of the metal on the surface to Cu(II), followed by diffusion of the Cu2+ions back into the exchanger, took place. This was shown by the change in X-ray patterns from that of A-ZrP to ZrCu(PO& Water probably formed at the surface by the action of the displaced protons on the surface oxygen species. This reaction will be reported on separately. La Ginestra et a1.l1 reported that both Cu(0) and Cu(1) can be obtained by hydrogen reduction under different conditions. Similar oxidation-reduction reactions have been detailed for Cu(11)-exchangedzeolites and related supports.12-17 These

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(1)Bond, G. C. “Catalysis by Metals”; Academic Press: New York, 1962. (2) Moss, R. L. ‘Experimental Methods in Catalytic Research”; Anderson, R. B., D a m n , P. T., Eds.; Academic Press, New York; 1976;Vol. 2, p 43. ‘ (3)Jacobs, P. A. “Carboniogenic Activity of Zeolites”;Elsevier: Amsterdam, 1977. (4)Clearfield, A.; Smith, G. D. Inorg. Chem. 1969,8,431. (5)Troup, J. M.; Clearfield, A. Inorg. Chem. 1977,16, 3311. (6)Clearfield, A.; Kalnins, J. M. J.Inorg. Nucl. Chem. 1976,38,849. (7) Kalman, T. J.; Dudukovic, M.; Clearfield,A. Ado. Chem. Ser. 1974, 133,654. (8)Clearfield, A.; West, P. B., unpublished results. (9) Clearfield, A.; Pack, S. P. J. Catal. 1978,51,431. (10)Clearfield, A,; Pack, S. P. J. Inorg. Nucl. Chem. 1975,37,1283. (11)La Ginestra, A.;Ferragina, C.; Massucci, M. A.; Tomassini, N.; Tomlinson, A. A. G. Int. Conf. Thermal Anal., R o c . , 5th, 1977 1977,424. (12)Jacobs, P. A,; Tielen, M.; Linard, J.; Uytterhoeven, J. B.; Beyer, H. J. Chem. SOC.,Faraday Trans. I 1976,72,2793. (13)Herman, R. G.; Lunsford, J. H.; Beyer, H.; Jacobs, P. A.; Uytterhoeven, J. B. J. Phys. Chem. 1975,79,2388. (14)Naccache, C. M.; Ben Taarit, Y. J. Catal. 1971,22,171. (15)Misono, M.; Hall, W. K. J. Phys. Chem. 1973,77,191. (16)Maxwell, J. E.; Drent, E. J. Catal. 1976,41,412. (17)Tester, J.; Storme, D. H.; Herman, R. G.; Klier, K. J.Phys. Chem. 1977,81,333.

0 1982 American Chemical Society

Metal Dispersions on Zirconium Phosphates


reactions are undoubtedly related to the catalytic behavior exhibited by ZrCu(P04),. This provided a further incentive for examination of the hydrogen-reduction reactions.

Experimental Section Preparation of Catalysts. A zirconium phosphate gel was prepared by the method of Clearfield et al.la and refluxed in 12 M H3P04for 100 h. The resultant solid was filtered off and washed free of excess phosphoric acid. It was then treated exhaustively with copper(I1) acetate to obtain ZrCu(P04),*4H20.BThis sample was air dried and characterized by TGA, X-ray diffraction, EPR, and ESCA measurements. Samples for reduction were heated to about 400 "C under vacuum for 17 h to obtain the anhydrous s01id.l~ Apparatus. The apparatus in which the reductions were carried out consisted of a U-tube reactor connected to a recirculatory loop, which in turn formed a part of a vacuum system with oil diffusion and mechanical pumps, and a cold trap maintained at liquid-nitrogen temperature to arrest the oil vapors from the pump. The U-shaped Pyrex glass reactor had a thermocouple well at the middle of the catalyst bed to register the temperature of the reaction zone, and a side-arm Pyrex tubing (narrow) for collecting the sample, in situ, without being exposed to the atmosphere. In order to minimize the dead space, most of the assembly (loop) was made of capillary tubing. The reactor was heated by means of a well-insulated tubular furnace; the heating rate and the temperature were regulated by using a Weathermeasure temperature controller. The temperature was controlled within f0.1 "C. The pressure changes occurring during the progress of reduction were monitored by a Mensor quartz manometer connected to a Linear single-pen recorder. The precision of the quartz manometer was fO.OO1 torr. The volume of the vacuum system was obtained as follows. A portion of the apparatus was filled with water and weighed so as to determine its volume. This portion was then filled with gas (H, or He) at a specific pressure, which was then allowed to expand into the remainder of the apparatus. The ideal gas law was used to calculate the total volume. Reduction Procedure. A weighed amount of sample (-0.3 g) was placed inside the reactor and supported over glass wool to expose maximum surface. The reactor was mounted onto the vacuum system and evacuated at room temperature for 15 min. The sample was then slowly heated and degassed at about 350 "C for 16 h so as to remove traces of water which may have been present. From the amount of water trapped, the weight of dehydrated sample was computed. The temperature was then adjusted to the desired value at which the reduction was to be carried out. After the steady state of temperature was attained, the reduction was started by bleeding in a known amount of hydrogen which was kept in the outer chamber at a known pressure. The hydrogen uptake was followed by recording the decrease in pressure with time. At the end of reaction, when no further decrease in pressure could be detected, the reactor was evacuated at the reaction temperature for 1 h and the sample cooled to room temperature under vacuum. The reduced sample was transferred to the side-arm sample tube and sealed off. The amount of Cu(I1) reduced was calculated from the hydrogen consumed. The degree of reduction of Cu(I1) to Cu(1) is p1 while that of Cu(1) to Cu(0) is 0,. Instrumentation. A Varian E-6S EPR spectrometer was used to monitor the changes in the magnetic characteristics (18) Clearfield, A.; Stynes, J. A. J . Inorg. Nucl. Chem. 1964,26,117. (19) Clearfield, A.; Pack, S. P. J . Inorg. Nucl. Chem. 1980, 42, 771.

The Journal of Physical Chemlstry, Vol. 86, No. 4, 1982 501

of the samples which occurred during the reduction. DPPH was used as an internal standard (g N 2.0028). A Hewlett-Packard 5950-A ESCA spectrometer was used to record the XPS spectra of various ZrCu(P04), samples before and after reduction. The excitation radiation was A1 K a (hu = 1486.6 eV). The sample was gently ground under dry nitrogen inside the glovebox attached to the spectrometer, and mounted on the sample holders. The carbon 1s line was used as the reference line with binding energy = 285.0 eV.,O The sample was slowly inserted through the sample preparation chamber, (maintained at a vacuum of 4 X lo-' torr) into the analyzer chamber (vacuum of the order of lo4 torr). An electron flood gun was used to control charging effects. Sufficient scanning was carried out so as to obtain peak heights of about 10 K counts. A Cahn vacuum electrobalance RG system (Model No. 2002) was used to perform the thermogravimetric analysis, the main aim being to find a suitable temperature of dehydration of the copper-exchangedzirconium phosphate, Z ~ C U ( P O ~ ) ~ . ~ HThe ~ O .experiments '~ were conducted under dynamic conditions in a nitrogen atmosphere. The powder diffraction patterns of several samples (fresh, dehydrated, and reduced) were obtained by using a Philips X-ray diffractometer with Cu K a radiation. Cu (111) and (200) lines were used for zero-angle correction. The particle size determination yas carried out from line-broadening measurements on (111) and (200) reflections of Cu(0) (reduced samples), with the aid of the Schemer equation:

D = K X / ( Pcos e)

(3)

where D is the mean crystallite dimensions in A, K the crystallite shape constant, h the X-ray wavelength, 0the corrected line breadth, and 8 the Bragg angle. Corrections were made according to Rau's method;21 errors due to strain were not considered. The particle size values thus determined are probably lower than the actual values because of neglect of strain, but they are meaningful as a basis of comparing the data.

Results Thermogravimetric analyses show that, under atmospheric pressure, anhydrous ZrCu(P04), can be obtained at temperatures above 450 "C. Full details were supplied in previous paper^.'^^^^ In vacuo a temperature of 350 "C was sufficient. We attempted to characterize the samples at various degrees of reduction so as to obtain information about structural changes occurring in these samples with the progress of reduction. The sample completely reduced at an initial hydrogen pressure of 120 torr is characterized by an X-ray pattern which contains none of the original reflections. From the hydrogen-uptake data, the white color of the sample, and ESCA studies described below, it was concluded that this is a Cu(1) phase of a-zirconium phosphate. Investigations are underway to prepare this phase by ion exchange followed by dehydration. The X-ray patterns of samples reduced at higher hydrogen pressure (400 torr) show X-ray lines characteristic of X-ZrP (composition Zr(HPO,),) and strong reflections due to Cu (200) and (111) planes, while partially reduced samples also contain Z ~ C U ( P OX-ray ~ ) ~ line-broadening (20) Johansson, G.; Hedman, J.; Brendtsson, A.; Klasson, M.; Nilsson,

R. J . Electron Spectrosc. Relat. Phenom. 1973,2, 295.

(21) Rau, R. C. Adu. X-Ray Anal. 1961,5,104. (22) Allulli, S.; Ferragina, C.; La Ginestra, A.; Massucci, M. A.; Tomassini, N. J . Chem. SOC.,Dalton Tram. 1976, 2115.

502

The Journal of Physical Chemlstry, Vol. 86,No. 4, 1982

0.8,

TABLE I : Particle Size of Cu o n Surface of ZrP as Measured b y X-ray Line Broadening

_____ __

temp of reduction, "C

particle size, A

216 270 300

21 3 241 262

Ciearfield et al.

--

temp of reduction, "C

particle size, A

320 345

332 3 54

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947

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Figure 2. Variatlon of ESCA intensities I c u / I n as a function of the particle size of Cu(0).

65y0R_ed ~95%Re?

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Flgure 1. ESCA spectra of Cu 2p3,2 level: (a) ZrCu(P0,)2 dehydrated at 320 OC, (b) ZrCuH(PO,),, reduction carried out at 120 torr, (c) reduction to 65 % of completion at 400 torr, (d) 95 % level of reduction, and (e) complete reduction. Note that Cu(I1) is present in all cases except e.

measurements conducted on the (200) and (111) planes of copper metal in the samples show that the particle size is a function of the temperature, at which reduction is carried out. The results are presented in Table I. Figure 1 shows the ESCA spectra of Cu 2~312from the Z ~ C U ( P Osamples ~)~ (in the ionic Cu(I1) and reduced forms). Spectrum a represents the original Z ~ C U ( P O ~ ) ~ sample, which has a main peak at a binding energy value 0 8 16 of 935.5 eV, accompanied by a satellite peak on the higher Time t ( m i n ) binding energy side. The reduced samples are characterized by a new peak with a binding energy of 934.4 eV, Flgure 3. Dependence of Cu(I1) Cu(1) reduction rate on temperature at 100 torr of initlai hydrogen pressure: (A)142, (0) 196, (0) while the Cu(I1) peaks gradually disappear with the 240, @j300, and (0)335 OC. progress of reduction, in accordance with the reported literature.%% Since the 2p level binding energy of copper copper loadings of these samples, we could not detect any metal is the same as that of Cu(I), the Auger line L3M6M6 hyperfine structure. The decrease in EPR intensity upon was used to distinguish Cu(0) from C U ( I ) . The ~ ~ relative reduction of the sample parallels similar reductions reintensities (Icu/Ia) were calculated by the equation proported in the l i t e r a t ~ r e . ' ~ - ' ~No ~ ~signal ~ ~ ~ 'was detected posed by Penn2' and Carter et a1.;28the values of phoin the sample completely reduced at low pressure (below toionization cross sections used were from the data given 150 torr) as well as at higher pressures. by S c ~ f i e l d .The ~ ~ ESCA spectra of samples reduced at By varying the initial pressure Po, we found that redifferent temperatures were also recorded. Relative induction to Cu(1) only took place at pressures less than tensities (Icu/IB)obtained for these samples were plotted about 150 torr. Thus, it was possible to separate the reagainst the particle size of copper metal (determined by action into two stages: X-ray line broadening) in Figure 2. we had shown that the intensity of In our earlier Z~CU(PO,)~ + 1/2H2 ZrCuH(P04):, (4) the EPR signal, due to Cu(I1) species, decreases as the samples are progressively reduced. Similar results were k2 Z ~ C U H ( P O+~1/2H2 )~ Zr(HP04)2+ Cu(0) (5) obtained in the present case. However, because of the high

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(23) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975,47, 2208. (24) Ikeniatoital, I. Bull. Chem. SOC.Jpn. 1973, 46, 2237. (25) Schoen, G. Surf. Sci. 1973, 35, 96. (26) Laraon, P. E. J.Electron Spectrosc. Relat. Phenom. 1974,4, 213. (27) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (28) Carter, W. J.; Schweitzer, G. K.; Carlson, T. A. J . Electron Spectrsco. Relat. Phenom. 1974, 5 , 827. (29) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8, 129.

The kinetic curves for the reaction represented by eq 4 as a function of temperature are shown in Figure 3. The (30) John, C. S.; Leach, H. F. J . Chem. SOC.,Faraday Trans. 1 1977, 10, 1595.

(31) Romand, M.; Roubin, M.; Deloume, J. P. J . Electron Spectrosc. Relat. Phenom. 1978, 13, 229.

The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 503

Metal Dispersions on Zirconium Phosphates u.y

I

I

Time t (min.) Flgure 4. Dependence of Cu(I1) Cu(1) reduction rate on initial pressure of hydrogen at 300 OC: (0)50, (A)60, (X) 88, and (0)100 torr. Downloaded by UNIV OF NEBRASKA-LINCOLN on August 29, 2015 | http://pubs.acs.org Publication Date: February 1, 1982 | doi: 10.1021/j100393a017

-+

Time t (min.) Flgure 6. Dependence of Cu(I1) Cu(0) reduction rate on temperatwe at 500 torr of initial hydrogen pressure: (1) 335, (2) 315, (3) 263, (4) 210, (5) 187, (6) 151, (7) 142, (8) 100 OC. +

I

I

0

4 Time t (min 1

0

-

Time t h n )

Flgure 5. Dependence of Cu(1) Cu(0) reduction on temperature and initial hydrogen pressure: (0)303 OC, 447 torr: (A)303 OC, 229 torr; (0)303 OC, 182 torr: (0)275 OC, 182 torr; (X) 240 OC, 182 torr; (A)200 OC, 182 torr.

Flgure 7. Logarithmic plots for the variations of fraction reduced (p,) with time at various temperatures and pressures; symbols same as those in Figures 4 and 5.

We applied each of these equations to our curves inreaction was dependent upon initial hydrogen pressure as cluding those for autocatalytic reactions.39 The best fit shown by the curves of Figure 4. In these figures, PIis was obtained with the Elovich equation in the form the degree of reduction of Cu(I1) to Cu(1) as measured by hydrogen uptake according to eq 4. Increasing the pressure d@/dt = k exp(-a@) (6) allowed reaction 5 to take place, and the representative where a and k are constants, and @ is the degree of recurves for this reaction (Cu(1) Cu(0)) are given in Figure duction. Integration of equation 6 yields 5. It was also possible to reduce Cu(I1) directly to Cu(0) at high enough temperature and pressure, as shown in p = ( 2 . 3 / a ) log ak + ( 2 . 3 / a ) log (t to) (7) Figure 6. Strictly speaking to = l / a k , but initially it was treated as Hydrogen-reduction curves have been found. to fit a an empirical constant. At large values of t, eq 7 is very variety of kinetic equations including first-order (for cupric nearly to cuprous ion),12second-order (for overall reduction of Cu(I1) to Cu(0),l"le and diffusion-controlled mechanisms @ = ( 2 . 3 / a ) log ak + ( 2 . 3 / a ) log t (8) (for Cu(1) to C U ( O ) ) ' and ~ ~ ~the ~ Elovich e q ~ a t i o n . ~ ~ - ~ ~

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+

~~

~~~~~~

(32)Delmon, B. "Introduction a la Cinetique Hererogene";Technip.: Paris, 1969. (33)Voge, H. H.; Atkim, L. T. J. Catal. 1962,1,171. (34)Inui, T.;Ueda, T.; Suchiro, M.; Shingu, H. J.Chem. SOC.,Faraday Trans. 1 1978,74, 2490. (35) Taylor, H. A.; Thon, N. J. Am. Chem. SOC.1952,74, 4169.

(36)Aharoni, E.;Tompkins, F. C. Adu. Catal. 1970,21,1. (37)Ritchie, A. G.J. Chem. SOC.,Faraday Trans. 1 1977,73, 1650. (38)Iwaki, T.; Komuro, M.; Hirosawa, K.; Miura, M. J. Catal. 1974, 39,324. (39)Boudart, M. "Kinetics of Chemical Processes"; Prentice-Hall: Englewood Cliffs, NJ,1968.

504

The Journal of Physical Chemistry, Vol. 86, No. 4, 1982

Clearfleld et ai. 30 *O

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1

t

7

10.0

N

1

IO

0 01 C 0 01

O

01

20

IO

,


pH*

Figure 9. Logarithmic plot for variation of k , and k , with initial hydrogen pressure.

TABLE 111: Values of a,, to,and k, at Various Temperatures and Initial Hydrogen Pressures for the Reaction Cu(1) Cu(0) --f

temp of reduc-

Time t + to (min) Flgwe 8. Semilogarithmic plots for the variations of fraction reduced (j3,) with time at various temperatures and pressures (Elovlch plot); symbols same as those in Figures 4 and 5.

TABLE 11: Values of a, t o ,and k, at Various Temperatures and Initial Hydrogen Pressures for the Reaction Cu(I1)-+ Cu(1) temp of initial reducH, tion, press., "C torr 142 196 240 300 335 300 300 300 300 300

100 100 100 100 100 50 60 91 120 142

k,,

kl', min-'

a

to

min-'

atm-"

4.26 3.46 2.76 3.55 2.99 3.78 3.34 3.32 3.46 3.68

5.21 3.07 2.18 0.63 0.53 3.12 2.16 1.31 0.61 0.51

0.045 0.094 0.165 0.444 0.653 0.085 0.138 0.229 0.473 0.535

1.6 3.3 5.8 10.9 23.2 10.2 12.0 9.6 12.0 10.2

and a plot of p vs. log t should be linear. However, the curves are exponentially concave at low values of t, deviating from the straight-line plot. To obtain a preliminary value of to, one can plot log @ vs. log t on the assumption that an equation of the type p = k"t" (9) holds at low as well as high values oft; k"and n are constants. The log B1 vs. lot t plot is shown in Figure 7. The induction period towas aasumed to be the intercept of the straight line at P1 = 0.01. These values are substituted as the initial values of to in eq 7 and then were adjusted by successive plots to give the best straight-line fit to the data. The results are shown in Figure 8. Extrapolation of these lines to = 0 provides the value of kl (the initial rate constant) from the fact that at this point to = (l/ukl), to was found to decrease exponentially, while k1 increased with increase in temperature. Following the method of

C

initial H, press., torr

a,

303 303 303 303 200 240 275

170 182 229 447 182 182 182

1.92 2.06 2.26 1.55 1.59 1.90 1.77

tiy,

k,',

k,,

min-'

to

min-'

atm-"

0.63 0.52 0.25

0.828 0.932 1.75 4.57 0.276 0.464 0.666

11.6 11.5 14.5 11.6 3.4 5.7 8.2

0.14 2.27 1.13 0.85

Taylor and T h ~ nwe , ~observed ~ that kl is approximately proportional to PH: where P H 2 is the initial hydrogen pressure. The relevant kinetic data for reaction 4 are collected in Table II. In order to account for the pressure dependence, we assumed that dp/dt = k' exp(-a@)pHt

(10)

and by comparison with eq 6 lZl =

kl'PHt

(11)

The plot (Figure 9) of log kl vs. log P H 2 yields the value of kl' (a constant independent of pressure) = 10.9 and n = 1.76 at 300 "C. Values of kl' at other temperatures are collected in Table 11. Relevant kinetic data for the second reduction step (eq 5), as derived from Figures 5,9, and 10, are given in Table 111. The value of n for this reaction is 1.77. Figure 11gives the Arrhenius plots of In kl' and In k2' against 1/T for the reduction reactions. Activation energies calculated from the slopes of these lines are as follows: Cu(11) Cu(I), 7.1 kcal/mol; Cu(1) Cu(O), 5.7 kcal/mol.

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Discussion Dehydration studies have shown that temperatures in the neighborhood of 450 "C are required to prepare anhydrous a-ZrCu(POq)2from the tetrahydrate.1g.22In the present study we used a temperature of 350 "C, the difference being that the dehydration was carried out at a

The Journal of Physical Chemistry, Vol. 86, NO. 4, 1982 505


Metal Dispersions on Zirconium Phosphates

Time t + t,(min.l Figure 10. Semllogarithmlc plots for the varlatlons of fraction reduced (&) with time at various temperatures and pressures: symbols same as those in Figure 6.

F

0 01 16

20

24

1 x 103 T Figure 11. Arrhenius plot of log k , and log k, against 1 I T .

reduced pressure of about lod torr. The presence of water in the exchanged zirconium phosphate was found to retard the rate of reduction so that proper dehydration is essential. Reduction at pressures below 150 torr led to the formation of Z ~ C U H ( P OThe ~ ) ~color changes from blue to white in the process, and the distinctive Cu(I1) EPR spectrum was no longer obtained. Furthermore, 0.5 mol of hydrogen was consumed per mole of Cu(I1) in the original solid. In the second step, with the initial hydrogen pressure above 180 torr, another 0.5 mol of Hz was consumed. X-ray patterns taken at various stages of the reduction show that A-ZrP is obtained early on and at the completion of the reaction all of the zirconium phosphate exists in this form. Thus,the metal must be on the surface since A-ZrP can be obtained by treating exchanged phases

of a-ZrP with gaseous HC1.1° Two main differences between the fresh and reduced samples are observed by ESCA. Firstly, the primary Cu 2p3/2begins to shift from 935.5 eV for the Cu(I1)-containing sample to -934.4 eV for the reduced samples. Chemical shift to a lower binding energy side indicates either the presence of copper ions with an oxidation state lower than P 3 t 4 0 or less ionic character of bonding in the reduced samples as compared to the original Z ~ C U ( P O The ~)~ latter effect was observed by Romand et al.31 in ESCA studies of copper compounds. Secondly, the intensity of the satellite band on the higher binding energy side of the principal Cu 2p3/2 line, which is prominent in ZrCu(P0,)2, gradually decreases with the progress of reduction. It is no longer observed in completely reduced samples. These results are in accordance with the reported literature on copper compounds.23-2sSatellite bands are often observed in transition metal compounds and are attributed to unfilled d shells, which leads to a chargetransfer ligand metal shake-up transition.l6 The ESCA spectra of partially reduced samples show a broader peak at about 934-935 eV, which could be deconvoluted into two peaks, one due to Cu(I1) and the other to Cu(1) or Cu(0) species. Since the ESCA binding energies of Cu(1) and copper metal are the same, the Auger line (L3M4M4J was used to differentiate Cu(1) from copper metal. It has been shown by Minachev et al.@that, on reduction of divalent copper in zeolites, the relative intensity of Cu 2p3/, peak increases; this phenomenon was attributed to the migration of metallic copper from the bulk to the surface. In the present case, we found that the relative intensity (Icu/IzI.)decreases with increase in the temperature of reduction and also with the progress of reaction. Vidriene et aL9 reported a decrease in relative intensity of the ratio INi/ISi upon reduction, which they ascribed to the formation of agglomerates. Fun$2 also observed such an effect, which was explained in terms of sintering or agglomeration of copper metal particles at the reaction temperature. Our results suggest that the copper metal, formed during reduction, tends to undergo a sintering process, which results in the formation of larger particles. X-ray line-broadening measurements support the formation of large particles (-200-300 & and Figure 3 shows that the ESCA relative intensity (Icu/Ia)decreases with increase in particle size. The reduction reactions are characterized by an initial induction period in which the rate is accelerating (ARP)34 followed by a maximal rate period during which the rate decreases slowly with time and, finally, a declining rate period (DRP). The AFtP is most evident in Figure 6. For the reduction of CuO, Voge and at kin^^^ applied eq 9 and found n to be 2. They proposed a disklike spread of metallic copper on the outer surface of the oxide particles. The enhanced rate was then attributed to increased sorption of Hzby the metal and its subsequent migration to the surface of the oxide particle where reaction occurs. Inui et al.% found n = 1.6 and postulated the presence of some retardation factor, such as the delay, due to desorption of water. In the present case, for the reduction of Cu(I1) to Cu(I), no metal particles form, nor is a second product present. Thus, other explanations than those given for CuO reduction must be responsible for the accelerating rate pe(40)Minachev, Kh. M.; Antoshin, G . V.; Shpiro, E. s.; Yusifov, Y. A. h o c . Int. Congr. Catal., 6th, 1976 1977,B2. (41)Vedrine, J. C.;Hollinger, G.; Duc, T. M. J.Phys. Chem. 1978,82, 1515. (42)Fung, S. C.J. Catal. 1979,58,454.


506

The Journal of Physical Chemistty, Vol. 86,No. 4, 1982

Clearfield et al.

riod. This subject will be treated in more detail in a subsequent paper. For the present, we will deal only with the maximal rate period. In order to develop a proper mechanism, it is necessary to describe some features of the structure a-ZrP. The crystals have a layered structure4p5with a nonporous surfacesG The layers are situated relative to each other such that six-sided cavities are formed between the layers. There is just 1 mol of these cavities per formula weight of a-ZrP, so that, when fully loaded with Cu2+,there is one cation per cavity. However, only half-cavities are present on the surface with the copper ions occupying every other half-cavity. This requires copper ions to be in adjacent half-cavities in one direction, but in every other cavity in the other direction. The intercavity distance is about 5.2 A. Since the Elovich equation is obeyed, it can be considered that the process of hydrogen adsorption and diffusion to the reaction site determines the rate of reduction. This is borne out by the very low activation energies and the high hydrogen-pressure dependence of the reactions. Once the hydrogen has reached the active site, the reaction may be visualized in terms similar to the process occurring in zeolites.12

. H-H, (Cu2+O-), + H, *Cuz+'

-0-

(12)

+ OH

(13)

H6--H6+ c u 2+ *

'O-*(Cu2+H-)

Cu2+H-

+ Cu2+ e 2Cu+ + H+

(14)

or 2(Cu2+:H-)

2Cu+

+ H2

(15)

In reaction 14, the H+would immediately diffuse to an

01-site and form an OH group. Both reactions 14 and 15 require additional diffusion processes, that is, diffusion of the two copper species toward each other brought about by oxygen sites close to them being protonated, and diffusion of protons to negative oxygen sites. In zeolite Y, the slow step is thought to be reaction 14 and, in fact, the reaction is considerably slower than the reductions reported in this work. Our view is that ion movemenh in a-ZrP are more rapid than those in zeolites. For example, the kinetics of reaction 2 were examinedu and half-times of the order of a few seconds were observed in the same temperature range as the reactions in the present study. Reaction 2 must occur on the surface since all of the NaCl is present on the surface and there is no mechanism for chloride anion diffusion between the layers of a-ZrP. Thus, HCl at the surface ionizes to form protons which then diffuse into the interior. Sodium ions are displaced and diffuse to the surface, and this counterdiffusion of ions was found to be rate determining.u We feel that a similar process occurs (43) Homely, S. E.;Nowell, D. V. J. Colloid Interface Sci. 1974, 49, 394. (44) Jerue, P.; Clearfield, A. J. Znorg. Nucl. Chem. 1981, 43, 2117. (45) Clearfield, A.; Daux, W. L.;Medina, A. S.;Smith, G.D.; Thomas, J. R. J.Phys. Chem. 1969, 73, 3424.

in the reduction of Cu2+. Reaction 12 is rate determining with the other steps being rapid. As the Cu+ ions form, they, along with protons, diffuse into the interior and Cu2+ diffuses out to the surface. The foregoing remarks need some qualification, as they apply to the maximal rate period. When reaction 14 or 15 is completed, two Cu+ and H+ions exist in adjacent cavities (or close proximity), and these would diffuse in pairs into the crystal lattice. Thus, the two Cu2+,which replace these ions, would be in the proper position to react. Such sites may be termed activated and are responsible for the rapid rate in the maximal rate period. Initially, there are no (or few) active sites as there is a low probability that all of the species are in their proper positions. It is this generation of active sites which must take place in the accelerating rate period. According to this idea, the lower the concentration of surface cations, the slower the rate should be in the ARP. In a subsequent paper dealing with Ag+ reduction, quantitative data to substantiate this idea will be presented. In the second reduction step, where Cu(1) is reduced to Cu(O),the rates are about equal to the fust stage, as shown by the near correspondence of kl' and k i values. Thus, the mechanism for the second stage is about the same as depicted for the first stage. The main difference is that only protons diffuse into the interior, while Cu+ diffuses to the surface and copper metal accumulates on the surface. However, the metal must be porous enough to allow H2 molecules free access to the surface. Sorption of additional hydrogen by the metal seems to play no role in enhancing the reaction rate. Furthermore, the fact that A-ZrP forms in both this reaction and reaction 2 provides indirect evidence that the reduction reaction occurs on the surface. The sticking ratio for H2 on the a-ZrP surface must be low so that only a small fraction of the hydrogen molecules are available to diffuse to the reaction sites. However, as the pressure increases, the number of H2molecules sticking to the surface increases proportionately. Thus,the average distance traveled by an H2 must decrease and the ratk increase proportionately. At sufficiently high pressures Cu(I1) is reduced directly to metal, and this may come about by having hydrogen molecules sufficiently close to the already positioned Cu+ pairs to react before they can diffuse away. A second possible reaction path involves diffusion of H2 into the interior of the crystals with reduction in place. This mechanism seems less likely for a number of reasons. It would require that metal atoms formed inside the cavities diffuse to the surface. The cavities are quite narrouP5 and would be expected to restrict the motion of large neutral atoms. Furthermore, in our work with Ag+ reduction (to be reported later), which also follows the Elovich equation, it was found that the rate increases in direct proportion to the surface area. This is what would be expected if the reaction occurred at the surface. Acknowledgment, We gratefully acknowledge the financial support of this study by the National Science Foundation under Grant CHE-79-16160and Gulf Research and Development Co.

zirconium phosphate - American Chemical Society

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