The preparation of palladium metal catalysts supported on carbon part II: Deposition of palladium and metal area measurements

Carbon Vol. 26. No. 6. pp. 815-823. 1988 hinted in Great Britain.

CM-&6223/88 S3.00+ .CKI

Copyright0 1988PergamonPressplc

THE PREPARATION OF PALLADIUM METAL CATALYSTS SUPPORTED ON CARBON PART II: DEPOSITION OF PALLADIUM AND METAL AREA MEASUREMENTS G.

Department

R. HEAL and L. L. MKAYULA*

of Chemistry and Applied Chemistry, University of Salford, The Crescent, Salford M5 4WT, U.K. (Received 10 April 1988; accepted 23 May 1988)

Ahstraet-Four techniques for Pd deposition were compared and various carbons from Part I were used. Increasing burnoff produced higher area of the support and higher available metal area. However, excessive removal of carbon resulted in a decrease in Pd area for a given loading. Although HNO, treatment did not produce an increase in carbon area, there were significant increases in catalyst areas for some deposition techniques. Carbon deposition appeared to block micropores to some extent, but was disappointing in the areas of Pd found on these samples. A favorable technique would seem to be to use 15 to 20% burnoff followed by HNOJ acid washing and the impregnation adsorption deposition method. Key

Words-Palladium,

catalyst, carbon.

1. INTRODUCTION

In Part I[ 11, the treatment of some commercial carbons was described, together with the measurement of surface properties by nitrogen adsorption. The next stage was to deposit palladium on the surface and to analyze the resulting catalyst. These experiments were to determine the available metal area after reduction of any oxide present. The methods of preparation of supported palladium catalysts have been described previously and are similar to techniques for other precious metals[2-81. Selective chemisorption of a gas onto the active component of a supported catalyst is widely used for measuring surface areas[9]. Among the gases used extensively are hydrogen, oxygen, and carbon monoxide. An extensive review of such determinations has been made for supported metals and metal oxides by Farrauto(lO]. The apparatus used is usually a static volumetic system, but continuous flow systems have also been described[ll,l2]. Both adsorption and desorption may be studied by adjusting conditions. Usually an inert carrier is used and the concentration of the active gas measured by a thermal conductivity detector. The advantage of the continuous flow technique is rapid determination[l3]. In a pulse technique, a precise volume of gas of known composition is injected into a stream of inert carrier that passes over the catalyst bed. The thermal conductivity detector monitors the gas composition and shows a peak as the pulse passes. Several injections have to be applied before saturation is

achieved, each producing a peak on a recorder. The sum of the decreases in peak area (relative to the nonadsorbed peak after saturation) enables calculation of the amount of gas adsorbed[l4-161. A disadvantage of this technique is that the catalyst may not be able to hold weakly chemisorbed gas and this may lead to desorption and to low area results. Metal area is given by: A,

= nf,, X,ln,

(1)

where n; is the monolayer coverage (saturation uptake), X,,, is the number of surface metal atoms associated with the adsorption of each adsorbate molecule at monolayer coverage, and n, is the number of metal atoms per unit area. At ambient temperature and pressure, palladium is inert to CO/H2 reactions. Carbon monoxide is more strongly adsorbed than hydrogen and expels all chemisorbed hydrogen[ 171. Palladium catalysts may be reduced in a stream of pure hydrogen at relatively low temperatures of 50 to lOO”C, to protect palladium from sintering. The catalyst may be cooled in the same stream of hydrogen, and CO injected into the hydrogen carrier until saturation is reached. The state of subdivision of a metal in a catalyst may be defined by the ratio of the total number of surface atoms per unit weight of catalyst, n,, to the total number of metal atoms present per unit weight of catalyst, r+ This quantity, metallic dispersion D, is thus given by: DPd = n,ln,

(2)

The values of D, are affected by variation in X,,,. If crystallite size is small and, if for hydrogen X, is less

*Present address: Department of Chemistry, University of Dar es Salaam, Tanzania. 815

G. R. HEAL and L. L. MKAYULA

816

than 2 (more than one hydrogen atom per surface metal atom), then Dpdcan exceed the expected upper limit of unity. Small values of Dpdresult if there is strong metal-support interaction rendering the surface metal atoms inaccessible to the adsorbate. The mean particle size of the crystallites may be found from the metal surface Apd and the total volume of the dispersed metal V. If the particles are spherical: ;i__ ”

I

=6v

(3)

-4,

The same formula may be used if cubic particles are assumed when d+ has the meaning of edge length of a cube, (4)

this solution, 0.132 g of NaCl was added and stirring started, then 5% NaOH solution was added dropwise over a 15-min period until the pH was about 6.8. If a precipitate appeared the solution was discarded.

To about 2.36 g carbon, 20 ml of deionized water was added and stirring started. The chloropalladite solution obtained above was added rapidly and stirred for 15 min. Then 4 ml of 5% Na,H,PO, were added rapidly to the mixture and stirred for 15 min. The carbon was filtered off and washed with about 2 1 of water until Cl- free. The resulting paste was dried overnight at 110°C. For other Pd loading the quantities were varied accordingly. 2.3 The LPRF method As for the above method substituting hyde for hypophosphite.

formalde-

where ni is the number of particles of side dp

2. EXPERIMENTAL

2.1 Codes used Samples chosen from the set described in Part I[ l] were used for the deposition of Pd and the measurement of metal area by a pulse flow of CO. Three methods for the introduction of Pd were used: 1. A deposition method, code named LPRH, which stands for Liquid Phase Reduction of palladium metal salt (sodium chloropalladite) solution with sodium Hypophosphite solution. Alternatively the LPRF method used Formaldehyde for reduction. 2. An adsorption method, code named IMA, a short form of IMpregnation by Adsorption, which is not to be confused with the impregnation method proper. 3. The Ion Exchange Technique which was code named IET. The details of the preparations

are as follows.

2.2 The LPRH method Quantities to produce about 2.48 g of 4.8% Pd-C catalyst. Weight of PD: 0.2 g of 59.96% Pd Cl*. Volume of sodium hypophosphite: 4 ml of 5% NaH,PO, solution. Weight of carbon 2.36 g of pretreated activated carbon. Approximately 0.2 g of palladium chloride was dissolved in 15 ml of 0.2M HCl solution. This volume of the acid was slightly in excess of that required by about 1.1 mmol of Pd present. The resulting solution, initially yellow, turned brownish red, due to the excess acid. This would possibly indicate the formation of hydrochloropalladous acid, H,PdCl,. To

2.4 The IMA method Starting from palladium chloride, a solution of hydrochloropalladous acid solution was prepared as above. About 2.36 g of carbon were added to 20 ml of deionized water and stirred. To this, H,PdCI, solution was added to make about 35 ml of carbon slurry. Stirring was continued for 30 min and the slurry then left to stand at room temperature for 40 hours. The carbon was filtered off and washed Clfree and dried as before. 2.5 The IET method A solution of tetraammine palladium(B) chloride was prepared by treating 2.5 g PdC& with 75 ml of a solution containing equal amounts of water and concentrated ammonia. The solution was evaporated in a steam bath until only a faint odor of ammonia was noticeable. The resulting light yellow solution was filtered, and the volume adjusted to about 50 ml. The solution was cooled, poured into a RIO-ml volumetric flask, and the flask filled to the graduation mark to provide 0.141 molar [Pd(NH&]*+ solution. A quantity of 2.4 g of carbon was added to 20 ml of deionized water and stirred. After the addition of 8 ml of the [Pd(NH,),)12+ solution, stirring was continued for 30 min, followed by standing for 100 hours. The paste was filtered, washed Cl- free and dried at 177°C overnight. The chemicals employed were: Sodium hypophosphite-BDH, assay not less than 98%. Formaldehyde-BDH, general purpose reagent Hydrochloric acid-May and Baker, AR grade Sodium hydroxide pellets-Vickers, assay (98%) Sodium chloride-Hopkins and Williams, AR grade Ammonia-Fisons, SLR grade assay 35% and Palladium chloride-Johnson Matthey

Palladium on carbon II: Deposition and area 2.6 The gas chemisorption apparatus and experimental procedures The gas chemisorption equipment was similar to that previously described[ll-131 and essentially consisted of a gas handling system, a thermal conductivity cell and associated electronics, a linear temperature programmer, a chart recorder, a sample reactor, a furnace, cold traps, and a microcomputer. 2.7 Gas-handling system Oxygen-free nitrogen gas was provided from a cylinder. Hydrogen was produced by a PYE Unicam HGlO generator. Nitrogen and hydrogen were both purified to remove moisture by gas purifiers (GP) packed with indicating Drierite and molecular sieve 5A, which were supplied by Alltech Associates. Nitrogen gas was further purified to remove traces of oxygen by an on-line OXY-TRAP from Alltech Associates. 2.8 Gas detector (CC) The effluent gases from the reactor were analysed by the thermal conductivity cell forming part of a conventional PYE 104 gas chromatograph. 2.9 Reactors, furnaces, and temperature control The reactor tubes used were quartz and could sustain temperatures up to lOOCK, well above the operating temperature range in all the experiments. The sample rested on a sinter near the bottom of the tube, below which the carrier gas flowed in. This allowed time for the mixture to attain the reactor’s temperature while it descended through the outer tube of the reactor, before reaching the sample. A small Cressal electric heating furnace was used to heat the reactor. This was controlled by a Newtronic linear temperature programmer. A CromelAlumel thermocouple junction was used in conjunction with a digital multimeter to measure temperature. 2.10 The microcomputer-based CC integrator A GC integrator using a microcomputer[l8,19], Rockwell AIM 65, was constructed. A signal from the thermoconductivity detector (TCD) of the GC was fed into a lo-mV full-scale chart recorder. Also, in parallel, the signal was fed to a high-input impedance differential amplifier. The lo-mV full-scale signal was amplified to 2 V, the optimum input range of the analog-to-digital converter (ADC) used. An analog signal from a Chromel-Alumel thermocouple was amplified to 2 V range and was picked up by another ADC and, together with the TCD signal, were converted into digital codes via controls as instructed by the AIM 65 microcomputer at predetermined intervals. The microcomputer had on board a versatile interface adapter (VIA) that contained two separate timers for controlling the data-logging interval and had also a built-in printer for recording data. CAR26:6-D

817

A detailed description of the application of the Rockwell AIM 65 is given elsewhere[ 18,191. The data obtained was smoothed by the SavitzkyGolay method[20] and the area under the peaks found by numerical integration. 2.11 Experimental procedures 2.11.1 Carbon monoxide (CO) pulse injectionfor the determination of palladium metal surface area. Carbon monoxide chemisorption on palladium was carried out at 27°C. It was considered that, at this temperature[ 141, CO chemisorption was strong enough to ensure full coverage and that there was no chemisorption by the support. The amount of sample normally used was between 0.10 and 0.20 g. The catalyst sample that had been previously dried at 110°C was weighed into one of the reactor tubes. The sample was then connected to the apparatus and nitrogen passed over at a rate of 50 ml min-I. After all the air was flushed out, usually over a period of about 15 min, the sample was placed in the furnace where it was reduced in 100% hydrogen for 90 min at 27°C. The flow of hydrogen was then adjusted to 133 ml per minute. A lecture bottle was used to provide CO of 99.96% purity. About 0.3 to 0.4 ml of CO (in excess of what could be adsorbed by the sample) was drawn through a septum by a Dynatech Pressure-Lok Syringe (0.5 ml capacity). This amount of CO was then injected through another septum into the hydrogen stream in front of the catalyst bed. The CO pulse passed first through the catalyst bed and then through the measuring side of the conductivity cell. For high surface area samples, several injections were necessary to obtain final peaks that were reproducible and had equal areas corresponding to zero CO uptake. 3. RESULTS

The injections of CO were continued until a constant area A, was obtained, in arbitrary units, from the integrator. In early injections the area was smaller, A?. The uptake was thus proportional to AI - A,. If the injected volume was Vi, cm3 then the total volume adsorbed in a number of injections V,, was given by:

2 (A, - 4)

vex= V,n

Al

cm3

(5)

The temperature T and pressure P of this gas was at ambient conditions for the syringe used and was corrected to STP for 1 g of catalyst by v

= CO

P

273 Vex

760 e T

w cm3 (STP) g-l

(6)

where W is the weight of catalyst sample used. The value of n; (i.e., number of molecules adsorbed per

G. R. HEAL and L. L. MKAYULA

818

gram) is then given by VN molecules g-’ nl, = 3 22414

(7)

where NA is Avogadro’s number. The value of X,,, was taken as 2 (i.e., assuming a bridged CO form). The number of palladium atoms per unit area, n,, used was 1.2 x 10lg, this being the mean value for the crystallographic planes (loo), (llO), and (111). This is based on the work of Lanyon and Trapnell[21]. The metal area API was then calculated using eqn (1). Dispersion values D, were found from eqn (2) and the mean particle size $_, from eqn (3). Since the main expense of a precious metal catalyst lies in the cost of the metal, cost effectiveness is important. This may be represented as specific surface area per gram of Pd metal, ALd. The results for carbons type 5S, 6S, and 3s treated in various ways, as shown in Part I, and with various deposition techniques are given in Tables 1 to 3. The effect of various loadings of Pd was tested for a restricted set of carbons and the results shown in Table 4. In Table 6 the effects caused by increasing the reduction temperature are shown for one carbon and one deposition method only.

As well as the main tests carried out on the three carbons described above, tests were carried out on other carbons described in Part I. These results are in Table 5, together with figures for samples bought from Koch-Light or donated by Johnson-Matthey and used as commercial standards for comparison. The Johnson-Matthey sample was quoted as having an area of 19 to 22 mz g-l. 4. DISCUSSION

4.1 The effect of pretreatment of the carbons on AM The effect of %CBO of the carbons on the surface area, &, dispersion and mean particle size, &-,, may be seen in Tables 1 to 3. It is seen in Table 1, for instance, that AW for the palladium catalyst prepared by the LPRH method increases with increasing burnoff, then stays almost constant before increasing with further gasification. The maximum AM reached is for the 18.5% CBO sample and is 1.4 times higher than for the original untreated carbon. However, APddecreases 0.5-fold for the 54% CBO sample (51) despite an increase in the total surface area, ABET,of the support of almost 2.5-fold. The IMA method tends to give relatively higher values of AW than the LPRH method. The IET method appears worst, giving the lowest values.

Table 1. Summary of the computed free-metal areas, dispersions, and mean particle sizes of the carbon 5SO (NORIT SKI GA3/33) supported Pd catalysts

Sample-Carbon code Wt % Pdlcarbon, and method of preparation

Percent burnoff and treatment

5A + 4.78% Pd LPRH 5A + 4.78% Pd LPRH 5A + 4.8% Pd IMA

0 8

5B 5A 5C 5D 5D 5E 5F 5F 5F 5G 5H 51 55 5J 5J 5K 5K 5L 5M 5N

+ + + + + + + + + + + + + + + + + + +

4.85% 4.83% Pd LPRH IET 4.85% Pd LPRH 4.85% Pd LPRH 4.88% Pd IMA 4.83% Pd LPRH 4.86% Pd LPRH 4.86% Pd IMA 4.82% Pd IET 4.86% Pd LPRH 4.84% Pd LPRH 4.84% Pd LPRH 4.89% Pd LPRH 4.9% Pd IMA 4.82% Pd IET 4.84% Pd LPRH 4.87% Pd IMA 4.85% Pd LPRH 4.86% Pd IMA 4.82% Pd IET

FFA-treated

0 0 0 0 0

4.7 0 9.5 18.5 18.5 21.0 23.0 23.0 23.0 25.0 37.0 54.0 + HNO3 + HNO, + HN03 + FFA + FFA

18.5 18.5 + FFA HNOj 0 + HNOl

Pd surface area, AM Dispersion, DPd

Pd surface area A& (m2Pd g-’ Pd)

12.2 12.7 12.5

0.54 0.56 0.55

255 266 260

16.9 0.5 14.2 17.4 20.2 16.3 17.1 18.5 0.2 13.1 11.2 5.6 7.7 27.5 6.3 8.7 14.8 11.9 29.5 8.0

0.74 0.02 0.62 0.76 0.88 0.72 0.75 0.81 0.01 0.57 0.49 0.25 0.33 1.19 0.28 0.38 0.64 0.52 1.29 0.35

3:: 293 359 414 337 352 381 4 270 231 116 157 561 131 180 304 245 607 166

(m:)g’

with polyfurfuryl alcohol and recarbonized.

Keyfor all Tables: LPRH-Liquid Phase Reduction of Palladium Salt by Hypophosphite LPRF-Liquid Phase Reduction of Palladium Salt by Formaldehyde IMA-Impregnation Adsorption IET-Ion Exchange Technique

Mean particle size from CO ad>orption, dCmr nm 1.96 1.88 1.92 48.30 1.43 1.71 1.39 1.21 1.48 1.42 1.31 120.08 1.85 2.16 4.31 3.18 0.89 3.83 2.78 1.65 2.04 0.82 3.01

Palladium on carbon II: Deposition and area

819

Table 2. Summary of the computed free-metal surface areas, dispersions, and mean particle sizes of the carbon 3S0 (Degussa Eponit 113K special)-supported Pd catalysts.

Sample-Carbon code Wt % Pdlcarbon and method of preparation

Percent

Degussa eponit 113K

burnoff and

Special

treatment

3A + 4.8% Pd LPRH 3A 3A 3B 3B 3C 3D 3D 3E 3F 3F 3F 3G 3H 31 33

+ + + + + + + + + + + + + + +

4.8% Pd IMA 4.8% Pd LPRF 4.86% Pd LRPH 4.89% Pd IMA 4.85% Pd LPRH 4.86% Pd LPRH 4.86% Pd IMA 4.86% Pd LPRH 4.82% Pd LPRH 4.85% Pd IMA 4.82% Pd IET 4.84% Pd LPRH 4.85% Pd LPRH 4.87% Pd IMA 4.83% Pd IET

FFA--treated

0 8 20.0 20.0 40.9 47.6 47.6 O+FFA 0 + HNO, 0 + HN03 0 + HNOx 40.9 + FFA 20.0 + FFA 20.0 + HNO, o+HNo,

Mean particle

Pd surface area, APd (m::‘.,“-’ 12.1 16.1 4.3 17.4 23.0 18.4 16.3 21.9 14.1 10.0 25.5 11.4 17.1 7.7 27.3 4.2

size from CO

Dispersion,

Pd surface area A i,,

&

(m2Pd g-l Pd)

d,_, nm

252 335 89 358 470 380 335 451 296 207 526 237 353 159 561 87

1.98 1.49 5.63 1.40 1.06 1.32 1.49 1.11 1.69 2.41 0.95 2.11 1.42 3.15 0.89 5.75

0.53 0.71 0.19 0.76 1.00 0.81 0.71 0.96 0.63 0.44 1.11 0.50 0.75 0.34 1.19 0.19

ad>orption,

with polyfurfuryl alcohol and recarbonized.

The mean particle sizes show the inverse trend. However, from the &--s values shown in the tables mentioned above, it may be deduced that the LPRH method produces catalysts with palladium crystallites situated mainly in the wide pores with widths greater than about 1.2 nm. It has already been seen that the treatment of SSO with a deposit of carbon from furfuryl alcohol decreased the values of most quantities indicated in Table 3 in Part I[l], except AWL*,A,,, Tp, and YCLpj.

Although there was some micropore blocking, this did not result in an increase of AM. The nitric acid treatment (5J) gives lower AW values for the LPRH method. This treatment seems to give better values if the IMA method is used. The IMA method is not significantly improved by prior gasification of the carbon, as is indicated by sample SM in Table 3 in Part I[ l] or in Table 1 in this article. The dispersion, Dw, of the palladium catalyst prepared by the IMA method is greater than the

Table 3. Summary of the computed free-metal surface areas, dispersions, and mean particle sizes of the carbon 6S0 (NORIT GSX A1339)-supported Pd catalysts Sample-Carbon code Wt % Pdlcarbon, and method of preparation Norit GSX Al339 6A 6A 6A 6A 6B 6B 6C 6C 6C 6D 6D 6D 6E 6E 6F 6F 6F 6G 6H 61

+ + + + + + + + + + + + + + + + + + + +

4.83% Pd LPRH 4.85% Pd LPRF 4.85% Pd IMA 4.83% Pd IET 4.85% Pd LPRH 4.83% Pd IMA 4.87% Pd LPRH 4.87% Pd IMA 4.82% Pd IET 4.89% Pd LPRH 4.9% Pd IMA 4.84% Pd IET 4.81% Pd LPRH 4.89% Pd IMA 4.84% Pd LPRH 4.85% Pd IMA 4.82% Pd IET 4.86% Pd LPRH 4.83% Pd IMA 4.83% Pd IET

FFA--treated

Percent burnoff and treatment 0 0 0 0

0 0 0 0 0 14.3 14.3 0

14.3 14.3 25.8 25.8 25.8 36.0 36.0 36.0 + FFA + FFA + HNO, + HNO, + HNO, + FFA + HNO, + HNO3

Pd surface area, A, Dispersion, DPd 17.8 8.8 21.4 0.6 12.7 18.3 20.9 24.5 0.4 17.1 18.4 0.3 9.1 13.0 9.6 30.9 6.8 11.5 29.9 10.6

with polyfurfuryl alcohol and recarbonized.

0.78 0.38 0.93 0.03 0.56 0.80 0.91 1.07 0.02 0.74 0.80 0.01 0.40 0.56 0.42 1.35 0.30 0.50 1.31 0.47

Pd surface area ALd (m*Pd g-l Pd) 369 181 441 12 262 379 429 503 35: 376 6 189 266 198 637 141 236 619 219

Mean particle size from CO ad_sorption, dc_, nm 1.36 2.76 1.13 40.30 1.19 1.32 1.17 0.99 60.30 1.43 1.33 80.70 2.64 1.88 2.52 0.78 3.54 2.11 0.81 2.28

G. R. HEALand L. L. MUYULA

820

Table 4. The effect of Wt% Pd loading on the surface area and mean particle sire of nalladium AW (m* Pd g-l cat.); Ah (m’ Pd g-l Pd) in brackets; and di_, (nm) in parentheses

Sample-Carbon code and method of preparation

13.2%

2.5%

4.8%

9.2%

3.35

12.7

34.5

11351 (3.73)

[2651 g9)

i3751 g3)

3SO-LPRH

[252] 1’8918)

[168] $.9;1)

3SO-IMA

[335] $.;9)

(2361 (2.12) 30.5

41.1

[3321 (1.51)

13111 (1.61)

1%

SSO-LPRH

6SO-LPRH

2.53

12.1

12531 (1.98)

P841 (1.03)

theoretical maximum value of one. According to Scholten[22], even if D,, were nearly one, the metal is not dispersed atomically; and palladium crystallites with a diameter of 1 nm and containing 63 Pd atoms have nearly 90% of their atoms at the surface. Taken in the context of the present work, the values of Dw > 1 should imply a highly dispersed catalyst, despite the uncertainty as to the chemisorption stoichiometry of CO on such a highly dispersed palladium catalyst[23,24]. In Table 2 here and Table 4 of Part I[ 11, a similar trend in Ap,, and &-, is observed, for type 3 carbons. Both the LPRH and IMA methods show an improvement of 1.4 times in Apd for the 20 to 40% CBO range, relative to the original untreated 3S0

[3il] (1.35)

(2$ (1.83)

16.9% 35.3

[207 (2.39)

[3::] (1.59)

material contrasting with the worsening effect of the FFA treatment. The nitric acid treatment improves A, 1.6-fold if the IMA method is used. The dispersion on supports 3F and 31 (IMA method) in Table 2 is once again greater than one. In Table 5 of Part I[ l] the nitrogen adsorption and porosity characteristics of the 6s carbons produce the same effect on API, DPd, and &_ as already seen in respect of 5s and 3S carbons. However, as shown in Table 3 here, a 14.3% CBO of 6S0 resulted in a slight decrease in APd. Both the LPRH and IMA method produced low values for the 14.3% bumoff sample. This was reversed to higher values of APd for the 25.8% CBO sample (6B) and reduced slightly for the 36% CBO sample (6D). The FFA treatment

Table 5. Total surface area, free-metal areas, and mean particle sizes of commercial Pdlcarbon catalysts and some other laboratory prepared catalysts Sample-Commercial cat. carbon code wt% Pdlcarbon, method of preparation 5% Pd/C-JMC423K” 5% Pd/C-JMC-373K” 5% PdlC-JMC-373K” 0 5% Pd/C-Koch-Light-373K” I, I, 1SO + 4.83% Pd LPRH 1S (PWD) + 4.83% PD LPRH 1S (PWD-HNOI) + 4.83% Pd IET -’ 2S0 + 4.85% Pd LPRH 2S0 + 4.85% Pd LPRF 2S0 + 4.84% Pd IMA 2S (PWD) + 4.85% Pd LPRH 2S (PWD + HNOs) + 4.83% Pd IET 4S0 + 4.84% Pd LPRH 4S (HNO,) + 4.83% Pd IET

Support Pd surface surface area ABET area, Apd (m’g-’ cat.) (m*g-’ cat.) 765b 638b v I, 710b ,, !I 695 778

823 V ;85

1129 -

Dispersion, Dpd

Mean particle size, ;is_ nm

18.2 18.4 19.0 18.4 13.6 14.0 14.2 1.2 12.1

0.77 0.78 0.81 0.78 0.58 0.59 0.60 0.05 0.53

1.37 1.36 1.32 1.36 1.84 1.79 1.76 20.10 2.00

6.4 34 0.5 13.5 17.6

0.28 0.15 0.02 0.77 0.59

3.77 7.13 48.50 1.38 1.80

9.8 8.8 3.7

0.43 0.39 0.16

2.46 2.75 6.53

‘Refers to the temperature at which the catalyst was degassed overnight prior to ABETmeasurements. bABETcalculated over the entire BET relative pressure range (0.05-0.35). Some carbons were granular and were ground to a powder in a pestle and mortar and sieved through a 60 BSS. These are designated (PWD).

Palladium on carbon II: Deposition and area again reduced area, so did the HNO, treatment in the case of the LPRH method of catalyst preparation. It is the IMA technique following HNO, treatment of the carbons that results in a much improved surface area of the palladium on the support. Taken in the context of the present work, the IMA method on I-IN03 treated carbon supports is superior as it gives higher free-metal surface areas, smaller particle sizes, and hence greater dispersion of the palladium for a given catalyst loading. In contrast, the LPRH method responded poorly on nitric acidtreated carbons. The LPRH method was also surpassed by the IMA method, even on the high burnoff carbon samples. As a result, it is apparent that treatment of the commercial activated carbons is required, to increase the free-metal surface area of the catalysts. The treated samples are sometimes better than the original but not as often for carbon 6s. 4.2 The effect of metal loading on API High metal surface areas may be obtained by (1) modifying the method of preparation so that the metai is more completely dispersed, or (2) increasing the total amount of the active metal. Table 4 shows the effect of increasing the metal loading for the LPRH and IMA methods of preparation on 3S, 5S, and 6s carbons. It is noted that AW expressed in m* g-l catalyst (cat.), is still rising at 13 to 17% loading for two carbon supports (i.e., 5SOand 6SO). With metal loading above 9%, the catalyst based on 5SO shows only a moderate increase in metal area, while the catalyst based on 6S0 continues to increase linearly. However, when A;, expressed in m* g-’ Pd, is considered, the situation is altered in such a way that now (see Table 4) a rise to a maximum is noted for 5SO at 9.2% Pd, for 3S0 at 4.8% and for 6S0 at 2.5%. While Ai,+ for 5S0 falls above 9.2% Pd loading, Si, for 6S0 remains almost constant above that. This strongly indicates that 5S0 and 6S0 carbon supports should be loaded with palladium only up to 4.8 or 9.2%, respectively, but that 6S0 can take higher loadings to produce a lower total catalyst bulk. This should be considered when purchasing a catalyst for, although the surface area per gram catalyst may be high, that same area per gram palladium may be too low to be considered good value. 4.3 The effect of different methods of catalyst preparation on AW Many preparations undertaken in this work gave higher Pd areas for loadings similar to those of commercial catalysts (Table 5). Catalysts prepared from untreated original carbon samples gave areas very much comparable with commercial samples (e.g., the area of 6A + 4.83% Pd (LPRH) was 17.8 m* g-’ cat., the area of 5A + 4.78% Pd (LPRH) was 12.7 m2 g-’ cat., and the area of 3A + 4.8% Pd (LPRH) was 12.1 m* g-’ cat.). This compares with 5% Pd/C(JMC) of area 19.0 m* g-’ cat. and 5% Pd/C (Koch-Light) of area 14.2 m* g-l cat.

821

On the other hand, the higher AW values obtained in the present work for some other preparations suggests the value of the techniques here described. Results on other CAC are presented in Table 5, showing that ABETof pulverized (PWD) 1SO and 2S0 has not changed significantly. But AW on the same pulverized carbons is increased from 1.2 to 12.1 m* g-l cat. for the lS(PWD) carbons. Likewise, AW on ZS(PWD) is increased from 3.4 m* g-’ to 13.5 m* g-’ cat. Furthermore, even for the same method of preparation, such as the LPRH, an increase A*,, for a given metal loading cannot be explained in terms of the total surface area increase of the support alone. Other important factors, such as surface heterogeneity may be introduced or increased by grinding or by gasification of the carbon[25]. The increase in surface heterogeneity is due to an increase in the width and depth of crystallite boundaries and enlargement of the diameter and depth of point defects in the basal plane of the crystallite. It is known[24] that gasification of graphitized carbon blacks increases the total surface areas, ABET, as well as the active surface area, A ASA(i.e., the concentration of the carbon sites at the edges of the crystallites). In the present work, gasification was brought about by a high temperature reaction of the carbon with CO*. The high temperature CO, treatment leaves the carbon surface with no combined oxygen[26], provided an inert atmosphere is employed during cooling. The resulting oxygen-free carbon forms a basic surface oxide on exposure to atmospheric oxygen at room temperature or during cooling. The basic oxide gives the carbon the capability of raising the pH of a neutral or acidic solution. It is known that other materials such as chlorine and ammonia[27] also impart some basic character to the carbons when treated under similar conditions of high temperature. By contrast, nitric acid treatment of the carbon covers the surface of the latter with an acidic oxide. The nature of the resulting surface group is inferred from the CO2 evolved when the sample is degassed at high temperatures[28]. According to Puri[28], the group containing the CO,-complex involved in the liberation of CO, is also involved in determining the surface acidity. The acidic complex is thought to consist of two oxygen atoms attached per surface carbon atom lying at the edge of the crystallite. 4.4 The effect of surface heterogeneity of carbons on AW

It is now evident that the difference in the observed A,, by the two methods of catalyst preparation may be linked with the nature of the surface of the carbon support-which in turn can be varied by altering the method of pretreatment. Activated carbons[29] are known to disrupt certain complexes such as platinum and osmium compounds, and to selectively adsorb certain constituent parts, preferably the metal. Salts of some metals (e.g., silver and gold) undergo partial or complete

G. R. HEAL and L. L. MKAYULA

822

reduction to metals[29,30] when adsorbed. The origin of the reducing power of activated carbons is still poorly understood[30]. It is possible that the phenolate groups are involved in a quinone-hydroquinone-type redox-couple or that chromene groups may be involved. Furthermore, it is possible that the graphitic structure of carbon may contain a vast amount of electrons that are delocalized over the aromatic structure and are thus available for reducing other species[27]. It is known[31] that the surface functional groups of an activated carbon when oxidized with chlorine or a mixture of HNOJ and H,SO, at 353 K destroy the ability of the carbon to adsorb gold cyanide or metallic gold from a solution of Au(Cl,)- anions. The carbon bumoff, on the other hand, seems initially to have enhanced the reducing power of the carbons by increasing the number of active sites (i.e., the active surface area, AAS*). Further gasification to a high percentage burnoff as for the 5H and 51 carbons may have led to a reduction in A,,. The differences in AW for the different methods of preparation remain to be accounted for. There appears to be no clear-cut picture, the general impression being that, in the LPRH method, the reduction of Na,PdCl, on the carbon is assisted by sodium hypophosphite. This has the advantage of speeding up the reduction process in a limited time (30 min), but may have had the disadvantage of allowing the reaction to proceed too quickly and only in the wide pores. In contrast, in the IMA method the support is allowed much longer (40 hours) to come into contact with the H,PdC1, solution. During this time the ion has sufficient time to penetrate deep into the micropores and therefore to increase dispersion. The IET method, as pointed out in earlier sections, was not very successful. The method depends on the presence of ion-exchangeable species, such as hydrogen ions, on the carbons. The hydrogen ions can then be exchanged with palladium ammine complex cations. Ion-exchangeable activated carbons were certainly formed, especially by the HNOJtreated carbons, as indicated by the slightly raised values of S, on carbons 5J and 5N in Table 1, carbons 3F and 3J in Table 2 and carbons 6F and 61 in Table 3 (relative to untreated carbon). The ion-exchangeable species were insufficient in number to produce high free-metal surface areas as obtained in other methods of preparations.

Ehrburger et al. [32] obtained some interesting experimental results with carbon supported platinum catalysts, finding differences in the degree of dispersion of platinum supported on a graphitized carbon black that had been subjected to varying levels of carbon bumoff in air. They observed that the greater the extent of gasification of the carbon black, the smaller was the particle size of the supported platinum; hence, its greater dispersion for a given catalyst loading. They interpreted the results by linking the greater gasification with an increase in surface heterogeneity. The latter phenomenon they considered to be responsible for providing a high potential energy barrier to diffusion of metal atoms or crystallites during the reduction of the catalyst at 773 K, thus preventing particle growth in accordance with Flyn and Wanke’s[33] postulate of sintering. Flyn and Wanke[33] attribute sintering to loss of atoms or molecular metal species from crystallites followed by surface diffusion and recapture by other crystallites. Phillips et a1.[34] proposed a model for the morphological changes occurring in gold particles with diameter less than 10 nm on amorphous carbons, considering there to be a random distribution of preferred trapping sites on the heterogeneous surface of the amorphous carbons. Metallic particles trapped in these sites, which act as twodimensional potential wells, will escape only if their thermal energy is greater than the energy of the potential wells. The carbon supported palladium catalysts prepared in this work, were dried in air at 383 K then reduced at 300 K in hydrogen. The only sintering observed is presented in Table 6; the effect of sintering starts at a temperature of 457 K. More results are needed at intermediate temperatures in order to obtain a better picture. The decrease in APd for the extensively gasified carbons may have been caused by the loss in AAsA which, as seen above, means a decrease in surface heterogeneity (decrease in the potential energy of the wells) that enabled the small metal crystallites to escape from would-be trapping sites. In the LPRH method, this could occur during the critical stage when the reduced metal particles must be trapped immediately or adsorbed before they get the chance to agglomerate. In the IMA method, the metal solution containing the active component was allowed longer to penetrate deeper into the smaller pores so that, if reduction took place, the reduced metal

Table 6. The effect of reduction temperature on the free-metal surface area, dispersion and mean particle size of palladium catalysts Sample

6S0 + 4.83% Pd LPRH

Reduction temperature (K)

AW (m*Pd g-’ cat.)

298

17.8

457 538 780

4.11 1.69 0.26

D,

;iG_ (nm)

0.78

1.36

0.18 0.07 0.01

5.88 14.3 92.9

823

Palladium on carbon II: Deposition and area

would be trapped on the active sites of the carbons. The larger the number of the active sites, the higher the dispersion, thus the greater the surface area Apd of palladium, as was observed. For the IMA method, the pretreatment of the carbons with HN03 enhanced the dispersion of palladium, possibly indicating that the additional CO,- complexes affected the dispersion by providing greater potential energy barriers to the diffusion of metal atoms or crystallites across the surface, thus hindering particle growth. 5. CONCLUSIONS Increasing burnoff of the carbons produces increased area and pore volume, but, although freemetal area initially rises, excessive gasification leads to a decrease in Ar,,. Gasification is thought to give carbons a surface heterogeneity and it may be that this rises to a maximum and then decreases again as carbon is removed. Although micropore volume and wide pore volume rises with burnoff, the average micropore width remains almost constant, perhaps indicating that pore development involves the creation of new micropores, while some micropores grow to form wider pores. The LPRH method of preparation yields catalysts with metal located mainly in the wider pores. The evidence for this comes from the mean particle size values. In contrast the IMA allows a longer time for penetration deeper into the pore structure and leads generally to smaller particle size. Fine tuning of the variables such as time of impregnation for this technique could lead to adjustment of catalyst properties. The treatment of carbons by a deposit of carbon from furfuryl alcohol, so as to block micropores, does not produce a significant increase in palladium area. In contrast, the HN03 treatment is very effective especially if IMA is used. The IET technique is generally worst, but even this method is improved by prior HNOJ washing. Many preparations in this work gave higher freemetal area than those of similar commercial catalysts with similar metal loadings. The commercial catalysts correspond roughly to the catalysts prepared using untreated original commercial carbons, so that the treatments described here are worth following. Study should be concentrated on 15 to 25% bumoff, nitric acid washing, and the impregnation adsorption technique for deposition. Preferential blocking of micropores by carbon from polyfurfuryl alcohol does not appear to have succeeded.

AcknowledgmenrsL.

L. Mkayula wishes to thank the Norwegian Agency for International Development for the provision of a grant enabling this research to be carried out.

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