Temperature-programmed reduction study on calcination of nano-palladium

Article abstract of DOI:10.1021/jp011170g

Pd membranes with high mechanical strengths are used to purify hydrogen in industrial scale for feedstock in petrochemical processes. The effects of calcinations treatment on samples of mono-metallic Pd and Pd₇₇-Ag₂₃ alloy with primary particles ～ 8 nm were studied with the TPR technique. A hydrogen-consumption at low temperature and a hydrogen-desorption at high temperature were observed from obtained TPR traces. The extent of oxidation on both samples increased with the calcinations temperature, i.e., oxygen adsorption on the surface of primary particles at T_o < 373 K, formation of surface oxide at 473 K, and incorporation of oxygen into sublayer at T_o > 573 K. The temperature required for a reduction of oxidized palladium by hydrogen was generally low. T_r = 150 K for oxygen chemisorbed on nano-palladium but raised over 260 K for oxide anions in the PdO structure. Primary particles of nano-palladium retained their size of 8 nm upon calcinations at T_o < 673 K but dramatically sintered under severe calcinations at high temperatures. The freshly prepared alloy was inhomogeneous in composition but became homogeneous upon calcination at T_o > 673 K. Incorporation of oxygen into sublayers of Pd crystallites was promoted by alloy with silver.

Full text of DOI:10.1021/jp011170g

J. Phys. Chem. B 2001, 105, 9113-9117
9113
Temperature-Programmed Reduction Study on Calcination of Nano-Palladium
Chi-Wei Chou,† Shou-Juan Chu,‡ Hui-Jean Chiang,§ Chien-Yu Huang,§ Chiu-jung Lee,†
Shyang-Roeng Sheen, Tsong P. Perng,‡ and Chuin-tih Yeh*,†
Department of Chemistry, Department of Material Science, and Material Science Center,
National Tsinghua UniVersity, Hsinchu, Taiwan 30043, Republic of China, and Department of
Science Education, Hsinchu Normal College, Hsinchu, Taiwan 30043, Republic of China
ReceiVed: March 28, 2001; In Final Form: June 26, 2001
Effects of calcination treatment on samples of mono-metallic Pd and Pd77-Ag23 alloy with primary particles
around 8 nm were studied with the temperature-programmed reduction technique. Temperature profiles of
hydrogen consumption for calcined samples from a stream of 10% H2 in N2 were monitored by a thermal
conductivity detector. Two distinct peaks, i.e., a consumption of hydrogen for PdO reduction and a subsequent
desorption of hydrogen from bulk palladium hydride, were observed. The extent of palladium oxidation upon
calcinations increased with the temperature of calcination (To): i.e., chemisorption of oxygen on particle
surface upon calcination at To < 373 K, reconstruction into a surface PdO structure at 473 K and incorporation
into sublayers to form bulk PdO structure at high To. Minimum temperature (Tr) required for reduction of
oxidized palladium by the hydrogen stream was generally low (150 K < Tr < 320 K) and increased with the
extent of oxidation. A quantitative measurement of hydrogen desorbed from reduced samples suggested a
formation of alloy phase in freshly prepared Ag77-Pd23 primary particles. The freshly prepared alloy was
inhomogeneous in composition but became homogeneous upon calcination at To > 673 K.
Introduction
programmed reduction (TPR). This technique is especially
Palladium metal permeates the hydrogen gas selectively.1 In
laboratories, hydrogen gas of ultrahigh purity is achieved by
permeating low graded gas through palladium thimbles. Nev-
ertheless, palladium membranes with high mechanical strengths
are still sought by engineers to purify hydrogen in industrial
scale for feedstock in petro-chemical processes.2,3
Currently, the preparative technology of palladium-coated
stainless steel tubes4 and other porous hollow supports5 for
purification of hydrogen in industrial quantity, is actively
pursued. One of the palladium coating methods is to press
palladium crystallites onto porous stainless steel substrate. A
stainless steel plate pressed with crystallites of nano-palladium
has recently been demonstrated to exhibit not only an excellent
selectivity toward hydrogen permeation (99.99% relative to
nitrogen), but also a decent stability against repeated treatments
of hydrogen absorption.
An alloying of 23% silver to palladium may improve its
diffusibility of hydrogen and its handicap of hydrogen em-
brittlement. Hence, a substitution of Pd nano-crystallites with
Pd-Ag crystallites to coat the stainless steel tubing may be of
an additional advantage. Nano-crystallites of Pd-Ag alloy for
such a use have been successfully prepared with a wet-reduction
technique.6,7 TEM examination indicated that the prepared
samples are aggregates formed from primary particles with a
diameter of d ∼ 8 nm.6 An extensive characterization of this
prepared nano-alloy becomes pertinent for further application.
A convenient technique for characterizing chemical environ-
ment of metal atoms in solid samples is the temperature-
talented toward probing oxidation states of noble metals.
Samples of platinum dispersed on different porous supports have
been recently studied.8-10 Reductive phenomena of platinum
depend heavily on the extent of sample oxidation and the
property of support. From variations in the reduction temperature
(Tr) and the amount of hydrogen consumption noticed in TPR
traces, three stages of oxidation, i.e., the chemisorption, surface
structures of PtO and PtO2, have been successfully distinguished
on increasing the oxidation temperature. The interest of this
study is to study the effect of calcination on prepared Pd nano-
metal and Pd-Ag nano-alloy with the TPR technique developed
in our laboratory.
Experimental Section
Sample Preparations. Powdered Ag2O was prepared by
dehydration, at room temperature through evacuation of AgOH
precipitated out from aqueous AgNO3 by alkalizing with NaOH.
A similar procedure was used to prepare PdO powders from
Pd(NO3)2. Prepared oxides were used as standard samples for
TPR characterization.
Nano-crystallites of mono-metallic Pd and bi-metallic Pd77-
Ag23 were prepared with a wet reduction method described in
the literature.5 Briefly; research grade Pd(NO3)2 and AgNO3
were dissolved in distilled water and mixed with excessive
formaldehyde. Metal particles in the nanometer size were then
precipitated by adding the mixed solution with a 3 M aqueous
solution of NaOH until the solution became transparent.
Obtained precipitates were sequentially washed with distilled
water and acetone, and dried in a vacuum at the ambient
temperature. Evacuated samples were stored in capped bottles
as fresh samples. According to TEM characterization, the fresh
samples are aggregates (∼100 nm in size) of primary particles
* To whom correspondence should be addressed. Fax: 886-3-5711082.
E-mail: ctyeh@mx.nthu.edu.tw.
† Department of Chemistry, National Tsinghua University.
‡ Department of Material Science, National Tsinghua University.
Material Science Center, National Tsinghua University.
§ Department of Science Education, Hsinchu Normal College.
10.1021/jp011170g CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/24/2001

9114 J. Phys. Chem. B, Vol. 105, No. 38, 2001
Chou et al.
TABLE 1: A Correlation between the Tra Required for
Complete Reduction of Metal Oxides with the Eo b of
Corresponding Metal Ions
oxides
Au2O PtO PdO Rh2O3 Ag2O CuO NiO CoO
Tr/K 20011 25010 280
38012
380
0.80 0.34 0.23 0.28
50016 60017 65018
Eo/V 1.50
1.20 0.99 0.80
a Minimum temperature. b Standard reductive potential.
H2 + Ag2O f2Ag + H2O
(1)
Two peaks were noticed in TPR trace of PdO, i.e., a
hydrogen-uptake peak at T ) 280 K and a hydrogen-desorption
peak at 360 K. The uptake peak may have resulted from a
consumption of hydrogen on reduction of oxidized palla-
dium:13
Figure 1. TPR traces for Ag2O and PdO prepared by calcination of
hydroxide precipitates from alkalizing AgNO3 and Pd(NO3)2 with
NaOH.
H2 + PdO f Pd + H2O
(2)
The palladium crystallites, thus reduced, may absorb hydrogen
to form PdH (R or â) phase in hydrogen environments. The
R-PdH phase contains less hydrogen (adsorbed only on the
surface of crystallites) than the â-PdH phase (with hydrogen
atoms absorbed into the bulk of palladium). Both phases are in
fcc structure but with different lattice parameters (0.39 and 0.40
nm, respectively). The dominant phase of PdH depends on the
pressure of hydrogen (PH) and the system temperature.
which are spherical in shape and exhibit a homogeneous
diameter of d ∼ 8 nm.6,7
Prepared Pd77Ag23 bimetallic crystallites have been character-
ized with XRD. A single (111) peak around 2θ ) 40.0° was
noticed from the diffraction spectrum. The peak position
indicates an alloy phase was formed in the sample. The half-
width (B1/2 ) 1.8°) of peak confirms the nanosize nature (d ∼
8 nm) of primary particles found in TEM7 for the freshly
prepared sample. The peak retained at the same diffraction
position, but its width was narrowed after the bimetallic sample
was severely heated. The primary particles should have gradually
sintered to large sizes upon the heating at To > 673 K.
TPR Characterizations. Each metallic sample was separated
into several portions (∼10 mg) and calcined for 1 h at different
To temperature in air. The effect of calcination on the sample
was studied by TPR using a subambient-temperature routine
(SATR). The SATR routine is a specially designed experimental
procedure that we use in our laboratory to investigate reduction
phenomena possibly occurring at subambient temperatures. This
routine has been satisfactorily used to find the minimum
temperature (Tr) required for reduction of oxygen species
adsorbed on noble-metal crystallites of gold (Tr ∼ 200 K),11
rhodium (245 K),12 and platinum (around 250 K)8-10 dispersed
on different substrates.
According to the phase diagram of the Pd-H2 system,1
palladium crystallites tend to absorb hydrogen to form â-PdH
at low temperatures under PH ) 1.0 × 104 Pa (the partial
2
pressure of hydrogen in H2-N2 flow of TPR system), i.e.,
yH2 + 2Pdb WwPdbHy
(3)
In reaction 3, superscript b denotes palladium atoms in the
bulk of palladium crystallites reduced from reaction 2, and
subscript y is a temperature-dependent stoichiometry (y ∼ 0.51
at T ) 360 K).1 On raising the temperature over 360 K, â-PdH
in the flow of H2-N2 tends to desorb the hydrogen absorbed,
and shift spontaneously to the R-phase (through the reversed
reaction 3). The hydrogen-desorption peaks noticed in the TPR
traces of PdO in Figure 1 are therefore attributed to the â-to-R
phase transition.
Transition metals of groups VIII and IB in the periodic table
are generally catalytically active toward hydrogenation reactions.
Reduction of salts into metallic state with hydrogen is an
essential step for activation of these metallic catalysts. However,
the temperature selected for the reduction was regarded as an
art in the preparation. A reduction at high temperatures generally
causes sintering and therefore a loss in activity to the prepared
crystallites. On the contrary, a reduction at low temperatures
may be insufficient for the activation. A minimum temperature
(Tr) required for a complete reduction varies with the inherent
activity of the metal.
In the SATR routine, oxidized samples were inserted into a
U-shape tubing of quartz and pre-cooled to 183 K in a nitrogen
stream. At this temperature, the nitrogen stream was replaced
with a flow of 10% hydrogen in nitrogen for TPR measurement.
The thermal conductivity of the flow at downstream of the
sample was subsequently monitored by a TCD (thermal
conductivity detector). After 5 min of the baseline stabilization,
the sample temperature was linearly raised from 183 to 500 K
at a rate of 10 K min-1. A positive peak generally indicates a
consumption of hydrogen (a decrease in the thermal conductiv-
ity) by the sample, while a negative peak (an increase in the
thermal conductivity) indicates a desorption of hydrogen from
the sample.
TPR is a talent method to characterize the Tr of different
metallic salts. Figure 1 indicates that Ag2O and PdO may be
completely reduced at Tr ) 380 and 280 K, respectively. In
our laboratory the Trs have been pursued for many oxides of
transition metal. Table 1 lists Trs accumulated thus far in our
laboratory for reduction of several metal oxides prepared from
dehydration of metal hydroxides. Following trend of increasing
Tr is noticed:
Results and Discussion
Reduction of Ag2O and PdO. Figure 1 depicts a series of
TPR results obtained from the SATR routine on the prepared
blank samples of Ag2O and PdO. Sample Ag2O exhibited a
single hydrogen-consumption peak at 380 K. This peak may
be assigned to a reaction of
Au2O < PtO < PdO < RhO, Ag2O < CuO < NiO < CoO

Calcination of Nano-Palladium
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9115
TABLE 2: A Summary for TPR Results from the
Mono-metallic Nano-palladiuma Oxidized at Different
Oxidation Temperatures (To)
To/K
Td/K
NH,d/NPd
Tr/K
NH,r/NPd
300
373
473
573
673
773
358
358
368
363
363
353
0.47
0.47
0.47
0.45
0.51
0.49
150b
0.07b
ndc
ndc
228
238
263
293
0.10
0.96
1.40
1.45
a D ) 11%. b Data from the sample precalcined at 300 K were
observed from ULTR routine. Those from the sample precalcined at
high temperatures were obtained from the normal SATR routine. c Not
detected.
kJ mol-1) than the absorbed hydrogen (∆Hab ∼ 38 kJ mol-1).14
Accordingly, the adsorbed hydrogen remained on the sample
surface as the temperature of the TPR experiment was raised
to 360 K.15
The amount of hydrogen desorbed (NH,d) in the reverse of
reaction 3 should therefore vary with the dispersion of palladium
according to formula II,
NH,d ) 0.51(1 - D)NPd
(II)
Figure 2. Variation of TPR traces (from SATR routine) of palladium
nano-crystallites precalcined for 1 h at different Tos: (a) 298 K
(especially with ULTR routine); (b) 373 K; (c) 473 K; (d) 573 K; (e)
673 K; (f) 773 K. The vertical scale of traces e and f has been decreased
to a factor of 1/2.
The third column of Table 2 indicates that calculated ratios
of NH,d/NPd were ∼0.47 in TPR traces of To < 600 K. The
calculated ratio is in fair agreement to a value of 0.46 estimated
from formula I (for DPd ) 0.11).
The obtained NH,d/NPd ratio, however, increased slightly from
0.47 to 0.50 on increasing the To of the oxidation treatment
over 600 K. This increase in ratio may have resulted from
sintering of primary particles of palladium during high-temper-
ature oxidations. It is noteworthy that palladium metal has a
melting point of Tm ) 1825 K which indicates a Huttig
temperature (Tm/3, a temperature over which surface atoms of
solid become mobile) ∼600 K.
Noteworthy, this sequence agrees well with the standard
reductive potential, E°, for their corresponding cations in
aqueous solution,13 i.e.,
M
n+ + n e- f M
(4)
Evidently, the Tr required for reduction of these metal oxides
increases with the potential E° of the corresponding cations
according to a linear equation of:
Both the Tr (column 4 in Table 2) required for a reduction of
oxidized palladium and the amount of the hydrogen consumption
(NH,r in the last column in Table 2) during the reduction are
found in Figure 2 to increase with the oxidation temperature
(To). This trend agrees well with our previous results on an
oxidation of platinum dispersed on alumina.8 Observed oxidation
of the dispersed platinum was limited at the surface layer of
platinum crystallite but led to different oxidized species. Oxygen
atoms were chemisorbed (to a stoichiometry of PtsOc) on the
surface of dispersed platinum at a mild oxidation at To ) 300
K. Upon a severe oxidation at To > 373 K, a surface structure
of PtsO was formed through a surface reconstruction. The
temperature required for a reduction of oxygen in the chemi-
sorbed PtsOc and in the PtsO structure was found at Tr ) 300
and 325 K, respectively. Conceivably, the temperature required
for a reduction of oxidized palladium (reaction 1) should also
vary with the extent of oxidation.
The last column of Table 2 lists the hydrogen consumption
(NH, r) in the reduction of nano-palladium oxidized at different
To. Obviously, the oxidation was limited at the surface of nano-
palladium upon To < 473 K but gradually extended into its bulk
at high To temperatures. However, the maximum NH,r/NPd ratio
(1.45) found at To ) 773 K is still smaller than a theoretically
stoichiometric ratio of 2.0 for a complete conversion to PdO.
Probably, a sintering of nano-palladium occurred during severe
calcination and the expected penetration of oxygen into the inner
core of sintered palladium might be kinetically hindered.
Oxygen atoms should have chemisorbed on the surface of
the nano-palladium on the oxidation treatment at To ) 373 K:
Tr/K ) 600-280V
(I)
Calcination of Mono-metallic Palladium. Figure 2 depicts
a series of TPR results obtained from the SATR routine on a
fresh sample of mono-metallic palladium preoxidized at increas-
ing To. Two major peaks i.e., a hydrogen-uptake peak at T e
300 K and a hydrogen-desorption peak around 360 K, were
noticed from these traces. According to a former assignment in
Figure 1, the uptake peak came from a combination of reactions
2 and 3 and the desorption peak was resulted from the reverse
of reaction 3.
Column 3 of Table 2 summarizes amounts of the desorbed
hydrogen (NH,d) found in TPR experiments from the nano-
palladium sample preoxidized at increasing To. Calculated NH,d
/
NPd ratio stays very close to a value of 0.47 for samples calcined
at To < 573 K. This ratio is slightly smaller than a ratio of 0.51
reported1 for â-PdH at temperature 360 K. The difference
between these two ratios may be attributed to the particle size
of palladium sample. Our nano-palladium crystallites have a
diameter φ ∼ 8 nm. A substantial fraction (11%) of palladium
atoms was exposed to the surface of the nano-palladium. These
atoms tended to chemisorb hydrogen atoms:
H2 + 2Pds f 2PdsHc
(5)
where Pds and Hc denote palladium atoms exposed to the surface
of crystallites and hydrogen adsorbed on Pds, respectively. The
adsorbed hydrogen has a higher bonding enthalpy (∆Had ∼ 60

9116 J. Phys. Chem. B, Vol. 105, No. 38, 2001
2Pds + xO2 f 2PdsOc
Chou et al.
(6)
The Oc in this equation denotes chemisorbed oxygen atoms.
To our surprise, no reduction peak was found from the 373 K
trace of Figure 2. There is scarce information available in the
literature reporting on the Tr required for a reduction of oxygen
atoms chemisorbed on unsupported palladium by hydrogen, i.e.,
3/2H2 + PdsOcx f PdsHc + xH2O
(7)
Speculatively, Oc atoms might have been reduced by the H2-
N2 flow at 183 K during stabilization the TCD baseline before
raising the sample temperature for the TPR monitoring.
Figure 3. Schematic diagram for consecutive changes of 8 nm
palladium during calcination in air on raising the temperature: (a) To
< 373 K, adsorption of oxygen on surface; (b) To ) 473 K, formation
of a surface layer of PdO; (c) To ) 573 K, incorporation of oxygen
into sublayers; and (d) To > 673 K, sintering of the primary particles.
Accordingly, the Tr of oxygen chemisorbed on the surface
of nano-palladium was pursued on purpose in this study with a
specially designed ultralow-temperature routing (ULTR). In this
routine, a sample of palladium chemisorbed with oxygen
(pretreated by a TPR reduction to 500 K and a subsequent
oxidation in air at room temperature) was intentionally cooled
to 110 K in a nitrogen stream before switching to the H2-N2
flow for the TPR measurement. The TPR trace (with To ) 298
K in Figure 2) agreed well with our expectation. Besides the
peak of hydrogen desorption at 360 K, a hydrogen consumption
peak was indeed noticed at 150 K. The amount of hydrogen
consumed in the reduction peak is slightly larger than that
desorbed in the peak at 360 K (by an amount of 0.07NPd).
Apparently, the 150 K peak should be contributed to a
combination of reactions 7 and 3. The amount of hydrogen
consumed in reaction 7 was therefore 0.07NPd.
The fourth column of Table 2 lists variation of Tr with To of
oxidation treatment. Observed Tr generally increased with the
extent of oxidation, i.e., Tr remained less than 230 K for oxygen
chemisorbed on palladium surface and gradually increased to
290 K for oxygen in PdO structure. Conceivably, PdO reduction
with hydrogen is a thermodynamically favored reaction
(∆G° ∼ -28 kJ mol-1). The rate of reduction depends on the
structure of PdO produced in calcination treatment. Oxygen
chemisorbed on a palladium surface at mild calcination is
thermodynamically possible and kinetically easy. However, an
energy barrier for reconstruction of crystallite becomes necessary
to reduce oxygen ions in the rigid PdO lattice formed at severe
calcinations.
Figure 3 schematically illustrates the physical change of nano-
palladium on increasing To of calcination treatment. The
aggregated nano-palladium (represented by big circles) chemi-
sorbed oxygen atoms (small dots) on surface at light calcination
at To < 373 K. Upon raising the To over 473 K, a layer of PdO
was formed. The thickness of the layer gradually increased with
To and a sintering of PdO became evident when To was raised
over 873 K.
Calcination of Pd-Ag Bi-metallic Crystallites. Figure 4
presents a series of TPR traces obtained from a Pd77Ag23 (in
atomic ratio) sample preoxidized at different Tos. Similar to TPR
results from the sample of nano-palladium (Figure 2), each trace
in Figure 4 also exhibits a reduction peak (at low temperature)
as well as a hydrogen-desorption peak (at Td > 380 K).
However, the expected reduction peak was absent in trace a of
this figure for the sample calcined at To ) 373 K. The absence
may, similar to the case of monometallic palladium, be attributed
also to a low Tr (<183 K) for this sample. The chemisorbed
oxygen should have been reduced at 183 K, during stabilizing
the baseline of trace a, by the H2-N2 flow before raising the
system temperature.
Figure 4. Variation of TPR traces from samples of nano-Pd77Ag23
precalcined for 1 h at different Tos: (a) 373 K; (b) 473 K; (c) 573 K;
(d) 673 K; (e) 773 K; and (f) 873 K.
Since reduced Pd-Ag alloys also have the ability of
absorbing hydrogen into bulk, the hydrogen-desorption peaks
in traces of Figure 4 should come from the â-to-R phase
transition of bimetallic crystallites. However, a bimetallic alloy
of Pd with Ag slightly raised the transition temperature to Td )
380 K instead of 360 K noticed in Figure 2 for mono-metallic
Pd. A slight shift of equilibrium of reaction 3 toward right by
alloy palladium with silver is indicated.
A ratio of NH,d/NPd ) 0.31 was noticed in the third column
in Table 3 for the fresh sample. This ratio is significantly smaller
than the 0.51 of the nano-palladium. The difference may be
attributed to a decrease in the amount of hydrogen absorbed
into bulk palladium by alloying with silver. A ratio of NH,d
/
NPd ) 0.25 has been reported for bulk Pd74Ag26 alloy at a
transition temperature of 300 K.1 This ratio is smaller than the
0.31 observed in the present study for Pd77Ag23. The discrepancy
suggests that the composition in the alloy of freshly prepared
Pd77Ag23 was inhomogeneous.
The NH,d/NPd ratio of the bimetallic sample was decreased
from 0.31 to 0.20 after a calcination at To > 673 K. Obviously,
the composition of bimetallic sample became homogenized due
to an inter-diffusion of constituent components upon severe

Calcination of Nano-Palladium
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9117
TABLE 3: A Summary for TPR Results from Pd-Ag
Bimetallic Crystallitesa Preoxidized at Different Oxidation
Temperatures (To)
surface of primary particles at To < 373 K, formation of surface
oxide at 473 K, and incorporation of oxygen into sublayers at
To > 573 K.
2. The temperature required for a reduction of oxidized
palladium by hydrogen is generally low. Tr ) 150 K for oxygen
chemisorbed on nano-palladium but raised over 260 K for oxide
anions in the PdO structure.
3. Primary particles of nano-palladium retain their size of 8
nm upon calcination at To < 673 K but dramatically sinter under
severe calcination at high temperatures.
4. Pd-Ag bimetallic crystallites freshly prepared are inho-
mogeneous in composition. However, the composition gradually
homogenized because of inter-diffusion upon calcination at
To > 673 K.
To/K
Td/K
NH,d/NPd
Tr/K
ndb
210
273
280
>310
300
NH,r/NPd
ndb
0.12
1.08
1.47
2.96
2.85
373
473
573
673
773
873
400
393
390
380
390
400
0.31
0.26
0.31
0.31
0.20
0.18
a DPd ) 11%. b Not detected.
oxidation treatments. However, silver was slightly segregated
toward the external surface of the crystallites during the diffusion
because XPS study indicated that the surface of calcined sample
was enriched with Ag (40%).7 Observed segregation probably
reflected the difference in the surface energy between Ag and
Pd (0.39 and 1.50 J m-2, respectively).
The extent of oxidation on these bimetallic crystallites also
increased with the calcination temperature. The oxidation was
limited to surface (chemisorption) for To < 373 K but extended
to sublayers at high temperatures. At To > 773 K, a hydrogen
consumption ratio of NH,r/NPd ) 2.7 was reached for reaction
1. This ratio suggested not only that silver atoms in the alloy
had oxidized to Ag2O, but also that the oxidation had extended
to the center of crystallites.
Similar to the reduction of calcined nano-palladium (Figure
2), the Tr found in Figure 4 for a complete reduction of Ag-Pd
nano-alloy increased with calcination temperature. A comparison
of Trs found in Tables 2 and 3 indicates that an alloy of silver
to palladium does not significantly alter the Tr of oxidized
palladium. The absence of a 400 K peak, assigned to a reduction
of Ag2O (reaction 1), suggests that hydrogen was effectively
spiltover from reduced palladium to Ag2O during the reduction
of PdO in bimetallic sample.
5. Incorporation of oxygen into sublayers of palladium
crystallites is promoted by alloy with silver.
Acknowledgment. The authors are grateful to the Chinese
Petroleum Corporation of the Republic of China for the financial
support of this study.
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Effects of calcination on nanocrystallite samples of mono-
metallic palladium and bimetallic Pd-Ag have been carefully
studied with TPR technique. Two peaks, a hydrogen-consump-
tion at low temperature and a hydrogen-desorption at high
temperature, were observed from obtained TPR traces. The
following features can been concluded from variations in the
area and temperature of these two peaks:
1. The extent of oxidation on both samples increases with
the calcination temperature, i.e., adsorption of oxygen on the