Structure and activity of Pd/V2O5/TiO2catalysts in Wacker oxidation of ethylene

Article abstract of DOI:10.1016/j.apcata.2016.10.024

The present study concerns preparation of Pd/V₂O₅/TiO₂catalysts and activity of the catalysts in selective gas phase oxidation of ethylene by O₂in presence of water at atmospheric pressure and in the temperature

Full text of DOI:10.1016/j.apcata.2016.10.024

Accepted Manuscript
Title: Structure and activity of Pd/V2O5/TiO2 catalysts in
Wacker oxidation of ethylene
´
´
´
Author: Robert Barthos Andras Hegyessy Gyula Novodarszki
´
´
´
Zoltan Paszti Jozsef Valyon
PII:
S0926-860X(16)30519-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.10.024
APCATA 16040
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-6-2016
7-10-2016
23-10-2016
´
´
´
Please cite this article as: Robert Barthos, Andras Hegyessy, Gyula Novodarszki,
´
´
´
Zoltan Paszti, Jozsef Valyon, Structure and activity of Pd/V2O5/TiO2
catalysts in Wacker oxidation of ethylene, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.10.024
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

1
Structure and activity of Pd/V2O5/TiO2 catalysts in Wacker oxidation of ethylene
Róbert Barthos*, András Hegyessy, Gyula Novodárszki, Zoltán Pászti, József Valyon
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences,
Hungarian Academy of Sciences, Magyar tudósok körútja 2, Budapest 1117, Hungary
*Corresponding author. Tel:+36 1 3826861
E-mail addresses: barthos.robert@ttk.mta.hu (Róbert Barthos), hegyessy@gmail.com (András
Hegyessy), gyula.novodarszki@ttk.mta.hu (Gyula Novodárszki), paszti.zoltan@ttk.mta.hu
(Zoltán Pászti), valyon.jozsef@ttk.mta.hu (József Valyon)

2
Highlights
- Steps of consecutive ethylene oxidation over heterogenized Wacker catalyst are described.
- Redox properties of TiO2-supported vanadia and Pd/vanadia is discussed.
- High stability and Wacker activity of Pd/V2O5/TiO2 catalyst is shown.
- Wacker activity is related to surface vanadia forms of Pd/V2O5/TiO2 catalysts.

3
Abstract
The present study concerns preparation of Pd/V2O5/TiO2 catalysts and activity of the
catalysts in selective gas phase oxidation of ethylene by O2 in presence of water at
atmospheric pressure and in the temperature range of 100-200 oC. The influence of palladium
and vanadia loading and the partial pressures of the reactants on the yield of oxidation
products (acetaldehyde and acetic acid) were examined. The surface-bound vanadia forms
were identified by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and
FT-Raman spectroscopy. The best activities were achieved with catalysts having near to
monolayer vanadia coverage of the support. Time-on-stream catalytic tests and chemical
analysis of the fresh and used catalysts proved the structural and catalytic stability of the
Pd/V2O5/TiO2 preparations. It was shown that under the conditions of Wacker-oxidation
primary product acetaldehyde could be further oxidized to CO2, whereas no consecutive
oxidation of product acetic acid proceeded.

4
1. Introduction
The oxidation of ethylene to acetaldehyde by contacting ethylene and oxygen gases
with a catalyst system, dissolved in water was first described by Schmidt et al. [1]. The
aqueous solution contained PdCl2, and, in higher concentration, CuCl2. The copper and
palladium was shown to act together in initiating and maintaining the catalytic oxidation
process. In the aldehyde-forming reaction the ethylene reduces Pd2+ to Pd0. The role of Cu2+
ions is the in situ selective regeneration of Pd2+. The final step of the catalytic cycle is the
oxidation of CuCl to CuCl2 by O2 [2]. It was shown that the rate-controlling step of the
process is the third-order reaction between dissolved oxygen and CuCl, which reaction is
favoured at very low pH values, i. e., at high HCl concentrations [3]. However, the high
chloride concentration leads to formation of undesired chlorinated by-products from olefins,
especially from higher olefin reactants. Additional drawbacks of the reaction system are the
corrosiveness of the liquid phase reaction medium, formation of noxious copper waste by
disproportionation of Cu+ to Cu0 and Cu2+, precipitation and aggregation of Pd, leading to Pd
loss and, thereby, to extra expenses.
As a matter of fact, heterogeneous catalyst was used when the selective oxidation of
olefin to carbonyl compounds was first recognized. Based on these early results a pilot plant
was built for carrying out the reaction. Moist ethylene/oxygen mixture was passed through a
tube reactor, loaded by solid catalyst. However, deactivation and short lifetime of the catalyst
turned attention toward homogeneous, liquid-phase catalyst, which was then exploited in
industrial practice [4]. In order to overcome the drawbacks of homogeneous Wacker catalyst
system the use of chlorine-free terminal oxidants were suggested to be used in absence or in
presence of oxygen using Fe2(SO4)3 oxidizing agent [5] or phosphomolybdic acid and
benzoquinone as catalyst [6, 7], respectively.

5
Although Wacker plants using dissolved catalyst system are operating commercially,
the intent of realizing heterogeneous catalytic olefin partial oxidation is not diminishing. The
solid Wacker-type catalysts, studied up to now, can be classified as (i) microporous materials,
(ii) heteropoly compounds, and (iii) supported transition metal oxides [8]. The microporous
materials were usually zeolites. In the zeolite Wacker catalyst the negatively charged zeolite
framework corresponds to the chloride anions of the common solution catalyst, whereas
metals, corresponding to the active redox pair in the solution catalyst (Pd and Cu), balance the
framework charge. Espeel et al. [9] found zeolite to be efficient 'solid solvent' and showed
that palladium and copper exchanged zeolite Y converts ethylene to acetaldehyde in exactly
the same way as homogeneous Wacker catalyst. Park and Lee [10] reported that Cu,Pd,H-
mordenite is active in the Wacker-type oxidation of carbon monoxide. Layered clays also
allow for intercalation of metal cations between negatively charged layers. Mitsudome et al.
[11] reported high conversions and selectivities (80-95%) in the selective oxidation reactions
of C3-C18 olefins over Pd2+-exchanged montmorillonite clay in N,N-dimethylacetamide
solvent, containing CuCl2 co-catalyst. Palladium salts of heteropolyacids of the Keggin-series
(H3+nPVnMo12-nO40) supported on silica were successfully applied for selective oxidation of
ethylene [12] and 1-butene [13]. However, in both systems the conversion and selectivity in
the formation of carbonyl compounds showed fast decrease and the catalytic activity was
rapidly lost. Vanadia, supported on various oxides (SiO2 [14], Al2O3[15,16] and TiO2 [17])
and doped with palladium showed good activity in vapour phase oxidation of alkenes. Studies
showed that TiO2 supported Pd/V2O5 redox system is more active in the Wacker oxidation of
propylene to acetone [14] or 1-butene to butanone [17] than the corresponding SiO2 or Al2O3
supported catalysts. Nevertheless, the Pd/V2O5/TiO2 catalyst was hardly studied in
heterogeneous Wacker-oxidation. Evnin et al. [15] ascertained that titanium acts on the V2O5
phase as a p-type dopant. As such, it should lower the Fermi level of the system and create a

6
depletion layer near to the surface, thereby, increase the electron mobility and make the whole
system more “reducible”. Stobbe-Kreemers et al. [17] proved the higher reducibility of V2O5
on TiO2 than on Al2O3 by means of X-ray photoelectron spectroscopy (XPS) and temperature-
programmed reduction (TPR) measurements. The XPS spectra gave information about the
steady-state surface concentration of Pd2+ and Pd0 for catalysts used in Wacker oxidation of
olefin. A significant amount of zero valent palladium was detected in the alumina-supported
catalysts indicating that the re-oxidation of Pd0 by the vanadium oxide layer under steady-
state reaction conditions is slow. In contrast, no Pd0 was observed over the titania-supported
catalysts, suggesting that the re-oxidation of Pd0 by the V2O5 was either fast on this support
or there was hardly any formation of Pd0.
Studies reveal that V2O5 supported on TiO2 is more active in gas phase non-Wacker
oxidation reactions by O2 than the V2O5 on Al2O3 or SiO2 support. Deo et al. [18] studied the
partial oxidation of methanol to formaldehyde and observed that the turnover frequency
(TOF) of the reaction was two order of magnitude higher over V2O5/TiO2 than over
V2O5/Al2O3 and three order of magnitude higher than over V2O5/SiO2 catalyst. Similarly, the
TiO2 appeared to be the support of the most active supported V2O5 catalyst in the oxidation of
formaldehyde to formic acid [19].
The outstanding activity of V2O5/TiO2 catalysts was recently reviewed by Haber [20].
The formation of the surface vanadium oxide layer on the support before the formation of
crystalline V2O5 phase is a consequence of the high surface mobility of vanadium oxide,
which stem from lower surface free energy of crystalline V2O5 than that of the TiO2 [21]. The
migration of vanadium oxide was observed only over the surface of the anatase form titania.
The reactivity (and reducibility) of V2O5/TiO2 is supposed to be related to the V-O-Ti bond
strength [18]. The spreading of vanadia over the surface of the support increases the number
of V-O-Ti bonds and leads to more reducible and active catalyst.

7
When a solid catalyst is contacted with a liquid phase reaction mixture it is often
questioned whether the activity could be associated with the effect of surface active site
ensembles because leaching of metal particles can result in an active homogeneous catalyst
[22]. Heterogeneous catalytic effect can be proved by filtration separation of the supported
catalyst and testing activity in the filtrate. In vapour phase reactions catalyst leaching is less
probable. However, the boiling point (vapour pressure) restricts the number of olefins which
can be converted in gas phase.
The aims of present study are to gain better understanding of the catalytic effect of
Pd/V2O5/TiO2 catalysts in the Wacker-type oxidation of ethylene, the influence of palladium
and vanadia loading and reactant partial pressures on the activity and selectivity of the
catalysts. The consecutive oxidation of partially oxidized products, acetaldehyde and acetic
acid, is also discussed.
2. Experimental
2.1 Catalyst preparation
The catalysts were prepared using solution containing decavanadate (V10O566-) ions,
-
formed from solution, containing metavanadate (VO3 ) ions. Ten grams of ammonium
metavanadate (NH4VO3, VEB Laborchemie, Apolda, 99.0 % purity) was dissolved in 1 dm3
distilled water and, in order to obtain decavanadate ions, the pH of the solution was adjusted
to pH=4 by addition of 0.1 mol/dm3 HNO3 solution. Calculated amounts of decavanadate
solution was added to previously dried TiO2 (Aeroxide TiO2, P-25, Evonik Industries AG) to
obtain 2.1, 4.2 and 8.4 wt% V2O5 in V2O5/TiO2 preparations. In the next step the solvent
water was evaporated, and the obtained solid was dried at 120 °C overnight, and calcined at
400 °C for 4 h. Using Pd(NH3)4(NO3)2 (5.0 wt% Pd as solution, Strem Chemicals Inc.) a
solution was made containing 4 g/dm3 Pd. Catalysts of about 0.01, 0.1, 0.4, and 0.8 wt% Pd

8
content were obtained by impregnating the above described V2O5/TiO2 preparations with
calculated amount of the Pd solution. Preparations were dried and calcined at 400 °C for 4 h.
The vanadium contents of the preparations were expressed in V2O5 equivalents. The
vanadia/titania samples are designated with a number giving the V2O5 content in weight
percent and letter V, such as 2.2V, 4.6V and 8.6V. An example for the designation of the Pd-
containing samples is 0.4Pd4.6V, where the number in front of the Pd symbol gives the Pd
content in weight percent.
Characterization of catalysts
2.2.1. Specific surface area
Specific surface area (SSA) of the catalysts was obtained by the BET method from N2
adsorption isotherm determined at -195°C by using Quantachrome NOVA Automated Gas
Sorption Instrument. Before measuring adsorption isotherms samples were outgassed by
vacuum at 150 °C for 24 h.
2.2.2. Elemental analysis
The palladium and vanadium content of catalysts was determined by means of
Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) equipped with
polychromator, array detector, and applying axial viewing of the plasma (SPECTRO
Analytical Instruments GmbH). For the measurements 100 mg of catalyst sample was
rendered soluble by digesting it in 50 ml of boiling concentrated sulphuric acid for 2 hours.
Individual calibration curve was recorded for each component in 50 vol% sulphuric acid
solution in order to eliminate errors that could be caused by the use of common nitric acidic
standards, having densities different from that of the sample solutions.
2.2.3. X-ray powder diffraction

9
X-ray patterns were recorded by Philips PW 1810/3710 diffractometer applying
monochromatized Cu Kα radiation (40 kV, 35 mA). The patterns were recorded at ambient
conditions between 3° and 65° 2Θ, in 0.02° steps, counting in each step for 0.5 s.
2.2.4. FT-Raman spectroscopy
Raman spectra were recorded with a dynamically aligned BIO-RAD Digilab Division
dedicated FT-Raman spectrometer equipped with a Spectra-Physics Nd-YAG-laser (1064 nm)
and high sensitivity liquid-N2 cooled Ge detector. The laser power used was about 250 mW at
the samples. The resolution of the Raman instrument was 4 cm−1. A backscattered geometry
was used. For each spectrum 256 individual spectra were averaged. The obtained spectra were
normalized to the most intense band of the TiO2 support at 145 cm-1.
2.2.5. Temperature-programmed reduction by hydrogen (H2-TPR)
A flow-through microreactor made of quartz tube (I.D. 4 mm) was used. About 100
mg of catalyst sample (particle size: 0.63–1.00 mm) was placed into the reactor and was
treated before the measurement in a 30 cm3 min-1 flow of O2 at 350 °C for 1 h. Then the
sample was cooled to room temperature in N2 flow, flushed for 30 min and contacted then
with a 30 cm3 min-1 flow of 10% H2/N2 mixture. The reactor temperature was ramped up at a
rate of 10 °C min-1 to 600 °C and kept at this temperature for 1 h while the effluent gas was
passed through a trap, cooled by liquid nitrogen, and a thermal conductivity detector (TCD).
Data were collected and processed by computer. The hydrogen consumption was calculated
from the area under the H2-TPR curve. The system was calibrated by determining the H2-TPR
curve of CuO reference material.
2.2.6. X-ray photoelectron spectroscopic (XPS) measurements
Spectra were measured using an electron spectrometer made by OMICRON
Nanotechnology GmbH (Germany). The photoelectrons were excited by MgK (1253.6 eV)
radiation. Spectra were recorded in constant analyzer energy scan mode of EA125 Energy
Analyser with 30 eV pass energy resulting in a spectral resolution of 1 eV.

10
Pellets, pressed from powdered samples, were fixed to standard OMICRON sample
plates. Samples were either annealed in high vacuum or treated in the high-pressure chamber
of the spectrometer in 300 mbar H2 at elevated temperatures. Spectra of vanadium(V)oxide
and vanadium (III)oxide were determined and used as reference to assign XPS peaks of the
catalyst samples to vanadium species in different chemical states.
The non-reduced samples were charging heavily. A fortunate situation is that in both
vanadium oxides [23] and TiO2 [24-26] the leading contribution to the O 1s spectrum appears
around 530.0 eV, thus a relatively reliable binding energy scale (containing no more
uncertainity than 0.1-0.2 eV) can be obtained by setting the binding energy of the maximum
of the O 1s envelope to this value. According to the literature, this calibration is much better
than the one using the binding energy of the C 1s peak of hydrocarbon contamination as
reference value [27].
Data were processed using the CasaXPS software package [28] by fitting the spectra
with Gaussian-Lorentzian product peaks after removing a Shirley background. Nominal
surface compositions were calculated using the XPS MultiQuant software package [29,30]
with the assumption of homogeneous depth distribution for all components. XPS databases
were used to identify the chemical states of vanadium [31,32].
2.2.7. Catalytic activity measurements
Catalytic test reactions were carried out at atmospheric pressure in a fixed-bed,
continuous flow tubular microreactor. Prior to the reaction the catalysts were activated in
oxygen flow (20 cm3 min-1) for 1 h at 350 °C. The same treatment was applied to re-activate
used catalysts. In the catalytic test C2H4/O2/H2O/He gas mixture was fed on 500 mg of
catalyst sample (particle size 0.63-1.00 mm). The partial pressures of ethylene, oxygen, and
water were 3.4, 13.5, and 27 kPa, respectively. In oxidation tests of acetaldehyde and acetic

11
acid, the partial pressure of latter reactants was kept at 1.7 kPa, whereas the oxygen and water
partial pressures were the same as during ethylene oxidation tests. The effect of the partial
pressures on the activity was studied by varying the partial pressure of oxygen and water in
the ranges of 6.4-52.2 and 0-56.4 kPa, respectively, on expense of the partial pressure of the
helium carrier gas The space time of reactants ethylene, acetaldehyde, and acetic acid was
either 205 h gcat. mol-1reactant or 410 h gcat mol-1reactant, except when the space time dependence
of ethylene or acetaldehyde conversion was studied. In the measurements of partial pressure
and space time dependence 100 mg of catalyst, diluted with 400 mg of inert γ-Al2O3 was
used. The total flow rate of the reaction mixture was always 30 cm3 min-1. All gas lines of the
apparatus were heated to 120 °C in order to avoid condensation of water and reaction
products. The reaction products were analysed by on-line Shimadzu GC-2010 gas
chromatograph (GC) equipped with a 30-m HP-PLOT-U column, thermal conductivity and
flame ionization detectors (TCD and FID). The calibration of the GC for each reactant and
product compound was carried out separately. The conversion of ethylene was calculated
from the ethylene concentrations in the feed and effluent. Selectivities were calculated from
molar product composition.
3. Results and discussion
Specific surface area and contents of palladium and vanadia of fresh and used catalysts
are given in Table 1. Deposition of V2O5 and Pd on TiO2 resulted in catalysts, having only
slightly smaller specific surface areas (SSA) than the pure titania support (SSA=55 m2 g-1).
The catalytic activity and selectivity of the fresh catalysts could be reproduced using re-
activated catalysts. Therefore, the effect of reaction conditions on the activity could be studied
over a single catalyst charge that was always re-activated before reaction conditions were
changed. Neither the activity nor the selectivity changed in 20 h time-on-stream. After several

12
days in contact with the reacting mixture under varying conditions the chemical composition
of the catalysts was determined again. Results suggest that no significant compositional
change occurred (Table 1).
The catalytic activity strongly depends on the dispersion and structure of the supported
vanadium-containing species. Monolayer surface coverage corresponds to approximately 7 to
8 VOx species on a square nanometer of titania surface [33]. In case of titania P-25 the
monolayer coverage amounts to about 6 wt% V2O5 content. The V2O5 content of our catalyst
preparations was either far below the monolayer coverage (2.2 wt%) or strongly exceeded it
(8.6 wt%). One of our catalysts contained 4.6 wt% V2O5, i. e., the V2O5 loading of the titania
support was near to that corresponding to monolayer coverage (Table 1). The XRD patterns in
Fig. 1 show that crystalline V2O5 was present in the sample of highest vanadia content only.
Reflections of anatase and rutile appear in the XRD pattern of each sample.
The Raman spectra of titania-supported V2O5 samples are presented in Fig. 2. In the
wavenumber region below 800 cm-1 only the intense absorption bands of TiO2 support (~ 145,
397, 515 and 638 cm-1) are present (not shown). Between 800 and 1200 cm-1, except for the
catalyst with the lowest vanadia coverage, a single band appeared at 995 cm-1. This band can
be assigned to crystalline V2O5 particles. The strength of the band increases with increasing
vanadia loading. The V2O5 reflections and Raman band are somewhat stronger for the 8.6V
sample prepared by mechanically mixing V2O5 and TiO2 than those of the chemically
prepared 8.6V preparation, indicating that vanadia is present in slightly different forms in the
two samples. The finding that the Raman band of crystalline V2O5 at 995 cm-1 appears also in
the spectrum of 4.2 wt% V2O5/TiO2 catalyst can be explained by the much larger Raman
scattering cross section of crystalline V2O5 than that of dispersed monovanadate and
polyvanadate structures [34]. Traces of crystalline V2O5 gave Raman band but could not
generate X-ray reflection signals.

13
H2-TPR measurements were carried out to characterize the reducibility of catalysts.
The TPR profiles of V2O5/TiO2 and Pd/V2O5/TiO2 samples are shown on Fig. 3. In absence of
palladium the reduction of V2O5 phase proceeds between 300 and 600°C for each sample. The
change in the oxidation state of the vanadium atoms upon reduction was determined by
integration of the H2-TPR peaks. Table 2 shows that the H2/V values obtained for the
V2O5/TiO2 samples vary between 1.13 and 1.27, corresponding roughly to the reduction of
V5+ to V3+. On the profiles of palladium-containing samples two reduction peaks are present:
a sharp low-temperature peak below 200°C and a broad high-temperature peak overarching
the whole temperature range of the measurement. The reduction of palladium, supported on
TiO2 takes place around 0°C [35]. Thus, the formation of metallic palladium occurs
immediately when the N2 flow is switched to H2/N2 flow. The Pd/TiO2 (not shown) and
0.4Pd2.1V samples present negative peaks in region 50-180 °C, which are assigned to release
of hydrogen absorbed in Pd0 crystallites at lower temperature. At low vanadia loadings the
released hydrogen could mask hydrogen consumption but at higher loadings no negative H2
desorption peaks could be discerned. Van der Heide [36] showed that the low-temperature
reduction of V2O5 proceeds via a hydrogen spill-over process in which hydrogen is
dissociatively chemisorbed at the palladium surface and chemisorbed hydrogen atoms move
from palladium to the surrounding vanadium oxide. The spill-over experiments showed no
formation of water at 373 K, only the uptake of hydrogen. Moreover, hardly any weight
change was observed during hydrogen uptake. They concluded that the V2O5 monolayer is
transferred into a monolayer of a hydrogen bronze of vanadium (HxV2O5 x=1.4 to 2) and not
into lower valence vanadium-oxides. In our experiments H2/V values between 0.49 and 0.66
were obtained for palladium-containing samples (Table 2) that suggests only one-electron
reduction (V5+→V4+). Latter result is in conflict with our earlier findings with different oxide-

14
supported Pd/vanadia catalysts, where always near to two-electron reduction was attained in
similar H2-TPR measurements [37, 38].
In order to find explanation for the unexpected one-electron reduction we
accomplished a detailed XPS study on the 8.6V and 0.4Pd8.6V catalysts. V2O5 and V2O3
were used as reference materials (Figure 4A). The XPS spectrum of the reference materials
was recorded first in their “as received” state, i.e., without applying any thermal or reductive
treatment. For both samples V5+ was found to be the dominant surface vanadium species. The
V 2p spectrum of V2O5 is dominated by the narrow 2p3/2 line of V5+ at 517.1 eV, separated
from the main O 1s contribution by 12.8-12.9 eV. These data are in good agreement with the
XPS characteristics of these compounds published earlier [39]. In addition to the main V 2p3/2
peak, a small contribution is also present at about 515.5 eV. It is assigned to the photoelectron
peak of V4+ ions, the presence of which is interpreted as a result of photoinduced V2O5
reduction (Figure 4A, lowermost spectrum).
The V2O3 reference sample was studied also after 2 hours of vacuum annealing at
230°C. The treatment transformed the surface layer of the “as received” sample into a mixed
valence V6O13 –like compound as suggested by the shape of the valence band spectrum [40].
The spectrum of the O 1s and V 2p region of the sample is shown in Figure 4A (uppermost
spectrum). The narrow V 2p3/2 line, characteristic of the V2O5 sample, did not show up but a
broad band appeared shifted towards lower binding energies. This band was deconvoluted
into a narrow band at 517.1 eV and another rather broad feature centered at 516.1 eV. The
narrow and the broad bands were shifted from the main O 1s contribution by 12.8 eV (V5+)
and 14.0 eV, respectively. Latter can be identified as contribution from V4+ ions [39].
Presence of V4+ ions is confirmed also by the appearance of a weak peak in the valence band
region, above the O 2p-related edge, in the vicinity of the Fermi level, which arises from filled
V 3d states.

15
The V 2p region of the 8.6V sample in the “as received” state shows a narrow V 2p3/2
component at a binding energy of 517.1 eV, what indicates that the most abundant vanadium
species in this sample is V5+. Nevertheless, the low binding energy component at 516.0 eV
(V4+) is clearly stronger than that in the spectrum of the V2O5 reference sample (Figure 4A). It
is to be noticed that the sample is somewhat unstable against X-ray exposure, as this V4+
component clearly increases during the measurement. In order to minimize this effect, in all
experiments the V 2p region was recorded prior to any other spectral region. Titanium was
found to be in its fully oxidized state (Ti4+). The Ti 2p3/2 binding energy corresponds to
literature values. No reduction of titanium was observed during the measurements.
Figure 4A also shows the XPS spectra of 8.6V V2O5/TiO2 sample, obtained after H2 treatment
at different temperatures. Spectra were recorded without exposing the sample to air after the
treatments. Treatment at 100°C left the majority of vanadium in V5+ state. The V4+
contribution was not enhanced by the treatment. Composition was calculated from the XPS
data. The vanadium content was expressed in V2O5 and is given in weight percents.
Accordingly, the surface layer, available for XPS analyses, contains 19 wt% V2O5, indicating
that vanadia covers large fraction of the titania surface and prevents it from being detected by
XPS.
The treatment at 200 °C did not affect the chemical state of Ti4+ but a clear increase
was noticed in the intensity of the V4+ component of the V 2p3/2 band.
The H2 treatment at 300°C caused significant changes relative to the previously
recorded XPS spectrum of the 8.6V sample. The V 2p3/2 band became broad and without
structure. No sharp V5+ contribution could be identified by either simple inspection or curve
fitting. The band center shifted obviously to binding energy that is lower than the 516.0 eV
binding energy of V4+. According to the literature, the broad V 2p3/2 band of V3+ appears

16
roughly around a binding energy of 515.2 eV [39]. Most probably vanadia, containing
mixture of V4+ and V3+ ions, is present on the titania surface after the applied treatment.
The lowering oxidation state of vanadium was paralleled by the decreasing amount of
XPS detectable vanadium. The obtained 13 wt% V2O5 content indicated that the surface layer
was depleted from vanadium. It may be either due to diffusion of vanadium ions into deeper
layers of the TiO2, due to decrease of dispersion of the vanadium-oxide or due to evaporative
loss, if volatile oxide- or hydroxide-like vanadium compound was formed. It is to be noted
that no vanadium loss was detected during catalytic tests, which were carried out at
temperatures below 200 oC (Table 1).
Upon reduction at 400°C the rather weak but still broad V 2p3/2 peak shifted to
515.2 eV, which corresponds to the binding energy reported for V2O3. The detected 6 wt%
V2O3 content shows that the treatment enhanced the depletion of vanadium from the surface
of the 8.6V sample.
Fig. 4B shows XPS spectra of the Pd-containing V2O5/TiO2 catalyst 0.4Pd8.6V. The
V 2p region of the “as received” sample contains a narrow V 2p3/2 component at binding
energy 517.1 eV, along with a much broader peak around 516.2 eV, indicating that the V4+
contribution is already significant. The sample was strongly unstable against X-ray exposure,
as this V4+ component rapidly increased and became dominant during the measurement. The
bulk vanadium content of the 0.4Pd8.6V sample was essentially the same as that of the
corresponding Pd-free sample, whereas that of the surface layer was equivalent with a
concentration of 18 wt% V2O5. Titanium maintained its fully oxidized state during the
measurements, i.e., the Ti 2p3/2 peak appeared and remained at the binding energy published
for Ti4+ in the literature.
The 1h reduction at 100°C resulted in no significant change in the surface
composition. The vanadium chemical states were, however, notably influenced. The V 2p3/2

17
region shows a broad and asymmetric envelope which can be deconvoluted into a V5+
component at 517.1 eV and a broad peak at 516.1 eV, which can be attributed to V4+ ions
(Fig. 4B).
The hydrogen treatment at 200°C resulted in a further reduction of vanadia. The peak
shape of the broad V 2p3/2 feature and the corresponding binding energy resemble now to
those of the Pd-free sample reduced at 300°C. The surface vanadium concentration, expressed
in V2O5, decreased to 15% (Fig. 4B).
The reduction at 300°C did not bring about noticeable change in the surface
composition (Fig. 4B). The broad V 2p3/2 peak around 515.4 eV indicated that the vanadium
was mostly in V3+ ionic state. Reduction of TiO2 was still not detectable.
The reduction at 400°C resulted in a minor shift of the broad V 2p3/2 peak to 515.3 eV
and decreased the surface V2O5 concentration to 12 wt% (Fig. 4B).
The relative amounts of vanadium in high (V5+) and lower oxidation states (V4+, V3+)
determined from the spectra of Fig. 4, are visualized in Fig. 5. The 8.6V sample is
significantly more resistant to reduction than the 0.4Pd8.6V sample. At temperature whereon
the V5+ ions of the Pd-free sample only began to get reduced, all the V5+ ions of the Pd-
containing sample became reduced to V4+ and V3+.
For the above characterized samples X-ray induced Auger spectra of vanadium are
shown in Fig. 6. As the vanadium L23M23M45 Auger transition involves electrons from the
valence band, the obtained line shape is very sensitive for the electron population of the 3d
orbitals that is increasing as the oxide undergoes reduction. For V2O5 a relatively symmetric
line was found at kinetic energy 466.2 eV. Sambi et al. [25] reported that reduction of
supported vanadia shifts the maximum of the Auger line towards higher energies giving rise
to a new component at 472-473 eV, which is indicating the appearance of filled vanadium 3d
orbitals. Our results are in accordance with these findings. The kinetic energy and shape of the

18
L23M23M45 Auger line suggests that V5+ is the major vanadium species of the 0.4Pd8.6V
sample in the “as received” state, although the V4+ contribution is already significant. Upon
H2 treatment at 100°C no significant changes occurred. After reduction at 200°C the V
L23M23M45 Auger band appeared with center around 469 eV together with a very significant
contribution around 472 eV. The line shape starts to resemble the two-peaked feature reported
for V2O3 what indicates that the sample at this point contains a mixture of V4+ and V3+ ions,
with the probable dominance of the latter species. The spectrum recorded after reduction at
300°C corresponds to the shape of that reported for V2O3 suggesting that V3+ is the prevailing
vanadium species. On effect of reduction at 400°C the Auger line broadens towards the low
kinetic energy side. Similar spectrum was obtained for sample 8.6V, reduced at or above
300°C (Fig. 6).
In accordance with the results of H2-TPR measurements the results of XPS
measurements suggest that milder reduction conditions are required to reduce V5+ to V4+ and
V3+ if the sample contains also palladium than in absence of palladium Because of the
sensitivity of the Pd/V2O5/TiO2 catalysts to become reduced upon X-ray exposure the
measured oxidation states might not represent the initial oxidation state of the samples.
However, considering that the X-ray-related changes occur on a time scale of several minutes
we still believe that the initial V4+ content of the Pd-loaded catalyst was higher than that of the
Pd-free system (Fig. 5, lowest temperature data points). However, the relatively minor
difference of the initial vanadium oxidation state of the samples, detected by XPS, does not
explain the large difference measured by TPR. The pre oxidized and N2 purged V2O5/TiO2
sample consumed near to 1 H2/V, whereas the Pd/V2O5/TiO2 catalysts, which were more
easily reducible, consumed only about 0.5 H2/V (Table 2). The oxidation state of vanadium in
the deeply-reduced palladium-doped samples was found by XPS to be V3+, i.e. the same as in
the case of the Pd-free catalyst (Fig. 4). Therefore, results substantiate that the vanadium of

19
the Pd-loaded catalyst already was not in its fully oxidized V5+ state when recording of the H2
consumption in the H2-TPR measurements was started. This can be explained by oxygen loss
during N2 flushing and cooling of the sample after high-temperature pre-treatment in O2 flow.
As a consequence, lower amount of H2 was consumed to reach the final V3+ state than was
needed starting the reduction from V5+ state. As another explanation we considered the
possible hydrogen uptake of vanadia via spill over mechanism from the H2/N2 gas mixture
before starting temperature ramp-up in the TPR measurement. Latter explanation was rejected
because a Pd/VOx/TiO2 sample, which was pre-treated similarly in O2 flow, consumed H2 in
an amount that corresponded to H2/V=1.15, if sample was cooled to room temperature before
it was contacted with H2/N2 flow (Table 2). This result suggests that no hydrogen spillover
proceeded at room temperature. Thus, spill over cannot be responsible for the unexpected
TPR results.
The Wacker oxidation of ethylene was studied over Pd/V2O5/TiO2 catalysts, having
different Pd loadings. When the Pd loading of the 2.1V support was raised from 0.01 to 0.1
wt% the conversion of ethylene approached to a much higher stable level (Fig. 7). As a result
higher acetaldehyde yield was obtained even at decreasing acetaldehyde selectivity. Much
higher Pd content hardly affected product distribution. The selectivity of CO2 slightly
increased on expense of the selectivity of partially oxidized products. Based on these results
V2O5/TiO2 catalysts of 0.4 wt% Pd content were chosen for detailed catalytic examination.
Fig. 8 shows the effect of oxygen and water partial pressure on the conversion and
selectivity of ethylene oxidation over 0.4Pd4.6V catalyst at 125 °C. At low oxygen partial
pressures (< 15 kPa) the conversion and selectivity of products is almost constant. At higher
oxygen partial pressures both the ethylene conversion and the acetic acid and CO2 selectivity
are higher but the acetaldehyde selectivity is lower (Fig. 8A). These results are in accordance
with the picture of a consecutive oxidation process where acetaldehyde is the primary product

20
that becomes further oxidized to acetic acid or CO2. In absence of water in system ethylene
becomes converted to CO2 only (Fig. 8B). At higher water partial pressure higher
acetaldehyde selectivity is obtained at relatively low ethylene conversion. Above about
40 kPa water partial pressure the selectivities are virtually independent from the water content
of the reaction mixture. Only slight increase of ethylene conversion was observed, whereas
the CO2 selectivity dropped to about 5%.
The effect of space time on conversion of ethylene and acetaldehyde and on the yield
of the main products was examined using 0.4Pd4.2V catalyst at 150 °C (Fig. 9). The yield of
acetaldehyde (Fig. 9A) passes through maximum indicating that this product is intermediate
of a consecutive oxidation process. In the Wacker conversion of acetaldehyde crotonaldehyde
has a concentration maximum at low space times, indicating that aldol condensation of
acetaldehyde takes place. No concentration maxima were observed for acetic acid suggesting
that it is not intermediate to CO2 formation (Fig. 9B).
Results of ethylene Wacker oxidation over Pd/V2O5/TiO2 catalysts with different
vanadia contents are shown as function or reaction temperature in Fig. 10. The main reaction
products are acetaldehyde, acetic acid and CO2. The formation of methane and acetone was
also detected but in negligible amounts. At low temperatures (100-125°C) acetaldehyde is the
main product irrespectively of vanadia content. At higher temperatures the selectivities of
acetic acid and CO2 rapidly increases and at 200°C the oxidation to CO2 becomes the main
reaction. Between 125 and 175°C the formation of acetic acid and CO2 is almost
equimolecular. The catalysts of higher vanadia content show similar ethylene conversion
activity (Fig. 10). The highest acetaldehyde yields were obtained with the catalyst having
close to monolayer vanadia coverage on the titania support (~65 % at 100 °C, see Fig. 10B).
For Wacker activity the intimate contact of Pd and the VOx particles is essential. The structure
of surface vanadia has also importance. Fig. 10C demonstrates that a fraction of the primarily

21
formed acetaldehyde becomes converted to acetic acid and CO2 over Pd/vanadia redox pair at
low temperatures. Low vanadia content corresponding to sub-monolayer coverage of titania
support (Fig. 10A) results in lower ethylene conversion and higher acetaldehyde selectivity,
whereas higher vanadia content, corresponding to about monolayer coverage results in higher
ethylene conversion and lower acetaldehyde selectivity. The product distribution at 175-200
°C is almost the same for all catalysts.
Results of Wacker oxidation of ethylene is shown in Fig. 11A as function of reaction
temperature over 0.4Pd4.6V in comparison with the conversion of the partially oxidized
products, such as acetaldehyde (Fig. 11B) and acetic acid (Fig. 11C) over the very same
catalyst under identical conditions. At low temperatures (100-125 °C) the conversion of
acetaldehyde is low. At higher temperatures (150 °C) the acetaldehyde is converted to acetic
acid and CO2 in molar ratio close to 1 to 1. Formation of methane can be also observed with 5
to 10 % selectivity. Forni et. al. [41] substantiated that CO2 formation takes place not only
from acetaldehyde and acetic acid but, at least in part, directly from ethylene. Based on the
results, shown on Fig. 11, direct oxidation of ethylene to CO2 cannot be excluded, however, it
is obvious that oxidation of acetic acid starts only above about 175 °C (Fig. 11C).
4. Conclusions
Pd/V2O5/TiO2 catalysts gave high acetaldehyde yield in the selective oxidation
of ethylene in presence of water at 100 °C. The highest acetaldehyde selectivity was achieved
over catalyst with vanadia content corresponding to sub-monolayer coverage of the titania
support. The conversion was low because of the small number of active Pd/VOx redox
couples. At high vanadia content V2O5 crystallites were formed on the surface of the support.
Palladium on V2O5 crystallites has high oxidation activity and it converts the primarily
formed acetaldehyde to acetic acid and CO2 even at temperature as low as 100 °C. Regarding

22
the acetaldehyde yield the favorable vanadia content is about equivalent with the amount
needed to attain monolayer V2O5 coverage of the titania support. The results suggest that
acetic acid and CO2 are products obtained from primarily formed acetaldehyde. Oxidation of
acetic acid to CO2 occurs only above 175 °C.
Acknowledgement
This work was financially supported by the Hungarian Scientific Research Fund
(OTKA, contract no. K 100411)
References
[1] J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Rüttinger, H. Kojer, Angew.
Chem. 71 (1959) 176-182.
[2] J. Smidt, W. Hafner, R. Jira, R. Sieber, J. Sedlmeier, A. Sabel, Angew. Chem. Int.
Ed. 1 (1962) 80-88.
[3] A.S. Jhaveri, M.M. Sharma, Chem. Eng. Sci. 22 (1967) 1-6.
[4] R. Jira, in: B. Cornils and W.A. Herrmann (Eds.), Oxidation of Olefins to Carbonyl
Compounds, Viley-VCH Verlag GmbH, Weinheim, 2002, pp. 386-405.
[5] R.A. Fernandes, D.A. Chaudhari, J. Org. Chem. 79 (2014) 5787−5793.
[6] S.F. Davison, B.E. Mann, P.M. Maitlis, J. Chem. Soc. Dalton Trans. 6 (1984) 1223-1228.
[7] P. Teo, Z.K.Wickens, G. Dong, R.H. Grubbs, Org.Lett. 14 (2012) 3237-3239.
[8] G. Centi, F. Cavani, F. Trifirò, Selective Oxidation by Heterogeneous Catalysis, first ed.,
Springer Science+Business Media New York, 2001.
[9] P.H. Espeel, M.C.Tielen, P.A.Jacobs, J.Chem. Soc. Chem.Commun.10 (1991) 669–671.
[10] E.D. Park, J.S. Lee, J. Catal. 180 (1998) 123–131.
[11] T. Mitsudome, T. Umetani, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda, Tetrahedron
Lett. 47 (2006) 1425-1428.
[12] K. Nowińska, D. Dudko, Appl. Cat. A. 159 (1997) 75-87.
[13] A.W. Stobbe-Kreemers, G. van der Lans, M. Makkee, J.J.F. Scholten, J. Catal. 154
(1995) 187-193.
[14] M. Li, J. Shen, React. Kinet. Catal. Lett. 72 (2001) 263-267
[15] B. Evnin, J.A. Rabo, P.H. Kasai, J. Catal. 30 (1973) 109–117.
[16]A.W. Stobbe-Kreemers, M. Soede, M. Makkee, J.J.F. Scholten, Appl. Catal. A 131
(1995) 367–381.

23
[17] A.W. Stobbe-Kreemers, M. Makkee, J.J.F. Scholten, Appl. Catal. A 156 (1997) 219–238.
[18] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 323-334.
[19] E.V. Danilevich, G.Ya. Popova, T.V. Andrushkevich, V.V. Kaichev, I.G. Danilova,
Yu.A. Chesalov, V.A. Rogov, V.I. Bukhtiyarov, V.N. Parmon, Appl. Catal. A 475 (2014) 98–
108.
[20] J. Haber, Catal. Today, 142 (2009) 100-113.
[21] I.E. Wachs, Dalton T. 42 (2013) 11762-11769.
[22] R.A. Sheldon, M. Wallau, I.W.C.E. Arends, U. Schuchardt, Acc. Chem. Res. 31 (1998)
485-493.
[23] J. Mendialdua, R. Casanova, Y. Barbaux, J. Electron Spectr. Rel. Phenom. 71 (1995)
249-261.
[24] Q. Guo, S. Lee, D.W. Goodman, Surf. Sci. 437 (1999) 38-48.
[25] M. Sambi, M. Della, Negra, G. Granozzi, Surf. Sci. 470 (2000) L116-L122,
[26] J. Biener, M. Bäumer, R.J. Madix, Surf. Sci. 432 (1999) 178-188.
[27] J. Mendialdua, R. Casanova, Y. Barbaux, J. Electron Spectr. Rel. Phenom. 71 (1995)
249-261.
[28] N. Fairley (2006). http://www.casaxps.com/.
[29] M. Mohai, Surf. Interface Anal. 36 (2004) 828–832.
[30] M. Mohai (2003). XPS MultiQuant: Multi-model X-ray photoelectron spectroscopy
quantification program, Version 3.00.16
http://www.chemres.hu/aki/XMQpages/XMQhome.htm.
[31] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr.,
NIST X-ray Photoelectron Spectroscopy Database, Version 3.4, National Institute of
Standards and Technology, Gaithersburg, MD (2003) http://srdata.nist.gov/xps/,
[32] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben: Handbook of X-ray Photoelectron
Spectroscopy, Perkin-Elmer Corp. Eden Prairie, Minnesota, USA, 1992
[33] I.E. Wachs, B. M. Weckhuysen, Appl. Catal. A 157 (1997) 67–90.
[34] S. Xie, E. Iglesia, A. T. Bell, J. Phys. Chem. B 105 (2001) 5144-5152.
[35[ C. Zhang, Y. Li, Y. Wang, H. He, Environ. Sci. Technol. 48 (2014) 5816-5822.
[36] E. van der Heide, M. Zwinkels, A. Gerritsen, J. Scholten, Appl. Catal. A 86 (1992) 181–
198.
[37] R. Barthos, A, Hegyessy, Z, May, J. Valyon, Catal. Lett. 144 (2014) 702-710.
[38] R. Barthos, A, Hegyessy, Sz. Klébert, J. Valyon, Micropor. Mesopor. Mater. 207 (2015)
1-8.
[39] G. Silversmit, D. Depla, H. Poelman, G. B. Marin, R. De Gryse, J. Electron Spectr. Rel.
Phenom. (2004) 135, 167-175.
[40] M. Demeter, M. Neumann, W. Reichelt, Surf. Sci. 454-456 (2000) 41-44
[41] L. Forni, G. Terzoni, Ind. Eng. Chem. Process. Des. Dev. 16 (1977) 288-293.
Figure legends
Fig. 1 XRD patterns of V2O5/TiO2 preparations with different vanadia loadings. The
diffractograms of the TiO2 support (P25) and its mechanical mixture with 8.6 wt% V2O5 are
given for comparison.
Fig. 2, FT-Raman spectra of V2O5/TiO2 preparations with different vanadia loadings. The
spectra of the mechanical mixture of TiO2 and 8.6 wt% V2O5 is given for comparison.

24
Fig. 3. H2-TPR profiles of V2O5/TiO2 (dashed lines) and Pd/V2O5/TiO2 (full lines) catalysts.
Fig. 4. X-ray photoelectron spectra of the (A) 8.6V and (B) 0.4Pd8.6V samples in their
untreated (as received) and treated states. Treatment involved heating the samples in 300
mbar H2 successively at 100, 200, 300 and 400 °C for 1h at each temperature. Spectra are
labelled by the treatment temperature only. The spectra were corrected for the MgK3,4
contribution. In panel (A) also spectra of V2O5 and a V6O13-like compounds are shown for
reference. Latter compound was obtained by vacuum annealing V2O3 at 230 oC. The reference
spectra include both V 2p and O 1s contributions.
Fig. 5. Relative amounts of vanadium in V5+ and lower oxidation states as function of the
reduction temperature for the 8.6V and 0.4Pd8.6V samples. Data were derived from the
spectra of Fig. 4.
Fig. 6. X-ray induced vanadium L23M23M45 transition Auger spectra of samples 0.4Pd8.6V,
8.6V, and reference samples V2O5, and V6O13 compounds. Spectra were obtained in the
experiments described in the legend of Fig. 4. Spectra of sample 0.4Pd8.6V are labelled by
the treatment temperature only.
Fig. 7. The activity of Pd/V2O5/TiO2 catalysts as a function of Pd content in the Wacker
oxidation of ethylene at 150 °C. The 2.1V preparation was used to make Pd2.1V catalysts of
different Pd contents. The space time was 205 h gcat. mol-1ethylene. The partial pressures of
ethylene, oxygen, and water in the C2H4/O2/H2O/He gas mixture were 3.4, 13.5, and 27 kPa,
respectively.
Fig. 8. Effect of partial pressure of (A) oxygen and (B) water on the selective oxidation of
ethylene at 125°C over 0.4Pd4.6V catalyst. 100 mg of catalyst was diluted by 400 mg of inert
γ-Al2O3 to get a catalyst bed. The space time of ethylene was kept constant at
41 h gcat. mol-1ethylene. The partial pressure of oxygen or water was changed on expense of the
partial pressure of the He carrier gas.
Fig. 9. The Wacker oxidation of (A) ethylene and (B) acetaldehyde with O2 in presence of
water at varied reactant space time over 0.4Pd4.6V at 150 °C. 100 mg of catalyst was diluted
by 400 mg of inert γ-Al2O3 to get a catalyst bed. The partial pressure of reactant acetaldehyde
was kept at 1.7 kPa, The partial pressures of ethylene, oxygen and water were the same as
those given in the legend of Fig. 7.
Fig. 10. Effect of temperature in selective oxidation of ethylene over catalysts with different
vanadia loadings (A) 0.4Pd2.2V (B) 0.4Pd4.6V (C) 0.4Pd8.6V. The space time was
205 gcat. mol-1ethylene. The partial pressures of ethylene, oxygen and water were the same as
those given in the legend of Fig. 7.
Fig. 11. Effect of temperature on the selective oxidation of (A)ethylene, (B) acetaldehyde and
(C) acetic acid over 0.4Pd4.6V catalyst. The space time for ethylene was
205 h gcat. mol-1reactant and for aceteldhyde or acetic acid was 410 h gcat. mol-1reactant. The partial
pressures of ethylene and acetaldehyde or acetic acid were 3.4 and 1.7 kPa, respectively.

25
Table 1. Specific surface area and active phase composition of titania-supported catalysts.
Table2. Results of H-TPR measurements.
Table 1. Specific surface area and active phase composition of titania-supported catalysts.
Catalysta
SSA, m2/g
Pd content, wt %
V2O5 content, wt %
fresh
used
n.d.c
n.m.b
0.39
fresh
2.84
2.26
2.18
4.59
8.57
n.m. b
used
n. mb
2.34
2.16
4.56
8.40
n.m. b
0.01Pd2.1V
0.1Pd2.1V
0.4Pd2.1V
0.4Pd4.2V
0.4Pd8.4V
0.8Pd17V
n.m.b
n.m.b
55
n.d.c
0.11
0.40
0.37
0.42
n.m. b
53
0.39
50
0.38
n.m. b
45
a In the designation of the Pd-containing V2O5/TiO2 samples V stands for V2O5/TiO2 the number before the letter
symbols gives the Pd and V content of the catalyst in weight percent.bNot measured. c Non detectable.

26
Table 2. Results of H2-TPR measurements.a
Catalyst
Low-temperature peak
High-temperature peak
∑H2/V
H2 (μmol g-1) H2/V
H2 (μmol g-1) H2/V
0.4Pdb
17
-
-
(0.46)a
-
2.2V
-
-
311
120
537
163
1114
136
406
-
1.27
0.49
1,13
0.50
1.17
0.66
1.16
0.4Pd2.2V
4.6V
-
-
0.49
-
-
-
0.4Pd4.6V
8.6V
91
0.18
0.32
-
-
-
0.4Pd8.6V
0.8Pd17Vc
168; 306
0.18; 0.33
0.15
1 763
0.94
0.21
a Before TPR run samples were pre-treated in O2 flow at 350 °C for 1 h, then cooled to room temperature in N2
b
flow and flushed by N2 for additional 30 min. Pd content of the sample is 0,39 wt% (38 μmol/g); H2
c
consumption 17 μmol/g, i. e., H2/Pd = 0.46. Before TPR run sample was pre-treated in O2 flow at 350 °C for 1
h then cooled to room temperature in O2 flow and flushed by H2/N2 for additional 30 min.