Journal of Catalysis 397 (2021) 183–191Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier .com/locate / jcatCharacterization of Pt-based oxidation catalyst – Deactivated
simultaneously by sulfur and phosphorushttps://doi.org/10.1016/j.jcat.2021.03.026
0021-9517/
 2021 The Author(s). Published by Elsevier Inc.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑ Corresponding author.
E-mail addresses:mari.honkanen@tuni.fi (M. Honkanen), mika.huuhtanen@oulu.
fi (M. Huuhtanen), marja.karkkainen@oulu.fi (M. Kärkkäinen), tomi.kanerva@ttl.fi
(T. Kanerva), kimmo.lahtonen@tuni.fi (K. Lahtonen), avh@dinex.fi (A. Väliheikki),
kki@dinex.fi (K. Kallinen), riitta.keiski@oulu.fi (R.L. Keiski), minnamari.vippola@tu-
ni.fi (M. Vippola).Mari Honkanen a,⇑, Mika Huuhtanen b, Marja Kärkkäinen b, Tomi Kanerva c, Kimmo Lahtonen d,
Ari Väliheikki b,e, Kauko Kallinen e, Riitta L. Keiski b, Minnamari Vippola a,f
a Tampere Microscopy Center, Tampere University, P.O. Box 692, 33014 Tampere University, Finland
b Environmental and Chemical Engineering, University of Oulu, P.O. Box 4300, 90014 University of Oulu, Finland
c Finnish Institute of Occupational Health, P.O. Box 40, 00032 Työterveyslaitos, Finland
d Faculty of Engineering and Natural Sciences, Tampere University, P.O. Box 692, 33014 Tampere University, Finland
eDinex Finland Oy, Vihtavuorentie 162, 41330 Vihtavuori, Finland
f Faculty of Engineering and Natural Sciences, Materials Science and Environmental Engineering, Tampere University, P.O. Box 589, 33014 Tampere University, Finland
a r t i c l e i n f o a b s t r a c tArticle history:
Received 22 December 2020
Revised 19 March 2021
Accepted 22 March 2021
Available online 31 March 2021
Keywords:
Deactivation
Environmental catalysis
Analytical transmission electron microscopy
Fourier transform infrared spectroscopy
X-ray photoelectron spectroscopySimultaneous poisoning of sulfur + phosphorus on a platinum-based diesel oxidation catalyst was studied
to gain a deeper understanding on the catalyst deactivation. Compared to a single poisoning (sulfur or
phosphorus), the simultaneous poisoning had a severe effect on the catalyst activation: light-off temper-
ature for 90% conversion of propene was not reached, this of carbon monoxide was much higher than
expected, and the maximum conversion of nitrogen monoxide collapsed. With very comprehensive struc-
tural characterization by various methods (e.g. STEM-EDS, XPS, DRIFTS) used, we achieved to conclude an
explanation for this. In the case of the S + P-poisoning of the catalyst, formed aluminum phosphate was
found to block adsorption sites for sulfur species on alumina and sulfur adsorbs mainly on cerium oxides.
In addition, sulfur species remain with and in the vicinity of the platinum particles blocking active sites.

 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).1. Introduction
An exhaust gas purification system in a diesel engine consists of
a diesel oxidation catalyst (DOC), a particulate filter, and a nitrogen
oxides (NOx) storage catalyst or a selective catalytic reduction
(SCR) system [1]. The function of the DOC is to oxidize CO and
HC to CO2 and water and to oxidize NO to NO2 [2,3]. Pt-based cat-
alysts, working at low temperatures (190–250 C) [4], are widely
used in the exhaust aftertreatment systems in diesel engines and
alumina is the most common support material used [3].
Performance of the catalysts decreases over the time; thus, to
extend a catalysts lifetime, it is important to study deactivation
phenomena. Catalytic converters have been used since the 1970s
in vehicles and stationary applications, however, their deactivation
is still a major problem and it is not fully understood. In the ther-
mal deactivation, the active surface area of the catalyst decreasesbecause of structural changes of the catalyst support and/or active
metal sites. In the poisoning, the active surface area of the catalyst
decreases because poisoning elements will chemisorb and foul the
active metal or support surface preventing the desired reactions
[3]. The deactivation of DOCs by sulfur is one of the most important
challenges in the diesel engine exhaust aftertreatment [5]. Sulfur
originates from low-quality fuels and lubricants and even a very
small amount of sulfur can decrease the performance of the cata-
lyst [6]. Under oxygen-rich conditions in the diesel engine, sulfur
compounds are oxidized to SO2 followed by oxidation to SO3 on
the Pt particles in the DOC [7,8]. In the presence of oxygen, plat-
inum is the very active catalyst oxidizing SO2 to SO3, and the reac-
tion starts already at ~ 150 C [9,10]. However, the formation of
platinum sulfate is not favored [4] and formed SO3 strongly inter-
acts with the support components, e.g. alumina, ceria, and
ceria + zirconia, forming metal sulfates [11,12]. Al2(SO4)3 starts
to form at 600 C and is catalyzed by Pt. Ce(SO4)2 forms at 195
C [11]. Thus, Pt improves the storage of sulfur into sulfating sup-
ports oxidizing sulfite to sulfate [3]. For example, in the case of the
alumina support, formed Al2(SO4)3 covers the alumina surface and/
or plug its micropores [8]. Sulfur species are formed on ceria and
mixed ceria + zirconia also without Pt because they are able to oxi-
dize SO2 without any catalyst [12]. CeO2-based materials can be
Fig. 1. The photo of the catalyst sample.
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191used as a NOx storage and release component and may having
some promotion effect on the light-off performance and selectivity
in the DOC and thus, the sulfation of CeO2 decreases the catalyst
performance [13]. In addition of sulfur, it is known that phospho-
rus, originating e.g. from lubricating oils, deactivates Pt-based cat-
alysts [14]. Because of affinity between Al and P, they will form
dense and amorphous AlPO4 in the case of the alumina-based sup-
port [15,16]. Phosphate overlayers cause fouling of the support
surface, clogging of the pores, and occlusion of the noble metal par-
ticles decreasing significantly the catalyst performance [13,17–19].
Sulfur and phosphorus adsorb differently onto the catalyst
monoliths. Sulfur goes mainly uniformly throughout the catalyst
length and support depth [14,20,21], while, phosphorus exists
mainly in the inlet part of the catalyst monolith and on the top
layer of the support [14,17,18,22,23]. Beckman et al. [14] have
noticed that ~ 5% of sulfur fed into the system was absorbed in
the catalyst after the poisoning treatment. The amount of sulfur
in the catalyst depended on the sulfur content in fuel (increasing
with increasing sulfur content). In addition, the amount of sulfur
depended also on the phosphorus content (increasing with
decreasing phosphorus content). The phosphorus content in the
catalyst depended directly on its amount in the lubricating oil
and even 70% of the amount of phosphorus in the system could
be found from the catalyst. Buwono et al. [13] have studied the
adsorption of SO2 and phosphorus oxides on the Rh/Al2O3 and
Rh/AlPO4 catalysts. They reported that nearly all SO2 in the gas feed
was adsorbed on the Rh/Al2O3 until a saturation point was reached.
While, only a very small amount of SO2 was adsorbed on the Rh/
AlPO4 because of the lack of basic sites on the surface of AlPO4 were
present. Phosphorus oxides formed amorphous AlPO4 layers with
Al2O3 while the AlPO4 surface was less reactive with sulfur oxides.
In our earlier studies [18,21], we have noticed that sulfur or
phosphorus presenting in the catalyst decreases the performance
of the Pt-based DOCs. Here, we observed that the simultaneous
sulfur + phosphorus poisoning causes more severe deactivation
than sulfur and phosphorus separately. In addition, their interac-
tion with catalyst components varies depending on the existence
of sulfur only or both sulfur and phosphorus. With very compre-
hensive structural characterization by various methods, we
achieved to conclude an explanation for this. Thus, in the
laboratory-scale accelerated poisoning tests, it is very important
to use co-treatments to simulate the real conditions wherein many
poisoning elements exist at the same time. Based on our knowl-
edge, only in a few articles [24,25] the co-effect of sulfur and phos-
phorus on the catalyst performance by accelerated laboratory-
scale tests has been studied. In our earlier article [25], the DOC
studied was Pt/SiO2-ZrO2, thus with very different support material
than here we have studied. Dahlin et al. [24] have studied the co-
effect of sulfur and phosphorus on the SCR catalysts (Cu-SSZ-13
and V2O5-WO3/TiO2).2. Material and methods
A catalyst studied was a metallic DOC containing platinum
(50 g/cft) supported on the alumina-based washcoat including
additives (Ti, Ce, Si, and Zr oxides). It was provided by Dinex Fin-
land Oy (formerly Ecocat Oy). The inlet and outlet part of the cat-
alyst was studied as fresh and after laboratory-scale accelerated
phosphorus and sulfur treatments (Fig. 1). In the treatment, the
catalyst was placed in a vertically positioned tubular quartz reac-
tor, and the total flow was 1 dm3/min and the gas hourly space
velocity (GHSV) was 21 000 h
1 [25]. The sample (marked as
PSW) was poisoned for 5 h at 400 C. The feed contained 10%
H2O with phosphorus c((NH4)2HPO4) = 0.13 M (solution fed by a
peristaltic pump), and 100 ppm SO2, 10% air, and N2 balancing184gas. The PSW-treated sample was compared to a water-treated
(marked as W) [18] and to single phosphorus- (marked as PW)
[16,18] and sulfur- (marked as SW) [21] treated samples to deeply
understand the co-effects of phosphorus and sulfur. The PW and
SW treatments were carried out similarly than the PSW treatment,
just leaving out SO2 and (NH4)2HPO4, respectively, and theW treat-
ment leaving out both SO2 and (NH4)2HPO4. The accelerated
laboratory-scale treatments used in this study have been devel-
oped and validated against the information achieved from the cat-
alysts used in the real conditions [17,21,22]. Thus, the used
treatment conditions have been selected so that the amount of poi-
sons in the laboratory-scale-treated catalysts corresponds with the
analyzed values from the real ones.
Laboratory-scale diesel oxidation activity measurements were
carried out for the PSW-treated catalyst in lean reaction conditions
using the following gas mixture: 1000 ppm NO, 500 ppm CO,
300 ppm C3H6, 12 vol% O2 and 10 vol% H2O, and N2 as balance
gas. The total gas flow was 1 dm3/min, resulting in a gas hourly
space velocity (GHSV) of 31,000 h
1 for a monolith. The measure-
ments were carried out at atmospheric pressure in a horizontally
aligned tubular quartz reactor. The temperature was increased
from room temperature up to 300 C with a linear heating rate of
5 C/min under the reaction gas mixture flow H2O adding was
started at 110 C with a peristaltic pump. The catalyst was kept
in a steady state for 15 min at 300 C, and after that, the furnace
was cooled down to room temperature under the N2 flow. The pro-
cedure was repeated twice to determine the repeatability. Gas flow
rates were controlled by mass flow controllers (Brooks 5280S). The
outlet gas composition was measured by a GasmetTM FT-IR gas
analyzer. Oxygen concentration was determined with a paramag-
netic oxygen analyzer (ABB Advanced Optima).
The catalyst samples were characterized at microscale by a field
emission scanning electron microscope (FESEM, Crossbeam 540,
Zeiss) equipped with an energy dispersive spectrometer (EDS,
XMaxN silicon drift detector, Oxford Instruments). For imaging,
an energy selective backscattered (EsB) detector was used to max-
imize a compositional contrast. Cross-sectional samples for FESEM
studies were prepared by a conventional metallographic method
including molding the sample to resin followed by grinding and
polishing and finally by carbon coating to avoid a sample charging
during the FESEM-EDS studies. The catalyst samples were studied
at nanoscale by a cold field emission gun (scanning) transmission
electron microscope ((S)TEM, JEM-F200, Jeol) equipped with EDS
(dual EDS system for F200, Jeol). For imaging, a STEM dark field
(DF) detector was used to maximize a compositional contrast.
STEM samples were prepared by scraping the surface layer of the
catalyst material from the monolith followed by crushing the
scraped catalyst powder between two laboratory glass slides and
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191dispersing the powder with ethanol onto a copper grid with a
holey carbon film. The scraped catalyst powder was also used for
X-ray diffraction analysis (XRD, Empyrean with PIXcel3D detector,
PANanalytical, using Cu Ka radiation with wavelength
0.15418 nm). Phases were identified from XRD patterns using the
database (PDF-4+ 2020) from International Centre for Diffraction
Data (ICDD).
The state of the elements in the catalysts was studied by X-ray
photoelectron spectroscopy (XPS). The lens-defined selected-area
XPS (SAXPS) was performed employing non-monochromatizedFig. 2. The fresh catalyst, (a) the cross-sectional FESEM image (EsB) and the la
Table 1
Light-off temperatures for 90% conversions (T90) and maximum NO conversion over
the PSW-treated catalyst compared to the fresh, W-, SW-, and PW-treated catalysts.
T90 [C] Max conversion [%]
Catalyst C3H6 CO NO
Fresh [18] 186 160 50
W-treated [18] 192 172 48
SW-treated [21] 206 182 46
PW-treated [18] 295 196 17
PSW-treated [this study] Not reached 230 6
185broad illuminating DAR400 X-ray source (AlKa, 1486.6 eV, 300 W)
and Argus hemispherical electron spectrometer (Omicron Nan-
otechnology GmbH) equipped with micro-channel plate electron
multipliers and a 128 channel stripe anode detector. The core level
spectra were collected in normal emission angle with a pass energy
of20 eV,highmagnification lensmode, and in-lens apertureyielding
circular analysis area of 2.93mm2 (£1.93mm). The surface compo-
sition was identified by analyzing core level spectra using CasaXPS
software (Version 2.3.19 PR 1.0) [26]. Due to surface charging, the
binding energy scale was calibrated according to C 1s C–C/H peak
at 284.8 eV. The background subtracted XPS peaks were least-
squares fitted with a combination of G–L component line shapes.
The relative atomic concentrations were calculated using Scofield
photoionization cross sections and an experimentally measured
transmission function of the Argus analyzer.
The specific surface area, pore size, and pore volume of the
PSW-treated catalyst were measured using the Micrometrics ASAP
2020. Specific surface areas were determined from the N2 adsorp-
tion isotherms at 
196 C (77 K) according to the standard
Brunauer-Emmet-Teller (BET) method. Pore size and pore volume
distributions of the catalysts were calculated from N2 desorption
isotherms by the Barret-Joyner-Halenda (BJH) method.yered FESEM-EDS map, (b) the STEM DF images, and (c) the XRD pattern.
Fig. 3. The cross-sectional FESEM image (EsB) and FESEM-EDS maps from the inlet part of the PSW-treated catalyst. More color indicates higher amount.
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191A Fourier transform infrared (FT-IR) spectrometer (Bruker, Ver-
tex V80) equipped with a diffuse reflectance infrared Fourier trans-
form (DRIFT) unit (Harrick) and a liquid-nitrogen-cooled mercury
cadmium telluride (MCT) detector was utilized to study poisoning
compounds on the scraped catalyst sample. The DRIFT measure-
ments were carried out at room temperature under ambient atmo-
sphere conditions. A background spectrum was measured using a
mirror. Spectra were recorded by using a resolution of 4 cm
1.
3. Results and discussion
3.1. Effect of sulfur and phosphorus on catalyst performance
The activity test over the PSW-treated catalyst was carried out.
In our earlier studies [18,21], the effect of water and single poisons
(S and P) on the performance of the DOC studied (CO, C3H6 and NO
oxidation) has been investigated (Table 1). In the case of the PSW-
treated DOC, it was found that light-off temperature for 90% con-
version (T90) of C3H6 was not reached and this of CO was much
higher than expected (Table 1). In addition, a drastic effect was
detected in the maximum conversion of NO. Thus, the results indi-
cate that in the presence of both impurities (S and P) simultane-
ously, a severe catalyst deactivation occurred. However, the
reason or mechanism has not been yet clearly explained. Therefore,
several characterization methods were used to find out the poison-
ing mechanism.
3.2. Fresh catalyst
To study the effect of sulfur and phosphorus on the catalyst, it
was essential to comprehensively characterize the fresh catalyst.
The cross-sectional FESEM image (EsB) and the FESEM-EDS layered
elemental map are presented in Fig. 2a. The catalyst consisted of
aluminum-rich areas, cerium-rich particles, and titanium and sili-
con containing areas. Pt particles were too small to be detected
by FESEM. The STEM DF images with various magnifications are
presented in Fig. 2b showing Pt particles which lattice fringes cor-
respond to metallic Pt. Sequential tilt-series STEM DF images (±55,186in steps of 2) were collected from the fresh catalyst and the
images were constructed to a video (Video 1), bright spots indicate
Pt particles. The XRD (Fig. 2c) and XPS results indicated that Pt par-
ticles were as metallic form and support material as oxide form (Pt
4f7/2 at 70.2 eV and Al 2p3/2 at 74.6 eV).3.3. Microscale distribution of sulfur and phosphorus
A cross-sectional FESEM image (EsB) and FESEM-EDS maps col-
lected from the inlet part of the PSW-treated catalyst are presented
in Fig. 3. Phosphorus existed mainly on the top of the catalyst sup-
port and sulfur in the bottom area. More detailed micro-scale dis-
tribution of phosphorus and sulfur was studied by FESEM-EDS line
analyses collected from the inlet and outlet parts of the PSW-
treated catalyst (Fig. 4). In the inlet part (Fig. 4a), phosphorus
amount on the top layer of the catalyst was 10–15 wt% clearly
favoring alumina-rich areas. The highest sulfur amount, <5 wt%,
was detected on the ceria-rich areas (Fig. 4a). However, in the bot-
tom area of the catalyst where in the amount of phosphorus was
small, sulfur (<5 wt%) existed more uniformly in the both ceria-
and alumina-rich areas. In the outlet part (Fig. 4b), only a very
small amount of phosphorus was detected. Sulfur (<5 wt%) existed
thoroughly in the outlet part and it was determined mainly to be in
the alumina-rich areas and not as much in the ceria-rich areas
(Fig. 4b).
To compare the effect of the single- and co-treatments, FESEM-
EDS line analyses were collected also from the SW- and PW-treated
catalysts (Supplementary material). In the SW-treated sample, a
small amount (<5 wt%) of sulfur existed in the inlet and outlet
parts thoroughly and its distribution was uniform regardless of
alumina- and ceria-rich areas. In the inlet part of the PW-treated
catalyst, phosphorus (10–15 wt%) existed mainly on the top layer
of the catalyst support and it seemed to favor alumina-rich areas
and avoid ceria-rich areas. Only a very small amount of phosphorus
was detected in the outlet part of the PW-treated catalyst. The
average amount of phosphorus and sulfur was similar in the both
single- and co-treatments and a saturation point seemed to be
10–15 wt% for phosphorus and < 5 wt% for sulfur.
Fig. 5. The STEM DF image and STEM-EDS maps collected from the inlet part of the PSW-treated catalyst. In the maps, brighter color indicates higher amount. Scale bar
marked in the STEM DF image is 100 nm.
Fig. 4. FESEM-EDS line analyses from the PSW-treated catalyst, (a) two measurement points (areas 1 and 2) from the inlet part and (b) two measurement points (areas 1 and
2) from the outlet part.
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191
187
Fig. 6. STEM-EDS results for PSW-treated catalyst. (a) STEM DF image and STEM-EDS maps, brighter color indicates higher amount, scale bar marked in the STEM DF image is
20 nm. (b) STEM BF image and STEM-EDS line analyses over three Pt particles, intensity values for Pt and S are normalized.
Fig. 7. DRIFT spectra of the fresh and PSW-treated catalysts.
Fig. 8. XPS spectra of the fresh, SW-treated and PSW-treated catalysts.
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–1913.4. Nanoscale distribution of sulfur and phosphorus
To study nanoscale distribution of sulfur and phosphorus after
the simultaneous poisoning treatment, STEM-EDS elemental maps188were collected from the inlet and outlet parts of the PSW-treated
catalyst. By STEM-EDS, it was possible to detect phosphorus also
on the top layer of the outlet part of the catalyst although it was
challenging to detect with bulk analyses by FESEM-EDS. The
STEM-EDS results collected from the inlet and outlet parts of the
PSW-treated catalyst were similar. The maps collected from the
Table 2
Crystallite size of Pt particles determined from XRD patterns and particle size determined from STEM images for the fresh, PSW-, PW-, and SW-treated catalysts.
Fresh SW inlet PW inlet PSW inlet
Crystallite size [nm] 4 4 8 11
Particle size [nm] 6.1 ± 1.8 6.5 ± 1.8 7.2 ± 2.2 6.1 ± 2.4
Fig. 9. Histograms and example STEM DF images (bright spots indicate Pt particles, scale bar is 50 nm) showing Pt particle size distributions measured from STEM images
(300 particles) for (a) the fresh catalyst, (b) the SW-treated catalyst, (c) the PW-treated catalyst, and (d) the PSW-treated catalyst.
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191inlet part are presented in Fig. 5: phosphorus seemed to follow
alumina-rich areas while sulfur existed all over the catalyst
support.
The EDS maps collected from the PSW-treated catalyst with
higher magnification than in Fig. 5 are presented in Fig. 6a and line
analyses in Fig. 6b: sulfur seemed to locate mainly with and in the
vicinity of the Pt particles. Based on the data collected from the
SW-treated sample, sulfur existed more uniformly in the support.
Phosphorus behaved similarly in the both PW- and PSW-treated
samples, favoring aluminum-rich areas. Based on the literature,
the deactivation of the noble metal particles is much faster if the
support is not adsorbing sulfates and more sulfur species remain189on the Pt surface blocking active sites [3]. Adsorption of SO2 is
not favored on the AlPO4 because of the lacking basic sites on the
AlPO4 surface [13]. According to our earlier study on a DOC from
a heavy-duty vehicle (used for 80 000 km), sulfur followed ceria-
rich areas and phosphorus favored alumina-rich areas [22].
3.5. Composition of sulfur and phosphorus
Based on the DRIFT results of the PSW-treated catalyst (Fig. 7),
both sulfur and phosphorus species were detected. The peak
1640 cm
1 on both fresh and PSW-treated samples is assigned to
moisture (–OH) on the catalyst surface e.g. [27]. In the spectrum
M. Honkanen, M. Huuhtanen, M. Kärkkäinen et al. Journal of Catalysis 397 (2021) 183–191of the fresh catalyst, the peaks 960 and 1240 cm
1 are due to mate-
rial vibrations, such as Al-O [28]. The peaks on the PSW-treated
sample at 1210–1280 cm
1 are assigned as sulfates, most probable
O = S = O and O 
 SO3 vibrations of Al2(SO4)3 [17,20,21,29–32], but
1280 cm
1 can be also caused by PO2 vibration, e.g. [33,34]. In
addition, it is challenging to interpret the specific adsorption sites
(Al or Ce). In the case of phosphorus, a strong peak at 900 cm
1
indicates P 
 O 
 P bonds and at 500–650 cm
1 as P 
 O vibrations
thus it can be assumed that phosphorus is in the form of phos-
phates [16,18,33–35]. The gained results are in good agreement
with the XPS studies.
According to the XPS measurements (Fig. 8), sulfur was in the
form of sulfates on the SW-treated (S 2p3/2 at 169.8 eV) and
PSW-treated (S 2p3/2 at 169.0 eV) catalyst surfaces. Phosphorus
had a high oxidation state on the PSW-treated sample (P 2p3/2 at
134.6 eV) suggesting phosphate. Platinum was always in the
metallic state in the fresh (Pt 4f7/2 at 70.2 eV), SW-treated (Pt
4f7/2 at 71.1 eV), and PSW-treated (Pt 4f7/2 at 71.1 eV) samples
(Supplementary material). The observed 0.9 eV difference in the
binding energy of Pt 4f7/2 between the fresh and PSW-treated cat-
alysts suggests changed surrounding of the Pt atoms in the case of
PSW, but not actual Pt–O bonding. Pt oxides should be located at
higher binding energies, between 72.4 and 74.9 eV. Also, the Pt
crystallite/particle size and the degree of crystallinity could affect
both the XPS peak position and width. The concentration ratio of
Al:P was 1:1 after the PSW treatment indicating that within the
XPS sampling depth the surface contained mainly Al1P1Ox species,
and not anymore pure Al2O3. A drastic change was observed in the
relative amount of sulfur at the same time. The atomic concentra-
tion of S decreased to one tenth from 2.43 at% on the SW-treated
catalyst to 0.25 at% on the PSW-treated catalyst. This can be
explained either by decreased absolute amount of sulfur caused
by reduced adsorption during the poisoning or the sulfur species
were mainly buried below XPS sampling depth by AlPOx layer.3.6. Effect of sulfur and phosphorus on catalyst morphology
The crystallite size of Pt particles for the fresh catalyst and after
the SW-, PW-, and PSW-treatments was determined from the XRD
patterns by Scherrer equation (Table 2). Corresponding Pt particle
sizes (300 particles) were measured from STEM DF images (Table 2
and Fig. 9) by the TEM Center software (TEM Operation & Acquisi-
tion System for JEM-F200). STEM studies indicated that Pt particles
were mainly single crystals. Based on the XRD measurements, the
Pt crystallite size increased in the PW- and PSW-treatments.
According to STEM studies, growing was very slight. Based on
the histograms (Fig. 9), in the poisoned catalysts, there were some
larger particles compared to the fresh catalyst. However, poisoning
treatments caused only a slight Pt particle growing which is not
exclusively explaining the severe deactivation of the PSW-treated
catalyst.
Specific surface area (SBET), total pore volume, and average pore
size of the fresh and treated catalysts are presented in Table 3. TheTable 3
Specific surface area (SBET), total pore volume, and average pore size for the fresh and
treated catalysts.
Specific surface
area [m2/g]
Total pore
volume [cm3/g]
Average pore
size [nm]
Fresh [18] 225 0.41 7.5
W-treated [18] 218 0.40 7.3
SW-treated [21] 192 0.36 7.6
PW-treated [18] 137 0.26 7.4
PSW-treated
[this study]
108 0.20 7.6
190W-treatment had a negligible effect on the catalyst [18]. SBET and
total pore volume values decreased in the order of SW- [21], PW-
[18] and PSW-treatments, however, the pore sizes remained
unchanged regardless of the treatment. BET isotherms (not shown)
followed the IUPAC classification type IV(a) indicating material
having quite large mesopores with size 2–50 nm [36,37]. Forma-
tion of the microporous aluminum phosphate layer on the alumina
surface (in the PW- and PSW-treated catalysts) is assumed to influ-
ence on the total pore volumes and specific surface area.4. Conclusions
Catalyst deactivation caused by simultaneous sulfur and phos-
phorus exposure in the Pt-based catalyst with an alumina-rich
support was studied. Compared to a single poisoning (sulfur or
phosphorus), the simultaneous poisoning (PSW) caused severe
effects on the catalyst activation: the light-off temperature (T90)
of propene was not reached, T90 of carbon monoxide was higher
(30–70 C) than this of the fresh- and single-element poisoned,
and the maximum conversion of nitrogen monoxide collapsed
below 10%. By several characterization methods utilized to find
out the deactivation mechanism, it can be concluded that phospho-
rus favors formation of aluminum phosphate. Formed aluminum
phosphate acts as a non-sulfating support hindering the migration
of sulfur oxides from the Pt particles to the support leading more
severe poisoning of the Pt particles. Thus, in the absence of phos-
phorus, sulfur distributes evenly throughout the catalyst support
as aluminum sulfate, but in the presence of phosphorus, sulfur
adsorbs mainly on mixed cerium and zirconium oxide. In the case
of the PSW poisoning, the support becomes poisoned by both sul-
fur and phosphorus species, and, in addition, Pt particles are cov-
ered by sulfur species. To gain deeper understanding on the
catalyst deactivation phenomena, introducing poisoning com-
pounds simultaneously on the catalyst surface is essential.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
Electron microscopy work made use of Tampere Microscopy
Center facilities at Tampere University.Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2021.03.026.
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