Chem. P oc. 2020, 2, 13; doi:10.3390/ECCS2020-07529 www.mdpi.com/jou nal/chemp oc
P oceedings
Syn hesis and Cha ac e iza ion o Pd o e No el TiO
2
Mix u es: Insigh s on Me al–Suppo In e ac ions
†
Ma ías G. Rinaudo
1,
*, Ana M. Bel án
2
, Ma ía A. Fe nández
3
, Luis E. Cadús
1
and Ma ia R. Mo ales
1
1
Facul ad de Química, Bioquímica y Fa macia, Ins i u o de In es igaciones en Tecnología Química
(INTEQUI-CONICET), Uni e sidad Nacional de San Luis (UNSL), Almi an e B own 1455, 5700 San Luis,
A gen ina; [email p o ec ed]om (L.E.C.); m mo [email protected] (M.R.M.)
2
Depa amen o de Ingenie ía y Ciencia de los Ma e iales y del T anspo e, Escuela Poli écnica Supe io ,
Uni e sidad de Se illa, Vi gen de Á ica 7, 41011 Se illa, Spain; [email p o ec ed]
3
Ins i u o de Ciencia de Ma e iales de Se illa, CSIC-Uni e sidad de Se illa, A da. Amé ico Vespucio 49,
41092 Se illa, Spain; [email protected]
* Co espondence: [email p o ec ed]
† P esen ed a he Fi s In e na ional Elec onic Con e ence on Ca alysis Sciences, 10–30 No embe 2020;
A ailable online: h ps://eccs2020.sci o um.ne .
Published: 9 No embe 2020
Abs ac : Palladium nanopa icles we e suppo ed on unusual mix u es o ana ase, TiO
2
(II) and
u ile i ania phases by we imp egna ion, ob aining ca alys s wi h me al con en s o ca. 0.25 w %
labeled Pd/Ti5, Pd/Ti45, and Pd/Ti120. C ys alline s uc u es we e con i med by X- ay di ac ion.
Pd pa icle sizes in he ange o 4–20 nm we e obse ed by scanning- ansmission elec on
Mic oscopy. Ex e nal su ace a eas (S
BET
) in he ange 10–17 m
2
g
−1
we e highe enough o achie e a
good dis ibu ion o palladium o e i anium oxide ou e su ace, as e idenced by ene gy-
dispe si e X- ay spec oscopy elemen al p o iles. Pd
0
/Pd
δ+
a omic a io measu ed by X- ay
pho oelec on spec oscopy showed a dec ease om Pd/Ti5 o Pd/Ti120, in line wi h he dec ease in
ana ase phase p esen in he ca alys s. This beha io sugges ed ha palladium ended o o m mo e
TiPd
x
O s uc u es in Pd/Ti5 whils PdO
x
s uc u es we e mo e likely o be p esen on suppo s wi h
g ea e amoun s o TiO
2
(II) and u ile, due o he dis inc me al–suppo in e ac ions. An inc ease
in educibili y and oxygen mobili y om Pd/Ti5 o Pd/Ti120 was obse ed by empe a u e
p og ammed measu emen s and associa ed o he di e en high-ene gy ball milled suppo s.
Ca alys s wi h imp o ed p ope ies epo ed he ein could exhibi an excellen pe o mance in
oxida ion eac ions, e.g., glyce ol selec i e oxida ion.
Keywo ds: i anium oxide; palladium ca alys ; me al–suppo in e ac ions; suppo e ec ; edox
p ope ies
1. In oduc ion
Palladium-based ca alys s ha e been ex ensi ely used in oxida ion eac ions, whe e bo h
dispe sion and pa icle size o me al a e conside ed he dominan ac o s o a good ac i i y and
selec i i y [1,2]. Howe e , he ole o suppo s is no limi ed o be ca ie s o me al pa icles [3,4].
Se e al imes, he in e ace be ween me al and suppo is a be e ac i e si e han he me al i sel and
hus, an app op ia e selec ion o a ca alys suppo could educe he amoun o expensi e me als
needed (e.g., noble me als) [3,5,6]. Mo eo e , me al–suppo in e ac ion is a well-known
phenomenon a ibu ed o elec onic and geome ic e ec s ha induce cha ge ans e be ween me al
pa icles and suppo [5,7]. These in e ac ions migh no only enhance he adso p ion capaci y o
ac i e si es bu also inhibi he sin e ing o me al pa icles du ing eac ion [4]. A a ional design is
Chem. P oc. 2020, 2, 13 2 o 7
he e o e c ucial o ob ain sus ainable me al-suppo ed ca alys wi h high ac i i y, eusabili y, and
low a e o deac i a ion [3,7].
TiO2 is a ecognized suppo o me als, which shows di e en p ope ies depending on he
polymo phs p esen [3]. Since mos eac ions in he e ogeneous ca alysis ake place on he su ace o
me al ac i e phase o he in e ace be ween me al and suppo , he p esence o a ious c ys alline
phases could enhance ca aly ic p ope ies [3,8]. To he bes o ou knowledge, ca alys s wi h modi ied
TiO2 suppo s o med by de ec i e s uc u es and unusual polymo phic mix u es ha e no been used
o oxida ion eac ions un il now. The e o e, we p opose he s udy o Pd ca alys s suppo ed on no el
mix u es o ana ase, TiO2 (II) and u ile (no comme cially a ailable) ob ained by high-ene gy ball
milling in ou p e ious wo k [9] o e alua e he di e en me al–suppo in e ac ions and hei
po en ial ole in liquid-phase glyce ol selec i e oxida ion.
2. Ma e ials and Me hods
2.1. Ca alys s P epa a ion
We imp egna ion me hod was used o deposi palladium on h ee di e en TiO2 suppo s
ob ained in ou p e ious wo k [9]. Aqueous solu ions o HCl wi h app op ia e amoun s o PdCl2
(99.99%, Sigma Ald ich, Buenos Ai es, A gen ina) we e added o i ania suppo s in o de o achie e
palladium loadings o 0.25 w % on a me al basis. Reco e ed solids we e d ied in an o en unde
acuum a 70 °C o e nigh . Then, samples we e calcined in a mu le u nace unde ai a 500 °C o
4 h using a hea ing amp o 10 °C min−1 o elimina e he es s o me al p ecu so . Final Pd suppo ed
ca alys s we e labeled Pd/Tix, whe e x co esponds o he milling ime o p e iously syn hesized
suppo s.
2.2. Cha ac e iza ion
Ca alys s we e cha ac e ized by means o X- ay di ac ion (XRD), scanning- ansmission
elec on mic oscopy (STEM), ene gy-dispe si e X- ay spec oscopy (EDS), B unaue –Emme –Telle
(BET) me hod, induc i ely coupled plasma–op ical emission spec oscopy (ICP-OES), scanning
elec on mic oscopy (SEM), X- ay pho oelec on spec oscopy (XPS), and empe a u e p og ammed
measu emen s (TPM).
3. Resul s and Discussion
XRD analysis was pe o med o e alua e he c ys alline s uc u e o suppo ed ca alys s. We
obse ed (Figu e 1A) ha samples we e o med o di e en mix u es o i ania polymo phs. Pd/Ti5
showed in ense e lec ions associa ed o ana ase phase (PDF 00-21-1272) bu also wo b oad and low-
in ensi y e lec ions a 27.54 ° and 31.53 ° a ibu ed o (110) u ile e lec ion (PDF 00-21-1276) and
(111) high-p essu e TiO2 (II) e lec ion (PDF 00-72-0021), espec i ely. These obse a ions con i med
he p esence o an almos pu e ana ase phase in his ca alys , as expec ed o i s suppo [9]. On he
o he hand, Pd/Ti45 exhibi ed an inc ease in bo h TiO2 (II) and u ile e lec ions bu a dec ease in
ana ase e lec ions. Pd/Ti120 also showed he e lec ion a ibu ed o TiO2 (II) phase and he
appea ance o se e al high-in ensi y u ile e lec ions. An incipien signal o (101) PdO e lec ion
(PDF 01-06-0515) a 34.04° was de ec ed in Pd/Ti5 ca alys . This beha io sugges ed he p esence o
la ge palladium c ys alli es deposi ed on his ca alys , as also con i med in u he TEM analysis.
The absence o his signal in Pd/Ti45 and Pd/Ti120 ca alys s could be associa ed o smalle pa icles
inely dispe sed on he suppo s a o ed by he g ea e numbe o de ec s and he in e aces be ween
polymo phs p esen in hese suppo s and unde he de ec ion limi o he equipmen [9,10].
We also made a ough calcula ion o he a e age c ys alli e size o ca alys s by means o Sche e
equa ion and he ela i e in ensi ies o he mos in ense i ania e lec ions o each c ys alline phase,
(101) ana ase, (110) u ile, and (111) TiO2 (II), as sugges ed by Dulian e al. [11]. We obse ed (Table
1) a educ ion end acco ding o he ex en o milling in he suppo s, which is in line wi h he pa icle
e inemen caused by milling p ocess [9,12]. These esul s we e also in line wi h he inc ease in SBET
alues (Table 1) obse ed in a u he analysis. We es ima ed he ca alys s composi ion using he mos
Chem. P oc. 2020, 2, 13 3 o 7
in ense B agg e lec ions o each i ania phase and he equa ions epo ed by Du a ci e al. [13],
ob aining he unusual mix u es o i ania phases (no comme cially a ailable) p esen in he suppo s
(Table 1).
STEM and EDS analyses we e ca ied ou o ha e an insigh on he size o palladium species.
We could obse e Pd pa icles o 20 nm in Pd/Ti5 ca alys (Figu e 1B) and in he ange 4–6 and 5–10
nm o Pd/Ti45 and Pd/Ti120 espec i ely ( igu es no shown). These obse a ions sus ained p e ious
XRD esul s, whe e PdO e lec ion could only be seen in Pd/Ti5 ca alys due o he bigge size o
palladium pa icles. We also made a ough es ima ion o palladium dispe sion by means o he
equa ions p o ided by Maha a e al. [14] and he a e age sizes o palladium pa icles, ob aining
alues o 6%, 22%, and 15% o me al dispe sion o Pd/Ti5, Pd/Ti45, and Pd/Ti120 espec i ely.
Figu e 1. XRD spec a o ca alys s (A) and STEM image and EDS analysis o Pd/Ti5 sample (B).
F om ex u al analyses, we obse ed speci ic su ace a ea (SBET) alues (Table 1) in acco dance
wi h he milling ime o each i ania suppo . I is no ewo hy ha hese su ace a eas we e lowe
han ypical alues o me als suppo ed on educible oxides (a leas 50 m2 g−1). Howe e , he small
amoun s o palladium used du ing p epa a ion (0.25 w %) and he enhanced i ania suppo s
allowed a good me al dispe sion, as obse ed by STEM-EDS p e ious analysis. Po e diame e s a ied
in he ange 9–21 nm and o al po e olumes loca ed in he ange 0.04–0.1 cm3 g−1 (Table 1) as expec ed
o he low po osi y o i ania [8]. I is impo an o highligh ha po e diame e s ob ained a e highe
enough o a oid in e nal di usion limi a ions o big eac an molecules, such as glyce ol (a e age
molecule size o 0.62 nm) [15]. Fu he mo e, he low po osi ies obse ed may indica e ha SBET alues
co esponded almos en i ely o ex e nal su ace a ea [6,9]. This ac could induce he p e e en ial
loca ion o palladium species on he ex e nal su ace o suppo s, a o ing he access o eac an s o
s a he mechanism o eac ion [6].
Table 1. Weigh ac ion (%) o i ania polymo phs, a e age c ys alli e size, and ex u al p ope ies o
he ca alys s.
Ca alys Ana ase
w %
TiO2 (II)
w %
Ru ile
w %
A g
C ys alli e
Size (nm)
SBET
(m2 g−1)
Po e
Diame e
(nm) a
To al Po e
Volume
(cm3 g−1) b
Pd/Ti5 99.2 0.3 0.5 54 10 21 0.05
Pd/Ti45 48.4 43.3 8.3 44 15 26 0.1
Pd/Ti120 23 49.4 27.6 42 17 9 0.04
a BJH adso p ion b anch a e age po e diame e . b Quan i y o N2 adso bed a ela i e p essu e o 0.98.
Chem. P oc. 2020, 2, 13 4 o 7
Mo phology o ca alys s was s udied by SEM (mic og aphs no shown). Sample Pd/Ti5 showed
pa icles wi h sphe ical shape and diame e s be ween 120–220 nm and Pd/Ti45 ca alys p esen ed
g ape-like clus e s o med by a couple o pa icles, wi h a e age diame e s be ween 100–200 nm.
Obse ed agglome a ion migh be a consequence o elec os a ic e ec s on ine pa icles (wi h
inc eased su ace ene gy) as epo ed by o he au ho s [16]. Smalle sizes we e ound in case o
Pd/Ti120 wi hin a ange be ween 80–180 nm. Mo eo e , he smalle pa icle sizes co ela ed well wi h
he inc ease in SBET alues (Table 1) om Pd/Ti5 o Pd/Ti120.
Pd loadings measu ed by ICP-OES we e ound o be in he ange 0.2–0.36 w % compa ed o he
heo e ical 0.25 w %. Di e ences we e a ibu ed o expe imen al e o s du ing syn hesis and/o
diges ion o he samples.
Su ace oxida ion s a e and me al–suppo in e ac ions we e s udied by means o XPS (Table 2).
A he Ti 2p le el we ound Ti 2p3/2 and Ti 2p1/2 double s no mally loca ed a binding ene gies o 458.3
and 464 eV and a ibu ed o Ti4+ [9]. No signals associa ed o Ti3+ we e obse ed. XPS spec a o O 1s
we e decon olu ed in o wo signals (ca. 529.4 and 530.1 eV) associa ed o la ice oxygen (Ola ) and
oxygen adso bed in oxygen acancies (Oads) espec i ely [17]. The con en s o oxygen acancies
Oads/(Ola + Oads) we e in he same o de as obse ed in he suppo s, wi h a sligh a ia ion due o
palladium imp egna ion and u he calcina ion [9]. These low alues we e p e iously explained by
he mig a ion o oxygen acancies om he su ace o he bulk in o de o achie e polymo phic
ans o ma ion du ing he syn hesis o suppo s [9].
Table 2. XPS analysis o ca alys s
Ca alys
Binding Ene gy (eV) a %, XPS
Ti
2p3/2
Ti
2p1/2
O 1s
(Ola )
O 1s
(Oads)
Pd 3d5/2
(Pd0)
Pd 3d5/2
(Pdδ+) Pd0/Pdδ+ Oads
/
(Ola +
Oads) Pd/Ti
Pd/Ti5 458.81 464.47 529.75 531.24 334.68 336.62 64.45 13.76 0.39
Pd/Ti45 458.4 464.05 529.65 531.29 334.28 336.42 50.2 9.48 0.51
Pd/Ti120 458.35 464 529.59 531.15 334.62 336.32 30.55 10.93 0.64
A he Pd 3d le el we obse ed Pd 3d5/2 and Pd 3d3/2 double s which we e decon olu ed in o ou
peaks. We selec ed Pd 3d5/2 signals o he in e p e a ion o palladium oxida ion s a e (Table 2). I is
well-known ha Pd 3d5/2 le el binding ene gy o Pd0 is a ound 335.2 eV [2,6]. Signals wi h posi i e
shi s espec o he Pd0 signal a e associa ed o ca ionic species (Pdδ+ wi h δ ≥ 2) and/o Pd/PdOx
in e aces, while nega i e shi s a e a ibu ed o anionic species and s uc u es such as TiPdxO, whe e
palladium s a e could be mo e nega i e han ze o [6]. Pd0/Pdδ+ a omic a io (Table 2) showed a
dec ease om Pd/Ti5 o Pd/Ti120, in line wi h he dec ease in ana ase phase p esen in he ca alys s.
This beha io could be explained by he g ea e numbe o de ec s and oxygen mobili y in he
suppo s milled o longe ime, ha would lead o elec on-de icien palladium species due o he
ans e o elec on densi y om palladium o i anium [1,2,4]. Pd/Ti a % we e in all cases highe
han he heo e ical calcula ion o he bulk (0.19 a %), which con i med he good dispe sion o Pd
on he ex e nal su ace o suppo s [3].
H2-TPR measu emen s we e pe o med o u he s udy me al–suppo in e ac ions and edox
p ope ies o ca alys s. Signals obse ed a a ound 52 °C (Figu e 2A) in all ca alys we e associa ed o
he o ma ion o β-PdHx species on su ace me allic palladium, ha akes place unde educ ion a
ambien empe a u e [6]. Nega i e signals obse ed a a ound 72 °C we e a ibu ed o he
decomposi ion o o med hyd ides, eleasing he H2 [1]. The b oad educ ion signal wi h a maximum
a 260 °C and he shoulde a 138 °C obse ed in his ca alys was associa ed o he educ ion o PdO
s ongly in e ac ing wi h he i ania suppo [18]. In case o Pd/Ti45 and Pd/Ti120, well-de ined
signals a 141 °C and 160 °C ( espec i ely) we e a ibu ed o he enhanced educ ion o PdO species.
This inc ease in educibili y, was ela ed o he g ea e numbe o de ec s, la ice oxygen mobili y,
and amoun o PdOx species p esen in he ca alys s [1]. S ong signals ound in he ange 533–578 °C
we e asc ibed o he educ ion o su ace TiO2 pa icles, as obse ed p e iously in he suppo s [9].
Pd/Ti120 also showed shoulde s a 307 °C and 558 °C which could be associa ed o he educ ion o
Chem. P oc. 2020, 2, 13 5 o 7
Ti4+ species loca ed in de ec i e s uc u es o he polymo phs, due o he longe ex en o milling o
his ca alys suppo [9].
Figu e 2. H2−TPR (A) and O2−TPD (B) o ca alys s.
H2-up akes (Figu e 2A) inc eased om Pd/Ti5 o Pd/Ti120. This beha io was expec ed due o
he longe milling ime o he suppo s, which gene a es s uc u es wi h imp o ed oxygen di usion
and educibili y [9].
We pe o med O2-TPD measu emen s (Figu e 2B) o ob ain in o ma ion ega ding oxygen
mobili y in he ca alys s. Deso p ion signals ound below 100 °C we e ela ed o oxygen species
weakly bonded o he su ace, as epo ed by o he au ho s [19]. Signals in he ange 174–220 °C we e
a ibu ed o oxygen species adso bed on oxygen acancies [19]. Fu he mo e, signals a ound 450 °C
we e asc ibed o su ace la ice oxygen while deso p ion peaks abo e 500 °C we e associa ed o bulk
la ice-oxygen elease [19]. Well-de ined signals obse ed in case o Pd/Ti45 and Pd/Ti120 ca alys s,
wi h mo e ca ionic species, we e ela ed o he deso p ion o mo e ac i e-oxygen species om he
su ace o i s mo e de ec i e suppo s [9]. Weigh -co ec ed a eas (Figu e 2 B) ollowed he same end
as H2 up akes, inc easing om Pd/Ti5 o Pd/Ti120. This ac could be asc ibed o he g ea e amoun
o PdOx species p esen in hese ca alys s, in line wi h ou p e ious XPS and H2-TPR analyses.
4. Conclusions
Pd-based ca alys s epo ed in his wo k showed dis inc i e p ope ies associa ed o he unusual
mix u es o i ania phases p esen in he suppo s, p e iously p epa ed by high-ene gy ball milling
[9]. Me al nanopa icles we e well dispe sed on he ou e su ace o TiO2, as con i med by XPS and
STEM-EDS analyses. Pd/Ti5 ended o o m mo e anionic Pd species in he o m o TiPdxO s uc u es,
which seemed o be less exposed on he su ace due o s ong in e ac ion wi h he suppo , whils
Pd/Ti45 and mainly Pd/Ti120 o med mo e ca ionic PdOx species loca ed on he su ace o ca alys s,
as obse ed by XPS measu emen s. In he case o glyce ol oxida ion, ca ionic species o palladium
would bene i he adso p ion o hyd oxides and successi e glyce ol dep o ona ion, accele a ing he
mechanism o eac ion. Achie ed me al–suppo in e ac ions could imp o e edox p ope ies,

Chem. P oc. 2020, 2, 13 6 o 7
inducing low- empe a u e educibili y and an inc ease in he mobili y o eac i e oxygen species, as
e idenced by TPM analyses, ac o s well-known o bene i oxida ion eac ions. P esen ca alys s a e
being es ed in liquid-phase glyce ol selec i e oxida ion in o de o s udy he in luence o me al–
suppo in e ac ions on he ca aly ic pe o mance.
Au ho Con ibu ions: Concep ualiza ion, M.G.R., M.R.M., and L.E.C.; In es iga ion, M.G.R.; Visualiza ion,
M.G.R.; W i ing–o iginal d a , M.G.R.; W i ing– e iew, M.G.R., A.M.B., M.A.F., L.E.C., and M.R.M.; W i ing–
edi ing, M.G.R.; TEM measu emen s and analysis, A.M.B. and M.A.F.; Valida ion, L.E.C. and M.R.M.;
Supe ision, L.E.C. and M.R.M.; Resou ces, M.R.M. All au ho s ha e ead and ag eed o he published e sion
o he manusc ip .
Funding: The au ho s acknowledge he Uni e sidad Nacional de San Luis (UNSL), Agencia Nacional de
P omoción Cien í ica y Tecnológica (ANPCyT) and Consejo Nacional de In es igaciones Cien í icas y Técnicas
(CONICET) o hei inancial suppo . Also, hanks a e gi en o ANPCyT o he pu chase o he SPECS
mul i echnique analysis ins umen (PME8-2003) whe e XPS analyses we e made. The au ho s also acknowledge
he Spanish Minis y o Science, Inno a ion and Uni e si ies, co inanced by EU FEDER unds, unde g an
RTI2018-093871-B-I00; he Uni e si y o Se ille o he use o i s gene al esea ch se ice (CITIUS); he CSIC
suppo unde g an PIE-201760E002; and he use o he LANE (ICMS) acili ies o TEM da a analysis.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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