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Catalysis Today 197 (2012) 256– 264
Contents lists available at SciVerse ScienceDirect
Catalysis Today
j ourna l ho me p ag e: www.elsev ier .com/ lo cate /ca t tod
ctivation of metallic open-cell foams via washcoat deposition of Ni/MgAl2O4
atalysts for steam reforming reaction
inzia Cristiania,∗ , Elisabetta Finocchiob , Saverio Latorrataa , Carlo Giorgio Visconti c , Enrico Bianchic ,nrico Tronconic, Gianpiero Groppic, Paolo Polleseld
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, P.zza Leonardo da Vinci 32, 20133 Milano, ItalyUniversità di Genova, Dipartimento di Ingegneria Chimica e di Processo, P.le Kennedy 1, 16129 Genova, ItalyPolitecnico di Milano, Dipartimento di Energia, P.zza Leonardo da Vinci 32, 20133 Milano, ItalyENI Spa, Divisione Refining & Marketing, Via Maritano 26, 20097 San Donato Milanese, Italy
r t i c l e i n f o
rticle history:eceived 22 April 2012eceived in revised form 4 September 2012ccepted 17 September 2012
a b s t r a c t
Preparation of active washcoats of Ni/MgAl2O4 steam-reforming catalysts on FeCrAlloy foams is reportedin this work. The MgAl2O4 powdered support was prepared via co-precipitation method, and Ni/MgAl2O4
was obtained via dry impregnation of 10% (w/w) of Ni. After full characterization of the powders,the deposition of the catalyst over the foams was performed by percolation followed by air-blowing
vailable online 16 October 2012
eywords:tructured catalystsashcoatingetallic foams
(percolation-blowing). The resulting washcoat layers were quite homogeneous, uniform, and well adher-ent to the metallic support. Prototype catalytic foam samples, tested at the lab-scale, were found to beactive in the steam reforming process.
© 2012 Elsevier B.V. All rights reserved.
eforming
. Introduction
Structured catalysts and reactors for process intensification areeceiving a large interest from the modern chemical engineeringommunity [1]. It is now well recognized that monoliths, open-ell foams and other structured catalysts (e.g. metallic gauzes andbres, filamentous mats), due to their geometrical characteris-ics, enable to maximize interfacial areas and therefore enhancehe rates of fluid/solid mass transfer processes, also grantingn improved energy efficiency (lower pressure drop) because ofigh porosities. Moreover, the adoption of catalytic layers of suit-ble thickness ensures an optimized catalyst effectiveness, oftenssociated with beneficial results in terms of selectivity [2,3].uch advantages are well established in the case of monolithiconeycomb structures, but the potential of other promising and
nnovative structures have been pointed out in the literature [4].his is particularly the case of systems based on open-cell foamsalso known as sponges), innovative materials increasingly studiedn recent years and expected to be ready for the industrial take-
p [5,6]. Historically, structured catalysts were first developed fornvironmental applications where thermal effects are essentiallyegligible due to the extremely low concentration of reactants in∗ Corresponding author. Tel.: +39 02 23993248; fax: +39 02 70638173.E-mail address: [email protected] (C. Cristiani).
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2012.09.003
the feed stream (see e.g. the catalytic converters for vehicles). Thecurrent growing interest in their application to other processes isrelated to their properties also in relation to heat transport [7].Among many examples of processes that can possibly be improvedby the adoption of structured reactors, an important one, in viewof its industrial impact, is the production of synthesis gas (H2/COmixtures) from hydrocarbons [8], particularly via steam methanereforming (SMR). In fact, SMR is an endothermic process, and hightemperatures are necessary to activate the hydrocarbons, whichrequire high heat transfer rates. All the group VIII metals of theperiodic system are active for the process. In particular, Ni/MgAl2O4has been reported to be a good catalyst for SMR [9]. Ni-supportedcatalysts are the materials of choice commercially used in this pro-cess. Ni is more economic than noble metals, iron is oxidized atthe reaction conditions and metallic cobalt is unstable due to thehigh H2O/H2 ratios in the reaction mixture [10]. Industrial reform-ers consist of banks of tubes containing pelletized catalysts andlocated in a furnace. In this configuration, temperature gradientsinside the reactor tubes are strong both longitudinally and radi-ally [11]. Therefore, structured catalysts with thermally conductivesupports represent an alternative to conventional packed beds.
Many methods have been proposed to deposit the catalytic layer
on the surface of the structured support [12]. Typically, catalystdeposition requires two main steps: (1) support pretreatment, viaanodic oxidation, chemical activation or thermal treatment fol-lowed, in some cases, by a primer deposition, and (2) coating of the
sis Today 197 (2012) 256– 264 257
cd(lu
wuooflci2pdlctd
sdnosrfirtcni[tbc“ctpda
priepbnoTptic
2
pms
Rheological
measurementsDeposition:
Dip-Blowing or Percolation-Blowing
Load
AdhesionWashcoat
Powders
Foam pre-treatment:
Calcination at 900°C (10h)
Determination
of surface
sites and
pores volume
Slurry preparation:
Powder + Acid + Water
+ ball-milling 24 h
Flash drying
Thermaltreatments
pH value of 6–6.2 was measured. Then, the powders were filtered,washed and dried overnight under re-circulating air.
Table 1Characterization of foam samples.
Foam ε dc (m) dp (m) ds (m) SV (m−1)
C. Cristiani et al. / Cataly
atalyst on the metallic supports via dip coating, chemical vaporeposition (CVD), plasma spraying and electrophoretic depositionEPD) [12]. Among these methods, dip-coating from a sol or a slurryiquid phase is the most cost-effective and the simplest one to besed in practice.
In a previous paper the preparation of Pd/Al2O3-based catalystsashcoated onto metallic foams was reported by Giani et al. [13],sing a sol–gel-type procedure. The method makes use of a sol–gelf pseudoboehmite as a precursor of �-Al2O3. The open structuref the foam is filled by percolation while the excess of material isushed away with an air jet. The influence of solid content, acidontent and ageing on the sol–gel rheological behavior was stud-ed to find dispersions with an appropriate viscosity to deposit a0 �m thick washcoat layer. Foam samples of different nominalorosities were coated with this method and activated with palla-ium using a wet impregnation procedure (3%, w/w of palladium
oading). Despite the interesting results thus obtained, this methodould present some drawbacks, such as the need of a precursor ofhe sol–gel system and the use of a wet impregnation reaction toeposit the active phase.
Alternatively the washcoating can be performed starting from alurry of the solid powders with optimized rheology, followed byrying and calcination steps [14–23]. The preparation of the slurryeeds the dispersion of the powders using a ball-milling processr a high shear mixer in presence of a dispersant [17,20–23]. Theupport geometry determines the washcoating technique, whichequires a proper slurry rheology, which in turn is obtained byormulation [17,20–23]. The properties of the final washcoats, load-ng, thickness and adhesion, depend on the physico-chemical andheological properties of the suspensions and on the nature ofhe surface of the geometrical support [17]. The slurry rheologi-al behavior is governed by many parameters, such as the chemicalature of the solid, particle dimensions of the powder, powder load-
ng, dispersant concentration, temperature, viscosity modulators17,24,25]. On this basis, it is clear that a slurry is a complex sys-em, considered as a sort of “black-box”, where the relationshipsetween composition and physico-chemical properties are not solear. Accordingly, the slurry formulation is often a time-consumingtrial and error” process. Recently, attempts to rationalize this pro-ess have been reported in the literature. It has been found thathe viscosity of the suspensions can be designed by knowing theore volume and the surface acidity/basicity of the dispersed pow-ers, this last property being determined by both titration and FT-IRnalysis [20].
Accordingly, in this work the development of a simpleercolation-blowing method to deposit active, well adherent andobust washcoats, of Ni/MgAl2O4 catalysts, on FeCrAlloy is stud-ed, in view of the application of metallic open-cell foams as novelnhanced structured catalysts for methane steam reforming. Asrepared MgAl2O4 and Ni/MgAl2O4 powders were characterizedy pore volume measurements and by FT-IR and acid titration tech-iques in order to assess the acid/base and the charging propertiesf the surface so to obtain guidelines for the slurry formulation.he washcoated foams were characterized so to evaluate activehase loading and layer adhesion. CH4 steam reforming tests overhe washcoated foams were finally performed using diluted feedn order evaluate the catalytic activity under thermally controlledonditions.
. Experimental
Catalytic activation of a metallic foam (Fig. 1) is a multi-steprocedure that implies: (1) support characterization and pretreat-ent; (2) preparation and characterization of the powders, (3)
lurry preparation; (4) slurry deposition and thermal treatment.
Fig. 1. Scheme of the washcoating procedure.
In the following the experimental details adopted in this work foreach step are illustrated.
2.1. Supports characterization and pre-treatment
FeCrAlloy open-cell foams (supplied by Porvair), had a nominalcell density of 12 (S12) and 30 (S30) pores per inch (ppi) and a nom-inal relative density of 0.05. Foam void fractions and geometrieswere characterized by gravimetric analysis and optical microscopy,respectively. Results are reported in Table 1 along with estimatesof specific surfaces obtained according to the revisited tetrakaidec-ahedra model proposed in [26].
Before the coating process, the foams were calcined in air at900 ◦C for 10 h (heating/cooling rate 2 ◦C/min) to enable the migra-tion of �-Al2O3 to the surface [13,25,27], thus making the surfacemore suitable for washcoat deposition.
2.2. Powders preparation and characterization
The MgAl2O4 support (MgAl) was prepared via a co-precipitation method described elsewhere [28]. In a typicalpreparation, soluble nitrates of the constituents (magnesium andaluminum nitrates, Aldrich) are precipitated at pH = 7.5 withammonium carbonate (Aldrich) as precipitating agent. Upon filtra-tion, washing, overnight drying at 110 ◦C and calcination in air at900 ◦C for 10 h (heating/cooling rate 2 ◦C/min) the final material isobtained.
For the preparation of Ni/MgAl2O4 (Ni = 10%, w/w, (NiMgAl))powders two different routes were used for Ni ions deposition,namely wet and classical incipient wetness impregnation tech-niques. For the wet impregnation reaction, according to previouswork [13], the MgAl2O4 powder was contacted with a 10−3 M solu-tion of Ni(NO3)2 in a thermostated jacketed cell, at 60 ◦C undervigorous stirring for 3 h; 3%, 5% and 10% (w/w) of Ni nominal contentin the final catalyst was considered. For all the reactions a constant
S12 0.940 5.09 × 10−3 2.20 × 10−3 4.5 × 10−4 551S30 0.931 1.55 × 10−3 0.86 × 10−3 1.4 × 10−4 1476
ε, hydraulic void fraction; dc, cell diameter; dp, pore diameter; ds, strut diameter;SV, volumetric surface.
2 sis Tod
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adm
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amd
tFurotNcPCaa[
swm
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58 C. Cristiani et al. / Cataly
In the case of the dry impregnation, an aqueous solution of nickelitrate was dropped on the support up to the total pores filling andubsequently dried overnight at 110 ◦C in re-circulating air.
The dried powders were crushed in a mortar and were fired inir at 800 ◦C for 10 h, heating and cooling rate 2 ◦C/min.
Ni loading was determined by elemental chemical analysis withtomic absorption (Varian AA110 instrument) on the solid uponissolution for the dry-impregnated catalyst, and on both solid andother liquors for the wet-impregnated ones.Calcined samples were characterized by X-ray powder diffrac-
ion (XRPD) with a Bruker D8 instrument, using graphiteonochromated Cu K� radiation; the diffraction patterns were col-
ected in the 2� range 10–80◦ with a step of 0.05◦ and a countingime of 12 s per step.
Surface area and pore volume were determined by nitrogendsorption–desorption at 77 K (Micromeritics Tristar 3000 instru-ent). Pore volume was also determined by water adsorption after
rying the powders overnight at 110 ◦C in re-circulating air.Powders surface characterization was performed both by a titra-
ion method and FT-IR spectroscopy of adsorbed probe molecules.T-IR spectra have been recorded by Nicolet 380 FT instrument,sing a conventional IR cell connected to a gas manipulation appa-atus. The sample powders were pressed into self supporting disksf around 30 mg, activated in vacuum at 500 ◦C and cooled at roomemperature prior to any adsorption experiment. Reduction of theiMgAl powder has been carried on in the IR apparatus, using twoycles of pure hydrogen (500 Torr) and vacuum treatment at 500 ◦C.ivalonitrile (PN) adsorption, CO2 adsorption and low temperatureO adsorption were carried out to characterize surface acidic char-cter. Maximum surface charging of the materials was determinedccording to the titration methodology reported in previous work20,29].
Diffuse reflectance spectra (DR–UV–vis–NIR) of pure self-upported sample powder disks have been recorded in theavenumber range of 50,000–4000 cm−1 by a Jasco V-570 instru-ent at room temperature in air.
.3. Slurry preparation
Slurries with rheological behavior appropriate for the selectedoating technique were prepared by ball-milling the Ni/MgAl2O4owders using HNO3 as dispersant (HNO3/powder = 2.27 mmol/g),emineralized water as diluent (water = 110% of the powder poreolume), ZrO2 balls as grinding bodies (ZrO2/powder = 10 g/g), andollowing the experimental procedures previously developed byome of us [17]. The optimal amount of dispersant was determinedy titration to evaluate the maximum surface charging of the pow-er, while the water content was calculated taking into account theore volume of the powder [20].
The rheological behavior of the prepared slurry was character-zed in a rotational stress controlled rheometer Stresstech 500 fromheologica Instruments. The instrument used flat plates of 40 mmiameter and all the measurements were done at 20 ◦C. The flowurves were collected both on the fresh slurry, e.g. immediatelyfter the ball-milling process, and after some days of in-pot ageingo verify the slurry stability.
.4. Washcoat deposition and thermal treatments
The washcoat was deposited onto the foams by dip-blowingDB) and percolation blowing (PB) [13,30,31]. In the case of dip-lowing process, foams were dipped in the slurry, while in the
ercolation blowing technique the foams were placed inside a tubehere an excess of slurry was poured. The slurry was drained at aontrolled velocity while percolating through the foam. The lin-ar velocity of percolating slurry was 1.5 cm/min, controlled with
ay 197 (2012) 256– 264
a vacuum pump. In both processes the excess of slurry trappedin the foam cells was removed by an air jet pumped at 5 bar for10 s (blowing rate 1.2 m/s), which left a thin layer around the foamstruts. Coated foams were flash dried in a preheated oven at 553 Kfor 5 min and then calcined at 1173 K for 10 h.
Adhesion was quantified by measuring the weight loss aftertreatment in the ultrasonic bath for 30 min according to [17,22].
Finally, the morphology of the washcoated foams was analysedby optical microscopy using an Olympus SZ4045TR instrumentwith 40× maximum magnification.
In the following the washcoated foams will be identified as S12-DB, S12-PB and S30-PB.
2.5. Activity tests in steam reforming reaction
Activity tests of the washcoated samples in the steam methanereforming reaction were run in a 10 mm inner diameter quartztubular microreactor, externally heated by a three zones Carbo-lite oven. The reactor was loaded with a single foam sample, whichtightly fitted the quartz chamber. The upper part of the reactor wasfilled with quartz granules to improve reactants mixing and flowdistribution. The temperature of the catalyst was measured by afixed k-type thermocouple located just at the foam inlet.
Space velocity was kept at 130,000 N cm3/h/gcat (referred tothe washcoat load) and pressure at 1 bar. Before the activity tests,loaded foams were reduced in a 5% (v/v) H2 flow in N2, adoptinga space velocity of 140,000 N cm3/h/gcat. During the reduction, thetemperature was increased up to 1123 K (heating rate 5 K/min),kept at this level for 3 h, and then decreased to 293 K. Reformingtests were carried out following this protocol: the pre-reduced cat-alyst was heated up from room temperature to 923 K in a 5% (v/v) H2flow in N2, the reacting mixture (CH4 = 1.5% (v/v), H2O = 4.5% (v/v),N2 complement) was fed, the catalyst was gradually heated up to1123 K and from here the temperature was gradually decreaseduntil 673 K according to a temperature step procedure, measuringthe composition of the outlet stream at constant T-intervals (50 ◦C).Products composition was measured by means of an on-line micro-GC (3000 A, Agilent Technologies) equipped with 2 TCDs connectedto a Molecular Sieve 5 A column (180 ◦C, Ar carrier gas) and a Plot Qcolumn (160 ◦C, N2 carrier gas) for the identification of N2, H2, O2,CH4, CO, CO2 and H2O, respectively. In all the runs, the balances ofC, H and O in the product gas closed to within ± 5%.
3. Results and discussion
3.1. Characterization of the slurry precursor powders: MgAl2O4
The first attempt to obtain activated foams to be tested in theSMR reaction was performed following the procedure reportedby Giani et al. [13], which implies the deposition of the morpho-logical support onto the structured carrier and the subsequentwet-impregnation of the active phase.
Before proceeding with the washcoating process, characteriza-tion of the powders was performed.
Upon thermal treatment at 900 ◦C MgAl consists of a crys-talline MgAl2O4 single phase [JCPDS 21-1152] (Supplementaryinformation 1) with surface area of 144 m2/g and pore volume of1.1 g/cm3 (Table 2).
Skeletal FT-IR spectra and surface species spectra(Supplementary Information 2) are completely consistent withthe spinel structure of this material. Moreover, the activated MgAl
sample spectrum shows peaks due to stretching vibration of freehydroxyl groups centred at 3728 cm−1 (split), with shouldersat 3770 and 3790 cm−1. Similar features have been reported byMorterra et al. [32], Rossi et al. [33] and Busca et al. [34] and are
C. Cristiani et al. / Catalysis Today 197 (2012) 256– 264 259
Table 2Morphology, phase composition and surface charging of MgAl2O4, Ni/MgAl2O4 and Al2O3 as reference (* [20]).
Sample Surface Area (m2/g) Vp (cm3/g) Phase composition (XRD, FT-IR, UV–vis) Surface charging (C/m2)
MgAl 144 1.1 MgAl2O4 1.5
NiMgAl (Ni = 10%, w/w) 132 0.72 MgAl2O4 (bulk) 1.5
AlO* (Puralox-140) 140 1.2
0,05 a.u.
Abs
orba
nce
12001400160018002000
Wavenumbers (cm-1 )
NiMgAl
AlO
MgAl
Fof
apcciacshbtpcsa
Fa
ig. 2. FT-IR subtraction spectra of surface species arising from CO2 adsorption andutgassing at room temperature over MgAl2O4 and Ni/MgAl2O4 catalysts and Al2O3
or comparison. The activated surface spectrum has been subtracted.
scribed to OH bound to octahedral Al ions, possibly split by theresence of OH groups coordinated over Mg2+. The high frequencyomponents (3770 and 3790 cm−1) are likely assigned to OHoordinated over exposed tetrahedral Al ions and due to the partialnversion of the spinel structure [33,34]. A further broad absorptiont 3690–3670 cm−1 can be assigned to bridging OH groups. Thehemisorption of CO2 as spectroscopic probe molecule for basicites, leading to surface carbonate and bicarbonate formation,as been deeply discussed by several authors [33,35] and appliedy us to better understand the behavior of catalyst powders inhe formulation of slurries for washcoating processes [36]. In a
revious work [20], some of us have shown that different surfaceharging can be related to the different amount of surface basicites, as revealed by CO2 adsorption. These sites are those that canctually be titrated by HNO3 aqueous solution.0.02 a.u.
Abs
orba
nce
215022002250230023502400
Wavenumb
2
22932281
2235
Abs
orba
nce
a) MgAl
PN at r.t.
outgassed r.t. 30’
outgassedr.t.
ig. 3. FT-IR subtraction spectra of surface species arising from pivalonitrile (PN) adsorptctivated surface spectrum has been subtracted.
NiAl2O4, NiO (surface)
�-Al2O3 1.1
Over MgAl, CO2 adsorption at room temperature gives rise tothe spectrum shown in Fig. 2. The main sharp bands at 1652, 1436and 1228 cm−1 are due to stretching and deformation vibrationalmodes of HCO3
− species adsorbed at the surface, formed by theinteraction of CO2 with nucleophilic centres (hydroxyls and O2−
basic sites), while weaker and broader components at 1595 and1400 cm−1 are assigned to carbonate species, coordinated at thesurface as bidentate and bridging species. All these bands are resis-tant to outgassing up to 100 ◦C. Taking into account the intensityof the band at 1228 cm−1 for the determination of relative basicityof our different metal oxides, the comparison with the spectrumof CO2 adsorbed over pure alumina (AlO, Fig. 1) points out that Mgaluminate surface is only slightly more basic than alumina [32–35].
Adsorption of PN over the activated catalyst gives rise to thespectra reported in Fig. 3(a). In the 2400–2100 cm−1 region, bandsdue to CN stretching mode of the coordinated nitrile molecule overacidic centres fall. The sharp band at 2235 cm−1, decreasing uponoutgassing is assigned to PN weakly interacting with OH groups.The quite complex band with two maxima at 2293 and 2281 cm−1 isresistant to outgassing up to 200 ◦C. The frequencies are consistentwith the assignation to Lewis acidic centres of different strength,in agreement with data reported by Morterra et al. [32] on pyri-dine adsorption over aluminate surface. The former component isconsistent with the detection of Al ions exposed at the surface,and its frequency almost match the CN stretching frequency ofPN adsorbed over transitional alumina AlO [36] whereas the lattercomponent is ascribed to weaker Lewis sites. These findings furtherconfirm the defective structure of the spinel surface, in agreementwith data from OH region analysis. Over the same sample, low tem-perature CO adsorption gives rise to a main band at 2181 cm−1
and to a shoulder at 2154 cm−1, which is due to CO weakly inter-acting with the surface hydroxyls, and readily disappearing uponoutgassing (Fig. 4(a)). The detection of negative bands in the OH
stretching region confirms this assignment. The former compo-nent decreases in intensity following outgassing upon warmingand shifts to higher frequency (up to 2200 cm−1), showing a typ-ical behavior of CO linearly coordinated over surface oxides. Theers (cm-1 )
0.05 a.u.
21502200225023002350400
2290
2283
2235
b) NiMgAl
outgassedr .t. 30’
PN at r.t.
outgasse d r.t.
ion and outgassing at room temperature over (a) MgAl2O4 and (b) Ni/MgAl2O4. The
260 C. Cristiani et al. / Catalysis Today 197 (2012) 256– 264
0,02 a.u.
Abs
orba
nce
190020002100220023002400
Wavenumbers(cm -1 )
2154
2181
2200-10°C
-140°C
0,05 a.u.
Abs
orba
nce
190020002100220023002400
15°C
-140°C
2155
2180
2184
2190
2075
21280,005 a.u.
Abs
orba
nce
190020002100220023002400Wavenumbers(cm-1)
2186a) MgAl b) NiMgAl
F liquidN agnifi
qC[u
Fat
es(scsross
i
ig. 4. FT-IR subtraction spectra of surface species arising from CO adsorption ati/MgAl2O4 catalysts. The activated surface spectrum has been subtracted (inset: m
uite large range band width is consistent with the coordination ofO molecules both over Mg ions (reported frequency: 2200 cm−1)32] and over Al ions (around 2180 cm−1) acting as coordinativensaturated surface centres.
Surface charging (q), measured by solid titration, is reported inig. 5. The maximum surface charging of MgAl, 1.5 C/m2, is reachedt pH below 1. This value is similar to that of 1.1 C/m2 measured forhe AlO reference [20].
Considering the close similarity between MgAl and AlO ref-rence samples (see Table 1, and surface characterization), theame slurry formulation was adopted (e.g. H2Oexternal/powderg/g) = 0.86 and HNO3/powder (mmol/g) = 2.16). In case of AlO, thislurry composition resulted in a quasi-Newtonian fluid with vis-osity of 40 mPa s (at shear rate 10 s−1), while, in the case of MgAllurry, higher viscosity values were measured in all the investigatedange (370 mPa s, at shear rate 10 s−1), too high to be used in dip-r percolation-blowing techniques. Further attempts to dilute the
lurry resulted in unstable systems in which powders progressivelyeparated from water.Furthermore, to select the operating conditions for the wetmpregnation of Ni ions, some experiments were performed on
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
MgAl
q [C
/m2 ]
pH
NiMgAl
Fig. 5. Surface charging of MgAl2O4 (triangles) and Ni/MgAl2O4 (squares).
nitrogen temperature and outgassing upon warming over (a) MgAl2O4 and (b)cation of the carbonyl stretchimg region).
MgAl powder using 3%, 5% and 10% (w/w) of Ni nominal catalystloadings. A constant Ni adsorption of about 2% (w/w) was obtained,independently of the initial Ni amount. Considering that a reactionpH of 6.2, relatively high, was measured in all the experiments, itcan be concluded that these reaction conditions are not effective forthe hydrolysis of Ni ions. Moreover, even the careful addition of astrong base, namely KOH, to increase pH, or the increase of the reac-tion temperature, parameters that are known to favor hydrolysisreaction, did not result in any further improvement of Ni adsorp-tion.
On these bases, this washcoating procedure was abandoned,and an alternative one, that consists of the deposition of the pre-impregnated Ni/MgAl2O4 powder, was attempted.
3.2. Characterization of the slurry precursor powders: NiMgAl
NiMgAl fresh powders were fully characterized, too.
NiMgAl powders consist of a crystalline MgAl2O4 single phase[JCPDS 21-1152] (Supplementary information 2). The reflectionsof the impregnated powder are totally overlapped to those of the
101 100 1000
1
10
100
1000
10000
1 weak
4 days
24 h
0 h
visc
osity
[mPa
*s]
shear rate [s-1]
range of interest
Fig. 6. Flow curves of Ni/MgAl2O4 slurry at different ageing times.
C. Cristiani et al. / Catalysis Today 197 (2012) 256– 264 261
ms w
sp
m
bf5ss4rs6dp
NN
w
otsth1obht1p
mncnrbotdp
Cia
Fig. 7. Optical microscopy images (20×) of the foa
upport material, suggesting that a high dispersion of the activehase was obtained.
A surface area of 132 m2/g, and a pore volume of 0.7 g/cm3 wereeasured (Table 2).In the FT-IR skeletal spectra of the NiMgAl sample, the main
ands due to the spinel structure are slightly shifted to lowerrequencies and a further absorption seems to grow below00 cm−1. These modifications suggest the presence of an inversepinel NiAl2O4 phase [37]. Moreover, the subtraction [NiMgAlpectrum] − [MgAlO spectrum] evidenced a broad band around50 cm−1 which can be associated with some NiO formation notevealed by XRD [38] (Supplementary information 3). On the otheride, the DR–UV–vis analysis of this sample shows two peaks at00 and 634 nm, attributed to Ni2+ in octahedral sites and tetrahe-ral sites, respectively, confirming the formation of a Ni aluminatehase (Supplementary Information 4) [39].
All these results confirmed the presence of a highly dispersedi ions that was already pointed out by the absence of crystallinei-phases in the XRD measurements.
Also in this case, the surface characterization of the fresh catalystas performed by both FT-IR and titration analysis.
Surface OHs, whose IR bands are well defined in the spectrumf NiMgAl catalyst, are not perturbed by the dry impregna-ion process. Like for the CO2 adsorption, addition of Ni in theample results in a quite complex spectrum showing, besideshe same spectroscopic features described above and due toydrogenocarbonate/carbonate species, several components in the700–1800 cm−1 range, which have been assigned to a mixture ofrganic-like carbonate species [36] (Fig. 2). Moreover, the mainands of hydrogenocarbonate species show weak shoulders atigher frequencies, possibly due to slightly different coordina-ion sites at the surface. The intensity of the diagnostic band at230 cm−1 allows us to evaluate the basicity of the surface nucleo-hilic groups to be almost the same as the Mg-aluminate support.
On the other side, PN adsorption leads to the detection of twoain bands at 2283 and 2235 cm−1, the latter component due to
itrile molecules H-bound to hydroxyl groups, whereas the formeromponent due to nitrile molecules coordinated over Lewis sites,amely exposed Al3+ ions. The comparison of the two spectral serieseported in Fig. 3(b) evidences that the split of the high frequencyand, well resolved in the MgAl spectra, is barely detectable, andnly after prolonged outgassing, in the NiMgAl spectra. Moreover,he frequency of this band is slightly lowered. According to theseata, the Lewis acidity could be slightly reduced by Ni deposition,ossibly by the formation of Ni aluminate phase.
The spectra of surface species arising from low temperatureO adsorption over the fresh NiMgAl sample are also reported
n Fig. 4(b). Although the hydrogen-reduction step is usuallypplied in this kind of experimental set up, CO adsorption over the
aschoated via the percolation-blowing procedure.
outgassed surface “as such” leads to useful information on surfacespecies, providing it is carried out at liquid nitrogen temperature[40,41]. In this case, we have chosen to perform CO adsorptionover the freshly prepared catalyst powder to compare results onexposed ions with results obtained on the bare support, from thepoint of view of acidic–basic properties.
At the lowest temperature, two different bands were detectedat 2184 and 2155 cm−1, ascribed to CO linearly coordinated overLewis sites and H-bound to hydroxyl groups. At decreasing cover-age the band located at the highest frequency clearly splits in twocomponents: the shoulder at 2184 cm−1 considerably decreases,shifting to 2200 cm−1, while the shoulder at lower frequenciesappears more resistant to outgassing and shifts to 2186 cm−1. Theyare assigned to CO species interacting with Lewis acidic Al3+ cationsof medium strength. On the other side, bands due to carbonyls overNi2+ ions (�-bonds and �-type backbonding) like of NiO particlesor NiAl2O4, reported in the range of 2180–2160 cm−1 could also fallin this spectral region [40].
The detection of a CO peak at lower frequency with respect tothe Mg aluminate surface, points out that the addition of Ni to themixed metal oxides results in a moderate decrease of Lewis acidity,in agreement with data from PN adsorption and as reported byOtero Arean et al. for Ni aluminate catalysts having different Nickelcontents [42].
The magnification of the lower frequency region (Fig. 3(b) inset)shows two additional components at 2075 and 2128 cm−1 asso-ciated with the formation of Ni+(CO)2 complexes, characterizedby adsorption bands at 2145–2130 cm−1 and 2100–2081 cm−1 andreported to be much more stable than Ni2+-CO complexes [43]. Thedetection of these species, formed after outgassing at 500 ◦C, pointsout the existence of easily reducible Ni species exposed at the cat-alyst surface and possibly due to Ni oxide. In order to study thesurface Nickel species active in the tested steam reforming reaction,CO adsorption has also been performed over the reduced pow-der and the corresponding spectra are reported and discussed inSupplementary information 3. In sum, FT-IR results suggest theexistence of various Nickel species exposed at the surface, hav-ing different reducibility properties. Ni ions are still detected, quiterefractory to reduction, possibly in the aluminate phase, togetherwith atomically dispersed zerovalent nickel and small Ni clustersstrongly interacting with the surface, characterized by polycar-bonyl bands in the IR spectra [41]. The formation of such speciesis in agreement with the not exceedingly high Ni loading, how-ever, a small fraction of larger Ni particles can also be detected,characterized by bridging carbonyl band below 2000 cm−1.
Surface charging was quantified by titration with HNO3. Accord-ing to FT-IR characterization, similar surface charging behaviorswere found for NiMgAl and MgAl (Fig. 5). In particular, the samevalue of maximum surface charging, q = 1.5 C/m2, is measured
262 C. Cristiani et al. / Catalysis Today 197 (2012) 256– 264
(a) (b)
F nt reG
bidp
3
dtosidf(wtrd
psppp
d1h
3
gtm
TCp
ig. 8. Reactant conversions (a) and product distributions (b) obtained at differeHSV = 130,000 N cm3/h/gcat, H2O/CH4 = 3 mol/mol, yN2 = 0.94).
elow pH = 1. This suggests that the nature of the surface of NiMgAls primarily determined by the support and the presence of highlyispersed Ni ions does not affect its behavior towards charginghenomena.
.3. Preparation of the NiMgAl slurry
The NiMgAl slurry was prepared according to the procedureetailed in the Section 2. HNO3 was added to the powders to reachhe maximum dispersion, the H2O amount was initially set to 110%f the total pore volume, and the mixture was ball-milled for 24 h. Aemi-solid material, was obtained. Accordingly, to reach the viscos-ty values typical of dip- or percolation blowing process, a stepwiseilution of the slurry was performed by repeated addition of waterollowed by 1 h of ball-milling each. The final slurry of compositionH2Oexternal/powder) = 1.38 g/g and (HNO3/powder) = 2.27 mmol/gas finally ball-milled for 24 h. Flow curves at different ageing
ime are reported in Fig. 6. The curve at time = 0 h (triangles) rep-esents the rheological behavior obtained immediately after theilution-milling process described above.
Upon the dilution-milling process, a slightly non-Newtonianseudo-plastic slurry is obtained, with viscosity of 60–70 mPa s athear rate 10 s−1, that is suitable for both DB and PB washcoatingrocess. The viscosity of the slurry progressively increases upon in-ot ageing possibly due to the evolution of the sol–gel system thatartially forms upon the milling process [22].
Differently from MgAl sample, the NiMgAl slurry is very stable: itoes not separate when diluted and it does not separate even after
week of in-pot ageing. However, for the deposition techniquesere selected, the slurry at t = 0 h has the right viscosity.
.4. Deposition of the NiMgAl slurry
Slightly different viscosities were needed due to the differenteometry of the foams, namely lower viscosity are required forhe foams with the higher cell density. Acceptable coverage of the
etallic structures was obtained for both geometries (Fig. 7).
able 3oating load and adhesion as function of support characteristics, operating parameters aercolation-blowing).
Sample Foam nominaldensity (ppi)
�SR 10 s−1 (mPa s) Deposition techniqu
S12-DB 12 60 DB
S12-PB 12 60 PB
S30-PB 30 10 PB
action temperatures in steam-reforming runs (experimental conditions P = 1 bar,
Loads and adhesions of the washcoats were determined on thewashcoated foams calcined at 800 ◦C, results are summarized inTable 3. Both deposition techniques resulted in reasonable wash-coat loadings (about 5–6%, w/w) with acceptable adhesion (weightlosses about 10%) on foams with the same geometry (comparedS12-DB and S12-PB in Table 3). On increasing the pores density(S30-PB) a higher washcoat load was obtained (11%, w/w), whichcorrelates well with the higher surface to volume ratio of the 30 ppifoam. Also for this sample a good adhesion was found, being theweight loss upon sonication about 11%, w/w.
Considering that the dip-blowing process is less reproducibleand hardly applicable to large samples, the highly reproduciblepercolation-blowing process seems the technique of choice forfuture industrial applications and the dip-blowing procedure con-fined to laboratory practice.
3.5. Catalytic activity tests
The washcoated S30-PB foam with the highest catalyst load wastested in CH4 steam reforming runs.
Tests were performed under diluted conditions in order to avoidstrong temperature variations in the reactor so to obtain informa-tion on the activity of the catalyst as a function of the reactiontemperature.
Reactant conversions and product distributions obtained atdifferent reaction temperatures are shown in Fig. 8(a) and (b).Conversion of CH4 closely approached thermodynamic equilibriumalready at 450 ◦C, evidencing a very good steam reforming activ-ity of the washcoated catalyst. Also H2O conversion approachedthe equilibrium curve in all the investigated temperature rangeindicating that global equilibrium, including water gas shift, wasreached. This was also evidenced by product distributions showinga maximum in H2 production at about 600 ◦C, whereas CO outlet
concentration monotonously increased with temperature. Due toits endothermicity (�HR = 206 kJ/mol), in fact, in according to the LeChatelier’s principle, the steam methane reforming chemical equi-librium is increasingly shifted toward the products upon raisingnd the deposition technique (catalyst powder: Ni/MgAl2O4; DB, dip-blowing; PB,
e Blowing rate (m/s) Washcoat loading(%, w/w)
Weight loss aftersonication (%, w/w)
1.2 6.0 101.2 4.5 111.2 11 10
C. Cristiani et al. / Catalysis Today 197 (2012) 256– 264 263
after
tiarc
dVttuik
4
(
(
(
(
[
[
[[
[
[[
[
[[
[
[[
Fig. 9. Optical microscopy image of the S30-PB foam
he reaction temperature. On the contrary, due to its exothermic-ty (�HR = −41.1 kJ/mol), the water gas shift extent of reaction (topproach the chemical equilibrium) decreases with increasing theeaction temperature, thus limiting the amount of CO and H2Oonsumed to form H2 and CO2.
After unloading of the used foam sample, weight measurementsid not show significant variations with respect to the fresh sample.isual inspection of the sample (Fig. 9) confirmed the integrity of
he washcoat layer along with blackening possibly due to Ni reduc-ion. No traces of catalyst dusts were found in the reactor afternloading and no significant weight losses were detected upon son-
cation of the tested foam, proving the ability of the washcoat toeep adhesion upon reduction and reaction treatment.
. Conclusions
1) A methodological approach previously proposed has beenapplied and implemented to deposit active washcoats ofNi/MgAl2O4 on complex structures such as FeCrAlloy open-cellfoams. The washcoating process requires proper slurry rheo-logy and stability that can be obtained via formulation of theslurry composition. A combined bulk and surface characteriza-tion of the material can allow an “a priori” design of the slurryrheology, starting from the knowledge of the pore volume andof the surface acidity/basicity of the powders. In particular, thecombined use of quantitative acid/basic sites titration and FT-IRsurface characterization of the fresh powder allows the calcu-lation of the dispersant amount, in this case HNO3, Indeed, it ismore difficult to be determined “a priori” than water, which iseasily correlated to the pore volume. This simple approach isapplicable to a wide class of catalytic powders and structuredsupports.
2) The washcoat layer can be deposited on FeCrAlloy foamsvia both dip- and percolation-blowing techniques. The highlyreproducible percolation-blowing process seems the techniqueof choice for future industrial applications of this kind of geo-metrical supports.
3) Good washcoat loadings can be obtained that result in anacceptable catalytic covering of the metallic foam matrix andin a good adhesion of the active layer. No weight variationsafter activation and catalytic tests were measured and negligi-ble amounts of dust were found in the reactor after the tests,
suggesting a reasonable adhesion of the washcoat layer to thesupport during testing under reaction conditions.4) The prepared catalysts were found to be active in the methanesteam reforming process, allowing approach to thermodynamicequilibrium above 450 ◦C with high space velocity.
[
[
[
catalytic tests. Left panel (10×); right panel (20×).
Acknowledgment
Student Giulia Baracchini is acknowledged for her help in theexperimental work.
Appendix A. Supplementary data
Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.cattod.2012.09.003.
References
[1] E. Tronconi, G. Groppi, Catalysis Today 147 (2009) S1.[2] M.T. Kreutzer, F. Kapteijn, J.A. Moulijn, Catalysis Today 111 (2006) 111.[3] G. Groppi, A. Beretta, A. Tronconi, in: A. Cybulski, J.A. Moulijn (Eds.), Structured
Catalysts and Reactors, 2nd ed., Taylor & Francis, Boca Raton, 2006, p. 243.[4] A. Cybulski, J.A. Moulijn, Structured Catalysts and Reactors, Taylor & Francis,
Boca Raton, 2006.[5] L. Giani, G. Groppi, E. Tronconi, Industrial and Engineering Chemistry Research
44 (2005) 4993.[6] J. Grosse, B. Dietrich, G.I. Garrido, P. Habisreuther, N. Zarzalis, H. Martin, M.
Kind, B. Kraushaar-Czarnetzki, Industrial and Engineering Chemistry Research48 (2009) 10395.
[7] Z. Dai, K. Nawaz, Y.G. Park, J. Bock, A.M. Jacobi, International Communicationsin Heat and Mass Transfer 37 (2010) 575.
[8] L.L. Smith, H. Karim, M.J. Castaldi, S. Etemad, W.C. Pfefferle, Catalysis Today 117(2006) 438.
[9] H.S. Roh, K.Y. Koo, J.H. Jeong, Y.T. Seo, D.J. Seo, Y.S. Seo, W.L. Yoon, S. Bin Park,Catalysis Letters 117 (2007) 85.
10] J.R. Rostrup-Nielsen, in: J.R. Anderson, M. Boudart (Eds.), Catalysis, Science andTechnology, Springer, Berlin, 1984, p. 5.
11] F. Basile, P. Benito, G. Fornasari, V. Rosetti, E. Scavetta, D. Tonelli, A. Vaccari,Applied Catalysis B: Environmental 91 (2009) 563.
12] V. Meille, Applied Catalysis A: General 315 (2006) 1.13] L. Giani, C. Cristiani, G. Groppi, E. Tronconi, Applied Catalysis B: Environmental
62 (2006) 121.14] C. Agrafiotis, A. Tsetsekou, Journal of the European Ceramic Society 20 (2000)
815.15] C. Agrafiotis, A. Tsetsekou, Journal of Materials Science 35 (2000) 951.16] B.J. Briscoe, A.U. Khan, P.F. Luckham, Journal of the European Ceramic Society
18 (1998) 2141.17] M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi, P. Forzatti, Catalysis
Today 69 (2001) 307.18] J. Davies, J.G.P. Binner, Journal of the European Ceramic Society 20 (2000) 1539.19] L.C. Guo, Y. Zhang, N. Uchida, K. Uematsu, Journal of the European Ceramic
Society 17 (1997) 345.20] C. Cristiani, C.G. Visconti, E. Finocchio, P.G. Stampino, P. Forzatti, Catalysis Today
147 (2009) S24.21] C. Cristiani, A. Grossale, P. Forzatti, Topics in Catalysis 42–43 (2007) 455.22] C. Cristiani, M. Valentini, M. Merazzi, S. Neglia, P. Forzatti, Catalysis Today 105
(2005) 492.23] A.A. Tracton, Coatings Technology Handbook, Taylor & Francis, Boca raton, FL,
2005.24] H. Mollet, A. Grubenmann, Formulation Technology: Emulsions, Suspensions,
Solid Forms, Wiley-VCH, Weinheim; New York, 2001.25] V. Meille, S. Pallier, G.V.S.C. Bustamante, M. Roumanie, J.P. Reymond, Applied
Catalysis A: General 286 (2005) 232.
2 sis Tod
[
[
[
[
[
[
[
[
[[[
[
[[[
[
64 C. Cristiani et al. / Cataly
26] A. Inayat, H. Freund, T. Zeiser, W. Schwieger, Chemical Engineering Science 66(2011) 1179.
27] J.S. Jia, J. Zhou, H.U. Zhang, Z.S. Yuan, S.D. Wang, Applied Surface Science 253(2007) 9099.
28] G. Groppi, C. Cristiani, P. Forzatti, M. Bellotto, Journal of Materials Science 29(1994) 3441.
29] W. Janusz, Electrical double layer at oxide-solution interfaces, in: Encyclopae-dia of Surface and Colloid Science, Marcel Dekker, New York, 2002, p. 1687.
30] C.G. Visconti, E. Tronconi, L. Lietti, G. Groppi, P. Forzatti, C. Cristiani, R. Zennaro,S. Rossini, Applied Catalysis A: General 370 (2009) 93.
31] C.G. Visconti, Mileston in Chemistry Supplement to Chemistry Today 29 (2011)
9.32] C. Morterra, G. Ghiotti, F. Boccuzzi, S. Coluccia, Journal of Catalysis 51 (1978)299.
33] P.F. Rossi, G. Busca, V. Lorenzelli, M. Waqif, O. Saur, J.C. Lavalley, Langmuir 7(1991) 2677.
[
[
ay 197 (2012) 256– 264
34] G. Busca, V. Lorenzelli, G. Ramis, R.J. Willey, Langmuir 9 (1993) 1492.35] J.C. Lavalley, Catalysis Today 27 (1996) 377.36] A. Vimont, J.C. Lavalley, A. Sahibed-Dine, C.O. Arean, M.R. Delgado, M. Daturi,
Journal of Physical Chemistry B 109 (2005) 9656.37] G. Busca, V. Lorenzelli, V.S. Escribano, R. Guidetti, Journal of Catalysis 131 (1991)
167.38] G. Busca, V. Lorenzelli, V.S. Escribano, Chemistry of Materials 4 (1992) 595.39] N. Bayal, P. Jeevanandam, Journal of Alloys and Compounds 516 (2012) 27.40] G. Garbarino, V. Sanchez Escribano, E. Finocchio, G. Busca, Applied Catalysis B:
Environmental 113 (2012) 281.41] M. Garcia-Diéguez, E. Finocchio, M.A. Larrubia, L.J. Alemany, G. Busca, Journal
of Catalysis 274 (2010) 11.42] C.O. Arean, M.P. Mentruit, A.J.L. Lopez, J.B. Parra, Colloid Surface A 180 (2001)
253.43] K. Hadjiivanov, H. Knozinger, M. Mihaylov, Journal of Physical Chemistry B 106
(2002) 2618.
