Journal of Catalysis 311 (2014) 153–160

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Journal of Catalysis

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Palladium-catalyzed selective hydrogenation of nitroarenes: Influenceof platinum and iron on activity, particle morphology and formationof b-palladium hydride

0021-9517/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcat.2013.11.019

⇑ Corresponding author. Fax: +49 6181 59 3554.E-mail address: peter.albers@aqura.de (P.W. Albers).

K. Möbus a, E. Grünewald a, S.D. Wieland a, S.F. Parker b, P.W. Albers c,⇑a Evonik Industries, Business Line Catalysts, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germanyb ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdomc AQura GmbH, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germany

a r t i c l e i n f o

Article history:Received 5 August 2013Revised 7 November 2013Accepted 19 November 2013Available online 24 December 2013

Keywords:Catalyst–hydrogen interactionSelective hydrogenationNitroarenesPalladiumAlloying

a b s t r a c t

The influence of alloying supported palladium particles of 2 nm size with platinum and iron on the cat-alytic activity for the selective hydrogenation of nitrobenzene (NB) was studied. We show that the use ofa carbon black support of enhanced sp2 character has a marked ‘templating’ effect on the location of thepalladium particles, which are preferentially deposited at the edge sites of the carbon support surface.Alloying with platinum and iron leads to disaggregation down to isolated primary particles, and thishas a major effect on the catalytic activity (Pd 6.6, PdPt 36.9, PdPtFe 23.1 mmolNB min�1), as well as onthe relative amounts of b-palladium hydride formed (normalized peak integrals: Pd 127.0, PdPt 87.6,PdPtFe 27.2). The results enable a greater understanding of how better performance can be obtained incatalysts by appropriate choice of support, particle size, alloying and adjustment of hydrogen storagecapability.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

The catalytic hydrogenation of nitroarenes, for example, ofnitrobenzene to aniline or of dinitrotoluenes (DNT) to toluene dia-mines (TDA), is a major industrial activity [1–3]. TDA is a key com-ponent in the production of toluene diisocyanate (TDI) for themanufacture of polyurethane (PU) [4]. These reactions illustratethe need and economic impact of steering and optimizing catalystselectivity while maintaining adequate catalyst activity. It can beeconomically favorable to lower the reaction pressure andtemperature by improving the catalyst’s properties. The reactionrate and selectivity in the hydrogenation can be enhanced bychanging from nickel- to palladium-based catalysts. Usage of othernoble metals such as platinum and additional promoters can fur-ther improve reaction efficiency and lower the amount of catalystneeded [5]. The addition of platinum to palladium increases activ-ity, and the addition of iron increases selectivity: the enhancementof activity by the addition of platinum is moderated by iron inorder to avoid aromatic ring hydrogenation [6–8]. The addition ofPt and Fe results in better selectivity and long-term stability onthe large-scale production level as was reported by DuPont in thelate 1960s that a Pd–Pt–Fe/C trimetallic catalyst was becoming a

‘workhorse for the hydrogenation of aromatic nitro compounds’[6]. Optimization of the carbonaceous support materials has ledto the use carbon black-based catalysts which have a better pre-cious metal–support interaction and improved catalytic activityas compared to the usual activated carbon-type supports [9].

The aim of the present work was to improve the understandingof the factors that affect the activity in palladium-based catalystsused for the hydrogenation of nitrobenzene. This requires knowl-edge of the subtle interdependence between the composition, thesize and the size distribution function of the supported preciousmetal entities. In particular, the impact of alloying promoting addi-tives on the catalyst particle morphology and the correspondinghydrogen interaction properties was the focus of this investigation.The transparency of most materials combined with the selectivityto hydrogen in neutron scattering experiments allows the determi-nation of the hydrogen-related properties of macroscopic amountsof hydrogenated catalyst and support material. In the presentinvestigation, the inelastic incoherent neutron scattering (IINS)measurements of 13–19 g of material provide statistically mean-ingful data on catalysts which have been produced and appliedfor decades. The multi-technique approach adopted here using IINSin combination with transmission electron microscopy (TEM), en-ergy dispersive X-ray nano-analyses (EDX), electron energy lossspectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) andcatalyst testing addresses all these aspects to build a comprehen-sive picture of the role of each of the components of the catalyst.

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2. Experimental

2.1. Materials

The catalysts were manufactured by Evonik Industries bystrictly following the wet precipitation procedure reported andpatented in [10]. All the catalyst samples in this study were pre-pared using the same lot of carbon black as support and usingthe same preparation procedure. Aniline (99%) and nitrobenzene(99%) were purchased from Sigma–Aldrich. Hydrogen (>99.999%)was supplied by Linde.

2.2. Hydrogen content

The hydrogen content of the carbon black-type catalyst supportwas determined by hot extraction using a LECO TCH600instrument.

2.3. Surface area

The surface area was determined by the Brunauer, Emmett andTeller method (BET).

2.4. Transmission electron microscopy (TEM)

A Jeol 2010F field emission transmission electron microscopewas operated at 200 keV acceleration voltage. For spot analysesof the supported metal particles and characterization of binary orternary alloying effects, energy dispersive X-ray (EDX) spot analy-ses at the nanoscale of the supported particles and the support par-ticles were performed using a Noran SiLi detector with a 30-mm2

crystal and a Noran System Six device. For examination, a catalystsample was dispersed in chloroform and transferred onto holeycarbon foil supported by a 200-mesh copper grid. For the statisticalevaluation of the primary particle sizes of the supported primaryparticles, the I-TEM software of Soft Imaging Systems (SIS, Mün-ster, Germany) was utilized. The average primary particle sizes ofthe supported Pd-based catalyst particles were determined by sta-tistical evaluation of 2000 particles in TEM images per catalystsample. In the catalysts Pd1–Pd3, no additional fraction of coarseprecious metal particles was observed in the TEM. The quality, sta-bility and calibration of the TEM system were ensured by the use ofthe Magical No. 641 standard (Norrox Scientific Ltd., Beaver Pond,Ontario, Canada).

2.5. X-ray photoelectron spectroscopy (XPS)

A Leybold MAX100 instrument with an EA200 electron energyanalyzer was operated at 72 eV in the fixed analyzer transmissionmode. A catalyst sample was introduced into a differentiallypumped pre-chamber as a loose powder. Integral XPS spectra ofan area of 2 mm � 2 mm each were recorded.

2.6. Electron energy loss spectroscopy (EELS)

A Zeiss 912 LaB6 TEM instrument equipped with an in line Ome-ga filter was used to probe the average sp2 character throughoutthe carbon primary particles and aggregates in transmission bymeans of the parallel-EELS technique to complement the TEMand XPS results.

2.7. Inelastic Incoherent Neutron Scattering (IINS)

Each catalyst was loaded into a thin-walled (0.5 mm) stainlesssteel (1.4571) can which was sealed by a Conflat™ flange with a

stainless steel pipe and a welded bellows valve (Nupro) via an oxy-gen-free high conductivity (OFHC) copper gasket. A sealed can con-taining macroscopic amounts (ca. 15 g, precise values in Table 1) ofcatalyst was evacuated using a turbo-molecular pump which wasbacked by a dual-stage rotary pump with a zeolite trap to avoidback-diffusion of oil and other potential molecular contaminants.Additional information on IINS of related finely divided palladiumentities is given in [11]. Each catalyst was subjected to slow, care-ful cycles of hydrogenation (99.999% hydrogen) and dehydrogena-tion at room temperature to avoid fast local heating due to aspontaneous excessive release of the heat of the dissociativeabsorption of hydrogen in the palladium in fast-step dosing. Noheating during degassing of a hydrogenated catalyst by pumpingdown a sample can was performed, in order to avoid particlegrowth induced by heat in the presence of residual pressure ofhydrogen. The hydrogen uptake of the supported palladium wasmonitored by capacitive pressure transducers (MKS/Baratron).

Four cycles of hydrogenation/dehydrogenation were applied toeach sample. Since in these cycles, the outgassing was restricted toroom temperature, complete decomposition of the hydride phaseswas not achieved; thus, there is residual hydrogen in the samples.Each sample was measured at the MAPS spectrometer [12] at ISIS(STFC Rutherford Appleton Laboratory, UK) with incident energiesof 5000 cm�1 (C–H stretch region of the carbon support and also tocheck for hydroxyls), 2000 cm�1 (surface Pd–H vibrations) and1200 cm�1 (b-PdHx). We have measured the catalysts in the hydro-genated state under 750 mbar hydrogen equilibrium pressure (Ta-ble 1) and after thermal decomposition of the hydride phases tocheck for the individual contribution of palladium hydride (ca.480 cm�1: b-phase, ca. 552 cm�1: a-phase). To maximize sensitiv-ity, we have used the S-chopper of MAPS rather than the A-chopperwe used previously. This provides a factor of approximately threein sensitivity, albeit at a factor of two in resolution [11,12].

Background spectra were recorded after the removal of thehydrogen from samples Pd1 and Pd3 (Table 1) by a turbo-molecu-lar pump at room temperature and subsequent heating to 120 �Cwith continuous evacuation. An evacuated empty can and the purecarbon black support which was used to manufacture the catalystswere also measured for background subtraction.

For quantitative evaluation of the weight-corrected and nor-malized neutron scattering peak integrals, the following steps wereused:

(1) Subtraction of the low background of the stainless steel can(only relevant at 50–300 cm�1).

(2) Subtraction of the carbon support.(3) Normalization to 1 g Pd.(4) Integration over the range 350–600 cm�1 using the end-

points as the baseline.

This procedure enables the relative amounts of palladium hy-dride in the catalyst samples to be reliably compared.

2.8. Catalyst activity tests

Hydrogenation of nitrobenzene was carried out in a jacketedsemi-batch stirred titanium autoclave (500 ml, Medimex)equipped with thermocouple, circulation thermostat, pressure sen-sor, gas inlet and outlet, sampling port, nitrobenzene supply portand gas entrainment stirrer (1700 rpm). At the beginning of eachexperiment, the reactor vessel was loaded with 50 mg of the cata-lyst and 150 ml of a mixture of aniline and water (stoichiometricmixture of the reaction products). After purging with nitrogen,the reactor was filled with hydrogen and heated to the operationaltemperature of 80 �C; the hydrogen pressure was adjusted to avalue of 40 bar. Nitrobenzene was fed into the reactor by a Gison

Table 1Sample numbers, composition, weight and final hydrogen equilibrium pressure in the IINS sample cans.

Sample No. Composition (wt.%) IINS Sample weight (g) PH2 (mbar)

Pd1 Pd(5%)/C 14.72 712Pd2 Pd(4.5%)Pt(0.5%Pt)/C 16.72 760Pd3 Pd(4.5%)Pt(0.5%)Fe(5%)/C 18.65 745

C Unloaded carbon black support 13.05 –

Fig. 1. Top: TEM images of the carbon black used as support for the preparation of the hydrogenation catalysts Pd1–Pd3. Aggregates of intergrown primary particles withenhanced internal sp2-ordering. Bottom: EELS spectra of the carbon K-absorption edge recorded at different spots in different aggregates.

K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160 155

307 pump. Hydrogen pressure was kept constant using a pressflowgas controller (bpc 9901, Büchi Glas Uster AG). Hydrogen con-sumption was monitored online and was a measure of hydrogena-tion rate and, thus, catalytic activity. Catalytic activity [ml H2 s�1]was calculated for each pulse by linear regression of hydrogen con-sumption plotted against time within the linear range of the plot.

3. Results and discussion

The TEM images of the pure carbon black support in Fig. 1shows that typical aggregate structures are present [13,14]. Theinter-grown primary particles that are forming the aggregates

show graphite-type layers and edges at the surface and extendedsurface-near graphitic ordering.

Plane-like oriented graphiticity, turbostratic disorder, corannu-lene-type curvature and sp2-sphericity is present which is charac-teristic of graphitized grades of carbon black. This is in line withthe low hydrogen content of 0.018 ± 0.002 wt.% H, which is onthe order of that of pure graphitic carbon. Details on the role andimportance of non-hexagonal rings in the formation and sphericalordering of primary particles and aggregates of carbon blacks up tofaceting are reviewed in [15]. The high degree of surface orderingand the absence of significant quantities of aliphatic matter in thiscarbon black support provide stable and uniform surface condi-tions for precious metal loading.

156 K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160

The primary particle size of the black is in the usual range of30–40 nm, the average size of the aggregates is 300–650 nm, andthe BET surface area is 65 m2/g1 [14]. Parallel-EELS in the TEMprovides an average picture of the electronic state of the primaryparticles and the aggregates of the carbon black and thus spectro-scopically complements the high-resolution TEM images and thesurface-related results of XPS. Typical results are depicted inFig. 1: the absorption edge as measured at different spots in differ-ent aggregates shows a strong p⁄ excitation band below 290 eVand the characteristic overall profile of sp2-type carbon. In agree-ment with the TEM results, there is no evidence for the presenceof significant amounts of incorporated sp3-type carbon. Thephysical background of these characteristic fine structures isoutlined in Ref. [16].

In analyzing the XPS C1s signals of the carbon black and the cat-alysts, a pronounced plasmon loss feature [17–19] around 290.7 eVcontributed to the whole C1s region with ca. 7–8% of the integratedintensity as a signature for the high degree of graphiticity at thesupport’s surface and for the absence of significant amounts of ad-sorbed sp3 matter. Increasing sp2-ordering at the surface and in thesurface-near ‘selvedge’ region of carbon black leads to a muchmore uniform and narrow distribution function of the surface en-ergy, as shown by inverse gas chromatography at finite concentra-tion conditions and static gas adsorption techniques [20–22].Different fractions of different adsorption sites are discussed inliterature: (I) graphitic planes (sp2), (II) amorphous carbon (sp3),(III) crystallite edges, (IV) slit-shaped cavities in the carbon blackstructure [Fig. 3 in Ref. 22]. Fig. 1 suggests that for the given carbonblack type (I) and type (III) surface sites dominate. It follows that anarrow surface energy distribution function should be present.This leads to a more homogeneous and narrow adsorption site dis-tribution and hence to a uniform deposition of particles onto thissp2-type carbon support, as compared to more disordered, para-crystalline and porous carbonaceous materials such as activatedcarbon [9]. Fig. 2 compares TEM images of a typical 5% Pd/activatedcarbon catalyst (left) and the 5% Pd/carbon black catalyst Pd1(right). In the case of the activated carbon support, the Pd particlesare comparatively more randomly distributed and more isolated.In the case of the carbon black support, the palladium particlesare preferably located or targeted at the surface (Fig. 2, right, andFig. 3) sites in edge decoration of type (III) sites [22]. This indicatesan impact of a narrow adsorption site distribution of a carbon blacksupport of enhanced sp2 character on the deposition of palladiumentities onto certain surface sites. The high sp2-surface orderingsuggests a templating effect of the support in the precious metaladsorption and deposition, nucleation, particle formation, growth

Fig. 2. Left: TEM image of a 5% palladium on activated carbon catalyst. DN: 2.10 nm; SDN

Right: catalyst Pd1: 5% palladium on carbon black.

resulting in aggregation to chain-like entities during catalyst prep-aration. The high graphiticity of the carbon black (Fig. 1) provides ahigher degree of local ordering, larger basic structural units andlower amount of micro- or ultramicroporosity (type IV sites) com-pared to typical microporous carbon blacks [23].

In the Pd(4.5%)Pt(0.5%)/C catalyst Pd2, much more isolated par-ticles are encountered shown by TEM at different magnifications(Fig. 4, Table 2).

EDX nanospot analyses reveal that all the supported particlesare formed by a binary Pd/Pt alloy (Table 2). It appears that dueto the intensive alloying even with small amounts of platinum,the surface free energy of the particles is changed compared tothe palladium-only catalyst. A strong separation of the primaryparticles and a lower degree of aggregation are achieved onaverage. No isolated platinum-only or palladium-only entitieswere detected by TEM/EDX. Also chain-like alloy aggregatesoccur with lengths of up to 20 nm. It can be estimated that inthe case of the palladium-only catalyst, about 90% of theparticles are arranged as chain-like aggregates, whereas for thebinary alloy catalyst, about 90% of the particles are isolated(Table 2).

In the catalyst Pd3, Pd(4.5%)Pt(0.5%) Fe(5%)/C, the metal ismostly present as small isolated primary particles and as fewchain-like aggregates (up to ca. 50 nm length) (Fig. 5).

The relative amount of chain-like aggregates is also much lowerthan in the case of the palladium-only catalyst. Furthermore, manyof the alloy particles are present in spherical clustering, agglomer-ation or nucleation zones. This suggests different interactions be-tween the pure palladium and the carbon black supportcompared to the binary or the ternary palladium alloy particles.

Broad area XPS (2 mm � 2 mm) reveals an overall Pd/Pt/Fe com-position of 1.8/0.1/1.0 (at.%) in the topmost surface region whichsuggests a partly lower dispersion of Fe compared to Pd. EDX inthe TEM indicates that for the case of the binary alloy catalystPd2, a rather constant local Pd/Pt composition was achieved forthe isolated particles as well as for the linear aggregates (Table 2).For the ternary system Pd3 nanospot, EDX analyses show that a lar-ger variation in local Pd/Pt/Fe stoichiometry is observed; however,phases such as Fe3Pt are not present [24,25]. The occurrence ofspherical clustering is correlated with enhanced local concentrationof Fe. According to the DN and SDN values (Table 2), it can be con-cluded that the addition of iron to form a ternary alloy led – at thestatistical average (2000 particles) – to smaller primary particlesizes and a narrowing of the primary particle size distributionand, hence, to the largest calculated TEM surface area for the metalcomponent.

: 0.20 nm; DA: 2.17 nm; EMS: 230 m2/g (see Table 2); catalyst characterized in [11].

Fig. 3. TEM images of the catalyst Pd1: Pd(5%)/C. Predominantly chain-like aggregates of about 2-nm-sized primary particles on the graphitic planes and edges of the support.

Fig. 4. TEM images of the binary alloy catalyst Pd2: Pd(4.5%)Pt(0.5%)/C. Predominantly isolated alloy particles of about 2.2 nm size. For the typical local elementconcentrations of Pd and Pt see Table 2.

Table 2Catalyst composition, hydrogenation activity (nitrobenzene (NB) to aniline), primary particle size, morphology and local composition (EDX, TEM), and relativeamounts of supported b-phase palladium hydride (IINS).

Catalyst Pd1a Pd2b Pd3c

Catalytic activity (mmolNB min�1) 6.6 36.9 23.1TEM average primary particle size DN (nm)d 2.0 2.2 1.9Standard deviation SDN (nm)d 0.24 0.31 0.19TEM average primary particle size DA (nm)e 2.1 2.3 2.0EMS (m2/g)f 243 216 254

TEM particle morphology(1) isolated particles (%), ca. 10 90 60(2) chain-like aggregates (%), ca. 90 10 10(3) spherical clustering (%), ca. 30EDX/TEM nano-composition, ranges (wt.%) Pd 100 Pd/Pt 85/15 to 93/7 Pd/Pt/Fe 71/5/24 to 86/6/8Normalized IINS peak integrals of b-phase Pd-hydride (arb. units) 127.0 87.6 26.3g/27.2h

a Pd(5%)/C.b Pd(4.5%)Pt(0.5%)/C.c Pd(4.5%)Pt(0.5%)Fe(5%)/C.d DN and SDN: primary particle size (arithmetical average) and its standard deviation, DN = (Rnidi)/N.e DA: primary particle size averaged over the surface, DA ¼ ðRnid

3i Þ=ðRnid

2i Þ.

f EMS: calculated electron microscopic surface, EMS= 6000/(DA � q), qPd: 11.99 g cm�3.g After subtraction of the normalized spectrum of the pure carbon black support;h After subtraction of the normalized spectrum of the dehydrogenated sample (12 h, 473 K, turbo-molecular pump).

K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160 157

In summary:

(1) edge (type III) and surface (type I) [22] site decoration andtwo-dimensional, linear aggregation in the palladium-onlycatalyst.

(2) predominantly isolated particles in the binary alloy case.

(3) predominantly isolated particles and three-dimensional,spherical clustering/agglomeration over the whole sp2-type sup-port in the ternary alloy catalyst.

The overall loading capacity of the support’s surface (65 m2/g)compared to ca. 1000–1500 m2/g of nitrogen area for typical

Fig. 5. TEM images of the ternary alloy catalyst Pd3: Pd(4.5%)Pt(0.5%)Fe(5%)/C. Predominantly isolated alloy particles, spherical agglomerates and small amounts of chain-likeaggregates.

Fig. 6. IINS spectra of the hydrogenated catalysts Pd1-Pd3 (top), recorded under ca.750 mbar equilibrium pressure, and of the carbon black support (bottom). Thesignal around 480 cm�1 is due to b-palladium hydride: vibrations of the interstitialprotons in the octahedral sites of the supported palladium particles. The signalaround 120 cm�1 is due to the rotational transition of physisorbed molecularhydrogen at the catalyst surface. It is IINS allowed because neutron scatteringresults in a nuclear spin flip.

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activated carbons is lower due to the low porosity and surface areaof the carbon black. This can explain the effects of spherical clus-tering at enhanced metal loading in Pd3 (Fig. 5).

The proton dynamics of Pd1–Pd3 were compared by means ofIINS. The high sp2-character and very low hydrogen content ofthe dry, non-porous carbon black support used for these catalystsenables direct, nondestructive investigation of the hydrogen stor-age capabilities in the ca. 2 nm mono-, bi- or tri-metallic particlespresent by IINS without the spectral interference from the hydrog-enous entities found in an activated carbon support [11]. The highpenetrating power of the neutron allows each catalyst to be inves-tigated under in situ conditions: sealed in a stainless steel cuvetteand hydrogenated up to ca. 750 mbar sorption equilibrium pres-sure or more. The normalized spectra reveal distinct differencesin hydrogen absorption capacity between the catalyst samples(Fig. 6 and Table 2). The question arises as to how relevant thesedifferences are to the catalyst under the operation conditions of40 bar hydrogen pressure and 80 �C. In the case of palladium, thea-/b-phase transition region and the corresponding hydrogen stor-age window at constant hydrogen equilibrium pressures, well be-low 750 mbar, are very broad [26–30]. The b-hydride phase isformed and saturated at pressures also well below the 750 mbarhydrogen pressure used in our measurements. Further, the iso-therms show a very steep increase with pressure once the b-phaseis obtained up to 100 bar and more. Very similar isotherms are ob-tained at both 20� and 80 �C [Fig. 3.4 in Ref. 29 and Fig. 7 in Ref. 30].This holds for coarse as well as for finely divided palladium. The re-sult is that our measurements of the relative amounts of b-phasehydride present in the catalysts at 20 �C and 750 mbar are essen-tially those that would be obtained at 40 bar hydrogen pressureand 80 �C and hence are directly relevant to the industrialconditions.

At the surface of the palladium particles, the hydrogen mole-cules are dissociatively chemisorbed [31,32,29] and the hydrogenatoms then diffuse into the bulk. The band at ca. 480 cm�1

(Fig. 6) represents the vibrations of the electronically screenedinterstitial protons on the octahedral sites of the highly loadedb-phase [33,34]. The lattice expansion due to hydrogen uptake asa geometric factor is of influence on surface reactivity [35]. Thewidth, fine-structure and filling of the d-band are also influencedby changes in the lattice constants and elastic interactions [36]due to hydrogenation as well as by alloying [37]. For the hydroge-nation of ethyne, it is reported that the transition from b- to a-hydride phase increases ethane selectivity [30] illustrating theelectronic effect.

Due to the high diffusion coefficient of hydrogen in palladium[29,38 and literature cited therein] and the small primary particle

size in the catalysts Pd1–Pd3 and, therefore, the rate of hydrogenuptake and release, it can be expected that the different hydrogenstorage capacities observed are of relevance for the different cata-lytic properties.

The numerical results from evaluating the vibrational mode re-gion of the b-palladium hydride phase are compared in Table 2.The signal assignment follows conclusions from the literature forthe IINS signal of compact and of finely divided palladium hydrides[33,34,39–45].

It is known that:

(A) The solubility of hydrogen in bulk and in supported palla-dium decreases linearly with the percentage of dispersion[46,47]. Furthermore, it is known that the hydrogen solubility insingle-sized palladium clusters (stabilized by surfactant or poly-mer-embedding) decreases significantly with decreasing size ofthe isolated primary nanoparticles (5 ? 3 ? 2 nm): the shape of

Fig. 7. Hydrogenation activity of Pd1–Pd3 in selective hydrogenation of nitroben-zene to aniline: addition of Pt to Pd strongly increases hydrogenation activity;addition of Fe leads to slightly lower activity compared to the binary system. Thehydrogen consumption rates [ml H2 s�1] were measured. Activity values are notedin Table 2.

K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160 159

the hydrogen absorption isotherms and the relative proportions ofa- and b-phase palladium hydride are strongly affected [48–52].The width of the phase transition region is much smaller than inthe case of larger palladium particles [26–29,53]. It is also knownthat the hydrogen absorption capability of nanosized primary par-ticles of palladium is lower than that of palladium aggregateswhich are formed from intergrown primary particles of compara-ble size (ca. 3–4 nm) [11].

(B) For the case of larger, isolated, stabilized Pd/Pt particles(about 6 nm size and more), the hydrogen solubility of the alloywas found to be higher than for pure Pd particles of equal size;however, differences were observed in comparing solid solutionversus core/shell particles [54,55]. However, at higher Pt concen-tration (50% and more), the hydrogen absorption capability is low-ered [56].

(C) For the Pd/Pt system, it is reported that melting tempera-ture, melting enthalpy and catalytic activation energy decreasewith size. The physicochemical properties including surface energyof bulk material, of individual constituents and of bimetallic nano-particles differ [57,58].

Fig. 6 indicates that for the ca. 2 nm primary particles in catalystPd2 and Pd3, the influence of A) (low primary particle size of thePd-based entities) together with the disaggregation effect causedby alloying with Pt and Fe does overcome the potential influenceof B) (enhancement of hydrogen storage capability [54,55]) but stillat a much lower level than for bulk palladium entities [26–29,53]which may be more significant at larger particle sizes. The catalystsPd1–Pd3 were tested for the selective hydrogenation of nitroben-

Table 3Ratios of normalized IINS peak integrals: quantification of the effect of alloying, disaggregformation of b-palladium hydride.

Catalyst Cause of change in hydrogen storage capa

Pd1/Pd2 Disaggregation by alloying with Pta

Pd2/Pd3 Moderating Pd/Pt with Fea

Pd1/Pd3 Disaggregation, alloying and moderation wLindlar catalyst Poisoning Pd with Pbb

Pd/CaCO3/Pd/C Aggregates vs. isolated primary particlesc

a Calculated from IINS data in Table 2, support: carbon black.b Pd(5%)/CaCO3/Pd(5%)Pb(3.5%)/CaCO3, both 10- and 15-nm-sized aggregates of ca. 3.c 10–15-nm-sized aggregates of ca. 3.7 nm primaries versus isolated ca. 3.2 nm prima

zene to aniline in order to examine the influence of the additionalcomponents (Pt and Fe) on catalytic activity (Fig. 7). The activity ofthe Pd-only catalyst was the lowest. The Pd–Pt catalyst exhibitedthe highest average activity among the three catalysts. The activityof the Pd–Pt–Fe catalyst was significantly higher than the activityof the Pd-only catalyst but lower than that of the Pd–Pt catalyst.Clearly, the catalytic activity of supported Pd for the hydrogenationof nitroarenes can be optimized by alloying with Pt and moderat-ing with Fe [6–8]. While this has been known (and used commer-cially) for nearly half-a-century, the underlying reasons for thisbehavior were unknown. In this work, we have shown that differ-ent templating influence of a carbon black support of enhanced sp2

character has a tremendous influence on the catalyst morphologyin the case of the deposition of Pd-only entities on the one handand of binary or ternary alloy particles on the other.

For the given ternary catalyst, it is found that alloying Pd withsmall amounts of Pt and, furthermore, with Fe leads to a markeddisaggregation effect, to a much higher amount of isolated sup-ported primary particles in the 2-nm size regime and this has amajor effect on the catalytic behavior: as the relative amounts ofisolated metal or alloy particles are changed from 10 ? 90 ? 60%and the primary particle size distribution function is narrowed,in parallel the catalytic activity changes from 6.6 ? 36.9? 23.1 mmolNB min�1 (Pd ? Pd–Pt ? Pd–Pt–Fe). For this catalystsystem, there is a subtle balance between the disaggregation effectand the huge increase in activity by adding Pt and the moderationand stabilization of activity and selectivity by Fe. Undoubtedly,part of the moderating effect of Fe is the marked reduction inhydrogen storage capacity that it causes due to the incorporationof Fe into the crystal structure of the PdPt alloy: the long rangephase coherence in b-phase hydride formation is strongly affected.There is a clear parallelism between the proportion of isolated par-ticles, alloying and the catalytic activity (Table 2), suggesting thatthese are the most active components. This implies that it is con-ceivable that the same activity could be obtained with a reducedmetal loading, provided that the metal particles are sufficientlydispersed and alloyed.

4. Conclusions

The catalytic activity of supported Pd for the hydrogenation ofnitroarenes can be optimized by alloying with Pt and moderatingwith Fe [6–8]. While this has been known (and used commercially)for nearly half-a-century, the underlying reasons for this behaviorwere unknown. In this work, we have shown that the use of a car-bon black support of enhanced sp2 character has a marked ‘tem-plating’ effect on the location of the catalyst particles, which arepreferentially located at the edges of the carbon particles. The de-gree and type of aggregation depends on the alloying metal(s) and,in particular, changes the proportion of isolated metal particles inthe catalyst, which are strongly correlated with the activity. Alloy-ing reduces the availability of hydrogen in all cases, as shown by

ation, moderation and controlled poisoning in supported palladium catalysts on the

bility Ratio of decrease in the formation of b-PdHx

1.53.2

ith Fea 4.72.23.5

7 nm primary particles, from Ref. [11].ries, from Ref. [11]; C-support: activated carbon.

160 K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160

the quantity of b-phase hydride present. With Pt, this effect isoutweighed by the increased activity of the catalyst, the additionof Fe so drastically reduces the amount of hydrogen available thatit moderates the activity.

In Table 3, numerical results on the relative impact of differentinfluences on the hydrogen storage capability in supported palla-dium-based nanoparticles are summarized. The comparison ofthe values indicates that the addition of Fe to form a ternary alloyhas an even stronger impact on decreasing b-phase hydride forma-tion than the addition of lead in the Lindlar catalyst (factor 3.2 vs.2.2). Together with the disaggregation of Pd by alloying with Pt,this leads to a factor of 4.7, whereas the enhancement of the pro-portion of isolated primary particles of palladium in the case of aporous activated carbon support leads to a factor of 3.5. Knowledgeof these effects in finely divided supported particles offers clearpaths for further catalyst optimization. The choice of support andalloying metal enables fine-tuning of the catalytic activity bychanging the morphological and chemical properties of the sup-ported particles.

Acknowledgments

Professor D.K. Ross (Salford University, Manchester, UK) isgratefully acknowledged for many fruitful discussions on hydrogenin metal systems. The STFC ISIS facility (Rutherford Appleton Lab-oratory, Chilton, UK) is thanked for access to neutron beamfacilities.

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