Individual Heat of Adsorption of Adsorbed CO Species on Palladium

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Individual Heat of Adsorption of Adsorbed CO Species on Pd and Pd-Sn Nanoparticles Supported on Al2O3 by using Temperature Programmed Adsorption Equilibrium Methods. Imen Jbir, Julien Couble, Sihem Khaddar-Zine , Zouhaier KSIBI, Frederic Meunier, and Daniel Bianchi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02749 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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Individual Heat of Adsorption of Adsorbed CO Species on Pd and Pd-Sn Nanoparticles Supported on Al2O3 by using Temperature Programmed Adsorption Equilibrium Methods. AUTHOR NAMES Imen Jbir,1,2 Julien Couble,1 Sihem Khaddar-Zine,2 Zouhaier Ksibi,2 Fréderic Meunier,1 Daniel Bianchi1* AUTHOR ADDRESS 1

Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256

CNRS, Université Claude Bernard Lyon I, Bat. Chevreul, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne-France.

2

Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Sciences de Tunis, Université de

Tunis EL Manar, 2092 Tunis.

Submitted to ACS-Catalysis

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2 ABSTRACT: The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x=2 or 5) in the 300-713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using FTIR spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted B) which are the dominant species on Pd° particles and (b) a significant increase in the Pd sites forming linear CO species (denoted L). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3 in parallel to the formation of two strongly adsorbed B CO species, two linear L1Pd° and L2Pd° CO species are formed which exhibit different heats of adsorption. For the dominant L1Pd° CO species, the heat of adsorption decreases linearly with the increase in its coverage from 92 kJ/mol to 54 kJ/mol at coverage 0 and 1 respectively while that of the L2Pd° species is > 165 kJ/mol at coverage 1. On the two Pd-Sn containing particles two linear CO species are formed denoted L12Pd-xSn and L22Pd-xSn with x= 2 or 5. The L12P-xSn species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (that is slightly dependent on x) linearly varies with its coverage: ≈ 90 kJ/mol and ≈ 50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1Pd° indicates that the electronic effect of tin is very modest as compared to it geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd3Sn and Pd2Sn are present and (b) the impact of tin on the H2 chemisorption on Pd° sites are presented. KEYWORDS : Adsorption, Pd, Pd-Sn alloy, alumina, carbon monoxide, FTIR, heat of adsorption,


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3 TEXT: 1. Introduction Extensive literature data dating from the 1950s show that supported bimetallic nano particles offer enhanced performances as compared to the monometallic particles in numerous catalytic reactions as summarized by periodic review articles.1-5 Research on bimetallic particles started in the 1950s with the attempt to provide experimental data to support or not the electronic theory of catalysis that was under debate.1,3 However, bimetallic catalysts have rapidly found an interest in different industrial processes in particular by the net improvement of the selectivity of different catalytic reactions.1-6 From these early studies, the adsorption properties and/or catalytic activity of supported bimetallic particles M1-M2 have been ascribed to two effects (a) ensemble (or geometric) and/or ligand (or electronic) effects.1-5 The ensemble effect considers that the insertion of M2 in the M1 particles modifies the superficial sites by a dilution like process without modifying the electronic structure of the M1 sites that is the basis of the ligand effect.1-3 Numerous experimental data using particularly the change in the IR bands of adsorbed CO species by the formation of bimetallic particles were consistent with the ensemble effect without any significant electronic effect.2-3 For instance, for the adsorption of CO, the insertion of Ag in Pd particles deposited on SiO2 (total weight percentage of metal of 9 wt% with atomic Pd/Ag ratios decreasing in the (9.75/0.25)-(3.5/6.5) range)7 (a) led to the progressive disappearance of Pd° sites adsorbing CO as bridged species (denoted B CO and identified by IR bands in the range 1960-1920 cm-1) ascribed to a geometric effect and (b) had no impact on the position of the IR band of the linear CO species (denoted L CO species) observed at 2060 cm-1 whatever the Pd/Ag ratio indicating the absence of any ligand effect.7 However, the position of an IR band such as the linear CO species on Pd° sites is dependent of different parameters particularly the coverage of the adsorbed CO species and the particle size. Different authors have proposed to consider the


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4 modification of the heats of adsorption of CO as a second criterion revealing the presence or not of electronic effect in bimetallic particles.2, 3 Using conventional methods such as temperature programmed desorption and microcalorimetry, the measurement of the individual heats of adsorption of adsorbed CO species is not straightforward (particularly for bimetallic particles) because the experimental data can be due to the overlap of different processes (i.e. formation of several adsorbed species, parallel reactions such as CO dissociation and surface reconstruction). This situation was the driving force for the development of an original analytical method denoted Adsorption Equilibrium InfraRed Spectroscopy (AEIR method) which provides the individual heats of adsorption of coadsorbed species characterized by an IR band.8-10 This method that has been developed considering the different adsorbed CO species (linear-, or/and bridge- or/and three-fold- bonded CO species) on supported monometallic particles such as Pt°,8-10 Pd°,11,12 Cu°,13 Au°,14 Ag°15 and Ru°16 is based on the quantification of the decrease in the intensity of an IR band characteristic of each adsorbed species during the increasing in the adsorption temperature Ta in isobaric condition. This gives the experimental evolutions of the coverage of each adsorbed species with Ta in isobaric condition.8-16 These experimental data provide the individual heats of adsorption of the different adsorbed CO species as a function of their coverage by the comparison with theoretical curves obtained by an adsorption model (often either Temkin or Langmuir model) considering localized adsorbed species (see below).8-16 For bimetallic M1-M2 particles, this means that the AEIR method may permit from the same experiment (a) to study the evolutions of the IR bands of the L and B CO species with the M1/M2 ratio (as described for Pd/Ag)7 which are relevant of the ensemble effect and (b) to measure the modification of their heats of adsorption by the AEIR method which is relevant of the ligand effect.2,3 This constitutes the main aim of the present study considering Pd-Sn particles supported on Al2O3. A similar use of the AEIR method


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5 has been recently reported indicating that the heat of adsorption of the linear CO species on Pt supported particles at low coverage was divided by 2-fold by the insertion of tin.17 The choice of Pd-Sn bimetallic particles for the present study has been selected for the following reasons: (a) numerous preparation methods lead to the formation of Pd-Sn particles showing improved properties for different catalytic hydrogenation reactions such as the hydrogenation of citral,18 cinnamaldehyde,19 and hexadienes,20,

21

(b) it is well known that the

adsorption of CO on monometallic Pd° particles leads to the formation of L and B CO species favoring the evidence of a geometric effect by insertion of Sn considering the change in the L/ B ratio similarly to Pd-Ag bimetallic particles7 (contrary to platinum mostly leading to L CO species on mono8-10 and bimetallic particles)17 and (c) few studies18, 21 have been dedicated to the characterization by FTIR spectroscopy of the adsorbed CO species on supported Pd-Sn particles providing experimental data favoring the development of the present study. For instance, the adsorption properties of Sn and SnO2 for CO are limited18,

21

preventing an overlap of the IR

bands of adsorbed CO species on Pd and Sn sites which is favorable to the AEIR method. For instance, a recent FTIR study22 on the adsorption of CO on Pd-Cu bimetallic particles (atomic ratio Cu/Pd=10) indicates the disappearance of the characteristic IR bands of the B CO species on Pd° particles leading only to the presence of the L CO species. However, there characteristic IR band observed at 2050 cm-1 is strongly overlapped with two strong IR at 2115 and 2101 cm-1 due to linear CO species Cu+ and Cu° sites. This is not a favorable situation to study of the heats of adsorption of the L CO species on the Pd° sites of bimetallic particles by using the AEIR method. The measurements of the individual heats of adsorption of the adsorbed CO species on pure and bimetallic metal particles supported on alumina have a second interest in line with the experimental microkinetic approach (denoted EMA) of heterogeneous catalytic reactions implying CO such as CO/O2 on Pt/Al2O323, 24 and CO/H2 on Co/Al2O3.25 The heats of adsorption


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6 allow characterizing the first and key surface elementary steps of the reaction constituted by the adsorption/desorption of one of the reactant. The present study constitute the first stage of the EMA of the CO/H2 reaction on Pd and Pd-Sn particles supported on alumina which is underway in our group. The interest of the CO/H2 on noble metals containing particles, particularly Pt and Pd supported on alumina, is that their selectivity to CH4 is ≈ 100%,26 and this limits the number of surface elementary steps to be considered in the EMA as compared for instance to Co and Fe particles. .

2. Experimental. 2.1 Preparation of the Supported Mono and Bimetallic Pd of Pd-Sn Particles. The main aim of the preparation method was to prevent ambiguities in the interpretation of the FTIR spectra of the adsorbed CO species on the bimetallic particles. For instance, after the adsorption of CO, the presence of IR bands in the range 1980-1900 cm-1 on Pd-Sn containing solids can be interpreted as due to the heterogeneity in the elemental compositions of the metal particles from monometallic to different Pdx-Sny particles. This situation can be prevented if the B CO species are not detected on the bimetallic particles and literature data18, 21 (see more detail in the supporting information section) lead to the views that this is favored by (a) Sn/Pd weight ratio in the 1-2.5 range and (b) a high reduction temperature in hydrogen (i.e. 713 K). Moreover, to favor the quantification of the IR spectra the palladium loading must be neither too high nor to low and a 2 weight % of Pd have been selected for the mono and bimetallic Pd-Sn solids. Similarly to previous reports on monometallic supported particles,11-15 the Pd and Pd-Sn nano particles supported on Al2O3 (Degussa, 110 m2/g) were prepared (3 g of each solid) by using the incipient wetness impregnation method with aqueous solution of precursors. A 2 wt% Pd/Al2O3


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7 solid was prepared using Pd(NO3)2 x H2O (Alfa Aesar). Two bimetallic particles 2 wt% Pd-2wt% Sn/Al2O3 and 2 wt% Pd-5wt% Sn/Al2O3 (atomic ratio Sn/Pd of 0.896 and 2.24 respectively) were prepared by co-impregnation with an aqueous solution of Pd(NO3)2 x H2O and SnCl4 x H2O (Alfa Aesar). The appropriated amounts of aqueous solutions of the precursors were added to the alumina support contained in a Teflon beaker. A vigorous mixing with a spatula allowed obtaining homogeneous pastes which were dried in air 24 h at room temperature and then 24 h at 393 K. After crushing, the powders were heated (3 K/min) in air to 743 K with a stage at 573 K during 1 h and maintained at 743 K during 4 h to decompose the precursors. After cooling to 300 K in air the solids were reduced in pure H2 (250 cm3/min) at 713 K (5 K/min) during 1 h. They were cooled in H2 to 300 K and then after a purge in helium they were stored in air (freshly prepared solids). Before use the solids were reduced at 713 K in hydrogen on the different analytical setups. 2.2 Analytical Methods Conventional analytical methods have been used to characterize the prepared solids allowing a comparison with literature data such as (a) elementary analysis using ICP-OES (Active from Horiba Jobin Yvon) which has ascertained the Pd and Sn loadings and (b) N2 physisorption at 77 K for the measurement of the BET area (Micromeritics ASAP 2020) which indicated that the impregnation does not modify significantly the BET area of the support. Others specific analytical methods are provided below with more details. Structure and Composition of the Pd-Sn Particles The averaged crystalline structure of the freshly prepared bimetallic particles was analyzed at RT and ambient atmosphere using a Bruker D8 Advance A25 diffractometer (CuKα radiation at 0.154184 nm) equipped with a Ni filter and 1-D fast multistrip detector (LynxEye,


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8 192 channels on 2.95°). The diffractograms were collected at 2θ with steps of 0.02° from 4° to 80° for a total acquisition time of 32 min. Phase identification was performed using the Diffrac.Eva software (Bruker) and the ICDD-PDF4+ database of the International Center for Diffraction Data. The local sample microstructure was examined using a Ly-EtTEM microscope, a latest generation ETEM (Titan 80–300 kV from FEITM, scanning transmission electron microscopyhigh angle annular dark field (STEM-HAADF)) operated mainly at 300 kV with 1 Å best resolution (few analysis have been performed at lower kV) and equipped with an imaging aberration corrector and an energy-dispersive X-ray (EDX) analyzer (SDD X-Max 80 mm2 from Oxford InstrumentsTM) used for elemental chemical analysis. The fresh bimetallic Pd-Sn solids were crushed in ethanol and the solution was ultrasonically stirred before dropping it on a holey carbon-covered copper TEM grid, followed by drying. In some cases, the solids were studied after in-situ reduction consisting in heating the as-prepared samples to 673 K (5 K min-1, 2 h plateau) in pure H2 flow (total pressure 7 mbar) followed by a cooling stage in H2 to 300 K. The temperature was measured with a thermocouple placed around the furnace and not directly in contact with the grid: this gave an estimation of the sample temperature. IR Cell in Transmission Mode The FTIR characterizations that constitute the main original contribution of the present study, have been performed using a Nicolet-6700 FTIR spectrometer equipped with a small pathlength (≈2 mm) home made stainless steel IR cell in transmission mode using CaF2 windows and described in more detail previously.8 Briefly, it allowed in-situ treatments of a compressed disk of solid (Φ= 1.8 cm, m≈ 40-80 mg), in the temperature range of 293-800 K with a controlled gas flow rate in the range of 150-2000 cm3 min-1 at atmospheric pressure selected using different


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9 valves and purified by different traps in particular cold traps for H2O impurities (77 K using helium and CO/He gas mixtures). The thermal inertia of the IR cell led to a low cooling rate after a pretreatment procedure: 1 h to cool down from 713 K to 300 K leading to the readsorption of minute traces of H2O on the solids. Note that several successive CO adsorption cycles in the 300713 K range were performed with the same pellet of solid without and with a H2 reduction procedure at 713 K before the CO adsorption at 300 K. This led to a gradual modification of the IR spectra of the adsorbed CO species which will be discussed. Volumetric Measurements using Mass Spectrometry To support the exploitation of the IR spectra, the amounts of adsorbed CO and hydrogen species on the different solids (in isothermal and temperature programmed conditions) have been quantified with an analytical system for transient experiments using a quartz microreactor and a mass spectrometer as described previously,27 providing the rates of either formation of a product or consumption of a reactant with time on stream: R(t) (see Supporting Information for more details). The low thermal inertia of the microreactor that allowed a fast cooling from 713 K to 300 K (∼4 min) and the amount of solid (200-300 mg) limited the impact of H2O traces on the different measurements. In particular, this system has been employed to study how the insertion of Sn modifies the adsorption capacity of palladium for CO and H2 chemisorption.

2.3 Heats of Adsorption of Adsorbed CO Species using the AEIR and TPAE Methods. The procedures for the measurement of the individual heats of adsorption of adsorbed CO species on metal particles using the AEIR8-9 and TPAE27 methods have been previously described in details. Briefly, in the AEIR method the evolutions of the intensity of the individual IR bands of each Xads species are following during the increase in the adsorption temperature Ta in isobaric


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10 conditions Pa. This provided the evolution of the coverage θX(Ta) of each Xads adsorbed CO species with Ta according to:8-16 θX(Ta) = AX(Ta)/AX(M)

(1)

Where AX(Ta) and Ax(M) are the area of an IR band characteristic of the Xads species at Ta and at saturation of the sites respectively. The saturation of the sites was ascertained by verifying that the IR band area was unmodified by increasing either Pa at 300 K or Ta at a constant pressure. The heats of adsorption of the Xads species at different coverages were determined comparing the experimental curve θX(Ta) to a theoretical curve which was often obtained from the generalized expression of the Temkin’s model (that considers that the heat of adsorption linearly increases with the decrease in the coverage):8-16

θ=

RTa 1 + K X (0).Pa ln( ) ∆ E X 1 + K X (1).Pa

(2)

where KX(0) (EX(0)) and KX(1) (EX(1)) are the adsorption coefficients (heats of adsorption) of the Xads species at θ= 0 and 1 respectively, ∆EX= EX(0)-EX(1) and R is the perfect gas constant. The EX(0) and EX(1) values were obtained considering that Xads species is localized and that the adsorption coefficients KX(θ) was provided by the statistical thermodynamics:8-16 K X (θ ) =

h3 (2 π m) 3 / 2

1 (k T a)

5/2

exp(

ED X (θ ) − EA X (θ ) ) R Ta

(3)

Where h and k are Planck’s and Bolztmann’s constants, m is the mass of the molecule and EDX(θ) and EAX(θ) are the activation energies of desorption and adsorption respectively (EX(θ)= EDX(θ)-EAX(θ) with EAX(θ)= 0 because CO adsorption is non activated). The EX(0) and EX(1) values were those leading to theoretical curves θXth= f(Ta) (from eqs 1-3) consistent with the experimental data. The accuracy of the EX(θ) values was ≈± 5 kJ/mol.


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11 Moreover, we have shown28 that the area A(Ta) of a common IR band of two adsorbed species X1 and X2 can be used to obtain the average coverage of the two species as a function of Ta and Pa. This curve provides the individual heats of adsorption using:

θ A (Ta ) =

A(Ta ) θ X 1 = AM

A1M + θ X 2 AM

A2 M

= x1 θL1(Ta) + x2 θL2(Ta)

(4)

where θX1(Ta) and θX2(Ta) are the theoretical coverages of the X1 and X2 adsorbed species respectively provided by eqs 2 and 3 and x1=A1M/AM and x2=A2M/AM are their contributions (in fraction) to their common IR band at 300 K respectively. The TPAE method was similar to the AEIR procedure except that the average coverages of the adsorbed species were determined by a volumetric method.S3 After the adsorption equilibrium at Ta= 300 K using a x% CO/x% Ar/He mixture, Ta was linearly increased (α K/min): the progressive decrease in the adsorption equilibrium coverage was followed by the net CO desorption rate: Ri(t) in µmol/(g×s) measured using the M.S. The integration of R(t) with time on stream provided θex(Ta) in quasi isobaric condition according to: ta

∫ R i (t ) dt 0 ( ) = 1 − θ ex T a Qs

(5)

where ta (in s) is the time to obtain the adsorption temperature Ta starting for 300 K at ta=0 and Qs (µmol/g) is the amount of adsorbed CO at saturation of the sites. The experimental curve θex(Ta) led to the individual heats of adsorption of the adsorbed species according to eqs 2-4. It must be noted that the agreement between the heats of adsorption obtained by the AEIR and TPAE procedures validates eq. 1 (linear relationship between the area of the IR band characteristic of a Xads species and its amount on the surface). The original contribution of the present study regards (a) the modification of the nature of


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12 the adsorbed CO species by insertion of Sn in palladium particles supported on alumina for the evaluation of the geometric effect and (b) the measurement of the individual heats of adsorption of different adsorbed CO species on Pd° and Pd-Sn bimetallic particles for the evaluation of the electronic effect of Sn and the characterization of the first surface elementary steps of the CO/H2 reaction according to an experimental microkinetic approach of the global rate of the reaction on the two types of particles.

3. Results and Discussion. 3.1 Adsorbed CO species and their Individual Heats of Adsorption on 2% Pd/Al2O3. IR Bands of the Adsorbed Species and Impact of the Ageing of the Solid The nature and individual heats of adsorption of adsorbed CO species on Pd° particles supported on metal oxides have been studied in detail in previous works in particular on a reduced 1.4 wt% Pd/Al2O3 obtained by a wet impregnation.11,

12

However, we have revisited

these measurements before the presentation of the impacts of the insertion of Sn considering that (a) the position and intensity ratio of the IR bands of the L and B CO species on the present 2% Pd/Al2O3 solid are slightly different that on 1.4 wt% Pd/Al2O3 and (b) a recent study29 using single crystal microcalorimetry has shown the significant impact of the size of Pd particles (1.8-8 nm range) deposited on Fe2O3/Pt(111) oxide film on the heat of adsorption of CO. Spectrum a in Figure 1 shows the IR bands of the adsorbed CO species at 300 K using 1% CO/He on the freshly prepared solid after reduction at 713 K. This spectrum is similar to those observed in numerous literature data (Ref. 7, 11, 12, 18, and references cited). The IR band at 2087 cm-1 (observed previously at 2080 cm-1 on 1.4 wt% Pd/Al2O3)11 is ascribed to a linear CO species (denoted L1Pd° CO) adsorbed on low-coordinated (edges, corners) Pd° sites.7, 11, 12,

18, 30-32

detected on Pd (100)33 and Pd (111)34, and some authors31,

believe that on supported


35, 36

However, it is also

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13 catalysts, the sites may be situated in the middle of the crystal planes. This shows that an IR band detected around 2080 cm-1 after adsorption of CO on Pd-supported catalysts can be attributed to two types of linear CO species as considered by Liotta et al.31 The IR bands at 1987 and 1954 cm1

in spectrum a of Figure 1 (observed previously at 1985 and 1942 cm-1 on 1.4 wt% Pd/Al2O3)11

can be ascribed to two bridged CO species denoted B1Pd° and B2Pd° CO species on Pd° sites situated on different planes Pd (100) or Pd (110) and on Pd (111), respectively.30,

31, 35, 37

.

However, others assignments of these IR bands have been proposed such as (a) µ2 bridge-bonded CO on steps/edges and terraces38, 39 and (b) B CO species on Pd+ sites.40 Using the AEIR method, the individual heats of adsorption of the L1Pd° and BPd° CO species on Pd° (the B1Pd° and B2Pd° CO species are not differentiated due to the difficulties in the decomposition of their IR bands) are obtained from the evolutions of the IR bands of the adsorbed CO species during a progressive increase in Ta from 300 to 713 K in isobaric condition (i.e. 1% CO/He). However, different processes may contribute to the evolution of the intensity of an IR band in parallel to the modification of the adsorption equilibrium and this imposes a careful design of the AEIR experiments. For instance, spectrum b in Figure 1 has been recorded after two cycles constituted by the adsorption of 1% CO/He from 300 K to 713 K on the reduced 2% Pd/Al2O3 catalyst, followed by H2 reduction at 713 K (denoted A/R cycle). The comparison with spectrum a of Figure 1 shows that the two A/R cycles decrease the intensity of the IR band of the L1Pd° and BPd° CO species probably due to a sintering of the Pd° particles. The following A/R cycles have no impact and the solid is considered stabilized. However, on a stabilized solid, the adsorption of 1% CO/He in the range 300-713 K leads to a modification of the IR bands of the adsorbed CO species. For instance, spectrum c in Figure 1 has been recorded at 300 K after heating to 713 K and cooling to 300 K in 1% CO/He (denoted H(TaM)/C(300 K) cycle where TaM is the highest


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14 adsorption temperature) showing that the profiles of the IR bands are modified particularly the decrease in the intensity of the IR band of the B2Pd° CO species. Similarly to our previous study,10,11 this is ascribed to a surface reconstruction. However, different processes are relevant of reconstruction as commented by Ertl.41 The author defines two types of reconstruction (a) “displacive” reconstruction which requires transport of substrate atoms only over very short distances and may hence proceed even at fairly low temperatures and (b) “true” reconstruction associated to a substantial variation in the density of substrate atoms within the unit cell by mass transport over larger distances.41 The present reconstruction is probably “displacive” (in the following we do not mention “displacive”) modifying the composition of the surface sites. After the reconstruction, the following heating/cooling cycles in 1% CO/He have no significant impact as shown by spectrum d in Figure 1. Note that after spectrum d, the reduction of the solid at 713 K in H2 leads after adsorption of 1% CO/He at 300 K to an IR spectrum overlapped with spectrum b. The heats of adsorption of the adsorbed CO species on Pd° sites have been measured (as described in the following section) on a stabilized solid (after two A/R cycles) and on a reconstructed surface after a first H(713 K)/C(300 K) cycle in 1% CO/He. Individual Heats of Adsorption of the L and B CO Species on 2% Pd/Al2O3. Figure 2 shows the evolution of the IR bands of the L and B CO species during the increase in Ta in 1% CO/He on a stabilized solid (after two CO A/R cycles) and reconstructed Pd° surface (after a first H(713 K)/C(300 K) cycle). It can be observed that the IR band of the L1Pd° CO species decreases for Ta> 300 K associated with its shift to lower wavenumbers. However for Ta > ≈ 580 K, the IR band remains unmodified (intensity and position at 2062 cm-1). This has been ascribed to the presence of a small amount of a strongly adsorbed linear CO species (denoted L2Pd° CO species) with an IR band overlapped at 300 K with the strong IR band


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15 of the L1Pd° CO species more weakly adsorbed.11 The IR bands of the B CO species are not significantly modified by the increase in Ta in the 300-340 K range. In the 340-480 K range (spectra b-f in Figure 2), the B1 and B2 IR bands shift to lower wavenumbers and become broader leading to an unmodified area of their IR band whereas their maximum decrease. For Ta> 480 K, the area of the B1 and B2 IR bands decrease progressively: it can be observed that the B1 IR band is more affected than B2 IR band and for Ta> 580 K a single broad IR band is detected at 1922 cm-1. According to the AEIR procedure the decrease in the area of the IR bands of the L and B CO species (the B1 and B2 CO species are not differentiated because the decomposition of the two IR bands is difficult) during the increase in Ta for PCO= 1 kPa is used to follow the decrease in their coverage (θL and θB) according to Eq. 1. The ALM and ABM values (i.e. area at saturation of the sites) in Eq. 1 are obtained as follows: the increase in Pa at Ta= 300 K according to the switch 1% CO/He → 5% CO/He has no impact on the intensity and position of the IR bands of the L and B CO species indicating the full coverage of their sites for Pa= 1 kPa. This allows us using the area of the IR bands of L and B CO species at 300 K for Pa= 1 kPa as AL(M) and AB(M) values respectively. Symbols  and in Figure 3 give the evolutions of θL and θB respectively from the FTIR spectra of Figure 2. Note that the coverage of the L CO species remains roughly unmodified at ≈ 0.17 for Ta ≥ 560 K due to the presence of the small amount of L2Pd° CO species. The contribution of the IR band of this CO species can be subtracted at each temperature to the area of the IR band of the L CO species leading to the evolution of the coverage of the L1Pd° CO species during the increase in Ta for Pa= 1 kPa (symbols in Figure 3). The reproducibility of the data has been ascertained by performing a new H(713 K)/C(300 K) cycle in 1% CO/He after symbols and  (without any H2 reduction of the solid): this provides symbols and in Fig. 3 for the LPd° and L1Pd° CO species respectively which are overlapped with symbols  and


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16

respectively. According to the AEIR method,8-15 curves a and b which overlap the experimental coverages of the BPd° and L1Pd° CO species are obtained considering the Temkin model (Eq. 2) for localized adsorbed species (Eq. 3) using the following heats of adsorption: EL1(1)= 54 and EL1(0)= 92 kJ/mol at coverage 1 and 0 respectively for the L1Pd° CO species and EB(1)= 90 kJ/mol and EB(0)= 175 kJ/mol at coverage 1 and 0 for the BPd° CO species. These values (reported in Table 1 for comparison with the Pd-Sn bimetallic particles) are very similar to those measured on a 1.4 wt% Pd/Al2O3: 54 kJ/mol and 92 kJ/mol for the L1Pd° CO species and 92 kJ/mol and 168 kJ/mol for the BPd° CO species.11 Note that for the BPd° CO species the Temkin model for one adsorbed CO species fits the average coverage θB of the B1Pd° and B2Pd° CO species without considering Eq. 4. This is due to the fact that (a) the average coverage remains high at 713 K (θB= 0.58) for PCO= 1 kPa and (b) the heats of adsorption of the two species are similar. This prevents discriminating the contribution of the more strongly adsorbed CO species (as was observed for the case of the L CO species). Note, that the fact that two B CO species adsorbed on different sites have similar heats of adsorption is consistent with DFT calculations on the adsorption of CO on Pd(210)42 which indicate that there are two bridge sites (denoted E and C) being energetically almost degenerate with a heat of adsorption of CO of ≈ 179 kJ/mol (difference between the two species of 0.02 eV) consistent with the present study.

3.2 Adsorption of CO on 2% Pd-2% Sn/Al2O3 and Associated Surface Processes. Spectrum a in Figure 4A has been recorded after adsorption of 1% CO/He at 300 K on the fresh 2% Pd-2% Sn/Al2O3 solid reduced in H2 at 713 K: a strong IR band at 2075 cm-1 is detected due to a linear CO species (denoted L12Pd-2Sn) indicating that the introduction of Sn suppresses the Pd° sites forming the B CO species as compared to the monometallic solid (Figure 1). This is consistent with different literature data indicating that the intensity of the IR band of the B CO


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17 species decreases with the increase in the Sn loading.18, 21, 43, 44 However, to our knowledge only Sales et al.21 mention the total disappearance of this IR band (as observed in Figure 4A) for a 5.1 %Pd-3.8% Sn/Al2O3 solids. Moreover, the presence of B CO species on Pd-Sn containing solid is mainly ascribed to an heterogeneity in the PdxSny particles with y≥0.43 This suggests that the preparation method of the present Pd-Sn particles prevents the formation of mono metallic Pd° particles. Similarly to 2% Pd/Al2O3, there is an ageing of the Pd-Sn particles according to the numbers of A/R cycles as shown in Figure 4A which indicates the decrease in the intensity of the IR band of the L12Pd-2Sn species associated with a slight shift to lower wavenumbers. At the difference of the monometallic Pd particles each new A/R cycle decreases slightly the intensity of the IR band of the L12Pd-2Sn CO species: a clear pseudo steady state is not observed even after six A/R cycles. However, this has a limited impact on the measurement of the heats of adsorption of the L12Pd-2Sn species as shown below because the decrease due to the ageing is very small as compared to the impact of the adsorption temperature. Moreover, whatever the ageing (Figure 4A) a second surface process is operant during the first increase in Ta. For instance, after a first A/R cycle, Figure 4B compares the intensity of the IR band of the L1Pd-Sn CO species at 300 K using 1% CO/He (a) on the reduced solid (spectrum a) and (b) after two successive H(713K)/C300 K) cycles in 1% CO/He (spectra b and c). It can be observed that the first H(713 K)/C(300 K) cycle (spectrum b) leads to a significant increase (by 23%) in the intensity of the IR band of the L12Pd-2Sn species associated to a shift to higher wavenumbers. This is ascribed to a reconstruction of the Pd-Sn surface in the presence of CO similarly to the reconstruction of Sn/Pd intermetallic compounds on Pd(110).45 However, in Figure 4B, the area of the IR band in spectrum b is only slightly modified (increase by ≈ 3%) as compared to spectrum a, indicating that the reconstruction leads mainly to a sharper IR band without increasing strongly the number


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18 of adsorption Pd sites. The second H(713 K)/C(300 K) cycle (spectrum c) leads to a very small decrease in the intensity of the IR band. Whatever the number of A/R cycles the first H(713 K)/C(300 K) cycle in 1% CO/He is associated with a reconstruction process as shown in Figure 5A for three A/R cycles. Note that this reconstruction does not lead to the detection of the IR band of the B CO species indicating that the Sn fraction of the surface of the Pd-Sn particles remains very high (there is no segregation of Pd° to the surface of the Pd-Sn particles).

3.3 Heat of Adsorption of the L12Pd-2Sn Species on 2% Pd-2% Sn/Al2O3 The measurement of the heat of adsorption of the L12Pd-2Sn species on the 2% Pd-2% Sn/Al2O3 solid has been performed after two A/R cycles and one H(713 K)/C(300 K) cycle in 1% CO/He. Inset A of Figure 6 shows the evolution of the IR band of the L12Pd-2Sn species for 1% CO/He (Pa= 1 kPa) during the increase in Ta. It can be observed that the intensity of the IR band of the L12Pd-2Sn species decreases progressively with the increase in Ta in the range 300-713 K associated with a shift to lower wavenumbers: 2080 and 2065 cm-1 at 300 and 628 K respectively. Insert B in Figure 6 provides similar data after a new A/R cycle showing the good reproducibility of the experiment. However, the successive A/R cycles lead to the detection of a shoulder at 2034 cm-1 for high Ta values (see inset B Figure 6) indicating the formation of a small amount of a new linear CO species (denoted L22Pd-2Sn) with a heat of adsorption slightly higher than that of the L12Pd-2Sn. According to Eq. 1, the estimation of the coverage of L12P-2Sn at each temperature imposes the measurement of the area of the IR band at full coverage of the sites. For instance, Figure 5B shows that the switch 1% CO/He (spectrum a) → 4% CO/He (spectrum b) increases slightly the intensity of the IR band. For PCO> 4 kPa the IR spectra are overlapped with spectrum b. This indicates that the saturation of the sites of the L12Pd-2Sn is obtained for PCO= 4 kPa


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19 providing the values of AL12Pd-2Sn(M) in Eq. 1. Then the area of the IR band of spectrum a in Figure 5B and Eq. 1 indicate that the coverage of the L12P-2Sn CO species is θL12Pd-2Sn= 0.95 at 300 K for PCO= 1 kPa. The impact of Pa on the IR band of the L12Pd-2Sn species after different (a) A/R and (b) H(713 K)/C(300 K) cycles in 1% CO/He indicates that the coverage of the L12Pd-2Sn species is always of 0.95 at 300 K for Pa= 1 kPa. The overlapped symbols and in Figure 6 give the evolution of the coverage of the L12Pd-2Sn CO species from the data in insets A and B of Figure 6 respectively showing the good reproducibility of the data for two A/R cycles. Curve a in Figure 6 provides the theoretical evolution of θL12Pd-2Sn considering the Temkin model and assuming localized adsorbed species (Eqs. 2-3) with the following heats of adsorption EL1-2Pd-2Sn(1)= 49 kJ/mol and EL1-2Pd-2Sn(0)= 108 kJ/mol which indicate slight modifications as compared to those of the L1Pd° CO species on the monometallic particles (see Table 1). It is clear that the main impact of the insertion of Sn in the palladium particles of the 2% Pd-2% Sn/Al2O3 solid is a geometric effect which suppresses the ensemble of Pd° sites forming the B CO species. This creates “more isolated” Pd sites for the formation of linear CO species. For instance, Figures 1 and 4 indicate that for two pellets of similar weight, the ratio of the intensities of the IR bands of the L12Pd-2Sn and LPd° CO species is of ≈ 10. However, quantitative measurement using the M.S system (see below) show that the total number of Pd° sites on the surface of the particles decreases up the insertion of Sn. Figures 4 and 5 indicate that the number of A/R and H(713 K)/C(300 K) in 1% CO/He cycles modifies the IR band of the L12P-2Sn CO species due to ageing and reconstruction processes respectively. However, these two processes are subdued whenever applying a limited increase in Ta as shown by the inset of Figure 5B that compares on the fresh reduced solid (first reduction) the intensities of the IR band of the L12P-2Sn CO species at three adsorption temperatures during


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20 the heating stage to 383 K (spectra a, b, c) and the cooling stage to 300 K (spectra c, d, e) in 1% CO/He. It can be observed that the intensities are only slightly higher during the cooling stage suggesting the absence of reconstruction. This allows us measuring the heat of adsorption of the L12Pd-2Sn CO species on the fresh and unreconstructed Pd-Sn surface. Symbols and  in Figure 6 show (from the inset of Figure 5B) the evolutions of the coverage of the L12Pd-2Sn species during the heating and cooling stage in 1% CO/He (Eq. 1) respectively. Curve b which overlaps the experimental data is obtained (Eqs 2-3) considering the following heats of adsorption 50 kJ/mol and 90 kJ/mol at high and low coverage respectively. This shows that the reconstruction process at high temperatures has a limited impact on the heat of adsorption at low coverage (108 kJ/mol after reconstruction curve a in Figure 6). It is shown below using the 2% Pd-5% Sn/Al2O3 solid that the difference is probably due to the formation by reconstruction of a second type of L CO species having a higher heat of adsorption than the L12Pd-2Sn CO species and associated to the broad shoulder at 2034 cm-1 in inset B of Figure 6. Note that the decrease in the coverage of the L CO species on the freshly reduced sample is limited to 0.6. However, we have shown that this has no impact on the determination of the heats of adsorption at high and low coverages (i.e. the value at low coverage fixes the slope of the linear section of the isobar which is clearly observed in Figure 6).9 Quantitative measurements on the amounts of CO and H2 chemisorption on 2%Pd2%Sn/Al2O3 provide supplementary data on the modifications of the surface properties of Pd° sites by insertion of Sn (see below).

3.4 Heat of Adsorption of the Adsorbed CO Species on 2%Pd-5%Sn/Al2O3. The adsorption of 1% CO/He at 300 K on the reduced 2% Pd-5% Sn/Al2O3 solid leads to spectrum a in inset A of Figure 7 with an IR band at 2066 cm-1 ascribed to a linear CO species


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21 (denoted L12Pd-5Sn) which confirms that the insertion of Sn suppresses the Pd sites forming the bridged CO species. However, the comparison of spectra a and b in inset A of Figure 7 obtained for two pellets of similar weight of 2% Pd-5% Sn/Al2O3 and 2% Pd-2% Sn/Al2O3 respectively shows that the increase in the Sn loading decreases significantly the intensity of the IR band of the L CO species (factor 6.5) associated with a shift to lower wavenumbers. The evolutions of the IR band of the L2Pd-5Sn species with the number of either A/R or/and H(713 K)/C(300 K) cycles in 1% CO/He are qualitatively similar to those of the IR band of L2Pd-2Sn CO species. In particular, after a first H(713 K)/C(300 K) cycle in 1% CO/He the intensity of the IR band of the L12Pd-5Sn species increases as shown by spectrum c in inset A of Figure 7 due to a surface reconstruction. However, on 2% Pd-5% Sn/Al2O3, the area of the IR band is also significantly increased (factor ≈ 1.9 between spectra c and a in inset A of Figure 7 as compared to 1.03 on 2%Pd-2%Sn/Al2O3). This suggests that the reconstruction increases significantly the number of Pd° adsorption sites for the sample which the largest Sn content. Similarly to the 2% Pd-2% Sn/Al2O3 solid, the heat of adsorption of the L12Pd-5Sn CO species has been measured after a first A/R cycle and a first H(713 K)/C(300 K) cycle in 1% CO/He. At 300 K, the impact of PCO indicates that the full coverage of the sites is obtained for 4 kPa and that the coverage for 1 kPa is 0.95. According to Eq. 1, symbols in Figure 7 give the evolution of the coverage of the L12P-5Sn species during the cooling stage of the second H(713 K)/C(300 K) cycle in 1% CO/He (see the FTIR spectra in Figure S1 of the supporting information section). Curve a in Figure 7 has been obtained considering Eqs 2-3 with the following heats of adsorption: EL12Pd-5Sn(1)= 52 kJ/mol and EL12Pd-5Sn(0)= 92 kJ/mol. Note that at high temperatures the experimental coverages are slightly higher than the theoretical curve. This is due to the contribution of a second more strongly adsorbed linear CO species


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22 characterized by an IR band at 2022 cm-1 (see Figure S1) denoted L22Pd-5Sn and probably formed during the reconstruction process. Indeed, the contribution of this adsorbed CO species increases with the numbers of A/R cycles. For instance, insert B of Figure 7 shows the evolution of the IR band of the linear CO species on the reconstructed 2% Pd-5% Sn/Al2O3 after three A/R cycles. It can be observed that the decrease in the main IR band at 2075 cm-1 reveals a broad IR band at 2022 cm-1 due to the L22Pd-5Sn species with a heat of adsorption higher than that of the L12Pd-5Sn species. The accurate decomposition of the two IR bands is difficult and symbols in Figure 7 are obtained from the evolution of the area of the two IR bands in inset B of Figure 7. It can be observed that symbols overlap the theoretical curve a for Ta< 480 K and they differ significantly for higher temperatures due to the increase in the contribution of the L22Pd-5Sn species. This allows us using Eq. 4 to measure the heats of adsorption of the two species and their contribution to the IR band. This gives curve b in Figure 7 with the following parameters: the contributions of the two adsorbed L12P-5Sn and L22P-5Sn to the area of the global IR band are x1= 0.85 and x2= 0.15 respectively whereas their heats of adsorption at high and low coverage are (a) EL12P-5Sn(1)= 52 kJ/mol and EL12P-5Sn(0) = 87 kJ/mol and (b) EL22P-5Sn(1)= 80 kJ/mol and EL22P5Sn(0)=

120 kJ/mol respectively. A similar L2 CO species is also present on the 2%Pd-2%Sn (see

inset B in Figure 6 with the broad shoulder at 2034 cm-1) but in smaller proportion preventing using equation 4. However, the contribution of this broad shoulder which may indicate the formation (during the reconstruction) of an heterogeneity in the Pd sites forming the linear CO species is included in the averaged heats of adsorption at low coverage (108 kJ/mol). Finally the data in Table 1 show clearly that the heat of adsorption of the L CO species on Pd° and Pd-Sn particles are slightly different suggesting a very limited electronic effect of Sn whereas its geometric effect is strong considering the disappearance of the Pd sites forming the B CO species on the Pd° particles.


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23

3.5 CO and H2 Chemisorption and TPAE Method using the M.S. Quantitative CO and H2 Chemisorption These measurements allow the comparison of the adsorption capacity of the mono and bimetallic solids for CO and H2 that are complementary to the FTIR data. Figure 8 shows the evolution of the molar fractions of Ar (tracer) and CO at the outlet of the quartz microreactor during a switch He → 1% CO/1% Ar/He at 300 K on a freshly reduced 2% Pd-2% Sn/Al2O3 solid. The total amount of adsorbed CO from the difference between the Ar and CO curves is of 17.3 µmol of CO/g of catalyst. The inset in Figure 8 gives similar data using a switch He → 1% H2/1% Ar/He on the reduced solid cooled to 300 K in helium, indicating a very small amount of H2 chemisorption: 1.2 µmol. H2/g. The adsorption of 1% CO/1% Ar/He on the freshly reduced solid has been also measured at 383 K: 11.9 µmol/g. Similar experiments have been performed on the freshly reduced 2% Pd/Al2O3 and 2% Pd-5% Sn/Al2O3 solids and the different values are indicated in Table 2. It is clear that the increase in the Sn loading from 2% to 5% decreases significantly the CO adsorption capacity whereas that for H2 of the solid with 5% Sn is too low to be measured in the present experimental conditions. The data in Table 2 provide different information on the adsorption properties of the solids and the impact of the insertion of Sn in the Pd particles. Considering the 2% Pd/Al2O3 solid, Table 2 shows that the amount of H2 and CO chemisorbed are similar. Assuming dissociative hydrogen chemisorption, this confirms that the adsorption of CO gives mainly the B CO species on the monometallic particles as observed in Figure 1. The amounts of Pd° sites forming the LPd° and BPd° CO species can be obtained as follows: (a) the decrease in the amount of adsorbed CO by the increase in Ta from 300 to 383 K


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24 is 0.7 µmol/g (Table 2) and (b) Figure 3 indicates that the coverage of the BPd° CO species is not modified in this range of temperature whereas that of the adsorbed L1Pd° CO species is decreased from 1 to 0.65. This indicates that at full coverage of the L1Pd° CO species its amount is 0.7/(10.65)= 2 µmol/g which quantifies the amount of Pd° sites adsorbing the L CO species (the small amount of strongly adsorbed L2 CO species is neglected in the calculation) whereas the amount of Pd° sites (couple of Pd° sites) adsorbing the BPd° CO species is ≈(13.7-2)= 11.7 µmol/g. Assuming a dissociative H2 chemisorption on the different Pd° sites these values indicate that the Pd° sites adsorbing CO may adsorb (11.7+ 2/2)= 12.7 µmol of H2 /g which is consistent with the H2 chemisorption value measured at 300 K: i.e. 13.5 µmol/g (Table 2). The larger amount of H2 chemisorption is maybe due to a slight diffusion of hydrogen atom in the Pd° particles forming hydride.46 However this activated process46 cannot contribute significantly to the present measurements due to the short period of adsorption (< 2 min in inset of Figure 8). The total amount of Pd° sites on the surface of 2% Pd/Al2O3 is of 25.4 µmol of Pd° site/g, which leads to a Pd dispersion of D= 0.135 and a particle size of 8.1 nm considering the relationship between the particles size d (in nm) and the palladium dispersion d= 1.13/D.47,48 This particle size is consistent with that measured on a reduced 5.05% Pd/Al2O3 by Sales et al.48 using different analytical methods (TEM, XRD and H2 chemisorption): ≈ 11 nm. Table 2 shows that the introduction of 2% Sn increases significantly the amount of CO chemisorption at 300 K (by 3.6 µmol/g of catalyst) due to the transformation by dilution of Pd sites forming the B CO species into Pd sites forming the L CO species. Taking into account that the coverage of the L2Pd-2Sn CO species at 300 K for PCO= 1 kPa is of 0.95 the total number of Pd° sites for the 2% Pd-2% Sn/Al2O3 solid is of 17.3/0.95= 18.2 µmol/g that is an amount lower than on 2% Pd/Al2O3: 25.4 µmol Pd° site/g. Moreover the insertion of Sn is strongly detrimental to the


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25 H2 chemisorption which decreases from 13.5 to 1.2 µmol/g of solid on the Pd and Pd-Sn particles respectively. This strong reduction of the hydrogen chemisorption capacity of Pd-Sn particles has been previously mentioned by Sales et al.48 This can be ascribed to the disappearance of the couples of Pd° sites adsorbing the B CO species on the monometallic Pd° particles that seem favorable to the dissociative hydrogen chemisorption. The strong decrease in the amount of hydrogen chemisorption of the Pd-Sn particles is not inconsistent with the use of these particles in hydrogenation reactions. However, others surface processes linked to the bimetallic particles can be more favorable. For instance, on Pd-Cu particles, it is considered that (a) hydrogen spillover (strongly favored on these particles) is implicated in the performances on the solid for unsaturated hydrogenation39, 49, and (b) DFT calculations suggest that single Pd atom situated at a edges position of the bimetallic particles considerably reduce the energy barrier for hydrogen dissociation.50 These different surface processes must be considered for the understanding of the global rate of hydrogenation reactions on mono and bimetallic particles such as CO/H2 on Pd and Pd-Sn particles on Al2O3 which is underway in our group by an experimental microkinetic approach. Note that the increase in the amount of L CO species by the insertion of Sn from 2 µmol/g on 2% Pd/Al2O3 to 18.2 µmol/g on 2% Pd-2% Sn/Al2O3 is consistent with the difference in the intensity of the IR band of the L CO species on the two solids (factor ≈ 10, Figures 1 and 4) showing that the IR molar absorption coefficient of the L CO species are not strongly different on the two solids. The data in Table 2 concern the freshly reduced 2% Pd-2% Sn/Al2O3 solid and the amounts of CO adsorbed at 300 and 383 K allow a comparison of the coverage of the L12Pd-2Sn at 383 K obtained from M.S and FTIR data. The full coverage of the L12Pd-2Sn corresponds to 18.2 µmol CO/g indicating that the coverage at 383 K is of 10.9/18.2= 0.6 which is the value determined by FTIR on the none reconstructed solid (symbol in Figure 6). This validates Eq. 1


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26 for the L12Pd-2Sn species. Table 2 shows that the increase in the Sn loading from 2% to 5% decreases strongly the adsorption capacity for CO and the total number of Pd° sites on the surface: (4.5 µmol/g)/0.95 = 4.7 µmol of Pd sites/g This suggests that the excess of Sn blocks the Pd sites of the Pd-Sn bimetallic particles which is consistent with the TEM images of the solids shown below. Note that the strong decrease in the amount of CO chemisorption at 300 K on 2% Pd- 5% Sn/Al2O3 is consistent with the lower intensity of the IR band of the L2Pd-5Sn species compared to that of the L2P-2Sn species (Figure 7 inset A). TPAE method applied to the adsorption of CO on 2% Pd-2% Sn/Al2O3. As described in more details in the supporting information section, the TPAE method is a complementary analytical procedure to the AEIR method. It is similar to the conventional TPD method in flowing gas conditions using a microreactor except that there is a molar fraction of the desorbing gas in the inlet gas flow rate. The advantage of this procedure is that the adsorption equilibrium at a quasi constant adsorption pressure Pa is maintained during the increase in Ta facilitating the mathematical exploitation of the experimental data and suppressing several difficulties linked to the design of the TPD experiments.51,52 However, TPAE imposes a careful design of the experiments (weight of the sample and choice of the heating rate) to prevent the following situations: (a) a large net desorption rate (high amount of adsorbed species in the reactor and/or high heating rate) may prevent assuming that Pa remains quasi constant (this condition is fulfilled if the increase in the Pa is < ≈30 % of the inlet partial pressure)27 and (b) a low net desorption rate (small amount of adsorbed species in the reactor and/or low heating rate) cannot be quantified with an acceptable accuracy using the MS system. For instance, after the adsorption equilibrium at 300 K on the reduced 2% Pd-2% Sn/Al2O3 using a 0.2% CO/0.2%


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27 Ar/He (results not shown similar to Figure 8), a first H(713 K)/C(300 K) cycle in the presence of CO is performed (leading to a reconstructed surface). Figure 9 gives the evolution of the CO molar fraction during the second H(580K)/C(300 K) cycle in 0.2% CO/0.2% Ar/He constituting the experimental part of the TPAE method. It can be observed that (a) the increase in Ta leads to a net CO desorption rate due to the decrease in the adsorption equilibrium coverage of the L CO species and (b) the highest CO molar fraction is 0.218: allowing us to validate the assumption of isobaric conditions. The red curve in the inset of Figure 9 gives the evolution of the coverage of the adsorbed CO species during the increase in Ta from Eq. 5 of the supporting information section. From the FTIR data the coverage of the L12Pd-2Sn CO species at 300 K for Pa= 0.2 kPa is of 0.89. The blue curve in the inset of Figure 9 gives the theoretical evolution of the coverage from Eqs 2-3 (adsorption pressure Pa= 0.2 kPa) with the following heats of adsorption 49 kJ/mol and 105 kJ/mol at high and low coverages that are consistent with those obtained using the AEIR method for the L CO species on 2% Pd-2% Sn/Al2O3 (Table 2). The difference at low coverage between AEIR and TPAE methods comes from the accuracy of the TPAE measurements at high temperatures using the M.S due to the small difference between the inlet and outlet molar flow rates in the microreactor. Note the decrease in the CO molar fraction during the fast cooling stage (Figure 9) which is due to the high net CO adsorption rate. Finally, the good agreement between the heat of adsorption from the TPAE (heat of adsorption of CO) and AEIR (heat of adsorption of the L CO species) methods confirms the linear relationship between the area of the IR band of the L2Pd-2Sn species and its amount on the surface (Eq. 1) as observed previously for different adsorbed CO on supported Pt°,53 Cu°54 and Ir°55 particles and NH3 species on TiO2.28 3.6 Averaged and Local Structure/Composition of the Pd-Sn Particles The FTIR spectra after adsorption of CO on Pd/Al2O3 and Pd-Sn/Al2O3 solids show clearly that the introduction of Sn leads to the total disappearance of the Pd° ensembles forming


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28 the B CO species. However this can be due to either the deposition of Sn on the surface of the Pd° particles (whereas the excess of Sn is deposited as SnOx on the alumina support) or to the formation of bimetallic particles PdxSny of defined structure/composition. Averaged and localized structures/compositions of the bimetallic particles have been studied by XRD and with the LyEtTEM microscope. Figure 10 provides the X-ray diffractograms of the alumina support and of the three Pd containing solids. To facilitate the identification of the different phases of the Pd and Pd-Sn particles, the inset in Figure 10 gives their XRD patterns after subtraction of those of alumina support. Table S1 of the supporting information gives the XRD patterns of different phases (except alumina) which can be present after the preparation of the three Pd containing solids according to the ICDD-PDF-4+ database. On 2% Pd/Al2O3 the three XRD patterns at 2θ= 40.2° and 46.9° and 67.9° (Inset of Figure 10) are ascribed to the cubic phase of Pd° particles similarly to literature data on reduced Pd/Al2O3 or Pd/SiO2 solids of similar Pd loading (≤5%).19,48,56 The two Pd-Sn containing solids present same XRD patterns at 2θ= 39.5°, 45.8°, 46.5° and 67.7°. According to Table S1, none of them can be ascribed to the SnO2 phase whereas the XRD pattern at 2θ= 45.8° and 46.5° can be ascribed to the cubic phase of Pd3Sn and the orthorhombic phase of Pd2Sn respectively with their common contribution to the patterns at 39.5° at 67.7°. It cannot be ruled out that the pattern at 46.5° corresponds partially to a small amount of Pd° cubic phase. However, the presence of Pd2Sn and Pd3Sn phases are mentioned in different studies (a) for reduced x% Pd-y% Sn supported either on Al2O3 or SiO2 with x and y values similar to the present study19, 48, 57, 58, 59 and (b) on model surface alloys such as PdSn/Pd(111).60, 61 Finally, the XRD patterns of the three solids lead to the conclusion that the insertion of Sn decreases strongly the amount of the cubic Pd° phase by formation of different PdxSny particles in particular Pd3Sn


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ACS Catalysis

29 and Pd2Sn indicating an heterogeneity in the composition of the Pd-Sn bimetallic particles. The size and the local structure/composition of the Pd-Sn particles have been studied with the Ly-EtTEM microscope. For the structure/composition of the bimetallic particles video of the TEM analysis indicate that the particles change under the electron beam probably due to the vaporization of tin which is often observed for dispersed Sn-containing particles and this may contribute to modifying the Pd/Sn ratio. TEM indicates that the particle sizes of the freshly prepared 2% Pd-5% Sn/Al2O3 solid are mainly in the 2-6 nm range with few larger particles: ≈ 10 nm (see Figure S2) which is consistent with literature data on solids of similar composition48, 56, 57, 58

whereas significantly higher particular size are mentioned on a SiO2 support: 20-60 nm

range.59 The EDX analysis of five regions of the sample (See Figure S1) indicate that four are of composition PdxSny and one contained only Sn probably SnOx species formed by the excess of tin. The identification of the structure of a bimetallic particle (see Figure S3 of the supporting information section) is not straightforward. The experimental parameters may correspond to different PdxSny phases (such as Pd3Sn2, PdSn and PdSn2) as shown in Table S2-S3 of the Supporting Information section. Moreover, TEM reveals that the PdxSny particles of 2% Pd-5% Sn/Al2O3 are often either partially (see Figure S3) or totally (see Figure S4) surrounded of an amorphous phase of SnOx. This is not observed for the 2% Pd-2% Sn/Al2O3 catalyst (see below) and it is consistent with the significant decrease in the intensity of the IR band of the L CO species with the increase in the Sn (inset A Figure 7). The 2% Pd-2% Sn/Al2O3 solid has been analyzed by TEM either as prepared or after in situ reduction at 673 K in a H2 flow at the total pressure of 7 mbar followed by cooling the solid in H2 to 300 K without observing significant differences in the particle sizes of the bi-metallic particles. For instance, Figure S5 recorded at 300 K after in situ reduction shows that the particles


ACS Catalysis

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30 are in the 2-4 nm range without large particles and that they are rarely surrounded by SnOx. This may contribute to the high intensity of the IR band of the L2Pd-2Sn species as compared to that of the L2Pd-5Sn species. EDX analysis (see Figure S6 and Table S4 of the supporting information section) of different regions of the sample indicates the close proximity of Pd and Sn with Pd/Sn ratio in the 2.32-6.1 range (the vaporization of tin under the electron beam may contribute to the high Pd/Sn ratios). The structure of the bimetallic particles of the 2% Pd-2% Sn/Al2O3 have been studied by TEM as shown in Figure S6-S8 and the experimental parameters can be tentatively ascribed to Pd3Sn (see Tables S4-S6). However, it must be noted that they are also close to those of the cubic phase of Pd° (see Tables S4-S6) showing the difficulty in the clear identification of the local structure of the Pd-Sn bimetallic particles. Finally, XRD and Ly-EtTEm indicate that (a) PdxSny particles of different compositions are present on 2% Pd-x% Sn/Al2O3 (x=2 or 5) such as Pd3Sn, Pd3Sn2 and Pd2Sn structure, (b) the particle size increases with the Sn loading and (c) for the highest Sn loading the bimetallic particles are often surrounded (either partially or totally) by an amorphous phase of SnOx. The two last points are consistent with the higher (a) intensity of the IR band of the L CO species and (b) adsorption capacity at 300 K for x= 2 as compared to x= 5. Moreover, the strong increase in the IR band of the L CO species on 2% Pd-5% Sn/Al2O3 by the reconstruction process during the CO adsorption at high temperatures can be linked to the modification of the coverage of the bimetallic particles by the SnOx layer.

3.7 Comparison of the Heats of Adsorption of the L2Pd-2Sn and L2Pd-5Sn Species to Literature Data. The comparison of the heats of adsorption of the LPd° and BPd° CO species obtained by using the AEIR method to literature data has been discussed in previous works.11, 12 Recent works are in line with our previous finding. For instance, using single crystal microcalorimetry, Flores-


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ACS Catalysis

31 Camacho et al.29 have shown for model Pd/Fe3O4/Pt(111) particles of different sizes (1.8- 8 nm range) and Pd(111) that, whatever the sample, the heat of adsorption of CO linearly varies with the coverage in agreement with the Temkin model used in the present study and (b) that the heat of adsorption at low coverage is in 110-140 kJ/mol depending on the particle size (the higher the particles size the higher the heats of adsorption) which is consistent with the averaged values of the LPd° and BPd° CO species in the present study. Moreover, Loffreda et al.62 have performed DFT calculations on the heats of adsorption of the adsorbed species formed on Pd(111). At a coverage of 0.33 ML, they determine that the B CO species are more strongly adsorbed than the L (Top) CO species with heats of adsorption of 1.81 eV (174 kJ/mol) and 1.36 eV (131 kJ/mol) which are consistent with the values in the present study at a coverage of 0.33: 147 and 84 kJ/mol respectively considering the difference in the approaches (theoretical versus experimental) and materials. Note that the authors show that the most strongly adsorbed CO species on Pd(111) is the threefold coordinated species not observed on the present Pd/Al2O3 catalyst (IR band below 1850 cm-1).62 Finally, the DFT calculations of Lischka et al.42 on the adsorption of CO on Pd(210) indicate that there are two types of sites forming bridge-bonded CO species of very similar heats of adsorption (almost degenerated) ≈ 179 kJ/mol (difference between the two species of 0.02 eV) which is consistent with the fact the total surface area of the two IR bands of the B CO species (Figure 2) can by represented by the Temkin model for a single adsorbed species (curve a in Figure 3). . To our knowledge there are only few experimental and theoretical studies dedicated to the measurement of the heats of adsorption of CO on Pd-Sn particles. For instance Hill et al.44 have compared by microcalorimetry the differential heat of adsorption of CO on reduced 4.27% Pd/SiO2 and 3.81% Pd-1.39% Sn/SiO2 solids. They indicate that the insertion of Sn decreases the


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32 heats of adsorption of CO from 132 kJ/mol to 120 kJ/mol at low coverage. However they considered that the values on Pd° is underestimated due to the absence of adsorption equilibrium in the sample explaining their difference with single crystal microcalorimetry which indicates 163 kJ/mol. This value is similar to that of the B CO at low coverage obtained by the AEIR method on the presence 2% Pd/Al2O3 catalyst (Table 1). The value of 120 kJ/mol on PdSn/SiO2,44 is consistent with the present study: i.e. 108 kJ/mol (Table 1). However in Ref. (44) the FTIR data indicate that the insertion of Sn decreases the amount of B CO species without their total disappearance as in the present study indicating that 120 kJ/mol is probably the average of the heats of adsorption of B and L CO. More recently Tsud et al.45 have compared by TPD procedure the CO chemisorption on Pd(110) and after Sn deposition at 300 K and 120 K forming Pd3Sn and Pd2Sn phases. The presence of Sn decreases the temperature at the maximum of the TPD peaks from TM≈ 480 K to 370 K. Assuming, that TM is proportional to the activation energy of desorption ED they estimate that roughly ED decreases from ≈ 130 kJ/mol to 80 kJ/mol before and after Sn deposition respectively. This decrease is consistent with the present study which shows that this is due to the suppression of the Pd° adsorption sites forming strongly adsorbed B CO species on Pd° leading only to the L CO species which are weakly adsorbed. Pick63 has performed DFT calculations on the adsorption of CO at low coverage (1/6) on Pd(110) and on different Sn containing surfaces: PdSn and PdSn2. He shows that the insertion of Sn modifies the CO adsorption sites from short bridge CO species on Pd(110) to atop CO species on Sn containing surface. This is consistent with the FTIR data of the present study after adsorption of CO on 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 with x= 2 or 5 (compare Figures 1 and 4). Moreover Pick determines that the heat of adsorption of the short bridge and atop CO species on Pd(110) are 1.44 and 1.19 eV (134 and 115 kJ/mol) respectively whereas their position


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ACS Catalysis

33 of their IR bands are 1967 and 2116 cm-1. On different structures of Sn containing surface the heat of adsorption of the atop is in the 0.6-0.85 eV (58-82 kJ/mol) range with an IR band position in the 2107-2103 cm-1 range. The slight shift of the IR band of the atop species in the presence of Sn is consistent with the present study 2095 (Figure 1) and 2080 cm-1 (Figure 4) on stabilized and reconstructed Pd° and Pd-Sn particles respectively. The heat of adsorption of 82 kJ/mol on a Sn containing surface in Ref (63) is consistent with the heat of adsorption of the L12Pd-2Sn(0) CO species before reconstruction (curve b in Figure 6): 91 kJ/mol and (b) with EL12Pd-5Sn(0)= 84 kJ/mol obtained using Eqs 4.

4- Conclusion The present study is a contribution to the evaluation of the geometric (ensemble) and electronic (ligand) effects due to the insertion of Sn in palladium particles to form bimetallic particles using the modifications of the properties of Pd sites for the chemisorption of CO: nature and heats of adsorption of the adsorbed CO species on the Pd° sites. Three solids have been studied (weight %) 2% Pd/Al2O3, 2% Pd-2% Sn/Al2O3 and 2% Pd-5% Sn/Al2O3. On the monometallic solid (particle size ≈ 8 nm) linear and bridged CO species (denoted L and B CO species) have been formed with significantly different heats of adsorption. The L CO species is weakly adsorbed (54 kJ/mol and 90 kJ/mol at high and low coverages) as compared to the B CO species (90 kJ/mol and 175 kJ/mol at high and low coverages). XRD and TEM/EDX indicated that PdxSny bimetallic particles have been formed particularly with x= 3 and 2 and y=1. It has been shown that the main impact of Sn is the total disappearance of the palladium sites/ensembles forming the B CO species due to a geometric effect. The heats of adsorption of the L CO species on the PdxSny particles are slightly lower than that on the Pd° particles indicating that the


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34 electronic effect is very modest. Moreover, “displacive” reconstruction41 of the PdxSny particles in the presence of CO leads to the formation of a small amount of a second linear CO species (roughly 15% of the L CO species) with a heat of adsorption slightly higher than the Lpd° CO species. In parallel to the study of the geometric and electronic effects in Pd-Sn bimetallic particles, the present study constitutes the first stage of the experimental microkinetic approach ‘denoted EMA) of the global rate of the CO/H2 reaction on Pd and Pd-Sn particles supported on alumina by the determination of the parameters controlling the adsorption of CO on the two surfaces. It has been shown that the heat of adsorption of the L CO species on Pd sites on Pd and Pd-Sn particles are not strongly different. So the global rate of hydrogenation of CO on the two types of particles must differ due to the modification of the Pd sites forming the B CO species: suppression of a type of adsorbed species and decrease in the dissociative hydrogen chemisorption. This decrease seems a priori unfavorable to the hydrogenation reaction. However, others favorable surface processes can be operant on the bimetallic particles such as hydrogen spillover,22, 39,49 dissociation on single metal atom50 and specific sites geometry64, 65 which must be taken into account in the development of the EMA.


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35

FIGURES:

1987 1954 2087 a

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2095

b

0.01 1957

c

d 2200

2100

2000

Wavenumber

1900

(cm-1)

1800

Figure 1: Comparison of IR spectra after adsorption of 1% CO/He at 300 K on 2% Pd/Al2O3 after different pretreatments: (a) after reduction of the fresh solid in H2 at 713 K, (b) after two A/R cycles (see the text for more details), (c) after one H(713 K)/C(300 K) cycle after spectrum (b) and (d) after a second H(713 K)/C(300 K) cycle after spectrum (c).


ACS Catalysis

36

1987 2095

a Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 51

1974 1957

0.004

1960

a

1922

2083

l

2072

2062

2200

2100

1938

l

l a

2000 1900 Wavenumber (cm-1)

1800

Figure 2: Evolution of the IR bands of the L and B CO species on the stabilized 2% Pd/Al2O3 solid during the increase in the adsorption temperature Ta for 1% CO/He after a first H(713 K)/C(300 K) cycle: (a)-(l) Ta= 300, 340, 370, 400, 430, 460, 490, 520, 550, 580, 650, 680 K.


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37

Coverage of the adsorbed CO species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1

0.8

0.6

0.4 a 0.2

0

b

300

400 500 600 700 800 Adsorption temperature (K)

900

Figure 3: Heats of adsorption of the L and B CO species on 2% Pd/Al2O3 using the AEIR method : and  experimental coverages θB and θL respectively from the FTIR spectra of Figure 2, evolution of the coverage of the L1Pd° CO species after subtraction of the contribution of the L2Pd° CO species (see the text for more details); and same as symbols

 and after a second increase in Ta; curves (a) and (b) theoretical evolution of the coverage of the B and L1Pd° CO species respectively according to the AEIR method considering the Temkin model with heats of adsorption of 90 and 175 kJ/mol and 54 and 92 kJ/mo respectively at high and low coverage (see the text for more details).


ACS Catalysis

38

2080 2075

A

B

a

d

2200

2100

a

Absorbance

2072

2000

Wavenumber (cm-1)

1900

2200

b c 2074

0.1

0.1 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2100

2000

Wavenumber (cm-1)

1900

Figure 4: Comparison of the IR band of the L CO species on 2% Pd-2% Sn/Al2O3 after different pretreatments for the adsorption of 1% CO/He at 300 K. Part A: impact of the number of A/R cycles: a)- d) First, second, third and fifth A/R cycles. Part B: impact of the number of H(713 K)/C(300 K) cycles in 1% CO/He after two A/R cycles: (a) fresh solid, (b) and (c) after the first and second cycles respectively.


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39

A

b a 2075 e

2075 Absorbance

0.1

2080 b c d

a

0.1

Absorbance

2080

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

B

a d b

0.1 2073

c 2200

2200

2100 2000 1900 2200 Wavenumber (cm-1)

2100 2000 1900 Wavenumber (cm-1)


1900

Figure 5: Comparison of the IR band of the L CO species on 2% Pd-2% Sn/Al2O3 after different pretreatments for the adsorption of 1% CO/He at 300 K Part A: impact of the number of H(713 K)/C(300 K) cycle in 1% CO/He after three A/R cycles: (a) fresh solid, (b)-(d) after the first, second and third cycles respectively. Part B: Impact of the adsorption pressure at 300 K after the third H(713 K)/C(300 K) cycle in 1% CO/He: (a) 1% CO/He (1 kPa) and (b) 4% CO/He (4 kPa).

Inset: comparison of the IR spectra of the L12P-2Sn CO species on the fresh reduced solid during the heating and cooling stage to 383 K in 1%CO/He: (a)-(c) Ta= 300, 320 and 383 K (heating) and (d)-(e) Ta= 320 and 300 K (cooling).


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40

A

1

2080

a 0.1

Absorbance

0.8 2080

B a

a

0.6

0.4

0.2

Absorbance


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 51

0.1

2074

2070

2073

i i

2069 2034

b

2200

2065 2100

2000

Wavenumber (cm-1)

2065

0

2200 Wavenumber (cm-1)1900 300

400

500

600

700

800

900

Adsorption temperature (K) Figure 6: Heats of adsorption of the L CO species on 2% Pd-2% Sn/Al2O3 using the AEIR method. and experimental coverages on the stabilized solid during the second and third H(713 K)/C(300 K) cycles in 1% CO/He,  and experimental coverage during the heating and cooling stage (300-373 K range) in 1% CO/He of the fresh reduced solid; (a) and (b) theoretical evolution of the coverage according to the AEIR method with heats of adsorption of 49 and 108 kJ/mol (curve a) and 50 and 90 kJ/mol (curve b) at high and low coverages (see the text for more details). Inset A: Evolution of the IR band of the L CO species on the stabilized 2% Pd-2% Sn/Al2O3 solid (two A/R cycles) during the heating stage of the second H(713 K)/C(300 K) cycle with 1% CO/He: (a)-(i) Ta= 300, 350, 390, 430, 470, 510, 550, 590 and 630 K. Inset B: Evolution of the IR band of the L CO species on the stabilized (three A/R cycles) 2% Pd-2% Sn/Al2O3 during the heating stage of the third H(713 K)/C(300 K) cycle with 1% CO/He: (a)-(i) Ta= 300, 350, 390, 430, 470, 510, 550, 590 and 630 K.


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41

2075

1

a

a 0.8

B Absorbance

2075

b 0.1

0.6

0.4

Absorbance


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

A

2067

k c

0.2

2078 2066

2200 b

2200 2100 2000 300

2022

2061

a 0

0.04

400

500

600

1900 2100 2000 Wavenumber (cm-1) 700

Adsorption temperature (K)

800

900

Figure 7: Heats of adsorption of the L CO species on 2% Pd-5% Sn/Al2O3 using the AEIR method. and experimental coverages on the stabilized solid after two and three A/R cycles, (a) and (b) theoretical evolution of the coverage of L2Pd-5Sn CO species considering one (Eqs. 2-3) adsorbed species with heat of adsorption of 52 and 92 kJ/mol at high and low coverages) and two adsorbed species (Eqs 2-4 with x1= 0.85, EL12Pd-5Sn (1)=52 kJ/mol, EL12Pd-5Sn (0)= 87 kJ/mol, x2= 0.15, EL22Pd-5Sn (1)= 80 kJ/mol, EL22Pd-5Sn (0)= 120 kJ/mol) (see the text for more details) respectively. Inset A: comparison of the IR spectra after adsorption of 1% CO/He at 300 K on (a) and (b) the fresh reduced 2% Pd-5% Sn/Al2O3 and 2% Pd-5% Sn/Al2O3 respectively and (c) after one H(713 K)/C(300 K) cycle on 2% Pd-5% Sn/Al2O3. Inset B: Evolution of the IR band of the L CO species on the stabilized (second A/R cycles) 2% Pd-5% Sn/Al2O3 during the heating stage of the second H(713 K)/C(300 K) cycle with 1% CO/He: (a)-(k) Ta= 310, 340, 360, 380, 400, 420, 440, 470, 530, 580, 630 K.


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42

1 He

|

1% CO/1% Ar/He

0.8 Ar × 60 0.6

0.4 CO × 60 0.2

Molar fractions of the gases


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 51

1 1 He | 1% H2/1% Ar/He 0.8 Ar × 60 0.6 0.4

H2 × 60

0.2 0

0

5

0 0

20

40

60

80 Time in s

10

15

20

Time (s) 100 120

25

140

30

160

Figure 8: Adsorption of 1% CO/1% Ar/He at 300 K on the fresh reduced 2% Pd-2% Sn/Al2O3 using the M.S system. Inset: Adsorption of 1% H2/1% Ar/He at 300 K on the fresh reduced 2% Pd-2% Sn/Al2O3 solid using the M.S system.


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43

0.4

600

0.35

550 500

CO × 100

0.25

450 0.2 Coverage of the L2Pd-2Sn species


T 0.3

0.15 0.1 0.05 0

0

200

400 1

Theoretical curve

0.8

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

350

0.6 0.4

300

0.2 Experimental 0

400

data

300 400 500 600 700 Temperature (K)

600 800 Time (s)

1000

1200

1400

1600

Figure 9: Temperature programmed adsorption equilibrium method (TPAE) using 0.2% CO/0.2% Ar/He on the 2% Pd-2% Sn/Al2O3 solid after one H(713 K)/C(300 K) cycle. Inset: red curve: experimental coverage from the data in Figure 9; blue curve: theoretical coverage according to Eqs 2-3 with heats of adsorption of 54 and 104 kJ/mol at high and low coverages respectively.


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44

Pd° Intensity (µA)

Pd3Sn

1200

See

text for more details

2%Pd-5%Sn/Al2O3

800

400

2%Pd-2%Sn/Al2O3

2%Pd/Al2O3

0

30

1600

Intensity (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 51

1200

40

2θ (°)

50

60

800 400

70

2%Pd-5%Sn/Al2O3 2%Pd-2%Sn/Al2O3 2%Pd/Al2O3 Alumina support

0

30

40

50 2θ (°)

60

70

Figure 10: XRD patterns of the three Pd containing solids and of the alumina support. Inset: XRD patterns on the three Pd containing solid after subtraction of the alumina patterns.


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ACS Catalysis

45 TABLES:

Table 1: Individual heats of adsorption of adsorbed CO species on mono Pd and bimetallic PdSn supported particles on alumina using the AEIR method. * L and B correspond to the averaged heats of adsorption of two linear and two bridged CO species respectively. ** This value includes the contribution of a small amount of a second strongly adsorbed linear CO species similar to the L22Pd-5Sn species. *** This value is underestimated: it does not allow fitting the experimental data at high temperatures.

Adsorbed CO species

Heat of adsorption (kJ/mol) High coverage

Low coverage

L1Pd° on 2%Pd/Al2O3

54

92

BPd°* on 2%Pd/Al2O3

90

175

L12Pd-2Sn on non reconstructed

50

90

49

108**

52

92***

52

87

80

120

2%Pd-2%Sn/Al2O3 L12Pd-2Sn on reconstructed 2%Pd-2%Sn/Al2O3 L2Pd-5Sn* on reconstructed 2%Pd-5%Sn/Al2O3 L12Pd-5Sn on reconstructed 2%Pd-5%Sn/Al2O3 L22Pd-5Sn on reconstructed 2%Pd-5%Sn/Al2O3


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46

Table 2: Amount of adsorbed CO and H2 at 300 and 383 K on Pd° and Pd-Sn particles for PCO= PH2= 1 kPa. solid

2%Pd/Al2O3

CO at 300 K for PCO= 1 kPa µmol/g 13.7

H2 at 300 K for PH2= 1 kPa µmol/g 13.5

CO at 383 K for PCO= 1 kPa µmol/g 13

2%Pd-2%Sn/Al2O3

17.3

1.2

10.9

2%Pd-5%Sn/Al2O3

4.5

Not measurable


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ACS Catalysis

47 AUTHOR INFORMATION

Corresponding Author *: To whom correspondence should be addressed. Tel: 0033472431419 E-mail: [email protected]

ACKNOWLEDGMENT We thank Mrs Mimoun Alouine for the TEM analysis of the Pd-Sn solids. D.B thanks the “Institut de Chimie de Lyon” for the purchase of the mass spectrometer in the framework of the “Contrat de Projets Etat-Région” Rhône-Alpes (2007-2013). D.B dedicates this article to the memory of Carroll O. Bennett, former Professor of University of Connecticut, Storrs USA who passed away January 9, 2016 in Paris, France. C.O.B was an expert in the field of the characterization of the surface processes associated to gas/solid catalytic reactions using transient experiments. A stay in his group and several years of collaboration, have strongly contributed to the orientation of my research.

SUPPORTING INFORMATION AVAILABLE: Catalyst preparations, FTIR spectra of the first adsorption of CO on 2% Pd-5% Sn/Al2O3, XRD data on Pd and Pd-Sn phases, TEM images, EDX and structures of Pd-Sn particles using the Ly-EtTEM microscope. This information is available free of charge via the Internet at http://pubs.acs.org


ACS Catalysis

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Heats of adsorption of a L1 CO species on 2% Pd-2% Sn/Al2 O3 PCO = 1 kPa 2080 AEIR method Ta (K) Temkin Model: EL1=f(θ θ) 300

Absorbance

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ACS Catalysis

EL1 (1)= 49 kJ/mol EL1 (0)= 108 kJ/mol

390 470 550

2070

0.1 630 2200


Graphical abstract


Individual Heat of Adsorption of Adsorbed CO Species on Palladium

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