H2 Reaction on

J. Phys. Chem. C 2008, 112, 17183–17192

17183

An Operando Spectroscopic Investigation of the NO/H2 Reaction on LaCoO3 and Pd-modified LaCoO3 s Influence of O2 on Catalyst Performances and Structure of Adsorbed Species C. Dujardin,* I. Twagirashema, and P. Granger Unite´ de Catalyse et de Chimie du Solide, UniVersite´ des Sciences et Technologies de Lille, UMR CNRS 8181, Baˆtiment C3, 59655, VilleneuVe d’Ascq Cedex, France ReceiVed: April 7, 2008; ReVised Manuscript ReceiVed: July 18, 2008

The catalytic reduction of NO by hydrogen has been studied over palladium-based catalysts supported on LaCoO3. Particular attention has been paid to the observation of chemisorbed N-containing species from operando infrared spectroscopic experiments during the NO/H2 reaction in the presence and in the absence of excess oxygen. X-ray photoelectron spectroscopy coupled to a catalytic chamber was used to examine parallel changes in oxidation state and chemical environments of Co and Pd after exposure to various reactive conditions. Complementary atomic information were obtained from the examination of the N 1s photopeak and compared to those obtained from IR observations. Dinitrosyl species adsorbed on oxidic cobalt species were observed at low temperature, whereas nitrate/nitrite species and NHx species predominate at high temperature. The presence of dinitrosyl species has been discussed for elucidating surface changes in the presence of oxygen and related modifications in the selectivity behavior of prereduced Pd/LaCoO3. The formation of adsorbed N-atoms and NH2 species as intermediate was observed correlatively to ammonia formation on LaCoO3 and on Pd/LaCoO3 catalysts in the absence of oxygen. Monodentate nitrates were characterized and could be involved in the selective reduction of NO by hydrogen on Pd/LaCoO3 in excess O2. 1. Introduction Removal of nitrogen oxides from industrial and automotive exhaust gas is a great challenge, especially in the presence of excess oxygen. As an alternative to the selective catalytic reduction of nitric oxides (NOx) by ammonia implemented for stationary sources, the use of hydrogen has been proposed to promote the reduction of NO at low temperature.1-6 For example, Costa et al.2 reported a high activity for the NO/H2 reaction under lean conditions with Pt supported on perovskites (ABO3) inducing a high selectivity toward nitrogen production (temperature range: 100-200 °C). Recent investigations have shown that Pd/LaCoO3 could also be potentially interesting for NOx reduction in the presence of excess oxygen.4,7 Generally, these perovskite materials are sensitive to reductive atmospheres according to the nature of the transition metal ion in B-sites, such as cobalt. Reductive pretreatment in H2 is usually achieved for stabilizing metallic noble metal particles that exhibit a high intrinsic activity. Simultaneously, an extensive reduction of LaCoO3 to CoOx/La2O3 (with x < 1) may occur in severe temperature conditions and was previously evidenced by in situ X-ray diffraction (XRD) experiments in the presence of hydrogen at 500 °C, with this transformation being assisted by the presence of palladium. Previous investigations of the NO/ H2 reaction on preactivated Pd/CoOx/La2O34,7 in the presence of excess oxygen showed a different behavior due to surface modifications in the course of the reaction above 300 °C. In such conditions, redispersion of oxidic palladium species correlatively to the simultaneous reconstruction of the perovskite structure occurs.8 Both processes generate different interactions * Corresponding author. Phone: +33 3 28 77 85 29; fax: +33 3 20 43 65 61; E-mail: [email protected].

between oxidic palladium and the in situ reconstructed perovskite as compared to what is usually observed when PdO clusters weakly interact with LaCoO3,9 mainly associated to a partial replacement of Co3+ by oxidic palladium species stabilized in an unusual oxidation state. In this case, a better understanding of the role of cobalt and palladium species as active phase in such catalysts is interesting. Only few vibrational spectroscopic techniques (infrared, Raman) give access to information dealing with adsorbed species during reactions in lean conditions. The interaction of NO with the surface of solids containing cobalt has largely been reported in the literature using Fourier transform infrared (FTIR) spectroscopy, especially in the case of Coexchanged zeolites for the selective catalytic reduction (SCR) of NO by hydrocarbons.10-12 Several potential intermediates or adsorbates were characterized depending on the catalyst compositions. By way of illustration, the interaction between NO and isolated cobalt ions could lead to the formation of mononitrosyl (Co3+-NO) and dinitrosyl (Co2+-(NO)2) species.10,13,14 Mononitrosyl (Co2+-NO) species were also characterized by an IR band located at 1857 cm-1.15 The effective role of nitrosyl/dinitrosyl species is unclear because dinitrosyl species or dimeric species of NO can behave either as intermediate species or as spectator species depending on the reaction mixture or catalytic system. Steady state isotopic transient kinetic analysissdiffuse reflectance infrared Fourier transform spectroscopy (SSITKA-DRIFTS) experiments may provide more relevant information for assigning given structures of NOx as active intermediates or not. Such an attempt was recently reported on Pt/MgO-CeO2 catalyst with the observation of active adsorbed NOx species formed at the metal-support interface.5 Subsequent addition of palladium or platinum usually induces significant rate and selectivity enhancements to the

10.1021/jp802993d CCC: $40.75  2008 American Chemical Society Published on Web 10/09/2008

17184 J. Phys. Chem. C, Vol. 112, No. 44, 2008 production of nitrogen.2 For supported palladium-based catalysts, spectroscopic studies have shown the existence of positively charged NO [Pd-NO+], linear NO [Pd0-NO] and bent NO [Pd-NO-] species.16 Previous kinetic investigations of the NO/ H2 reaction in stoichiometric conditions suggested different types of mechanisms, depending on the nature of the support. Some of them assumed competitive adsorptions of NO and H2 17,18 and successive surface reactions involving only Pd dispersed on inert Al2O3 supports. Subsequent formation of N2 and N2O predominantly involved the intermediate formation of chemisorbed N-atoms from NO dissociation according to mechanism schemes earlier proposed.17,18 Alternately, stronger interactions between noble metal particles and reducible supports, such as perovskite, may lead to cooperative effects. Such effects are usually characterized under lean conditions with an usual beneficial effect of oxygen probably associated to changes in the nature of intermediates and active sites. Such a statement seems to be in line with a recent investigation on Pd/LaCoO3 under excess O2 that suggests that bidentates nitrates or nitrosyl interacting with cobalt can be efficiently transformed into nitrogen when reacting with hydrogen dissociatively adsorbed on palladium.19 The aim of this present operando DRIFTS-mass spectroscopy and in situ X-ray photoelectron spectroscopy (XPS) study is to contribute to a better understanding of active sites and intermediates involved in the reduction of NO by hydrogen in the absence or in the presence of excess oxygen on supported Pd on LaCoO3. 2. Experimental Section 2.1. Catalyst Preparation. The preparation procedure of LaCoO3 (20 m2g-1) was described elsewhere7,20 using a socalled sol-gel method involving a citrate route. Supported palladium catalysts on LaCoO3 calcined beforehand at 600 °C were prepared according to a classical wet impregnation route using palladium nitrate solutions with adjusted concentrations in order to obtain 1 wt % Pd. The impregnated samples were finally calcined in air at 400 °C. After prereduction in pure H2 overnight at 500 °C, H2 chemisorption measurements on Pd/ LaCoO3 performed at 100 °C, to minimize the formation of bulk hydrides,21 and the Pd dispersion was approximately equal to 0.16 assuming an atomic H/Pd ratio equal to 1. The average particle size of approximately 60 Å was estimated. 2.2. Catalytic and Spectroscopic Measurements. XPS experiments were performed using a Vacuum Generators Escalab 220XL spectrometer equipped with a monochromatized aluminum source for excitation. The preactivation step and the reaction were performed at atmospheric pressure in a catalytic chamber coupled to the spectrometer and the sample was then transferred in the analysis chamber under ultra high vacuum (∼10-10 Torr) for XPS analysis. Before reaction, all samples were activated in H2. The temperature was gradually increased with a constant heating rate of 3 °C/min at 500 °C and then immediately cooled down at room temperature before exposure to the reaction mixture under various temperature conditions. After XPS analysis, the catalyst was transferred in the catalysis chamber and then submitted to reaction conditions. Binding energy (BE) values were referenced to the BE of La 3d5/2 (833.7 eV). The quantification of cobalt in different chemical environments was achieved using a mixed Gaussian/Lorentzian peak fit keeping BEs and half-widths (fwhm ( 0.1 eV) constant for all decomposition spectra. Catalytic and spectroscopic FTIR were performed with a commercial DRIFT cell (Harrick). The calcined samples (20

Dujardin et al. mg) were grinded and loaded in the cell. An additional thermocouple placed inside the catalytic bed allowed us to measure the temperature inside the catalytic bed during temperature-programmed experiments. The IR spectra were recorded in the course of the reaction with a spectral resolution of 4 cm-1 and 256 scans (acquisition time 52 s) using a Thermo Nicolet 460 Prote´ge´ FTIR spectrometer equipped with a MCT detector. A KBr spectrum was used as background for the reprocessing of spectra. The catalyst was reduced in situ overnight at 390 °C in pure H2. Temperature-programmed experiments were performed during spectroscopic measurements. The total flow rate was 0.9 L · h-1. The DRIFT cell was connected to a quadrupole mass spectrometer (Balzers Omnistar) to analyze the composition of the exhaust gas. The signals corresponding to m/z ) 2 (H2), 28 (N2), 30 (NO), 44 (N2O), and 46 (NO2) were recorded. The overall conversion of NO (XNO), H2 (XH2), and that related to the consumption of H2 by reaction with NO (XH2+NO) leading to the production of N2, N2O, and NH3 was calculated according to the following set of reactions:

2NO + 2H2 ) N2 + 2H2O

(1)

2NO + H2 ) N2O + H2O

(2)

2NO + 5H2 ) 2NH3 + 2H2O

(3)

XN2, XN2O, and XNH3were the conversion of NO to nitrogen, nitrous oxide, and ammonia, respectively. F0i was the inlet molar flow rate of the reactant i (i ) NO or H2), whereas FN2, FN2O and FNH3 were the outlet molar flow rate of N2, N2O, and NH3. p0i was the inlet partial pressure of the reactant i.

XNO )

2(FN2 + FN2O) + FNH3 0 F NO

XH2total ) XH2+NO )

F H0 2 - FH2

2FN2 + FN2O + 5FNH3 F H0 2

) XN2 + XN2O + XNH3 (4)

(5)

FH0 2

(

) XN2 +

XN2O 2

+

)

0 5XNH3 pNO 2 p0 H2

(6) 3. Results 3.1. XPS Investigation of the NO/H2 Reaction. Surface changes have been examined by XPS on LaCoO3 and Pd/ LaCoO3 catalysts after various in situ thermal treatments under controlled atmosphere. Co 2p, Pd 3d, and N 1s photopeaks have been carefully examined. 3.1.1. LaCoO3 Catalysts. Thermal treatment under hydrogen at 500 °C on LaCoO3 leads to modifications on the Co 2p photopeak (Figure 1A, traces a and b), with a shift of the binding energy toward lower values from 780 to 778.1 eV correlated to the disappearance of the typical shakeup structure of Co3+ at 789.5 eV.9 As indicated in Table 1, these observations are related to a surface reduction of Co(III) species, in B sites, into Co(II) and, predominantly, into metallic cobalt species. Interestingly, successive exposure to the reaction mixture at 25 °C under rich conditions (with molar H2/NO ratio of 3.3) induces an increase of the relative intensity of the signal located at 780 eV at the expense of the low BE contribution. The shakeup structure at 785.6 eV characteristic of Co2+ simultaneously develops. Clearly, a reoxidation process of Co0 to Co2+ starts at 25 °C under reactive conditions and is complete at 200 °C. Subsequent

Influence of O2 on Pd/LaCoO3 in NO/H2 Reactions

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17185

Figure 1. XPS spectra of Co 2p (A), N 1s (B), and Pd 3d (C) photopeaks on fresh sample (a and g)), reduced sample (b and h) and during temperature-programmed reaction for 0.15% NO + 0.5% H2 reaction mixture at 25 °C (c and i), 200 °C (d and j), 300 °C (e and k), and 500 °C (f and l) for LaCoO3 and 1% Pd/LaCoO3 catalysts.

TABLE 1: XPS Measurements of LaCoO3 and Pd/LaCoO3 during Reduction and after Successive Steps of the Reaction under 0.15% NO + 0.5% H2 at Various Temperatures atomic quantification of Co analyzed step

% Co3+

fresh reduction 500 °C reaction 25 °C, 30 min reaction 200 °C, 30 min reaction 300 °C, 2 h reaction 500 °C, 2 h fresh reduction 500 °C reaction 25 °C, 30 min reaction 200 °C, 30 min reaction 300 °C, 2 h reaction 500 °C, 2 h

100

atomic quantification of Pd

% Co2+

% Co0

40 79 100 100 100

60 21

41 88 100 100 66

59 12

100

% Pd2+

% Pd0

100

reoxidation into Co3+ is not observed during the NO/H2 reaction. The reoxidation process can be easily explained by the dissociation of NO onto the surface. Parallel to these observations, additional information on the N 1s photopeak appears due to subsequent interactions between gaseous NO and the surface. As shown in Figure 1B, a contribution, previously assigned to atomic adsorbed nitrogen species,22-24 initially appears at 398.7 eV and then develops up to 300 °C. However, the presence of adsorbed NHx species with typical binding energy values previously reported at 398.5 and 400.5 eV may also contribute.25,26 Subsequent increase in temperature leads to a significant attenuation of the N 1s photopeak, but the latter contribution is still observable at 500 °C, which indicates the stabilization of strongly chemisorbed nitrogen atoms and/or N-containing species on the partially reduced catalyst. Atomic quantifications for Co, Pd, and atomic ratio Co/La are summarized in Table 1. We can notice that surface atomic Co/La values on LaCoO3

34

10 100

100 100 100 90

atomic ratio Co/La 0.49 0.54 0.86 0.83 1.03 0.69 0.49 0.62 0.76 0.69 0.59 0.47

after NO/H2 exposure exceed those measured before and after H2 reduction with a maximum at 300 °C of 1.03. 3.1.2. Pd/LaCoO3 Catalysts. Similar XPS experiments were repeated on Pd/LaCoO3. Regarding the Co 2p photopeak, no noticeable change occurs after in situ H2 reduction at 500 °C and then subsequent exposure to reaction mixture in the temperature range 25-300 °C. As shown in Figure 1A and Table 1, Co3+ is initially completely reduced to Co2+ and Co0 after H2 exposure at 500 °C with no significant modifications in the relative surface composition. A reoxidation process also takes place at 25 °C with the formation of an unique Co(II) species at 200 °C after NO/H2 exposure. On the other hand, the surface properties of the catalyst completely change at 500 °C associated to the development of a low BE signal on the Co 2p photopeak located at 778.1 eV at the expense of the 780 eV contribution. Such an observation emphasizes the fact that a reversible transformation associated to the reduction of Co2+

17186 J. Phys. Chem. C, Vol. 112, No. 44, 2008

Dujardin et al.

Figure 2. XPS spectra of Co 2p (A), N 1s (B), and Pd 3d (C) photopeaks on fresh sample (a and g), reduced sample (b and h), and during temperature-programmed reaction for 0.15% NO + 0.5% H2 + 3% O2 reaction mixture at 25 °C (c and i), 200 °C (d and j), 300 °C (e and k), and 500 °C (f and l) for LaCoO3 and 1% Pd/LaCoO3 catalysts.

into Co0 takes place at high temperature in the presence of Pd, contrary to previous observations on LaCoO3 (see Figure 1A). Now, the examination of the Pd 3d photopeak reveals the reduction of oxidic palladium species in the presence of hydrogen accompanied by a decrease of the binding energy from 336.2 to 335.1 eV, characteristic of Pd2+ and Pd0 species, respectively (Figure 1C). Successive NO/H2 exposures do not lead to significant changes on the Pd 3d photopeak until 200 °C. Above that temperature, a shift of the Pd 3d photopeak toward higher BE values indicates the stabilization of oxidic palladium species. The BE value at 336.2 eV on the Pd 3d5/2 spectrum recorded at 500 °C corresponds to what is usually observed for Pd2+, which is stabilized as PdO.27 All these observations are listed in Table 1. Interestingly, some cobalt species become reduced as Co0 parallel to the stabilization of oxidic Pd species as PdO at high temperature. Subsequent examination of the atomic Co/La atomic ratio shows a surface Co enrichment when cobalt segregates as Co2+ under reactive conditions. On the other hand, the reverse tendency occurs when metallic Pd species forms. Additional examination of the N 1s photopeak reveals a broad and weak signal at 398.7 eV previously assigned to chemisorbed N atoms and/or NHx species and an additional contribution discernible at 500 °C at 404.1 eV, corresponding to the formation of nitrite species.24 Let us note that the relative intensity of the N 1s photopeak is lower than that recorded on LaCoO3 in the temperature range of this study, which may suggest either a weaker stabilization of N-containing species after palladium incorporation. As a matter of fact, these changes in relative intensity could simply mean that the surface concentration of chemisorbed NHx is lower on LaCoO3 than on Pd/LaCoO3 due to different reactivity in the reaction

conditions. However, no final conclusion could be drawn due to the fact that changes in surface concentration of chemisorbed N-containing species could be the results of a significant desorption or reaction during the transfer in the UHV chamber and/or during X-ray irradiation. Hence, the adsorbate composition under UHV conditions may differ from that under reactive conditions. 3.2. Influence of Oxygen on XPS Features in the Course of the NO/H2 Reaction. The surface properties of prereduced LaCoO3 and Pd/LaCoO3 have been examined after exposure to 0.15% NO, 0.5% H2, and 3% O2 at different temperatures in the range 25-500 °C. As illustrated in Figure 2, the presence of oxygen induces changes in spectral features of the Co 2p photopeak due to an extensive surface cobalt oxidation into Co3+ above 300 °C associated to the development of the characteristic shakeup structure at 789.5 eV. The complete oxidation of Co0 to Co3+ at 300 °C has been previously discussed and associated to surface reconstruction of LaCoO3, whereas bulk reconstruction was characterized above 500 °C from XRD.8 In parallel to these observations, significant changes also occur on the photopeak N 1s associated to the initial appearance of a single signal at 403.4 eV at room temperature. Subsequent rise in temperature leads to the development of an additional contribution at 407.2 eV at the expense of the former low BE signal. According to literature data,7,9 these BE values characterize, respectively, the presence of nitrites (403.4 eV) and nitrates (407.2 eV) stable at 500 °C on LaCoO3. Now regarding the Pd 3d photopeak, it is noticeable that intermediate BE values recorded at 500 °C, between those currently associated to Pd4+ and Pd2+, correlatively to the formation of a narrow photopeak, suggest the stabilization at the surface of a unique species, which agree with the insertion of oxidic palladium species inside the



Figure 4. FTIR spectra recorded on prereduced LaCoO3 catalyst (a) and in the course of the NO/H2 reaction at 30 (b), 64 (c), 118 (d), 127 (e), 136 (f), 151 (g), 246 (h), 257 (i), 396 (j), amd 426 °C (k).

Figure 3. Temperature-programmed reaction curves (A) obtained on the DRIFT cell for 0.15% NO + 0.5% H2 reaction mixture and selectivity (B) on LaCoO3 prereduced at 390 °C overnight. (C) Comparison of overall conversion of H2 (XH2) and theoretical conversion of H2 + NO reaction (XH2+NO).

perovskite structure.7,28 Subsequent palladium incorporation does not lead to significant modification on the Co 2p photopeak (see Figure 2A). However, it is worthwhile to mention that Pd incorporation could affect the stability of nitrate and nitrite species. As illustrated in Figure 2B, these species completely disappear after exposure to reaction mixture at 500 °C. Binding energy recorded for the Pd 3d core level progressively shifts to higher values from 335.1 to 337.5 eV. 3.3. Operando Infrared Spectroscopic Investigation. 3.3.1. NO/H2 Reaction. 3.3.1.1. LaCoO3 Catalysts. Changes in adsorbate compositions and catalytic performances in terms of activity and selectivity have been simultaneously recorded during operando investigations of the NO/H2 reaction at various temperatures. Temperature-programmed conversion and selectivity curves on prereduced LaCoO3 in H2 at 390 °C are presented in Figures 3, panels A and B. Different NO conversion ranges can be distinguished in Figure 3A. Below 250 °C, an increase in NO

conversion takes place and then stabilizes with no correlative H2 conversion. Such behavior highlights a reoxidation process from NO dissociation with the subsequent formation of N2O and N2 formation in the gas phase and oxygen accumulation onto the surface of the prereduced LaCoO3 according to previous XPS characterization. Above 250 °C, NO conversion decreases with the appearance of a minimum conversion at ≈ 368 °C. Above that temperature the NO/H2 reaction predominantly occurs. Subsequent comparisons of the overall H2 conversion profile (XH2) and that corresponding to the reduction of NO (XH2+NO) provides more precise information (Figure 3C). As observed, the superimposition of both temperature-programmed conversion curves above 400 °C means that the NO/H2 reaction only takes place, leading to N2, N2O, and H2O formation. Below 200 °C, the theoretical H2 conversion curve (XH2+NO) that would correspond to its reaction with NO does not match to the H2 conversion curve owing to the reoxidation of the solid through the dissociation of NO into N2 and N2O. In the intermediate temperature range, the conversion of NO to N2O and N2 is not related to H2 conversion. Similar catalytic features occur below 250 °C which indicate that NO decomposition could essentially occur. As observed, only N2O and N2 are observed during the reoxidation process, whereas an additional ammonia formation starts above 250 °C and accentuates above 370 °C when the NO/H2 reaction becomes significant. The corresponding IR spectroscopic information is reported in Figure 4 with tentative assignments according to earlier investigations summarized in Table 2. After reductive thermal pretreatment, a broad and intense signal appears in the range 1300-1500 cm-1 with apparent maxima at 1456 and 1385 cm-1 assigned to monodentate carbonates probably due to residues from the perovskite synthesis and/or subsequent carbonate formation during storage in air. Monodentate lanthanum carbonates stabilized at the surface could be held responsible for the


Dujardin et al.

TABLE 2: Assignment of IR Bands wavenumbers /cm-1 catalyst

assignments 2+

Co-ZSM-5

Co (NO)2

Co-ZrO2

Co3+(NO) Co2+(NO) Co2+(NO)2 NO2- (bridging nitro) NO3- (monodentate nitrate) NO3- (bidentate nitrate) NO3- (bridged nitrate)

Pd/Al2O3 Co(Ph3PO)2(NO2)2 Co(Ph3AsO)2(NO2)2 Pd/TiO2-Al2O3

La2O3

Pd · · · NO+ linear Pd · · NO bent Pd · · · NObridging bidentate nitrito NH2 NH3(ads) NH4+(ads) monodentate CO32-

broad signals around 1456, 1385, 1060, and 850 cm-1.29,30The introduction of the reaction mixture at 25 °C leads to the appearance of two additional bands at 1848 and 1755 cm-1, in the usual frequency range of nitrosyl species. These signals have been associated to Co2+(NO)2 according to previous assignments.31 Both IR bands progressively broaden and shift toward lower wavenumbers values with an increase in temperature and then disappear above 151 °C. The deviation ∆σ between symmetric and asymmetric stretching vibrational modes of the gem-dinitrosyl complex remains constant and is equal to ≈93 cm-1. A broad and weak band appears above 250 °C at 1555 cm-1 and could be related to adsorbed monodentate nitrate (NO3-),31 bridging nitro (NO2-), and/or NH2 species.32,33 No significant signal that could correspond to complementary vibrational modes of nitrogen-containing species developed in the range of 1300-900 cm-1 during the temperature-programmed reaction (Figure 4B). 3.3.1.2. Pd/LaCoO3 Catalysts. Figure 5 illustrates changes in the temperature-programmed conversion and selectivity curves after subsequent addition of palladium. As previously described on LaCoO3, a reoxidation process starts at similar temperatures with a significant NO consumption, whereas no correlative H2 conversion can be observed. NO conversion is complete from 180 till 430 °C. No minimum NO conversion as previously observed on LaCoO3 appears at high temperatures. The conversion of H2 begins at 164 °C and is accompanied with a significant formation of NH3. Figure 5C compares the overall conversion of H2 and that which would correspond to its reaction with NO (XH2+NO). Both conversion profiles only coincide above 300 °C, which emphasizes the fact that the NO/ H2 reaction predominantly occurs. On the other hand, the divergence observed below that temperature signifies that the reoxidation via NO decomposition predominates. All these observations emphasize the fact that the NO/H2 reaction occurs more readily after palladium incorporation. It is worthwhile to

vibrational modes

literature (reference)

ν(s)NO ν(as)NO νNO νNO ν(s)NO ν(as)NO ν(as)NO2 ν(as)NO2 ν(s)NO2 νNO ν(as)NO2 ν(s)NO2 νNO ν(as)NO2 ν(s)NO2 νNO νNO νNO ν(as)NO2 ν(s)NO2 δ(s)NH2 δNH3 δ(as)NH4+ δ(s)NH4+ ν(as)OCO ν(s)OCO νCO πCO3

1894 (15) 1811 (15) 1857 (15) 1940 (14) 1855 (31) 1777 (31) 1545-1530 1580-1540 1292-1270 1615 (32) 1229 (32) 1040 (32) 1640 (32) 1220 (32) 1060 (32) 1803 (16) 1754 (16) 1650 (16) 1314-1266 1203-1176 1510 (33) 1270 (33) 1680 (33) 1460 (33) 1463 (30) 1376 (30) 1029 (30) 849 (30)

this study 1848-1885 1750-1810

(32) (32) (32)

( 13, 40) ( 13, 40)

1848-1885 1750-1810 1555 1555

1747 1656 1373 1260 1555

1456 1385 1060 850

note that significant changes also occur in the product distribution due to Pd incorporation, particularly above 400 °C. As illustrated, an increase of N2 production correlated to a decrease in ammonia formation occurs. Such an observation seems to be relevant because a reversible behavior was observed in this temperature range on LaCoO3. As a matter of fact, such changes could be related to modifications in surface properties, as previously highlighted from XPS measurements that show a significant Pd oxidation to Pd2+ in parallel to the reduction of Co2+ to Co0. Such surface changes could partly explain modifications in the catalytic performance of Pd/LaCoO3 above 400 °C with a lower ammonia formation connected to an extensive segregation of Pd2+ at the surface. IR spectra in Figure 6 show the development of two bands at 1850 and 1750 cm-1 after exposure to reaction mixture in the temperature range 56-120 °C previously assigned to dinitrosyl species interacting with Co2+ cations. Their intensity declines when NO conversion becomes complete at high temperature. The corresponding value of ∆σ ) 100 cm-1 slightly varies in comparison to previous observations on LaCoO3 and does not indicate a significant effect of palladium on the spectral features of these species. Nitrosyl species interacting with palladium species are usually observed between 1650 and 1803 cm-1.13,16 Previous NO exposures at 83 °C on Pd/Al2O3 led to the observation of NO linearly coordinated to Pd0 with possible negatively charged NO species on Pd mainly at lower frequencies (results not shown). Other investigations already mentioned the appearance of linear NO on Pd0 at 175416 and 1733 cm-1.34 However, the relative intensity ratio between the bands at 1850 and 1750 cm-1 does not vary significantly for LaCoO3 and Pd/LaCoO3 catalysts. Consequently, no conclusive argument can be drawn relative to the presence or the absence of these species on Pd/LaCoO3. Nevertheless the potential contribution of nitrosyl species adsorbed on palladium may remain minor.



Figure 6. FTIR spectra recorded on prereduced Pd/LaCoO3 catalyst (a) and in the course of the NO/H2 reaction at 25 (b), 56 (c), 120 (d), 161 (e), 193 (f), and 312 °C (g).

Figure 5. Temperature-programmed reaction curves (A) obtained on the DRIFT cell for 0.15% NO + 0.5% H2 reaction mixture and selectivity (B) recorded on prereduced Pd/LaCoO3. (C) Comparison of overall conversion of H2 (XH2) and theoretical conversion of H2 + NO reaction (XH2+NO).

Now, let us examine spectral features observed above 140 °C when the reduction of NO with H2 starts. Figure 6B shows significant changes in relative intensity of IR bands in the range 900-1600 cm-1 compared to previous observations on LaCoO3. The most important observation is related to the lack of significant accumulation of carbonates at the surface after H2 reduction with weak IR bands at 1384 and 1458 cm-1. Consequently, the IR band at 1555 cm-1 related to nitrates, nitro species,31,32 and/or NH2 adsorbed species33 clearly appears at significantly lower temperatures (193 °C vs 250 °C on LaCoO3) and intensifies. These species still remain at 312 °C and are observed exactly in the temperature range when NH3 production occurs on both LaCoO3 and Pd/LaCoO3 catalysts. Hence, this signal could reflect intermediate adsorbed species such as NH2 and NH3 ad-species formed before ammonia desorption. The intensity of the broad signal in the region of carbonates increases between 193 and 312 °C with the development of a band around 1414 cm-1 (not assigned) and the intensification of the signal at 1458 cm-1. 3.3.2. NO/H2/O2 Reaction. Oxygen addition leads to competitive adsorptions at the surface of the catalyst that strongly

influences catalyst activity. It has been previously reported that, in the absence of noble metal, NO conversion on LaCoO3 starts at 150 °C but remains below 10% in a wide range of temperature.8 In parallel, FTIR spectra were recorded during temperature-programmed experiments in the presence of oxygen in the reaction mixture. A broad and intense signal due to the stabilization of carbonates probably overlaps the characteristic IR bands of nitrite and nitrate species, as evidenced by XPS analysis. The lack of observation of dinitrosyl species could be explained by a stronger O2 adsorption than NO at the surface of the solid, which would inhibit the observation of nitrogencontaining species on Co and Pd. Ivanova et al.35 reported a similar evolution of cobalt dinitrosyl species disappearing in oxygen atmosphere. More interesting catalytic and spectroscopic features have been obtained after palladium addition. In the presence of excess O2, a competition between the NO/H2 and H2/O2 reactions is expected. Figure 7 compares the conversion profiles recorded during operando measurements in the DRIFT cell and when using a classical integral fixed bed flow reactor in similar conditions (partial pressures) except for the gaseous hourly space velocity (GHSV, W/F0) of, respectively, 0.825 and 1.733 g · h · mol-1. It was found that change in the maximum conversion at low temperature is correlated to change of GHSV. Temperature-programmed conversion curves according to the above-mentioned conditions reflect the same catalytic behavior with the observation of a narrow NO conversion range at low temperature between 80 and 200 °C due to the competitive NO/ H2 and H2/O2 reactions in excess O2. NO conversion does not exceed 30% at high temperatures. As previously observed, the comparison of the overall H2 conversion profile with that corresponding to the conversion of H2 by reaction with NO (XH2+NO) enables a more precise examination of the selectivity to the NO/H2 reaction. Interestingly, the NO/H2 reaction occurs


Dujardin et al.

Figure 8. FTIR spectra recorded on prereduced-Pd/LaCoO3 catalyst (a) and in the course of the 0.15% NO + 0.5% H2 + 3% O2 reaction at 42 (b), 74 (c), 83 (d), 90 (e), 105 (f), 109 (g), 113 (h), 123 (i), 147 (j), 263 (k), and 332 °C (l).

and 1260 cm-1 develop between 260 and 320 °C, which probably reflect the accumulation of bridged bidentate nitrite species or ionic nitrate species36 at the surface of the catalyst. Figure 7. Temperature-programmed reaction curves (A) obtained on the DRIFT cell (full symbols) and on fixed bed reactor (open symbols) for 0.15% NO + 0.5% H2 + 3% O2 reaction mixture on prereduced Pd/LaCoO3. Selectivity (B) and comparison of overall conversion of H2 (XH2) and theoretical conversion of H2 + NO reaction (XH2+NO) (C) obtained on the DRIFT cell.

more readily and predominantly in the presence than in the absence of oxygen, which underlines the beneficial effect of oxygen on the NO conversion at low temperatures. Subsequent increase in temperature shifts the selectivity in favor of the H2 + O2 reaction. The presence of oxygen also suppresses the formation of ammonia. Now, let us examine FTIR spectra; Figure 8 shows two bands at 1885 and 1810 cm-1 appearing at low temperatures (T ≈ 80 °C). A shift of these two IR bands toward lower wavenumbers occurs with an increase in temperature and finally leads to IR bands located at 1870 and 1789 cm-1. The deviation between the asymmetric and symmetric NO stretching band slightly varies (∆σ ) 75-81 cm-1) in the temperature range of the study. Such wavenumber values are in good agreement with the formation of dinitrosyl species adsorbed on Co2+.32 The shift of wavenumbers toward lower values may reflect slight changes in the oxidation state of cobalt in the presence of O2, H2, and NO. This shift is observed at the same time as N2O selectivity decreases (90-140 °C). A maximum in intensity of these bands corresponds to a maximum of NO conversion at low temperatures. At higher temperatures, additional IR bands around 1373

4. Discussion Previous investigations on the kinetics of the NO/H2 reaction, in the presence and in the absence of oxygen, have reported different activity and selectivity behavior, particularly when noble metals are supported over reducible supports.2,17,37 Dhainaut et al.17,38 showed a significant inhibiting effect of oxygen on the rate of NO reduction by H2 on Pd/Al2O3 and a poor selectivity due to a significant rate enhancement of the H2/O2 reaction. These kinetic features have been quantitatively explained based on a classical Langmuir-Hinshelwood mechanism reported elsewhere on Pd- and Rh-based catalysts18,38,39 where O2, NO, and H2 competed for adsorption on noble metal sites in weak interaction with alumina or silica. On the other hand, different catalytic features can be observed in this present study when noble metals are deposited on reducible LaCoO3 support. This is well illustrated in Figures 5 and 7 where O2 induces a significant rate enhancement of NO reduction by H2 at low temperature. Correlatively, a better selectivity toward the NO/H2 reaction is observed on Pd/LaCoO3 than on Pd/Al2O3 in that temperature range.38 Such an observation is in agreement with previous steady-state kinetic measurements performed in our laboratory at 150 °C on the same Pd/LaCoO3 catalyst with an apparent order with respect to oxygen of +0.48 versus -0.1 on Pd/Al2O3 at 158 °C.38 Such a beneficial effect has been reported earlier for the hydrocarbon-selective reduction of NOx and was mainly assigned to the involvement of adsorbed nitrite and/or nitrate species as intermediates. Similar tentative explanations have been recently reported by Chiarello et al.19 on Pd/

Influence of O2 on Pd/LaCoO3 in NO/H2 Reactions LaCoO3 and by Machida et al.37 on Pd/MnOx-CeO2. These authors observed a significant consumption of nitrite species during H2 exposure on MnOx-CeO2 and proposed the occurrence of a spill-over process between hydrogen species dissociated on metallic Pd sites and activated NOx species on the support for depicting the NO/H2 reaction. Recent work on Pt/ MgO-CeO2 also provides additional arguments for a Hspillover and the location of active NOx on the support with SSITKA-DRIFTS and SSITKA-MS experiments.5 As a matter of fact, these comparative XPS and FTIR experiments may in a certain way provide additional atomic and molecular information for clarifying the nature of the metal/ support interface and changes in the nature of intermediates involved during the NO/H2 reaction in the presence and in the absence of oxygen. Interestingly, the observation of dinitrosyl species on oxidic cobalt species at low temperatures can be considered as surface probe for explaining the effective role of oxygen on the kinetics of the NO/H2 reaction. 4.1. NO/H2 under Rich Conditions. Catalytic and spectroscopic observations during the NO/H2 reaction on LaCoO3 and Pd/LaCoO3 were obtained under rich conditions with H2/NO ) 3.3 on preactivated catalyst after H2 exposure at 390 °C. XPS measurements reveal on prereduced catalysts the predominant formation of metallic Co species with the coexistence of Co2+ in agreement with previous bulk characterization from XRD analysis, which led to the conclusion that LaCoO3 extensively reduced to Co0/CoO/La2O3.8 As illustrated in Figure 3 these reduced materials exhibit a low activity in the NO/H2 starting above 250 °C. Similarly, Pd interacts with Co0/CoO/La2O3, and the NO/H2 reaction occurs more readily, starting at 180 °C, and is accompanied with the production of N2, N2O, and ammonia. XPS measurements in Figure 1 suggest that NO dissociates on Co0/CoO/La2O3 at relatively low temperatures. Subsequently, strong accumulation of adsorbed nitrogen atoms up to 300 °C may induce inhibiting effects on the reaction rate. Alternately, the presence of Pd drastically attenuates the observation of these strongly adsorbed N atoms. In parallel to that observation, a significant rate enhancement is observed at low temperature, but the activity stabilizes then decreases above 350 °C. Accordingly, Pd incorporation induces changes in the adsorptive properties and/or reactivity of the intermediate, which could partly explain modifications in the activity and selectivity behavior above 300 °C with a loss of H2 conversion and a significant increase in N2 production at the expense of ammonia, with these tendencies being more accentuated above 350 °C. The partial transformation of Pd to Pd2+ would partly suppress its ability to dissociate hydrogen and explain the lower ammonia formation. In parallel to these observations, the weak signal at 1555 cm-1 assigned to adsorbed NH2 species observed above 246 and 161 °C, respectively, on LaCoO3 and Pd/LaCoO3 could be involved in the formation of ammonia because the appearance of these adsorbed species coincides with the formation of ammonia. Let us note that Pd addition promotes the formation of NH3. An open question arises concerning the nature of the interaction between nitrites and Pd and/or Co species. As shown in Figure 1 no signal at 404 eV, related to nitrites, appears above 300 °C, with no detection at 500 °C on LaCoO3 catalyst. Consequently, the formation of nitrite species on Pd2+ cannot be completely ruled out and could also explain the loss of activity by site blocking. Such an interpretation seems to be in good agreement with previous kinetics investigation dealing with that reaction under stoichiometric conditions, which concluded that the elementary steps leading to N-containing products

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17191 involved the presence of CoOx species and Pd sites where the vicinity of Pd particles assists the dissociation of NO.18 4.2. NO/H2 in Excess O2. Now regarding the influence of oxygen on the NO/H2 reaction, different key points can be identified relative to (i) the role of palladium, depending on the nature of its chemical environment and its oxidation state, and (ii) the nature of activated ad-NOx species as intermediate or spectator, which may drastically differ from those involved under rich conditions. As observed in Figure 7, O2 strongly enhances the conversion of NO at low temperature in the temperature range 90-150 °C without ammonia formation. Such an observation is not related to the involvement of reoxidation process monitored by the presence of oxygen. In this temperature range Pd preserves its metallic character, whereas Co2+ and Co3+ coexist according to XPS measurements. Interestingly, the N 1s photopeak at 403.4 eV only reveals the presence of nitrites at room temperature. Subsequent increase in temperature favors the formation of nitrates parallel to extensive formation of Co3+. In comparison to previous experiments performed under rich conditions, the disappearance of the 1555 cm-1 band previously assigned to intermediate NH2 adsorbed species is in accordance with the absence of ammonia formation on LaCoO3 and Pd/ LaCoO3 at high temperature. Thus, the most important IR observations are probably related to the formation of gemdinitrosyl species on oxidic cobalt species exhibiting different spectral features than those previously evidenced in rich conditions with symmetric and asymmetric stretching vibrational modes, respectively, at 1870 and 1789 cm-1 versus 1850 and 1750 cm-1 in the absence of oxygen. Harrison41 explained the shift of the wavenumbers from 1874 and 1785 cm-1 to 1840 and 1765 cm-1 on Co/SnO2 when cobalt particles became partly reduced as Con+ with n < 2. XPS and in situ IR observations on Pd/LaCoO3 are in good agreement with these previous observations associated to the predominant segregation of Co2+ under rich conditions, whereas Co2+ and Co3+ coexist in the presence of excess O2. Consequently, the spectral features of dinitrosyl species could be a probe for analyzing surface changes and explaining the continuous shift to lower wavenumbers. However, the location of these dinitrosyl species is not solely affected. As observed, ∆σ also varies to a significantly higher value, as well as, the relative intensity ratio Ias/Is ratio shifting from 2.7 to 2.2 in the presence of oxygen. Such changes are sometimes difficult to explain unambiguously. Dictor42 previously developed some arguments for explaining changes in location and relative intensity of asymmetric and symmetric stretching vibration of gem-dicarbonyl complex on rhodium and proposed that the involvement of electronic and/or geometric effects may partly explain these modifications in spectral features. Such effects cannot be completely ruled out. However, changes observed on the spectral features can also be explained by interferences with the characteristic nitrosyl species on Pd related to positively charged NO species and neutral ones, respectively, at 1803 and 1754 cm-1. Consequently, the relative predominance of neutral NO species on Pd under rich conditions and the progressive formation of Pd-NO+ at the expense of the former ones under lean conditions could simply explain our results. However, such an explanation is not satisfactory for explaining the beneficial effect of oxygen on the rate of NO reduction with the formation of stable and less reactive Pd-NO+ toward the dissociation. As a matter of fact, previous findings reported by Kantcheva et al.32 during NO adsorption on Co/ ZrO2 may in a certain way provide some guidelines for explaining our spectroscopic information. These authors ob-

17192 J. Phys. Chem. C, Vol. 112, No. 44, 2008 served during NO adsorption at room temperature the occurrence of nitrosyl species overlapping the characteristic IR bands at 1876 and 1781 cm-1. Interestingly, an increase in time exposure leads to a strong attenuation of these signals at the expense of new contributions at 1906 and 1545-1530 cm-1 associated to the following NO-Co2+-NO2- complex. Accordingly, the following elementary mechanism was proposed for fitting these observations.

As previously mentioned, different information have been obtained on Pd/LaCoO3 in the course of the NO/H2 reaction under lean conditions. Nevertheless, the positive effect of oxygen, the appearance of nitrite species, and then the disappearance of dinitrosyl species above 140 °C seem to be in good agreement with spectroscopic observations made by Kantcheva et al.32 and with mechanistic information provided by Machida et al.37 suggesting the involvement of nitrite species in the NO/ H2 reaction under lean conditions. In fact, oxygen would induce an optimal balance between Co2+ and Co3+ for the stabilization of these nitrite species. Above a critical temperature, the extensive formation of Co3+ would induce the preferential formation of stable nitrates less reactive than nitrites. Consequently, similar mechanistic information previously reported by Machida et al. on Pd/MnOx-CeO237 could be suggested on Pd/LaCoO3 with active ad-NOx species as nitrite species on oxidic Cobalt interacting with metallic Pd particles, where hydrogen can dissociate and then spills over onto the surface of the solid, allowing the reduction of nitrite species at the metal support interface. On the other hand, different catalytic features probably occur at high temperatures. Previous investigations revealed that oxidic palladium species prevail at the surface7 and that reconstruction of the perovskite took place at high temperature with partial Pd insertion into the perovskite structure. 5. Conclusion The transformation of NO in the presence of hydrogen was studied in the presence and absence of excess oxygen over palladium supported on LaCoO3-noble-metal-based catalysts after a preactivation thermal treatment under H2. Complementary in situ XPS and FTIR operando experiments allowed us to follow the surface of the catalyst and the nature of adsorbed species and metal/support interactions while the reaction proceeded. We have established that, in rich conditions, a reoxidation process occurs on reduced catalysts at low temperature, and the NO/H2 reaction is catalyzed on palladium at higher temperatures with the formation of N2, N2O, and NH3. Oxygen addition enhances the conversion of NO at low temperatures on Pd/LaCoO3 catalyst. Whereas dinitrosyl species could reflect changes in the nature of cobalt sites, nitrite species could be involved in the reduction of NOx by hydrogen in lean conditions at high temperature on Pd/LaCoO3 catalyst. Acknowledgment. We wish to thank M. Michel Cle´ment from UCCS for the modification of the DRIFT cell as well as

Dujardin et al. Dr. L. Gengembre and M. Frere from UCCS for the XPS measurements. References and Notes (1) Burch, R.; Coleman, M. D. Appl. Catal., B 1999, 23, 115. (2) Costa, C. N.; Stathopoulos, V. N.; Belessi, V. C.; Efstathiou, A. M. J. Catal. 2001, 197, 350. (3) Costa, C. N.; Efstathiou, A. M. J. Phys. Chem. B 2004, 108, 2620. (4) Engelmann-Pirez, M.; Granger, P.; Leclercq, L.; Leclercq, G. Topics Catal. 2004, 30/31, 59. (5) Costa, C. N.; Efstathiou, A. M. J. Phys. Chem. C 2007, 111, 3010. (6) Costa, C. N.; Savva, P. G.; Fierro, J. L. G.; Efstathiou, A. M. Appl. Catal., B 2007, 75, 147. (7) Twagirashema, I.; Engelmann-Pirez, M.; Frere, M.; Burylo, L.; Gengembre, L.; Dujardin, C.; Granger, P. Catal. Today 2007, 119, 100. (8) Twagirashema, I.; Frere, M.; Gengembre, L.; Dujardin, C.; Granger, P. Topics Catal. 2007, 42-43, 171. (9) Dacquin, J. P.; Dujardin, C.; Granger, P. J. Catal. 2008, 253, 37. (10) Montanari, T.; Marie, O.; Daturi, M.; Busca, G. Appl. Catal., B 2007, 71, 216. (11) Lonyi, F.; Valyon, J.; Gutierrez, L.; Ulla, M. A.; Lombardo, E. A. Appl. Catal., B 2007, 73, 1. (12) Kozyra, P.; Da Costa, P.; Baudin, F.; Datka, J.; Dje´ga-Mariadassou, G. Stud. Surf. Sci. Catal. 2004, 154 (3), 2509. (13) Hadjiivanov, K. Catal. ReV.-Sci. Eng. 2000, 42, 71. (14) Gora-Marek, K.; Gil, B.; Sliwa, M.; Datka, J. Appl. Catal., A 2007, 330, 33. (15) Hadjiivanov, K.; Ivanova, E.; Daturi, M.; Saussey, J.; Lavalley, J.-C. Chem. Phys. Lett. 2003, 370, 712. (16) Almusaiteer, K.; Chuang, S. S. C. J. Catal. 1999, 184, 189. (17) Dhainaut, F.; Pietrzyk, S.; Granger, P. Appl. Catal., B 2007, 70, 100. (18) Dhainaut, F.; Pietrzyk, S.; Granger, P. Catal. Today 2007, 119, 94. (19) Chiarello, G. L.; Ferri, D.; Grunwaldt, J.-D.; Forni, L.; Baiker, A. J. Catal. 2007, 252, 137. (20) Taguchi, H.; Matsu-ura, S. I.; Nagao, M.; Choso, T.; Kabata, K. J. Solid State Chem. 1997, 129, 60. (21) Di Monte, R.; Fornasiero, P.; Ka˜spar, J.; Ferrero, A.; Gubitosa, G.; Graziani, M. Stud. Surf. Sci. Catal. 1998, 116, 559. (22) Sugai, S.; Watanabe, H.; Kioka, T.; Miki, H.; Kawasaki, K. Surf. Sci. 1991, 259, 109. (23) Rainer, D. R.; Vesecky, S. M.; Koranne, M.; Oh, W. S.; Goodman, D. W. J. Catal. 1997, 167, 234. (24) Mamede, A. S.; Leclercq, G.; Payen, E.; Granger, P.; Gengembre, L. Grimblot, J. Surf. Interface Anal. 2002, 34, 105. (25) Kulkarni, G. U.; Rao, C. N. R.; Roberts, M. W. J. Phys. Chem. 1995, 99, 3310. (26) Laksono, E.; Galtayries, A.; Argile, C.; Marcus, P. Surf. Sci. 2003, 532, 37. (27) Brun, M.; Berthet, A.; Bertolini, J. C. J. Electron. Spectrosc. Relat. Phenom. 1999, 104, 55. (28) Uenishi, M.; Taniguchi, M.; Tanaka, H.; Kimurac, M.; Nishihata, Y.; Mizuki, J.; Kobayashi, T. Appl. Catal., B 2005, 57, 267. (29) Rosynek, M. P.; Magnuson, D. T. J. Catal. 1977, 48, 417. (30) Klingenberg, B.; Vannice, M. A. Chem. Mater. 1996, 8, 2755. (31) Tsyntsarski, B.; Avreyska, V.; Kolev, H.; Marinova, Ts.; Klissurski, D.; Hadjiivanov, K. J. Mol. Catal. A 2003, 193, 139. (32) Kantcheva, M.; Vakkasoglu, A. S. J. Catal. 2004, 223, 352. (33) Qi, G.; Yang, R. T.; Rinaldi, F. C. J. Catal. 2006, 237, 381. (34) Maillet, T.; Barbier, J.; Gelin, P.; Praliaud, H.; Duprez, D. J. Catal. 2001, 202, 367. (35) Ivanova, E.; Hadjiivanov, K.; Klissurski, D.; Belvilacqua, M.; Armaroli, T.; Busca, G. Microporous Mesoporous Mater. 2001, 46, 299. (36) Lesage, T.; Verrier, C.; Bazin, P.; Saussey, J.; Malo, S.; Hedouin, C.; Blanchard, G.; Daturi, M. Top. Catal. 2004, 30/31, 31. (37) Machida, M.; Kurogi, D.; Kijima, T. J. Phys. Chem. B 2003, 107, 196. (38) Dhainaut, F.; Pietrzyk, S.; Granger, P. Top. Catal. 2007, 42-43, 135. (39) Hecker, W. C.; Bell, A. T. J. Catal. 1985, 92, 247. (40) Goodgame, D. M. L.; Hitchman, M. A. Inorg. Chem. 1956, 4, 721. (41) Harrison, Ph.; Thornton, E. J. Chem. Soc. Faraday Trans. 1978, 74, 2703. (42) Dictor, R. J. Catal. 1988, 109, 89.

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H2 Reaction on

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