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Chlorine influence on palladium doped nickel catalysts in levulinic acid hydrogenation with formic acid as hydrogen source Emilia Soszka, Hanna Reijneveld, Marcin Jedrzejczyk, Izabela Irena Rzeznicka, Jacek Grams, and Agnieszka Malgorzata Ruppert ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03211 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Sustainable Chemistry & Engineering

Chlorine influence on palladium doped nickel catalysts in levulinic acid hydrogenation with formic acid as hydrogen source Emilia Soszka†, Hanna M. Reijneveld†, Marcin Jędrzejczyk†, Izabela Rzeźnicka‡, Jacek Grams†, Agnieszka M. Ruppert†* †

Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of

Technology, Żeromskiego 116, Łódź 90-924, Poland ‡

Shibaura Institute of Technology, Graduate School of Science and Engineering, 3-7-5

Toyosu, Tokyo 135-8548, Japan KEYWORDS: Internal hydrogen source, Formic acid, Nickel, Bimetallic catalysts, γvalerolactone, levulinic acid *Corresponding author: [email protected]

ABSTRACT Levulinic acid (LA) is one of platform molecules, and its valorization towards biofuel additives like γ-valerolactone or tetrahydrofuran is considered as important step in planning future biorefinery schemes. In this study various Ni-based catalysts were studied for the LA hydrogenation with formic acid (FA) used as a hydrogen source. Two different ways of catalytic activity improvement are discussed (nickel loading vs. addition of dopants). The influence of Ni doping by small amount of noble metals (Pt, Pd, Ru, Rh) showed that Ni-Pd is

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the most active catalyst. Its high catalytic performance is attributed to the synergic effect between two metals and interaction with chlorine. The effect of chlorine on catalytic performance and properties of the catalysts was evaluated by variety of surface and bulksensitive characterization methods. It was shown that addition of chlorine is one of the key factors required for high catalytic performance. Chlorine influences distribution of metals on the surface of the catalyst, their interaction with support and facilitates the formation of small crystallites which is beneficial for reaching high catalytic activity.

INTRODUCTION Lignocellulosic biomass is one of the most abundant sources of renewable carbon1,2. That is why, in the last decade, the attention of the researchers was focused on its selective transformation to platform molecules. Catalytic conversion of levulinic acid (LA) or ethyl levulinate

(AL)3

towards

important

chemicals

like

γ-valerolactone

(GVL)

and

methyltetrahydrofuran (MTHF) can constitute one of the crucial steps in the design of future solutions of new biorefinery schemes. Thanks to its properties, γ-valerolactone can be used as green solvent and chemical intermediate that can be further hydrogenated to produce fuel additives (2-methyltetrahydrofuran-MTHF or added-value chemicals like 1,4-pentadiol)1,4,5. Recent studies have focused on the use of noble metal based catalysts for the hydrogenation of LA with external hydrogen supply6-8 with Ru catalysts being the most frequently applied due to its high activity9-11. High temperatures, presence of organic solvents and relatively high hydrogen pressure are often used for the production of GVL, but such conditions generate high energy costs and limits the sustainability of the process12-14. Involvement of non-noble metal active phases would improve the economy of the process by increasing resistance of the catalyst to impurities present in the biomass. There are only few ACS Paragon Plus Environment

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examples of the application of non-noble metals in the hydrogenation of LA with the external hydrogen source13,15. Due to lower activity of non-noble metals, harsh reaction conditions are usually necessary to reach high yield of products. Hengst et al.12 investigated the influence of the preparation method and the effect of solvent in the LA hydrogenation using an external hydrogen source for the 15% Ni/γ-Al2O3 catalyst. For reaching a high GVL yield of 91%, harsh reaction conditions with a temperature of 200°C and a 50 bar hydrogen pressure were necessary using dioxane as a solvent. Kumar et al.16 obtained a similar GVL yield of about 90% over 20% Ni/SiO2 catalysts when carrying out the reaction at 270ºC. As described above, the reaction in the gas phase requires a high temperature or high pressures which increases the cost of the process. In order to improve the sustainability of the process, various H-donors (solvent or reaction byproducts) are used for the hydrogenation reaction. Hengne et al.17 applied isopropyl alcohol as a hydrogen source for nickel catalysts dispersed on various supports. In order to achieve high activity, a high nickel loading (50% Ni/montmorillonite) and additional N2 pressure were necessary. On the other hand, the application of formic acid as hydrogen donor constitutes an elegant solution as formic acid is co-produced together with levulinic acid by the acid hydrolysis of cellulose. Therefore, one of the greatest challenges is to use this crude solution, obtained directly from biomass hydrolysis, for the conversion of LA into GVL. Literature reports concerning the use of formic acid as a hydrogen source for the hydrogenation of LA on non-noble metal based catalysts are scarce, particularly in a one pot reaction when the same catalyst has to be used for both processes. Active catalysts should possess active sites for both FA decomposition towards hydrogen and subsequent hydrogenation of LA (FALA reaction). Recently Upare et al.7 used thermally stable nickelpromoted copper-silica nanocomposite catalysts for performing the LA hydrogenation in the presence of 1,4-dioxane using formic acid as hydrogen source. Different reaction conditions ACS Paragon Plus Environment


and LA/FA molar ratio were tested, and a temperature as high as 285ºC was needed in order to obtain a high GVL yield. The group of Fujitani tested copper catalysts supported on SiO2 in the vapor-phase hydrogenation of LA to GVL using formic acid as a hydrogen source at 250oC. The high activity obtained was attributed to the presence of monomeric, partially oxidized copper species, strongly interacting with the support18. Very nice approach was also shown in the work of the group of Luque for the production of MTHF from LA in a one pot system with copper based catalysts19. So far, there are only a scarce number of reports concerning the hydrogenation of levulinic acid using formic acid as a hydrogen source in water, with cost effective non-noble metal catalysts with a low metal loading. In this work we evaluated the activity of Ni - based catalysts doped with small amount of noble metals in the hydrogenation of LA with FA as an internal source of hydrogen. The influence of the preparation method and the addition of small amounts of chlorine on the catalyst activity was studied in details. This study shows that small amount of chlorine has a strong promoting effect in FALA reaction. The role of chlorine on both the catalyst surface properties and the catalyst activity was investigated and explained.

EXPERIMENTAL SECTION Catalyst synthesis. All catalysts were prepared by wet impregnation method using γ-Al2O3 as a support. Bimetallic catalysts were loaded with 4% Ni and doped with 1% of other metals (Pd, Pt, Ru, Rh). Ni(NO)3·6H2O and the respective chloride precursors of metals were used (more details in SI). For Ni-Pd catalysts prepared from nitrate precursors (Ni-Pd/γ-Al2O3),


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Pd(NO3)2 and Ni(NO3)2·6H2O salts were used. For the catalysts prepared with chlorine precursor (Ni-Pd(Cl)/γ-Al2O3), PdCl2 diluted in HCl (35 vol%) was used as Pd source, and nitrate salt as Ni precursor. For the catalysts prepared with HCl addition, the appropriate amount of HCl (35 vol%) (1:1, 1:2, 1:0.25 in respect of the molar amount of chlorine used in the case of the 4%Ni-1%Pd(Cl)/γ-Al2O3 catalyst) was added together with Pd(NO3)2 and Ni(NO3)2·6H2O during the first step of the synthesis. The resulting catalysts are labeled as NiPd(a1HCl)/γ-Al2O3 for 1:1 ratio, Ni-Pd(a2HCl)/γ-Al2O3 for 1:2 ratio and Ni-Pd(a0.25HCl)/γAl2O3 for 1:0.25 ratio. All samples were calcined at 500°C for 5 h under static air conditions with a temperature ramp rate of 5°C min-1, and further reduced under H2 flow for 1 h at 650°C, directly before the reaction. Catalytic test in LA hydrogenation with FA as a hydrogen source (FALA). 1 g of LA, 0.4 mL of FA, 0.6 g of catalyst and 30 mL of water were combined in a stainless steel autoclave (Berghof, Germany), equipped with teflon insert having volume of 45 mL. The temperature was maintained at 190°C for 2 h. At the end of the reaction, the reactor was cooled down, the remaining pressure was released and the reaction mixture was centrifuged to separate the catalyst and liquid products. Reaction product analysis. Liquid products were analyzed by high-performance liquid chromatograph (Agilent Technologies 1260 Infinity, Perlan Technologies) equipped with refractive index detector and Rezex ROA column, 0.0025 mol/L H2SO4 was used as an eluent. The details concerning the materials used, the preparation of monometallic catalysts, the reaction conditions in separate reactions as well as the characterization methods (TPR, BET, SEM, TEM) are described in Supporting Information (SI).



RESULTS AND DISCUSSION Catalytic activity The catalytic activity tests of Ni-based catalyst in the hydrogenation of levulinic acid using formic acid as a hydrogen source are summarized in Table 1. As it was pointed out in our earlier study, the decomposition of formic acid occurs in the first step of the reaction and constitutes the driving force for a subsequent hydrogenation process20. All Ni monometallic catalysts displayed activity only in the FA decomposition. When comparing different Ni loadings on γ-Al2O3, it is possible to notice that the catalyst containing 4% of Ni displayed a low formic acid conversion of 26%, while the FA conversion strongly increased up to around 86% for catalysts containing 15-30% of Ni. For doped M-Ni (M= Pd, Pt, Ru, Rh) bimetallic catalysts, a more pronounced effect on the catalytic performances was observed. For all the catalysts, the activity was higher in comparison to that of 4%Ni/γ-Al2O3 monometallic counterpart. The lowest activity boost was noticed for the Ru-doped system (only 42% of FA conversion was observed), followed in term of FA conversion by Rh- and Pt- doped nickel based bimetallic catalysts. The highest activity in FALA reaction among the bimetallic catalysts was obtained using 4%Ni-1%Pd/γAl2O3 catalyst, with a 51% GVL yield. Only in this latter case, a significant yield of GVL was observed.


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Table 1. Influence of noble metal doping of Ni catalysts on the catalytic performance in LA hydrogenation with FA used as hydrogen source.

FA

LA

GVL

[%]

[%]

[%]

4% Ni/γ-Al2O3

26

5

0

15% Ni/γ-Al2O3

86

0

0

20% Ni/γ-Al2O3

87

0

0

30% Ni/γ-Al2O3

85

0

0

Ni-Pt(Cl)/γ-Al2O3

97

6

0

Ni-Pd(Cl)/γ-Al2O3

100

56

51

Ni-Ru(Cl)/γ-Al2O3

42

5

0

Ni-Rh(Cl)/γ-Al2O3

88

7

0

Catalyst

Reaction conditions: 190ºC; 2 h; 0.6 g of catalyst; 1 g LA; 0.4 ml FA and 30 ml water .

In the next step, the influence of the palladium precursor was investigated (Table 2). Two palladium precursors were used, nitrate (N) and chloride (Cl). It can be seen that the Ni-Pd (Cl)/γ-Al2O3 catalyst highly outperformed the one derived from the nitrate precursor, with a 51% GVL yield much higher than the 2 % GVL yield obtained on the Ni-Pd(N)/γ-Al2O3 catalyst.



It is worth noting that all noble metals doped catalysts were prepared from chloride precursors, so in the following step the influence of the chlorine addition was investigated. Chlorine was added in different amounts to the samples prepared from the nitrate precursor in order to understand its promoting effect on the activity of the catalysts (Table 2). Interestingly, adding a similar amount of chlorine that was used in preparation of the NiPd(Cl)/γ-Al2O3 catalyst resulted in a Ni-Pd(a1HCl)/γ-Al2O3 catalyst with nearly the same catalytic activity (54% of LA conversion). Reducing the amount of chlorine to 0.25 (in respect to the original Cl content present in the Ni-Pd(Cl)/γ-Al2O3 material) diminished the LA conversion to 41 % LA, while a stronger decrease to 31% of LA was observed when doubling the amount of chlorine. This difference was less visible for the monometallic catalysts. Indeed, for Pd/γ-Al2O3, only a very small increase in LA conversion and GVL yield was observed at complete FA conversion, whereas this was more pronounced in the case of Ni/γ-Al2O3, with a slightly more visible FA conversion increase from 26% to 38%. The reaction conditions were chosen in a way to be far from full conversion in order to evidence differences in terms of catalytic activity. The activity can however be increased by adjusting the reaction conditions, and eg. 83% LA conversion was achieved for 4 h, reaction in the case of Ni-Pd(Cl)/γ-Al2O3.


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Table 2. Influence of chlorine addition on the catalytic performance in LA hydrogenation with FA used as hydrogen source.

Conversion [%]

GVL

FA

LA

Yield

Ni-Pd(Cl)/γ-Al2O3

100

56

51

Ni-Pd(N)/γ-Al2O3

99

10

2

Ni-Pd(a2HCl)/γ-Al2O3

100

31

26


100

54

50

Ni-Pd(a0.25HCl)/γ-Al2O3

100

41

36

Pd(a1HCl)/γ-Al2O3

100

11

4

Pd(Cl)/γ-Al2O3

100

14

8

Pd(N)/γ-Al2O3

100

12

4

Ni/γ-Al2O3

26

5

0

Ni(a1HCl)/γ-Al2O3

38

7

0

100

83

79

Catalyst

*

Ni-Pd(Cl)/γ-Al2O3

Reaction conditions: 190oC; 2 h; 0.6 g of catalyst; 1 g LA; 0.4 ml FA and 30ml water; *4h reaction. Temperature-programmed reduction tests The temperature-programmed reduction (TPR) profiles of the investigated catalysts are illustrated in Figure 1. 4%Ni/γ-Al2O3 displayed a hydrogen consumption peak starting at



around 450°C with a long tail extending towards high temperatures. This suggests a strong interaction of NiOx with the support or the possible formation of a spinel interfacial phase21. By contrast, the reduction profile for 4%Ni(a1HCl)/Al2O3 is well-defined, having a clear peak with maximum at much lower temperatures (450°C). This indicates that chlorine weakens the interaction between Ni and the support, causing Ni to be reduced at much lower temperatures. The influence of the chlorine was also visible in the reduction profiles recorded on the monometallic Pd catalysts. In the case of Pd(N)/γ-Al2O3, a negative H2 consumption peak was observed due to decomposition of -PdH that is characteristic for large Pd particles22. In the case of the monometallic Pd catalysts prepared in the presence of chlorine, well defined maximum at the temperatures around 120oC was visible. The absence of negative reduction peak could be related to the presence of smaller Pd particles22,23. Bimetallic Pd-Ni(N)/γ-Al2O3 prepared from the nitrate precursor exhibited a TPR profile similar to that of the monometallic nickel catalyst, which can suggest a stronger interaction of Ni with the support in comparison to other Ni-Pd systems. Thus the presence of the palladium is only visible by the appearance of the negative peak. By contrast, TPR profiles for bimetallic catalysts prepared with chlorine addition were similar to each other. In the TPR profiles, the first low temperature effect is likely due to the reduction of the Pd species, while the second peak observed at higher temperature can be related to the reduction of NiOx species. In the case of the catalysts prepared with the lowest amount of chloride, ie. Ni-Pd(a0.25HCl)/γ-Al2O3, three reduction regions were identified. The first one below 200°C is similar to that observed for other chlorine-containing catalysts, while more intense hydrogen consumption was observed at about 300°C, along with a broad signal in the temperature range from 330 to 500°C.


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Figure 1. TPR profiles of mono- and bimetallic catalysts.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements In order to characterize the changes that may occur on the surface of the catalysts, ToF-SIMS measurements were performed (Table 3). Intensity ratios were calculated for the chosen ions identified on the mass spectra collected from the catalyst surface. It was noticed that the intensity of Ni and Pd on the surface was different for mono and bimetallic catalysts and was also influenced by presence of chlorine. Generally, the intensity of Ni on the surface was higher for the chlorine-containing bimetallic catalysts, which may suggest a better nickel dispersion20. On the other hand, an opposite trend was observed for Pd ions. The highest intensity was recorded for the monometallic catalysts (Pd+/total ratio varying from 10.4 to 14). Additionally this value was almost similar for all bimetallic, chlorine-containing catalysts, with a Pd+/total



ratio varying from 6.0 to 6.9. What is more, the lowest intensity was observed for the NiPd(N)/γ-Al2O3 catalysts (Pd+/total ratio of 0.86), which can suggest lower dispersion of palladium on the surface20. The comparison of the chlorine content showed that the surface of monometallic samples contained the lowest amount of chloride (Cl-/total ratio varying from 8.0 to 9.5) in contrast to that of the bimetallic ones (Cl-/total ratio varying from 11.6 to 16.9). Interestingly for all bimetallic catalysts for which Cl- was introduced from HCl, the Cl-/total ratio was similar. However, despite similar chlorine content, different amounts of metal-chlorine species were formed on the surface. When it comes to NiCl2 type species, the highest amount was identified on Ni-Pd(a0.25HCl)/γ-Al2O3, whereas on other samples containing chlorine the value was similar (NiCl2-/total ions was from 9.9 to 11.6). The lowest value of NiCl2- was observed on the monometallic Ni catalyst. When it comes to PdCl- intensity, the lowest value was observed for Ni-Pd(a1HCl)/γ-Al2O3 and Ni-Pd(Cl)/γ-Al2O3 catalysts. A bit higher value was observed for two other bimetallic catalysts, whereas the intensity of PdCl- ions was significantly higher for monometallic catalysts. This latter case suggests that more chlorine is involved in the interaction with Pd. Interestingly, the presence of Ni2Pd species, which suggests a strong Pd-Ni interaction or the alloy formation, was identified on catalysts prepared from nitrate precursors and on Ni-Pd(a0.25HCl)/γ-Al2O3.


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Table 3. Normalized intensity of selected ions identified on the surface of the catalysts.

Ni-

Pd+

Cl-

/total

/total

/total

/total

total

/total

x10-4

x10-4

x10-2

x10-4

x10-4

x10-5

4%Ni(N)/γ-Al2O3

6.3

-

-

-

-

-

4%Ni(a1HCl)/γ-Al2O3

3.7

-

8.0

5.0

-

-

1%Pd(Cl)/ γ-Al2O3

-

14.0

9.5

-

9.9

-

1%Pd(N)/ γ-Al2O3

-

20.6

-

-

-

-

1%Pd(a1HCl)/γ-Al2O3

-

10.4

8.6

-

6.8

-

Ni-Pd(N)/γ-Al2O3

4.8

0.9

-

-

-

27.1

Ni-Pd(a0.25HCl)/γ-Al2O3

11.3

6.1

11.6

24.2

4.6

6.2

Ni-Pd(Cl)/γ-Al2O3

7.1

6.9

16.9

11.6

3.1

-


9.9

6.0

11.9

10.5

2.3

-


9.3

6.3

11.8

9.9

4.1

-

Catalyst

NiCl2- PdCl-/ Ni2Pd+

FTIR measurements The spectra of carbon monoxide adsorption on monometallic nickel, palladium and bimetallic catalysts are shown in Figure 2.



A

B

C

D

Figure 2 FTIR spectra of CO adsorbed on the surface of A) monometallic Ni catalysts, B) monometallic Pd catalysts and C-D) bimetallic catalysts.

In the case of nickel catalysts, the band at 2145 cm-1 corresponds to CO physically adsorbed on the surface of the catalyst. Two strong bands at ≈2055 cm-1 and 2020 cm-1 are assigned to the linear CO adsorption on metallic nickel crystallites24-28. The band at higher frequency may correspond to subcarbonyl nickel species Ni0(CO)4, while shoulders at 2020 cm-1 can be attributed to isolated linearly adsorbed CO24,26,27. In the case of monometallic Pd, the bands at ≈2080 cm-1 and ≈1930 cm-1 with the shoulder at 1970 cm-1 are assigned to the linear and the bridge forms of CO adsorbed on palladium species, respectively27, 29-31. They are more intense for chlorine-containing catalysts. The band at 1970 cm-1 corresponds to CO bridge adsorption on Pd species having lower coordination number while the band at 1930 cm-1 is assigned to the bridge adsorption on palladium species


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having higher coordination number29-31. In the spectra of Pd(N)/γ-Al2O3, an additional band at 2054 cm-1 is visible, which indicates the presence of a second type of palladium centers, probably with higher coordination29,32. Comparing the spectra of monometallic palladium catalysts, it is evident that adsorption centers at the surface of the Pd(Cl)/γ-Al2O3 and Pd(a1HCl)/γ-Al2O3 catalysts have similar properties, while the amount of centers on the Pd(N)/γ-Al2O3 catalyst remains highly limited. The depletion of the palladium adsorption centers is related to the strong interaction of the metal with the support for a reduction temperature above 600°C33. The presence of chlorine inhibits this interaction, which confirms the increase in the intensity of the CO adsorption bands in the spectrum of the Pd(N)/γ-Al2O3 catalyst. The spectra for bimetallic catalysts shows two bands, one at 2054 cm-1 with a shoulder at 2020 cm-1 and another with low intensity around 1940 cm-1. The bands at 2054 cm-1 and 2020 cm-1 correspond to the linear forms of CO adsorption on nickel particles, and have a noticeably higher intensity in comparison to monometallic ones. A broad band below 1940 cm-1 is associated with CO bridging adsorption, most probably occurring on palladium crystallites, which is less intense in comparison to that observed on monometallic Pd. It indicates that for bimetallic catalysts, the surface availability of Ni is much higher and that of Pd is much lower, in comparison to their surface availability in the respective monometallic catalysts. The CO adsorbtion FTIR results were confirmed by the CO chemisorption study, demonstrating that bimetallic catalysts enabled higher CO adsorption than their monometallic counterparts (Table S2).



SEM and TEM measurements The morphology and chemical composition of the bimetallic catalysts prepared from the nitrate and chloride precursors were observed by scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) analysis. At low magnification, typical flowerlike morphology of γ-Al2O3 was observed in images recorded on both samples (Figure S3). The γ-Al2O3 crystallites had intact flat planes indicating their resistance to morphological changes under the conditions of the catalyst preparation. At higher magnification, groups of nanoparticles and isolated clusters were observed on the surface of Al2O3 (Figure 3A and B). The image and EDX line profile for the catalysts prepared from the chloride precursor in the presence of additional chlorine ions are shown in Figure 3A.

Figure 3 SEM images of A) Ni-Pd(Cl)/γ-Al2O3, B) Ni-Pd(N)/γ-Al2O3, and TEM images; C) Ni-Pd(Cl)/γ-Al2O3, D) Ni-Pd(N)/γ-Al2O3. The bright features corresponded to the changes in nickel content over the surface. No detectable signal from Pd was observed which could indicate that Pd is organized into small


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nanoclusters. The opposite trend was observed on samples prepared from the nitrate precursor (Figure 3B). In this case, the bright features corresponded to the changes in Pd content, and no detectable nickel signal was recorded. The enrichment of the surface by Pd for bimetallic catalysts prepared from the nitrate precursor has already been reported by Miegge et al.34. A more detailed analysis of the surface using transmission electron microscope (TEM) equipped with EDX showed a presence of small bright particles with size below 10 nm for the catalysts prepared from the chloride precursor. The dark field image for this case is shown in Figure 3C. The EDX analysis of Ni-Pd(Cl)/γ-Al2O3 revealed that small crystallites of Ni were located in very close proximity of Pd. Typical TEM image of the sample prepared from the nitrate precursor is shown in Figure 3D. Mainly large particles attributed to Pd were observed. This is in accordance with the XRD analysis (Figure S5) as only in this case the reflex corresponding to metallic Pd was identified.

Role of Pd addition and chlorine presence The role of noble metal doping of Ni catalysts is usually explained by the synergic effects of a second metal and the formation of new active sites. Our results clearly indicated that the conversion of LA can be tuned by the addition of an appropriate amount of chlorine ions. In the literature, the role of chlorine is often overlooked or neglected, and eventually is often considered as a poison causing blocking of the catalyst’s active sites. On the other hand, chlorine can be responsible for redispersion of the metal particles in the case of Pt and Ir35. Its exact impact is however still under debate.



We showed that the presence of chlorine ions facilitates the reduction of Ni and Pd, and influences the distribution of the metals on the surface of the support (TPR Figure 1, ToFSIMS Table 3, SEM Figure 3). This might be explained by a strong competitive adsorption of chlorine species on the alumina during the impregnation step, which further limits the affinity of the nickel precursor to the surface and prevents the formation of spinel. In contrast, Ni has a strong affinity to the alumina when nitrate precursor is used. At high reduction temperature, this effect leads to strong metal-support interactions and spinel formation. As a consequence, large aggregates of Pd are formed (TEM Figure 3D). Our results demonstrated that the addition of chlorine facilitates the homogeneous distribution of metals on the surface of γ-Al2O3, which is in agreement with the literature36. The positive role of palladium in bimetallic Ni-Pd catalysts is attributed to favorable hydrogen molecule dissociation and hydrogen spillover onto Ni37. This in turn favors the NiOx reduction at lower temperatures. The interaction of Pd with Ni was evidenced by stronger CO chemisorption (FTIR Figure 2 and Table S2), which was also confirmed by the literature observations38. The aforementioned properties have a direct influence on catalytic performance. With the help of density functional theory (DFT) calculations, we have recently showed that alloying Ni with noble metals like Au can strongly modify its behavior in FA decomposition39. The alloying modifies the strength of FA adsorption, which in consequence facilitates the activity of the catalyst. It also prevents strong adsorption of H2 and CO, which favors the H2 production and increases the stability of the catalysts40. Similar phenomena could take place with Pd-doped Ni catalysts.


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CONCLUDING REMARKS We demonstrated that among the different noble metal dopants of Ni catalyst, the most active in FALA reaction was Pd. The performance of bimetallic Ni-Pd catalysts could be additionally tuned by the choice of preparation methods. We showed that traces of chlorine can change the way the metal is interacting with the support and the metal distribution at the surface. Particularly this effect is based on diminishing the affinity of Ni to the surface, so that its reduction is facilitated at lower temperatures. In turn, the dispersion of Ni particles becomes higher, while Pd (introduced from PdCl2) interacts with the surface more strongly due to the strong affinity of chlorine to alumina, therefore preventing the formation of large Pd aggregates. Last but not least the role of Pd is also crucial as it interacts with Ni and modifies the strength of the adsorption of probe molecules. This could consequently facilitate the adsorption of the reaction intermediates and increase the activity in the tested reaction. Those findings can shed the light on other processes as the scope of applications of Pd-Ni systems are continuously broadening to other hydrogenation or hydrogenolysis reactions e.g. hydrogenolysis of model lignin molecules for which similar catalysts displayed very high activity41.

Supporting Information Additional figures and data including preparation of catalysts, description of characterization techniques, test protocol for separate reactions, XRD patterns, BET surface area, SEM-EDX characterization, TPR profiles of other mono and bimetallic catalysts, activity of catalysts in separate reactions (FA decomposition and LA hydrogenation) are placed in SI.



ACKNOWLEDGMENTS The authors gratefully acknowledge that this work was financially supported by a grant from the National Center of Science (NCN) in Krakow (Poland) (2016/22/E/ST4/00550). A part of this work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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GRAPHICAL ABSTRACT

Key role of chlorine in designing active, non-noble metal based catalysts for levulinic acid hydrogenation with the formic acid used as hydrogen source.


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Chlorine Influence on Palladium Doped Nickel ... - ACS Publications

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