Novel Pd / CeO 2 and Pd-NiO / CeO 2 nanocomposites ’ catalytic activity in glycerol oxidation processes

In this work novel Pd supported on cerium oxide nanocomposites in the liquid phase glycerol oxidation with molecular oxygen was studied. Pd/CeO2 and Pd-NiO/CeO2 composites with different Pd loading were prepared using the extractivepyrolytic method. In the glycerol oxidation processes it was found that Pd/CeO2 and Pd-NiO/CeO2 composites are catalytically active. Besides, NiO additives can significantly increase Pd/CeO2 catalysts’ activity and selectivity to the main product – glyceric acid. Several glycerol oxidation parameters like NaOH initial concentration, glycerol/Pd molar ratio, oxygen pressure and temperature were varied. The best yield of glyceric acid reached 71–75 mol% with full glycerol conversion.


INTRODUCTION
* With the extensive search for alternative renewable energy sources and development of the biodiesel industry, glycerol has been produced in large amounts reaching considerable surplus.Glycerol is the main by-product in vegetable oil transesterification process to biodiesel and its yield is about 10 wt% [1].Due to the high abundance of glycerol, low price and excellent functionality, glycerol utilization gains a lot of attention.Liquid phase catalytic oxidation of glycerol is one of the most promising routes to produce some high-value chemicals [2].The most common products obtained by glycerol oxidation are glyceric acid, lactic acid and dihydroxyacetone, which find potential applications in polymer and fine chemical industries [3].Catalytic oxidation with molecular oxygen in the presence of supported noble metal catalysts is environmentally friendly and mild glycerol utilization method.Catalysts can be reused [4][5][6].
Nanoscale palladium-based catalysts have been found to be very active in the glycerol oxidation processes.Compared to Au catalysts, which are the most studied catalysts in the glycerol oxidation field, Pd is relatively cheap, abundant and it can be used in neutral solutions [2].Up to now several different Pd supports, mainly metal oxides (C, Al 2 O 3 , Y 2 O 3 , SiO 2 , TiO 2 , combined oxide of Zr and Ce) have been used and it has been found that Pd containing catalysts' activity and product distribution strongly depend on the nature of the support [7][8][9][10].
In this work novel Pd supported on cerium oxide composites' catalytic activity in the liquid phase glycerol oxidation with molecular oxygen was studied.CeO 2 is a versatile material, its wide use is attributed to ceria unique redox features, which enables the oxide to act as an excellent oxygen storage material [11].In the glycerol oxidation related processes bimetallic Au-Pt and bimetallic Ag-Au, Ag-Pt, Ag-Pd catalysts supported on CeO 2 until now have been investigated [12,13].Also mixed cerium-zirconium oxide as support for gold and copper or for gold and ruthenium monometallic and bimetallic catalysts have been investigated [14,15].During this work several novel Pd/CeO 2 and Pd-NiO/CeO 2 nanocomposites with different Pd loading were prepared using the extractive-pyrolytic method.Composite compositions' as well as glycerol oxidation parameters' influence on glycerol oxidation results was investigated.

Composites' preparation
Supported palladium composites were prepared by extractive-pyrolytic method described in [16,17].In the case of monometallic Pd/CeO 2 nanocomposites, the composites' preparation started with the production of an organic precursor by the liquid extraction method.In order to obtain precursor, 20 mL of 1.0 mol/L tetrachloride palladium acid solution in 2 mol/L hydrochloric acid solution was added to 50 mL of 1 mol/L trioctylamine solution in toluene.After shaking the mixture for 5 min, the organic phase was separated from the water phase and filtered.The obtained organic phase, which was a 0.4 mol/L [(C 8 H 17 ) 3 NH] 2 PdCl 4 solution in toluene, was the composites' precursor and it was added to the support (CeO 2 ).The amount of support was calculated as having a final palladium loading on the composite (0.4-2.8 wt% by weight of support).The obtained system was stirred for 10 min, during which support was impregnated by precursor.After impregnation the support and precursor mixture was dried for 20-60 min at room temperature.The dry mixture was then calcinated at 300 ºC for 5 min at the atmospheric pressure.During the calcination stage, all the Pd compound in the precursor was reduced to Pd (0).
For the preparation of Pd-NiO/CeO 2 composites, at first NiO/CeO 2 composite was prepared.NiO/CeO 2 composite's preparation started with production of nickel-containing organic extract (precursor), which is described in details in [18].After organic extract was prepared, support (CeO 2 ) was impregnated in it.Following sample was dried at room temperature and calcinated at 300 ºC for 30 min.After NiO/CeO 2 composite was produced, it was impregnated by palladium precursor, dried and calcinated in the same way as it was described in the Pd/CeO 2 composites' preparation.Loading of NiO was kept constant in all Pd-NiO/CeO 2 composites and was 5.0 wt%, while Pd loading varied from 0.4-2.8wt%.

Equipment
The characterization of novel nanocomposites' morphology, crystallization, chemical content and surface area was done by scanning electron microscopy (SEM TESCAN LYRA3), X-ray diffraction (XRD), X-ray fluorescence (XRF) and BET surface area analysis method.The phase composition was determined by XRD analysis with D8 Advance, Bruker AXS system.The Pd crystallite size d Pd was calculated from broadening of diffraction maxima using the Scherrer Equation (software Topas 3).Chemical analysis was performed by means of the S4 Pioneer X-ray Spectrometer (Bruker AXS).The BET specific surface area (SSA) of the composites was determined by nitrogen adsorption at -196 ºC with a HROM-3 chromatograph.
The oxidation experiments of aqueous glycerol solutions with molecular oxygen in the presence of novel composites was performed in an autoclave and in a thermostated slurry bubble column reactor operated in batch mode.Glycerol oxidation process parameters like NaOH initial concentration c 0 (NaOH) = 0-1.5 mol/L, glycerol and palladium molar ratio n(glycerol)/n(Pd) = 300-1000 mol/mol, reaction temperature 45-60 ºC and oxygen pressure from 1 to 6 atm were varied.Glycerol initial concentration c 0 (glycerol) was 0.3 mol/L.Analysis of the reaction mixture was performed by high-performance liquid chromatograph Shimadzu Nexera equipped with UV-Vis SHIMADZU SPD-20A (UV 210 nm) and ELSD-LTII detectors.A Waters IC-PAC Ion-Exclusion column (300 mm × 7.8 mm) (75 ºC) was used with aqueous trifluoroacetic acid 0.045 vol% as the eluent.

Composites' characterization
Figure 1 shows a typical SEM image and EDS spectra of the 1.4 wt% Pd-NiO/CeO 2 composite.The data testify that the composite particles form agglomerates with irregular shapes and sizes from 1-100 μm.The EDS spectrum shows the presence of palladium and nickel in the produced composite.
In our early work [18] it was found that after thermal treatment at 300 ºC NiO/CeO 2 composite was x-ray amorphous.XRD pattern of the 1.4 wt% Pd-NiO/CeO 2 and 2.8 wt% Pd-NiO/CeO 2 composites shown in Fig. 2 approves the presence of Pd.X-ray diffraction analysis presents that with the increase of Pd loading, the characteristic Pd (111) peak becomes more apparent.
In Table 1 results of specific surface area and size of Pd crystallites d Pd measurements for some Pd/CeO 2 and Pd-NiO/CeO 2 composites as well as support are shown.As it can be seen from Table 1, surface area decreases with increasing content of Pd in both cases -promoted with NiO and non-promoted Pd composites.Surface   areas of non-promoted and promoted Pd composites with the same Pd loading are similar.Pd crystallite sizes determined by XRD for 1.4 and 2.8 wt% Pd containing composites are also similar (10-15 nm).

Glycerol oxidation
Glycerol oxidation with molecular oxygen tests in the presence of novel Pd supported composites showed that these composites are catalytically active for the glycerol conversion into other products.By comparing Pd/CeO 2 and Pd-NiO/CeO 2 composites, it was found that NiO additives can significantly increase Pd/CeO 2 composites' activity.From Fig. 3 it can be seen, that regardless of Pd loading glycerol conversion in the presence of NiO containing composites was greater.This could be explained by NiO promotional effect caused by synergy, which occurs between Pd and NiO particles reported in [19].Testing composite NiO/CeO 2 without Pd in the glycerol oxidation experiments, it was found that it doesn't show catalytic activity.
The main glycerol oxidation product over all Pd containing composites was glyceric acid.By-products were lactic, tartronic, glycolic, oxalic, acetic and formic acids.Comparing Pd containing composites' activity depending on Pd loading, from Fig. 3 it can be seen that, in the case of Pd-NiO/CeO 2 composites, glycerol conversion was similar in all cases (74-82 mol%), while non-promoted composites activities' dependence on Pd loading was ambiguous.
Using one of the promoted composites (1.4 wt% Pd-NiO/CeO 2 ) as well as one of non-promoted composites (2.8 wt% Pd/CeO 2 ), influence of NaOH was determined (Fig. 4 (left) and (right), respectively).At first, it was experimentally tested that non-promoted as well as promoted Pd containing composites are inactive in base-free glycerol solutions, so base is needed to initiate the reaction.NaOH initial concentration was varied in the range 0.3-1.5 mol/L.From Fig. 4 it can be seen that optimal NaOH initial concentration in the case of promoted composite was 0.6 mol/L, when full glycerol conversion was reached.If NaOH concentration was increased until 1.5 mol/L, glycerol conversion decreased by almost 20 mol%, which could be related with greater media viscosity determined by NaOH and also with  lower oxygen dissolution in the liquid medium [20].
In the presence of Pd/CeO 2 composite it was found out that optimal concentration of NaOH was 0.3 mol/L.Selectivity of Pd containing composites to the main product -glyceric acid -was similar for both composites -selectivity didn't depend on the change of NaOH concentration, neither on glycerol conversion.
In further experiments only the most active composite -1.4 wt% Pd-NiO/CeO 2 -was used.
Investigating the influence of composites' amount in the reaction mixture, glycerol/Pd molar ratio in the range from 300 to 1000 mol/mol was varied (Table 2).Glycerol/Pd molar ratio increase from 300 to 500 didn't influence glycerol oxidation results, when oxidation was carried out at atmospheric pressure.Obtained glycerol conversion and glyceric acid selectivity was similar at both ratios.Glycerol/Pd molar ratio increase until 1000 mol/mol, as suspected, significantly reduced glycerol conversion.Conversion decreased more than two timesfrom 74-32 mol%.
Using small amounts of 1.4 wt% Pd-NiO/CeO 2 composite (n(glycerol)/n(Pd) = 1000 mol/mol), oxygen pressure influence on glycerol oxidation was also studied (Table 2).Oxygen pressure was varied in the range from 1 to 6 atm.As it can be seen from Table 2, high oxygen pressure (above 1atm) causes deactivation of catalyst surface and therefore is not allowed.It coincides with the literature where it is said that one of the significant platinum group metal disadvantages is their overoxidation at high oxygen pressures.Over-oxidation, which is the coverage of surface sites by oxygen, leads to catalyst deactivation and a slowdown of alcohol oxidation [21,22].
Glycerol/Pd molar ratio of 500 mol/mol and oxidation at atmospheric pressure was found to be optimal conditions.Extending glycerol oxidation duration from 3-5 h at mentioned conditions, full glycerol conversion was reached with glyceric acid selectivity of 74 mol%.
In this work also influence of oxidation temperature at pO 2 = 1 atm was investigated.Oxidation temperature was varied in the range from 45-60 ºC.Glycerol conversion and glyceric acid yield dependence on temperature using 1.4 wt% Pd-NiO/CeO 2 composite as catalyst is shown in Fig. 5 (left) and (right), respectively.

Oxidation time (h) Oxidation time (h) Glycerol conversion (mol%)
Glyceric acid yield (mol%) It was concluded that higher oxidation temperature leads to greater glycerol conversion.Increasing temperature from 45-60 ºC, glycerol conversion rose by 40 mol%.Selectivity to glyceric acid at different temperatures and different glycerol conversions was similar.Thereby the greatest yields of glyceric acid (see Fig. 5 (right)) was reached at the 60 ºC temperature when glycerol conversion was complete.The best yield of glyceric acid was 74 mol%.Obtained glyceric acid yield was greater than that reached previously in [23], where in the presence of similar Pt containing catalyst

CONCLUSIONS
It was found that Pd/CeO 2 and Pd-NiO/CeO 2 composites are catalytically active in the glycerol oxidation processes.NiO additives can significantly increase Pd/CeO 2 catalysts' performance and yield of the main productglyceric acid.Decreasing NaOH initial concentration from 1.5-0.6 mol/L, glycerol conversion was improved when Pd-NiO/CeO 2 composite was used.The change of glycerol/Pd molar ratio showed that the ratio 500 mol/mol was optimal.Oxygen pressure above 1 atm was not allowed, because of catalyst's deactivation.The best yield of the main product -glyceric acid -was 74 mol% with full glycerol conversion and it was achieved in the presence of 1.4 wt% Pd-NiO/CeO 2 composite at the following oxidation parameters: c 0 (glycerol) = 0.3 mol/L, c 0 (NaOH) = 0.6 mol/L, n(glycerol)/n(Pt) = 500 mol/mol, pO 2 = 1 atm, 60 °C.
(4.8 wt% Pt-NiO/CeO 2 ) yield of glyceric acid was 68 mol% (glycerol oxidation conditions were as follows: c 0 (glycerol) = 0.3 mol/L, c 0 (NaOH) = 1.5 mol/L, n(glycerol)/n(Pt) = 1000 mol/mol, pO 2 = 6 atm, t = 70 ºC, oxidation time 3 h).Besides oxidation in the presence of novel 1.4 wt% Pd-NiO/CeO 2 composite was milder method -it required lower temperature and oxygen pressure, smaller initial NaOH concentration.Fig.6represents an Arrhenius type plot of reaction rate versus temperature.Apparent activation energy was calculated from the Arrhenius type plot and was found to be about 27 ± 3 kJ/mol.The low value of activation energy testifies that the oxidation process occurs in the transition region, where the mass transfer rate approximately equals to the chemical reaction rate.

Table 1 .
Specific surface areas and size of Pd crystallites of some synthesized composites

Table 2 .
Glycerol/Pd molar ratio and oxygen pressure influence on the glycerol oxidation results in the presence of 1.4 wt% Pd-NiO/CeO 2 composite