The Application of Copper-Gold Catalysts in the Selective Oxidation of Glycerol at Acid and Basic Conditions

The crude glycerol is produced during the transesterification of animal fats and vegetable oils, but it is a by-product of this process. Currently, its elimination is a problem in the chemical industry. The main goal of this work was the preparation, characterization and application of mesoporous cerium-zirconium oxide as supports for copper and gold species and the comparison of selected factors on the properties of catalysts in glycerol oxidation in the liquid phase. The samples were characterized using adsorption and desorption of nitrogen, XRD, UV-vis, XPS, TEM, SEM, and STEM-EDXS. The obtained results of glycerol oxidation show that the bimetallic copper-gold catalysts are more active and selective to glyceric acid in this reaction than analogous monometallic gold catalysts. Additionally, bimetallic catalysts are also characterized by the catalytic stability, and their application leads to the increase of selectivity to glyceric acid during their reusing in glycerol oxidation in alkali media. In this work, the influence of selected factors, e.g., oxygen source and its pressure, solution pH, and base content on the catalytic activity of bimetallic catalysts is discussed.


Introduction
Glycerol is a chemical compound obtained during the transesterification of oils and fats [1,2]. The increase of biodiesel production leads to the surplus of crude glycerol, which cannot be assimilated into traditional glycerol outlets. Glycerol is a highly functionalized reagent that can be applied in the synthesis of fine chemicals such as fuel sources, pharmaceuticals, biodegradable polymers, etc. [3][4][5]. Various catalytic processes can be used to obtain the valuable chemicals: oxidation, hydrogenolysis, dehydration, etherification, transesterification, etc. [6]. The studies of the catalytic selective oxidation of glycerol over supported Au containing catalysts started in the 1990s [7].
The catalytic oxidation of glycerol is a reaction that can run through multiple pathways and gives various valuable chemicals [8]. The products of glycerol oxidation are, e.g., glyceric acid, tartronic acid, 1,3-dihydroxyacetone and glyceraldehyde [9][10][11][12]. The challenge of the petrochemical industry is the development of catalysts for glycerol oxidation, which will be characterized by high activity, selectivity to selected products and catalytic stability.
The proposal mechanisms of glycerol oxidation were reported in References [13,14] and these mechanisms were based on the transformation of glycerol to glyceric (GLA) and tartronic acid (TA), without C-C cleavage, which was explained by the formation of H 2 O 2 during the reaction. At the beginning, glycerol molecules are oxidized to glyceraldehyde. In the presence of alkali media, this substrate is rapidly consumed and is transformed to glyceric acid. It was shown that the catalysts modified with noble metals during glycerol oxidation in the basic aqua solution [15][16][17][18], can lead to the formation of the products presented in Scheme 1.
It was described in Reference [12] that the composition of reaction mixture can influence the catalytic properties of catalysts. It was shown that the reaction selectivity over gold catalysts can be controlled by the particle size of noble metal [19] and the properties of the support [20]. It was shown in Reference [21] that the size of gold nanoparticles and their content can lead to high activity in glycerol oxidation. The authors of this research explained this phenomenon that low Au loadings favor the formation of smaller homogenous gold nanoparticles, because these particles are stronger adsorption sites for glycerol molecules. It was also reported that other catalysts, based on noble metals, can be effective for the oxidation of glycerol to glycolic acid in the liquid phase, e.g., about 85% glycerol conversion and 57% selectivity to glycolic acid were obtained over the Ag/Al 2 O 3 catalyst [22].
The catalysts modified with copper species in the form of CuO supported on selected oxides such as SiO 2 , Al 2 O 3 , CaO, SrO or ZrO 2 were studied for the synthesis of lactic acid from glycerol in the liquid phase without using reducing or oxidizing agents [23][24][25]. The studied catalysts achieved high glycerol conversion and the selectivity to lactic acid-the highest selectivity to this acid was obtained over CuO/ZrO 2 catalyst (94.6% yield). It is worth noting that lactic acid and its derivatives are used in the synthesis of biodegradable fibers and other biobased chemicals [23,26].
The catalytic glycerol oxidation has been studied in the aqueous solution at acidic or basic conditions, over the monometallic catalysts, e.g., modified with Au, Pd or Pt or the bimetallic, e.g., Pt-Au or Pd-Au catalysts [27][28][29]. Regardless of the supporting material, size and morphology of catalytic, metallic gold species, the oxidation of glycerol usually produces fairly high selectivity toward the formation of glyceric acid, and also in smaller concentrations of other acids: glycolic acid, oxalic acid, tartronic acid, lactic acid, etc. [23,25,26,[30][31][32][33].
Glyceric acid and its derivatives are promising fine chemicals for pharmaceutical or biochemical industry because there are applied, e.g., in the production of medicaments for ethanol and acetaldehyde oxidation [34]. Glyceric acid and its derivatives can also be used in the synthesis of biodegradable surfactants, novel functionalized phospholipid derivatives and phosphatidylcholine analogues [35].
It has been reported in References [13,[36][37][38][39][40] that mono, bi and tri-metallic supported catalysts modified with gold species can be applied as the catalysts in this reaction. It was evidenced in References [41,42] that the catalytic efficiency of catalysts in glycerol oxidation can be improved for glycerol oxidation after the addition of monometallic catalyst, the second noble metal such as Au, Pd or Pt. It has been described in Reference [42] that the introduction of Cu and Ag species on the surface of Au/ZnO catalyst can generate strong metal-metal interactions. It was reported in Reference [43] that the introduction of the second metal to the active phase of catalysts can induce the structural modifications of gold particles. The modification of gold catalysts by the addition of the second metal can improve the selectivity to valuable chemical compounds in oxidation processes because the second metal can play the role of a promoter and it stabilizes the metallic gold particles before their agglomeration. It was presented in Reference [44] that the oxygen adsorption capacity of lactic acid containing perovskite supports for AuPt nanoparticles had a significant influence on the reaction selectivity. An according to the literature data, Au-Pd [45][46][47][48][49][50][51], Au-Pt [46,49] and Au-Cu [52][53][54] catalysts were characterized by a positive interaction between two metals, which can lead to the growth of activity in oxidation processes and the catalytic stability of catalysts.
The catalytic activity of catalysts can be controlled by selected factors of reaction conditions, e.g., solution pH, pressure [55][56][57][58] and the physicochemical properties of the catalyst can have a significant effect on the selectivity of each reaction pathway [20].
The main aim of this study is to show that mesoporous cerium-zirconium oxides, and their modification with copper species, can lead to obtaining the active catalysts in selective glycerol oxidation to fine chemicals. The novelty of this work is shown that the factors such as: pH solution, oxygen source and its pressure had an influence on the activity of mesoporous cerium-zirconium oxides modified with copper and gold species in glycerol oxidation. Additionally, the catalytic activity in glycerol oxidation and the selectivity to selected products achieved over the bimetallic copper-gold catalysts were compared to the results obtained over monometallic gold catalysts. The catalysts were deeply characterized before and after their performance in the oxidation process using selected analytical techniques: XRD, XPS, UV-vis spectroscopy and STEM-EDXS.

General Characterisation
Selected analytical techniques, e.g., XRD, adsorption and desorption of nitrogen at 77 K, UV-vis, XPS, TEM, SEM, and STEM-EDXS, were applied to the characterization of the structure and texture properties of materials, before and after their performance as catalysts in selective glycerol oxidation. These studies were also performed to obtain information about the chemical composition of catalysts after their reusing in the oxidation process and to estimate the oxidation state of metal species before the application of catalysts in the glycerol oxidation. The comparison of catalysts is described in the paragraph titled "the changes in materials after reaction" (which is the point of Section 2.3 in this work), but the general information about the properties of calcined materials before their application in the catalytic tests are described in this paragraph.
XRD patterns were carried out to obtain information about the crystallinity of oxides, the coordination of selected oxides and the size of metallic gold particles. XRD patterns performed for materials confirmed that all oxides were characterized by crystal phase (Figure 1). Ceria was characterized by the crystal cubic phase. In the case of catalysts with pure zirconia as the support, XRD patterns were characterized of two phases: the tetragonal and monoclinic phase. The XRD patterns of CeZrO x confirmed the domination of reflections due to the crystal structure of zirconia. It can suggest that, in mixed ceriumzirconium oxide, ceria crystals were located in the structure of zirconia particles. The XRD patterns recorded for bimetallic catalysts showed that the addition of copper species leads to the creation of the crystal phase of metallic gold particles, which has a size bigger than 5 nm (the reflections at 2θ =~38.2 • ). based on CeZrOx revealed much lower surface areas (about 65 m 2 g −1 ) after their modification with copper and gold species. It can confirm the incorporation of gold and copper species to pores of CeZrOx. More details about the results of the texture parameters of catalysts before their use in glycerol oxidation were reported in Reference [56] and they are shown in this work to outline the general information about the morphology of materials. Figure 1. X-ray diffraction (XRD) patterns recorded for materials based on CeO2, CeZrOx and ZrO2 before and after their application as catalysts in glycerol oxidation in alkali media. The reflections were marked using symbols:  means the crystal phase of ceria in cubic coordination, ○ and ☐ are due to the crystal phases of zirconia in tetrahedral (T) and monoclinic (M) coordination, respectively,  is due to the crystal phase of metallic gold particles. 20   . X-ray diffraction (XRD) patterns recorded for materials based on CeO 2 , CeZrO x and ZrO 2 before and after their application as catalysts in glycerol oxidation in alkali media. The reflections were marked using symbols: • means the crystal phase of ceria in cubic coordination, and are due to the crystal phases of zirconia in tetrahedral (T) and monoclinic (M) coordination, respectively, is due to the crystal phase of metallic gold particles.
The nitrogen adsorption and desorption isotherms were applied to estimate the texture parameters of materials ( Figure S1). The specific surface area of the catalysts depended on the nature of the supports, and the increase of zirconium content in the oxide distribution led to the growth of this parameter in the following order: CeO 2 (24 m 2 g −1 ) < CeZrO x (103 m 2 g −1 ) < ZrO 2 (121 m 2 g −1 ). The modification of catalysts with gold and copper species slightly changed the surface area of monometallic oxide supports, whereas the catalysts based on CeZrO x revealed much lower surface areas (about 65 m 2 g −1 ) after their modification with copper and gold species. It can confirm the incorporation of gold and copper species to pores of CeZrO x . More details about the results of the texture parameters of catalysts before their use in glycerol oxidation were reported in Reference [56] and they are shown in this work to outline the general information about the morphology of materials.
TEM images ( Figure S2) were recorded for selected materials before their use in glycerol oxidation. These images confirmed the presence of mesopores in the bimetallic catalysts and monometallic gold and/or bimetallic copper-gold particles on the surface of oxides. The TEM images proved that the addition of copper species leads to the decrease of the size of metallic gold particles and/or bimetallic copper-gold particles. The obtained gold or copper-gold particles were between 1.6 nm in CuAu/ZrO 2 and 3.6 nm in CuAu/CeO 2 . The full information about the particle size distribution of gold and copper-gold particles on the surface of oxides was presented in Reference [56] and it was described in this paragraph to outline the general information about this parameter in the studied oxides.

The Influence of Selected Conditions on the Catalytic Activity and Selectivity
It was reported in References [57,58] that high conversion during the oxidation in the gas phase of different chemical compounds (e.g., methanol or carbon dioxide) can be achieved over the bimetallic copper-gold catalysts. The novelty in this work is the presentation of the influence of selected factors (e.g., oxygen source and its pressure, pH solution, etc.) on the conversion of glycerol in its oxidation in the liquid phase and the selectivity of products over catalysts based on ceria, zirconia, cerium-zirconium oxides modified with gold and copper species.

The Influence of Oxygen Source and pH Solution
The comparison of results in Figures 2 and 3 shows that bimetallic Cu-Au catalysts can achieve high glycerol conversion (>70%) in its oxidation and high selectivity to glyceric acid (65-79%) if the source of oxygen is pure oxygen and the reaction runs in an alkali media. In these conditions, other products created in the liquid phase, e.g., glycolic acid or formic acid, but the selectivity of these compounds were much lower. The results confirm the possibility of creating these products in glycerol oxidation in the presence of alkali media and pure oxygen [13,[59][60][61][62].
The comparison of the results of glycerol oxidation over copper-gold catalysts in the alkali media using gas oxygen or hydrogen peroxide ( Figure 2) as the oxygen source shows that the source of oxygen has a strong influence on the catalytic activity of catalysts. The differences in the mechanism of hydrogen peroxide decomposition can explain this phenomenon. Hydrogen peroxide is unstable in aqua solution which pH is around 13 and it decomposed to water and molecular oxygen [2 H 2 O 2 →2 H 2 O + O 2 ] [63]. The oxygen molecule is created in situ during the reaction of glycerol oxidation and can be a friendly oxidant for the environment. The process of hydrogen peroxide decomposition is more effective at basic conditions than in acid aqua solution, which has a pH around 1. The partial reaction of hydrogen peroxide with nitric acid can explain this process. This reaction leads to the creation of peroxynitric acid [H 2 O 2 + HNO 3 →H 2 O + HNO 4 ] [64]. At basic conditions, the reaction between hydrogen peroxide and sodium hydroxide leads to the attack of anions such as OH − and HO 2 − and H 2 O 2 decomposition. HO 2 −• is a vital reactive intermediate, which is created mainly during the reaction between H 2 O 2 and NaOH [65].
was studied in glycerol oxidation at 333 K for 5 h at 1000 rpm in the presence of other acids-hydrochloric acid (HCl) in the presence of hydrogen peroxide as a source of oxygen. The change of acid did not have an influence on the glycerol conversion and the selectivity of the products of oxidation, because the conversion was 24% and the main product of this reaction was carbon dioxide (99% of selectivity and traces to glyceraldehyde and lactic acid). The achieved results show that chloride ions (Cl − ) can take part in electron transfer during glycerol oxidation and they can generate the AuCl − species, which can play a role in basic centers [66].  Hydrogen peroxide is known as a strong oxidant applied in the oxidation of chemical compounds in the liquid-phase under very mild reaction conditions. The interaction between hydrogen peroxide and the bimetallic copper-gold active phase of catalysts lead to the creation of gaseous compounds (carbon dioxide and water) during glycerol oxidation. Depending on the properties of the catalyst surface, the different types of O-O bond cleav-  The conversion of glycerol (in the range of 31-47%) in its oxidation over coppergold catalysts using hydrogen peroxide as the source of oxygen was lower than when pure oxygen was applied and the main product of this process was carbon dioxide. The highest glycerol conversion in its oxidation by hydrogen peroxide was achieved over CuAu/CeZrO x . It can evidence the presence of a positive interaction between cerium and zirconium species based on the electron transfer from oxygen in the crystal structure of oxides and/or hydrogen peroxide molecule to the cations of cerium (Ce 3+ and Ce 4+ ) and/or zirconium Zr 4+ . The UV-vis and XPS study proved the presence of these cerium and zirconium species. The obtained results suggest that, in the selective oxidation of glycerol to glyceric acid, the transport of oxygen and its form in aqua solution has an influence on the rate and pathway of glycerol transformation. At acid conditions, glycerol conversion in its oxidation over copper-gold catalysts by hydrogen peroxide was lower (in the range of 16%-28%) than in the alkali media. It is worth noting that, in the case of CuAu/CeO 2 , higher glycerol conversion in its oxidation at acid conditions was achieved in the presence of H 2 O 2 (28%), than in pure oxygen (18%) (Figure 3). This can suggest the positive interaction based on the electron transfer between reduced cerium species (Ce 3+ ) and oxido nitrate anions (NO 4 − ). The presence of reduced cerium species (Ce 3+ ) on the external surface of CuAu/CeO 2 catalyst after its application in glycerol oxidation using hydrogen peroxide was confirmed using XPS analysis ( Figure S3). The distribution of Ce 3+ species before and after oxidation process was 23% and 25%, respectively. The CuAu/CeO 2 catalyst was studied in glycerol oxidation at 333 K for 5 h at 1000 rpm in the presence of other acids-hydrochloric acid (HCl) in the presence of hydrogen peroxide as a source of oxygen. The change of acid did not have an influence on the glycerol conversion and the selectivity of the products of oxidation, because the conversion was 24% and the main product of this reaction was carbon dioxide (99% of selectivity and traces to glyceraldehyde and lactic acid). The achieved results show that chloride ions (Cl − ) can take part in electron transfer during glycerol oxidation and they can generate the AuCl − species, which can play a role in basic centers [66].
Hydrogen peroxide is known as a strong oxidant applied in the oxidation of chemical compounds in the liquid-phase under very mild reaction conditions. The interaction between hydrogen peroxide and the bimetallic copper-gold active phase of catalysts lead to the creation of gaseous compounds (carbon dioxide and water) during glycerol oxidation. Depending on the properties of the catalyst surface, the different types of O-O bond cleavage in H 2 O 2 could occur (homolytic or heterolytic), which generate selected oxidative species. The results of glycerol oxidation to glyceric acid suggest that it was a process run on the catalyst surface, but the formation of this acid was possible mainly in the presence of a base (NaOH) and pure oxygen (O 2 ) molecules. The glycerol conversion in its oxidation by H 2 O 2 was lower, and additionally, glycerol was oxidized to carbon dioxide. It can suggest that the direct introduction of hydrogen peroxide to the basic reaction mixture leads at the beginning to the reaction between H 2 O 2 molecules and OHions and the total oxidation of glycerol molecules to carbon dioxide and water.
The results of glycerol oxidation in the presence of hydrogen peroxide and in basic conditions show that the main product of this reaction was carbon dioxide at the selectivity to this product increased from 69% after 30 min to 85% after 300 min, but the highest growth of selectivity is observed in the first two hours (Figure 4). The decrease the selectivity to glyceric and glycolic acid and the slight increase of selectivity to formic acid was also observed. Formic acid and glycolic acid are the products of glyceric acid oxidation and these acids are probably strongly adsorbed on the surface of catalysts and they are oxidized to carbon dioxide.  •-glycerol conversion; ▲-selectivity to glyceric acid; ■-selectivity to glycolic acid; ○selectivity to formic acid; ◊-selectivity to carbon dioxide.

The Influence of Catalyst Amount
The influence of different amounts of catalysts on the catalytic activity over coppergold catalysts was also studied in glycerol oxidation at 333 K for 5 h in alkali media. The results, collected in Table 1, show that the twice smaller amount of catalyst (15 mg) did not lead to the significant drop of glycerol conversion, irrespective of the chemical composition of support, but the smallest decrease (from 80% to 66%) is observed in the case of CuAu/CeZrOx. It can suggest that the presence of the interaction based on the electron transfer between cationic cerium (Ce 3+ /Ce 4+ ), zirconium (Zr 4+ ) species and metallic species such as metallic gold (Au 0 ) and copper oxides (mainly CuO) can improve the oxygen transfer on the surface of the catalyst to the active centers. In the case of CuAu/CeO2, the smaller amount of catalyst can lead the growth of selectivity to the product of the total oxidation of glycerol-carbon dioxide. The most interesting results were achieved for CuAu/ZrO2 catalyst, because the selectivity to formic acid at the lower amount of catalyst was 23%, and it can be explained by the presence of small gold particles (the average size of gold particles in CuAu/ZrO2 catalyst was 1.6 nm) and the influence of porosity on the diffusion of reagents. This catalyst was characterized by the high specific surface area (110 m 2 /g) ( Figure S1). These parameters could have an influence on the slower diffusion glyceric acid and the increase of selectivity of its oxidation at the beginning to glyceric acid, which was oxidized to formic acid.  1 Reaction conditions: 15.00 g distilled water, NaOH:glycerol = 2:1, 3.0 mmol NaOH, 1.5 mmol glycerol, 15 or 30 mg od catalyst; 5 h at 333 K at 1200 rpm, pO2 = 6 bars; 2 GLA-glyceric acid, GCA-glycolic acid, FA-formic acid, TA-tartronic acid. •-glycerol conversion; -selectivity to glyceric acid; -selectivity to glycolic acid; -selectivity to formic acid; ♦-selectivity to carbon dioxide.

The Influence of Catalyst Amount
The influence of different amounts of catalysts on the catalytic activity over coppergold catalysts was also studied in glycerol oxidation at 333 K for 5 h in alkali media. The results, collected in Table 1, show that the twice smaller amount of catalyst (15 mg) did not lead to the significant drop of glycerol conversion, irrespective of the chemical composition of support, but the smallest decrease (from 80% to 66%) is observed in the case of CuAu/CeZrO x . It can suggest that the presence of the interaction based on the electron transfer between cationic cerium (Ce 3+ /Ce 4+ ), zirconium (Zr 4+ ) species and metallic species such as metallic gold (Au 0 ) and copper oxides (mainly CuO) can improve the oxygen transfer on the surface of the catalyst to the active centers. In the case of CuAu/CeO 2 , the smaller amount of catalyst can lead the growth of selectivity to the product of the total oxidation of glycerol-carbon dioxide. The most interesting results were achieved for CuAu/ZrO 2 catalyst, because the selectivity to formic acid at the lower amount of catalyst was 23%, and it can be explained by the presence of small gold particles (the average size of gold particles in CuAu/ZrO 2 catalyst was 1.6 nm) and the influence of porosity on the diffusion of reagents. This catalyst was characterized by the high specific surface area (110 m 2 /g) ( Figure S1). These parameters could have an influence on the slower diffusion glyceric acid and the increase of selectivity of its oxidation at the beginning to glyceric acid, which was oxidized to formic acid.

The Effect of Oxygen Pressure
The influence of different oxygen pressures on the catalytic activity over copper-gold catalysts was also studied in glycerol oxidation at 333 K for 5 h in alkali media. The results, collected in Figure 5, show that the decreasing of oxygen concentration used to glycerol oxidation from 6 to 3 bars leads to the decrease of conversion. At lower oxygen pressure (3 bars), the selectivity to glyceric acid for all copper-gold catalysts and glycolic acid, excluding CuAu/CeZrO x , decreases and the selectivity to carbon dioxide increases. At lower oxygen contents, cerium-zirconium oxide positively effects glycerol conversion and it can suggest the presence of the interaction between cerium and zirconium species, which improves the catalytic properties of the active phase (the bimetallic Cu-Au system) in the oxidation process. The increase of oxygen content in the autoclave improves not only glycerol conversion in its oxidation, but also the selectivity to glyceric acid, especially over catalysts with cerium species. It suggests that cerium species lead to the increase in the adsorption rate on the surface of catalysts of oxygen molecules and their migration to metallic gold particles. This phenomenon confirms the results presented earlier in References [57,58].

The Effect of Oxygen Pressure
The influence of different oxygen pressures on the catalytic activity over copper-gold catalysts was also studied in glycerol oxidation at 333 K for 5 h in alkali media. The results, collected in Figure 5, show that the decreasing of oxygen concentration used to glycerol oxidation from 6 to 3 bars leads to the decrease of conversion. At lower oxygen pressure (3 bars), the selectivity to glyceric acid for all copper-gold catalysts and glycolic acid, excluding CuAu/CeZrOx, decreases and the selectivity to carbon dioxide increases. At lower oxygen contents, cerium-zirconium oxide positively effects glycerol conversion and it can suggest the presence of the interaction between cerium and zirconium species, which improves the catalytic properties of the active phase (the bimetallic Cu-Au system) in the oxidation process. The increase of oxygen content in the autoclave improves not only glycerol conversion in its oxidation, but also the selectivity to glyceric acid, especially over catalysts with cerium species. It suggests that cerium species lead to the increase in the adsorption rate on the surface of catalysts of oxygen molecules and their migration to metallic gold particles. This phenomenon confirms the results presented earlier in References [57,58]. The changes during the catalytic process in the molar ratio between oxygen and selected reagents (glycerol, glyceric acid and glycolic acid) were shown in Figure 6 and these correlations were presented for the CuAu/CeO2 catalyst after its application in glycerol oxidation at 333 K for 5 h at 1200 rpm. At the beginning of catalytic oxidation was observed the highest consumption of glyceric acid and it was represented by the growth of  The changes during the catalytic process in the molar ratio between oxygen and selected reagents (glycerol, glyceric acid and glycolic acid) were shown in Figure 6 and these correlations were presented for the CuAu/CeO 2 catalyst after its application in glycerol oxidation at 333 K for 5 h at 1200 rpm. At the beginning of catalytic oxidation was observed the highest consumption of glyceric acid and it was represented by the growth of the molar ratio between pure oxygen and glyceric acid. After 180 min of reaction, the content of pure oxygen decreased, because it was consumed in the next steps of oxidation processes, e.g., in the oxidation of glyceric acid to glycolic acid. It is worth noting that, in the case of subsequent reactions, the oxidation of glycerol to glyceric acid and then glyceric acid to glycolic acid, the decrease in the molar ratio between glyceric acid and glycolic acid from 1.90 (after 30 min of reaction) to 1.37 (after 300 min of reaction) was observed, but this ratio decreased to 1.41 after 180 min of reaction. The lack of other products of glycerol oxidation (e.g., formic acid or oxalic acid) can be explained by the strong adsorption of intermediates on the surface of the catalyst and their decomposition to gas products, mainly carbon dioxide. the molar ratio between pure oxygen and glyceric acid. After 180 min of reaction, the content of pure oxygen decreased, because it was consumed in the next steps of oxidation processes, e.g., in the oxidation of glyceric acid to glycolic acid. It is worth noting that, in the case of subsequent reactions, the oxidation of glycerol to glyceric acid and then glyceric acid to glycolic acid, the decrease in the molar ratio between glyceric acid and glycolic acid from 1.90 (after 30 min of reaction) to 1.37 (after 300 min of reaction) was observed, but this ratio decreased to 1.41 after 180 min of reaction. The lack of other products of glycerol oxidation (e.g., formic acid or oxalic acid) can be explained by the strong adsorption of intermediates on the surface of the catalyst and their decomposition to gas products, mainly carbon dioxide.

The Influence of Base Content
The correlation between the base concentration in the reaction solution and the catalytic activity was studied for the copper-gold catalysts in glycerol oxidation at 333 K for 5 h in the atmosphere of pure oxygen (6 bars). The results in Figures 2-7 show that a base in aqua solution has a positive effect on glycerol conversion and selectivity to glyceric acid. In the case of CuAu/ZrO2, the conversion of glycerol in its oxidation and the selectivity to glyceric acid did not change when the NaOH content was lower. It means that the first step of this reaction-the dehydrogenation via H-abstraction of one of the primary OH groups of glycerol molecule-does not depend on base concentration in the reaction solution. In the case of CuAu/CeO2, the increase of conversion and the decrease of selectivity are observed when the molar ratio between NaOH and glycerol is 1:1. The growth of selectivity to formic acid or glycolic acid is seen and the decrease of selectivity to glyceric acid at NaOH:glycerol = 1:1 (excluding CuAu/ZrO2). The decrease in the selectivity to glyceric acid and glycolic acid shows that the decrease of base concentration can lead to a change in the pathway of glycerol oxidation.

The Catalytic Stability
It was published in the literature [36,67] that the catalysts modified with gold species supported on activated carbon or oxides can be applied in glycerol oxidation several times without deactivation. It has been reported in Reference [67] that the best stability in recycling tests in glycerol oxidation was achieved over gold loaded on carbon (better than carbon catalysts modified with palladium or platinum species), showing neither deactivation nor metal leaching. The results of the reusing of gold and copper-gold catalysts in glycerol oxidation are shown in Table 1. It is worth emphasizing that each catalyst used

The Influence of Base Content
The correlation between the base concentration in the reaction solution and the catalytic activity was studied for the copper-gold catalysts in glycerol oxidation at 333 K for 5 h in the atmosphere of pure oxygen (6 bars). The results in Figures 2-7 show that a base in aqua solution has a positive effect on glycerol conversion and selectivity to glyceric acid. In the case of CuAu/ZrO 2 , the conversion of glycerol in its oxidation and the selectivity to glyceric acid did not change when the NaOH content was lower. It means that the first step of this reaction-the dehydrogenation via H-abstraction of one of the primary OH groups of glycerol molecule-does not depend on base concentration in the reaction solution. In the case of CuAu/CeO 2 , the increase of conversion and the decrease of selectivity are observed when the molar ratio between NaOH and glycerol is 1:1. The growth of selectivity to formic acid or glycolic acid is seen and the decrease of selectivity to glyceric acid at NaOH:glycerol = 1:1 (excluding CuAu/ZrO 2 ). The decrease in the selectivity to glyceric acid and glycolic acid shows that the decrease of base concentration can lead to a change in the pathway of glycerol oxidation.
in the first cycle of glycerol oxidation was simply recovered by decantation of the solvent after the reaction and then it was applied in the second cycle without any washing.

The Catalytic Stability
It was published in the literature [36,67] that the catalysts modified with gold species supported on activated carbon or oxides can be applied in glycerol oxidation several times without deactivation. It has been reported in Reference [67] that the best stability in recycling tests in glycerol oxidation was achieved over gold loaded on carbon (better than carbon catalysts modified with palladium or platinum species), showing neither deactivation nor metal leaching. The results of the reusing of gold and copper-gold catalysts in glycerol oxidation are shown in Table 1. It is worth emphasizing that each catalyst used in the first cycle of glycerol oxidation was simply recovered by decantation of the solvent after the reaction and then it was applied in the second cycle without any washing.
The data in Table 2 presents the values of glycerol conversion of achieved over bimetallic catalysts changed after the second cycle of oxidation. After the second cycle, higher selectivity to glyceric acid in glycerol oxidation was obtained over Au/CeO 2 and all bimetallic catalysts. Additionally, on another note, the decreases in the activity can be due to the loss of catalyst during the workup.  1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st

The Changes in Materials after Glycerol Oxidation
The catalysts were studied before and after their application in glycerol oxidation using UV-vis, XRD, TEM and XPS. The aim of these studies was evidenced by the changes in the structure of the catalyst surface after glycerol molecules adsorption and their transformation during oxidation in the liquid phase.

UV-Vis Study
UV-vis spectroscopy can be applied to estimate the oxidation state and the coordination of metals. The UV-vis spectra of studied catalysts before and after their application in glycerol oxidation and their reusing in this oxidation process are shown in Figure 8.
In the case of the UV-vis spectra recorded for the catalysts based on ceria, two bands are seen. They are characteristic of the ligand-metal electron charge transfer from oxygen ion (O 2− ) to cerium cation (Ce 4+ ). The UV-vis band at around 265 nm can be attributed to cerium cation (Ce 4+ ), which is in the tetrahedral coordination, and the band at around 350 nm can be assigned to cerium cation (Ce 4+ ), which has a coordination number higher than four [68,69]. In the case of catalysts based on CeZrO x support, only one UV-vis band at around 270 nm is visible with a shoulder at around 220 nm. It can mean that the coordination of cerium species on the surface in the ceria-zirconium mixed oxide is different than in the catalysts based on pure ceria.
The range characteristic of the ligand-metal electron charge transfer between oxygen ions and cerium cations is also typical of the electron charge transfer between ligands and gold cations (Au 3+ and Au + ) [70][71][72][73][74][75][76], but the examination of this region cannot be useful for characterization of cationic gold species. The bands are in the range of 500-560 nm and can be attributed to the optical absorption of light excited oscillating conductivity electrons of metallic gold particles called surface plasmon resonance [77][78][79][80][81] and around 540-560 nm can also originate from d-d electron transfer in metallic gold [79][80][81]. In the case of UV-vis spectra recorded for calcined catalysts, before their drying, the band around 530-560 nm was found only in the UV-vis spectra performed for copper-gold catalysts (Figure 8). This weak band can be found also in the UV-vis spectra recorded for gold catalysts after their drying. The bands at around 530-560 nm became well visible also in the spectra of gold catalysts after the first and the second cycle of oxidation. It is worth noting that these bands are more intense after the reusing of catalysts in glycerol oxidation (excluding CuAu/CeO 2 ). It can suggest that the migration of metallic gold species during the oxidation process on the external surface of support in gold catalysts. This phenomenon was confirmed by XPS measurements and it can explain the increase of glycerol conversion in its oxidation, especially over Au/CeO 2 (Table 1). In the case of Cu-Au catalysts, the bands at around 650 nm can be attributed to the ligand-metal electron charge transfer, e.g., in copper oxides nanoclusters [82,83], in (Cu-O-Cu) 2+ clusters [84][85][86], inter-valence charge transfer transitions between copper and other metal ions (gold, cerium and/or zirconium) in the bulk solid solution [87].
The obtained results show that the changes in the UV-vis spectra recorded for the catalysts before and after the application of materials in glycerol oxidation can explain the changes in the chemical composition and the coordination of metallic species in the catalysts during the oxidation process.

The Changes in Materials after Glycerol Oxidation
The catalysts were studied before and after their application in glycerol oxidation using UV-vis, XRD, TEM and XPS. The aim of these studies was evidenced by the changes in the structure of the catalyst surface after glycerol molecules adsorption and their transformation during oxidation in the liquid phase.

UV-Vis Study
UV-vis spectroscopy can be applied to estimate the oxidation state and the coordination of metals. The UV-vis spectra of studied catalysts before and after their application in glycerol oxidation and their reusing in this oxidation process are shown in Figure 8.

XRD Measurements
The XRD patterns were performed for samples before and after their application in glycerol oxidation in alkali media (Figure 1). The comparison of XRD patterns shows that the conditions of reaction slightly changed the crystal phase of materials because the intensity and positions of reflections are similar. It can explain the catalytic stability of catalysts in the second cycle of glycerol oxidation (Table 1).

TEM and STEM-EDXS Analysis
SEM image recorded for CuAu/ZrO 2 ( Figure S2, left side) confirms the presence of mesopores in the bimetallic catalyst. The TEM image recorded for CuAu/ZrO 2 catalyst ( Figure S2, right side) confirms the presence of metallic particles (gold and/or copper-gold) whose size is between 2 and 5 nm. It can suggest that the addition of copper species on the surface of gold catalysts leads to the protection of metallic gold particles by their agglomeration. The STEM images ( Figure S4) show the presence of bimetallic copper-gold particles in CuAu/CeZrO x , which are smaller than 10 nm, also after glycerol oxidation. It is worth noting that individual bimetallic particles, which increased to around 20 nm after the reaction, are seen. The results obtained using EDX spectroscopy ( Figure S4) evidence that the bimetallic copper-gold particles are mainly located on the external surface of materials and the content of gold species in the bulk of materials is much lower than copper species content. It can suggest the migration of copper species during materials modifications with copper and the strong interaction between copper and gold species.

XPS Measurements
The XPS method was applied to estimate the oxidation state of gold, copper and cerium species in the selected catalysts before and after their use in glycerol oxidation and the distribution of these species on the external surface ( Figures S5 and S6, Table 3, Tables S1 and S2). The chemical composition of supports has an influence on the changes in the position and intensity of bands in XP spectra.
In the gold catalysts before and after their application in glycerol oxidation, gold species on the external surface were in the metallic (Au 0 ) and cationic form (Au δ+ , where δ means 1 or 3), excluding Au/CeO 2 , because in this material, gold was only in the metallic form ( Table 3). The changes of gold distribution after glycerol oxidation in all monometallic gold catalysts were slight and the dominant form was metallic gold (Au 0 ) ( Figure S5). Cationic gold species (Au δ+ ) were formed on cerium-zirconium oxide and pure zirconia in monometallic gold samples and the part of cationic gold species were reduced in these catalysts to metallic form (Au 0 ) after glycerol oxidation ( Table 3). The reduction of a small part of cationic gold (Au δ+ ) to metallic gold (Au 0 ) and cationic cerium (from Ce 4+ to Ce 3+ ) can suggest that the products of glycerol oxidation (CO 2 and/or glyceric acid) had an influence on the partial reduction of gold and cerium species. In the case of bimetallic copper-gold catalysts, the main form of gold was metallic gold (Au 0 ), but metallic gold species with a partial negative charge on the external surface (Au 0 ) δ− appeared and cationic gold (Au δ+ ) species disappeared in CuAu/CeO 2 after glycerol oxidation. However, the distribution of metallic gold species with the partial negative charge on the external surface (Au 0 ) δ− decreased after the reaction in the case of bimetallic catalysts with zirconium species. It can suggest that (Au 0 ) δ− species are created on the external surface of ceria during glycerol oxidation, especially over the oxidized form of cerium (Ce 4+ ), because the addition of zirconium to the supports led to a bigger growth of reduced form of cerium species (Ce 3+ ) in the distribution of cerium species, especially after the reaction. In the case of CuAu/CeO 2 , the distribution of cerium species on the external surface before and after the reaction is very similar (23% of cerium is in the reduced form before and 24% after glycerol oxidation). In all oxides with cerium species, the main form of these species is in the oxidized form (Ce 4+ ) ( Table 3), but the presence of zirconium in the mixed oxide has a positive effect on the reduction of cerium species during the reaction of glycerol oxidation. The modification with copper species had an influence on the distribution of the reduced form of cerium species (Ce 3+ ) in bimetallic copper-gold catalysts because it was lower than in the analogues monometallic gold catalysts before their application in glycerol oxidation.
Generally, the presence of cerium and copper species led to the easier reduction of cationic gold in the catalysts before or after their performance in glycerol oxidation than in the case of analogues monometallic gold catalysts. In the monometallic gold and bimetallic copper-gold materials, cationic gold was observed in the case of all catalysts before or after glycerol oxidation (excluding Au/CeO 2 ), but its distribution was below 20%. The presence of copper species led to the decrease of cationic gold species in gold distribution (excluding CuAu/CeZrO x ). Additionally, the growth of copper species on the external surface of catalysts was correlated with the increase of the amount of metallic gold on supports. This can suggest that the co-existence of copper and gold species on the surface of catalysts can be responsible for enhanced core-hole shielding [87]. At the beginning, cetyltrimethylammonium bromide (CTMABr, Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in an aqueous acidic solution (pH = 2 controlled by 35-38% HCl, Chempur, PiekaryŚląskie, Poland) at room temperature. Then, the precursors of ceria and zirconia were cerium(III) nitrate (Ce(NO 3 ) 3 6 H 2 O, Sigma-Aldrich) and zirconium n-tetrapropanol (Zr(OC 3 H 7 ) 4 , 70% in 1-propanol, Fluka, Buchs, Switzerland), respectively. These precursors were added to the micellar solution of surfactant and it was assumed the molar ration between the surfactant/precursor of 0.33. After the thermal treatment of this solution at 313 K for 1 h, the synthesis solution was poured into a polypropylene bottle and heated at 353 K for 24 h. The surfactant was removed by ethanol extraction in a Soxhlet apparatus at 351 K for 30 h. The final material was obtained after drying using a vacuum rotator at 343 K and calcination at 673 K for 4 h (the temperature heating rate was 3 K min −1 ). The nominal (assumed) molar ratio in CeZrO x between ceria (CeO 2 ) and zirconia (ZrO 2 ) was 1:1. It was assumed that 100% of cerium was CeO 2 .

Preparation of Pure Ceria, CeO 2
The ethanol solution of Pluronic P-123 (polyethyl glycol-polypropyl glycol-polyethyl glycol, Sigma-Aldrich) was applied to the synthesis of ceria. At the beginning, in 12 cm 3 of ethanol (POCh), the surfactant (1.2 g of Pluronic P-123) and this solution was dissolved and mixed at room temperature for 1 h. Then, the precursor in the form of powder-cerium(III) nitrate (Ce(NO 3 ) 3 6 H 2 O, 99.99%, Sigma-Aldrich) was poured into the surfactant solution. The mixture was stirred for 1 h at room temperature and the final material was obtained after heating the solution to 673 K in air (using a heating rate of 1 K min −1 ), and at this temperature, the material was calcined for 10 h.

Preparation of Catalysts Modified with Gold Species
At the beginning, 137 mg of tetrachloroauric acid (HAuCl 4 3 H 2 O, Johnson-Matthey, London, UK) was dissolved in 217 cm 3 of distillate water. Then, the aqua solution of urea (99%, Fluka, 0.16 mol dm −3 ) was added to the solution with gold in such a volume to molar ratio between urea and gold was 100. Then, the powder support (4.500 g of selected oxide) was poured into the aqua urea solution with gold. The mixture was stirred at 353 K. After 4 h, the product was filtered and washed using 100 cm 3 of an aqua solution of ammonia (25%, NH 3 H 2 O, Chempur) and 850 cm 3 of distilled water to obtain pH = 7. The final materials-Au/CeO 2 , Au/CeZrO x , Au/ZrO 2 were dried at room temperature for 24 h under atmospheric pressure and calcined at 623 K for 3 h.

Modification of Gold Catalysts with Copper Species
Copper species were loaded on the surface of gold catalyst by incipient wetness impregnation. At the beginning, the catalyst modified with gold was dried using a vacuum rotator at 348 K. Then, the portion of 2.000 g of dried gold catalyst was impregnated by 2 cm 3 of aqueous solution of Cu(NO 3 ) 2 2.5 H 2 O (0.314 mol dm −3 , Sigma-Aldrich). The amount of copper nitrate used for the modification was calculated to achieve the loading of copper equal to 2.0 wt%. The CuAu/CeO 2 , CuAu/CeZrO x , and CuAu/ZrO 2 materials were dried under vacuum at 343 K for 1 h, then at 333 K for 16 h under atmospheric pressure and then calcined at 623 K for 3 h.

Catalysts Characterisation
In this work, the results of deep characterisation are presented using selected analytical techniques: XRD, adsorption and desorption of nitrogen at 77 K, UV-vis, XPS, TEM, SEM, STEM-EDXS. These techniques were used in the study of catalysts before and after their performance in glycerol oxidation.

X-ray Diffraction (XRD)
XRD measurements (Billerica, MA, USA) were performed using on a Bruker AXS D8 Advance diffractometer equipment with Cu K α radiation (λ = 0.154 nm), with a step size of 0.05 • and in the angle range of 2θ = 21-80 • .

The Adsorption and Desorption of Nitrogen
The adsorption and desorption of nitrogen was carried out using a Quantchrome Autosorb iQ Instruments (Odelzhausen, Germany) at 77 K. Before measurements, 0.200 g of powder sample was heated at 573 K for 4 h under high vacuum. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method (Odelzhausen, Germany). The Barrett-Joyner-Halenda (BJH) method was applied to estimate the volume of porous and mean pore size.

Ultra-Violet (UV-Vis) Spectroscopy
A Varian-Cary 300 Scan UV-Visible spectrophotometer (Santa Clara, CA, USA) was applied to record the UV-Vis spectra for the catalysts before and after their application in glycerol oxidation and after their drying at RT for 24 h in air atmosphere. In this order, the samples were placed in a cell equipped with a quartz window. The spectra were recorded in the range of 800-190 nm and Spectralon ® was applied as the reference material.

X-ray Photoelectron Spectroscopy (XPS)
X-ray Photoelectron (XP) spectra were recorded using an Ultra High Vacuum (UHV) System (Specs, Berlin, Germany) and the XP spectra were recorded and deconvoluted using OMNIC software (Waltham, MA, USA).

Transmission Electron Microscopy (TEM)
Before measurements using TEM, the powder samples were made in the form of a suspension in 1-butanol and this suspension was deposited on a grid covered with a holey carbon film. The TEM images of samples were carried out using a JEOL 2000 electron microscope operating at 80 kV and the size of the particles was estimated using ImageJ software (Bethesda, MD, USA).

Scanning Transmission Electron Microscopy Combined with Energy Dispersive X-ray Spectroscopy (STEM-EDXS)
The size and form of metallic gold particles were estimated using an aberrationcorrected dedicated STEM microscope HD 2700 CS (Hitachi, Chiyoda, Tokyo, Japan) at the operating potential of 200 kV. This microscope was equipment with a high-angle annular dark-field (HAADF) detector (Hitachi, Chiyoda, Tokyo, Japan). The size of the gold particles was estimated using ImageJ software.

Catalytic Test-Reaction of Glycerol Oxidation
The reaction of glycerol oxidation was carried out in a Berghof-25 (Darmstadt, Germany) autoclave reactor (25 cm 3 ) using a catalyst suspension in 15.00 g (when the source of oxygen was pure oxygen, O 2 , 5.0 N, Linde, Pfungen, Switzerland) or 14.25 g (when the source of oxygen was hydrogen peroxide, H 2 O 2 , 0.75 g, 30%, Fluka) of distilled water with 1.5 mmol of glycerol (138 mg, >99.0%, Sigma-Aldrich) and 1.5 or 3.0 mmol of NaOH (60 or 120 mg, >99.0%, Merck, Darmstadt, Germany) or 0.028 mmol of HNO 3 (2.7 mg, 65%, Sigma-Aldrich, Saint Louis, MO, USA). The reaction was conducted at 333 K at selected stirring speeds (1000 or 1200 rpm) at 3 or 6 bars of oxygen or in the presence of hydrogen peroxide. The assumed molar ratio between hydrogen peroxide and glycerol was 6.6:1.5 = 4.4. This molar ratio was similar to molar ratio between pure oxygen (6 bars) and glycerol, because in this case, the ratio was 5.4:1.5 = 3.6. The assumed content of hydrogen peroxide was higher, because the part of hydrogen peroxide (around 20%) was decomposed after its addition to the solution, before the addition of catalyst and main oxidation process. The liquid products and un-reacted glycerol were determined quantitatively by a HPLC system equipped with RI and UV-vis detectors.

Conclusions
The results of the selective oxidation of glycerol over the mesoporous ceria, zirconia or cerium-zirconium oxide modified with copper and gold species show that the bimetallic copper-gold catalysts can be attractive catalysts for the oxidation of organic compounds in the liquid phase. The most valuable results were achieved over the bimetallic catalysts in glycerol oxidation in the presence of pure oxygen under the pressure of six bars and a molar ratio between NaOH and glycerol was 2:1. The results show the influence of selected factors on the catalytic activity of copper-gold catalysts deposited on ceria, zirconia or cerium-zirconium oxide are collected in Table 4. The increase of zirconium content in the support has a positive effect on the catalytic stability and the growth of selectivity to the product of glycerol oxidation-glyceric acid. The highest selectivity to this acid (93%) was achieved in alkali media at 333 K at 1200 rpm over CuAu/ZrO 2 after the second cycle of glycerol oxidation. The results of this work show that the bimetallic catalysts can achieve higher activity than analogues gold catalysts not only in the oxidation process in the gas phase [88,89], but also in the liquid phase. Gold catalysts modified with copper species are characterized by the appearance of the electron interaction between both metals. The reduction of copper species was evidenced by the UV-vis and XPS study and the higher mobility of oxygen-promoting oxidative properties was confirmed by XPS measurements. Copper has the larger electron-donating ability and it can modify the electronic properties of metallic gold. The achieved results of glycerol oxidation show that the chemical composition of support, the texture and structure properties of catalysts and the reaction conditions such as the source of oxygen, pH solution, the concentration of OH − anions, and oxygen content can determine the effectiveness of glycerol oxidation.  Figure S2: The SEM (left side) and TEM (right side) images recorded for CuAu/ZrO 2 before its application in glycerol oxidation, Figure S3: The XP spectra of 3d Ce species region recorded for CuAu/CeO 2 after its application as catalyst in glycerol oxidation in the presence of hydrogen peroxide, Figure S4: STEM images and EDX spectra recorded for CuAu/CeZrO x before and after its application as catalyst in glycerol oxidation, Figure S5: XP spectra of 4f Au species region recorded for: (A) Au/CeO 2 ; (B) Au/CeZrO x and (C) Au/ZrO 2 before and after their application as catalysts in glycerol oxidation, Figure S6: XPS spectra of 4f Au species region recorded for: (A) CuAu/CeO 2 ; (B) CuAu/CeZrO x and (C) CuAu/ZrO 2 before and after their application as catalysts in glycerol oxidation, Table S1: The composition of catalysts (estimated using XPS) before and after their application in glycerol oxidation at 333 K for 5 h at 1200 rpm at basic conditions, Table S2: The BE of gold and copper species in the selected catalysts before and after their application in glycerol oxidation.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. Data is contained within the article or supplementary material. thank Maria Ziolek (Adam Mickiewicz University in Poznan, the Faculty of Chemistry) and Jeroen A. van Bokhoven (ETH Zurich) for a fruitful discussion and value remarks.