NiAu single atom alloys for the selective oxidation of methacrolein with methanol to methyl methacrylate

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Review

NiAu single atom alloys for the selective oxidation of methacrolein with methanol to methyl methacrylate Antonios Trimpalis1, Georgios Giannakakis1, Sufeng Cao, Maria Flytzani-Stephanopoulos

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Department of Chemical and Biological Engineering, Tufts University, Medford, MA, 02155, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Single atom alloys Nickel Gold Esterification Methacrolein Methyl methacrylate

The oxidative cross-coupling of unsaturated aldehydes and alcohols with methanol is catalyzed by O-activated gold surfaces in both ambient pressure and UHV conditions. From liquid-phase studies it has been found that either the addition of a base or the use of a basic support is necessary for high yields of the corresponding methyl ester. Here we demonstrate that addition of a small amount of Ni, at the single atom limit, on supported Au nanoparticles, without the aid of a base in the reaction solution or the use of a basic support, is sufficient to activate Au for the selective oxidative esterification of methacrolein with methanol. The Ni concentration and dispersion are followed by ICP-AES and in situ DRIFTS, respectively. The active site appears to be the same for both NiAu and Au NPs; namely, an [Au-Ox]- species, as similar apparent activation energies of the reaction were found in kinetic studies over these two catalysts. Ni was also shown to allow for lower methanol concentrations to be used without affecting the catalytic activity and selectivity to the desired product, methyl methacrylate. In situ vibrational spectroscopy studies were conducted in an ATR-IR cell, where the reaction progress was monitored by detecting the gradual conversion of adsorbed reactants on the catalytic surface to intermediates and finally to the desired product, which was produced at close to 100% selectivity at conversions exceeding 25% at 60 ℃ on NiAu single atom alloy nanoparticles.

1. Introduction

Gold-based catalysts have been considered as potential candidates for the MA-methanol oxidative esterification reaction, as they have been shown to be highly selective for various hydrogenation [5,6], dehydrogenation [7] and oxidation reactions [8–11]. In the case of oxidative cross-coupling of unsaturated aldehydes or alcohols with methanol, supports such as SiO2 [12,13], TiO2 [12,14,15], CeO2 [16,17], Al2O3 [18] have been used for Au NPs. However, in most of the cases, the use of a basic solution, such as K2CO3 or Na2CO3, was required to effectively improve catalytic performance [13–17]. An alternative that does not require the addition of free base has been reported, which instead involves the use of basic materials as supports for the Au nanoparticles such as MgO [19], hydrotalcite [20] and modified SiO2 [21,22]. In addition to the supported Au NPs, oxidative coupling reactions have been studied over unsupported nanoporous (np) Au(Ag) structures [23–25]. In unsupported form, nanoporous Au is offered for investigations of the role Au plays in reaction mechanisms, allowing for direct comparisons to UHV studies on gold single crystals. Nanoporous Au is mainly formed by dealloying AgAu or AuCu alloy ingots by means of a strong acid, such as HNO3 [24].

Methyl methacrylate (MMA) is an industrially important chemical that is mainly used for the production of acrylic plastics (poly-methylmethacrylate, PMMA) or polymer dispersions for paints and coatings [1]. The traditional and most widely used method for production of MMA is the Acetone Cyan-Hydrine (ACH) method [1]. The main drawbacks of this method are handling of the toxic hydrogen cyanide and the high cost for waste treatment of the by-product ammonium sulfide [1]. Recently, a much more environmentally benign method for producing MMA has been considered, involving the one-step esterification of methacrolein (MA) with methanol using molecular oxygen for the reaction [2]. Monometallic Pd supported catalysts have been tested for this reaction but with relatively low MMA yields [2]. To improve the catalytic performance, Pb was added and the formed PdPb alloy showed both increased MA conversion and high MMA selectivity [3,4]. However, this process still suffers from several drawbacks such as the toxicity of Pb and the need for additional liquid base additives, which must be removed from the product mixture [4].

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Corresponding author. E-mail address: maria.fl[email protected] (M. Flytzani-Stephanopoulos). 1 A. Trimpalis and G. Giannakakis contributed equally to this work. https://doi.org/10.1016/j.cattod.2019.04.021 Received 24 December 2018; Received in revised form 18 March 2019; Accepted 6 April 2019 0920-5861/ © 2019 Published by Elsevier B.V.

Please cite this article as: Antonios Trimpalis, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.04.021

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stirring for 2 h. The solution of EG-dispersed NiAu nanoparticles was added dropwise from the round bottom flask to the support suspension under sonication. The resulting solution containing NiAu/SiO2 was kept under stirring overnight. Supported Au NPs were prepared in a similar way, skipping the step of Ni precursor and reductant addition. The solution was filtered washing with ethanol and DI water, followed by drying under vacuum overnight and finally calcined in air at 400 ℃ for 5 h prior to use in the reaction. The latter step ensures the successful removal of the capping ligands (PVP) and facilitates the binding of the metal to the support. The gold metal loading on the support was estimated at 5% wt. on average, in all samples, based on inductively coupled plasma (ICP) elemental analysis.

The low reactivity of Au due to its weak binding of reactants can be improved by alloying it with a second metal. Systems that have demonstrated this effect include AgAu [26,27], AuCu [28,29], PtAu [30] and PdAu [31–34] for various selective hydrogenation and oxidation reactions. For the oxidative esterification of methanol with MA, a commercial catalyst with a core-shell Au@Ni structure has been developed by Asahi Industries [22]. This type of catalyst exhibits a yield of 57% MMA at conversion of 58% at 60 ℃ on suitable basic oxide supports. Gold, present as nanoparticles of 3 nm average size encapsulated by NiOx (Ni:Au = 2.95:1 atomic ratio at the nanoparticle surface), is found to enrich the shell surface after 2 h at reaction conditions. The conversion of MA increased 4 times, while selectivity to MMA was maintained at the same level as for Ni-free Au nanoparticles. NiOx was reported to be the active site for this reaction [22]. A mesoporous silica modified with basic oxides La2O3 and BaO was recently used as support of NiAu bimetallic catalysts studied for the same reaction [21]. Although a synergistic effect of Au and Ni was identified, no explanation was given regarding the role of Ni in the reaction. In addition, Au° was mentioned as the active site for oxidative coupling while more recent work clearly finds that neutral Au is inactive for such reactions [24,25]. From the above, the role played by Ni addition on Au NPs remains to be investigated and clarified. Since the first reports by the Sykes and Flytzani-Stephanopoulos groups of PdCu single atom alloys (SAAs) as catalysts for selective hydrogenations [35,36], there has been a growing interest in the application of SAAs in various reactions [36–42]. Regarding the Ni promoting ability, it has been reported that the presence of Ni at the single atom limit enhanced the reactivity of Cu surfaces for the selective dehydrogenation of ethanol [43,44]. In addition, SAAs of NiAu NPs supported on SiO2 (as well as unsupported nanoporous NiAu SAAs) were found to greatly improve the catalytic behavior of gold for the selective non-oxidative ethanol dehydrogenation to acetaldehyde and hydrogen [45]. Based on these findings, Ni is investigated here as a promising additive to Au for selective oxidation reactions. In the present study, we have identified and investigated the positive effect of Ni on the Au activity for the aerobic methanol esterification reaction when Ni is added in small amounts in Au NPs forming a single atom alloy with Au. Au and NiAu NPs were supported on SiO2. Mechanistic and structural investigations were conducted to elucidate the effect of nickel doping on the gold activity and selectivity.

2.2. Catalyst characterization Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were conducted on a Leeman Labs PS1000 instrument. Before each measurement, the samples were digested in an aqua regia solution (2–3 ml) overnight and then diluted by deionized water to obtain the desired concentration of the metal in a neutral pH solution. X-ray photoelectron spectra were obtained on a Thermo scientific KAlpha system equipped with an Al source and a double focusing hemispherical analyzer. The XPS system is also equipped with an argon ion sputter gun for depth-profile analysis. For all samples, 80 scans were collected for Ni 2p and 30 scans for Au 4f and the XPS data were analyzed using the Casa XPS software. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed on a Thermo Scientific Nicolet iS50 FTIR spectrometer equipped with a DTGS KBr detector, a Harrick praying mantis chamber and a HVC-DRP4 high temperature reaction cell equipped with ZnSe and quartz windows. CO was used as a probe molecule and adsorption on the various samples was examined at room temperature (RT). For CO DRIFTS, after each sample was placed in the cell, it was heated to 350 ℃ under He flow. At this temperature, the gas was switched to 5%H2/He gas mixture at a flow rate of 12 mL/min for 2 h. After reduction, the sample was purged with pure He gas at 350 ℃ for 10 min, cooled to RT under He flow (20 mL/min), and the background spectrum was recorded. A 10%CO/He gas mixture was introduced into the cell for 0.5 h at a flow rate of 10 mL/min and the spectrum was recorded under CO flow. Following this, the sample was purged with He at a flow rate of 20 mL/min for 0.5 h and spectra were recorded at certain time intervals during this period. The catalytic tests were performed in a stainless-steel Parr highpressure batch reactor, which had a maximum working pressure of 1000 psi and a maximum working temperature of 250 ℃. For each of the Au or NiAu NPs supported on silica, 660 mg of the sample were placed in a quartz beaker of maximum liquid volume of 35 mL designed to fit inside the bottom part of the reactor. Both the upper (cap) and bottom parts of the reactor were preheated in an oil bath at the desired temperature. A liquid mixture consisting of 20 mL methanol and 2.48 mL methacrolein was introduced along with a magnetic stir bar inside the beaker. The beaker was then inserted in the reactor and the latter was sealed. Once the desired temperature (measured by a K-type thermocouple inserted through the cap) was reached, a gas mixture of 20% O2 in N2 was introduced in the reactor until the pressure reached 130 psi (8.2 bar) which was the working pressure for all catalytic experiments. External diffusion phenomena were eliminated by using a high stirring rate equal to 600 rpm. After 3 h, which was the typical reaction time for these tests, the reactor was removed from the oil bath and cooled in ice. After the temperature dropped below 10 ℃ the gas composition was analyzed by gas chromatography to check for CO2 formation and the liquid was collected after filtration in a vial. The liquid mixture was then analyzed by HPLC (Agilent 1260 infinity) equipped with a refractive index detector. In another set of experiments, during the reaction period, liquid samples were collected at certain time intervals to check the progress of the reaction with time.

2. Experimental 2.1. Catalyst synthesis Nickel-gold alloys studied in this work were prepared as supported nanoparticles (NPs). The procedure used to prepare the silica supported NiAu NPs is the sequential reduction method, which was described in a recent publication [45]. Accordingly, the gold nanoparticles are first prepared, followed by Ni deposition in the Au surface to form the alloy, and then seeding with silica to deposit the nanoparticles. First, PVP-Au colloids were formed using PVP (MW = 58,000) and HAuCl4 at a molar ratio of 35:1, mixed in 50 mL of ethylene glycol (EG) [46] in a round bottom flask. The solution was stirred vigorously and kept under inert gas atmosphere (N2) for 1 h, followed by the addition of 0.5 g NaHCO3, which facilitates the reduction of the gold precursor. The temperature was set to 90 ℃ at 5 ℃ min−1 ramping rate and left for 2 h, before letting the mixture cool to ambient temperature. The formation of Au NPs was confirmed by the wine-red color of the suspension. In order to synthesize NiAu SAAs, the appropriate amount of nickel precursor (NiCl2) was added, hydrazine (1.6 mM) and NaOH 1 M (10 μL per mL of solution) were also dissolved in the mixture [47], followed by heating to 50 ℃ for 1 h before cooling to room temperature. Air-free conditions are ensured by constant flow of inert gas in the flask to displace the oxygen. Fumed silica was heated in air at 650 ℃ for 5 h, to pre-activate and remove organic impurities, and then suspended in ethanol under 2

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reported for the same NiAu NPs [45]. The structure of Ni in Au NPs was followed by in situ DRIFTS using CO as a probe molecule, which was adsorbed at RT after H2 treatment at 350 ℃ as discussed in the Experimental section. CO-DRIFT spectra for the undoped Au NPs supported on SiO2 in the CO related region (Fig. 2A), show one peak at 2117 cm−1, characteristic of atop Au°−CO modes [49]. CO is weakly adsorbed on the Au surface and its removal was complete after 20 min of purging with He. On the other hand, in the Ni0.005Au/SiO2 CO-DRIFT spectra (Fig. 2B), a shoulder appears around 2100 cm−1 in addition to the Au°−CO peak at 2117 cm−1. This shoulder can be ascribed to sub-carbonyl species Ni°(CO)x with x = 2 or 3 [50–52]. Typically, peaks related to oxidized Ni2+ or Ni+−CO adsorbed species appear around 2130-2200 cm−1 [50], however, such peaks are not present in the Ni0.005Au spectra. Thus, surface NiOx is effectively reduced to Ni° even though the reduction temperature was not higher than 350 ℃. In addition, the absence of sharp peaks in the 2050-2060 cm−1 region and broad bands in the 1800-1900 cm−1 region, which have been ascribed to linear CO and bridged CO adsorbed on Ni clusters, respectively [50], indicates that atomic Ni dispersion had been achieved during the synthesis of the samples when the Ni content was as low as 0.0075 Ni wt.%.

For quantification, calibration curves were made from liquid standard solutions of all reactants, products, and by-products in methanol. In situ and in operando vibrational spectroscopy studies were performed using an attenuated total reflection IR (ATR-IR) flow cell mounted on a Thermo Scientific Smart Ark accessory designed to fit in the aforementioned Nicolet FTIR spectrometer used for the CO DRIFTS; an MCT detector was used to record the ATR-IR spectra. The liquid phase esterification reaction was studied in situ in this apparatus. For each test, a slurry of 50 mg of sample in 1.5 mL ethanol was made and sonicated for several hours (5–6 h). The slurry was then added dropwise on top of a 45° ZnSe crystal and was left to dry at room temperature. In this way, a thin film of sample was created on top of the ATR crystal, which was then placed in the ATR-IR cell. The cell was mounted on the Smart Ark and He was used for purging. Subsequently, the cell was heated to 60℃ and kept at this temperature for 1 h under He gas flowing at a rate of 20 mL/min to dry the sample. A background spectrum was recorded at 60 ℃ and background spectra were also recorded at 50, 40 and 25 ℃ during stepwise cooling to room temperature. For in situ liquid reaction studies, the cell was used as a batch reactor. Initially, a liquid mixture of MA/methanol was introduced in the cell and after a spectrum was recorded at ambient pressure, it was pressurized to 130 psi with the same O2/N2 gas composition (21:79; 1.9 atm O2 partial pressure) used for the gas phase reaction. Surface changes after a reaction time of 1 h were monitored by collecting spectra at RT. The same procedure was followed for other reaction temperatures (40, 50, 60 ℃).

3.2. Catalytic reaction studies The catalytic performance of SiO2 supported Ni0.005Au SAAs for the oxidative esterification of MA with methanol was studied in a batch reactor. The methanol mole fraction was set at a value higher than 0.90 (XMeOH = 0.94), since cross coupling between MA and methanol has been found to be at a maximum around this concentration in similar reactions carried out in the gas phase [25]. Table 1 summarizes the catalytic performance of the supported Ni0.005Au SAAs and bare Au NPs, along with those from NiAu of higher Ni loading (Ni:Au atomic ratio 1:1), monometallic Ni/SiO2, and bare SiO2 for comparison. It has been reported that the use of a basic oxide as support is necessary to achieve high levels of MA conversion [19,22]. However, here, the use of a non-basic support, such as SiO2, was considered important to clarify the role the Au sites alone play in the reaction mechanism. When pure SiO2 was tested under the reaction conditions, neither conversion of MA nor methanol self-coupling (no production of methyl formate, MF) was observed. Ni-free Au NPs supported on SiO2 were found to be 100% selective for this reaction but the MA conversion was relatively low (11%). These results are in good agreement with UHV and gas phase studies in which unsupported nanoporous Au showed 100% selectivity for production of MMA [25,53]. When a small amount of Ni was added to the Au NPs at an atomic ratio of Ni/Au = 1/200 (Ni content 0.0075 wt.% as measured by ICP)

3. Results and discussion 3.1. Catalyst characterization Deposition of Ni in the form of isolated atoms in Au surfaces was reported to greatly improve the catalytic activity and stability of gold for the non-oxidative dehydrogenation of ethanol [45]. Here we investigate NiAu SAAs for the oxidative esterification of methacrolein with methanol. As will be discussed below (Section 3.2), at small amounts, Ni is atomically dispersed in the Au NPs. Fig. 1 shows HRTEM images of Ni0.005Au NPs (0.0075 Ni wt.%) supported on SiO2 after calcination in air at 400 ℃. The average particle size of gold was determined to be 13 ± 4 nm based on measurements performed on 200 particles. A typical TEM image is shown in Fig. 1A, along with the corresponding bar graph. Based on higher magnification TEM images (Fig. 1B) the lattice spacing was 0.25 nm, which matches that of the [111] lattice planes of Au [48], the predominant crystal plane present in these nanoparticles [45]. The NP size measured in the TEM images is also in good agreement with XRD data that have been

Fig. 1. TEM image of 400 ℃-calcined Ni0.005Au/SiO2 NPs (A) and Zoom on a Au NP and lattice spacing of Au {111} (B). 3

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Fig. 2. CO-DRIFT spectra of silica supported Au NPs (A) and silica supported Ni0.005Au NPs (B). CO adsorption took place at room temperature and spectra were recorded after removal of gaseous CO peaks upon He purge.

partly the role of the additional liquid base or of the basic support needed for achieving considerable MA conversions, since NiOx is also basic. In a different sample with a higher Ni:Au atomic ratio (1:1) where Ni clusters dominate the surface, the selectivity to MF increases dramatically at the expense of MMA. This finding differs from what has been reported for NiAu supported catalyst with an even higher atomic ratio of Ni:Au (4:1). A possible reason for this is the different preparation method used in that work [22]. In that case, a coprecipitation method led to an atomically thin Ni layer covering gold NPs of 3 nm particle size, while the sequential reduction method which was followed here, led to big Ni clusters when Ni was added in amounts that exceeded that of the single atom formation. Another reason for this difference, may be the use of a more basic support [22] which has been reported to stabilize the hemiacetal reaction intermediate [19]. In addition, when a monometallic Ni/SiO2 catalyst was tested under our reaction conditions, self-coupling of methanol to MF was the only reaction that took place. As has been reported recently, conversion of MA in liquid phase is expected to be higher than in the gas phase for the same reaction [54]. The reason for this is that adsorbed methacrolein, which in high surface concentrations poisons the catalyst, is either less stable or less dominant on Au surface in liquid phase catalytic tests. Indeed, the catalytic performance of both Au and Ni0.005Au NPs is better than that of nanoporous structures studied under gas phase oxidative coupling of MA with methanol [25,53]. For comparison, the catalytic tests of liquid phase (this work) and those of gas phase reaction from nanoporous structures are included in Table 2. In the gas phase studies, a maximum MMA yield of 9% was achieved (18% MA conversion and 50% MMA selectivity), when the methanol mole fraction was XMeOH = 0.98. It is noteworthy that the MMA production rate over pure Au NPs in the liquid phase is higher than the rate over np-Au in the gas phase, even though the latter studies were performed at much higher reaction temperature (150 ℃), compared to the liquid reaction (60 ℃). A higher selectivity to MMA could be achieved by lowering the methanol mole fraction to 0.92 but at this value, the MA conversion dropped to 1.2%, resulting therefore in low MMA formation rate (3.01 × 10−8) [25] (Table 2). Comparison of the normalized MMA yield calculated in MMA moles produced per surface area of Au per reaction time (molMMA/ m2Au·s) from this work (3.78 × 10-7) with that calculated for pure Au NPs supported on SiO2-TiO2 for the same reaction (3.64 × 10-7) on the commercial catalyst from data in ref. [22], corroborates the results for the Au NPs supported on SiO2 reported here as the two values are essentially the same. At the same time, the SAA catalysts (Ni0.005Au/SiO2 and Ni0.01Au/SiO2) yield comparable results to the commercial catalyst

Table 1 Catalyst performance in the oxidative esterification of MA with methanol in the liquid phase a. Catalyst

SiO2 Ni/SiO2 Au/SiO2 Ni0.005Au/SiO2 NiAu/SiO2b

MA conversion (%)

0 0 11 24 1.5

Selectivity (%) MMA

MF

CO2

– 0 100 100 2

– 98 0 0 98

– 2 0 0 0

a Reaction conditions: 660 mg catalyst in 20 mL methanol and 2.48 mL MA, O2/N2 (20:80 v/v, 8.2 bar total), 60 ℃, 600 rpm, 3 h-test. b Atomic ratio Ni:Au = 1:1.

Fig. 3. MA conversion at different temperatures for Au and Ni0.005Au NPs on SiO2, 660 mg catalyst in 20 mL methanol and 2.48 mL MA, O2/N2 (20:80 v/v, 8.2 bar), 600 rpm, 3 h-long tests.

for the Ni0.005Au/SiO2 sample, the catalytic activity of gold was greatly improved while its selectivity to MMA remained the same. Ni0.005Au SAA NPs were superior to Au NPs over the whole range of temperature studied for this reaction (Fig. 3). These findings indicate that addition of Ni atoms in Au NPs is essential to improve the gold activity without compromising the gold selectivity. Hence, the NiOx formed substitutes 4

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formation rates on the supported Au and Ni0.005Au SAA NPs. MMA formation rates are normalized by the surface area of Au in each catalyst. The calculated apparent activation energy from the Arrheniustype plots is 45 ± 1 kJ/mol for the Au NPs while for the Ni0.005Au NPs it is 42 ± 1 kJ/mol (Fig. 4). This value is in good agreement with what has been reported on Au NPs supported on a basic support [20]. When more Ni is added in such a way that its atomic dispersion was still preserved (Ni0.01Au NPs), the apparent activation energy did not change appreciably. Hence, adding a small amount of Ni on Au has hardly any effect on the apparent activation energy for this reaction, indicating a similar reaction pathway for both the doped and undoped Au NPs. As it will also be shown next from the spectroscopic studies, Au, not NiOx, appears to be the active site for the coupling of MA with methanol. This is in contrast with what was previously reported for the supported core-shell Au@Ni NPs [22]. In that work, activated surface NiOx was reported as the active site but as it was mentioned, Au was also present at the surface of the nanoparticle size, along with Ni. An atomic ratio Ni/Au of 2.95 was reported and therefore the interfacial activity at Au-Ox-Ni can be more correctly claimed. However, it remained unclear whether the active sites are [Au-Ox]- species rather than NiOx. In addition, it is possible that, even though most of Au is encapsulated in NiO, during reaction it diffuses to the surface and therefore more Au-Ox-Ni sites are formed, which further promotes the reaction. As will be shown below by XPS, when Ni is present as atoms in the Au surfaces, there is a higher number of the active sites, namely [Au-Ox]-, and this results in higher MMA yields. This is also the reason why the calculated reaction rate on Ni0.005Au NPs is more than twofold that of undoped Au NPs (8.33 × 10−7 and 3.78 × 10−7 molMMA/m2Au·s) for Ni0.005Au NPs and Au NPs, respectively). The finding that surface [Au-Ox]- is the active site for this reaction, is in agreement with gas phase and UHV studies of the reaction on nanoporous Ag-doped Au [25,53]. When higher Ni loadings were used, the selectivity to MMA dropped dramatically, the catalyst now being more “Ni-like” (Table 1). In addition to the above catalytic tests, the effect of the reaction mixture composition on the catalytic performance was also investigated under different reactant concentrations. As it was mentioned earlier, from gas phase studies the maximum MMA yield was achieved at a methanol mole fraction of 0.98 [25]. In that work, it was found that MA was more strongly adsorbed on the Au surface than methanol and therefore higher concentrations of the latter were needed in order to facilitate the adsorption of methoxy and the subsequent coupling with MA. In the current work, all catalytic tests were run at a methanol mole fraction XMeOH = 0.94. Addition of Ni could provide a site for adsorption of methanol, allowing for the MA-methanol coupling to take place even at lower methanol mole fractions than this value. Accordingly, Ni0.005Au NPs were subjected to four more reaction mixtures with different methanol mole fractions, XMeOH: 0.98, 0.67, 0.33 and 0.1 (Fig. 5). In contrast to what was reported in Refs [25,53], where selectivity was shown to be greatly affected by the reactant mixture concentrations, in this study not only the selectivity remained at 100% but there was also only a slight drop in MA conversion from 24 to 20% when the solution with methanol mole fraction of 0.67 was selected. Thus, it is plausible to surmise that there is an increased surface concentration of methoxy species due to additional adsorption of methanol on NiOx sites, which then spills over on the Au surface where MA-methanol coupling takes place. Further decrease of methanol concentration (XMeOH = 0.33) in the reaction mixture brought about a dramatic decrease in MA conversion to 2%. MA is more strongly adsorbed on the Au surfaces than methanol [25] and therefore it is the dominant adsorbate at the expense of adsorbed methanol even when the solution composition is 50:50 [25]. By contrast, the presence of Ni atoms prevents MA from covering the surface and leading to undesirable pathways. Instead, only the desirable coupling towards MMA is observed. Another difference from the gas-phase studies is that at very high MA concentrations, MA is oxidized to methacrylic acid [25]; while this was not observed in the range of MA:Methanol ratios used in the

Table 2 Comparison of liquid phase and gas phase reaction tests. Catalyst

MA conversion (%)

MMA selectivity (%)

MMA production rate (molMMA/m2Au·s)

Au/SiO2a Ni0.005Au/SiO2a Ni0.01Au/SiO2b Ni4Au/SiO2-Al2O3MgOc Ni4Au/SiO2-TiO2c Au/SiO2-TiO2c np-Aud np-Aue

11 24 10 58

100 100 100 98

3.78 × 10−7 8.33 × 10−7 1.42 × 10−6 3.87 × 10−6

29 6 18 1.2

96 89 50 100

1.90 × 10−6 3.64 × 10−7 6.08 × 10−8 3.01 × 10−8

* Formation Rate is normalized per m2 Au to allow comparison between active Au surface atoms on nanoparticles and nanoporous surfaces. a XMeOH = 0.94, T = 60 ℃, 3 h, mcat = 0.66 g; liquid phase (this work). b XMeOH = 0.94, T = 60 ℃, 3 h, mcat = 0.16 g; liquid phase (this work). c XMeOH = 0.94, T = 60 ℃, 2 h; liquid phase (ref. [22]). d For methanol mole fraction XMeOH = 0.98, T = 150 ℃; gas phase (ref. [25]). e For methanol mole fraction XMeOH = 0.92, T = 150 ℃; gas phase (ref. [25]).

(Ni4Au/SiO2-Al2O3-MgO) (Table 2). The catalytic activity is attributed to the use of a basic support in the latter [22]. Indeed, when a non-basic support was used (Ni4Au/SiO2-TiO2), the formation rate of MMA was similar to the SAA catalyst that contains almost two orders of magnitude less Ni (< 0.015 wt% compared to 1.1 wt%), Table 2. The same trend can be also noticed when comparing the bare Au catalyst on different supports, as the catalytic activity more than doubles on the basic supports (SiO2-Al2O3-MgO or SiO2-Al2O3) compared to SiO2-TiO2 [22]. Hence, the basicity is of importance in this reaction, as has been discussed in various reports [19–22,55]. In this work, we hypothesize that Ni atom doping can partially provide this needed basicity in the form of NiOx. Further contribution by Ni atoms to the catalytic activity, through an increase of the active Au-Ox sites, is discussed below in the Spectroscopic Studies section. Kinetic studies of the oxidative esterification reaction were performed on both Au/SiO2 and Ni0.005Au/SiO2. The contact time in these studies was kept short, to operate at low conversions (≤10%), therefore ensuring that the apparent activation energy values were calculated in the kinetic regime. Fig. 4 shows the Arrhenius-type plot of the MMA

Fig. 4. Arrhenius-type plots of the reaction rate normalized by the surface area of Au over monometallic Au NPs and NiAu SAA NPs tested under aerobic esterification conditions. 5

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Fig. 6. Stability test for Ni0.005Au/SiO2 catalyst. Reaction conditions: 660 mg in 20 mL methanol and 2.48 mL MA, O2/N2 (20:80 v/v, 8.2 bar), T = 60 ℃, 600 rpm, cyclic tests, 12 h- long each.

Fig. 5. MA conversion and product MMA selectivity on Ni0.005Au/SiO2 as a function of the mole fraction of MeOH/(MA + MeOH), Other reaction conditions: O2/N2 (20:80 v/v, 8.2 bar), T = 60 ℃, 600 rpm, 3 h test.

either the Au NPs or the NiAu SAA NPs. Thus, the catalyst is stable under reaction conditions.

present work, even at XMeOH = 0.1. Similarly, when Au NPs were tested in the reaction at such low methanol concentration, no methacrylic acid was detected in the solution. This difference in the reaction products between this work and that in ref [25] is therefore solely attributed to the catalyst being studied in a different phase. Testing of the commercial catalysts took place at a methanol mole fraction equal to 0.94 [22]. As mentioned above, higher concentrations of methanol are needed in order to allow for its adsorption on the surface and the subsequent formation of the necessary methoxy intermediate, ultimately leading to the formation of the ester. Further increase of methanol concentration results in excess methanol on the surface, leading therefore to self-coupling towards methyl formate. This was also verified on the present system; when the methanol molar fraction was above 0.94 (Fig. 5), MMA production was low (MA conversion similar to XMeOH = 0.94), while evidence of methanol self-coupling was observed. Overall, here we demonstrate that doping the Au surface with Ni atoms offers the chance to escape the limitations of previously examined systems and results in comparable product yields to those reported for the commercial catalysts, without the need for excess methanol present in the solution. Thus, the NiAu SAA catalyst offers a superior composition/structure material allowing for the possibility to work at lower methanol:methacrolein ratios, without methanol displacement by methacrolein, which would otherwise ultimately lead to catalytic surface poisoning and lower product yields [25]. The catalytic performance of the Ni0.005Au/SiO2 was evaluated in long-term stability tests. Accordingly, this sample was subjected to four consecutive 12-h long reaction cycles at the same reaction conditions. The results are shown in Fig. 6. At the end of the first cycle the MMA production rate was 4.2 × 10−7 mol/gcatal·s, while in the second cycle the production rate dropped to 1.5 × 10−7 mol/gcatal·s. This drop is attributed to partial loss of loosely bound Au NPs on SiO2, which were dislodged during the first reaction cycle or upon retrieving the sample from the reaction mixture at the end of the experiment [22]. No appreciable decrease was observed between the 2nd, 3rd and 4th cycles, Fig. 6. In addition, MA was catalytically converted to MMA with 100% selectivity for all four reaction cycles. From ICP measurements made on the fresh and used samples, the Au content was found to have decreased by 10% after the 1 st cycle and remained essentially the same after the 2nd cycle. A similar trend was found on the bare Au NPs between the 1st and the 2nd cycle (Fig. 6). No leaching of Ni took place during the reaction, as was shown by CO-DRIFTS on the used Ni0.005Au NPs, shown below, as well as by ICP measurements performed on the reaction solution. No deactivation due to carbon deposition took place, as verified by temperature-programmed oxidation (not shown here) on

3.3. Spectroscopic studies The in situ ATR-IR spectra of Au/SiO2 and Ni0.005Au/SiO2 were recorded under liquid reaction conditions in ∼8 bar O2 between 25 and 60 ℃ (Fig. 7). All IR-detectable adsorbates (reactants, intermediates and final products) are depicted in the reaction mechanism shown in Fig. 8. For Au/SiO2 the corresponding spectra at the different temperatures are shown in Fig. 7A. After injecting the MA-methanol solution at RT and increasing the pressure to ∼8 bar with the O2/N2 gas mixture, peaks in the high frequency region appeared at 2975 cm−1, 2930 cm−1 and 2835 cm−1. These are attributed to v(CH), vs(CH3) and 2δs(CH3) of surface methoxy and methacrolein species adsorbed both on Au (Fig. 8B) and uncovered SiO2 sites [56–58]. In the low frequency region, a peak at 1710 cm−1 corresponds to the carbonyl group v(C] O) of the adsorbed methacrolein molecule [58,59]. In addition, peaks that appear at 1450 cm−1 and 1315 cm−1 are attributed to δas(CH3) and CeH rocking modes of adsorbed methoxy and methacrolein species respectively [58]. A broad feature at ∼1350–1450 cm-1 results from various CeH bending modes of species adsorbed on different sites of Au or SiO2. With increase of the reaction temperature, both methoxy and methacrolein peaks decrease in intensity while the relative intensity of a peak centered at 1730 cm-1 increases. The latter is attributed to v(C] O) mode of adsorbed MMA (Fig. 8D) which is the main product from the esterification of methacrolein with methanol [58]. With further increase of reaction temperature, the ratio of the intensities of v(C] O)MMA/v(C]O)MA increases which is expected as more MA is converted to MMA at elevated temperatures. For Ni0.005Au/SiO2 the corresponding spectra at the different temperatures are shown in Fig. 7B. At RT, adsorbed methoxy and methacrolein peaks of various vibrational modes appear at 2975 cm−1, 2930 cm−1, 2835 cm−1, 1710 cm−1, 1450 cm−1 and 1315 cm−1 as in the case of Au/SiO2. One main difference from the Au NPs is that for the Ni0.005Au NPs formation of MMA happens already from RT since the 1730 cm−1 peak attributed to v(C]O) of adsorbed MMA appears in the respective spectrum. In addition, weaker peaks at 1330 cm−1 and 1300 cm−1 are also an indication of the increased MMA production, since they are attributed to asymmetric and symmetric CeOeC modes of ester molecules [58]. Another difference in these spectra is the appearance of a strong sharp peak at 1655 cm−1. This peak has been attributed to the presence of HeOeH mode of physisorbed molecular water either on the support or on the Au NPs (Fig. 8D) [57,60]. As already mentioned, the reaction takes place in the presence of O2 and 6

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Fig. 7. In situ ATR-IR spectra under oxidation esterification MA-methanol reaction for (A) undoped Au NPs on silica and (B) Ni0.005Au SAA NPs supported on silica, XMeOH = 0.94, O2/N2 (20:80 v/v, 8.2 bar), spectra resolution 4 cm−1.

be hindered by the broad feature at ∼1350–1450 cm-1. With reaction temperature rise, the methacrolein peaks are almost depleted while those of MMA and hemiacetal intermediate are still present. These observations are in good agreement with previous gas phase and theoretical studies for the coupling of unsaturated aldehydes with alcohols and the proposed reaction mechanism, supported also by this work, is presented in Fig. 8, which is an adaptation from ref. 53. Interestingly, in that reference, unsupported nanoporous gold is considered, but its activity derives from the presence of residual silver alloyed in it [62]. By-products such as methyl formate, MF, which is the product of self-coupling of methanol in the presence of O2, or methacrylic acid, MAA, which is the product of methacrolein oxidation, were not observed as was also confirmed from separate adsorption experiments, where the Ni0.005Au/SiO2 sample was subjected to separate solutions of MF and MAA in methanol at 8 bar and the corresponding spectra were recorded. The resulting absorbances are included in Table 3. No good match of these combinations of peaks was possible in the in situ ATR-IR spectra and therefore formation of either MF or MAA was not considered in the mechanism. From the above, it can be concluded that formation of the

produces MMA and water. The increased presence of this peak in the Ni0.005Au/SiO2 spectra may be the result of two factors; firstly, the increased production of MMA and water; and secondly, the stronger adsorption of the latter. Since water is weakly bound either on the bare support or the Au NPs [61] and for both Au NPs and NiAu NPs the Au loading is relatively low, the first factor seems to be more important, taking also into account that most of the catalyst surface in both cases consists of bare silica. A third difference of the spectra in Fig. 7B from those of Fig. 7A is the presence of two peaks at 1510 cm−1 and 1375 cm−1 on the former which are attributed to asymmetric and symmetric OeCeO modes indicating the presence of carbonate species [56,58]. These carbonates are possible intermediates in the reaction mechanism that takes place on Au NPs. It has been previously proposed that the hemiacetal CH2=C(CH3)C(OH)–O−CH3 is the intermediate product [2,19,55]. Here it is confirmed that an adsorbed hemiacetal of this form, adsorbed on the highly oxidized Au surface (Fig. 8C), may form these carbonate species and is indeed an intermediate of the reaction. Low formation of the adsorbed hemiacetal unit which leads to its low concentration on Ni-free Au NPs is a possible reason that these low-intensity peaks could not be observed. This observation may also

Fig. 8. Oxidative coupling mechanism of methacrolein with methanol over Ni-AuOx sites (adapted from ref. [53] where Ni-free gold was considered). 7

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Ni atoms were present in the Au NPs (0.08 for pure Au NPs versus 0.44 for Ni0.005Au). This is a strong indication that Ni brings about an increase in the number of [Au-Ox]- sites at the surface of the Au NPs. As mentioned above, [Au-Ox]- sites are considered necessary for the adsorption of the reactants on the Au surface and their presence was verified by the XP spectra both in unpromoted Au NPs, as well as in NiAu SAAs. However, the reason for their catalytic activity in these oxidative coupling reactions has not yet been fully explained. According to various DFT and surface science studies, zerovalent Au is expected to be inert for such reactions [64–66]. From another surface science study, even though the Au[211] surface was found to be the best candidate to dissociatively adsorb oxygen, this can only be realized either at high temperatures (900 K and 1 atm) or by ozone [67]. In addition, from gas phase and UHV studies of the oxidative coupling of MA with methanol on Au[111] surfaces, a small amount of ozone diluted in O2 gas mixture was necessary for Au activation [25,53]. However, the present work provides evidence that this reaction is activated without the need for ozone or high temperature treatment on supported Au NPs in the liquid phase under molecular O2 pressure, and similar results have also been reported in other work [22]. To our understanding, even though Au[111] facets were observed as the predominant ones by TEM, the presence of other facets where Au undercoordinated atoms should not be excluded. These atoms may be the ones that get oxidized by O2 and we were able to detect their oxidation state by XPS. The concentration of [Au-Ox]- sites was increased when Ni was doped into the Au surface as illustrated by the XP spectra, Fig. 10B. One explanation for this may be the tensile strain of the Au surface lattice as Ni atoms are embedded in it. Such sites are possible candidates for O2 dissociation as surface science studies have shown [68]. Another possibility is the presence of surface NiOx in close proximity to Au atoms which are oxidized, partially reducing the former. Enhanced stabilization of activated [Au-Ox]- species has also been reported for bimetallic NiAu catalysts for other reactions, such as CO oxidation [62]. X-ray Photoemission spectra collected for the used Au and Ni0.005Au NPs are presented in Fig. 11 A and B, respectively. After the reaction, Au is more oxidized in both the Au and Ni0.005Au NPs, with diminishing presence of Au°. As is clearly seen in Fig.12, after quantification of the relative peak areas of Au oxidation states when Ni atoms are present in the Au surface, Au gets more oxidized than in the Ni-free Au NPs. The reaction conditions bring about the same change in the Au oxidation state from Au° to AuI. As mentioned above, Ni atoms present in Au NPs do not change the apparent activation energy of the reaction but promote the catalytic activity by increasing the number of [Au-Ox]- species and the XP spectra of the samples after the reaction provide further proof of this. Coupling these results with that from the kinetic studies

Table 3 Frequencies of modes obtained from spectra recorded after adsorption of methyl formate and methacrylic acid on Ni0.005Au/SiO2 under pressure of 8 bar in O2/N2 gas. Adsorbed Species Modes Methyl formate v(C]O) δ(CH3) v(CeO) δ(CH) Methacrylic acid v(C]O) vas(OeCeO) vs(OeCeO) δ(CH)

v (cm−1)

1750 1450 1230 1190 1690 1610 1315 1200

intermediates and MMA is favored on Ni0.005Au NPs more than on monometallic Au NPs. However, Au forms the active sites, as found from the kinetic studies of these two catalysts, Fig. 4. A possible explanation is that Ni atoms in the Au surface enhance the formation of surface [Au-Ox]-species, that is the active sites for the oxidative coupling of MA with methanol. This hypothesis is supported from the XPS findings that are discussed below. Since for all steps of the proposed mechanism; namely, reactant adsorption, intermediate and final product formation, activated O sites on the Au NPs surface are necessary, these are more abundant at the Ni-Au atom interfaces. While NiOx will be the dominant phase in extended Ni surfaces under the aerobic reaction conditions, at the atom limit next to an Au atom Ni can be partially reduced while Au gets oxidized and the active [Au-Ox]- site is formed. Deconvolution of IR peaks areas of v(C = O) modes derived from adsorbed MA and MMA on the Au NPs and quantification of v (C = O)MA and v(C = O)MMA peak areas rendered the trends in concentration of these molecules and the extent the reaction was taking place in each sample more clear. The increased MMA formation in the case of Ni0.005Au NPs is clearly depicted in the deconvoluted relative peak areas of MA and MMA v(C = O) modes, Fig. 9A and B. Fig. 10A, B shows the photoemission spectra of Au 4f of pure Au and Ni0.005Au NPs respectively, before reaction (fresh). In both spectra two peaks are observed, which, after deconvolution, were found to be the cumulative result of a combination of other peaks. In the case of Au NPs (Fig. 10A), the deconvoluted peaks were those of Au° (83.8 and 87.8) and AuI (84.5 and 88.7) [63]. After calculation of all the Au peak areas it was found that the ratio, IAuox/IAutot (where IAuox the total area of oxidized Au and IAutot the total area of all Au peaks), was higher when

Fig. 9. Relative Peak Area of v(C = O) modes for MA and MMA calculated after deconvolution of the spectra of Au (A) and Ni0.005Au (B) NPs recorded in the full reaction temperature range. 8

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Fig. 10. Au 4f XP spectra of fresh Au/SiO2 (A) a nd Ni0.005Au/SiO2 (B).

above it can be concluded that [Au-Ox]- species are the active site where the esterification reaction takes place. It is noteworthy that the ratio between the percentage of AuI present on the Au surface (0.56:0.25) on NiAu SAAs and bare Au NPs, as measured by XPS (Fig. 11) correlates well with the measured production rate of MMA (8.33 × 10−7: 3.78 × 10−7 molMMA/m2Au·s), as shown in Table 2. Both ratios are equal to approximately 2.2, a result that further corroborates the dependence of the catalytic activity to the amount of available [AuOx]- species. Based on the above, it can be concluded that the improved catalytic performance introduced by the presence of Ni atoms is attributed to the following three reasons: i) NiOx being itself basic offers some basicity to the system, an essential component for improved reactivity, as has been discussed previously in the literature, ii) offers a binding site to methanol, which otherwise is displaced by methacrolein, owing to the stronger binding of the latter on Au surfaces, and iii) as shown by XPS (Fig. 11), the percentage of AuI increases significantly in Ni atom-doped catalysts. Post-reaction characterization of the samples was performed to identify any structural changes induced by the reaction conditions. Fig. 12 shows CO DRIFT spectra recorded on the used Ni0.005Au/SiO2. The characteristic peaks for Au°−CO and Ni°(CO)x can be clearly observed at 2115 and ∼2100 cm−1, respectively. Detection of the Ni°(CO)x peak provides evidence that Ni had not leached out in the reaction solution, which was verified by ICP measurements. No Ni was detected in the reaction solution, although the detection limit (0.1 ppm) could not exclude the possibility. More convincing data comes from the comparison of the relative area ratios of the Au°−CO and Ni°(CO)x linear carbonyl peaks (2115 and ∼2100 cm−1). More specifically, the peak area ratio of Ni/Au in the fresh Ni0.005Au/SiO2 sample CO-DRIFTS

Fig. 12. CO DRIFT spectra recorded from used silica supported Ni0.005Au NPs. CO adsorption took place at room temperature and spectra were recorded after removal of gaseous CO peaks upon He purge.

was 0.42 (Fig. 2) while the one calculated for the used sample was 0.38. This is strong evidence that no leaching of Ni took place during the reaction. In addition, absence of peaks around 2050 – 2060 cm−1 and broad bands at 1700 – 1800 cm-1 indicate that aggregation of Ni atoms to form clusters did not take place during the reaction. From that it can be concluded that the catalysts retain their atomic Ni dispersion under reaction conditions, which explains the high selectivity to MMA. Even

Fig. 11. Au 4f XP spectra of used Au/SiO2 (A) and Ni0.005Au/SiO2 (B). 9

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after the four 12-h long cycles shown in Fig. 6, there was no formation of MF, the side-product formed on the catalyst with higher Ni loading (such as the NiAu NPs with atomic Ni:Au = 1:1, where Ni clusters are present, Table 1).

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4. Conclusions In this work, we have investigated for the first time the effect of doping Au NPs with isolated Ni atoms and found them to boost the catalytic activity of gold (more than twofold) for the liquid-phase aerobic esterification of methacrolein with methanol. The dispersion of Ni in Au was followed by CO-DRIFTS while the oxidation state of Au was investigated by XPS. NiAu NPs were shown to be more active than pure Au NPs in the reaction temperature range 25–60 ℃. [Au-Ox]- active sites are formed in higher abundance in the presence of Ni than on undoped Au NPs. This is accompanied by increased formation of the hemiacetal reaction intermediate which in turn explains the increased activity of NiAu NPs. Kinetic studies indicate that the active sites are the same gold species, in both Au and NiAu NP SAAs. A key finding of this work is that NiAu SAAs are shown to withstand low concentrations of methanol and not be deactivated by the stronger adsorption of MA on the catalyst surface, owing to their unique and uniform surface structure. Overall, the role of Ni atoms is three-fold, as they offer a binding site for methanol; act as a base to stabilize the intermediate; and increase the number of active [Au-Ox]- sites, considerably boosting the catalytic activity, without affecting the high selectivity of gold. As such, NiAu SAAs are promising alternatives to other reported gold-based catalysts for this reaction; and a more environmentally benign process, with lower methanol recycle is envisioned.

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Acknowledgments

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This material is based upon work supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE- SC0012573.

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