Enhanced catalyst selectivity in the direct synthesis of H₂O₂ through Pt incorporation into TiO₂ supported AuPd catalysts

Abstract

The introduction of small quantities of Pt into supported AuPd nanoparticles is found to result in enhanced catalytic efficiency in the direct synthesis of H₂O₂. This is attributed to a combination of superior H₂O₂ synthesis rates, as determined through calculation of initial rates of reaction, and an inhibition of H₂O₂ degradation pathways, achieved through the modification of Pd oxidation states. Through gas replacement experiments we demonstrate that it is possible to reach concentrations of H₂O₂ approaching those produced during initial stages of the current industrial means of H₂O₂ production.

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Introduction

Hydrogen peroxide (H₂O₂) is a versatile, environmentally friendly oxidant that finds applications as a bleaching agent in the pulp and textile industry,¹ the treatment of waste streams^2,3 and is finding growing use in the production of both commodity and fine chemicals. With the demand from the chemical sector in particular driven by the growing need for both propylene oxide, via the integrated HPPO process,^4,5 and cyclohexanone oxime, a key intermediate in the production of Nylon-6.⁶ In recent years, global H₂O₂ production has exceeded 3 million tons per annum⁷ and is predicted to continue to grow at a rate of 4% per year to exceed 4 million tons per annum by 2020.⁸

Currently the global demand for H₂O₂ is met by the highly efficient anthraquinone oxidation (AO) or indirect synthesis process, first developed by BASF in 1939.⁹ The AO process has undergone numerous improvements since, but the underlying chemistry has changed little, utilising H₂, O₂ and an anthraquinone derivative, where the anthraquinone molecule undergoes sequential hydrogenation and oxidation steps to generate H₂O₂, while avoiding the risk of combining H₂ and O₂ directly. This process is able to initially yield H₂O₂ concentrations of 1–2 wt%, which through further distillation and purification steps can be raised to exceed 70 wt%; a concentration which can then be shipped and stored prior to dilution at point of use.

Despite the AO process being highly efficient there are some concerns regarding its carbon efficiency, with the over-hydrogenation of the anthraquinone carrier-molecule necessitating its replacement periodically. This coupled with the high infrastructure costs and complexity of the process has often prevented the large-scale generation of H₂O₂ at point of use. In addition, the instability of H₂O₂, undergoing rapid decomposition to H₂O at relatively mild temperatures or in the presence of weak bases requires the use of acidic stabilising agents, which result in additional purification steps to prevent contamination of product streams and decreased reactor lifetime due to corrosion, raising costs to the end user.

The catalysed direct synthesis of H₂O₂ from molecular H₂ and O₂ offers an attractive alternative to the current means of H₂O₂ production on an industrial scale and would allow for H₂O₂ production to be adopted at point of use. Since 1914¹⁰ Pd-based catalysts have received significant attention within both the academic^11,12 and patent^13,14 literature. However, issues around catalytic selectivity have prevented commercialisation of the direct synthesis process despite over 100 years of academic pursuit. The issue of catalyst selectivity is easy to understand given that that formation of H₂O is thermodynamically favoured compared to H₂O₂, as summarised in Scheme 1.

Scheme 1 Reaction pathways associated with the direct synthesis of H₂O₂ from H₂ and O₂.

In order to overcome limitations around selectivity, halide salts (e.g. NaBr)^15–17 or mineral acids (e.g. HCl, HNO₃)^18,19 have often been employed, with Pospelova et al.²⁰ first demonstrating increased yields of H₂O₂ through their application alongside a supported Pd catalyst. Although it is clear that the use of halide additives can greatly enhance catalytic selectivity, the means by which this effect is achieved is still ambiguous. Nevertheless, the use of halide and acid additives offers significant drawbacks to the user, akin to those associated with H₂O₂ generated via the anthraquinone process and significant attention has been placed on enhancing catalytic selectivity through catalytic design. With the incorporation of Au into Pd,^21–25 a particularly well-studied catalytic system for the production of H₂O₂, the need for acid or halide stabilising agents is removed. The means by which the incorporation of Au into Pd-based catalysts enhances catalytic activity is still of some debate, with electronic, structural and isolation effects all being cited as potential causes. However, conclusive evidence on the nature of catalytic enhancement is still lacking and it is likely that a combination of these factors are responsible for the observed synergy. More recently Freakley et al.²⁶ have demonstrated that it is possible to exchange Au with a range of secondary base metals to reach selectivity levels towards H₂O₂ in excess of 95% and this has prompted the further investigation of Pd modification with a range of non-precious metals.^27–30

Further studies have demonstrated that the incorporation of low concentrations of Pt into supported Pd or AuPd catalysts can greatly enhance catalytic activity towards the direct synthesis of H₂O₂. Indeed, a comprehensive study by Deguchi et al.³¹ revealed that the incorporation of Pt into a Pd-polyvinylpyrrolidone colloid resulted in a significant increase in catalytic activity, which was attributed to the ability of Pt to readily adsorb dissociated H₂. However, this rise in catalytic activity, with H₂O₂ formation rates doubling upon incorporation of 0.5 at% Pt, came at the expense of catalytic selectivity. By comparison we have previously demonstrated that the addition of Pt into AuPd catalysts in small concentrations enhances catalytic performance, through inhibition of H₂O₂ degradation pathways and leads to improved selectivity towards H₂O₂.^32,33 However, our previous studies have focussed on catalysts prepared by a conventional wet co-impregnation procedure, primarily due to the simplicity and industrial applicability of this methodology. It is important to note that a limitation of catalysts prepared via this procedure is the considerable variation in elemental composition with nanoparticle size, with larger particles generally being Au-rich, while smaller nanoparticles are predominantly Pd-rich. As such it has been difficult to determine the key parameter responsible for the enhancement in catalytic performance upon Pt incorporation into supported AuPd nanoparticles, with the modification of Pd oxidation state and changes in mean particle size both possible causes for the observed improvement. By comparison to catalysts prepared by wet impregnation, those produced via a sol-immobilisation methodology offer better control of particle size and elemental composition.³⁴ As such, this study now focusses on the efficacy of Pt introduction into AuPd catalysts prepared via a sol-immobilisation methodology.

Experimental methods

Catalyst preparation

Mono-, bi- and tri-metallic 1% AuPdPt/TiO₂ (total metal loading of 1 wt%) catalysts have been prepared (on a molar basis) by a sol-immobilisation procedure, based on methodology previously reported in the literature, which has been shown to result in enhanced precious metal dispersion by limiting particle growth and agglomeration.³⁴ The procedure to produce 1% Au₁Pd₁Pt₁/TiO₂ (1 g) is outlined below (where the Au : Pd : Pt molar ratio is fixed at 1 : 1 : 1) with a similar methodology utilised for mono- and bi-metallic catalysts.

Aqueous solutions of HAuCl₄·3H₂O (0.322 mL, 12.25 mg mL⁻¹, Strem Chemicals), PdCl₂ (0.356 mL, 6 mg mL⁻¹, Sigma Aldrich) and H₂PtCl₆·6H₂O (0.285 mL, 13.76 mg mL⁻¹, Sigma Aldrich) were added to deionised water (400 mL) under vigorous stirring conditions at room temperature. The resulting solution was allowed to stir for 2 minutes prior to the addition of polyvinylalcohol (PVA) (1.30 mL, 1 wt% MW = 9000–10 000 g mol⁻¹, 80% hydrolysed, Sigma Aldrich) such that the weight ratio of metal : PVA was 1 : 1.3. The resulting solution was stirred for 2 minutes prior to the addition of a freshly prepared solution of NaBH₄ (4.015 mL, 0.1 M), such that the molar ratio of NaBH₄ : (Au + Pd) was 5 : 1 and the molar ratio of NaBH₄ : Pt was 10 : 1. Upon the addition of NaBH₄ the mixture turned dark brown and was stirred vigorously for an additional 30 min followed by the addition of TiO₂ (0.99 g, Degussa P25). The solution was acidified to pH 1 via the addition of H₂SO₄ (>95%) and stirred for 1 h. Following this, the suspension was filtered under vacuum, washed thoroughly with distilled water, then dried (110 °C, 16 h) and calcined (400 °C, 3 h, 10 °C min⁻¹, static air).

Direct synthesis of H₂O₂

Hydrogen peroxide synthesis was evaluated using a Parr Instruments stainless steel autoclave with a nominal volume of 100 mL, equipped with a PTFE liner so that total liquid volume is reduced to 66 mL, and a maximum working pressure of 14 MPa. To test each catalyst for H₂O₂ synthesis, the autoclave liner was charged with catalyst (0.01 g) and solvent (5.6 g methanol and 2.9 g H₂O). The charged autoclave was then purged three times with 5% H₂/CO₂ (0.7 MPa) before filling with 5% H₂/CO₂ to a pressure of 2.9 MPa, followed by the addition of 25% O₂/CO₂ (1.1 MPa). A pressure of 5% H₂/CO₂ and 25% O₂/CO₂ are given as gauge pressures. The reaction was conducted at a temperature of 2 °C for 0.5 h with stirring (1200 rpm). The above reaction parameters are based on optimum conditions we have previously used for the synthesis of H₂O₂.³⁵ The H₂O₂ productivity was determined by titrating aliquots of the final solution after reaction with acidified Ce(SO₄)₂ (0.0085 M) in the presence of ferroin indicator. Catalyst productivities are reported as mol_H2O2 kg_cat⁻¹ h⁻¹.

The catalytic conversion of H₂ and selectivity towards H₂O₂ were determined using a Varian 3800 GC fitted with TCD and equipped with a Porapak Q column.

H₂ conversion (eqn (1)) and H₂O₂ selectivity (eqn (2)) are defined as follows:

The total autoclave capacity was determined via water displacement to allow for accurate determination of H₂ conversion and H₂O₂ selectivity. When equipped with the PTFE liner the total volume of an unfilled autoclave was determined to be 93 mL, which includes all available gaseous space within the autoclave.

Degradation of H₂O₂

Catalytic activity towards H₂O₂ degradation was determined in a similar manner to the direct synthesis activity of a catalyst. The autoclave liner was charged with methanol (5.6 g), H₂O₂ (50 wt% 0.69 g), HPLC standard H₂O (2.21 g) and catalyst (0.01 g), with the solvent composition equivalent to a 4 wt% H₂O₂ solution. From the solution, two 0.05 g aliquots were removed and titrated with acidified Ce(SO₄)₂ solution using ferroin as an indicator to determine an accurate concentration of H₂O₂ at the start of the reaction. The autoclave was pressurised with 2.9 MPa 5% H₂/CO₂ (gauge pressure). The reaction was conducted at a temperature of 2 °C, for 0.5 h with stirring (1200 rpm). After the reaction was complete the catalyst was removed from the reaction mixture and two 0.05 g aliquots were titrated against the acidified Ce(SO₄)₂ solution using ferroin as an indicator. The degradation activity is reported as mol_H2O2 kg_cat⁻¹ h⁻¹.

The reactor temperature was controlled using a HAAKE K50 bath/circulator using an appropriate coolant.

Catalyst reusability in the direct synthesis and degradation of H₂O₂

In order to determine catalyst reusability, a similar procedure to that outlined above for the direct synthesis of H₂O₂ is followed utilising 0.05 g of catalyst. Following the initial test, the catalyst was recovered by filtration and dried (30 °C, 17 h, under vacuum); from the recovered catalyst sample 0.01 g was used to conduct a standard H₂O₂ synthesis or degradation test.

Catalyst characterisation

The as-prepared aqueous sols, contained in a quartz cuvette, were optically characterised using a UV-vis spectrometer (V-570, JASCO) operating over the 200 to 800 nm wavelength range.

X-ray photoelectron spectroscopy (XPS) analyses were made on a Kratos Axis Ultra DLD spectrometer. Samples were mounted using double-sided adhesive tape and binding energies were referenced to the C(1s) binding energy of adventitious carbon contamination that was taken to be 284.8 eV. Monochromatic AlK_α radiation was used for all measurements; an analyser pass energy of 160 eV was used for survey scans, while 40 eV was employed for more detailed regional scans. The intensities of the Au(4f), Pt(4f) and Pd(3d) features were used to derive the Pd/Pt and Au/Pt surface composition ratios.

Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100 operating at 200 kV. Samples were prepared by dispersion in ethanol by sonication and deposited on 300 mesh copper grids coated with holey carbon film. Energy dispersive X-ray spectroscopy (XEDS) was performed using an Oxford Instruments X-Max^N 80 detector and the data analysed using Aztec software. Aberration corrected scanning transmission electron microscopy (AC-STEM) was performed using a probe-corrected Hitachi HF5000 S/TEM, operating at 200 kV. The instrument was equipped with bright field (BF), high angle annular dark field (HAADF) and secondary electron (SE) detectors for high spatial resolution STEM imaging experiments. This microscope was also equipped with a secondary electron detector and dual Oxford Instruments XEDS detectors (2 × 100 mm²) having a total collection angle of 2.02 sr.

Total metal leaching from the supported catalyst was quantified via inductively coupled plasma mass spectrometry (ICP-MS). Post-reaction solutions were analysed using an Agilent 7900 ICP-MS equipped with I-AS auto-sampler. All samples were diluted by a factor of 10 using HPLC grade H₂O (1% HNO₃ and 0.5% HCl matrix). All calibrants were matrix matched and measured against a five-point calibration using certified reference materials purchased from Perkin Elmer and certified internal standards acquired from Agilent.

DRIFTS measurements were taken on a Bruker Tensor 27 spectrometer fitted with a mercury cadmium telluride (MCT) detector. A sample was loaded into the Praying Mantis high temperature (HVC-DRP-4) in situ cell before exposure to N₂ and then 1% CO/N₂ at a flow rate of 50 cm³ min⁻¹. A background spectrum was obtained using KBr, and measurements were recorded every 1 min at room temperature. Once the CO adsorption bands in the DRIFT spectra ceased to increase in size, the gas feed was changed back to N₂ and measurements were repeated until no change in subsequent spectra was observed.

Results and discussion

Prior to immobilisation the as-synthesised Au–Pd–Pt colloids were analysed by UV-vis spectrometry (Fig. S1†) with no characteristic plasmon resonance band for Au being observed in the bi- and tri-metallic colloids, suggesting the formation of alloyed nanoparticles. Our initial studies, under conditions previously optimised for H₂O₂ synthesis, investigated the efficacy of supported monometallic (Au, Pd, Pt) and bi-metallic (AuPd, AuPt and PtPd) catalysts supported on TiO₂ for the direct synthesis of H₂O₂ and its subsequent degradation, via hydrogenation and decomposition pathways, as shown in Table 1. As previously reported, the activity of the immobilised Au-only catalyst towards H₂O₂ synthesis is limited (4 mol_H2O2 kg_cat⁻¹ h⁻¹). By comparison, the 1 wt% Pd/TiO₂ catalyst was observed to offer a marginally higher activity towards both H₂O₂ production_, with a higher synthesis rate (11 mol_H2O2 kg_cat⁻¹ h⁻¹) and subsequent degradation (59 mol_H2O2 kg_cat⁻¹ h⁻¹). In keeping with numerous previous studies, the co-immobilisation of Au and Pd is seen to result in an enhancement in catalytic activity towards H₂O₂ synthesis³⁵ (81 mol_H2O2 kg_cat⁻¹ h⁻¹), far greater than the activity observed over a physical mixture of the two mono-metallic catalysts (7 mol_H2O2 kg_cat⁻¹ h⁻¹). It should be noted that the H₂O₂ synthesis activity of the 1% AuPd/TiO₂ catalyst, prepared via the sol-immobilisation procedure is comparable to that observed for an analogous catalyst prepared via modified impregnation, where relatively high concentrations of HCl are utilized to enhance metal dispersion, (80 mol_H2O2 kg_cat⁻¹ h⁻¹)³⁵ and somewhat greater than that for the analogous catalyst prepared by conventional wet-impregnation (64 mol_H2O2 kg_cat⁻¹ h⁻¹). While these latter methodologies may be more attractive for catalyst synthesis on an industrial scale, they typically result in a wider variation in particle size and elemental composition than catalysts produced via a sol-immobilisation technique.³⁴ As such the sol-immobilisation procedure has clear advantage in producing model systems, where tight control of catalytic parameters are necessary.

Table 1 Catalytic activity of the various mono- and bi-metallic catalysts supported on TiO₂ towards the direct synthesis and subsequent degradation of H₂O₂

Catalyst	Productivity^a molH2O2 kgcat^−1 h^−1	Degradation^b molH2O2 kgcat^−1 h^−1
a H2O2 direct synthesis reaction conditions: catalyst (0.01 g), H2O (2.9 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 25% O2/CO2 (160 psi), 0.5 h, 2 °C, 1200 rpm. b H2O2 degradation reaction conditions: catalyst (0.01 g), H2O2 (50 wt% 0.68 g) H2O (2.22 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 0.5 h, 2 °C, 1200 rpm. c Reaction conditions identical to those outlined above, using 0.005 g of each catalyst.
1% Au/TiO2	4	27
1% Pd/TiO2	11	59
1% Pt/TiO2	9	340
1% Au1Pd1/TiO2	81	257
1% Au1Pt1/TiO2	30	243
1% Pd1Pt1/TiO2	18	316
0.5% Au/TiO2 + 0.5% Pd/TiO2^c	7	41
TiO2	0	0

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We have previously reported that an improvement in catalytic selectivity towards H₂O₂ can be achieved through the introduction of small quantities of Pt into AuPd nanoparticles, prepared by a conventional wet co-impregnation methodology, dispersed on a range of supports.^32,36 Additional studies have reported a similar enhancement in catalytic efficacy for a range of selective oxidation reactions, using supported AuPdPt catalysts prepared by a sol-immobilisation methodology.^37,38 Building on our initial findings, we next investigated the effect of Pt addition on the catalytic activity of 1% Au₁Pd₁/TiO₂ towards H₂O₂ synthesis (Fig. 1). In keeping with our previous studies, the addition of a small quantity of Pt (approx. 0.006 wt%) significantly enhances H₂O₂ synthesis rates, from 81 mol_H2O2 kg_cat⁻¹ h⁻¹ for the 1% Au₁Pd₁/TiO₂ catalyst to 112 mol_H2O2 kg_cat⁻¹ h⁻¹ for the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst. However, further addition of Pt is observed to lead to a decrease in catalytic activity towards H₂O₂ synthesis, with this metric decreasing to a value of 30 mol_H2O2 kg_cat⁻¹ h⁻¹ for the 1% Au₁Pd₁Pt₁/TiO₂ catalyst.

Fig. 1 The effect of Pt incorporation into 1% AuPd/TiO₂ on catalytic activity towards the direct synthesis of H₂O₂. H₂O₂ direct synthesis reaction conditions: catalyst (0.01 g), H₂O (2.9 g), MeOH (5.6 g), 5% H₂/CO₂ (420 psi), 25% O₂/CO₂ (160 psi), 0.5 h, 2 ° C, 1200 rpm.

The observation of a strong dependency between catalytic activity towards H₂O₂ synthesis and Pt content in the sol-immobilised materials, motivated us to further investigate the structure–activity relationships existing over the 1% Au₁Pd₁/TiO₂, 1% Au₁Pd₁Pt_0.01/TiO₂ and 1% Au₁Pd₁Pt₁/TiO₂ catalysts.

An assessment of catalytic selectivity towards H₂O₂ and H₂ conversion of the systematic set of 1% AuPdPt/TiO₂ catalysts is presented in Table 2. In keeping with the lower rates of H₂O₂ degradation and higher yield of H₂O₂, the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst displayed a selectivity towards H₂O₂ (37%) which was greater than that of the 1% Au₁Pd₁/TiO₂ (31%) or 1% Au₁Pd₁Pt₁/TiO₂ (15%) catalysts, while all catalysts displayed similar rates of H₂ conversion.

Table 2 Comparison of catalytic selectivity of the various catalyst formulations towards H₂O₂ and H₂ conversion

Catalyst	H2 conversion/%	H2O2 selectivity/%	Productivity/molH2O2 kgcat^−1 h^−1	H2O2 concentration/wt%	Degradation/molH2O2 kgcat^−1 h^−1
H2O2 direct synthesis reaction conditions: catalyst (0.01 g), H2O (2.9 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 25% O2/CO2 (160 psi), 0.5 h, 2 °C 1200 rpm. H2O2 degradation reaction conditions: catalyst (0.01 g), H2O2 (50 wt% 0.68 g) H2O (2.22 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 0.5 h, 2 °C 1200 rpm.
1% Au1Pd1/TiO2	39	31	81	0.16	257
1% Au1Pd1Pt0.01/TiO2	43	37	112	0.22	245
1% Au1Pd1Pt1/TiO2	44	15	30	0.10	271
1% Pt/TiO2	20	8	11	0.02	340

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Evaluation of the as prepared Pt incorporated 1% AuPd/TiO₂ catalysts by XPS can be seen in Table 3 (corresponding spectra in Fig. S.2†). Upon introduction of low quantities of Pt (approx. 0.006 wt%) the surface Pd : Au ratio remains unchanged, with further addition resulting in a minor decrease of the Pd : Au ratio. This can be attributed to a combination of significant decrease in mean particle size, as determined by TEM (Table 4) and the surface migration of Pt and disruption of the Pd-rich surface, which is a similar effect to that previously observed by Kondrat et al.³⁸ Perhaps more interesting is the significant decrease in the Pd²⁺ : Pd⁰ ratio upon Pt incorporation, with this value decreasing from a value of 1.3 for the bimetallic 1% Au₁Pd₁/TiO₂ catalyst to 0.9 for the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst (coinciding with an enhancement in catalytic selectivity towards H₂O₂) with Pd⁰ content continuing to increase upon further Pt incorporation. This may be surprising given the low selectivity of Pd⁰ species towards H₂O₂ that has been well reported in the literature.^39,40 However, Ouyang et al.⁴¹ have recently reported the enhanced selectivity and activity of supported Pd catalysts containing Pd⁰–Pd²⁺ ensembles in comparison to those catalysts with a predominance of Pd in either oxidation state. This improvement can be ascribed to the propensity of H₂ to dissociate on Pd⁰ and the enhanced stability of O₂ on Pd²⁺ surfaces, with the maintenance of the O–O bond required for the formation of H₂O₂ over H₂O. It is of note to highlight the similarity in Pd²⁺ : Pd⁰ oxidation ratio between the 1% PdPt/TiO₂ (1.1) and 1% AuPd/TiO₂ (1.3) catalysts, despite the significant differences observed in H₂O₂ synthesis activity, 18 and 81 mol_H2O2 kg_cat⁻¹ h⁻¹ respectively for these catalysts. This clearly highlights the importance of Au incorporation into precious metal catalyst, as well reported in the literature.^42–44

Table 3 Effect of Pt incorporation into supported 1% AuPd/TiO₂ of various compositions as determined by XPS

Catalyst	Au : Pt	Pd : Au	Pd^2+ : Pd^0
All catalysts calcined, 400 °C, 3 h, 10 °C min^−1 in static air.
1% Au1Pt1/TiO2	0.6	—	—
1% Pd1Pt1/TiO2	—	—	1.1
1% Au1Pd1/TiO2	—	1.9	1.3
1% Au1Pd1Pt0.01/TiO2	0.8	1.9	0.9
1% Au1Pd1Pt1/TiO2	0.5	1.9	0.6

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Table 4 Particle size measurements of 1% AuPdPt/TiO₂ catalysts, prepared by sol-immobilisation, as determined by analysis of bright field TEM micrographs

Catalyst	Mean particle size/nm (standard deviation)	Productivity/molH2O2 kgcat^−1 h^−1 (H2O2 wt%)
All catalysts calcined, 400 °C, 3 h, 10 °C min^−1 in static air.
1% Au1Pd1/TiO2	4.2 (0.98)	81 (0.16)
1% Au1Pd1Pt0.01/TiO2	3.7 (0.55)	112 (0.22)
1% Au1Pd1Pt1/TiO2	1.8 (0.56)	30 (0.10)

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It is therefore possible to relate the enhanced catalytic performance of the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst, compared to either the 1% AuPd/TiO₂ catalyst or Pt-rich analogue to the development of these Pd⁰–Pd²⁺ domains. It can be inferred that the increased degradation rates observed over the Pt-rich 1% Au₁Pd₁Pt₁/TiO₂ catalyst results from an increase in Pd⁰ content, at the expense of Pd²⁺.

The CO-DRIFTS spectra of the as prepared 1% AuPdPt/TiO₂ catalysts can be seen in Fig. 2. In the case of the 1% Au₁Pd₁/TiO₂ catalyst, the DRIFTS spectra are typically dominated by Pd–CO bands. The peak observed at 2090 cm⁻¹ represents linearly bonded CO to Pd atoms of low coordination (i.e., edge or corner sites) – denoted (Pd–CO) – while the broad feature that begins at 1950 cm⁻¹ represents the 2- and 3-fold adsorption of CO on Pd.⁴⁵ Upon the introduction of small quantities of Pt into AuPd, a small red-shift of the band related to the linearly bonded CO on Pd sites is observed, from 2090 to 2087 cm⁻¹. This shift is possibly a result of the charge-transfer to Pd d-orbitals, resulting in enhanced back donation to 2π CO molecular orbitals. In keeping with our observations, Ouyang et al.⁴⁶ have previously reported a similar transfer of electron density upon the alloying of Au and Pd with an associated suppression of O–O bond scission and enhancement in catalytic selectivity towards H₂O₂ synthesis.

Fig. 2 CO-DRIFTS spectra for selected sol-immobilised 1% AuPdPt/TiO₂ catalysts in the as-prepared state.

Catalytic activity towards H₂O₂ synthesis, in particular that of monometallic Pd catalysts, is widely reported to be dependent on particle size.^47,48 Indeed Tian et al.⁴⁹ have recently reported that an optimal particle size in the sub-nanometer range is desirable for achieving high activity and selectivity towards H₂O₂, with monodisperse atoms demonstrating an extremely low activity towards H₂O₂ formation, while larger nanoparticles offer greater activity towards the subsequent degradation pathways. Unlike with other means of catalyst preparation, such as wet-impregnation, the sol-immobilisation synthesis procedure allows for good control of mean particle size.³⁴ Measurements of mean particle size for the various 1% AuPdPt/TiO₂ catalysts (as determined from the bright field transmission electron micrographs presented in Fig. S3†) are shown in Table 4. This data reveals that the mean particle size is quite similar for the 1% Au₁Pd₁/TiO₂ (4.2 nm) and 1% Au₁Pd₁Pt_0.01/TiO₂ (3.7 nm) catalysts despite their distinctly different catalytic performances. Given the comparable particle size, it is therefore reasonable to propose that the enhancement in catalytic activity cannot be associated with metal dispersion and instead is related to the electronic modification of Pd, as indicated by XPS and CO-DRIFTS.

Further comparison of the as-prepared 1% AuPdPt/TiO₂ catalysts using complementary BF-, HAADF- and SE- AC-STEM imaging was also carried out (Fig. S4†) to illustrate the good size control and uniform dispersion of the alloy particles on the TiO₂ support, with EDX analysis demonstrating the presence of precious metals in keeping with nominal ratios (Table S1†). In addition, X-ray energy dispersive spectroscopy (X-EDS) spectrum imaging and point analyses of individual particles was performed as shown in Fig. 3 to demonstrate in all cases that intimate alloying of the constituent metallic elements has occurred. Previous studies of AuPdPt nanoparticles supported on TS-1 prepared by a conventional wet-impregnation methodology have demonstrated a strong correlation between nanoparticle size and elemental composition, with larger particles (>20 nm) typically being Au-rich.³² It is expected that the sol-immobilisation preparation methodology utilised here should facilitate better control over nanoparticle elemental composition.⁵⁰ In keeping with previous reports by Dimitratos et al.,⁵¹ we did not observe the development of any Au-core/Pd-shell morphologies in our sol-immobilised samples, that are typically found for AuPd nanoparticles prepared on oxide supports by impregnation methods in this case, the Au–Pd particles are a homogeneous random alloy as indicated by the STEM-XEDS elemental mapping (1% Au₁Pd₁/TiO₂, Fig. 3a). The Pt was not detectable by X-EDS mapping at very low concentrations (1% Au₁Pd₁Pt_0.01/TiO₂, Fig. 3b) but is clearly discernible at higher concentrations (1% Au₁Pd₁Pt₁/TiO₂, Fig. 3c) where it was found to be uniformly dispersed throughout the AuPd nanoparticles.

Fig. 3 Representative STEM-ADF micrographs and complementary XEDS elemental maps and integrated point spectra of individual alloy particles in (a) 1% AuPd/TiO₂, (b) 1% Au₁Pd₁Pt_0.01/TiO₂ and (c) 1% Au₁Pd₁Pt₁/TiO₂ catalysts.

Time-on-line studies comparing H₂O₂ synthesis rates over the bi-metallic 1% AuPd/TiO₂ and tri-metallic 1% AuPdPt/TiO₂ catalysts can be seen in Fig. 4a, with a stark difference in catalytic activity being observed between the bi- and tri-metallic variants. The greater catalytic activity of the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst is clear, with a H₂O₂ concentration of 0.22 wt% being reached over a time period of 0.5 h, which is significantly greater than that achieved over either the 1% Au₁Pd₁/TiO₂ (0.16 wt% concentration of H₂O₂) or 1% Au₁Pd₁Pt₁/TiO₂ (0.09 wt% concentration of H₂O₂) catalysts over the same reaction time. The enhanced activity of the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst is also highlighted through comparison of calculated reaction rates (Table S2†) at reaction times where there is assumed to be no contribution from subsequent degradation reactions. The rate of H₂O₂ synthesis over the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst was found to be over double that observed for the 1% Au₁Pd₁Pt₁/TiO₂ catalyst and 30% greater than the analogous bi-metallic 1% AuPd/TiO₂ catalyst. Furthermore, the H₂O₂ yield achieved over the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst remained stable at extended reaction times, reaching a H₂O₂ concentration of 0.25 wt% at a reaction time of 1.5 h, whereas this metric was significantly lower for both the 1% Au₁Pd₁/TiO₂ and 1% Au₁Pd₁Pt₁/TiO₂ catalysts, indicative of the comparatively higher H₂O₂ selectivity of the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst.

Fig. 4 Comparison of the catalytic activity as (a) a function of reaction time and (b) over sequential H₂O₂ synthesis reactions. H₂O₂ direct synthesis reaction conditions: catalyst (0.01 g), H₂O (2.9 g), MeOH (5.6 g), 5% H₂/CO₂ (420 psi), 25% O₂/CO₂ (160 psi), 0.5 h, 2 °C, 1200 rpm.

Evaluation of catalytic activity over multiple sequential H₂O₂ synthesis tests can be seen in Fig. 4b, with a marked enhancement in H₂O₂ concentration being observed for the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst compared to either the 1% Au₁Pd₁/TiO₂ or 1% Au₁Pd₁Pt₁/TiO₂ catalysts. After running the reaction eight consecutive times, the H₂O₂ concentration increased to a value of 0.97 wt%, over the 1% Au₁Pd₁Pt_0.01/TiO₂ material which is far superior to the yields of H₂O₂ achieved over the 1% Au₁Pd₁/TiO₂ (0.59 wt% H₂O₂) or 1% Au₁Pd₁Pt₁/TiO₂ (0.27 wt% H₂O₂) catalysts. Indeed the concentration of H₂O₂ achieved over the 1% Au₁Pd₁Pt_0.01/TiO₂ catalyst is comparable to that achieved in the initial stages of the current indirect method of industrial H₂O₂ production, prior to the use of multiple distillation steps to raise H₂O₂ concentrations to exceed ∼70 wt%.⁵²

With the requirement to re-use a catalyst successfully at the heart of green chemistry and the activity of homogeneous species towards H₂O₂ synthesis well known,¹¹ we next evaluated catalytic activity towards H₂O₂ synthesis and H₂O₂ degradation pathways (hydrogenation and decomposition) upon re-use. It can be seen that for all three catalysts evaluated the catalytic activity increased upon re-use compared to first use, under standard reaction parameters (Table 5), with a similar improvement in reaction rate at short reaction times, where the contribution from competitive degradation reactions are assumed to be negligible (Table S2†). We ascribe this to the rise in Pd⁰ content, at the expense of Pd²⁺ species, as determined by XPS (Table S3,† corresponding spectra in Fig. S2†). Numerous prior studies have reported an enhanced activity of Pd⁰-rich catalysts, towards both H₂O₂ synthesis and its subsequent degradation, compared to Pd²⁺ analogues.^53–55 As such, balancing the ratio of Pd species (Pd⁰ : Pd²⁺) is crucial to achieving a optimal catalytic performance. Analysis of the H₂O₂ synthesis reaction solution by ICP-MS (Table S4†) revealed the high structural stability of the supported 1% AuPdPt/TiO₂ catalysts during the H₂O₂ synthesis reaction. It should also be noted that a minor increase in mean particle size for all catalysts tested was observed after use in the direct synthesis of H₂O₂ (Table S5,† as determined from the bright field transmission electron micrographs presented in Fig. S4†).

Table 5 Catalyst re-usability towards direct H₂O₂ synthesis

Catalyst	Productivity molH2O2 kgcat^−1 h^−1		Degradation molH2O2 kgcat^−1 h^−1		Hydrogenation molH2O2 kgcat^−1 h^−1		Decomposition molH2O2 kgcat^−1 h^−1
	Fresh	Used	Fresh	Used	Fresh	Used	Fresh	Used
H2O2 direct synthesis reaction conditions: catalyst (0.01 g), H2O (2.9 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 25% O2/CO2 (160 psi), 0.5 h, 2 °C 1200 rpm. H2O2 degradation reaction conditions: catalyst (0.01 g), H2O2 (50 wt% 0.68 g) H2O (2.22 g), MeOH (5.6 g), 5% H2/CO2 (420 psi), 0.5 h, 2 °C 1200 rpm.
1% Au1Pd1/TiO2	81	117	257	355	165	206	92	149
1% Au1Pd1Pt0.01/TiO2	112	141	245	363	137	107	108	256
1% Au1Pd1Pt1/TiO2	30	37	271	429	48	119	223	310

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Conclusions

The addition of low quantities of Pt into AuPd nanoparticles results in a significant enhancement in catalytic selectivity and activity in the direct synthesis of H₂O₂ compared to AuPd or more Pt-rich AuPdPt analogues. This is attributed to a modification of Pd oxidation states and the formation of mixed Pd²⁺–Pd⁰ domains, which are well known to offer enhanced selectivity towards H₂O₂ compared to Pd⁰ or Pd²⁺ rich analogues. With increasing Pt addition to AuPd, the Pd⁰ content rises significantly with a corresponding loss of catalytic selectivity. The role of Pt in enhancing catalytic activity of supported AuPd nanoparticles can therefore be related to the electronic modification of Pd.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The China Scholarship Council (CSC) is gratefully acknowledged for the financial support given to X. G. The authors also acknowledge the National Key R&D Program of China (2016YFB0301600). We also wish to thank the Cardiff University electron microscope facility for use their instruments. The technical support from Mr Hiroaki Matsumoto and Mr Chaobin Zeng, Hitachi High-Technologies (Shanghai) Co. Ltd, for HR-STEM characterization is also greatly appreciated. This work is supported by National Natural Science Foundation of China (21673273, 21872163).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cy01079k

‡ These authors contributed equally to this work.

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