Controlling the performance of a silver co-catalyst by a palladium core in TiO2-photocatalyzed alkyne semihydrogenation and H2 production

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Highlights

  • Pd@Ag/TiO2 photocatalyst was prepared by using a two-step photodeposition method.

  • 4-Octyne was selectively hydrogenated to cis-4-octene in alcohol suspensions of Pd@Ag/TiO2.

  • H2 as the by-product was barely produced during the hydrogenation.

  • The activation energy for the hydrogenation was smaller than that for the H2 evolution.

  • Ag itself was inactive for the hydrogenation and Ag was activated by the presence of Pd core.

Abstract

Titanium (IV) oxide (TiO2) having palladium (Pd) core-silver (Ag) shell nanoparticles (Pd@Ag/TiO2) was prepared by using a two-step (Pd first and then Ag) photodeposition method. The core-shell structure of the nanoparticles having various Ag contents (shell thicknesses) and the electron states of Pd and Ag were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. The effect of the Pd core and the Ag shell was evaluated by hydrogenation of 4-octyne in alcohol suspensions of a photocatalyst under argon and light irradiation. 4-Octyne was fully hydrogenated to 4-octane over Pd/TiO2, whereas 4-octyne was selectively hydrogenated to cis-4-octene over Pd(0.2)@Ag(0.5)/TiO2. Further increase in the Ag content resulted in a decrease in the conversion of 4-octyne. Pd-free Ag/TiO2 was inactive for hydrogenation of alkyne and induced coupling of active hydrogen species (H2 production). Photocatalytic reactions at various temperatures revealed that the change in selectivity (semihydrogenation or H2 production) can be explained by the difference in values of activation energy of the two reactions. An applicability test showed that the Pd@Ag/TiO2 photocatalyst can be used for hydrogenation of various alkynes to alkenes.

Introduction

Metal nanoparticles have often been used as co-catalysts for semiconductor photocatalysts when the position of the conduction band bottom (CBB) is close to the reduction potential (RP) of target compounds [[1], [2], [3]]. The most popular co-catalyst is platinum (Pt), which has been used for hydrogen (H2) production through proton (H+) reduction (RP: 0 V vs NHE) with photogenerated electrons over titanium(IV) oxide (TiO2) with an anatase structure (CBB: ca -0.3 V vs NHE) [[4], [5], [6], [7], [8], [9]]. Metal nanoparticles are also utilized as co-catalysts for photocatalytic conversion of organic compounds. Palladium (Pd) exhibits excellent performance as a co-catalyst for dechlorination of chlorinated compounds [10] and hydrogenation of unsaturated Csingle bondC bonds [11] over a TiO2 photocatalyst. Silver (Ag) [12], copper (Cu) [13] and rhodium (Rh) [14] also show specific and selective performance for photocatalytic reductive conversion of organic compounds. These results of past studies indicate that the combination of TiO2 and metal nanoparticles widens the possibility of photocatalytic conversion.

Reductive conversion over a TiO2 photocatalyst with a co-catalyst can be roughly separated into two processes [[11], [12], [13], [14]]: 1) formation of active hydrogen species over the co-catalyst through the reaction of photogenerated electrons in the conduction band of TiO2 and H+ in the liquid phase and 2) reaction of a target compound and active hydrogen species over the co-catalyst. In other words, photocatalytic reduction over TiO2 is regarded as a combination of photoinduced electron production over TiO2 and catalytic reduction (hydrogenation) over the co-catalyst. As a side reaction, H2 evolution by coupling of H+ occurs if process 2) is slow over the co-catalyst. This means that the property of the co-catalyst loaded on TiO2 controls the performance (reactivity and selectivity) of the photocatalytic reduction.

There are several methods to enhance the catalytic performance of metal catalysts and the use of two metals (bimetallic system) is one of the effective methods [[15], [16], [17], [18], [19]]. A bimetallic system can be roughly classified into alloy type and core-shell type. In the former type, it is generally difficult to understand the function of each element because the mixing level greatly depends on the preparation method, and the distribution of two elements on the surface and in the bulk (depth direction) should be considered for discussion of the catalytic performance. On the other hand, due to the simpler distribution of two elements in the core-shell type, discussion of the catalytic performance of the bimetallic system would be easier than that of the alloy type. In our previous study, we achieved H2-free semihydrogenation of alkyne over a Cu-loaded TiO2 photocatalyst [13]. This photocatalyst converts alkynes to cis-alkenes selectively with complete suppression of isomerization and overhydrogenation of cis-alkenes, and the reaction rate greatly increased at a slightly elevated temperature with maintenance of a high level of cis-selectivity [20]. In photocatalytic semihydrogenation of alkynes, catalytic performance of Cu metal was greatly enhanced by introduction of a Pd core [21]. Inspired by the results obtained by using the Pd core-Cu shell, we were interested in another shell metal as the co-catalyst for semihydrogenation of alkynes. Silver metal exhibited negligible activity as a co-catalyst for photocatalytic alkyne hydrogenation [13]. In the field of catalytic (not photocatalytic) semihydrogenation under H2, Pd core-Ag shell nanoparticles (Pd@Ag) supported on hydroxy apatite hydrogenated alkynes to alkenes in high yields [15]. The authors of the paper reported that 1) H2 was supplied to the Ag shell from the Pd core, 2) the Ag shell worked as active sites for semihydrogenation, and 3) Ag played a role to inhibit overhydrogenation of alkenes over Pd. Since the mechanisms of catalytic hydrogenation in the presence of H2 and photocatalytic hydrogenation in the absence of H2 are different, investigation of photocatalytic hydrogenation over TiO2 having Pd@Ag (Pd@Ag/TiO2) and discussion of the functions of Pd and Ag in alkyne hydrogenation are valuable for designing photocatalysts having core-shell particles.

We prepared Pd@Ag/TiO2 by a two-step photodeposition method [22,23] and used it for photocatalytic hydrogenation of alkyne in an alcohol suspension without the use of H2 gas. We report here 1) characterization of Pd@Ag/TiO2 having various Ag contents, 2) evaluation of photocatalytic alkyne hydrogenation over Pd@Ag/TiO2 and 3) discussion of the functions of Pd and Ag in the alkyne hydrogenation based on the kinetics of Pd@Ag/TiO2.

Section snippets

Preparation of Ag/TiO2 and Pd/TiO2

Palladium(II) chloride (PdCl2) and silver(I) nitrate (AgNO3) (FUJIFILM Wako Pure Chemical Co., Tokyo, Japan) were used as received without further purification. In 10 cm3 of a 50 vol% aqueous 2-propanol solution containing PdCl2 or AgNO3 in a Pyrex test tube, TiO2 (P 25 supplied by Nippon Aerosil, Tokyo, Japan) was suspended, and the gas phase (air) of the test tube was replaced with argon (Ar). After plugging the test tube with a rubber septum, the suspension under continuous stirring was

Characterization of Pd(0.2)@Ag(X)/TiO2

Fig. 1(a) shows a TEM photograph of Pd(0.2)/TiO2, indicating that Pd nanoparticles were successfully loaded on TiO2. From the particle size distribution (Fig. S1(a)), the average diameter of the particles (Dave) was determined to be 2.5 nm. Nanoparticles were also observed in TEM photographs of Pd(0.2)@Ag(X)/TiO2 (Fig. 1(b)–(f)) and the values of Dave were larger than that of Pd(0.2)/TiO2 (Table 1), indicating that Ag was deposited on Pd nanoparticles. Table 1 also shows the thickness of the Ag

Conclusion

Pd@Ag/TiO2 having various Ag contents was prepared by using a two-step photodeposition method and used for photocatalytic hydrogenation of alkyne in an alcohol suspension without the use of H2 gas. Almost quantitative conversion of 4-octyne to cis-4-octene was achieved over Pd(0.2)@Ag(0.5)/TiO2, while evolution of H2 as the by-product barely occurred during the hydrogenation, resulting in high efficiency of AHS in photocatalytic hydrogenation. Reactions at various temperatures revealed that the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Shota Imai: Conceptualization, Investigation, Writing - original draft. Yasumi Kojima: Conceptualization, Investigation. Eri Fudo: Investigation. Atsuhiro Tanaka: Validation, Writing - review & editing. Hiroshi Kominami: Supervision, Validation, Writing - review & editing.

Acknowledgements

This work was partly supported by JSPS KAKENHI Grant Numbers 20H02527. A.T. is grateful for financial support from the Faculty of Science and Engineering, Kindai University.

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