The Effect of Residual Palladium Catalyst Contamination on the Photocatalytic Hydrogen Evolution Activity of Conjugated Polymers

Abstract

for this application. [1][2][3][4][5][6] Recently, however, new classes of materials are being investigated for hydrogen evolution from water, such as nitrogen doped graphene oxide, [7] covalent organic frameworks, [8][9][10] conjugated polymer networks, [11][12][13][14] and linear conjugated polymers. [15][16][17][18][19][20] With the notable exception of graphitic carbon nitride, [21] most organic semiconductors have required an additional metal co-catalyst to produce an appreciable H 2 evolution rate, often introduced by Pt photo deposition, [22] or the addition of a molecular catalyst. [8,23,24] However, recent reports of photocatalysis employing both linear and cross-linked conjugated polymers suggest that these systems are able to produce H 2 under visible light irradiation without any added co-catalyst, at a much faster rate than commercial graphitic carbon nitride, even when the latter is subjected to Pt photodeposition. [11] Activity without the need for co-catalysts, which are often precious metals or molecular complexes with limited stability, [25] could be a very attractive feature of such polymer photocatalysts with the potential to vastly reduce the cost and complexity of current photocatalytic systems.A feature of these linear and cross-linked conjugated polymers that has not yet been systematically studied is the presence of significant quantities of residual Pd originating from their synthesis via Pd catalyzed polycondensation reactions. [11,12,15,26] It has previously been shown that the molecular Pd catalysts used in these reactions can decompose via deligation and subsequent Ostwald ripening to form metallic Pd 0 particles, which are strongly retained within the polymer matrix and are difficult to remove via classical purification techniques. [27][28][29][30][31] Similar to Pt 0 , Pd 0 is a highly active proton reduction electrocatalyst. [32][33][34] Therefore, it is possible that the residual Pd, dispersed within these polymer structures, facilitates H 2 evolution and removes the need for additional co-catalysts. Much uncertainty remains about the nature of the active site in conjugated polymer systems, and there are conflicting views in the literature on whether or not the residual amounts of Pd play a significant role. No correlation was seen between Pd concentration and hydrogen evolution reaction (HER) rate in a range of conjugated micro porous polymers, [11] and in a series of linear polymersThe effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the h...

Photocatalytically Active Ladder Polymers

“…The catalytic activity of the materials does not correlate with the residual palladium content, but it is unclear whether the threshold for an effect of the residual palladium on the photocatalytic performance has been reached. 9,34 Since the TA spectrum of cLaP2 shows a persistent long-lived feature, which could be attributed to a deep-trapped charge ( Figure 5, right), we loaded in situ cLaP1 and cLaP2 with a platinum co-catalyst (1 wt. %).…”

“…However, it has been shown that residual palladium originating from the Suzuki-Miyaura coupling reaction can act as a co-catalyst. 9,34 Inductively coupled plasma mass spectrometry (ICP-MS) measurements show that the residual palladium content decreases from 0.83 wt. % for cLiP1 to 0.38 and 0.36 wt.…”

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

“…Instead, photogenerated electrons and holes are extracted at catalytic sites on the photocatalyst surface, which may be inherently present on the semiconductor or added in the form of additional cocatalysts such as transition metal or metal oxide nanoparticles (NPs). [243][244][245] Photogenerated electrons and holes are then transferred from their respective catalytic sites to oxidants and reductants in the surrounding matrix (Figure 9). If the overall redox reaction has a positive ΔG, then the photocatalyst can convert light to chemical energy that is stored in the reaction products.…”

“…Increasing the photocatalyst surface area by processing bulk semiconductors into NPs or nanofibers is a simple and commonly employed strategy to increase their EQE. [9,244,245,264,[275][276][277][278][279] Other strategies include designing porous semiconductors such as conjugated microporous polymers and covalent-organic frameworks (COFs) which can allow ingress of hole or electron scavengers into the semiconductor pores. [8,[280][281][282] If the photocatalyst needs to operate in a liquid matrix, efficient wetting of the pores by the solvent/scavenger solution is essential to maximize efficiency.…”

Designing conjugated porous polymers for visible light-driven photocatalytic chemical transformations

“…92 Linear conjugated polymers shown by Kosco et al demonstrated that the remaining Pd concentration in the polymers (B0.1 wt%) had a critical effect on the photocatalytic H 2 production rate, and the pure polymer after the complete removal of Pd species was inactive for H 2 evolution. 114 This implies that the residual Pd species should be considered to evaluate the photocatalytic behavior of CPPs. Incorporation of UV-active TiO 2 nanoparticles was recently carried out upon CPP photocatalysts.…”