Photocatalytic Activity of Carbon Nitride Modified with a Ruthenium(II) Complex Having Carboxylic- or Phosphonic Acid Anchoring Groups for Visible-light CO₂ Reduction

Abstract

Semiconducting C 3 N 4 powder modified with a catalytic Ru(II) complex having carboxylic acid groups as anchors showed higher activity for CO 2 reduction into HCOOH under visible light ( > 400 nm) at the initial stage of the reaction, whereas an analogue anchored with phosphonic acid groups exhibited higher stability.Photocatalytic reduction of CO 2 into energy-rich chemicals has long been studied as an important half cycle of artificial photosynthesis.110 CO 2 reduction using illuminated semiconductor particles as heterogeneous photocatalysts may become the ultimate goal for large-scale applications. We have proposed hybrid photocatalysts that consist of a metal complex and a polymeric C 3 N 4 semiconductor as potential heterogeneous photocatalysts for CO 2 reduction under visible light.10 For example, C 3 N 4 modified with trans(Cl)-Ru{(X 2 bpy) 2 (CO) 2 Cl 2 } (bpy: 2,2¤-bipyridine; X = PO 3 H 2 or CH 2 PO 3 H 2 ) was capable of photocatalytically reducing CO 2 into HCOOH under visible light. Besides, isotope tracer experiments using 13 CO 2 indicated that the produced HCOOH originated not from carbon contaminants and/or C 3 N 4 itself, but from CO 2 gas. 10aIn any hybrid system consisting of a semiconductor and a metal complex, anchoring groups in the metal complex play vital roles of determining the efficiency of electron transfer from one side to the other. The combination of a Ru polypyridyl complex and a metal oxide would be the best known system among metal-complex/semiconductor hybrids such as dyesensitized solar cells and visible-light-driven water splitting assemblies. 1116 It is known that a phosphonic acid group can form stronger chemical bonding with the metal oxide surface over a wide pH range, as compared to a carboxylic acid group. However, the stronger bonding via phosphonate does not always lead to higher performance. Actually, it has been reported that the incident photon-to-current conversion efficiencies of dyesensitized solar cells using phosphonate anchoring groups were approximately 30% lower than that of carboxylate-anchored ones.14b Conversely, the H 2 evolution photocatalytic activity of a Ru-complex/TiO 2 with phosphonate was much higher than that of a carboxylic acid one.15 Suzuki et al. recently investigated the effects of anchoring groups on photocatalytic CO 2 reduction using nitrogen-doped Ta 2 O 5 , a p-type semiconductor, modified with [Ru(X 2 bpy) 2 (CO) 2 ] 2+ (X = COOH or PO 3 H 2 ), 3b which is a well-known electrocatalyst for CO 2 reduction.17 According to that study, an analogue anchored by phosphonate exhibited a higher rate of HCOOH production and improved catalytic turnover number as compared to carboxylate anchored one. Although the reason for the different performances achieved in different applications has not been systematically understood, it is intriguing to compare such an anchoring effect on photocatalytic CO 2 reduction using C 3 N 4 , because the surface of C 3 N 4 is completely different from that of ordinary metal-based semiconductors. 18Here we compared...

Atomic‐Level Reactive Sites for Semiconductor‐Based Photocatalytic CO2 Reduction

“…The selectivity of HCOOH exceeds 80% . A similar result was also reported that the phosphonic acid groups are efficient in photocatalytic CO 2 reduction . The binuclear ruthenium complex can bind robustly with carbon nitride in basic aqueous solution and form a Z‐Scheme heterojunction .…”

“…[106] A similar result was also reported that the phosphonic acid groups are efficient in photocatalytic CO 2 reduction. [138] The binuclear ruthenium complex can bind robustly with carbon nitride in basic aqueous solution and form a Z-Scheme heterojunction. [139,140] These two ruthenium metals have two different effects: one is the photosensitizing unit to form the Z-Scheme with carbon nitride and the other is Adv.…”

Activation of the Carbon Nitride Surface by Silica in a CO‐Evolving Hybrid Photocatalyst

“…[48] DMF undergoes decompositiond uring photocatalytic reactionm ore or less, generating formic acid whereas DMA does not. [23][24][25][26][27][28][30][31][32] Therefore, TEOA musth ave acted as ah ole scavenger in this case. The hybrid gave rise to appreciable CO formationw ith at urnover number (TON CO )o f 27 based on RuRe used after 5h irradiation( entry 1).…”

“…For the development of artificial photosynthetic schemes, variousm etal complexes [1][2][3][4][5][6][7][8][9][10][11][12][13] and semiconductors, [14][15][16][17][18][19] as well as their hybrids, [20][21][22][23][24][25][26][27][28] have been examined as photocatalystsf or CO 2 reduction. For the development of artificial photosynthetic schemes, variousm etal complexes [1][2][3][4][5][6][7][8][9][10][11][12][13] and semiconductors, [14][15][16][17][18][19] as well as their hybrids, [20][21][22]…”

“…[21] The same binuclear complex can be combined with other semiconductors, that is, CaTaO 2 N [22] and carbonn itride (C 3 N 4 ), [23] for constructing Z-Scheme photocatalytic systems for CO 2 reduction to HCOOH. [20,[24][25][26][27][28] An exceptioni s C 3 N 4 modified with trans(Cl)-Ru{bpy(PO 3 H 2 ) 2 }(CO) 2 Cl 2 (bpy = 2,2'-bipyridine),w hich has been shown to selectively produce CO in a N,N-dimethylacetamide (DMA)/methanol solution under visible light. [23] Thus, several combinationso fm etal-complex/semiconductor hybridsh avee xhibited activity for visible-light CO 2 reduction, yieldingH COOHa st he major product.…”