Platinum Group Metal Phosphides as Efficient Catalysts in Hydroprocessing and Syngas-Related Catalysis

Abstract

Platinum group metal phosphides are reviewed as catalytic materials for hydroprocessing and syngas-related catalysis. Starting from synthetic procedures leading to highly disperse nano-particular compounds, their properties in the applications are discussed and compared with relevant benchmarks, if available. Regarding their mode of action, two confronting mechanistic scenarios are presented: (i) a cooperative scenario in which catalytic sites of different functionalities are active in hydroprocessing and (ii) single site catalysis, which appears to be the relevant mode of action in syngas-related catalysis and which occurs over “frustrated” active sites.

1. Introduction

Transition metal phosphides, although well explored from their solid-state chemistry, are in large parts under-exploited as catalytic materials. The prime focus of research on transition metal phosphides in catalysis was traditionally hydrogenolysis and hydrogenation reactions. Although the phosphide materials in many cases beat traditional benchmarks, up to now industrial applications are limited to nil. Catalysis beyond catalysis involving hydrogen has, up to now, only been sparsely reported; we try to shed light on that aspect by discussing options in syngas-related chemistry. In this review, we try to summarize the major findings for this materials class and, more specifically, for the platinum group metal phosphides, as they stand out for their catalytic properties compared to the other phosphide materials and alternative standards.

2. Structural Discussion of Platinum Group Metal Phosphides

Metals and phosphorus are known to form a large number of crystalline compounds, which display a high variety of metal to phosphorus ratios and a variety of crystalline structures [1]. The ICSD counts 760 entries for binary and 2300 entries for ternary phosphides. As expected, this variety in stoichiometry and structure can also be observed within the binary phosphides of the platinum group metals (PGM), where 23 binary compounds with 16 different structures are reported (

Table 1. Overview of the 23 reported binary phosphides, their 16 different structures, and the corresponding space groups of the platinum group metal (PGM) phosphides [1].

Comp.	Structure	Space Group	Ref.	Comp.	Structure	Space Group	Ref.	Comp.	Structure	Space Group	Ref.
RhP2	CoSb2	P 1 21/c 1	[4]	RuP	MnP	P b n m	[5]	Pd15P2	Pd15P2	R -3 H	[6]
IrP2	CoSb2	P 1 21/c 1	[4]					Pd6P	Pd6P	P 1 21/c 1	[7]
RuP2	FeS2	P n n m	[8]					Pd4.8P	Fe3C	P n m a	[9]
PdP2	NiP2	I 1 2/c 1	[10]					Pd3P	Fe3C	P n m a	[11]
PtP2	NiP2	P a -3	[12]					Pd5P2	Fe3C	P n m a	[9]
RhP3	CoAs3	I m -3	[13]					Pt5P2	Pt5P2	C1 2/c1	[14]
PdP3	CoAs3	I m -3	[13]					Pd7P3	Pd7P3	R 3 H	[15]
IrP3	CoAs3	I m -3	[13]					Ru2P	Co2P	P b n m	[16]
RuP3	RuP3	P -1	[17]					Rh2P	anti-CaF2	F m -3 m	[1,18]
RuP4	OsP4	P 1 21/c 1	[2,19]					Ir2P	anti-CaF2	F m -3 m	[13]
								Rh3P2	anti-PbFCl	P -4 m 2	[20]
								Rh4P3	Rh4P3	P n m a	[21]

). The phosphides of the PGM, similar to the transition metal phosphides, can be divided into three groups according to the metal to phosphorus ratio [1]:

• polyphosphides (with nP > nM),
• monophosphides (with nP = nM),
• metal-rich phosphides (with nP < nM).

The following review will start with a short overview of some compounds from the three groups, focusing on the phosphides of rhodium since catalytic materials containing rhodium phosphides have the largest amount of reported investigations as materials for catalytic applications in hydroprocessing and syngas-related catalysis. Further overviews of the structure of other binary and ternary metal phosphides can be found in Pöttgen [1], Kuz’ma [2], and Aronsson [3].

The polyphosphides of the PGMs are, as other polyphosphides, characterized by the formation of P–P bonds. The compounds RhP₃, PdP₃, and IrP₃ crystallize in the CoAs₃ structure [13] where the phosphorus atoms are coordinated to octahedrons. Further, the formation of P₄⁻⁴ rings can be observed (Figure 1e). Other binary polyphosphides crystallize in the pyrite structure and in other related structures. RuP₂ crystallizes in the FeS₂ structure (cubic) [8] and PtP₂ and PdP₂ crystallize in the NiP₂ structure [10,22]. RhP₂ (Figure 1d) and IrP₂ both crystallize in the CoSb₂ structure (monoclinic) [4]. In all three structures the metal coordinates the phosphorus to octahedrons, which are connected to each other by the formation P₂ dimers (Figure 1d).

The only reported monophosphide of the platinum metal group is RuP, which crystallizes in the MnP structure [5], just as the other transition metal phosphides (NiP, MnP). This structure can be distinguished by the formation of phosphorus zigzag chains along the c-axis.

Metal-rich phosphides are characterized by the formation of metal to metal bonds. Certain metal-rich PGM phosphides crystallize in a number of unique structure types compared to other transition metal phosphides that cannot be found in any other phosphide compound (Pd₁₅P₂, Pd₆P, Pt₅P₂, Pd₇P₃, and Rh₄P₃). Ru₂P crystallizes in the Co₂P structure, which is related to the anti-PbCl structure [16]. Characteristic of this structure is the formation of a distorted tetragonal prism ordered in chains along the b-axis (Figure 1f). Rh₂P and Ir₂P crystallize in the anti-CaF₂ structure [1,18]; in this structure, the metal coordinates the phosphorus atoms to tetrahedrons (Figure 1a).

The phosphides of rhodium are among the most investigated compounds of all the PGM phosphides in the field of heterogeneous catalysis. There are five different phosphides of rhodium (Rh₂P, Rh₃P₂, Rh₄P₃, RhP₂, and RhP₃) [23]. The compounds Rh₃P₂ and Rh₄P₃ both crystallize in specific structure types. The Rh₃P₂ structure is characterized by the coordination of the phosphorus to tetrahedrons and square pyramids; planes of tetrahedrons and pyramids are stapled over each other (Figure 1b). In the Rh₄P₃ structure, the phosphorus is coordinated to square pyramids (Figure 1c).

The structures of some rhodium phosphides are isomorphic with some other metal phosphides. The phosphides RhP₃, IrP₃, CoP₃, and NiP₃ all crystallize in the CoAs₃ structure. The phosphides RhP₂, IrP₂, and CoP₂ all crystallize in the CoSb₂ structure (Figure 1f). Finally, the metal-rich phosphides Rh₂P and Ir₂P both crystallize in the anti-CaF₂ structure; up to now, no ternary compounds have been reported. Common to all of the compounds is that with increasing metal to phosphorus ratio, the metallic properties of the phosphides increase. This can be observed especially regarding the distance between the rhodium atoms of the metal-rich rhodium phosphides. The distance between the rhodium atoms in Rh₄P₃ lies between 0.280 and 0.294 nm, in Rh₃P₂ between 0.282 and 0.285 nm, and in Rh₂P only 0.276 nm [1,3]. This last distance is just slightly larger than the distance between two rhodium atoms in metallic rhodium (0.27 nm).

3. Synthetic Pathways to Supported Platinum Group Metal Phosphides

As platinum metals are scarce and per unit mass expensive, the application of bulk materials for any application of commercial relevance can only have model character. A number of groups have embarked on developing ways to support nanoparticles of PGM phosphides. A variety of pathways for the preparation of transition and platinum group metal phosphides, either as a bulk material or as a support material for catalysis applications, have been reported. An overview of the different preparation pathways has been given by Aronsson et al. [3], Kuz’ma et al. [2,4], Pöttgen et al. [1], Prins et al. [5,24] and Oyama et al. [25,26]. Aronsson, Kuz’ma, and Pöttgen described the preparation of bulk binary and ternary phosphides and their structure while Prins and Oyama focused mainly on the preparation of supported transition metal phosphides for catalysis applications.

The three routes to be described next are the most frequently reported pathways in the literature for the preparation of supported platinum group metal phosphides:

• the incipient wetness impregnation method combined with reductive treatment,
• the conversion of supported metals or metal oxides with a phosphorus-containing compound combined with reductive treatment,
• the deposition of metal phosphide nanoparticles prepared via colloidal preparation routes on a support material.

Figure 2 gives a schematic overview of the three pathways and

Table 2. Overview of the methods applied for the preparation of supported phosphides.

Method	Compound	Support	Ref.
Incipient wetness impregnation	Pd3P	SiO2	[28]
	Pd4.8P	SiO2	[6,29]
	Rh2P	La2O3/Al2O3	[48]
		SiO2	[27,28,29,30,31,33,35]
		TiO2	[30]
		Al2O3	[30,33]
		MgO	[30]
		ZrO2	[30]
		MFI	[33]
		Na-Beta	[34]
		Na-MFI	[34]
		Na-MOR	[34]
	RuP	MCM-41	[36]
		SiO2	[37]
	Ru2P	MCM-41	[36]
		SiO2	[37]
Conversion of a supported metal or metal oxide precursor	Pd3P	SiO2	[41]
	Pd5P2	SiO2	[41]
		Active Carbon	[42]
	PdP2	Active Carbon	[42]
	Rh2P	Al2O3	[43]
	Ru2P	SiO2	[41]
	RuP	SiO2	[41]
Deposition of nanoparticles	Pd3P	SiO2	[28]
	Rh2P	SiO2	[28,47]

lists different supported PGM phosphides, the pathways by which they are prepared, and the relevant references from the literature.

The incipient wetness impregnation method (Figure 2a) consists in the preparation of a supported metal phosphate precursor that is then heated under a reductive atmosphere (H₂ or minor amounts of H₂ in N₂) to form the metal phosphide. First, the support material is impregnated with a solution containing the metal and phosphorus precursor. Nitrates and chlorides of the metals and (NH₄)₂HPO₄ are the most commonly used precursors. The samples are dried after the impregnation and further fired under air to decompose the nitrate or chloride compounds and to form the oxide and phosphate precursors. Last, the sample is reduced under hydrogen-containing atmosphere to form the corresponding phosphides.

The incipient wetness impregnation method combined with reductive treatment can be applied to prepare a broad range of metal phosphides, metal to phosphorus ratios (mainly metal-rich phosphides), and support materials for the preparation of dispersed PGM phosphides.

Hayes et al. [27] applied the incipient wetness impregnation for the preparation of Rh₂P on SiO₂ for the hydrodesulfurization (HDS) of thiophen with a rhodium (Rh) loading of 5 wt %. First, a solution of RhCl₃xH₂O and NH₄H₂PO₄ with an Rh:P = 1 in ultra-pure water is prepared and impregnated on the support material SiO₂, followed by a drying step at 318 K, calcination at 773 K, and reduction at 923 K under pure H₂ flow. The characterization via XRD shows the formation of Rh₂P without metallic Rh impurities. TEM investigations show well dispersed Rh₂P particles with an average size of approximately 5 nm.

Habas et al. [28] applied the method of the incipient wetness impregnation, among others, for the preparation of Rh₂P and Pd₃P on SiO₂ in order to compare the performance of the samples prepared by the different pathways as catalysts for the hydrodeoxygenation (HDO) of acetic acid as a model compound. Kanda et al. [6,29] applied the incipient wetness impregnation method for the preparation of Ru₂P, PtP₂, Rh₂P, and Pd_4.8P on SiO₂ to test the metal phosphides as catalysts in the HDS of thiophen. On later publications, Kanda et al. [30] focused on the preparation of Rh₂P on different support materials (SiO₂, TiO₂, Al₂O₃, MgO, and ZrO₂) and the effect of the reduction temperature and the effect of phosphorus loading [31] on the formation of Rh₂P. The compound Rh₂P can be successfully prepared on all the support materials investigated. Further, Kanda observed the formation of Rh₂P on SiO₂ at lower reduction temperatures (350 °C) with a high excess of phosphorus (P:Rh = 2), while higher temperatures are required (550 °C) to form phase-pure Rh₂P on SiO₂ samples with a lower excess of phosphorus (P:Rh = 1). This shows that the addition of P enhances the formation of Rh₂P at lower temperatures, but an excess of P leads to the formation of RhP₂ at higher reduction temperatures. Further, the TEM investigation shows that the average particle diameter of the Rh₂P particles on the catalysts increases with increasing reduction temperature and P content. A sample reduced at 550 °C with 1.5 wt % P loading has an average particle diameter of 6.4 nm, while a sample with 3.0 wt % has an average particle diameter of 8.8 nm. Kanda et al. [32] also compared the use of acetylacetonato (acac) precursors and triphenylphosphine (TPP) with the use of chloride and phosphate precursors for the preparation of Rh₂P and Pd_4.8P on SiO₂. When the acac precursor and TPP are employed, slightly lower reduction temperatures are needed for the formation of the phosphides. In the PXRD patterns, the group observed phase-pure Rh₂P on SiO₂ at a reduction temperature of 450 °C when the acac and TPP precursors are used, while a temperature of 650 °C is needed when chloride and phosphate are used as precursors.

Sawada et al. [33,34] prepared Rh₂P catalysts supported on SiO₂, Al₂O₃, and different zeolites (Na-beta, Na-MFI, Na-MOR) for the HDS of thiophen via the incipient wetness impregnation method. Sawanda et al. [33] investigated the effect of Na doping on the formation of Rh₂P. The addition of Na to samples lowers the required reduction temperature of the samples supported on zeolites or Al₂O₃ by weakening the interactions between the Al and the phosphate precursor [33].

Alvarado et al. [35] applied the incipient wetness method for the preparation of Rh₂P on SiO₂ and investigated the effect of the reduction temperature on the performance of the samples in the hydroformylation of ethene and propene. XRD patterns show the formation of phase-pure Rh₂P at temperatures of 500 °C that remain stable at temperatures of 900 °C. TEM images reveal a slight increase in the particle size by increasing the reduction temperature from 250 to 900 °C. Further, HRTEM analysis of a single particle from the sample reduced at 900 °C reveals high crystallinity of Rh₂P with cubic structure.

Guan et al. [36] and Wang et al. [37] also applied the method of the incipient wetness for the preparation of supported RuP and Ru₂P on MCM-41 and on SiO₂, respectively. Guan used RuCl₃·3H₂O and NaH₂PO₂·H₂O as precursors for the incipient wetness impregnation and a static Ar environment for the thermal treatment of the samples. From the results, Guan proposed a mechanism for the formation of the phosphides: first, PH₃ is formed by the decomposition of NaH₂PO₄, which reduces RuCl₃ to Ru. Further formation of PH₃ allows Ru to form Ru₂P or RuP, depending on the amount of PH₃ present.

Wang et al. [37] compared two similar preparation methods: the conventional method, which implies the use of RuCl₃ and NH₄H₂PO₄ as precursors for the incipient wetness impregnation; and a new method applying RuCl₃ and triphenylphosphine (TPP) dissolved in ethanol for a slurry impregnation. Both compounds, RuP and Ru₂P, can be prepared with the two methods by modifying the metal to phosphorus ratios; however, lower reduction temperatures are needed for the formation of the phosphide when TPP is used.

Later publications investigated the doping of Ni₂P with noble metals such as Ir, Rh, Ru, and Pt on SiO₂ [7,38] or Zeolites [39]. The groups applied the method of the incipient wetness, preparing first a solution with the two metals and the phosphorus precursor and impregnating it on the support material, followed by a drying, calcination, and reduction step. XRD patterns show the formation of Ni₂P with no impurities.

A further synthetic pathway for the preparation of supported metal phosphides consists of the conversion of supported metals or metal oxides with a phosphorus precursor to the corresponding phosphides (Figure 2b).

First, the preparation of a supported metal or metal oxide takes place. This can be carried out, for example, via incipient wetness of the support material with a solution of the metal precursor, followed by a calcination step and subsequent reduction to obtain the supported metal. For the preparation of the phosphides, a phosphorus precursor is added to the supported metal or metal oxide followed by a thermal treatment.

Muetterties et al. [40] described this method in 1974 for the preparation of the phosphides of Ru, Rh, Pd, Pt, and other transition metals (Ni, W, Co, Fe) on Al₂O₃. First, the metal or metal oxides are prepared on the support material. The supported metals and metal oxides are then treated under a mixture of He and PH₃ in a temperature range of 250–300 °C to form the phosphides. Unfortunately, the materials were not characterized by XRD to confirm the formation of the phases.

Bowker et al. [41] prepared first RuO₄ and PdO on SiO₂, which is then impregnated with NH₄H₂PO₂ and heated to 700 °C under a H₂ atmosphere for the formation of the phosphides. By changing the phosphorus to metal ratio during the preparation, the phosphides RuP or Ru₂P and Pd₅P₂ or Pd₃P can be prepared.

Kucernak et al. [42] used commercial Pd on active carbon, which is then heated in trioctylphosphine (TOP) to 300 °C for the formation of Pd₅P₂ and PdP₂. A similar method was applied by Kanda et al. [43] for the preparation of Rh₂P on Al₂O₃. First, Rh on Al₂O₃ is prepared via incipient wetness impregnation, calcination, and reduction. The Rh on Al₂O₃ is impregnated with a TPP/hexane solution, dried under inert conditions, and reduced under a 5% H₂ in N₂ flow at different temperatures. Kanda also prepared a further catalyst using the (NH₄)₂HPO₄ to compare the reduction temperature of the two samples. By using TPP as a phosphorus precursor instead of using a phosphate precursor, the reduction temperature for the formation of the phosphide can be decreased from 800 to 650 °C.

The last method discussed here consists in the deposition of phosphide nanoparticles, prepared via colloidal methods, on the support material (Figure 2c). First, the platinum group metal phosphide nanoparticles are prepared and, on a second step, deposited on the support material. The preparation of the nanoparticles beforehand seems to allow better control of the size and size distribution of the PGM phosphides. The preparation of phosphide nanoparticles can be carried out by preparing first the metal nanoparticles and then reacting them with PH₃ [44]_. Another highly reactive possible phosphorus precursor is P(SiMe₃)₃, which allows a one-pot preparation of the phosphide nanoparticles and has been successfully applied for the preparation of Ni₂P nanoparticles. A further, less reactive but versatile and more and more common phosphorus precursor for the preparation of phosphide nanoparticles is TOP. Carenco et al. [44] reviewed the preparation of nano-scaled metal phosphides where they reported the development of the synthetic pathways from highly active precursors such as PH₃ and metal carbonyls to more stable ones such as TOP and metal acetylacetonates. Henkes et al. [45] showed the versatility of TOP as a phosphorus precursor using it for the preparation of different metal phosphides (Cu, Fe, Ni, Co, Ag, In, Rh, Pd, Zn) as nanoparticles, powder, foils, wires, and thin films. Henkes et al. [46] also prepared Rh nanoparticles with different shapes and morphologies and converted them to Rh₂P using TOP. The phosphides keep the same shape as the metal templates, as shown on TEM images. A further method for the preparation of phosphide nanoparticles is the use of single-source molecular precursors such as Ni(PPh₃)₂(CO)₂, Rh(PPh₃)₂(CO)Cl, or Pd(PPh₃)₄, mixed in oleylamine and 1-octadecene, heated to 300 °C for 1 h and cooled to room temperature. The nanoparticles are then washed with ethanol and centrifuged. Single-source precursors were used by Habas et al. [28] and by Griffin et al. [47] for the preparation of Ni₂P, Rh₂P, and Pd₃P on SiO₂. The Rh₂P nanoparticles are formed using Rh(PPh₃)₂(CO)Cl mixed in an oleylamine and 1-octadecen solution, which is heated up to 300 °C. The nanoparticles are recovered, washed with ethanol, re-dispersed in CHCl₃, and added to a suspension of SiO₂ in CHCl₃, sonicated and dried to form the finished catalyst.

4. Rh₂P as a Prototypic Example for PGM Phosphides in Hydrotreating Reactions

Of all the rhodium phosphides, only Rh₂P has been reported to be a suitable catalyst material for different reactions, mainly the HDS and HDO of hydrocarbons. Most of the investigations regarding the application of metal phosphides in hydrotreating reactions have been driven by the motivation to find improved hydrotreating catalysts due to the increasing environmental regulations limiting the content of sulfur in transportation fuels and due to the diminishing quality of oil feedstocks [25,26,49]. The most investigated metal phosphide is Ni₂P [24,25,26,49], which shows high activity in the HDS of dibenzothiophene (DBT) where it hydrogenates preferably the sulfur compound than the aromatic ring [8,50]. The activity of the Ni₂P catalysts supported on SiO₂ is comparable to the activity of a commercially available Ni-Mo-S/Al₂O₃ catalyst. Further investigations show the higher activity of CoP for the HDN of chinolin [50], of MoP and Ni₂P for the HDO of palmitic acid [51], or Ni₂P for the HDO of benzofuran [50]. Within the frame of these investigations, different PGM phosphides have been tested as catalyst materials. Here, the Rh₂P/SiO₂ material shows a significantly higher activity in the HDS of DBT than Rh/SiO₂ and higher than the Ni₂P/SiO₂ material [27]. Further investigations have shown that Rh₂P has a higher activity in the HDS of thiophene than all the other PGM phosphides [29]. Due to the higher intrinsic activity of the Rh₂P catalyst, different publications have focused on the application of Rh₂P as a catalyst material for hydrotreating reactions [27,28,29,30,31,32,33,34,43,47,52], for the hydrogen evolution reaction [53], and for the hydroformylation [35]. Reviews focusing on the metal phosphides in general and their applications as catalyst materials can be found elsewhere [24,25,26,49,54,55].

The next section of this review deals with the application of Rh₂P as a catalyst material for the reactions of HDS and HDO. Investigations regarding the application of Rh₂P as a catalyst for the HDN have not been published yet to our knowledge.

4.1. Rh₂P as a Catalyst Material for HDS

Among the first investigations on the application of Rh₂P as a catalyst material in HDS are the articles published by Hayes et al. [27] who prepared a series of supported Rh-based catalysts (Rh₂P, Rh, sulfonated Rh on SiO₂, Rh loading of 5 wt %) and compared their performance in the HDS of DBT. The findings of their investigation show that the Rh₂P/SiO₂ catalyst material is more active in the HDS of DBT (fixed-bed reactor, temperature range of 225–300 °C, 3.0 MPa, Feed: decalin solution with 3000 ppm DBT) than the sulfonated Rh/SiO₂ and the Rh/SiO₂ catalysts; and is also more active than the commercially available Ni-Mo/Al₂O₃ catalyst used as a benchmark, as shown on Figure 3a.

Table 3. Comparison of the performance of different catalysts in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) in the investigation of Hayes et al. [9] (275 °C, 3.0 MPa, 3000 ppm HDS in decalin).

Catalyst	Conversion	BP 1	CHB 2	BCH 3
Rh2P/SiO2	99.0	4.1	52.1	43.8
Sulf. Rh/SiO2	70.1	52.7	46.1	1.2
Rh/SiO2	30.2	77.0	18.9	4.1
Ni-Mo/Al2O3	84.2	66.2	30.8	3.0

¹ BP: 4,4′-Biphenyl; ² CHB: Cyclohexylbenzene; ³ 1,1′-Bi(cyclohexyl).

shows the results of the investigation at 275 °C where Rh₂P/SiO₂ shows significantly higher conversion values (99.0%) than the benchmark catalyst Ni-Mo/Al₂O₃ (84.2%). The product distribution shows that the Rh₂P/SiO₂ catalyst favors the hydrogenation of the first aromatic ring and then the removal of S, and thus the high selectivity values for cyclohexane–benzene.

Further findings of the investigation are the higher tolerance of the Rh₂P catalyst toward H₂S than the Rh/SiO₂ catalyst. Both materials were tested before and after co-feeding of 50 kPa H₂S. The conversion of DBT at 275 °C decreases from 80% to 22% for the Rh₂P catalyst and from 65% to 12% for the Rh/SiO₂ catalyst. The normalization of the HDS activity per mol of metal allowed the authors to make a comparison with other catalyst systems such as a 25 wt % Ni₂P/SiO₂ catalyst. Hayes et al. showed that the per mol metal HDS activity of the Rh₂P/SiO₂ catalyst at 275 °C is approx. eleven times higher than the activity of the Ni₂P/SiO₂ catalyst, as Figure 3b and the values in

Table 4. Normalized HDS activity of different catalyst materials (275 °C, 3.0 MPa, 3000 ppm DBT in decalin) [9].

Catalyst	HDS Activity [µmol DBT/mol Me/s]
Rh2P/SiO2	330.4
Sulf. Rh/SiO2	241.0
Rh/SiO2	102.9
Ni2P/SiO2	32.5
Fe0.03P1.97	35.2

show, and so increasing the interest in Rh₂P as a suitable catalyst material for the HDS reaction.

Most of the work performed on Rh₂P as a hydrogenolysis active catalyst material for the HDS reaction has been reported by the group of Yasuharu Kanda. In a first publication [29], the performance of PGM metals and PGM phosphides supported on SiO₂ (Rh₂P, Pd_4.6P, Ru₂P, and PtP₂) were compared in the HDS reaction of thiophene. The findings of the investigation are that the Rh₂P/SiO₂ catalyst is the most active PMG phosphide in the HDS of thiophene, as shown by the results in

Table 5. Performance of different PMG metals and phosphides in the HDS of thiophene (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹) [29].

Catalyst	Reduction Temperature [°C]	Conversion [%]	C1–C3	Butanes	Butene	1,3-Butadiene	THT 1
Rh/SiO2	450	13.32	12.13	85.28	0.00	2.59	12.13
Rh2P/SiO2	550	54.95	20.82	75.54	0.00	3.53	20.82
Pd/SiO2	350	39.86	26.96	70.38	0.00	2.66	26.96
Pd4.8P/SiO2	500	45.85	10.04	87.48	0.00	1.42	10.04
Ru/SiO2	350	0.28	0.00	77.81	22.19	0.00	0.00
Ru2P/SiO2	650	12.66	4.94	86.85	0.00	8.21	4.94
Pt/SiO2	400	22.09	52.05	41.48	0.00	6.47	52.05
PtP2/SiO2	650	3.36	14.25	60.15	0.00	25.60	14.25
CoMoP/Al2O3	400	53.28	18.67	80.53	0.00	0.10	18.67

¹ THT: tetrahydrothiophene.

and Figure 4. The Rh₂P/SiO₂ catalyst shows the highest values for the conversion of thiophene in the HDS reaction. Further findings are that Rh₂P/SiO₂ is more active than the Ni₂P/SiO₂ catalyst; that Rh₂P/SiO₂ has a comparable activity to the commercial CoMoP/Al₂O₃ catalyst they use as a benchmark; that the PGM phosphides are superior to the PGMs themselves and, therefore, phosphide formation is essential for the activity of the material, as shown in the results in Table 5. The Rh/SiO₂ catalyst achieves a conversion value of 13.32% while Rh₂P/SiO₂ achieves a value of 54.95%; the only phosphide displaying a lower activity than the metal is PtP₂.

In investigations concerning the reaction mechanism, it was proven that direct desulfurization is favored over dehydrogenation of the double bonds of thiophene (reaction route 1 on Figure 5). The product with the highest selectivity is 1-butene, followed by butane and small amounts of tetrahydrothiophene, as seen in Table 5.

In a further publication [30], Kanda investigated the effect of the support material and reduction temperature on the performance of the Rh₂P catalysts on the HDS of thiophene. The activity of the sample changes with the support material. The catalysts on SiO₂, Al₂O₃, and TiO₂ are the most active followed by MgO and last ZrO₂, as shown in

Table 6. Effect of the support material on the performance of Rh₂P in the HDS of thiophene (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹) [30].

Catalyst	Support Material	Reduction Temperature [°C]	Conversion [%]	C1–C3	Butanes	Butenes	1,3-Butadiene	THT 1
Rh	SiO2	500	17.01	0.25	10.36	85.93	0.00	3.45
Rh	Al2O3	450	8.75	1.86	14.71	82.17	0.00	1.26
Rh	TiO2	400	17.94	0.80	13.64	82.60	0.00	2.96
Rh	MgO	500	6.61	0.27	10.53	85.52	0.00	3.68
Rh	ZrO2	400	20.03	0.09	12.29	82.28	0.10	5.24
Rh2P	SiO2	550	54.95	0.11	20.82	75.54	0.00	3.53
Rh2P	Al2O3	800	54.55	0.16	20.86	75.15	0.01	3.83
Rh2P	TiO2	650	54.64	0.06	21.46	74.77	0.00	3.70
Rh2P	MgO	650	25.67	0.00	11.45	82.87	0.00	5.66
Rh2P	ZrO2	750	22.53	0.15	7.74	78.46	0.00	13.64

¹ THT: tetrahydrofuran.

. Further, as mentioned above, the results show that the formation of the phosphides enhances the activity of the materials, as Table 6 shows. The highest conversion value for a supported Rh catalyst material is 20.03% (Rh/ZrO₂), while the Rh₂P material supported on SiO₂, Al₂O₃, or TiO₂ reaches conversion values of approx. 54%. Further, TPR experiments show that the reduction behavior of the phosphate precursors to the phosphides changes with the support material, due probably to metal-support interactions. A reduction temperature as high as 800 °C is needed to convert the phosphate precursors to Rh₂P on Al₂O₃, while the material on SiO₂ is converted to Rh₂P at a temperature of 550 °C. Last, the group investigated the effect of the reduction temperature. The materials are reduced at different temperatures in the range of 450–850 °C and their performance in the HDS of thiophene was compared. All materials show an increase in turnover frequency (TOF) with an increasing reduction temperature. The most significant increase is observed with the materials supported on Al₂O₃ (

Table 7. Effect of the reduction temperature on the TOF of the Rh₂P catalyst materials in the HDS of thiophene (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹) [30].

Catalyst	Support	Reduction Temperature [°C]	TOF 1 [h−1]
Rh2P	SiO2	450	99.0
	SiO2	700	198.1
Rh2P	Al2O3	450	22.9
	Al2O3	850	251.4
Rh2P	TiO2	450	182.9
	TiO2	700	601.9
Rh2P	MgO	450	15.2
	MgO	700	106.7
Rh2P	ZrO2	450	350.5
	ZrO2	750	502.9

¹ TOF calculated from the CO uptake.

, Figure 6). The sample reduced at 450 °C displays a TOF of only 22.9 h⁻¹ while the sample reduced at 850 °C shows a TOF of 251.4 h⁻¹. The increase in the TOF is due to the formation of Rh₂P at higher reduction temperatures, which is more active than Rh or the phosphate species.

A further investigation by Kanda et al. [31] focused on the preparation of Rh₂P on SiO₂ and on the effect of the P loading and the reduction temperature on the HDS of thiophene. The highest HDS activity is observed on the catalyst with a loading of 5 wt % Rh and 1.5 wt % P on SiO₂, reduced at a temperature of 550 °C, which achieves conversion values of up to 55.0 % (

Table 8. Effect of the P loading and reduction temperature on the performance of the Rh₂P/SiO₂ catalysts on the HDS of thiophene (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹) [31].

Catalyst	Support Material	Reduction Temperature [°C]	Conversion [%]	C1–C3	Butanes	Butenes	THT 1	TOF 2 [h−1]
Rh	SiO2	550	16.8	0.3	9.9	86.4	3.3	21
Rh-0.8P	SiO2	550	27.9	0.0	15.8	81.2	3.0	50
Rh-1.5P	SiO2	450	43.2	0.3	22.0	74.5	3.4	137
Rh-1.5P	SiO2	550	55.0	0.1	20.8	75.6	3.5	202
Rh-1.5P	SiO2	650	43.1	0.1	14.4	81.0	4.3	230
Rh-2.2P	SiO2	550	45.4	0.2	21.4	72.6	5.9	312
Rh-3.0P	SiO2	550	14.2	0.4	9.9	72.4	17.3	147

¹ THT: tetrahydrofuran; ² TOF calculated from the CO uptake.

) and arate constant of approximately 21 mmol h⁻¹ g⁻¹. The higher activity of the 1.5 wt % P loaded catalyst is attributed to the formation of small Rh₂P particles at a relatively low reduction temperature (550 °C), as shown by the results from the TEM and PXRD investigations. The authors assumed that catalysts with lower P loadings (0.8 wt %) need higher temperatures for the formation of Rh₂P with Rh impurities and that high reduction temperatures lead to sintering of the active phase. Catalysts with higher P loadings (2.2–3.0 wt %) show the formation of highly dispersed Rh₂P already at lower temperatures (350 °C), but the excess P covers the active phase. A moderate P loading of 1.5 wt % leads to the formation of well-dispersed Rh₂P at moderate temperatures without the drawback of excess P covering or sintering due to the high reduction temperature.

Sawanda et al. [33,34] investigated the HDS of thiophene with Rh₂P supported on zeolites. In the first investigation [10,33] the effect of adding Na to the Rh₂P catalysts supported on MFI zeolite, SiO₂, and Al₂O₃ on the HDS of thiophene was studied. The addition of Na to the zeolites increases the activity of the catalysts. The Rh₂P catalyst supported on NaMFI is higher than the catalysts supported on HMFI. The highest conversion values for the NaMFI catalyst is ca. 40% (Figure 7), while the HMFI reaches a maximum conversion value of ca 25% under identical conditions (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹). Sawanda suggested that the addition of Na decreases the interaction between the Al and the phosphate and thus enhances the reducibility of the phosphate species to Rh₂P and increases the activity of the catalysts. In a subsequent investigation [34], different Na-form zeolites (Na-beta, NaMFI, NaMOR) were used as supports for the Rh₂P catalyst and tested in the HDS of thiophen. The order of the thiophene conversion values is NaMFI (~35%) > Na-beta (~17%) » NaMOR (~8%) under identical reaction conditions (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹). Sawanda et al. attributed the low activity of NaMOR supported Rh₂P catalyst to the one-dimensional channel structure of NaMOR, so that the Rh and phosphate species do not easily diffuse into the micropores of the support material, leading to low CO uptake values and low catalyst activity. The catalyst supported on NaMOR displays CO uptake values between 2 and 7 μmol/g while the catalysts supported on NaMFI and Na-beta achieve CO uptake values as high as 40 μmol/g. Further, a significantly higher reduction temperature is needed for the formation of phase-pure Rh₂P supported on the Na-beta zeolite. Due to the high reduction temperature (850 °C), the structure of the support partially collapses, leading to lower activity of the Rh₂P/Na-beta catalyst.

Finally, Kanda compared the performance of two Rh₂P/Al₂O₃ catalysts prepared over two different pathways, the reduction of a supported phosphate precursor, and the conversion of supported Rh particles with TPP. The use of TPP reduces the reduction temperature from 800 to 450 °C for the formation of Rh₂P on Al₂O₃. The highest conversion values in the HDS of thiophene (fixed-bed reactor, 0.1 g catalyst, 350 °C, 0.1 MPa, H₂/thiophene = 30, W/F = 37.9 g h mol⁻¹) are achieved with catalysts prepared with TPP and reduced at 650 °C (the catalyst achieves a reaction rate of 38.5 mmol g⁻¹ h⁻¹, while the catalyst prepared over the phosphate reduction pathway achieves a reaction rate of 20.9 mmol g⁻¹ h⁻¹). At a reduction temperature of 450 °C, an excess of P covers the Rh₂P surface, which decreases the HDS activity of the catalysts. A reduction temperature of 650 °C eliminates the P excess and enhances the crystallinity of the Rh₂P nanoparticles and so increases the activity of the HDS. The higher activity of the catalysts prepared with TPP is attributed to the significantly smaller Rh₂P nanoparticles that have an average particle diameter of approx. 1.2 nm, while the particles prepared over the reduction of the phosphates have an average particle diameter of ca. 8.5 nm, according to the TEM studies performed in the investigation.

The investigations performed by Kanda and Sawanda and other research groups on the performance of supported Rh₂P catalysts and other supported noble metal phosphides as hydrotreating catalysts were reviewed in 2015 by Kanda and Uemichi [52].

4.2. Rh₂P as a Catalyst Material for HDO

In the last decades, transition metal phosphides have gained a lot of attention in hydroprocessing with the main focus on HDS [25,56]. PGM phosphides for HDO have only recently come into focus [24,25,57], also due to the emerging perspective of renewable resources for the production of fuels and chemicals. The most investigated phosphides in this field are non-noble metal phosphides such as Ni₂P, MoP, and Co₂P [56], and only recently Rh₂P and RuP have been considered as viable candidates.

Hydrodeoxygenation is a reaction that can be applied to different feeds and the specific process will determine the choice of the appropriate catalyst. Typically, HDO occurs parallel to HDS and HDN during hydrotreatment of conventional petroleum fractions. The oxygen content of most petroleum feedstocks is usually lower than 2% and sulfides of CoMo/γ-Al₂O₃ or NiMo/γ-Al₂O₃ [58] are the conventional and validated catalysts employed in this case (reaction condition range 3–10 MPa H₂ pressure, 300–500 °C). Bio-oils derived from biomass have a completely different composition, being remarkable for their high oxygen content (usually up to 40%) and water content (usually up to 30%) [59]. In the case of bio-oils, efficient HDO is relevant to improve their stability, increase the heating value, and decrease the viscosity [58]. However, conventional hydrotreating catalysts are usually not stable in the presence of feeds containing high oxygen content without an additional supply of a S source [58]. Up to now, the identification of optimal catalyst systems for HDO has been a challenging task and several solutions have been proposed and investigated. The main challenge for identifying an appropriate HDO catalyst is that a large versatility to convert different functional groups with different reactivity is demanded and stability under the corrosive reaction conditions is vital. Indeed, typical bio-oils are complex mixtures constituted of oxygenated components, such as phenolics, furans, ethers, carboxylic acids, aldehydes, ketones, and alcohols (an example of the reactivity scale for HDO is provided by Elliott [60]). Phenols [61] and dibenzofurans [62] are generally chosen as model compounds to discriminate and estimate the HDO reactivity of a given catalyst system since they are the least reactive compounds to HDO.

Transition metal phosphides are potential candidates in HDO. Especially, metal-rich phosphides that have excellent heat and electrical conductivities and high thermal and chemical stabilities can match the properties desired for hydrotreatment [25,54,63]. This can be attributed to their good hydrogen transfer properties that are enhanced by the presence of phosphorus [64,65].

As described in the case of Ni₂P, the presence of phosphorus has an important effect on the HDO activity [65,66]. Phosphorus has a ligand effect on the metal site of the phosphide, modifying the electron density on the metal cation (M^δ+) and resulting in an easier hydrogen dissociation on this surface, and the catalytic center can act as a Lewis acid [55,66]. The group P–OH, which is co-located on the metal phosphide phase as result of an incomplete reduction of phosphorus on the surface, constitutes a Brønsted acid of moderate acidity [63,66]. The synergy between the two groups (metallic and acidic) promotes catalyst activity comparable to that of a noble metal on an acidic support material and can be described as a cooperative mechanism over more than one neighboring catalytic center. The metal centers are active for hydrogenation, hydrogenolysis, and demethylation reactions [66], while PO-H groups can donate active hydrogen species as well, but they are much less active than the metal site. A higher d electron density of the metal is described as beneficial for an improved activity. A proposed HDO mechanism is reported in Figure 8. The oxygenated compounds and adsorbed H₂ are activated on the metal center and hydrogen atoms from the metal center and P–OH site react with the oxygenated species adsorbed on the surface; finally, water and the deoxygenated product species are formed [55].

Ni₂P is one of the most studied metal phosphides for hydrotreatment showing good HDO activity as well for model compounds and biomass-derived feeds [57,61,67]. Zhao et al. [61] demonstrated that Ni₂P shows superior activity compared to other base metals. The following sequence in activity was reported for several phosphides in the HDO of guaiacol in a fixed-bed reactor at 300 °C and atmospheric pressure:

Ni₂P/SiO₂ > Co₂P/SiO₂ > Fe₂P/SiO₂ > WP/SiO₂ > MoP/SiO₂

All the catalysts are less active than Pd/Al₂O₃, and the least stable system proved to be Co-MoS₂/Al₂O₃, representing an advantage of the phosphide materials. Another study related to phenol HDO underlines that phosphides can be active in hydrogenation as well as in isomerization [68]. For the phosphide materials, this results in higher selectivity for methylcyclopentane than cyclohexane during conversion of phenol. Koike et al. [57] explored Ni₂P/SiO₂ in the HDO of bio-oil produced from cedar chips in a two-stage fluidized bed reactor (0.1 MPa, 300 °C and 350 °C, residence time 0.64–1.0 s). Phenolic compounds were unreactive under the reaction condition used and a lower deoxygenation degree was recorded compared to the test with 2-methyltetrahydrofuran as the model compound.

Rh₂P has received less attention than Ni₂P but recent studies have demonstrated a high activity in HDO, superior also to that of supported noble metals in metallic form [25,47]. Griffin et al. [47] employed Rh₂P nanoparticles, obtained via solution-phase synthesis, supported on silica for guaiacol HDO. The catalytic tests were carried out in a reactor system allowing continuous flow at 350 °C, 0.5 MPa with a ratio 12:1 H₂ to guaiacol, and WHSV 5 h⁻¹. The synthesis method of the catalyst has an influence on the catalytic performance; a better selectivity towards deoxygenated species has been found for nanoparticles synthesized in solution-phase compared to incipient wetness (IW) impregnation (Figure 9). The activity of Rh₂P/SiO₂ in guaiacol conversion is not the highest among the tested catalysts but it has the highest selectivity for deoxygenated products such as cyclohexane and benzene (26% combined). Compared to the other catalysts (such as Rh/SiO₂, Ni/SiO₂, Ni₂P/SiO₂), Rh₂P/SiO₂ is more selective for C–O cleavage of the aryl–OH bond, giving benzene and anisole as the products.

Further studies on Rh₂P on different support materials were also carried out by Griffin et al. [69]. Similar reaction conditions compared to the previous study [47] were used with Rh₂P on TiO₂, Al₂O₃, and MgO as support materials in order to assess the influence of the support in HDO of guaiacol as the model compound. Rh₂P/TiO₂ and Ni/TiO₂ exhibit higher guaiacol conversion showing products with lower O:C ratio for C₅₊ fraction compared to the catalysts supported on active carbon. This is attributed to the properties of TiO₂, which displays reducible oxygen vacancies capable of facilitating C–O bond scission without aromatic ring hydrogenation. Ni/Al₂O₃ and Rh₂P/Al₂O₃ have also been found to be highly active catalysts, but they are less selective toward deoxygenation, and they also promote alkylation. MgO as support material produces the worst performance among the catalyst set investigated; it is speculated that the basic nature of the support is the main reason for the observation.

Habas et al. [28] investigated the activity of Ni₂P, Rh₂P, and Pd₃P on the HDO of acetic acid in a temperature range from 200 to 500 °C. The metal phosphide nanoparticles were synthesized by commercially available, air-stable metal−phosphine complexes in a one-pot reaction and supported on silica. Analogous catalysts were prepared by incipient wetness as a comparison. Both Rh₂P and Ni₂P show hydrogen-assisted decarbonylation reactions with CO, CH₄, and H₂O as products. Pd₃P does not show remarkable activity.

Cecilia et al. [38] were able to enhance the catalyst activity of Ni₂P by adding Rh, Ru, Ir, and Pt (molar ratio metal/Ni = 10%). The catalyst activity for HDO of dibenzofuran (P = 3.0 MPa, 175–300 °C, fixed-bed reactor) follows the sequence:

Ni_1.8Rh_0.2P > Ni_1.8Ru_0.2P > Ni_1.8Ir_0.2P >>> Ni₂P ≈ Ni_1.8Pt_0.2P

The structural interaction between the phosphide Ni₂P and the noble metal was not clarified but it was observed that Rh, Ru, and Ir could effectively enhance the catalyst activity and stability, while Pt provokes the opposite effect. The presence of a noble metal seems to improve the adsorption and activation of hydrogen in the metal support interface and it increases the presence of P–OH.

Bowker et al. [70] described the synthesis and HDO activity of silica-supported ruthenium phosphide (Ru₂P, RuP) catalysts. At 400 °C in a continuous reactor, Ru₂P and RuP are more active in HDO of furan than the conventional CoMo/Al₂O₃ catalyst, as reported in the sequence:

Ru₂P/SiO₂ > RuP/SiO₂ (phos.) > Ru/SiO₂ > RuP/SiO₂ (hypo.) > Ni₂P/SiO₂ > CoMo/Al₂O₃

Ru₂P/SiO₂ is the most active and stable catalyst of the set (TOS 48 h). Higher selectivity for C₄ hydrocarbons is attributed to the presence of phosphorus, compared to Ru metal that produces more C₃ compounds.

One of the biggest dilemmas concerning transition metal phosphides is their stability in contact with complex mixtures such as petroleum fractions or real bio-oils in the process reaction conditions. Only a few studies have reported the stability of these catalysts and offer insight into the eventual deactivation processes of these catalysts. Typically, water created in the reaction or present initially in the feed can deactivate or degrade the catalyst through the formation of phosphates or the respective metal oxides from the corresponding phosphides [66]. Griffin et al. [69] reported that metal phosphides show a tolerance to water under the acidic conditions present in pyrolysis vapors. Cecilia et al. [62] asserted that Ni₂P shows high resistance to deactivation towards coke, although the water produced as a byproduct could be responsible for the slight decrease in activity. Koike et al. [57] identified coke formation as the main source of deactivation for Ni₂P/SiO₂. Griffin et al. [69] reported also for Rh₂P an increase in the coke deposited on the catalyst surface during guaiacol HDO.

In conclusion, metal phosphides appear promising for HDO although their activity may be lower compared to their intrinsic activity in HDS or HDN. The advantage of these materials is that their properties can be tuned depending on the metal chosen or by varying the metal/phosphorus ratio or the particle size of the phosphides [54]. It will be interesting to see the advances in the field in order to design an optimal phosphide-based catalyst for HDO.

4.3. Other Succesful Examples of Catalysis with Rh₂P

The homogeneously catalyzed hydroformylation of olefins is one of the most important large-scale industrial processes. Cobalt carbonyl complex-based catalyst systems are still used today for the hydroformylation of mid- and long-chain olefins, while the inherently more active rhodium carbonyl complexes stabilized with phosphine or phosphite ligands are applied for the hydroformylation of short olefins. The application of expensive rhodium catalyst systems demands an effective separation of the catalyst from the reaction products to minimize the risk of rhodium loss and catalyst deactivation. This explains why rhodium is only applied for the hydroformylation of short olefins where the separation of the products by distillation is feasible or alternatively the biphasic process where the catalyst complex remains in the aqueous phase while the products are removed over the organic phase.

Technical solutions involving heterogeneous catalysts in the hydroformylation with the catalyst in the solid phase and the products in the gas or liquid phase could offer alternatives that may also lead to new solutions regarding the process design. Research activities in academia and in industry are focusing on the synthesis and development of heterogeneous or heterogenized molecular hydroformylation catalysts that have proven to be inferior to the current state of the art in homogeneous catalysis [71,72,73,74,75].

As shown in the previous section, PGM phosphides, and especially rhodium phosphide, have proven to be useful heterogeneous catalysts for hydrotreating reactions. Although the phosphides show high catalytic activity in different reactions, they have not been widely considered as candidates in reactions involving the activation of carbon monoxide such as the hydroformylation of lower olefins.

We have carried out tests with Rh₂P on SiO₂ as a catalyst for the gas phase hydroformylation of ethylene and propylene [35]. The preparation of these catalysts was carried out via the incipient wetness impregnation of phosphate precursors followed by a reductive thermal treatment at different temperatures. The samples show the formation of phase-pure Rh₂P particles on SiO₂ at a reduction temperature of 500 °C, as PXRD investigation illustrated; highly crystalline Rh₂P nano-particles could be retained even at a reduction temperature of 900 °C, as proven by HRTEM investigations.

The samples were tested in a fixed-bed reactor system for the gas phase hydroformylation of ethylene, where the best results are achieved at low H₂/CO ratios (50 bar; GHSV = 2773 h⁻¹; 170–240 °C; 10% C₂H₄; 10% H₂; 70% CO; 10% Ar). A further increase in the selectivity of the catalysts towards the aldehyde is observed when water is added to the feed. It is reasoned that by adding water to the system the formation of the hydrogenation product ethane and the aldol condensation of the aldehyde are suppressed so that the selectivity of the desired aldehyde increases.

A comparison between the performance of the catalysts reduced at different temperatures has proven that the most active and selective catalysts are the catalyst types reduced at the highest temperatures. Such catalysts are highly stable: the spent catalyst of the sample reduced at 900 °C displays phase-pure Rh₂P on SiO₂. Further evidence to rationalize the higher activity of the catalyst reduced at 900 °C can be taken from the IR absorption spectra after CO adsorption. The IR spectra of the catalyst reduced at 500 °C shows different signals in the region around 2000 cm⁻¹ that correspond to both the linearly and bridged bonded CO on Rh atoms. The sample reduced at 900 °C shows only one signal at 2700 cm⁻¹, which corresponds to the CO linearly bonded to the Rh atom, and which formation favors the hydroformylation of the olefin as reported by Chuang and Pien [73,74,75]. Considering these facts, the authors suggest that this finding can be taken as evidence that Rh₂P in the form of highly crystalline-supported nanoparticles features surface sites that allow catalytic functionality similar to that of a phosphine- or phosphite-modified rhodium complex [76].

Based on the results presented and the fact that single-site catalysis is the proposed mode of action for Rh₂P, the question may be raised as to what the nature of the catalytically active site is. From the evidence we have gathered [35,76] and the structural considerations, we can draw the following conclusions: Rh₂P crystallizes in the cubic CaF₂ phase prototype. The nano-particle catalysts are highly crystalline and one can take as first approximation the bulk structure as the model for potential surface terminations. The CO-adsorption experiments prove that the surface must be terminated with exposed metal species. Cuts through the cubic CaF₂ phase prototype, even along high index surfaces, suggest that for all geometries separation of rhodium sites by phosphorus is obtained. In light of the fact that the spectroscopic data indicate that the presence of single rhodium sites available on highly crystalline nano-particles is a prerequisite for good performance, we suggest that single-site catalysis is the mode of action relevant for the materials discussed. The rhodium sites are separated and coordinated by phosphorus and the prevailing motive of these solids can be summarized as “frustrated” single catalytic sites that do not cooperate, which is in contrast to the examples mentioned above in Rh₂P as hydroprocessing catalysis [76]. Although Rh₂P is present as an extended solid, the specific structure of the solid and the fact that a metal and a non-metal are the constituents do create an environment suitable for single site catalysis: the motif of “frustration” can be taken as the new descriptor for the development of alternative catalytic materials for single site catalysis.

Further, we also report the application of the catalysts in the hydroformylation of larger olefins, such as propylene, with a selectivity of 90% towards butyraldehyde with an n/iso ratio of 2.4 in the gaseous phase. Here again, a positive effect can be observed when water is added to the feed stream. Last, the catalysts are tested in the methoxycarbonylation of ethylene by exchanging hydrogen in the feed gas mixtures by methanol. Under the conditions applied, no ester formation could be observed. By adding small amounts of hydrogen, noticeable amounts of the product methyl propionate, together with major amounts of propionaldehyde and ethane, could be observed. From this observation, a reaction mechanism in which the ester is not formed via alkoxycarbonylation but via dehydrogenation of an intermediate hemiacetal is suggested.

5. Conclusions

Metal phosphides have been developed in the last years as hydrotreating catalysts. The reports show high activity of Ni₂P, MoP, and Rh₂P as catalysts for the reactions of HDS, HDN, and HDO. The development of the noble metal phosphide catalysts for hydrotreating catalysts shows that Rh₂P displays the highest activity in the HDS and HDO reactions. The main factors affecting the activity of Rh₂P are the preparation pathway and precursors, reduction temperature, and support material as shown by the broad investigations by Kanda et al. [29,30,31,32,33,34,43,52]. For deoxygenation reactions, cooperative mechanisms of multifunctional or adjacent sites are discussed as a major motive. Despite the high activity of the phosphides in hydrotreating reactions, only one publication [35] was found regarding the application of phosphides in CO activation reactions, such as the hydroformylation of short olefins. Here, Rh₂P appears to be an active, stable catalyst for the hydroformylation of short olefins. The characterization of the catalysts via XRD, TEM, and DRIFTS suggests that the highly crystalline Rh₂P nanoparticles on the support material features surface sites with a catalytic functionality similar to that of a phosphine- or phosphite-modified rhodium complex, which can be described as single-site catalysis or “frustrated” catalytic sites. This review gives an overview of the versatility of Rh₂P and its applications in heterogeneous catalyst systems. Here it shows high activity not only in hydrotreating reactions but also in CO activation reactions, such as the hydroformylation, expanding the possible applications of Rh₂P as a catalyst material.