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Article

Mechanism Insight into Catalytic Performance of Ni12P5 over Ni2P toward the Catalytic Deoxygenation of Butyric Acid

1
College of Chemical Engineering, Sichuan University, Chengdu 610065, China
2
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China
*
Author to whom correspondence should be addressed.
Submission received: 12 April 2022 / Revised: 12 May 2022 / Accepted: 17 May 2022 / Published: 21 May 2022
(This article belongs to the Topic Catalysis for Sustainable Chemistry and Energy)

Abstract

:
The Ni/P ratio of nickel phosphide has an important effect on the catalytic performance toward the deoxygenation of fatty acids to biofuel. The Ni12P5 cluster is preferred to model Ni12P5 catalyst with butyric acid as the reactant model of palmitic acid. The catalytic deoxygenation mechanism of butyric acid over Ni12P5 cluster has been theoretically investigated at GGA-PBE/DSPP, DNP level in dodecane solution. From butyric acid, the hydrodehydration is predominated to form n-butanal. Then, from n-butanal, low temperature benefits the hydroreduction to form butanol and then hydrodehydration to produce n-butane, whereas high temperature favors the direct decarbonylation to yield propane. n-Butane originates from n-butanol through hydrodehydration and not from n-butylene. Propane comes from n-butanal through decarbonylation and not from propanol and/or propylene. Additionally, CO stems from n-butanal through decarbonylation, whereas CO2 is ruled out from butyric acid through decarboxylation. Compared with Ni12P6 cluster, Ni12P5 cluster exhibits higher catalytic activity for the formation of butanal, n-butanol, and n-butane, while it displays lower catalytic activity toward the direct decarbonylation and dehydration to yield propylene. These results can be attributed to less negative charges of Ni-sites over Ni12P5 cluster, compared with Ni12P6 cluster.

1. Introduction

As a renewable energy, biofuel has aroused widespread concern, from the perspective of sustainable development of energy [1]. Catalytic deoxygenation of fatty acids is a promising method to produce high quality biofuel, due to the high oxygen content of fatty acids [2]. There are three deoxygenation methods, namely decarboxylation, decarbonylation and hydrodeoxygenation (HDO) [3]. Generally, there are plenty of kinds of raw materials for producing biofuel, such as palm oils and algal lipids [4,5]. Palmitic acid has commonly been preferred as a model compound of fatty acids, for the exploring the catalytic deoxygenation mechanism [6,7].
Various catalysts have been employed to produce high-quality biofuel from palmitic acid, including noble metals, base metals, and transition-metal phosphides (TMPs) [8,9]. For noble metal catalysts, they cannot be widely used because of the high cost, despite their good catalytic activity and selectivity in the deoxygenation of fatty acid [10,11]. Base metal catalysts have much lower cost than noble metal catalysts, and some of them exhibit good catalytic performance in the deoxygenation of fatty acid [12,13]. In particular, TMPs have attracted special attention, because of their activity and stability in the presence of hydrogen and oxygenated hydrocarbons, together with their cheap and accessible characteristics [14,15].
Noteworthily, nickel phosphide behaves as a promising catalyst toward the hydrodeoxygenation of palmitic acid [16,17]. Moreover, the dispersion and kinds of nickel phosphide with the different Ni/P ratios strongly affects the deoxygenation activities and selectivities of NixP catalysts [6,17]. Among them, the Ni/P molar ratios are usually be regulated by Ni2P and/or Ni12P5 compounds [6,17]. One can speculate that both Ni2P and Ni12P5 catalysts should exhibit different catalytic performance toward the deoxygenation of palmitic acid.
However, the current catalytic systems still face some problems, such as coke deposition, deactivation, instability, and/or large amount of hydrogen consumption [3]. These problems have urged researchers to study the selective deoxygenation mechanism of fatty acids. Encouragingly, some researchers have focused on the mechanism for the deoxygenation of different raw materials to hydrocarbon biofuels. Experimentally, Pt catalyst benefits decarboxylation [11]; MoP and Ni2P prefer decarbonylation [14,15]; and NiMo, CoMo and WP catalyst favor hydrodeoxygenation [12,15,16]. Particularly, our group have proposed that NixP facilitates both decarboxylation and decarbonylation [17], the coexistence and dispersion of both Ni2P and Ni12P5 attribute the deoxygenation activity for the palmitic acid [17], and Ni12P5 exhibits higher turnover frequency than Ni2P [18]. Newly, for the deoxygenation of butyric acid (as the model reactant) catalyzed by Ni12P6 cluster (as the Ni2P catalyst model), decreasing temperature is favorable to the formation of butyraldehyde, n-butyl alcohol, and n-butane, and increasing temperature is beneficial to the formation of propylene, propane, and butylene [19]. Nevertheless, toward the deoxygenation of fatty acids, the molecular mechanism of Ni12P5 catalyst is still unclear, especially its difference in catalytic performance with Ni2P catalyst.
In this work, comparing with Ni2P catalyst, the deoxygenation mechanism of palmitic acid over Ni12P5 catalyst has been theoretically investigated. In order to improve the computational efficiency, a Ni12P5 cluster is constructed to model the Ni12P5 catalyst, with butyric acid as the reactant model. The goals are the following: (a) to obtain the elaborated potential energy curves toward the hydrodeoxygenation, decarbonylation, and decarboxylation of butyric acid over Ni12P5 cluster, (b) to elucidate the optimal reaction pathways for the production of propane and n-butane, (c) to explore the temperature effect on the rate-determining reaction step, (d) to compare the catalytic performance of Ni12P5 cluster with Ni12P6 cluster, and (f) to explore the chemical nature for the difference in catalytic performance between Ni12P5 cluster with Ni12P6 cluster, which should facilitate the rational design of highly efficient catalytic system for the deoxygenation of fatty acids.

2. Computational Methods

At the beginning, a Ni12P5 cluster was carved from Ni12P5 crystal structure in the Inorganic Crystal Structure Database (ICSD) [20], which is preferred to model Ni12P5-based catalyst. The dodecane (the dielectric constant is 2.016) solvent effect was taken into account by using conductor-like screening model (COSMO) [21,22], because the deoxygenation of palmitic acid has been experimentally conducted in dodecane solvent [17].
In dodecane solution, all geometry calculations were run in Dmol3 module [23], using Materials Studio 7.0 package [24]. The generalized gradient approximation (GGA) and the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional were adopted [25,26]. For H, C, O, and P elements, all electrons were considered, while the core electrons were replaced by the density functional semicore pseudopotential (DSPP) for the metal Ni element [27]. The double numerical plus polarization (DNP) basis set was employed for the valence electron wave function of atoms [28], namely, GGA−PBE/DNP, DSPP.
The full geometry optimizations were performed for all species in the deoxygenation of butyric acid over Ni12P5 cluster. About the full geometric optimization, the convergence parameters were 2 × 10−3 Hartree per Å for force applied to each atom, 1.0 × 10−6 Hartree for total energy, and 5 × 10−3 Å for displacement. In addition, to improve the computational efficiency, a fermi smearing of 0.005 Hartree for orbital occupancy and a global orbital cutoff of 4.5 Å were used. The vibrational frequencies were calculated to confirm each stable intermediate or transition state, which have real frequencies for each intermediate and only one imaginary frequency for each transition state with the right vibration direction from reactant to product. These calculation parameters are identical to those in the deoxygenation mechanism of butyric acid over Ni12P6 cluster [19].
The established model of Ni12P5 cluster is shown in Figure 1. Unless additionally noted, the Gibbs free energies of species are relative to the original Ni12P5 cluster and reactants (butyric acid and H2) in dodecane solution at GGA−PBE/DSPP, DNP level, under the experimental temperature of 513.0 K [16].
For the entire catalytic cycle, the energetic span was employed to obtain both determining intermediate (DI) and determining transition state (DTS) of turnover frequency [29,30,31,32,33,34]. After that, as reported in our previous study [35], the rate constants were computed over the 413~613 K temperature range [19], based on the conventional transition state theory with tunneling correction.

3. Results and Discussion

Based on the literatures [7,19], there are three overall reactions (Reactions 1~3) for the deoxygenation of butyric acid, i.e.,
C3H7COOH → C3H8 + CO2
C3H7COOH + H2 → C3H8 + CO + H2O
C3H7COOH + 2H2 → C4H10 + 2H2O
As shown in Scheme 1, Reaction 1 is related to the direct decarboxylation of butyric acid to propane, for which there is only one reaction pathway (P-CO2) with only one reaction stage (S2). Reaction 2 is associated with both decarbonylation and dehydration of butyric acid with H2 to propane, for which there are four reaction pathways, i.e., through butanal (P-CO-nal) with two reaction stages (S3 + S4), through propanol, but not through propylene (P-CO-nol) with two reaction stages (S5 + S6), through both propanol and propylene (P-CO-nol-ene) with three reaction stages (S5 + S7a + S7b), through propylene but not through propanol (P-CO-ene) with two reaction stages (S(5/7a) + S7b). Reaction 3 is concerned with the hydrodehydration of butyric acid to n-butane, for which there are two reaction pathways, i.e., not through butylene (P-C4-non-ene) with three reaction stages (S3 + S8 + S9), and through butylene (P-C4-ene) with four reaction stages (S3 + F8 + F10a + F10b).

3.1. C3H7COOH → C3H8 + CO2

The potential energy curves and geometric structures for Reaction 1 of C3H7COOH → C3H8 + CO2 over Ni12P5 cluster are shown in Figure 2, the detailed energy data are shown in Table S1.
As shown in Figure 2, Reaction 1 includes only one reaction pathway P-CO2 with only one reaction stage S2. S2 is associated with the direct decarboxylation of butyric acid to propane. It mainly involves three reaction steps, i.e., (1) the cleavage of O–H bond via F2-TS1, (2) the cleavage of C1–C2 bond via F2-TS2, and (3) the formation of C2–H bond via F2-TS3.
The P-CO2 comprises the energy height of the highest point (EHHP) of 156.0 kJ mol−1 at F2-TS2, and the highest energy barrier (HEB) of 158.0 kJ mol−1 at the reaction step of F2-IM2 → F2-TS2 → F2-IM3 for the cleavage of C1–C2 bond.

3.2. C3H7COOH + H2 → C3H8 + CO + H2O

3.2.1. P-CO-nal (S3 + S4)

As shown in Scheme 1, P-CO-nal contains two reaction stages, i.e.,
C3H7COOH + H2 → C3H7CHO + H2O
C3H7CHO → C3H8 + CO
Here, S3 (Equation (4)) and S4 (Equation (5)) are related to the hydrodehydration of butyric acid to butanal, and the decarbonylation of butanal to propane, respectively. The potential energy curves and geometric structures for reaction stages of S3 and S4 over Ni12P5 cluster are depicted in Figure 3 and Figure 4, respectively. The detailed energy data are shown in Tables S2 and S3.
As shown in Figure 3, S3 mainly contains four reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the cleavage of C1–OH bond via F3-TS2, (3) the formation of C1–H bond via F3-TS3, and (4) the formation of HO–H bond via F3-TS4.
Moreover, as depicted in Figure 4, S4 mainly involves three sequential reaction steps, i.e., (1) the cleavage of C1–H bond via 4-TS1, (2) the cleavage of C1–C2 bond via F4-TS2, and (3) the formation of C2–H bond via five-membered F4-TS3.
The P-CO-nal with S3 and S4 includes the EHHP of 52.0 kJ mol−1 at F3-TS4, and the HEB of 95.1 kJ mol−1 at F3-IM7 → F3-TS4 → F3-IM8 reaction step for the HO–H bond formation.

3.2.2. P-CO-nol (S5 + S6)

As shown in Scheme 1, P-CO-hol contains two reaction stages, i.e.,
C3H7COOH → C3H7OH + CO
C3H7OH + H2 → C3H8 + H2O
Here, S5 (Equation (6)) and S6 (Equation (7)) are related to the direct decarbonylation of butyric acid to propanol, and the hydrodehydration of propanol to propane, respectively. The potential energy curves and geometric structures for reaction stages of S5 and S6 over Ni12P5 cluster are shown in Figure 5 and Figure 6, respectively. The detailed energy data are shown in Tables S4 and S5.
As depicted in Figure 5, S5 mainly contains three reaction steps, i.e., (1) the cleavage of C1–OH bond via 5-TS1, (2) the cleavage of C1–C2 bond via F5-TS2, and (3) the formation C2–OH bond via F5-TS3.
Additionally, as shown in Figure 6, S6 mainly includes four reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the cleavage of C2–OH bond via F6-TS2, (3) HO–H bond formation via F6-TS3, and (4) C2–H bond formation via F6-TS4.
The P-CO-hol with S5 and S6 comprises the EHHP of 130.4 kJ mol−1 at F5-TS3, and the HEB of 138.8 kJ mol−1 at F5-IM5 → F5-TS3 → F5-IM5 reaction step for the formation of C2–OH bond.

3.2.3. P-CO-nol-ene (S5 + S7a + S7b)

As shown in Scheme 1, the P-CO-nol-ene consists of three reaction stages, i.e., S5, S7a (Equation (8)) and S7b (Equation (9)),
C3H7OH → C3H6 + H2O
C3H6 + H2 → C3H8
As mentioned earlier, S5 is typical of the direct decarbonylation of butyric acid to propanol. Here, S7a and S7b are related to the dehydration of propanol to propylene and hydrogenation of propylene to propane, respectively. The potential energy curves and geometric structures for S7a and S7b over Ni12P5 cluster are depicted in Figure 7, respectively. The detailed energy data are shown in Tables S6 and S7.
As shown in Figure 7a, S7a mainly involves three reaction steps, i.e., (1) the cleavage of C2–OH bond via F7-TS1, (2) both the cleavage of C2–H bond and the formation of HO–H bond via F7-TS2, and (3) intramolecular [1,2]-H shift via F7-TS3.
As shown in Figure 7b, S7b mainly contains three reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the formation of C3–H bond via F7-TS4, (3) the formation of C2–H bond via F7-TS5.
The P-CO-nol-ene with S5, S7a and S7b comprises the EHHP of 130.4 kJ mol−1 at F5-TS3, and the HEB of 138.8 kJ mol−1 at F5-IM5 → F5-TS3 → F5-IM5 reaction step for the formation of C2–OH bond.

3.2.4. P-CO-ene (S5/7a + S7b)

As shown in Scheme 1, the P-CO-ene is composed of two reaction stages, i.e., S(5/7a) (Equation (10)) and S7b,
C3H7COOH → C3H6 + CO + H2O
Here, S(5/7a) is concerned with the direct dehydration and decarbonylation of butyric acid to propylene. As mentioned earlier, S7b is associated with hydrogenation of propylene to propane. Furthermore, S(5/7a) can be composed S5 and S7a, but without propanol. The potential energy curves and geometric structures the for S(5/7a) over Ni12P5 cluster are shown in Figure 5. As mentioned earlier, S7b is the hydrogenation of propylene to propane.
As depicted in Figure 5, S(5/7a) mainly consists of four reaction steps, i.e., (1) the cleavage of C1–OH bond via F5-TS1, (2) the cleavage of C1–C2 bond via F5-TS2, (3) the cleavage of C2–H bond and the formation of HO–H bond via F7-TS2, and (4) intramolecular [1,2]-H shift via F7-TS3.
The P-CO-ene with S(5/7a) and S7b includes the EHHP of 130.1 kJ mol−1 at F5-TS1, and the HEB of 121.6 kJ mol−1 at F2-IM1 → F5-TS1 → F5-IM2 reaction step for the cleavage of C1–OH bond.
Summarily, for Reaction 2, P-CO-nal is kinetically more favorable than P-CO-nol, P-CO-nol-ene, and P-CO-ene, owing to their lower EHHPs (52.0 vs. 130.4, 130.4, and 130.1 kJ mol−1) and lower HEBs (95.1 vs. 138.8, 138.8, and 121.6 kJ mol−1). In other words, P-CO-nal should be the optimal pathway for Reaction 2, which proceeds through sequential hydrodehydration and decarbonylation via butanal intermediate, whereas it does not accompany both propanol and propylene intermediates. This result is analogous to the experimental phenomena for the deoxygenation of palmitic acid over Ni12P5-based catalyst, in which both pentadecanol and pentadecene were not observed [18].

3.3. C3H7COOH + 2H2 → C4H10 + 2H2O

3.3.1. P-C4-non-ene (S3 + S8 + S9)

As shown in Scheme 1, the P-C4-non-ene comprises three reaction stages, i.e., S3, S8 (Equation (11)), and S9 (Equation (12)),
C3H7CHO + H2 → C3H7CH2OH + H2O
C3H7CH2OH + H2 → C4H10 + H2O
As mentioned earlier, S3 is typical of the hydrodehydration of butyric acid to butanal. Here, S8 is related to the hydroreduction of butanal to n-butanol, and S9 is concerned with the hydrodehydration of n-butanol to n-butane. The potential energy curves and geometric structures for S8 and S9 over Ni12P5 cluster are shown in Figure 8 and Figure 9, respectively. The detailed energy data are shown in Tables S8 and S9.
As shown in Figure 8, S8 mainly contains three reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the formation of O–H bond via F8-TS2, and (3) the formation of C1–H bond via F8-TS3.
In addition, as depicted in Figure 9, S9 mainly consists of four reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the cleavage of C1–OH bond via F9-TS2, (3) the formation of HO–H bond via F9-TS3, and (4) C1–H bond formation via F9-TS4.
The P-C4-non-ene with S3, S8, and S9 involves the EHHP of 84.9 kJmol−1 at F8-TS3 for the formation of C1–H bond, and the HEB of 115.2 kJmol−1 at F9-IM3 → F9-TS2 → F9-IM4 reaction step for the cleavage of C1–OH bond.

3.3.2. P-C4-ene (S3 + S8 + S10a + S10b)

As depicted in Scheme 1, the P-C4-ene consists of four reaction stages, i.e., S3, S8, S10a (Equation (13)), and S10b (Equation (14)),
C3H7CH2OH → C4H8 + H2O
C4H8 + H2 → C4H10
As mentioned earlier, S3 and S8 are typical of the hydrodehydration of butyric acid to butanal and the hydroreduction of butanal to n-butanol, respectively. Here, both S10a and S10b are related to the dehydration of n-butanol to butylene and hydrogenation of butylene to n-buane, respectively. The potential energy curves and geometric structures for S10a and S10b over Ni12P5 cluster are depicted in Figure 10. The detailed energy data are shown in Tables S10 and S11.
As shown in Figure 10a, S10a mainly includes three reaction steps, i.e., (1) the cleavage of C1–OH bond via F10-TS1, (2) both the cleavage of C2–H bond and the formation of HO–H bond via F10-TS2, and (3) intramolecular H-shift via F10-TS3.
In addition, as depicted in Figure 10b, S10b mainly contains three reaction steps, i.e., (1) the dissociation of H2 via F3-TS1, (2) the formation of C2–H bond via F10-TS4, and (3) the cleavage of C1–H bond via F10-TS5.
The P-C4-ene with S3, S8, S10a, and S10b comprises the EHHP of 109.0 kJmol−1 at F10-TS1 for the C1–OH bond cleavage, and the HEB of 112.6 kJmol−1 at F10-IM2 → F10-TS2 → F10-IM3 reaction step for both the cleavage of C2–H bond and the formation of HO–H bond.
In brief, for Reaction 3, P-C4-non-ene is preferable to P-C4-ene in kinetics, owing to its lower EHHP (84.9 vs. 109.0 kJ mol−1) and similar HEB (115.2 vs. 112.6 kJ mol−1). In other words, P-C4-non-ene is the optimal reaction pathway for Reaction 3, which goes through both butanal and n-butanol, and without through butylene. This result is similar to the experimental result, in which hexadecanol was observed without observation of hexadecene for the hydrodeoxygenation of palmitic acid over Ni12P5-based catalyst [18].

3.4. Catalytic Selectivity Dependent on Temperature

3.4.1. From Butyric Acid

As shown in Scheme 1, from butyric acid, there are four competitive reaction stages, i.e., S2, S3, and S5, and S(5/7a). In view of S2, the DTS and DI are ascertained to be F2-TS2 and F2-IM2, respectively. As shown in Figure S1, The formula of the rate constant for S2 (kS2) can be fitted as follows (15, in s−1):
k S 2 = 2.88 × 10 14 exp ( 172   166 / R T )
As to S3, the DTS and DI are determined to be (F3-TS4 + butanal) and F3-IM4, respectively. As shown in Figure S2, the formula of the rate constant for S3 (kS3) can be expressed as follows (16, in s−1):
k S 3 = 1.79 × 10 20 exp ( 174   054 / R T )
Taking S5 into accounts, the DTS and DI are computed to be (F5-TS3 + CO) and F5-IM4, respectively. As shown in Figure S4, the formula of the rate constant for S5 (kS5) can be adapted as follows (17, in s−1):
k S 5 = 1.23 × 10 21 exp ( 327   015 / R T )
Considering S(5/7a), the DTS and DI are ascertained to be (F7-TS2 + CO) and F5-IM4, respectively. As shown in Figure S5, the formula of the rate constant for S(5/7a) (kS(5/7a)) can be represented as follows (18, in s−1):
k S ( 5 / 7 a ) = 4.45 × 10 21 exp ( 306   230 / R T )
From the formulas i~iv, the selectivity of S3 is 100%, while the total selectivity of S2, S5, and S(5/7a) is nearly to zero, over the 413~613 K temperature range. As a result, S3 is predominated while S2, S5, and S(5/7a) should be ruled out. That is to say, both direct decarboxylation and direct decarbonylation should not be possible reaction pathway for the deoxygenation of butyric acid over Ni12P5 cluster. This result is analogous to the experimental result, in which CO2 was not observed during the hydrodeoxygenation of palmitic acid over Ni12P5-based catalyst [18].

3.4.2. From Butanal

As shown in Scheme 1, from butanal, there are two competitive reaction stages, i.e., S4 and S8, which result in the formation of (C3H8 + CO) and C3H7CH2OH, respectively. For S4, the DTS and DI are computed to be (F4-TS3 + CO) and F4-IM3, respectively. As shown in Figure S3, the formula of the rate constant for S4 (kS4) can be expressed as follows (19, in s−1):
k S 4 = 3.65 × 10 18 exp ( 175   375 / R T )
Additionally, for S8, the DTS and DI are calculated to be F8-TS3 and F8-IM3, respectively. As shown in Figure S6, the formula of the rate constant for S8 (kS8) can be described as follows (20, in s−1):
k S 8 = 1.00 × 10 12 exp ( 95   469 / R T )
In view of kS4 and kS8, their selectivities are 0.3~85.0% and 99.7~15.0%, respectively, over the 413~613 K temperature range. Among them, their selectivities are equal to 50.0% at about 553 K. It is inferred that S8 for the hydrogenation to n-butanol is governed at low temperature (413~553 K), whereas S4 for the decarbonylation to propane is dominated at high temperature (553~613 K). In other words, decreasing the temperature is advantageous to hydrogenation to n-butanol, and increasing the temperature is beneficial to the decarbonylation to propane.

3.4.3. From n-Butanol

As shown in Scheme 1, from n-butanol, there are two competitive reaction pathways, i.e., S9 and S10, which lead to the synergistic hydrodehyration to n-butane and the stepwise dehydration and hydrogenation to n-butane via butylene intermediate, respectively. For S9, the DTS and DI are ascertained to be F9-TS2 and F9-IM3, respectively. As shown in Figure S7, the formula of the rate constant for S9 (kS9) can be adapted as follows (21, in s−1):
k S 9 = 4.32 × 10 11 exp ( 100   816 / R T )
Alternatively, for S10, the DTS and DI are to determined be F10-TS1 and (F10-IM5 + H2O), respectively, in which the DTS (F10-TS1) lies before the DI (F10-IM5 + H2O). As shown in Figure S8, the formula of the rate constant for S10 (kS10) can be fitted as follows (22, in s−1):
k S 10 = 1.55 × 10 12 exp ( 156   411 / R T )
In view of both kS9 and kS10a, the selectivity of S9 is 100%, while the selectivity of S10 is close to zero, over the 413~613 K temperature range. It is indicated that n-butane stems from S9 (synergistic hydrodehydration), and it goes not through butylene.
As mentioned earlier, for the deoxygenation of butyric acid over Ni12P5 cluster, there are two kinds of alkanes, i.e., propane and n-butane. On the one hand, for the propane formation, the DTS and DI are computed to be (F3-TS4 + C3H7CHO) and F3-IM4, respectively, on the optimal reaction pathway P-CO-nal. Thereupon, for the propane formation, the corresponding rate constants of kC3H8 (23, in s−1) are identical to kS3, i.e.,
k C 3 H 8 = 1.79 × 10 20 exp ( 174   054 / R T )
On the other hand, for the n-butane formation, the DTS and DI are calculated to be (F8-TS3 + H2O) and (F3-IM4 + H2), respectively, on the optimal reaction pathway P-C4-non-ene. As shown in Figure S9, the rate constant kC4H10 can be fitted as follows (24, in s−1 mol−1 dm3):
k C 4 H 10 = 6.26 × 10 11 exp ( 103   468 / R T )
Based on the rate constants of kC3H8 and kC4H10, when the concentration of H2 is the standard concentration of 1.0 mol dm−3, the selectivity of C3H8 is calculated to be 25.2~99.6%, while the selectivity of C4H10 is computed to be 74.8~0.4%, over the temperature range of 413~613 K. Generally, in the experimental temperature range (513~553 K) [7,18], the selectivities of C3H8 and C4H10 are 94.9~98.4% and 5.1~1.6%, respectively. That is to say, at the normal experimental temperature, the selectivity of C3H8 is higher than that of C4H10. This result is qualitatively in agreement with the experimental result with 59.8% yield for C15 and 33.7% yield for C16 for the hydrodeoxygenation of palmitic acid over Ni12P5-based catalyst [18].
In short, there exist two main reactions for the deoxygenation of butyric acid over Ni12P5 cluster, i.e., Reaction 2 for the propane formation and Reaction 3 for the n-butane formation, while Reaction 1 for the decarboxylation should be ruled out. Herein, propane comes from P-CO-nal with S3 and S4 via butanal intermediate in Reaction 2. n-Butane stems from P-C4-non-ene with S3, S8 and S9 via both butanal and n-butanol intermediates and not via butylene in Reaction 3. From the crucial butanal intermediate, decreasing the temperature is favorable to the hydrodehydration to n-butane, and increasing the temperature is preferable to the decarbonylation to propane. The dominant reaction pathways with the temperature effect for the hydrodeoxygenation of butyric acid over Ni12P5 cluster are shown in Figure 11.

3.5. Comparison of Ni12P5 Cluster with Ni12P6 Cluster in the Catalytic Performance

As discussed above, for the C4H10 formation in Reaction 3, on the optimal reaction pathway, the rate constants of kC4H10 over Ni12P5 cluster are calculated to be 2~3 order of magnitude larger than that over Ni12P6 cluster [19], if the concentration of each reactant is the standard concentration of 1.0 mol dm−3. Therein, over Ni12P6 cluster, the crucial transition state F3-TS3 is associated with the formation of C1–H bond. The energy barrier of F3-IM5 → F3-TS3 over Ni12P5 cluster is 37.3 kJ mol−1, which is apparently lower than that (59.3 kJ mol−1) over Ni12P6 cluster. It is inferred that Ni12P5 cluster exhibits better catalytic activity than Ni12P6 cluster toward C1–H bond formation.
For the C3H8 formation in Reaction 2, on the typical reaction pathway P-CO-ene, the formation rate of rC3H8 over Ni12P5 cluster are computed to be 5~2 order of magnitude smaller than that over Ni12P6 cluster [19], if the concentration of each reactant is the standard concentration of 1.0 mol dm−3. Wherein, the C1–C2 bond cleavage is characteristic of F5-TS2. The energy barrier of F5-IM3 → F5-TS2 over Ni12P6 cluster is 31.0 kJ mol−1 [19], which is obviously lower than that (66.7 kJ mol−1) over Ni12P5 cluster. It is indicated that Ni12P6 cluster displays better catalytic activity than Ni12P5 cluster toward C1–C2 bond cleavage.
To gain insight into the chemical nature for the difference in catalytic performance between Ni12P5 cluster with Ni12P6 cluster, the corresponding Mulliken charges for F3-TS3 and F5-TS2 over Ni12P5 cluster and for 3-TS3 and 5-TS2 over Ni12P6 cluster are analyzed in Figure 12.
As shown in Figure 12a, over Ni12P5 cluster, the Mulliken charges of both Ni1~Ni12 moiety and P1~P5 moiety are −2.902 and +2.902, respectively. As depicted in Figure 12b, over Ni12P6 cluster, the Mulliken charges of both Ni1~Ni12 moiety and P1~P6 moiety are −3.217 and +3.217, respectively. It is indicated that the charge of catalytically active Ni-site over Ni12P5 cluster is less negative than that over Ni12P6 cluster. Over both Ni12P5 and Ni12P6 clusters, the valence electrons of P atom are partly filled into the empty orbit of Ni atom, making the Ni1~Ni12 moiety negative charge. That is to say, over NixPy cluster, the more P atoms, the more negative the charge of nickel atoms.
As displayed in Figure 12c,d, the charge of catalytically active Ni3-site in F3-TS3 is +0.107 over Ni12P5 cluster, whereas the charge of catalytically active Ni1-site in 3-TS3 is –0.118 over Ni12P6 cluster. Over Ni12P5 cluster, the Ni3–site on three-membered cyclic F3-TS3 is positive charge, which weakens the Ni3–H interaction and then induces it easy for the formation of C1–H bond. Alternatively, over Ni12P6 cluster, the Ni1-site on three-membered cyclic 3-TS3 is negative charge, which enhances the Ni1–H interaction and then makes it difficult for the formation of C1–H bond.
As depicted in Figure 12e,f, the charges of catalytically active Ni3- and Ni4-sites in F5-TS2 are +0.071 and +0.142 over Ni12P5 cluster, respectively, whereas the charge of catalytically active Ni2- and Ni3-sites in 5-TS2 are −0.048 and −0.071 over Ni12P6 cluster, respectively. Over Ni12P5 cluster, both Ni3- and Ni4-sites on four-membered cyclic F5-TS2 are positive charge, which abates the Ni3–C1 and Ni4–C1 interactions and then impels it difficult for the cleavage of C1–C2 bond. On the other hand, over Ni12P6 cluster, both Ni2- and Ni3-sites on five-membered 5-TS2 are negative charge, which boosts the Ni2–C1 and Ni3–C1 interactions and then causes it ready for the cleavage of C1–C2 bond.
Shortly, over NixPy cluster, the fewer the P atoms, the less negative the charge of Ni atoms, the weaker the Ni–H interaction, and the easier the formation of C–H bond. Alternatively, the greater the P atoms, the more negative the charges of Ni atoms, the stronger the Ni–C interaction, and the easier the cleavage of C–C bond. In a word, Ni12P5 cluster exhibits higher catalytic activity than Ni12P6 cluster toward the hydrodehydration of butyric acid to n-butane, owing to its less negative charge of Ni-sites. On the other hand, Ni12P6 cluster displays higher catalytic activity than Ni12P5 cluster toward both direct decarbonylation and dehydration of butyric acid to propylene, because of its more negative charge of Ni-sites.

4. Conclusions

Ni12P5 cluster is preferred to model Ni12P5-based catalyst, with butyric acid as the reactant model, for the deoxygenation of palmitic acid over Ni12P5-based catalyst. The conclusions are as follows.
There exist two main reactions, i.e., Reaction 2 of C3H7COOH + H2 → C3H8 + CO + H2O and Reaction 3 of C3H7COOH + 2H2 → C4H10 + 2H2O, whereas Reaction 1 of C3H7COOH → C3H8 + CO2 should be ruled out. Propane originates from P-CO-nal with S3 and S4 via butanal intermediate in Reaction 2, where the rate-determining reaction step is related to the formation of HO–H bond. n-Butane comes from P-C4-non-ene with S3, S8 and S9 via both butanal and n-butanol intermediates and not via butylene in Reaction 3, where the rate-determining reaction step is related to the formation of C–H bond. From the crucial butanal intermediate, decreasing the temperature is profitable to the hydrodehydration to n-butane, and increasing the temperature is beneficial to the decarbonylation to propane. In addition, CO roots only in n-butanal through decarbonylation, whereas CO2 is excluded from butyric acid through decarboxylation.
Compared with Ni12P6 cluster, Ni12P5 cluster displays higher catalytic activity toward the hydrodehydration of butyric acid to n-butane, thanks to its less negative charge of Ni-sites. Over NixPy cluster, the fewer the P atoms, the less negative the charge of Ni atoms, the weaker the Ni–H interaction, and the easier the formation of C–H bond. Alternatively, Ni12P5 cluster shows lower catalytic activity toward both direct decarbonylation and dehydration of butyric acid to propylene, because of its less negative charge of Ni-sites. The fewer the P atoms, the less negative the charges of Ni atoms, the weaker the Ni–C interaction, and the more difficult the cleavage of C–C bond.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/catal12050569/s1, Figure S1: Arrhenius plots of rate constants for the crucial reaction step (F2-IM2 → F2-TS2) in the reaction stage C3H7COOH → C3H8 + CO2 (S2) catalyzed Ni12P5 cluster; Figure S2: Arrhenius plots of rate constants for the crucial reaction step (F3-IM4 → F3-TS4 + C3H7CHO) in the reaction stage C3H7COOH + H2 → C3H7CHO + H2O (S3) catalyzed by Ni12P5 cluster; Figure S3: Arrhenius plots of rate constants for the crucial reaction step (F4-IM3 → F4-TS3 + CO) in the reaction stage C3H7CHO → C3H8 + CO (S4) catalyzed by Ni12P5 cluster; Figure S4: Arrhenius plots of rate constants for the crucial reaction step (F5-IM4 → F5-TS3 + CO) in the reaction stage C3H7COOH → C2H5CH2OH+ CO (S5) catalyzed by Ni12P5 cluster; Figure S5: Arrhenius plots of rate constants for the crucial reaction step (F5-IM4 → F7-TS2 + CO) in the reaction stage C3H7COOH → CH3CH=CH2 + H2O + CO (S5/7a) catalyzed by Ni12P5 cluster; Figure S6: Arrhenius plots of rate constants for the crucial reaction step (F8-IM3 → F8-TS3) in the reaction stage C3H7CHO + H2 → C3H7CH2OH catalyzed by Ni12P5 cluster; Figure S7: Arrhenius plots of rate constants for the crucial reaction step (F9-IM3 → F9-TS2) in the reaction stage C3H7CH2OH + H2 → C4H10 + H2O catalyzed by Ni12P5 cluster; Figure S8: Arrhenius plots of rate constants for the crucial reaction step (F10-IM5 + H2O → F10-TS1) in the reaction stage C3H7CH2OH + H2 → C4H10 + H2O (S10a + S10b) catalyzed by Ni12P5 cluster; Figure S9: Arrhenius plots of rate constants for the crucial reaction step (F3-IM4 + H2 → F8-TS3 + H2O) in the reaction stage C3H7COOH + 3H2 → C4H10 + 2H2O (S3 + S8 + S9) catalyzed by Ni12P5 cluster; Table S1: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO2↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO2) of various species with respect to the reactants for the reaction of C3H7COOH → C3H8 + CO2 catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S2: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree) and relative energies (Gr, kJ mol−1) of various species with respect to the reactants for the reaction of C3H7COOH + H2 → C3H7CHO + H2O catalyzed by Ni12P5 at GGA-PBE/DNP, DSPP level; Table S3: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO) of various species with respect to the reactants for the reaction of C3H7CHO → C3H8 + CO catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S4: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO) of various species with respect to the reactants for the reaction of C3H7COOH → C2H5CH2OH + CO catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S5: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO) of various species with respect to the reactants for the reaction of C2H5CH2OH + H2 → C3H8 + H2O catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S6: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO) of various species with respect to the reactants for the reaction of C2H5CH2OH → CH3CH=CH2 + H2O catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S7: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree), relative energies (Gr, kJ mol−1) and relative energies (Gr(CO↑), kJ mol−1, under the experimental condition of 10−5 atm pressure of CO) of various species with respect to the reactants for the reaction of CH3CH=CH2 + H2 → C3H8 catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S8: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree) and relative energies (Gr, kJ mol−1) of various species with respect to the reactants for the reaction of C3H7CHO + H2 → C3H7CH2OH catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S9: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree) and relative energies (Gr, kJ mol−1) of various species with respect to the reactants for the reaction of C3H7CH2OH + H2 → C4H10 + H2O catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S10: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree) and relative energies (Gr, kJ mol−1) of various species with respect to the reactants for the reaction of C3H7CH2OH → CH3CH2CH=CH2 + H2O catalyzed over Ni12P5 cluster at GGA-PBE/DNP, DSPP level; Table S11: Sum of electronic energies (Et, hartree), free energies (G0, hartree), sum of electronic and free energies (Gc, hartree) and relative energies (Gr, kJ mol−1) of various species with respect to the reactants for the reaction of CH3CH2CH=CH2 + H2 → C4H10 catalyzed by Ni12P5 cluster at GGA-PBE/DNP, DSPP level. Refs. [29,30,31,32,33,34,36] are cited in the Supplementary Materials.

Author Contributions

The manuscript was written through the contributions of all the authors. S.F. is responsible for the main computations and analysis, D.L., T.L. and L.L. are responsible for some computations and analysis, H.Y. is responsible for the design, analysis, and writing, and C.H. is responsible for the design and revision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No: 22073064) and the 111 Project (No: B17030).

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful for financial support by the National Natural Science Foundation of China (No: 22073064) and the 111 Project (No: B17030).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Ni12P5 cluster model, pink and purple balls represent P and Ni atoms, respectively. (a) Top view; (b) side view; and (c) for brevity, the atoms out of the reaction site are not shown.
Figure 1. Ni12P5 cluster model, pink and purple balls represent P and Ni atoms, respectively. (a) Top view; (b) side view; and (c) for brevity, the atoms out of the reaction site are not shown.
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Scheme 1. Deoxygenation pathways of butyric acid over Ni12P5 cluster.
Scheme 1. Deoxygenation pathways of butyric acid over Ni12P5 cluster.
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Figure 2. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S2 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO2↑ shows the CO2 pressure with 10−5 atm under experimental condition.
Figure 2. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S2 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO2↑ shows the CO2 pressure with 10−5 atm under experimental condition.
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Figure 3. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S3 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
Figure 3. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S3 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
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Figure 4. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S4 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
Figure 4. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S4 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
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Figure 5. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stages S5 (the yellow line) and S5/7a (the black line) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
Figure 5. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stages S5 (the yellow line) and S5/7a (the black line) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
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Figure 6. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S6 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
Figure 6. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S6 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
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Figure 7. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stages S7a (a) and S7b (b) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
Figure 7. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stages S7a (a) and S7b (b) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden. CO↑ shows the CO pressure with 10−5 atm under experimental condition.
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Figure 8. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S8 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
Figure 8. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) for reaction stage S8 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
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Figure 9. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) reaction stage S9 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
Figure 9. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) reaction stage S9 over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
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Figure 10. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) reaction stages S10a (a) and S10b (b) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
Figure 10. The potential energy curves with the relative Gibbs free energy (Gr, kJ mol−1) and the geometric structures with bond lengths (Å) reaction stages S10a (a) and S10b (b) over Ni12P5 cluster at GGA−PBE/DSPP, DNP level. For brevity, some hydrogen atoms are hidden.
Catalysts 12 00569 g010
Figure 11. The main reaction pathways for the deoxygenation of butyric acid over Ni12P5 cluster.
Figure 11. The main reaction pathways for the deoxygenation of butyric acid over Ni12P5 cluster.
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Figure 12. Mulliken charges for (a) Ni12P5 cluster, (b) Ni12P6 cluster, (c) F3-TS3 over Ni12P5 cluster, (d) 3-TS3 over Ni12P6 cluster, (e) F5-TS2 over Ni12P5 cluster, and (f) 5-TS2 over Ni12P6 cluster, respectively.
Figure 12. Mulliken charges for (a) Ni12P5 cluster, (b) Ni12P6 cluster, (c) F3-TS3 over Ni12P5 cluster, (d) 3-TS3 over Ni12P6 cluster, (e) F5-TS2 over Ni12P5 cluster, and (f) 5-TS2 over Ni12P6 cluster, respectively.
Catalysts 12 00569 g012
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Fu, S.; Li, D.; Liu, T.; Liu, L.; Yang, H.; Hu, C. Mechanism Insight into Catalytic Performance of Ni12P5 over Ni2P toward the Catalytic Deoxygenation of Butyric Acid. Catalysts 2022, 12, 569. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050569

AMA Style

Fu S, Li D, Liu T, Liu L, Yang H, Hu C. Mechanism Insight into Catalytic Performance of Ni12P5 over Ni2P toward the Catalytic Deoxygenation of Butyric Acid. Catalysts. 2022; 12(5):569. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050569

Chicago/Turabian Style

Fu, Shuai, Dan Li, Tinghao Liu, Lijuan Liu, Huaqing Yang, and Changwei Hu. 2022. "Mechanism Insight into Catalytic Performance of Ni12P5 over Ni2P toward the Catalytic Deoxygenation of Butyric Acid" Catalysts 12, no. 5: 569. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050569

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