Mechanistic and microkinetic study of nonoxidative coupling of methane on Pt-Cu alloy catalysts: From single-atom sites to single-cluster sites

doi:10.1016/S1872-2067(23)64408-0

Abstract

Abstract:

Desirable catalysts possessing the ability to selectively break C-H bond and controllably catalyze C-C bond formation are highly demanded for the nonoxidative coupling of methane (NOCM). Herein, a series of Pt-Cu alloy catalysts including Pt₁©Cu(111), Pt₂©Cu(111) and Pt₃©Cu(111) are deliberately designed and systematically studied for NOCM. Density functional theory calculations reveal that the Pt₁, Pt₂, and Pt₃ sites on Cu(111) can selectively break the C-H bond to generate CH₃, CH₂, and CH species, respectively. However, direct coupling of corresponding CH_x (x = 3, 2, 1) to form C₂H₆, C₂H₄, and C₂H₂ are favorable on Pt₃, Pt₁, and Pt₂ sites on Cu(111), respectively. The different reactivity trends of the three Pt sites mainly originate from the varying bonding abilities of CH_x species at the Pt sites. Microkinetic modeling manifests that the Pt₁©Cu(111) is the most active for methane dissociation (TOF = 2.98 s^-1 at 1000 K) and can selectively convert methane into ethylene with the highest selectivity up to 96.2% at 750 K. Moreover, Pt₁©Cu(111) also shows superb stability under reaction conditions. Overall, our studies not only provide a comprehensive understanding of the reaction mechanism of NOCM on Pt single-atom sites (SASs) and Pt single-cluster sites (SCSs) but also predict that Pt SASs are advantageous over Pt SCSs for NOCM.

Key words: Methane, Microkinetic modeling, Single-atom catalyst, Single-cluster catalyst, Density functional theory

Zheng-Qing Huang, Shu-Yue He, Tao Ban, Xin Gao, Yun-Hua Xu, Chun-Ran Chang. Mechanistic and microkinetic study of nonoxidative coupling of methane on Pt-Cu alloy catalysts: From single-atom sites to single-cluster sites[J]. Chinese Journal of Catalysis, 2023, 48: 90-100.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(23)64408-0

https://www.cjcatal.com/EN/Y2023/V48/I5/90

Figures/Tables 11

Fig. 1. Top views of the optimized structures of Pt1?Cu(111) (a), Pt2?Cu(111) (b), and Pt3?Cu(111) (c) surfaces. Only the first layer of the slab models is shown.

Fig. 2. Optimized structures of CH4 (a), CH3 (b), CH2 (c), CH (d), and C (e) adsorbed on the (111) surfaces of Pt1?Cu, Pt2?Cu, and Pt3?Cu, Cu, and Pt. The data without a bracket is the adsorption energy of CHx referring to the energy of CHx in the gas phase, whereas the data with a bracket is the adsorption energy of CHx referring to the energies of CH4 and H2 in the gas phase. The adsorption energies of CHx species and H atoms are also displayed in Table S1 and Fig. S1.

Fig. 3. (a) Linear scaling relationships between adsorption energies of CHx and that of C atom on the metal (111) surfaces of Pt1?Cu, Pt2?Cu, and Pt3?Cu, Cu, and Pt; (b) Intermediacy (α) of CHx adsorbed on the metal (111) surfaces. The fitting results and calculated intermediacies are listed in Tables S2 and S3, respectively. (c) The relative energy difference of CHx, ??E(CHx) = ?E(CHx) - ?E(CHx on Cu), on three Ptn?Cu catalysts referring to Cu. The energy difference of CHx is calculated as: ?E(CHx) = [E(CHx-1) - E(CHx)] - [E(CHx) - E(CHx+1)].

Fig. 4. (a) Energy profile of methane decomposition. (b) Br?nsted-Evans-Polanyi relations for methane decomposition on the metal (111) surfaces of Cu, Pt, Pt1?Cu, Pt2?Cu, and Pt3?Cu. The structures of corresponding intermediates are shown in Fig. 2 and the structures of corresponding transition states are displayed in Figs. S3 and S4.

Fig. 5. Optimized structures of transition states (TS) and final states (FS) in C-C coupling of two CH3 to form C2H6 (a), two CH2 to form C2H4 (b), and two CH to form C2H2 (c) on Pt1?Cu, Pt2?Cu, and Pt3?Cu (111) surfaces. The side-view structures are in the upper panel and the top-view structures are in the lower panel. The blue arrows in the top-view structure are the directions of the side-view structures.

Fig. 6. Br?nsted-Evans-Polanyi relations for CH3* coupling to form ethane (a), CH2* coupling to form ethylene (b), and CH* coupling to form acetylene (c).

Fig. 7. Reaction networks of nonoxidative coupling of methane to C2 hydrocarbons for microkinetic modeling. The asterisk (*) represents the Pt site, and the pound sign (#) represents the Cu site. The hydrogen involved in the reaction networks is not shown in this Figure.

Fig. 8. (a) TOF of methane consumption on (111) surfaces of Pt1?Cu, Pt2?Cu, Pt3?Cu and Cu. (b) Selectivity of C2H4 and C2H2 on (111) surfaces of Pt1?Cu, Pt2?Cu, and Pt3?Cu. The pressure of methane is 1.00 bar, and the pressures of other species are zero in (a) and (b). (c) TOF of methane consumption and selectivity of C2H6, C2H4 and C2H2 on Pt1?Cu(111). The data in solid dot is at P(CH4) = 1.00 bar, and P(H2) = 0.00 bar, and the data in hollow dot is at P(CH4) = 0.99 bar, and P(H2) = 0.01 bar.

Fig. 9. Advantageous reaction pathways of NOCM on Pt1?Cu(111) at 1000 K in pure methane [P(CH4) = 1.00 bar] (a) and in methane-hydrogen mixture [P(CH4) = 0.99 bar, P(H2) = 0.01 bar] (b). The numbers are the TOF in s-1 of corresponding elementary step. The elementary steps with TOF less than 10 s-1 in (a) and less than 0.10 s-1 in (b) are not listed in this Figure. The reactions represented by thicker red arrows are the advantageous pathways. (c) Degree of rate control analysis on methane consumption under different reaction conditions. (d) Degree of selectivity control analysis on ethylene production under different reaction conditions. States with absolute values greater than 0.02 in (c) and (d) are shown. The reaction rates of all the elementary steps and the labels representing the states are listed in Table S7.

Fig. 10. (a) First-principle energy of a nine-layer (3 × 3) Cu slab with one Cu atom replaced by a Pt atom in a vacuum or adsorbed by CH3, C, or H atom. The zero-energy reference is the Pt atom in the first atom layer of the Cu slab. (b) The top-view and side-view structures of the Cu slab with one Cu atom replaced by a Pt atom adsorbed by CH3, C, or H atom.

Fig. 11. (a) First-principle energy of transformation from two isolated Pt atoms to one Pt dimer in a vacuum or adsorbed by CH3, C, or H atom. (b) First-principle energy of the transformation from three isolated Pt atoms to one Pt trimer in a vacuum or adsorbed by CH3, C, or H atom.

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