Polyoxometalates on Nitrogen-Containing Carbon Supports as Catalysts for Selective Oxidation

Abstract

Polyoxometalates (POMs) represent an important class of metal oxygen clusters of the early transition metal elements. Supported POMs are good model catalysts for transition metal oxide catalysts because of their stable and well-known structure as well as the possibility of controlling acid site population in these catalysts. Physical, chemical and catalytic properties of POMs, e.g., H3PMo12O40 (HPM) supported on nitrogen-containing carbon materials were investigated in this work to understand support effects. Activated carbon (C), nitrogen-doped graphitic carbons, N-C-1000 (2 N-atom%) and N-C-600 (19 N-atom%), and mesoporous graphitic carbon nitride, mpgC3N4 (53 N-atom%) were used to study the influence of both nitrogen content and different nitrogen species.

Our results showed that the polyoxometalate framework was retained on all the four supports tested in this work, and the ability to disperse POMs without crystallite formation followed the trend N-C-600 < N-C-1000 ≈ activated carbon (C) < mpgC3N4. POMs preferentially interact with pyridinic nitrogen and surface amino groups; the latter lead to ammonium POM salt crystallites observed using X-ray Diffraction (XRD), while the former could be one of the reasons that mpgC3N4 can produce monolayer POM dispersions. At low coverage, POMs are molecularly dispersed on all four supports. At comparable POM coverages, the H+/POM ratio followed the trend C ≈ N-C-1000 > N-C-600 > mpgC3N4. The POM loading effect on the POM-support interaction was also investigated. On C, as POM loading is increased, both population and strength of the acid site increased as shown by ammonia temperature programmed desorption (NH3-TPD), while on N-C-600 and mpgC3N4 such increase was not observed.

C and N-C-1000 supported POM catalysts with comparable POM surface coverages showed similar dehydration/oxidation activities in methanol oxidation. However, N-C-600 and mpgC3N4 exhibited lower activities for both reactions. The simultaneous decrease for dehydration and oxidation activities as supports with higher nitrogen content were used confirmed that protons played an important part for methanol oxidation using supported POM catalysts. The selectivity for oxidation products can be improved when a small but finite number of acid sites exist on the catalyst surface. For supports used in this work, N-C-600 provides an optimal number of acid sites and thus results in the highest selectivity for the sum of all oxidation products (COX excluded), as well as the highest selectivity for formaldehyde (80%).

To further understand how different supports can affect the reaction outcome, mechanism-based kinetic models were constructed for both primary and secondary products in methanol dehydration and oxidation for the first time in the literature and are shown to have worked well for HPM/C and HPM/N-C-600 catalysts. Based on the proposed mechanism, formaldehyde (HCHO) and methyl formate (MF) may form on the same site (likely on the bridging O in the vicinity of H+); dimethyl ether (DME) and dimethoxymethane (DMM) may form on the same site (H+). The rate constant for the rate determining steps in DME and HCHO formation tracked the decreasing trend observed when catalysts were compared at similar POM surface coverage levels. N-C-600 supported catalysts exhibited lower rates of DMM formation, leading to higher HCHO selectivity.

This is the first systematic study of support effects for nitrogen-containing carbon support POM catalysts, which can provide guidance in using supports to tune the properties of supported POMs other than changing the identity of its center/framework atom and counter cations.