Introducing Porosity into Refractory Molybdenum Boride through Controlled Decomposition of a Metastable Mo–Al–B Precursor

Abstract

The high temperatures typically required to synthesize refractory compounds preclude the formation of highenergy morphological features, including nanoscopic pores that are beneficial for applications, such as catalysis, that require higher surface areas. Here, we demonstrate a low-temperature multistep pathway to engineer mesoporosity into a catalytic refractory material. Mesoporous molybdenum boride, α-MoB, forms through the controlled thermal decomposition of nanolaminate-containing sheets of the metastable MAB (metal−aluminum−boron) phase Mo 2 AlB 2 and amorphous alumina. Upon heating, the Mo 2 AlB 2 layers of the Mo 2 AlB 2 −AlO x nanolaminate, which is derived from MoAlB, begin to bridge and decompose, forming inclusions of alumina in a framework of α-MoB. The alumina can be dissolved in aqueous sodium hydroxide in an autoclave, forming α-MoB with empty and accessible pores. Statistical analysis of the morphologies and dimensions of the pores reveals a correlation with grain size, which relates to the pathway by which the alumina inclusions form. The transformation of Mo 2 AlB 2 to α-MoB is topotactic due to crystal structure relationships, resulting in a high density of stacking faults that can be modeled to account for the observed experimental diffraction data. Porosity was validated by comparing surface areas and demonstrating catalytic viability for the hydrogen evolution reaction.

Soft Chemistry of Hard Materials: Low-Temperature Pathways to Bulk and Nanostructured Layered Metal Borides

“…In this case, the high-energy thin alumina sheets between layers of metastable Mo 2 AlB 2 begin to collapse and form larger lower-energy inclusions (Figure c). Mo 2 AlB 2 simultaneously decomposes to the stable form of MoB, expelling Al that combines with residual oxygen and adds to the growing alumina inclusions, which concomitantly crystallize . This process results in MoB surrounding nanoscale inclusions of Al 2 O 3 (Figure d), which can be etched to form mesoporous MoB with empty nanoscopic pores (Figure e).…”

“…Panel (a) is adapted with permission from ref , copyright 2019 American Chemical Society. Panels (b–d) are adapted with permission from ref , copyright 2023 American Chemical Society.…”

“…Mo 2 AlB 2 simultaneously decomposes to the stable form of MoB, expelling Al that combines with residual oxygen and adds to the growing alumina inclusions, which concomitantly crystallize. 4 This process results in MoB surrounding nanoscale inclusions of Al 2 O 3 (Figure 7d), which can be etched to form mesoporous MoB with empty nanoscopic pores (Figure 7e). Heating collapses the planar sheets of Mo 2 AlB 2 by expelling the Al (conceptually analogous to Al deintercalation) and condenses the resulting MoB sheets (conceptually analogous to the topochemical condensation of Ti 2 InB 2 to TiB) to form a three dimensionally interconnected nanostructure.…”

“…Chem. Soc.202314514231432 , which is metastable, was further decomposed thermally into a network of MoB surrounding nanoscale pockets of alumina, which could be chemically etched to form mesoporous MoB.…”

Two-Dimensional Molybdenum Boride (MBene) Mo4/3B2Tx with Broadband and Termination-Dependent Ultrafast Nonlinear Optical Response

“…Inspired by MXenes, a new class of 2D materials derived from transition metal borides was recently synthesized, which are termed as MBenes. − However, the few-layer MBenes are not easy to obtain. On the one hand, the ordinary M-B layers prefer to form boron oxide in a long-term hydrofluoric acid (HF) etch. , On the other hand, the common MAB phase (the precursor of MBene) contains two layers of Al atoms between the M-B layers, which is difficult to be etched by HF, while the MAB phase with only one layer of Al atoms is thermodynamically metastable and challenging in preparation. , Very recently, by changing the precursor into a more reactive i-MAB phase (ternary laminated boride phases with in-plane chemical ordering) and shortening the HF etching time, Zhou et al obtained individual single-layer Mo 4/3 B 2 T x (T = O, F, OH) MBene with the ordered metal vacancies on their surface. As revealed, Mo 4/3 B 2 T x has shown application prospects in electrochemical hydrogen evolution, , surface-enhanced Raman spectroscopy, ultrafast photonics, and other fields.…”