Water denitration over titania-supported Pt and Cu by combined photocatalytic and catalytic processes: Implications for hydrogen generation properties in a photocatalytic system

https://doi.org/10.1016/j.jece.2022.107129Get rights and content

Highlights

  • Water denitration was carried out by combined photocatalytic and catalytic processes.

  • The in-situ generated solar H2 by water splitting reduces efficiently nitrate ions.

  • Deposition of well-defined Pt Np’s (~10 nm) onto Pt-Cu/TiO2 enhances production of H2.

Abstract

The present work provides a new approach to the water denitration process and a facile strategy of catalyst preparation for the efficient hydrogen generation. The main focus of the research reported here was nitrate removal from aqueous solutions and simultaneous hydrogen generation under UV-Vis-light irradiation. This is a very important aspect in case of practical applications, eliminating the external source of hydrogen. Aiming to get a deeper understanding on the role of surface structures (size and shape of nanoparticles) towards the reduction mechanism and to establish basic principles for an efficient process, Pt-Cu/TiO2 and Pt-Cu/TiO2 modified with well-defined Pt nanoparticles were employed. The synthesized materials were characterized using various physicochemical techniques and tested comparatively for: (i) nitrate catalytic reduction by hydrogen (dark reaction) and (ii) nitrate photocatalytic reduction by in-situ generated solar hydrogen. In order to enhance overall denitration reaction by combined photocatalytic and catalytic processes, photo-generated charges and in-situ generated H2 as reducing agent were used. Improvement of catalytic performances of nitrate hydrogenation reaction (~100% NO3¯ conversion) related to intimate contact between Pt and Cu was obtained. The in-situ generated H2 by water splitting over the studied catalysts reduces efficiently NO3¯ ions. Enhanced photocatalytic activity toward solar H2 production by deposited well-defined Pt nanoparticles (~10 nm) was achieved. In order to make possible decontamination of polluted waters using in-situ generated H2 under light exposure, the future optimization of such photo-catalytic systems looks promising.

Introduction

Nitrate (NO3¯) can be considered to be one of the most common problems regarding possible contaminants of groundwater. Water-soluble NO3¯ concentrations in the groundwater are rapidly increasing due to excessive fertilization in agriculture, industrial and domestic wastewater. Water pollution with nitrates results in eutrophication and could affect human health through various diseases (blue-baby syndrome, cancer, etc.) [1], [2].

One of the promising technologies capable of reducing NO3¯ to N2 by catalytic hydrogenation of nitrate, have been proposed for the first time by Vorlop et al. [3]. It was reported also [4], [5] the reaction mechanism based on catalytic reduction over bimetallic catalysts using a noble metal (Pd, Pt) and a transition metal (Cu, Sn, In) in the presence of hydrogen as the reductant [6], [7], [8]. Platinum-based catalysts have quite good activity and selectivity for nitrite reduction [9]. To reduce nitrate, it is necessary to activate the precious metal by addition of a promoting second metal [5], [9], [10]. The presence of Cu allows the conversion of nitrate to nitrite, while the further reduction of nitrite to nitrogen or ammonium occurs mainly on precious metal active sites (Pd) [9], [11], [12]. The metals may be present in the form of nanoparticles in order to expose a catalytically more active surface. Loading of TiO2 surface with noble metals such as Pt, Ag, and Au has been investigated from the early times of photocatalysis to increase the photocatalytic activity [13]. Some studies have pointed out the relevance of the structure and geometry of the metal particles in the final mechanism of nitrate reduction [10], [14], [15]. The ability of the (photo) catalytic process to succeed could largely depend on morphology of noble metal deposits (particle size and shape), therefore control of structural properties of metallic nanoparticles is critical to (photo)catalytic performances. Some studies have focused on photochemically produced hydrogen [16] or were concerned with understanding the effect of H2 generation on photoreduction of aqueous nitrate [17], [18].

The present work provides a new approach to the water denitration process and a facile strategy of catalyst preparation for the efficient hydrogen generation. We attempted to improve overall denitration process by using photogenerated charges and in-situ generated hydrogen as reducing agent. This is a very important aspect, because in case of practical application, there is no need for external hydrogen source. The technology becomes widely applicable and the operational costs can be thus reduced significantly. This research aims to tackle nitrate removal from aqueous phase by two approaches: (i) nitrate catalytic reduction by H2 in dark conditions and (ii) nitrate photocatalytic reduction by the in-situ generated solar H2. The present investigation aimed also to get a deeper understanding on the role of surface structures (size and shape of nanoparticles) towards the reduction mechanism and to establish basic principles for an efficient process.

For these purposes, (Pt-Cu)/TiO2 and (Pt-Cu)/TiO2 modified with well-defined Pt nanoparticles were synthesized, characterized using various physicochemical techniques and tested comparatively for nitrate photocatalytic and catalytic reduction reactions. The aim was to favor the selective reduction pathway of nitrate to molecular nitrogen. There are indications that in structure sensitive reactions N-N bond formation rates could be related to preferential orientation of platinum nanocrystal facets [19]. This principle should apply also to nitrate reduction to molecular nitrogen. The idea was to prevent the formation of ammonium ions (NH4+), which is known to be favored by deep hydrogenation of NO3¯ over nanometric sized Pt-Cu nanoparticles.

Section snippets

Catalysts preparation

Two methods of preparation were used: (i) successive impregnation of TiO2 support with metals precursors, and (ii) deposition of well-defined Pt nanoparticles obtained by the preparation procedure described elsewhere [20] onto previously impregnated bimetallic catalyst. In both cases, the calculated copper loading was 0.5 wt%. For comparison purposes, impregnated monometallic catalysts were prepared. The catalysts were prepared with support TiO2 P-25 (Aerosil, Japan, BET surface area 50 m2/g).

Structural property (SEM, TEM and XRD)

SEM images of the synthesized (Pt-Cu)imp/TiO2 and Ptnp-(Pt-Cu)imp/TiO2 are shown in Fig. 2-a and Fig. 2-b, respectively, displaying the morphologies of the calcined catalysts.

The SEM micrograph of (Pt-Cu)imp/TiO2 shows particles with uniform distribution, spherical morphology and slight agglomeration (Fig. 2-a). The SEM micrograph of Ptnp-(Pt-Cu)imp/TiO2 (Fig. 2-b) shows a higher degree of particles agglomeration.

EDAX provides localized elemental information in case of bimetallic nanoparticles.

Conclusions

The point ideas established from the present research are as follows:

  • (1)

    Overall denitration reaction improvement by combined photocatalytic (responsible for the in-situ H2 generation) and catalytic (accounting for reduction of nitrate by H2) processes, using photogenerated charges and in-situ generated H2 as reducing agent has been achieved.

  • (2)

    Improved catalytic performance is related to intimate contact between Pt and Cu. Almost 100% nitrate conversion was attained within 150 min over the (Pt-Cu)imp

CRediT authorship contribution statement

Anca Vasile: Conceptualization, Investigation, Writing – original draft. Florica Papa: Resources, Investigation, Supervision. Veronica Bratan: Investigation. Cornel Munteanu: Investigation. Mircea Teodorescu: Investigation. Irina Atkinson: Investigation. Mihai Anastasescu: Investigation. Daisuke Kawamoto: Investigation. Catalin Negrila: Investigation. Cristian D. Ene: Investigation. Tanta Spataru: Investigation. Ioan Balint: Conceptualization, Funding acquisition, Project administration,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support of the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI) Romania, Grants 46 PCCDI/2018 (MALASENT) and PN III-26PTE/2020 (DENOX), is gratefully acknowledged.

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